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Feedback Mechanism in Volume Support Ventilation

Updated: Oct 7, 2022


Volume support (VS) ventilation is a variable pressure support that targets a preset tidal volume. It is a spontaneously triggered mode where the patient determines the frequency and the duration of the breaths. In pressure support ventilation, the pressure is constant at the operator's preset value and the volume will be variable depending on the patient's efforts to overcome the airway resistance and the lung and chest wall elastance. In volume support ventilation, there is a feedback mechanism similar to PRVC, where the pressure support will be adjusted to ensure delivery of the targeted tidal volume.


Volume support ventilation is available on Servo ventilators and should not be confused with variable pressure support on Dragger ventilators, where the pressure support level is randomly changed to provide variable volume with each breath.


In volume support ventilation, once the target volume is set by the operator, a test breath (5 or 10 cm H2O dependent on the Servo model) is given initially, followed by increase in pressure during the next three breaths until target volume is achieved; the maximum available pressure is 5 cm H2O below upper pressure limit. Afterwards, if the delivered VT is lower than the target VT (due to decreased efforts, high resistance, or low compliance), then the pressure will be increased with each breath at 3 cm H2O increments until the target volume is delivered or the pressure limit is achieved. Conversely, if the delivered VT is higher than the set tidal volume, then the pressure will be decreased gradually until the preset VT is achieved.


In volume support, the patient triggers all breaths and the operator can determine the trigger sensitivity level. The flow is a decelerating flow waveform and cycling mechanism is a percentage of the maximum inspiratory flow that should be determined by the operator. If the patient goes into an apnea, then the ventilator will switch to PRVC once the apnea alarm is detected, then back to VS once the patient starts triggering again (automode).



This mode works based on a feedback mechanism, which begins once the patient triggers a breath; the ventilator delivers pressure based on the VT/C and maintains this pressure limit as long as the flow has not reached the cycling threshold (5% of the peak flow for example). Once the flow reaches the predetermined value, the ventilator will cycle off and terminate the breath. The respiratory system compliance is calculated based on the required pressure and the delivered tidal volume of the previous breath. If the delivered volume is equal to the set tidal volume, the machine will no longer make changes and will deliver the next breath with the same parameters. In case the delivered volume is higher (improved compliance) or lower (worsened compliance), the machine will calculate a new lower or higher-pressure limit as needed.


Looking through the literature, the only study I was able to find comparing VS to PSV as a weaning mode was published in the Turkish Journal of Anesthesiology & Reanimation. The total weaning time was shorter in VS compared to PSV among 60 subjects, each enrolled in a randomized fashion to receive volume support versus pressure support as means of weaning.


REFERENCES

1.Sancar NK, Özcan PE, Şentürk E, Selek Ç, Çakar N. The Comparison of Pressure (PSV) and Volume Support Ventilation (VSV) as a 'Weaning' Mode. Turk J Anaesthesiol Reanim. 2014;42(4):170-175. doi:10.5152/TJAR.2014.61687







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