Chapter 147: Principles of Mechanical Ventilation

INTRODUCTION

The pivotal importance of mechanical ventilation was highlighted in the early days of the COVID-19 pandemic when scientific societies issued guidelines on how to connect four patients to a single machine, and politicians pleaded with automakers to branch into the manufacture of ventilators.¹

Given the panic and pandemonium, the prediction that patients were being inappropriately intubated and ventilated was quickly borne out, as 90% or more of patients were intubated in some intensive care units.^2,3 The number of deaths that resulted from unnecessary use of invasive mechanical ventilation will never be known, but it is likely to have been very high.⁴ A sound understanding of the principles of mechanical ventilation is vitally important for every pulmonary and critical care physician.

OBJECTIVES AND INDICATIONS FOR MECHANICAL VENTILATION

The objectives of mechanical ventilation are listed in Table 147-1.⁵ Authors have listed criteria to guide intubation, but none has stood the test of rigorous investigation.^1,6 Measurements such as oxygen saturation help, but it is not possible to pick a saturation breakpoint at which the benefits of mechanical ventilation will decidedly outweigh its hazards across all patients.² The decision to insert an endotracheal tube is, instead, based on clinical judgment, gestalt, and tacit knowledge individualized to each particular patient.^1,6

Increased work of breathing is the most common reason for intubation.⁶ Accurate measurement of work of breathing involves inserting an esophageal balloon to measure inspiratory effort, which requires a certain technical skill.⁷ More practical is estimation of work of breathing based on meticulous physical examination. The key step is gently touching the sternomastoid muscle with the index finger, checking for phasic—not tonic—contraction, and then judging its magnitude qualitatively (Fig. 147-1).⁸ Next is placing the index finger on the cricoid cartilage and judging the presence and degree of tracheal tug (Fig. 147-2). Then, inspecting the suprasternal fossa and the intercostal spaces for recession is performed (Figs. 147-3 and 147-4). The physician integrates these physical signs into the clinical context and judges if respiratory work is sustainable, or if assisted ventilation (invasive or noninvasive) is more prudent.^1,6,8