Chapter 6: Traditional Modes of Mechanical Ventilation

Introduction

The relationship between breath types and phase variables is referred to as a mode of ventilation. During mechanical ventilation, the mode is one of the principal ventilator settings. Although many modes are available, the choice of mode is usually based on clinician's preference or institutional bias, since evidence is lacking that one mode is clearly superior to others. This chapter describes traditional ventilator modes (Table 6-1), which include continuous mandatory ventilation (CMV), continuous spontaneous ventilation (CSV), and synchronized intermittent mandatory ventilation (SMV).

Volume-Controlled Versus Pressure-Controlled Ventilation

The two general approaches to ventilatory support are volume-controlled ventilation and pressure-controlled ventilation. At any time, the ventilator controls either volume (flow) or pressure applied to the airway. Some volume-targeted modes such as pressure-regulated volume control and adaptive support ventilation actually adjust the level of pressure control to achieve the set tidal volume. Although the term volume control is usually used, in reality the ventilator controls the inspiratory flow. The important variables for volume-controlled ventilation are shown in Figure 6-1. During pressure-controlled ventilation, the inspiratory flow decreases as the alveolar pressure approaches the pressure applied to the airway. The important variables affecting pressure-controlled ventilation are illustrated in Figure 6-2.

Continuous Mandatory Ventilation

A minimal rate is set with CMV (Figure 6-3). The patient can trigger the ventilator at a more rapid rate, but every breath delivered is a mandatory breath type. Note that the mandatory breaths can be either volume-controlled or pressure-controlled. CMV is commonly called assist/control (A/C) ventilation—the terms CMV and A/C are used interchangeably. Note that, from the perspective of the ventilator, there ...