Ventilator Modes and Strategies

Background

• control vs support: in controlled modes, the ventilator is responsible for initiating and delivering each breath and assumes the entire work of breathing. In support modes, the patient initiates the breath but the work is partially offset.
• inspiratory breath characteristics
  • trigger - what is the variable the initiates the breath? Time (ventilator-triggered) or flow or pressure change (patient-triggered).
  • limit/target - the pressure, volume, or flow target during inspiration.
  • cycle - the variable the ends the inspiratory phase and signals the start of passive expiratory gas flow. This can be time, tidal volume, or decrease in flow to a percentage of the peak inspiratory flow.

Controlled Modes

Pressure Control Ventilation (PCV)

Pressure limited and time-cycled. Tidal volume is the dependent variable and changes based on the mechanics. Target pressure is achieved and flow then decelerates to maintain a constant pressure profile for the inspiratory time.

Volume Control Ventilation (VCV)

Volume limited and time-cycled. Predetermined tidal volume at set respiratory rate. Flow is usually a square waveform but can be decelerating or sinusoidal. Peak inspiratory flow or inspiratory time is set and determines the duration of inspiration. Guarantees minute ventilation; airway presures can vary based on the mechanics of the respiratory system.

High frequency oscillatory ventilation (HFOV)

Constant distending pressure (mean airway pressure MAP) is generated. Active inspiration and expiration with very small tidal volumes at 3-10 Hz (180 to 600 breaths per minute). Not been shown to improve mortality in ARDS with this lung-protective strategy.

Assist Modes

Assist Control Ventilation (ACV)

Etiher pressure or volume-limited. Essentially this is PCV or VCV except the patient may trigger the ventilator for any or all breaths. The breaths are delivered according to the pressure/volume settings. Guarantees a minimum number of breaths (i.e. there is a set back-up RR), and if the patient is not breathing this is indistinguishable from PCV or VCV.

Synchronized Intermittent Mandatory Ventilation (SIMV)

Similar to ACV, but in SIMV the patient is allowed to take unsupported breaths in between mandatory patient-triggered (or time-triggered) breaths. In the absence of additional PSV (see diagram below) or spontaneous breathing (e.g. in a fully sedated or paralyzed patient), this mode is essentially identical to ACV.

Pressure Support Ventilation (PSV)

Pressure limited and flow-cycled. Patient determines the respiratory rate and each breath is patient-triggered. Patient effort triggers gas flow increase until the pressure support level is reached. When inspiratory flow decreaes below a threshold, then the ventilator cycles to expiration.

Airway pressure release ventilation (APRV)

This is a time-cycled mode that alternates between two levels of airway pressure (Phigh and Plow). Most time is spent at Phigh to maintain alveolar recruitment, with brief cycling (<1 second) to Plow to allow for ventilation. The patient is meant to breath throughout the respiratory cycle (superimposed).

Lung-Protective Ventilation in ALI/ARDS

Ventilation can cause various mechanisms of lung trauma (1) barotrauma (2) volutrauma (3) atelectrauma (4) biotrauma which overall are termed "ventilator induced lung injury" (VILI). A lung-protective strategy aims to limit lung stress and strain (tidal voolumes, plateau pressures), and maintain PEEP to keep the lung recruited and to minimize atelectrauma.

Low tidal-volume ventilation

Generally used and proven in ARDS - see ARDSNet trial. May result in hypercapnia (can usually tolerate this in a permissive hypercapnic strategy).

Optimize PEEP

The optimal level of PEEP as part of a lung-protective strategy in ARDS is not definitely established. The ALVEOLI, LOVS, and Express trials failed to demonstrate a significant mortality benefit with higher PEEP, but higher PEEP patients overall required fewer rescue interventions for refractory hypoxemia. In meta-analysis, patients with ARDS at baseline (PF < 200) and higher PEEP were more likely to survive to hospital discharge.

HFOV

The OSCAR and OSCILLATE trials studied HFOV in ARDS - overall no mortality benefit in OSCAR, and actually showed increased mortality in OSCILLATE.

References

1. Ligori T. Modes of ventilation and ventilator strategies. In: McConachie I, Granton J, Fuller J, eds. Handbook of ICU Therapy. 3rd ed. Cambridge University Press; 2014:144-151. doi:10.1017/CBO9781107323919.014
2. Owens W. The Ventilator Book. First Draught Press; 2018.