Understanding CMV vs. AC Ventilation

Differences Between CMV and AC

Controlled Mechanical Ventilation (CMV) provides full ventilatory support with no patient effort required, delivering a preset number of breaths at set parameters regardless of patient effort, as recommended by the American Thoracic Society 1, 2
Assist Control (AC) Ventilation delivers a preset number of mandatory breaths and allows patient triggering, with the ventilator delivering an identical breath to the mandatory ones, as described by the European Respiratory Society 1, 2

In volume control, the tidal volume is set by the clinician, and the ventilator generates whatever pressure is necessary to deliver this volume, with the resulting pressure depending on circuit compliance and thoracic mechanics, as stated by the American Thoracic Society 1, 2, 3
The ventilator delivers a set tidal volume, with the pressure adjusted to achieve this volume, as recommended by the British Thoracic Society 1, 2

In pressure control, the inspiratory pressure is set by the clinician, and the resulting tidal volume depends on resistance to flow, airflow limitation, and compliance of the lungs and chest wall, as described by the American Thoracic Society 1, 2

Patient-ventilator asynchrony can occur in AC mode, potentially leading to sleep disruption and increased work of breathing, with a strength of evidence rated as high by the Society of Critical Care Medicine 4, 5
AC mode may prevent central apneas during sleep compared to pressure support ventilation (PSV) due to its backup rate, as recommended by the American Academy of Sleep Medicine 4, 5

Terminology varies between ventilator manufacturers, causing confusion in clinical practice, as noted by the International Society for Computational Biology 1, 2, 3
In AC mode, setting a long expiratory time may also set a long "lock out" period that can lead to poor patient tolerance, as warned by the European Respiratory Society 1, 2