What are the primary modes of mechanical ventilation?

Primary Modes of Mechanical Ventilation

The primary modes of mechanical ventilation are Controlled Mechanical Ventilation (CMV), Assist/Control Ventilation (AC), Pressure Support Ventilation (PSV), Synchronized Intermittent Mandatory Ventilation (SIMV), and Continuous Positive Airway Pressure (CPAP), with volume-cycled assist-control being the recommended initial mode for most patients requiring mechanical ventilation. 1

Core Ventilatory Modes

Controlled Mechanical Ventilation (CMV)

• CMV provides full ventilatory support with no patient effort required, delivering preset breaths at fixed intervals regardless of patient respiratory drive 2, 3
• The clinician sets either inflation pressure (pressure control) or tidal volume (volume control), along with respiratory frequency and timing of each breath 2
• In pressure control, the resulting tidal volume varies based on airway resistance, airflow limitation, and lung/chest wall compliance 2, 3
• In volume control, tidal volume is fixed and the ventilator generates whatever pressure is necessary to deliver this volume, determined by circuit compliance and thoracic mechanics 2, 3

Assist/Control Ventilation (AC)

• AC mode guarantees a preset number of mandatory breaths per minute while allowing patient-triggered breaths, with all breaths delivering identical preset parameters 1, 3
• The American Thoracic Society recommends starting with volume-cycled AC ventilation when initiating mechanical ventilation, as it provides complete ventilatory support immediately after intubation and prevents central apneas 1
• Patient triggering is permitted, but the ventilator delivers an identical breath to mandatory breaths, preventing hypoventilation if the patient becomes apneic 2
• A "lock out" period prevents excessive inflation through breath stacking; however, setting a long expiratory time may create a long lock out period leading to poor patient tolerance 2, 3

Synchronized Intermittent Mandatory Ventilation (SIMV)

• SIMV synchronizes patient-triggered breaths with machine-delivered breaths, delaying the next mandatory breath when a patient triggers 2
• This mode can achieve similar degrees of respiratory support as AC ventilation and may be used as an alternative 1
• Also referred to as spontaneous/timed (S/T) or IE mode on non-invasive ventilation machines 2

Pressure Support Ventilation (PSV)

• In PSV, the patient's respiratory effort triggers the ventilator both on and off, with the patient determining respiratory frequency and timing of each breath 2, 4
• PSV is not recommended as the initial mode but may be used during weaning or for prolonged ventilation in stable patients 1
• If the patient fails to make respiratory effort, no respiratory assistance occurs, though many manufacturers incorporate a backup rate of 6-8 breaths per minute 2
• The Intensive Care Medicine society warns that excessive support levels in PSV can cause hyperventilation, hypocapnia, and central apneas, especially during sleep 4

Continuous Positive Airway Pressure (CPAP)

• CPAP is employed to correct hypoxaemia by maintaining constant positive pressure throughout the respiratory cycle, recruiting underventilated lung similar to PEEP 2
• CPAP permits higher inspired oxygen content, increases mean airway pressure, and improves ventilation to collapsed lung areas 2
• In COPD exacerbations, CPAP offsets intrinsic PEEP, reducing ventilatory work and potentially lowering PaCO2 2
• Conventionally, CPAP is not considered respiratory support; its main indication is to correct hypoxaemia 2

Critical Initial Settings

Tidal Volume and Pressure Targets

• The American College of Chest Physicians recommends targeting 6 mL/kg predicted body weight (not actual body weight) to reduce mortality in ARDS and sepsis-induced respiratory failure 1
• Calculate predicted body weight using: Men = 50 + 2.3 × (height in inches - 60); Women = 45.5 + 2.3 × (height in inches - 60) 1
• Maintain plateau pressure ≤30 cmH₂O to prevent alveolar overdistension and ventilator-induced lung injury 1

Mode Selection for Specific Conditions

• For ARDS (severe: PaO₂/FiO₂ <100), use AC with low tidal volumes (6 mL/kg PBW) and plateau pressure ≤30 cmH₂O, adding prone positioning for >12 hours/day 1
• For post-cardiac arrest, avoid hyperventilation and target normocapnia with PaCO₂ 40-45 mmHg 1
• For sepsis-induced respiratory failure, follow the same lung-protective strategy as ARDS 1
• The British Journal of Anaesthesia suggests using volume-controlled ventilation for patients with obesity, as it is associated with lower peak airway pressures and less dead space ventilation 4

Common Pitfalls to Avoid

• Do not use actual body weight for tidal volume calculations—always use predicted body weight 1
• Do not hyperventilate patients, as this causes cerebral vasoconstriction, hemodynamic instability, and increased mortality 1
• Do not use high-frequency oscillatory ventilation (HFOV) routinely in ARDS—it does not improve outcomes 1
• Be aware that terminology for ventilation modes varies between ventilator manufacturers, potentially causing confusion 4, 3
• Patient-ventilator asynchrony can occur in AC mode, potentially leading to sleep disruption and increased work of breathing 3