What are the basic modes of mechanical ventilation?

Basic Modes of Mechanical Ventilation

Start with volume-cycled Assist/Control Ventilation when initiating mechanical ventilation, as it provides complete ventilatory support immediately after intubation and prevents central apneas. 1

Fundamental Classification: Volume vs. Pressure Targeting

Mechanical ventilation operates through two fundamental approaches that determine how breaths are delivered 2:

Volume-Targeted Ventilation

• The clinician sets a specific tidal volume and inspiratory time (Ti), and the ventilator generates whatever pressure is necessary to deliver this volume within the set time. 2
• The resulting pressure varies based on circuit compliance and thoracic mechanics (airway resistance, lung/chest wall compliance). 1, 3
• This mode ensures consistent tidal volume delivery regardless of changes in lung mechanics. 3

Pressure-Targeted Ventilation

• The clinician sets the inspiratory pressure level, and the delivered tidal volume becomes a function of lung impedance, airway resistance, and inspiratory time. 2, 3
• The Ti must be sufficiently long to achieve adequate volume and allow complete exhalation. 2
• This mode offers several advantages: constant pressure delivery avoids uncomfortable pressure spikes, compensates for air leaks (critical for non-invasive ventilation), and positive expiratory pressure (EPAP) flushes CO2 and prevents upper airway collapse. 2

Core Ventilatory Modes

Controlled Mechanical Ventilation (CMV)

• CMV provides full ventilatory support with zero patient effort required, delivering preset breaths at fixed intervals regardless of patient respiratory drive. 1, 3
• In pressure control CMV, tidal volume varies based on airway resistance and lung/chest wall compliance. 1
• In volume control CMV, tidal volume remains fixed while pressure adjusts to achieve delivery. 1
• This mode is used when complete control of ventilation is needed (deep sedation, paralysis, or absent respiratory drive). 3

Assist/Control Ventilation (AC)

• AC mode guarantees a preset minimum number of mandatory breaths per minute while allowing patient-triggered breaths, with all breaths—whether mandatory or triggered—delivering identical preset parameters. 1, 3
• This is the recommended initial mode when starting mechanical ventilation because it provides complete ventilatory support and prevents central apneas during sleep. 1, 4
• Critical pitfall: Setting a long expiratory time creates a long "lock out" period that can lead to poor patient tolerance and patient-ventilator asynchrony. 3
• AC prevents central apneas better than pressure support ventilation due to its backup rate, making it superior for nighttime ventilation. 4

Synchronized Intermittent Mandatory Ventilation (SIMV)

• SIMV synchronizes patient-triggered breaths with machine-delivered breaths, delaying the next mandatory breath when a patient triggers within a specified time window. 1
• Between mandatory breaths, patients can take spontaneous breaths that may or may not be supported (depending on whether pressure support is added). 5
• Research shows SIMV alone is less efficient than AC, but adding pressure support (10 cmH₂O) significantly improves minute volume and ventilatory equivalent. 5

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. 1, 4
• This mode requires intact respiratory drive and adequate respiratory muscle function. 4
• Major caveat: Excessive pressure support levels can cause hyperventilation, hypocapnia, and central apneas, especially during sleep. 4
• PSV is associated with more sleep fragmentation and central apneas compared to AC mode. 4

Continuous Positive Airway Pressure (CPAP)

• CPAP maintains constant positive pressure throughout the respiratory cycle to correct hypoxemia by recruiting underventilated lung units, functioning similarly to PEEP. 1
• This mode provides no ventilatory assistance—only continuous positive pressure. 1

Critical Initial Settings

Always target 6 mL/kg predicted body weight (not actual body weight) to reduce mortality in ARDS and sepsis-induced respiratory failure. 1

Essential Parameters

• Maintain plateau pressure ≤30 cmH₂O to prevent alveolar overdistension and ventilator-induced lung injury. 1
• For ARDS: Use AC with low tidal volumes (6 mL/kg PBW), plateau pressure ≤30 cmH₂O, and prone positioning >12 hours/day. 1
• For post-cardiac arrest: Avoid hyperventilation and target normocapnia with PaCO₂ 40-45 mmHg. 1
• Ensure adequate expiratory time (inspiratory-to-expiratory ratio of 1:2 or 1:3) to prevent air trapping. 4

Common Pitfalls to Avoid

• Never use actual body weight for tidal volume calculations—always use predicted body weight based on height and sex. 1
• Do not hyperventilate patients, as this causes cerebral vasoconstriction, hemodynamic instability, and increased mortality. 1
• Be aware that terminology for ventilation modes varies significantly between ventilator manufacturers, potentially causing dangerous confusion in clinical practice. 1, 3, 4
• Recognize that all mechanical ventilation modes are less efficient than spontaneous breathing and increase oxygen consumption. 5
• In the UK and many centers, volume ventilators are rarely employed for non-invasive ventilation outside specialist centers. 2

Practical Algorithm for Mode Selection

For initial intubation: Start with volume-cycled AC at 6 mL/kg PBW, respiratory rate 12-16, plateau pressure ≤30 cmH₂O. 1

For prolonged ventilation: Use AC at night to prevent central apneas and improve sleep quality; consider PSV during daytime with careful titration to avoid hyperventilation. 4

For weaning: Transition to PSV when patient demonstrates adequate respiratory drive, but screen for weanability through weaning predictor tests and T-tube trials to circumvent the impossibility of estimating work of breathing during pressure support. 6