7 - Mechanical Ventilation

Summary

Introduction

In many patients mechanical ventilation simply supports tidal breathing, for example, patients under general anesthesia or patients with trauma, drug overdose, sepsis, shock, or COPD. However, in ARDS patients, appropriate adjustment of mechanical ventilation may sometimes choose to support tidal breathing inadequately (permissive hypercapnia) to achieve more important goals. Mechanical ventilation of ARDS patients is complex and requires consideration of underlying pulmonary and cardiovascular physiology (Chapters 5 and 6), oxygen transport, beneficial and detrimental effects of positive pressure ventilation, the time course of airway pressure changes (e.g., assist-control mode vs. pressure control or other modes), cardiopulmonary interaction, and the effect of other nonpulmonary therapies (e.g., diuresis, inotropes). In this chapter the issue of mechanical ventilation of ARDS patients is approached from the perspective of the clinician – stepwise from the beginning. Issues that must be addressed in all patients, such as tidal volume, PEER and ventilator mode, are addressed first. This is then followed by “what if.” What if hypoxemia is still a problem? What if hypotension is a problem? Finally, important complications of mechanical ventilation are addressed.

Initiating Mechanical Ventilation

In patients with significant ARDS, one of two fundamental derangements typically leads the intensivist to begin mechanical ventilation: either hypoxemia no longer responds to oxygen (due to intrapulmonary shunt) or the work of breathing becomes excessive and unsustainable (due to reduced lung compliance, often compounded by a high minute ventilation). A subset of patients may be candidates for noninvasive ventilation, described later. Most will require endotracheal intubation (see Table 7.1).