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Objectives
Discuss the impact of respiratory muscle dysfunction on the need for mechanical ventilation in patients with chronic pulmonary disease.
Discuss the role of auto-positive end-expiratory pressure (auto-PEEP) that develops in patients with obstructive lung disease.
List indications for mechanical ventilation in patients with obstructive lung disease.
List the initial ventilator settings for obstructive lung disease.
Discuss monitoring and ventilator liberation for mechanically ventilated patients with obstructive lung disease.
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Obstructive pulmonary diseases include chronic obstructive pulmonary disease (COPD), asthma, bronchiectasis, and cystic fibrosis. Patients with this underlying pathology are a significant fraction of those requiring respiratory support. Although this chapter deals primarily with COPD and asthma, the principles related to mechanical ventilation are similar for other obstructive lung diseases.
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With COPD, flow limitation leads to air trapping with increased work-of-breathing and respiratory muscle dysfunction. Asthma is episodic and associated with airways inflammation and bronchospasm. COPD and asthma are chronic diseases that are often managed well in the community. But exacerbations of either can result in respiratory failure necessitating mechanical ventilation.
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Respiratory Muscle Dysfunction
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Because of the hyperinflation with COPD, the normal dome shape of the diaphragm is flattened and the zone of apposition is decreased. The result is less efficient diaphragmatic function. If the diaphragm is sufficiently flattened, during contraction the lateral rib cage moves inward instead of outward, leading to paradoxical breathing (Table 18-1). Accessory muscles of inspiration (intercostals, scalenes, sternomastoid, pectoralis, and parasternal) become the primary muscle groups for breathing. In patients with COPD where chronic respiratory muscle dysfunction has developed, reserve is limited and fatigue can occur with increases in respiratory muscle load.
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Auto-Positive End-Expiratory Pressure
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Auto-positive end-expiratory pressure (auto-PEEP) is end-expiratory alveolar pressure due to air trapping. Due to the heterogeneity that exists in the lungs, air trapping and auto-PEEP may differ between lung units. As a result, the auto-PEEP level measured is an average value. Long-time constants (Table 18-2) resulting from increased resistance and compliance in COPD necessitate more expiratory time to prevent air trapping and auto-PEEP. Auto-PEEP requires a greater inspiratory pressure to initiate flow into the lungs (difficulty triggering the ventilator) and the hyperinflation resulting from air trapping causes an increased work-of-breathing. The airways resistance with COPD is characterized by flow limitation.
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