Is Controlled Mechanical Ventilation (CMV) the same as Assist Control (AC), and how do they differ in volume control versus pressure control modes?

Understanding CMV vs. AC Ventilation and Their Differences in Volume vs. Pressure Control

Controlled Mechanical Ventilation (CMV) and Assist Control (AC) are distinct ventilation modes with key differences in patient interaction, though they share similarities in how they deliver breaths in volume versus pressure control settings.

Differences Between CMV and AC

• CMV (Controlled Mechanical Ventilation) provides full ventilatory support with no patient effort required. The ventilator delivers a preset number of breaths at set parameters (pressure or volume) regardless of patient effort 1.

• AC (Assist/Control Ventilation) delivers a preset number of mandatory breaths but also allows patient triggering. When the patient initiates a breath, the ventilator delivers an identical breath to the mandatory ones 1, 2.

• The key distinction is that CMV does not respond to patient effort, while AC permits patient-triggered breaths while maintaining a backup rate 2.

Volume Control vs. Pressure Control in Both Modes

Volume Control

• In volume control, regardless of whether using CMV or AC:

  • The tidal volume is set by the clinician 1, 3
  • The ventilator generates whatever pressure is necessary to deliver this volume 1
  • The resulting pressure depends on circuit compliance and thoracic mechanics 1
  • The clinician must set inspiratory flow, flow waveform, and inspiratory time 3

• During volume control, airway pressure may increase in response to reduced compliance or increased resistance, potentially increasing the risk of ventilator-induced lung injury 3.

Pressure Control

• In pressure control, regardless of whether using CMV or AC:

  • The inspiratory pressure is set by the clinician 1, 3
  • The resulting tidal volume depends on resistance to flow, airflow limitation, and compliance of the lungs and chest wall 1
  • The ventilator determines inspiratory flow and flow waveform to maintain a square inspiratory pressure profile 3
  • Pressure is limited by design, which may reduce the risk of ventilator-induced lung injury 3

• However, pressure control may result in variable tidal and minute volumes, requiring clinicians to titrate inspiratory pressure to achieve desired tidal volumes 3.

Clinical Implications and Considerations

• AC mode is generally preferred for patients who are not deeply sedated and can initiate breaths but cannot maintain adequate spontaneous ventilation 2.

• CMV is more appropriate for patients with severe neurological alterations, deep sedation, shock, or severe respiratory failure 2.

• In AC mode, patient-ventilator asynchrony can occur, potentially leading to sleep disruption and increased work of breathing 1.

• Studies have shown that AC mode may prevent central apneas during sleep compared to pressure support ventilation (PSV) due to its backup rate 1.

• When properly set to deliver the same tidal volume and peak inspiratory flow, different modes of assisted ventilation (including pressure and volume control) can provide similar work of breathing and gas exchange 4.

Common Pitfalls and Caveats

• Terminology varies between ventilator manufacturers, causing confusion in clinical practice 1.

• In AC mode, setting a long expiratory time may also set a long "lock out" period that can lead to poor patient tolerance 1.

• During pressure control, clinicians must monitor delivered tidal volumes as they may vary with changes in patient condition or effort 3.

• In volume control, failure to set appropriate inspiratory flow may increase work of breathing and cause patient discomfort 4.

• Adaptive pressure control modes have been developed to combine attributes of both volume and pressure ventilation, but they cannot guarantee a constant tidal volume 5.