Mechanical and Noninvasive Ventilation

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Chapter 9 Mechanical and Noninvasive Ventilation

Manuel Pardo, Jr. , MD

1 What are the primary indications for mechanical ventilation?

The primary indications for mechanical ventilation are inadequate oxygenation, inadequate ventilation, and elevated work of breathing.

2 What is the difference between conventional and noninvasive ventilation?

image The conventional approach to mechanical ventilation involves use of an airway device inserted directly into a patient’s trachea.

image Noninvasive ventilation is mechanical ventilation without the use of an endotracheal tube or tracheotomy. See Figure 9-1.

image

Figure 9-1 A demonstration of noninvasive positive-pressure ventilation with a nasal mask. A, The respiratory therapist is adjusting the inspiratory pressure support level and checking for proper mask fit. B, The mask must be fitted properly to minimize air leak but avoid excessive pressure on the skin.

3 What are the main types of mechanical ventilation?

The two main types are positive-pressure and negative-pressure ventilation. With positive-pressure ventilation, positive pressure is applied to the airway to inflate the lungs directly. This can be accomplished with an artificial airway (e.g., endotracheal tube) or with a nasal or face mask (noninvasive ventilation). With negative-pressure ventilation, negative pressure is applied to the abdomen and thorax to draw air into the lungs through the upper airway. This form of ventilation is typified by the iron lung negative pressure ventilators used after the polio epidemics in the first half of the 20th century. For the remainder of this chapter, noninvasive ventilation will refer to noninvasive positive-pressure ventilation.

4 What are the advantages of noninvasive ventilation compared with conventional mechanical ventilation with an endotracheal tube?

In most studies of noninvasive ventilation in the intensive care unit (ICU) setting, the main outcome measure has been reduced need for tracheal intubation. For those patients who avoid intubation, the main advantage is the avoidance of artificial airway complications. Several studies document a lower infection rate (e.g., nosocomial pneumonia and sinusitis) with noninvasive ventilation, which may be related to the maintenance of the protective glottic barrier. Other advantages include reduced need for sedative medications. Some noninvasive ventilation trials have demonstrated reductions in ICU length of stay, ICU mortality, or in-hospital mortality.

5 What are the disadvantages and contraindications of noninvasive ventilation?

Patient selection is crucial to ensuring success with noninvasive ventilation. Patients who are uncooperative, need immediate intubation, have upper airway pathologic conditions, are hemodynamically unstable, or have excessive secretions are not likely to have successful ventilation. Noninvasive ventilation should be contraindicated in these patients. In practical terms, noninvasive ventilation can initially require significant attention by the respiratory therapist and/or ICU nurse.

6 Which patients with acute respiratory failure are most likely to benefit from noninvasive ventilation?

Patients with acute exacerbations of chronic obstructive pulmonary disease are most likely to benefit, in terms of reduction in need for tracheal intubation and decreased mortality. Other populations likely to have a decreased intubation rate with noninvasive ventilation include patients with immune compromise from solid organ transplants or hematologic malignancies, cardiogenic pulmonary edema, respiratory failure after lung resection, and respiratory failure immediately after elective abdominal surgery. If noninvasive ventilation is used for patients that do not fit in these categories, consider limiting the time to 2 hours unless clear improvement is evident.

7 What are the most commonly used modes of mechanical ventilation?

For noninvasive ventilation, the most common mode is pressure support ventilation. For conventional ventilation, the three most commonly used modes are assist control ventilation, synchronized intermittent mandatory ventilation (SIMV), and pressure support ventilation.

8 How does pressure support ventilation work?

Pressure support ventilation requires the patient to trigger each breath. When a patient inhales and generates inspiratory gas flow, the ventilator rapidly delivers a preset inspiratory pressure, typically 5 to 20 cm H2O. The ventilator’s inspiratory flow stops when a certain threshold flow has been reached, typically 25% of the peak flow rate. During the expiratory phase of the respiratory cycle, positive end-expiratory pressure (PEEP) is often applied, typically between 5 and 10 cm H2O. The patient’s work of breathing is reduced, proportional to the amount of inspiratory pressure that is set. This is usually judged by the patient’s tidal volume, which can vary with each breath on the basis of patient effort and lung mechanics. Because minute ventilation is not guaranteed during pressure support ventilation, the patient may have hypoventilation if the pulmonary process is severe enough or if respiratory depressant sedatives are administered.

9 How do the assist control and SIMV modes differ?

Assist control and SIMV share some common features. Both deliver a preset tidal volume and respiratory rate. Thus a minimum minute ventilation is guaranteed, unlike pressure support ventilation, which requires the patient to have an intact respiratory drive.

Assist control and SIMV differ in how they manage patient-initiated inspirations above the preset respiratory rate. With assist control, every additional patient-initiated breath receives the preset tidal volume. During SIMV, the additional breaths depend on the patient’s respiratory muscles to generate the tidal volume.

10 Why is pressure support commonly combined with SIMV?

Because additional breaths above the preset respiratory rate depend on the patient’s effort and lung mechanics, the addition of pressure support can provide a reduction in the work of breathing. This may be desirable if added respiratory workload is imposed on the patient, for example, from a small-diameter endotracheal tube.

11 How does the clinician choose the initial ventilator settings?

The ventilator settings initially chosen depend on the reason for mechanical ventilation. Most patients with respiratory failure have some combination of hypoxemia, hypoventilation, or elevated work of breathing. The clinician has some choices to make, including:

image Mode of ventilation: assist control, SIMV, pressure support

image Tidal volume

image FiO2

image PEEP

For a patient who must have guaranteed minute ventilation and the greatest reduction in work of breathing, the assist control mode is most commonly chosen. Tidal volume and respiratory rate are selected to maintain arterial pH in the normal range. The inspired oxygen is initially set to 100%, and PEEP is set to at least 5 cm H2O. Both FiO2 and PEEP are adjusted to prevent hypoxemia, as judged by arterial PO2 or pulse oximetry.

12 What factors are used to select tidal volume?

For many years, clinicians selected tidal volumes of 10 to 12 mL/kg and adjusted respiratory rate to achieve normocarbia. The rationale for selecting larger than normal tidal volumes included prevention of atelectasis and ease of normalizing PCO2. However, the growing recognition of ventilator-associated lung injury led to alternate mechanical ventilation strategies, especially in the patient with acute respiratory distress syndrome (ARDS). A ventilation strategy using tidal volumes in the normal range of 6 mL/kg (based on ideal body weight) was shown to offer better clinical outcomes, including mortality, than ventilation using a higher tidal volume of 12 mL/kg. For patients without ARDS or lung injury, it is not clear whether the use of 6 mL/kg tidal volume is associated with improved outcomes, but a general trend has been to avoid use of high tidal volumes.

13 How is the optimal PEEP setting determined?

The determination of optimal PEEP in the patient with mechanical ventilation has been a source of controversy for decades. The goal of PEEP use is partly determined by the cause of respiratory failure. A patient with normal oxygenation who has mechanical ventilation may receive a low level of PEEP, 5 cm H2O, to help prevent atelectasis. A patient with pulmonary edema and hypoxemia may benefit from higher levels of PEEP (10 cm H2O or higher) to recruit alveoli that are collapsed; in such a patient there may be a reduced FiO2 requirement and improved lung compliance. Because high inspired oxygen concentration can itself cause lung injury, the use of PEEP may allow use of a lower FiO2.

14 What is auto-PEEP?

Auto-PEEP, also called intrinsic PEEP, is the development of PEEP because of incomplete exhalation before delivery of a positive-pressure breath. Auto-PEEP can be seen in patients with high minute ventilation requirement (e.g., ARDS) or diseases of airflow limitation (e.g., asthma). Physiologically, auto-PEEP behaves similarly to applied PEEP. Detection of auto-PEEP can be difficult, although modern ventilators often incorporate an expiratory hold (occlusion of expiratory valve before inspiration) to measure this otherwise-hidden pressure.

15 How can auto-PEEP be reduced?

Reduction of auto-PEEP involves treatment of the underlying disorder. Adjustment of the mechanical ventilator to maximize expiratory time may also provide some benefit. The approach may include increasing inspiratory flow rate and lowering minute ventilation by decreasing respiratory rate and/or tidal volume.

16 How does mechanical ventilation affect the cardiovascular system?

Positive-pressure ventilation results in positive intrathoracic pressure, which decreases venous return. The resulting decrease in cardiac output may lead to hypotension. The reduction in preload may be most dangerous in a patient with right-sided heart failure. In addition, the inflation of the lungs with positive pressure increases the right ventricular afterload, which is also potentially hazardous for the failing right ventricle. On the other hand, positive intrathoracic pressure increases the pressure gradient for aortic blood flow out of the thorax, thus facilitating some degree of left ventricular afterload reduction.

17 How are ventilator settings adjusted after the initial settings have been instituted?

Evaluation includes subjective assessment of the patient’s work of breathing and degree of respiratory distress. A patient who is dyssynchronous with mechanical ventilation may need adjustment of the ventilator mode, tidal volume, or intravenous sedation. PEEP and FiO2 are adjusted to avoid hypoxemia and hyperoxia (see question 13). Tidal volume and respiratory rate may also be adjusted on the basis of measured arterial pH and PCO2 (see question 12). pH is also affected by the presence of metabolic acidosis or alkalosis. Peak and plateau airway pressures are assessed to determine respiratory compliance. The peak pressure is measured at the end of inspiration, whereas the plateau pressure is measured after a brief inspiratory pause.

18 How can peak and plateau airway pressure be used to evaluate the patient’s respiratory system?

The peak airway pressure reflects respiratory resistance and compliance, whereas plateau airway pressure reflects only compliance. Therefore a large difference between peak and plateau pressure suggests a source of increased resistance (e.g., secretions, narrowed endotracheal tube, bronchospasm). An elevated peak and plateau pressure (with less than 5 cm difference between the two) implies reduced compliance (e.g., pulmonary edema, chest wall restriction).

19 What is permissive hypercarbia?

Traditionally, mechanical ventilation parameters were adjusted to normalize blood gases, including normal PCO2. For patients with severe bronchospasm (e.g., acute asthma exacerbation), the strategy of normalizing the PCO2 can lead to auto-PEEP, air trapping, and even life-threatening barotrauma. Similarly, a patient with severe ARDS has markedly elevated dead space, requiring extremely high minute ventilation to achieve normal PCO2. Ventilator-associated lung injury can result from repeated stretching of alveoli, with accompanying release of inflammatory mediators. A ventilation strategy that allows the PCO2 to become elevated (permissively) minimizes the risk of barotrauma in both types of patients and is a cornerstone of management, in addition to treating the underlying disease processes.

20 What are the complications of mechanical ventilation?

Major complications include ventilator-associated lung injury, barotrauma (including pneumomediastinum and pneumothorax), hemodynamic compromise, and pneumonia. Ventilator-associated pneumonia is now recognized as one of the most severe health care–associated infections in the critically ill patient. Other complications of mechanical ventilation can stem from the use of sedation, which is almost always needed in the patient with respiratory failure.

21 What are the complications unique to noninvasive ventilation?

Complications of noninvasive ventilation include pressure necrosis of the skin, aerophagia, and intolerance of the mask. The incidence of mask intolerance can be as high as 25%. In addition, the presence of the face mask may complicate nutrition because oral feeding requires mask removal and the presence of a nasoenteric tube may interfere with mask seal. Pulmonary toilet measures such as tracheal suctioning are also more difficult compared with those in the patient with a tracheal tube in place.

Key Points Mechanical Ventilationimage

1. The primary indications for mechanical ventilation are inadequate oxygenation, inadequate ventilation, and elevated work of breathing.

2. The patient populations most likely to benefit from noninvasive ventilation are those with acute exacerbations of chronic obstructive pulmonary disorder, cardiogenic pulmonary edema, and immune compromise.

3. The assist control mode provides a guaranteed minute ventilation and full support of all patient-initiated breaths.

4. Pressure support ventilation requires intact respiratory drive from the patient; tidal volume and minute ventilation are variable and depend on patient effort.

5. Low tidal volume mechanical ventilation can lead to improved outcomes in the patient with acute respiratory distress syndrome.

Bibliography

1 Acute Respiratory Distress Syndrome Network: Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. N Engl J Med. 2000;342:1301–1308.

2 Calfee C.S., Matthay M.A. Recent advances in mechanical ventilation. Am J Med. 2005;118:584–591.

3 Squadrone V., Coha M., Cerutti E., et al. Continuous positive airway pressure for treatment of postoperative hypoxemia: a randomized controlled trial. JAMA. 2005;293:589–595.

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