Mandatory Breaths

• Volume control (volume limited)- set TV and flow, pressure and inspiratory time are the dependent variables.

• Pressure control (pressure limited)- set inspiratory pressure and inspiratory time, volume and flow are the dependent variables.

What is the downside of VC and PC? You can only control one parameter, and the dependent variable varies based on the patient's lung mechanics. For a patient on VC, if their lungs become less compliant, delivering the same tidal volume will generate higher pressure, increasing the risk of barotrauma. For a patient on PC, if their lungs become less compliant, the target pressure will be reached at a lower volume, so there is a risk of decreased ventilation (↑PaCO2).

Pressure-regulated volume control (PRVC) is a hybrid mode that attempts to overcome this limitation. The target volume is delivered at the lowest possible inspiratory pressure by assessing the delivered tidal volume at the inspiratory pressure during each breath.

What about inverse ratio (IR, IRV-PC)? Increasing the inspiratory time relative to expiratory time increases mean airway pressure. This can be accomplished with pressure-controlled modes, where inspiratory time can be prolonged (normal ratio 1:2, IRV is when inspiratory time is greater than expiratory time). As discussed, MAP affects the surface available for oxygen exchange. This is why IR can be used to optimize oxygenation.

Mandatory and Spontaneous Breaths

• Synchronized intermittent mandatory ventilation (SIMV)- a variation on VC or PC. The machine delivers mandatory breaths, but the patient can also control spontaneous breaths in between the mandatory breaths.

Spontaneous Breaths

• Pressure support- spontaneous mode, the patient initiates breath, the ventilator provides support to overcome the resistance of breathing through the endotracheal tube, flow is adjusted to maintain the inspiratory pressure. The support is terminated when the flow decreases to <25% of peak flow. The patient controls duration and volume. *This is also a setting that can be adjusted in SIMV for assisting spontaneous breaths between ventilator breaths.

• Airway Pressure Release Ventilation (APRV)- invasive form of ventilation with BiPAP. The patient breaths spontaneously, alternating between a sustained time (time-high) at a set pressure (pressure-high) with a very brief release (time-low) of pressure (pressure low) to allow expiration. The goal is to maintain a higher MAP to optimize oxygenation.