Chapter 1: Physiologic Effects of Mechanical Ventilation

Introduction

Ventilators used in 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 an individual patient, this demands application of strategies that maximize the potential benefit of mechanical ventilation while minimizing the potential for harm. Due to 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.

Mean Airway Pressure

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 static transpulmonary pressure that can be generated normally during spontaneous inspiration is less than 35 cm H₂O.

Intrathoracic pressure fluctuations during positive pressure ventilation are opposite to those that occur during spontaneous breathing. During positive pressure ventilation, the mean intrathoracic pressure is usually positive. Intrathoracic pressure increases during inhalation and decreases during exhalation. Thus, venous return is greatest during exhalation and it may be decreased if expiratory time is too short or mean alveolar pressure is too high.

Many of the beneficial and adverse effects associated with mechanical ventilation are related to mean airway pressure. Mean airway pressure is the average pressure applied to the airway during the ventilatory cycle. It is related to both the amount and duration of pressure applied during the inspiratory phase (peak inspiratory pressure, inspiratory pressure waveform, and inspiratory time) and the expiratory phase (positive end-expiratory pressure [PEEP] and respiratory rate).

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 ...