Mode of Mechanical Ventilation: Volume Controlled Mode

It is important to understand the goals of mechanical ventilation (MV). The primary goal of ventilator support is the maintenance of adequate, but not necessarily normal, gas exchange, which must be achieved with min-imal lung injury and the lowest possible degree of hemodynamic impair-ment, while avoiding injury to distant organs such as the brain. Modes of MV are described by the relationships between the various types of breaths and by the variables that can occur during the inspiratory phase of ventilation. Each mode of ventilation is distinguished by how it ini-tiates a breath (trigger), how it sustains a breath (limit), and how it termi-nates a breath (cycle); these are referred to as phase variables. There are two basic modes of ventilation: ventilation limited by a pressure target and ventilation limited to the delivery of a specified volume. Volume-targeted ventilation modes can be categorized as follows: patient trigger or time trigger, flow-limited, volume-cycled assist/control, or synchronized intermittent mandatory ventilation (SIMV) modes. Volume-controlled mode In the volume-controlled mode, each machine breath is delivered with the same predetermined inspiratory flow–time profile. Because the area under a flow–time curve defines volume, the tidal volume (VT) remains fixed and uninfluenced by the patient’s effort. Volume-controlled ventilation (VCV) with constant (square wave) inspiratory flow is the most widely used breath Crit Care Clin 23 (2007) 161–167delivery mode. Alternative flow–time profiles such as decelerating or sinu-soidal inspiratory flow waveforms sometimes are used in the hope of reduc-ing the risk of barotraumas. Pressure is the dependent variable in the modes of ventilation in which volume is the target. Because pressure will vary i