adaptive support ventilation (ASV)
ASV is a closed-loop ventilation mode based on the concept of minimal work of breathing (WOB).
This idea was introduced by Otis in 1954. (--> See physics section)
The aim is to maintain oxygenation and ventilation while using optimal tidal volumes and respiratory rate to minimise WOB.
With ASV the ventilator can be set to deliver a percentage of minute ventilation from 25% - 350% of baseline. Setting a lower percentage will encourage spontaneous ventilation.
The tidal volume is set by the ventilator as it divides the minute volume by the optimal rate calculated using Otis' formula.
1. The optimal frequency will decrease with increasing non-elastic resistance and vice versa
2. The optimal frequency will decrease with increasing compliance (= greater time constant (Tau)) and vice versa
3. The optimal frequency will increase with increasing alveolar ventilation and vice versa
During spontaneous breathing, ASV only controls the amount of pressure support (PS).
ASV also controls Ti to maintain enough time for expiration on the basis of the time constant (Tau).
ASV in passive patients
In passive patients, ASV is a volume-targeted pressure controlled mode with automatic adjustment of inspiratory pressure, respiratory rate, and inspiratory/expiratory time ratio. Maximum tidal volume is controlled by setting a maximum inspiratory pressure. Expiratory time is determined according to the expiratory time constant in order to prevent dynamic hyperinflation.
ASV in active patients
In spontaneously breathing patients, ASV is a volume-targeted pressure support mode with automatic adjustment of pressure support according to the spontaneous respiratory rate. The automatic decrease of pressure support when the patient recovers their inspiratory strength is useful for weaning. ASV can also be used to perform a weaning trial before extubation.