Volume assist-control ventilation (ACV) is a ventilator mode in which the machine delivers the same tidal volume during every inspiration, whether initiated by the ventilator or by the patient. This occurs regardless of the mechanical load on the respiratory system and no matter how strenuous or feeble the inspiratory muscle effort. Current data indicate that ACV is still the most frequently used mode in intensive care units (ICUs).1 Nowadays, the main reason for patients being admitted to an ICU is the need for mechanical ventilation,2 and the most common reason to initiate mechanical ventilation is acute respiratory failure.1,3,4 Approximately 60% of intubated, ventilated patients receive ACV.5 This percentage is similar for patients ventilated for decompensated chronic obstructive pulmonary disease (COPD),5 and even higher for those ventilated for acute respiratory distress syndrome (ARDS).6
In ACV, mechanical breaths can be triggered by the ventilator or the patient. With the former, triggering occurs when a certain time has elapsed after the previous inspiration if the patient fails to make a new inspiratory muscle effort (Fig. 6-1). The frequency at which time triggering takes place is determined by the backup rate set on the ventilator. When patients trigger a mechanical breath, their spontaneous inspiratory effort is sensed by the machine, usually as a change in airway pressure or airflow. When such a change crosses the trigger-sensitivity threshold, the ventilator delivers the preset tidal volume. Chapter 3 provides a detailed explanation regarding the working principles of ventilators.
(From top to bottom) Tracings of airflow (FLOW), airway pressure (Paw), esophageal pressure (Pes), gastric pressure (Pga), and tidal volume (VOLUME). Each mark on the time axis denotes 1 second. These recordings were obtained in a passively ventilated patient. Each breath is time-triggered.
Mechanical breaths have precise mechanisms for being initiated (trigger variable), sustained (limit variable), and stopped (cycle variable). These are known as phase variables.7 In ACV, the mechanical breaths are limited by volume and/or flow and cycled by volume or time. The inspiratory flow-shape delivery is usually a square (constant) during ACV, although some ventilators also permit sinusoidal and/or ramp (ascending or descending) gas flows.
Mechanical ventilation is a lifesaving supportive treatment that improves gas exchange and decreases the mechanical workload of the respiratory muscles while buying time for the patient to recover. The way mechanical ventilation is used is central to its short-term and long-term effects. Ventilator settings are a major determinant of the physiologic and clinical effects of ACV. Chapters 36 and 37 address the physiologic effects of ACV on gas exchange and cardiovascular function.
In every assisted mode, the ventilator responds to a patient’s inspiratory effort. Both pressure-triggering and flow-triggering systems of modern ventilators offer high performance, ...