Ventilatory Workload or Demand

Patients who need mechanical ventilation often have a ventilatory workload or demand that exceeds their ventilatory capacity. This is the most common cause of ventilator dependence.1,2 The term ventilatory workload refers to the amount of work that the respiratory muscles are asked to perform to provide an appropriate level of ventilation. A patient’s total ventilatory workload is primarily determined by the following: (1) the level of ventilation needed; (2) the compliance of the lungs and thorax; (3) the resistance to gas flow through the airways, and (4) any imposed work of breathing (WOB_i) due to mechanical factors.^1,7

The level of ventilation required is determined by the following: (1) the metabolic rate; (2) the central nervous system (CNS) drive; and (3) the ventilatory dead space. Common causes of an increased demand for ventilation include increased carbon dioxide production (i.e., fever, shivering, agitation, trauma, or sepsis) and increased dead space (i.e., pulmonary emboli or chronic obstructive pulmonary disease [COPD]). Other common causes of increased ventilatory demand include metabolic acidosis, severe hypoxemia, pain, and anxiety.

Compliance is determined by the elastic nature of the lung–thorax system. Resistance is largely related to the nature of the conducting airways. Common causes of decreased lung compliance include atelectasis, pneumonia, pulmonary edema, acute lung injury, and acute respiratory distress syndrome. Thoracic compliance may be reduced because of obesity, ascites, or abdominal distention. Airway resistance increases with bronchospasm, excessive secretions, and mucosal edema.

Mechanical factors that can increase the work of breathing include artificial airways (i.e., endotracheal and tracheotomy tubes), partial obstruction of the airway, ventilator circuits, demand flow systems, auto-PEEP, and inappropriate ventilator flow and sensitivity settings. Factors that may increase ventilatory workload are summarized in Box 47-1.

Ventilatory Capacity

Ventilatory capacity is determined by CNS drive, ventilatory muscle strength, and ventilatory muscle endurance.¹⁸ Most patients who are being withdrawn from ventilatory support have a normal or an increased drive to breathe. Patients with neuromuscular disorders and those who are receiving sedatives, narcotics, or neuromuscular blocking agents may have a reduced drive to breathe or impaired neuromuscular transmission. Patients with metabolic alkalosis, hypothyroidism, and sleep deprivation also may have reduced ventilatory drive. Box 47-2 summarizes the factors that may reduce ventilatory drive.

Muscle strength is influenced by age, sex, muscle bulk, and overall health. Malnutrition, starvation, and electrolyte imbalances (especially involving calcium, magnesium, potassium, and phosphate) can lead to ventilatory muscle weakness.18,19 Critical illness myopathy, critical illness polyneuropathy, and the prolonged use of neuromuscular blocking agents are major causes of the development of ventilatory muscle weakness in the intensive care unit (ICU).²⁰ Controlled ventilation for prolonged periods can result in ventilatory muscle discoordination and atrophy.¹⁸ Ventilatory muscle endurance is a function of energy supply versus demand. Energy supply is related to nutrition, perfusion, and cell use, whereas demand is related to the amount of work performed and is a function of minute ventilation, compliance, and resistance. Figure 47-1 summarizes the relationship between ventilatory demands and capabilities.

Global Criteria for Discontinuing Ventilatory Support

Success with the discontinuation of ventilatory support is related to the patient’s condition in four main areas1,2,7:

Simply put, when ventilatory workload or demand exceeds ventilatory capacity, successful ventilator discontinuation is unlikely.1,2 Excessive ventilatory workload may lead to ventilatory muscle fatigue. When the ventilatory muscles fatigue, they must be rested for at least 24 hours to recover.^3,4,19 Ventilatory workload increases with decreased compliance, increased airway resistance, or an increased level of ventilation. Ventilatory capacity can be reduced by ventilatory muscle fatigue and by a loss of muscle strength and endurance.

Other factors that may contribute to ventilator dependence include inadequate arterial oxygenation, poor tissue oxygen delivery, myocardial ischemia, arrhythmias, low cardiac output, and cardiovascular instability.7,16 Neurologic problems that may contribute to ventilator dependence include decreased central drive to breathe and impaired peripheral nerve transmission.³ Psychologic issues that may contribute to ventilatory dependence include the fear of removal of the life support system, anxiety, stress, depression, and sleep deprivation.³ Box 47-3 summarizes the major factors that contribute to ventilator dependence.

The Most Important Criterion

The single most important criterion to consider when evaluating a patient for ventilator discontinuation or weaning is whether there has been significant alleviation or reversal of the disease state or condition that necessitated use of the ventilator in the first place.3,7,14 The clinician should determine whether the patient’s condition is improving, whether the initial reason for providing ventilatory support is improved or resolved, and whether the patient’s clinical condition is stable. The following specific questions for patient evaluation have been suggested³:

If the patient’s condition is improving, if the alleviation or reversal of the precipitating disease state or condition has occurred, if the patient is capable of spontaneous breathing, and if the oxygenation status and hemodynamic values are stable, then a formal evaluation for ventilator discontinuation should be performed.³

Weaning Indices

Mechanical ventilation is hazardous, and unnecessary delays in ventilator discontinuation increase the associated complication rate.3,4 Unfortunately, premature ventilator discontinuation may also cause serious problems, including difficulty with reestablishing the artificial airway and serious compromise of the patient’s clinical status.^3,4 Clinical judgment has been found to be a poor guide to determining whether a patient is ready for ventilator discontinuation, and more specific indicators have been sought.⁴ Ideally, specific indicators or weaning indices would clearly show whether a patient is ready to have the ventilator removed and would help to avoid inappropriate ventilator discontinuation. Unfortunately, none of the current weaning indices are capable of predicting readiness for ventilator discontinuance with a high level of accuracy.

Traditional discontinuation indices include the PaO₂/FiO₂ ratio, the alveolar-to-arterial partial pressure of oxygen difference [P(A–a)O₂], the maximum inspiratory pressure (MIP), the vital capacity (VC), the spontaneous minute ventilation (V_Esp), and the maximum voluntary ventilation (MVV).5,21 Newer indices include the rapid, shallow breathing index (f/V_t), airway occlusion pressure (P_0.1), and measures of WOB.^6–8 Although all of these values can be useful, there are enormous discrepancies in the literature regarding their accuracy with regard to the prediction of “weanability.”^3,6,7 With respect to the more traditional discontinuation indices, vital capacity and MIP can be highly variable, whereas minute ventilation, respiratory rate (f), and f/V_t tend to be more reliable.²² However, these measures may not correlate well with discontinuation success among all patients and especially among those receiving long-term ventilatory support, the elderly, and those with major pulmonary abnormalities.^5,18,23

A comprehensive evidence-based review identified a possible role for 66 specific measurements as predictors of weaning success.⁶ Of these, eight values were found to be the most useful for the prediction of successful ventilator discontinuation.^3,6 Useful predictive measures included spontaneous respiratory rate, spontaneous tidal volume, f/V_t, minute ventilation, MIP, P_0.1, P_0.1/MIP, and a combined index called the CROP score that included compliance, rate, oxygenation, and MIP.^3,6 Unfortunately, these measures all have limitations and relatively high false-positive predictions in specific settings.

It is doubtful that a single index will be found that can be used for consistent discrimination between discontinuation success and failure.⁷ Moreover, none of these traditional indicators alone has proved useful for the prediction of improvements in patient outcome or in the selection of a particular discontinuation method.^7,10 The likely explanation for this failure to identify any consistently powerful discontinuation predictor is that patients’ conditions vary greatly and, for research purposes, clinicians already fully consider information from predictors when choosing patients for trials of the reduction or discontinuation of ventilatory support.^10,23

Notwithstanding these limitations, the measurement of discontinuation indices in the difficult-to-wean patient may provide guidance with regard to the reasons that patients fail discontinuation trials. Many find it useful to trend these data on a daily basis for those patients who require lengthy weaning times.¹⁵ Specific values for respiratory indices that are used to predict the successful discontinuation of ventilatory support are found in Table 47-1.

TABLE 47-1

Indices That Are Used to Predict the Success of Weaning and Ventilator Discontinuation

Measurement	Criterion
Oxygenation
FiO2	≤0.40 to 0.50
PEEP (cm H2O)	≤5 to 8
PaO2 (mm Hg)	≥60
SaO2 (%)	≥90
SvO2 (%)	≥60
PaO2/PAO2 ratio	≥0.35
PaO2/FiO2 ratio	>150 to 200
P(A-a)O2 (mm Hg)	<350
(% shunt)	<15% to 20%
No lactic acidosis, adequate , blood pressure
Ventilation
PaCO2 (mm Hg)	<50
pH	≥7.35
Ventilatory Mechanics
Respiratory rate (f) (breaths/min)	12 to 30
Tidal volume (VT) (ml/kg)	>5
Vital capacity (VC) (ml/kg)	>10 to 15
Static compliance (ml/cm H2O)	>25
f/VT	<105
Respiratory Muscle Strength
Maximum inspiratory force (MIF) (cm H2O)	<−20 to −30
Ventilatory Drive (Demand)
Minute ventilation () for
Normal PCO2 (L/min)	<10
VDS/VT	<0.55 to 0.60
P0.1 (cm H2O)	<6
P0.1/MIP	<0.30
Work of Breathing
Spontaneous work of breathing	<1.6 kg·m/min (<0.14 kg·m/L)
Pressure–time index	<0.15 to 0.18
Ventilatory Reserve
Maximum voluntary ventilation (MVV) (L/min)	>20; more than twice the

Data from MacIntyre NR, Cook DJ, Ely EW, et al: Evidence-based guidelines for weaning and discontinuing ventilator support: a collective task force facilitated by the American College of Chest Physicians, the American Association for Respiratory Care, and the American College of Critical Care Medicine. Chest 120:375S–395S, 2001; AHRQ publication no. 01-E010, Rockville, MD, 2000, Agency for Healthcare Research and Quality; American College of Chest Physicians: Chest 104:1833, 1993; Burns SM et al: Am J Crit Care 4:4, 1995; Sharar S: Resp Care 40:239, 1995; Bassili HR, Deitel M: JPEN J Parenter Enteral Nutr 5:161, 1981.

Ventilation

Increased thoracic cage movement during spontaneous breathing and asynchronous chest-wall-to-diaphragm movement are related to an increased workload that may lead to ventilatory muscle fatigue and failure.¹⁹ Tachypnea (i.e., more than 30 to 35 breaths/min) is a sensitive marker of respiratory distress, but it can prolong intubation if it is used as an exclusive criterion.²⁵ Irregular spontaneous breathing or periods of apnea indicate that the patient is at risk for weaning failure. Asynchronous and rapid, shallow breathing patterns—although not definitive—suggest respiratory decompensation.¹⁹

The evaluation of patients for the presence of palpable scalene muscle use during inspiration, an irregular ventilatory pattern, palpable abdominal muscle tensing during expiration, and the inability to alter the ventilatory pattern on command can be helpful for the assessment of the potential for prolonged spontaneous ventilation. Patients with none of these signs have a 90% chance of success.²⁶ Patients with one or two of these signs usually need continued support. The presence of three or more of these signs can mean that the patient’s condition is unstable and that the patient has a poor prognosis for ventilator removal.

P_0.1 is the inspiratory pressure that is measured 100 milliseconds after airway occlusion.²¹ The P_0.1 is effort independent, and it correlates well with central respiratory drive.¹⁹ Ventilator-dependent patients with COPD who have a P_0.1 of more than 6 cm H₂O tend to be difficult to wean.²¹

The f/V_t is the ratio of spontaneous breathing frequency (breaths/min) to tidal volume (liters), and it has been found to be a good predictor of discontinuation success for the care of many patients who need mechanical ventilation.4,6,23 The f/V_t has less predictive power for the care of patients who need ventilatory support for longer than 8 days, and it may be less useful for predicting discontinuation success among elderly patients.^8,23,27 Adjusting the threshold value for f/V_t to 130 or less measured at 3 hours was very effective for predicting discontinuation success among patients 70 years old and older. Despite these limitations, an f/V_t of less than 105 can be an accurate and early predictor of weaning outcome, and an f/V_t of 80 is associated with an almost 95% probability of successful discontinuation.^4,23 The ratio must be calculated during 1 minute of unsupported spontaneous breathing, and the addition of pressure support significantly reduces the predictive value of the ratio.⁴

The P_0.1/MIP ratio has been found to be a good early predictor of discontinuation success,3,19 and it may be more useful than the MIP by itself. The f/V_t also has been found to be a better predictor of discontinuation success than the MIP alone.^2,3 However, even with f/V_t of less than 105, as many as 20% of patients have false-positive results (i.e., they cannot be discontinued from ventilation despite a favorable index) as a result of unpredictable factors such as congestive heart failure, aspiration, or the development of a new pulmonary lesion.²⁸ However, some patients can be successfully discontinued from ventilatory support despite poor f/V_t values.²⁹

WOB would seem to be an excellent way to gauge spontaneous ventilatory workload. Successful weaning has been found to be less likely among patients with spontaneous WOB levels of more than 1.6 kg/m/min (16 J/min) or 0.14 kg/m/L (1.4 J/L).⁵ However, WOB may not be predictive of weaning success for specific patients.⁷ This may be because WOB does not take into account ventilatory muscle capacity or fatigue. Consequently, WOB may be less accurate than other conventional discontinuation indices, and it is very difficult to measure at the bedside.³⁰

Oxygen cost of breathing (OCB) is the difference between oxygen consumption during spontaneous breathing and oxygen consumption during apnea (i.e., during full ventilatory support), which is determined as follows:

< ?xml:namespace prefix = "mml" />OCB=VO2sp−VO2(controlled ventilation)

After the OCB has been estimated, the relative proportion of oxygen consumed by the respiratory muscles as compared with the body as a whole can be calculated as follows:

%VO2(Resp)=(OCB/VO2sp)×100

Both OCB and %VO₂ have been correlated with the number of days required to wean patients.¹⁹ Patients with an OCB of 15% or less of the total VO₂ may be more likely to achieve discontinuation success. In one study, OCB was a better predictor of discontinuation success than was f/V_t.³¹

Pressure–time product (i.e., the area under the inspiratory pressure–time curve) and pressure–time index (PTI) may be the best measures of ventilatory workload for the care of patients who are receiving mechanical support.¹ The PTI can be calculated as follows:

PTI=(Mean inspiratory pressure/MIP)×Ti/Ttot

where MIP is maximum inspiratory pressure, T_i is the inspiratory time in seconds, and T_tot is the total respiratory cycle. The T_tot can be calculated by dividing 60 by the respiratory rate (f) (i.e., 60/f). A PTI of more than 0.15 to 0.18 has been associated with diaphragmatic fatigue,³² and a PTI of more than 0.15 cannot be sustained indefinitely.¹ There is currently no well accepted and reliable way to measure ventilatory muscle fatigue for patients who are receiving mechanical ventilation.⁵

Oxygenation

Poor oxygenation status is associated with weaning failure.3,7 Arterial blood gas (ABG) analysis, pulse oximetry, and continuous mixed venous oximetry have been used to monitor and assess the oxygenation status of patients before and during a discontinuation trial. In general, a PaO₂ of more than 60 mm Hg (or of more than 55 mm Hg for patients with COPD with carbon dioxide retention) with an FiO₂ of less than 0.40 to 0.50 and a PEEP of 5 to 8 cm H₂O or less should be adequate for ventilator discontinuation.^3,7 The PaO₂/FiO₂ ratio should be 150 to 200 mm Hg or more.³ With these values, a normal hemoglobin level, a normal oxygen saturation (SaO₂), and adequate cardiac output and tissue perfusion are assumed. Specific indices used to assess oxygenation status are found in Table 47-1.

Acid-Base Balance

Ideally the patient should have a normal acid–base balance (i.e., a pH of 7.35 to 7.45), and abnormalities in acid–base status should be corrected, if possible, before weaning. Patients with metabolic acidosis often have an increased ventilatory drive that can make weaning difficult. Patients who have metabolic alkalosis or those who have been mechanically hyperventilated for several days may have a reduced ventilatory drive. In these cases, a gradual method of discontinuing ventilatory support should be considered.

Metabolic Factors

Metabolic factors primarily affect discontinuation in those patients who require long-term ventilatory support. Although nutritional factors are important for all patients, they are unlikely to affect discontinuation in those who only require short-term ventilatory support. Nutrition should be adequate to maintain respiratory muscle mass and contractile force.¹⁶ Feeding should be adjusted according to individual patient needs; most patients need 1.5 to 2.0 times their resting energy expenditure.³³ In addition, protein intake should be between 1 and 1.5 g/kg per day. Excessive carbohydrate feeding can increase carbon dioxide production and may precipitate acute hypercapnic respiratory failure.¹⁶ Parenteral nutrition solutions that contain amino acid formulations (e.g., arginine/lysine) can cause metabolic acidosis and thus increase ventilatory demand.³³ Metabolic rate can increase as a result of fever or sepsis. Increased WOB, shivering, seizures, and agitation can also increase oxygen demand and should be evaluated.

Renal Function and Electrolytes

Adequate renal function is required to maintain acid–base homeostasis, electrolyte concentrations, and fluid balance.³⁴ The patient ideally should have an adequate urine output (i.e., more than 1000 ml/day), and there should be no inappropriate weight gain or edema.

Renal insufficiency can lead to metabolic acidosis, which increases respiratory drive. Electrolyte disorders can impair ventilatory muscle function.3,16 Key electrolytes should be normal (i.e., magnesium, 1.8 to 3 mEq/L; phosphate, 2.5 to 4.8 mEq/L; potassium, 3.5 to 5 mEq/L); see Chapter 16 for details. Fluid overload can lead to congestive heart failure and pulmonary edema, which may impair pulmonary gas exchange.

Cardiovascular Function

Adequate cardiovascular function is needed to provide sufficient oxygen delivery to the tissues. Cardiac rate and rhythm and blood pressure should be evaluated.3,16 Tachycardia (i.e., a heart rate of more than 100 to 120 beats/min) and bradycardia (i.e., a heart rate of less than 60 to 70 beats/min) should be controlled. The presence of arrhythmias, hypotension (i.e., a blood pressure of less than 90/60 mm Hg), and severe hypertension (i.e., a blood pressure of more than 180/110 mm Hg) should be evaluated carefully before the discontinuation of ventilatory support is considered.^3,19

Cardiac output and index measurements as well as central line and pulmonary arterial pressures (i.e., central venous pressure, pulmonary arterial pressure, and pulmonary capillary wedge pressure) may be helpful for the evaluation of cardiovascular function. Left ventricular dysfunction, myocardial ischemia, and cardiovascular instability are associated with decreased discontinuation success.3,21 Table 47-2 provides criteria for confirming cardiovascular stability. However, very few patients without primary cardiovascular dysfunction have pulmonary artery catheters in place. The assessment of cardiac output is usually by noninvasive means, and only arterial and central venous pressures are generally available.

TABLE 47-2

Criteria for Confirming Cardiovascular Stability

Criterion	Normal Value	Values That May Be Inconsistent With Weaning
Heart rate (beats/min)	60 to 100	<60, >120
Blood pressure (mm Hg)	90/60 to 150/90	<90/60, >180/110
(L/min)	4 to 8
Cardiac index (L/min−1·m2)	2.5 to 4	<2.1
Cardiac rhythm	No major arrhythmias present	Tachycardia, bradycardia, multiple premature ventricular contractions, heart block
Hemoglobin (g/dl)	12 to 15	Anemia, <10
Hematocrit	40% to 50%
No angina present
No lactic acidosis

Psychologic Factors and Central Nervous System Assessment

Adequate CNS function is needed to ensure stable ventilatory drive, adequate secretion clearance (i.e., coughing and deep breathing), and the protection of the airway (i.e., gag reflex and swallow). In addition, the level of consciousness, dyspnea, anxiety, depression, and motivation can affect discontinuation success.3,5

The patient ideally is awake and alert, free of seizures, and able to follow instructions.3,34 Patients should have an intact central drive to breathe and peripheral nerve function. Brainstem strokes, electrolyte disturbances, sedation, neuromuscular blocking agents, and narcotic drugs can impair the central neurologic control of ventilation.³ Mental status is a good predictor of discontinuation success, and patients who are not alert are at risk for upper airway obstruction, aspiration, and secretion retention. Obtunded patients should, at a minimum, have an adequate gag reflex and cough. Decreased levels of consciousness are associated with aspiration after extubation. The level of consciousness is affected by the use of narcotic, sedative, and analgesic drugs. Drugs with CNS depressant effects should be discontinued, if possible, before the withdrawal of ventilatory support and extubation.¹⁶ Protocols to reduce sedation and the daily cessation of sedative drugs may reduce weaning time^12,35,36 (see the section on Spontaneous Awaking Trials). The use of neuromuscular blocking agents to allow for controlled ventilation should be avoided, because they may prolong mechanical ventilation.³⁷

Psychologic factors may be among the most important nonrespiratory contributing factors that lead to ventilator dependence.³ Fear, anxiety, and stress should be minimized, and frequent communication among the staff, the patient, and the patient’s family can be helpful.³ Box 47-5 summarizes nonrespiratory factors that affect discontinuation success.

Integrated Indices

Many factors are associated with discontinuation success. In an analysis of 217 patients, discontinuation success could be accurately predicted in about 75% of the patients when multiple predictors were combined.²⁴ For all patients, regardless of diagnosis, days of mechanical ventilation (DMV), f/V_t; MIP, P_0.1, maximum expiratory pressure, and VC—when used in combination—were the best predictors.²⁴ For patients with acute respiratory failure, the best predictors were DMV, P_0.1/MIP, MIP, f/V_t, and age (76.1% accurate); for patients with COPD, the best predictors were DMV, f/V_t, P_0.1, P_0.1/MIP, MIP, and age (93.9% accurate); and for patients with neuromuscular disease, MIP, maximum expiratory pressure, f/V_t, and P_0.1 were the best predictors (73.9% accurate).²⁴ Integrated indices improve prediction by combining several measures of ability to breathe without ventilatory support. Current examples of integrated indexes include the CROP score, the Adverse Factor/Ventilator Score, the weaning index, and the Burns Weaning Assessment Program.^7,19 The CROP score combines measures of ventilatory load, respiratory muscle strength, and gas exchange.^7,23

The Adverse Factor/Ventilator Score combines ratings of 15 adverse factors, including hemodynamic values, infection, nutrition, and neurologic/psychiatric state, with ratings of six ventilator factors, including FiO₂, compliance, minute ventilation, and rate.¹⁹ The weaning index combines measures of ventilatory strength, endurance, and efficiency of gas exchange.¹⁹ A weaning index of less than 4 suggests successful discontinuation from mechanical ventilation.^18,32 The Burns Weaning Assessment Program is a 26-item assessment that combines 12 general and 14 respiratory factors into a single score.¹⁹ Although integrated indices appear promising, no single index has emerged as superior for use in diverse patient populations. Despite the success of these integrated indices in very specific settings, the best approach to determining if a patient can be successfully discontinued from ventilatory support is the patient’s performance on spontaneous breathing trials. All patients should be assessed daily, and their ability to breathe spontaneously should be the primarily variable to determine if the ventilator can be discontinued.

Evaluation of the Airway

The ability to maintain a patent natural airway and the likelihood of aspiration should be evaluated as a part of the process of discontinuing ventilatory support.

It is important for the clinician to separate the decision to discontinue ventilatory support from the decision to extubate.²⁹ The clinician must also be aware that most weaning indices do not evaluate airway patency or protection (see the section on Extubation). The inability to protect or maintain the natural airway is a clear contraindication to extubation. Some patients who can be successfully removed from a ventilator should not be extubated.²⁹ Tracheotomy may be considered for the care of patients who need artificial airways for an extended period.

Tracheotomy may improve patient comfort, allow for more effective suctioning, decrease airway resistance, enhance mobility, and allow the patient to eat and speak.³ Some patients also need high levels of sedation to tolerate the endotracheal tube.³ Consequently, tracheotomy should be considered for the care of patients who are likely to benefit and for those who need prolonged mechanical ventilatory support.³ See Chapter 33 for details about airway management and extubation.

Other patients with good airway patency and protection may be unable to maintain spontaneous breathing for prolonged periods without assistance.²⁹ These patients may be candidates for noninvasive ventilation (NIV) after extubation; see Chapter 46 for details.^18,38

Patients’ Psychologic and Communication Needs

As many as 47% of patients who spend more than 5 days in an ICU may experience psychologic disturbances.³⁸ The cause may be the stress of critical illness, the interruption of nighttime sleep, or the use of sedatives, tranquilizers, hypnotics, narcotics, and other drugs that affect the CNS.³⁹ The patient’s environment should be optimized, anxiety and depression should be evaluated, and communication should be maximized.^3,19,21,39

Environmental considerations that may improve the patient’s sense of well-being and outlook include reducing extraneous noise and providing clocks, calendars, pictures, a radio, a television, and, if possible, a room with windows.³⁸ Daily mobility should be considered, including getting the patient up in a chair or placing the patient in the hall in a chair.³⁹ Patients whose condition has been stable can sometimes even be helped to walk with the use of an E cylinder for oxygen, a manual resuscitator bag, and two or more health care workers for support.

A method for the patient to communicate with staff and visitors should be devised, if possible. Writing tablets, picture boards, and alphabet boards have been used effectively by patients to communicate with staff and visitors.

Anxiety and agitation should be minimized, and communication and encouragement may be helpful.³⁹ The patient should be told what is planned and be given some control over the situation. Patients should be asked to participate and help in the weaning process. Depression or a lack of motivation may increase weaning time.^5,39 If depression is present, assessment and treatment by an appropriate health care provider should be considered.³⁹

Rapid Ventilator Discontinuation

When the precipitating disease state or condition that necessitates support has been significantly alleviated or reversed, most patients can be rapidly removed from mechanical ventilation.16,21 These patients typically have acceptable blood gas levels with mechanical ventilation, adequate myocardial function, and no underlying cardiovascular, pulmonary, neurologic, or neuromuscular disorders.²¹

After a careful evaluation, patients in stable condition who have been treated with a ventilator for less than 72 hours and who have a good spontaneous respiratory rate and minute ventilation may undergo an SBT on the ventilator or with a T-piece for 30 to 120 minutes.3,7 If the patient does well, the endotracheal tube can then be removed if there is no reason to maintain an artificial airway.

Today, most SBTs are performed with the patient attached to a ventilator that is set on zero PSV and zero CPAP. This allows for a stable FiO₂ and ongoing monitoring. Others have recommended treating the patient with a low level of PSV (i.e., 5 to 7 cm H₂O) to overcome the resistance caused by the ventilator circuit, the demand flow system, and the artificial airway.² Others may prefer simply using CPAP (i.e., 5 to 7 cm H₂O).^2,3 These methods have the advantage of allowing the clinician to maintain ventilator alarm settings that can provide a margin of safety in the event of apnea or severe hypoventilation. Low levels of CPAP may be useful for maintaining lung volumes and overcoming intrinsic PEEP.² Some experts recommend PSV in combination with PEEP or CPAP.² However, the most widely accepted guidelines³ recommend a simple spontaneous breathing trial for the first SBT using the ventilator at zero CPAP and zero PEEP or a T-piece.

Patients Who Need Progressive Weaning of Ventilatory Support

Patients who have been receiving mechanical ventilation for more than 72 hours and those with marginal oxygenation, ventilatory, cardiovascular, or medical statuses may need a more prolonged period for ventilator discontinuation.19,21 The most common methods for accomplishing this are SBTs interspersed with continued ventilatory support, SIMV, and PSV.⁴

Spontaneous Breathing Trials

The oldest discontinuation method is an SBT via either the ventilator or T-piece, which allows for spontaneous breathing several times per day interspersed with periods of mechanical ventilatory support. For gradual weaning, initial SBTs may last only 5 to 10 minutes.4,7 Full ventilatory support is resumed for a 1- to 4-hour rest period in the assist-control or pressure-support mode. The time off of the ventilator is gradually increased until the patient is able to stay off of the ventilator for an extended period. SBTs can progress rapidly to ventilator removal.

When weaning is very difficult, the process can last days or even weeks. When a T-piece is used instead of the ventilator, the patient must be more carefully monitored, because none of the ventilator alarms are active during the SBT. If distress occurs, mechanical ventilation is resumed. In no case should the patient be overstressed during the SBT, because exhaustion can delay weaning.2,8 An example of an SBT is presented in Box 47-8.

In one major multicenter study, one or more daily SBTs resulted in three times faster extubation than with SIMV and two times faster than with PSV⁴⁰ (Figure 47-2). Results of another study favored PSV over T-tube trials or SIMV⁴⁷ (Figure 47-3). In a third study, SBTs were more effective than SIMV for weaning quadriplegic patients.⁴¹ Advantages of SBTs include the early and frequent testing of the patient’s ability to breathe spontaneously without support.^2,3 The use of SBTs several times per day has become unpopular, because it requires a great deal of time on the part of ICU staff, and a single daily SBT seems to be just as effective for most patients.^3,4,10 PSV and CPAP are sometimes used during the SBT, and patients who tolerate spontaneous breathing with a low level of PSV (i.e., 7 cm H₂O or less) should be able to tolerate extubation.⁴² Current recommendations suggest a single daily trial of spontaneous breathing may be preferable to other methods of weaning.³ Results of randomized, controlled studies have shown SBTs to be faster than SIMV and at least as effective as PSV.^3,4,10 Furthermore, performing an SBT one time per day is as effective as performing SBTs several times per day, and 30-minute trials are as effective as 120-minute trials.⁴

A collective task force representing the American College of Chest Physicians, the American Association for Respiratory Care, and the American College of Critical Care Medicine published evidence-based guidelines for weaning and discontinuing ventilatory support.³ These guidelines were based in part on a comprehensive review of the evidence collected under the direction of the U.S. Agency for Health Care Policy and Research.⁶

The task force made 12 specific recommendations (Box 47-9). These include the regular use of formal and carefully monitored SBTs to guide clinical decision making regarding the discontinuation of ventilatory support.³ Specifically, the task force recommended that, for the care of patients who need mechanical ventilation for more than 24 hours, a search for all possible causes of ventilatory dependence should be conducted and an attempt should be made to reverse or optimize each condition. Spontaneously breathing patients with evidence of reversal of the underlying condition that necessitated the ventilatory support, adequate oxygenation, and hemodynamic stability should be assessed with an SBT. Table 47-3 lists criteria to be used when determining whether patients who are receiving high levels of ventilatory support can be considered for SBTs.

A daily screen that includes the assessment of oxygenation, PEEP, and the use of vasopressors and sedatives should be conducted. A PaO₂/FiO₂ ratio of more than 150 to 200, a PEEP value of less than 5 to 8 cm H₂O, and a lack of a continuous infusion of vasopressors or sedatives (i.e., less than 5 mg/kg/min of dopamine) were good predictors of successful extubation and survival.⁴¹

The formal SBT may be performed in combination with low levels of PSV (i.e., 5 to 7 cm H₂O), CPAP (i.e., 5 cm H₂O or less), or automatic tube compensation applied to just offset the imposed WOB caused by the endotracheal tube.³ During the SBT, the patient is carefully observed for breathing comfort, anxiety, agitation, dyspnea, or diaphoresis. Respiratory rate, heart rate, blood pressure, and SpO₂ are monitored. The SBT is terminated if signs of distress are observed, including the following:

Patients with unsuccessful results of an SBT are returned to full ventilatory support for 24 hours to allow the ventilatory muscles to recover. During this period, the causes of failure are identified and corrected, if possible, and the patient is then reevaluated. If the criteria listed in Table 47-3 continue to be met, SBTs are repeated every 24 hours.³ Patients who tolerate the formal SBT for 30 to 120 minutes remain off of the ventilator, and extubation is considered. Box 47-10 is a sample protocol for an SBT for the discontinuation of ventilatory support.

Continuous Positive Airway Pressure

CPAP is used during an SBT in many facilities. CPAP has the advantages of maintaining the lung volume during the weaning phase and thus of improving the patient’s oxygenation status. Minimal levels of CPAP may be useful for reducing WOB and compensating for auto-PEEP, particularly in patients with obstructive lung disease.² CPAP is usually provided through the use of the CPAP mode that is available on most mechanical ventilators. By using the ventilator in the CPAP mode, the clinician can take advantage of the alarm systems that are available. This may provide an improved margin of safety as compared with the use of a T-piece. However, in one study, CPAP was no more effective as compared with T-piece trials for the weaning of patients who were recovering from coronary artery bypass graft surgery.⁴³

Synchronized Intermittent Mandatory Ventilation

SIMV has been advanced since the early 1970s as a method that can speed weaning and ventilator discontinuation.⁸ SIMV can be used to provide full or partial ventilatory support. Weaning from SIMV involves the gradual reduction of the machine rate on the basis of the results of ABG analysis and patient assessment.⁷ Early claims that SIMV allowed for faster weaning times⁸ have not been substantiated by subsequent studies.⁴⁴ However, SIMV is a commonly used mode of ventilation throughout the United States.^45,46

Patients who are receiving ventilation in the SIMV mode uncouple their breathing efforts from the support provided by the machine.² They continue to make spontaneous breathing efforts during the delivery of a “mandatory breath.” Evidence also suggests that, once the machine cycling rate is reduced to approximately 50% of the full ventilatory support value, the patient breathes approximately as hard per cycle as when ventilatory support is completely withdrawn.² SIMV provides a higher-level of WOB than control⁴⁶ or assist-control⁴⁷ ventilation, although the increase in WOB is not always evident.⁴⁶ This additional work can be overcome with the use of pressure support (i.e., 5 to 10 cm H₂O). However, the addition of pressure support further complicates the weaning process. For initial ventilator setup, the SIMV rate and the tidal volume usually are set at values that are equivalent to those used during volume control-continuous manditory ventilation (VC-CMV) or pressure control-continuous mandatory ventilation (PC-CMV). When the patient’s condition has been stabilized, one of two approaches can be used. Some clinicians prefer to continue at these settings until the patient’s precipitating disease state or condition has improved considerably.² At that point, the rate is reduced in a stepwise manner until complete spontaneous breathing can be achieved.² Other clinicians prefer to immediately reduce the level of mechanical ventilation, thereby forcing the patient to perform additional WOB.^1,2 From the beginning, attempts are made to reduce the SIMV rate, and the patient is challenged to provide a portion of the required ventilation.² The clinician adjusts the ventilator to make up the difference between what the patient is able to do and the total ventilation needed.² The rationale for this partial ventilatory support approach is that only the minimal level of mechanical support should be provided. The level of partial ventilatory support is titrated up or down the entire time that the patient is ventilated on the basis of changes in the patient’s condition.

Unfortunately, research indicates that SIMV prolongs ventilatory support and that it is the least effective method of weaning from ventilatory support as compared with either SBT or PSV.6,10,40,49

Pressure Support Ventilation

PSV is a mode of ventilatory support that allows the patient to have significant control over the process of ventilatory support. The only gas-delivery variable directly controlled by the ventilator is peak airway pressure; see Chapters 42 and 44 for details.

For initial ventilator setup in the PSV mode, the beginning pressure level can be adjusted to deliver an appropriate tidal volume, which is usually approximately 6 to 10 ml/kg of the ideal body weight based on the patient’s condition and the desired tidal volume. PSV is then gradually reduced to a minimal value that only compensates for the WOB imposed by the artificial airway.48–50 Generally this is about 5 to 8 cm H₂O. After this reduction is accomplished, extubation can be performed directly from the low level pressure support, or an SBT may be conducted for 30 to 120 minutes.^2,3

PSV allows the clinician to manipulate the level of patient work, but the benefit of this to weaning is questionable.1,2 In general, patients who can spontaneously breathe comfortably at 5 to 8 cm H₂O of PSV can be extubated without problems.⁴ However, if upper airway edema is present, WOB after extubation may be about the same as that caused by the endotracheal tube.⁴ In these cases, low levels of pressure support may give a false impression about the patient’s ability to tolerate extubation⁴ (Box 47-11).

Synchronized Intermittent Mandatory Ventilation With Pressure Support Ventilation

With SIMV, the addition of pressure support can overcome the WOB imposed during “spontaneous” breaths because of the presence of endotracheal and tracheostomy tubes, demand flow systems, and ventilator circuits. In this setting, pressure support is set to achieve the desired tidal volume during the spontaneous breaths (i.e., 6 to 10 ml/kg of the ideal body weight).

Mini Clini

Setting Pressure Support Levels

 Problem

An intubated patient is receiving mechanical ventilation in the SIMV mode with the following settings:

• V_T = 500 ml

• Rate = 12 breaths/min

• Peak inspiratory pressure = 40 cm H₂O

• Plateau pressure (P_plat) = 20 cm H₂O

• Ventilator inspiratory flow () = 60 L/min (1 L/s)

The patient is breathing spontaneously with a spontaneous rate of 12 breaths/min and a spontaneous peak inspiratory flow of 30 L/min (0.5 L/s). Find the level of PSV that is needed to overcome the imposed WOB.

Solution

PSV=([PIP−Pplat]/ V˙mach)×V˙Imax

PSV=([40 cm H2O−20 cm H2O]/1L/s)×0.5L/s=10 cm H2O

The calculated PSV level to overcome the imposed WOB for this patient is 10 cm.

Although it is clear that the addition of PSV during SIMV can reduce or eliminate imposed work caused by mechanical factors, it has not been shown how this affects weaning. In one study, SIMV with PSV increased tidal volume and reduced respiratory rate but did not significantly reduce weaning time or success as compared with SIMV alone.⁵¹ On the basis of all of the weaning trials, it can be concluded that this approach to weaning can only increase the length of ventilatory support.^38,49 Current guidelines recommend the use of SBT to rapidly wean patients from ventilatory support.³

Spontaneous Awaking Trials

Concern regarding the use of sedation in critically ill patients has increased markedly during the last few years. This concern has focused on the issue of delirium.⁵² The indiscriminate use of sedatives in the ICU is considered inappropriate patient management. In general, sedatives should always be considered last when trying to handle an agitated patient. Something has caused the patient’s agitation, and it should be addressed and corrected, if possible, before sedation is administered. In addition, it is becoming clearer that the use of excessive sedation lengthens the time of mechanical ventilation. A recent clinical trial compared an SBT alone with SBT with a spontaneous awaking trial (SAT). The SAT required that all of the sedation that a patient was receiving be stopped at midnight and the patient reassessed. If the patient did continue to require sedation, it was restarted at half the dose that the patient previously received. The use of the SAT along with the daily SBT resulted in the faster weaning of patients from ventilatory support as well as a decrease in mortality.⁵³ The authors argued that the decrease in morality was a result of patients being exposed for a shorter time to the complications of mechanical ventilation. Throughout the course of mechanical ventilation, sedation should be kept to the bare minimum necessary; patient sedation should be periodically stopped and the patient carefully assessed before additional sedation is given.⁵³

Adaptive Support Ventilation

ASV is a newer mode of ventilation that maintains a minimum minute ventilation with an optimal breathing pattern (tidal volume and rate) and that is based on the work of Otis;56,57 see Chapters 42 and 44 for details. ASV automatically adjusts inspiratory pressure and ventilator breath rate to achieve the target minute volume set. As the patient’s status improves, the target minute volume is reduced, and the level of pressure required each breath diminishes. When a minimal level is reached, the patient is considered weaned from ventilatory support and ready for extubation.

Preliminary studies of ASV for the care of patients who are recovering from cardiac surgery show a reduction in the duration of mechanical ventilation as compared with an SIMV weaning protocol.57,58 The ASV weaning protocol used in the study incorporated two 50% reductions in minute ventilation. Each reduction was followed by an assessment of patient tolerance according to criteria that were similar to those suggested for SBTs. Additional data about ASV indicates that it works very well for patients who are under controlled ventilation,⁵⁹ but additional data from a large heterogeneous group of patients will be necessary to determine if AVS truly improves the speed of ventilator discontinuation.

Computer-Based Weaning

Current versions of MMV and ASV are examples of computer-controlled mechanical ventilation. Several more complex systems have been developed, including Ventilation Manager, VQ-attending, ESTER, Continuous Respiratory Evaluator (CORE), KUSIVAR, and WEAN-PRO.¹⁹ The desire to develop computer-based weaning protocols is based on two factors. First, weaning is a time-consuming and labor-intensive process. If computer control can expedite or simplify this process, considerable time and money can be saved. Second, because most weaning decisions are based on objective data, computer-based weaning protocols are relatively easy to develop. In one study in which physician-directed weaning was compared with computer-based weaning, the computer-based system resulted in the less-frequent assessment of ABGs and shorter weaning times.¹⁹ Another study showed that a handheld computer weaning protocol for use by respiratory therapists was more effective than a paper-based weaning protocol.⁶⁰

The most successful computerized application for weaning is the Smart Care approach, which is based on data from Laurent Brochard’s group.⁶¹ The system adjusts pressure support on the basis of the patient’s tidal volume, respiratory rate, and end-tidal carbon dioxide level. When the pressure support has been decreased to a predefined level, the ventilator automatically begins an SBT. If the patient fails the SBT, which is determined by changes in the patient’s respiratory rate, tidal volume, and end-tidal carbon dioxide level, then the ventilator automatically reassumes ventilatory support. If the patient passes the SBT, the ventilator also returns to baseline ventilatory support but notifies the clinician that the patient is ready for ventilator discontinuation. In a randomized comparison with clinician-applied SBT, this system weaned patients faster and decreased the total number of days that patients were maintained in the ICU as well as the need for postextubation NIV. One criticism of this study was that the clinician-applied SBTs were not always performed properly; however, this is clinical reality, and if an automated system can wean patients faster, it will have a place in the care of critically ill patients.

Automatic Tube Compensation

ATC is an option on newer mechanical ventilators that compensates for the flow-dependent pressure decrease across the endotracheal tube during both inspiration and expiration; see Chapters 42 and 44 for details. ATC reduces WOB and may improve patient comfort.⁶² Because it compensates for the imposed WOB caused by the artificial airway, ATC has been referred to as electronic extubation. Patients who are able to breathe adequately with the addition of ATC at low peak airway pressure (i.e., less than 8 cm H₂O) should tolerate extubation. ATC is similar to pressure support in that an inspiratory pressure is used to compensate for imposed WOB; however, ATC varies the pressure, depending on the size of the endotracheal tube and the patient’s inspiratory flow rate. Alternatively, PSV delivers a preset inspiratory pressure that may overcompensate or undercompensate imposed WOB at any given point in time. However, no data is available to indicate that ATC weans patients faster than SBTs or PSV.

Volume Support

VC is a newer mode of ventilation that combines pressure support and volume ventilation by allowing a level of pressure support that is automatically adjusted to maintain a preset tidal volume. In the VC mode, inspiration is initiated by the patient. During the inspiratory phase, the pressure level is regulated to a value that is based on the previous breath’s pressure–volume relationship as compared with a preset tidal volume. The pressure-support level is automatically adjusted in a stepwise manner by ±3 cm H₂O from one breath to the next to maintain the preset tidal volume.

A major problem with VC is the excessive reduction of airway pressure in the presence of high ventilatory demand, thereby increasing patient effort.⁶³ With most ventilators offering this mode, no lower limit to ventilating airway pressure is set, and inspiratory airway pressure can decrease all the way to the PEEP level. Volume support may offer a sort of automatic weaning from pressure support. As the patient’s spontaneous tidal volume improves, the level of pressure support is automatically reduced. Clinical trials indicate no benefit to the use of VC to wean patients.⁶⁴

Noninvasive Ventilation

A number of groups have studied the use of NIV as an adjunct to ventilator discontinuance. Essentially, NIV has been used in this setting in three different manners:

The literature strongly supports the first two indications but does not support the third application. Refer to Chapter 45 for details regarding the application of NIV during weaning and the use of NIV in general.

Respiratory-Therapist–Driven Protocols

A number of randomized, controlled clinical trials have demonstrated that patients are weaned faster with the use of protocols than with individual physician orders.73–76 In all of these studies, the therapists, nurses, and physicians who normally manage patients in the unit developed the protocol. In addition, in the individual physician order group, the physicians who wrote the protocol also wrote the individual weaning orders. In all of these trials, patients who were randomized to the protocol group weaned faster and with less complications then the patients randomized to the individual physician group.^73–76

What these results do not mean is that therapists wean patients better than physicians. However, what they do mean is that, when an evidence-based approach to patient management is developed and followed precisely, care is generally better than with individual physician orders. The reason for this is that protocols empower the clinician at the bedside to advance care when the patient meets specific criteria without waiting for the physician to come and write an order. In many community hospitals, the wait for the physician could be an hour or a whole shift or more. Thus, with protocols, care processes rapidly on the agreed-upon course. In addition, the individual bias of the caregiver does not affect the manner in which care is provided. Thus, the approach does not change from day to day depending on which physician is at the bedside. An example of a respiratory-therapist–directed ventilatory discontinuance protocol is presented in Box 47-12.

Oxygenation

Continuous pulse oximetric (SpO₂) monitoring can provide a sensitive indicator of oxygenation status during weaning.⁵ Arterial blood gas analysis for PaO₂ and SaO₂ and the calculation of the oxygen content of the arterial blood (CaO₂) should be performed if doubt exists when assessing a patient’s oxygenation status during weaning.

Cardiovascular Status

Pulse, blood pressure, and cardiac rhythm should be monitored, and arrhythmias should be assessed to determine whether weaning should be continued. Tachycardia, bradycardia, and abnormalities in blood pressure should be promptly evaluated and the patient returned to ventilatory support or a higher level of support, if indicated. Silent myocardial ischemia may occur frequently in some postoperative patients during weaning.⁸² Table 47-6 summarizes changes that may occur during the withdrawal of ventilatory support.

TABLE 47-6

Changes During the Withdrawal of Ventilatory Support

Expected Change	Deleterious Change
Respiratory
Respiratory rate minimally increased	Respiratory rate of ≥35 breaths/min
Stable	Large increase or decrease in
SpO2 ≥ 90%	Decrease in SpO2 to ≤90%
5 to 10 mm Hg swing in PaO2	PaO2 of <60 mm Hg
5 to 10 mm Hg swing in PaCO2	An increase of >10 mm Hg in PaCO2
pH of >7.30 and <7.50	pH of <7.30
Minimal use of accessory muscles	Increased use of accessory muscles
No paradoxical breathing	Paradoxical breathing Diaphoresis Dyspnea
Cardiovascular
Heart rate increased by 15 to 20 beats/min	Persistent tachycardia of ≥120 to 140 beats/min
Blood pressure increased 10 to 15 mm Hg	Hypotension (blood pressure of <90/60 mm Hg) Hypertension (systolic blood pressure of >180 mm Hg)
Increased cardiac index	Decreased cardiac index
Increased stroke volume	Decreased stroke volume Angina New arrhythmias Increased pulmonary capillary wedge pressure
Other
Mental status good (i.e., awake, alert, responsive)	Anxiety, agitation, somnolence, coma