Chapter 1: Physiologic Effects of Mechanical Ventilation

Objectives

1. Compare intrathoracic pressure during spontaneous breathing and positive-pressure ventilation.

2. Describe the effects of positive-pressure ventilation on shunt and dead space.

3. Discuss the roles of alveolar overdistention and opening/closing on ventilator-induced lung injury.

4. Discuss the physiologic effects of positive-pressure ventilation on pulmonary, cardiac, renal, gastrointestinal, and neuromuscular function.

5. Describe the effects of sedation and delirium in the mechanically ventilated patient.

6. Discuss the effects of positive-pressure ventilation on nutrition, the upper airway, and sleep.

7. Describe methods that can be used to minimize the harmful effects of positive-pressure ventilation.

Introduction

Ventilators for adult acute care use positive pressure applied to the airway opening to inflate the lungs. Although positive pressure is responsible for the beneficial effects of mechanical ventilation, it is also responsible for many potentially deleterious side effects. Application of mechanical ventilation requires an understanding of both its beneficial and adverse effects. In the care of a patient, this demands application of strategies that maximize the potential benefit of mechanical ventilation while minimizing the potential for harm. Because of the homeostatic interactions between the lungs and other body systems, mechanical ventilation can affect nearly every organ system of the body. This chapter provides an overview of the beneficial and adverse physiologic effects of mechanical ventilation.

Intrathoracic Pressure Changes

During normal spontaneous breathing, intrathoracic pressure is negative throughout the ventilatory cycle. Intrapleural pressure varies from about −5 cm H₂O during exhalation to −8 cm H₂O during inhalation. Alveolar pressure fluctuates from +1 cm H₂O during exhalation to −1 cm H₂O during inhalation. The decrease in intrapleural pressure during inhalation facilitates lung inflation and venous return.

Transpulmonary pressure is the difference between proximal airway pressure and intrapleural pressure. The greatest transpulmonary pressure that can be generated normally during spontaneous inspiration is about 30 cm H₂O. Transalveolar pressure, also called alveolar stress, should be limited to 20 cm H₂O during positive-pressure ventilation.

Intrathoracic pressure fluctuations during positive-pressure ventilation are opposite to those that occur during spontaneous breathing. During positive-pressure ventilation, intrathoracic pressure is usually positive. Intrathoracic pressure increases during inhalation and decreases during exhalation.

Pulmonary Effects

Shunt

Shunt is perfusion (blood flow) without ventilation (Figure 1-1). Pulmonary shunt occurs when blood flows from the right heart to the left heart without participating in gas exchange. The result of shunt is hypoxemia. Shunt can be either capillary shunt or anatomic shunt. Capillary shunt results when blood flows past unventilated alveoli. Examples of capillary shunt are atelectasis, pneumonia, pulmonary edema, and acute respiratory distress syndrome (ARDS). Anatomic shunt occurs when blood flows from the right heart to the left heart and completely bypasses the lungs. Normal anatomical shunt occurs due to the Thebesian veins and the bronchial circulation. Abnormal ...