Goals of Mechanical Ventilation

The goals of mechanical ventilatory support are to maintain adequate alveolar ventilation and oxygen (O₂) delivery, restore acid-base balance, and reduce the work of breathing (WOB) with minimum harmful side effects and complications.¹ Mechanical ventilation also may reduce increased myocardial work secondary to hypoxemia and an increased WOB.¹ Other physiologic objectives of mechanical ventilatory support include increasing or maintaining lung volume with positive end expiratory pressure (PEEP) and continuous positive airway pressure (CPAP) for promotion, improvement, or maintenance of lung recruitment.¹

A lung protective ventilatory strategy is an approach to mechanical ventilation that includes the use of small tidal volume (V_T) and appropriate levels of PEEP.² This approach is usually employed in patients with acute lung injury (ALI) or the acute respiratory distress syndrome (ARDS). However, the concept of lung protection should be applied to all patients requiring ventilatory support for acute respiratory failure. Lung injury is primarily caused by an elevated transpulmonary pressure during positive pressure ventilation.³ Transpulmonary pressure is the difference between alveolar pressure and pleural pressure. A safe transpulmonary pressure during mechanical ventilation is not firmly established, but most clinicians would agree that the lower the transpulmonary pressure, the less likely the development of ventilator-induced lung injury.⁴ High transpulmonary pressures are associated with alveolar overdistention and lung injury.³

Plateau pressure (P_plat), the end inspiratory equilibration pressure, measures the mean peak alveolar pressure and is the best bedside clinical reflection of transpulmonary pressure.2,4,5 Although P_plat is not an accurate measurement of transpulmonary pressure, the transpulmonary pressure never exceeds the P_plat.⁴ P_plat provides an excellent bedside assessment of the level of potentially dangerous ventilating pressure. Limiting P_plat reduces the likelihood of ventilator-induced lung injury. Generally, the lower the P_plat, the better the patient outcome.^2,5 Ideally, P_plat should be less than 30 cm H₂O.⁵ However, a P_plat greater than 30 cm H₂O may be applied in patients with a decreased thoracic compliance without resulting in overdistention⁵ because a decrease in chest wall compliance (obesity, massive fluid resuscitation, abdominal distention, elevated bladder pressure) increases the pleural pressure, decreasing the transpulmonary pressure. Generally, the lowest possible P_plat is maintained by selecting a V_T of 4 to 8 ml/kg of ideal body weight (IBW). The higher the P_plat, the smaller the V_T should be. Generally, V_T greater than 10 ml/kg IBW is never indicated in critically ill patients.

Lung injury can also be caused by repetitive opening and closing of unstable lung units.⁶ The application of an appropriate level of PEEP ensures that unstable lung units are maintained in the open position reducing the likelihood of additional lung injury.

Specific clinical objectives of mechanical ventilation include reversal of hypoxemia, hypercapnia, and associated respiratory acidosis and prevention or reversal of ventilatory muscle dysfunction. The general trajectory of pH, PCO₂, and PO₂ during the progression of acute respiratory failure is depicted in Figure 44-1. Mechanical ventilation may be used to allow sedation or paralysis for certain procedures, to decrease myocardial and ventilatory muscle O₂ consumption to maximize O₂ delivery to the tissues, to decrease intracranial pressure acutely in the presence of closed head injury or cerebral edema (by reducing PaCO₂ to 25 to 30 mm Hg for a short period and promoting cerebral vasoconstriction), to prevent or reverse atelectasis, and to stabilize the chest wall in the case of a massive flail or chest wall resection. Table 44-1 lists the most common causes of acute respiratory failure leading to ventilatory support in the United States and Canada. Hazards of mechanical ventilation include decreased venous return and cardiac output, patient-ventilatory asynchrony, and ventilatory muscle dysfunction owing to inappropriate ventilator settings, ventilator-associated pneumonia, and ventilator-induced lung injury.¹ Box 44-1 lists the goals of ventilatory support, and Box 44-2 lists specific objectives of mechanical ventilation.

Ventilator Initiation

When the decision to begin mechanical ventilatory support is made, one must choose the mode of ventilation, select an appropriate device, and establish the initial ventilator settings. In the selection of initial ventilator settings, the goal is to optimize the patient’s oxygenation, ventilation, and acid-base balance, while avoiding harmful side effects. This goal is achieved by choosing an appropriate mode of ventilation, fractional inspired oxygen (FiO₂), V_T (volume ventilation) or pressure level (pressure ventilation), rate, peak flow and flow waveform, inspiratory time, and PEEP level. Appropriate trigger sensitivity, pressure limit, alarms, backup ventilation, and humidification must be selected. After initial ventilator setup, adjustments must be made on the basis of the patient’s response and the patient-specific clinical objectives of ventilatory support. Most patients who need mechanical ventilatory support receive invasive positive pressure ventilation; however, an increasing number of patients are being ventilated noninvasively (see Chapter 46). Next, the clinician must choose the mode of ventilation (e.g., volume assist/control [VA/C], pressure assist/control [PA/C], pressure support ventilation [PSV], pressure regulated volume control [PRVC], volume support, adaptive support ventilation, proportional assist ventilation [PAV], or neurally adjusted ventilatory assist [NAVA]) and initial ventilator settings (e.g., rate, V_T or pressure level, FiO₂, PEEP). Finally, the clinician must choose appropriate alarm and apnea settings. Box 44-3 summarizes key decisions that must be made as a part of initial ventilator setup.

Initial Ventilator Settings

Initial ventilator settings are chosen based on the patient’s clinical presentation and the need to provide full or partial ventilator support.

Mini Clini

Selecting Initial Ventilator Settings

 Problem

A 52-year-old man, 5 ft 10 in (178 cm) tall and weighing 200 lb (91 kg), is being returned from the operating room after coronary artery bypass surgery. He is being manually (bag-tube) ventilated with supplemental O₂ by the anesthesiologist en route to the ICU. He is apneic at this time. The patient has no history of lung disease and has never smoked cigarettes. Heart rate and blood pressure are stable, and SpO₂ during manual ventilation is 99%. What initial mode, V_T, rate, and FiO₂ should the RT select when starting ventilatory support for this patient?

Solution

The patient is apneic at this time but is likely to resume spontaneous breathing as the anesthetic wears off and sedation is reduced. Because the patient is expected to resume breathing spontaneously, volume ventilation or pressure ventilation in either assist-control or SIMV is appropriate.

Initial V_T and rate should be selected to provide full ventilatory support. Generally, initial V_T of approximately 6 to 8 ml/kg IBW or pressure control setting to establish this V_T with a rate of 12 to 20 breaths/min provides an adequate starting minute ventilation for most adult patients. The formulas for estimating IBW are:

< ?xml:namespace prefix = "mml" />IBW in kilograms(men)=[106+6(H−60)]/2.2

IBW in kilograms(women)=[105+5(H−60)]/2.2

where H is height in inches.

For this patient:

IBW=[106+6(70−60)]/2.2=75.5kg

On the basis of IBW of 75.5 kg, initial V_T can be set at about 600 ml. Initial inspiratory flow should be set at 60 L/min with a decreasing ramp flow waveform to achieve an inspiratory time of approximately 1 second. Initially, rate can be set at 12/min. Because the patient has a normal respiratory system and it is usual to return from the operating room with a below-normal body temperature, a low initial control rate is indicated. Trigger sensitivity (assist-control or SIMV) should be set so that minimal patient effort triggers the ventilator without autocycling.

Initial FiO₂ should be set at 1.0, but because of the patient’s history and the presence of normal lung function, it is expected that it will be reduced rapidly as the patient recovers. Initial PEEP is set at 5 cm H₂O. If the SIMV mode is chosen, PSV should be started at 10 cm H₂O and adjusted as needed when the patient resumes spontaneous breathing.

In summary, appropriate initial ventilator settings for this patient are:

Mode: Assist-control (volume or pressure) or SIMV (volume or pressure) with PSV

V_T: 8 ml/kg, 600 ml

f_mach: 12 breaths/min

PSV: 10 cm H₂O (SIMV mode only)

FiO₂: 1.0 followed by immediate assessment and SpO₂ observation with titration downward as indicated

Inspiratory flow and time: 60 L/min, decreasing ramp, inspiratory time approximately 1 second

Pressure limit: 30 cm H₂O and adjust to 10 to 15 cm H₂O above PIP after patient connection

Humidification: Heated humidifier to achieve temperature >35° C at the airway or an appropriate HME