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Objectives

  1. Discuss the impact of respiratory muscle dysfunction on the need for mechanical ventilation in patients with chronic pulmonary disease.

  2. Discuss the role of auto-positive end-expiratory pressure (auto-PEEP) that develops in patients with obstructive lung disease.

  3. List indications for mechanical ventilation in patients with obstructive lung disease.

  4. List the initial ventilator settings for obstructive lung disease.

  5. Differentiate the approach used to ventilate patients with chronic obstructive pulmonary disease (COPD) exacerbation and with severe acute asthma.

  6. Discuss monitoring and ventilator liberation for mechanically ventilated patients with obstructive lung disease.

Introduction

Obstructive pulmonary diseases include chronic obstructive pulmonary disease (COPD), asthma, bronchiectasis, and cystic fibrosis. Patients with this underlying pathology are a significant number 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.

Overview

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.

Respiratory Muscle Dysfunction

Because of the hyperinflation with moderate to severe COPD, the diaphragm is lowered and flattened, and the zone of apposition is decreased. The result is less efficient diaphragmatic function. If the diaphragm is severely flattened, during contraction the lateral rib cage moves inward instead of outward, leading to paradoxical breathing (Hoover’s sign) (Table 18-1). Accessory muscles of inspiration (intercostals, scalenes, sternomastoid, pectoralis, and parasternal) become the primary muscles for breathing. In patients with COPD where chronic respiratory muscle dysfunction has developed, reserve is limited and fatigue can occur with increased respiratory muscle load.

Table 18-1Characteristics of Normal Breathing Pattern and Paradoxical Breathing

Auto-Positive End-Expiratory Pressure

Auto-positive end-expiratory pressure (auto-PEEP) is positive end-expiratory alveolar pressure resulting from air trapping. Due to the heterogeneity in the lungs, air trapping and auto-PEEP may differ between lung units. Some lung units might have little auto-PEEP, whereas others might have markedly elevated auto-PEEP. The auto-PEEP measured on the ventilator is an average of the auto-PEEP among open lung units. Long-time constants (Table 18-2) resulting from increased resistance and compliance in COPD necessitate longer 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) ...

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