Purpose: Adaptive support ventilation (ASV) is a microprocessor-controlled, closed-loop mode of mechanical ventilation that adapts respiratory rates and tidal volumes (V(T)s) based on the Otis least work of breathing formula. We studied calculated V(T)s in a computer simulation model, and V(T)s delivered in a test lung setting as well as in clinical practice.
Materials and methods: In a computer simulation model using the Otis formula, V(T)s were calculated for increasing predicted body weights (from 50 to 80 kg) and increasing minute volumes (from 0.7 to 1.5 ml/kg). Different compliance-resistance combinations were chosen to mimic "acute lung injury (ALI)" (compliance 27 ml/cmH(2)O, resistance 20 cmH(2)0 l/s), "ALI using an open lung approach" (compliance 50 ml/cmH(2)O, resistance 20 cmH(2)0 l/s), "healthy lungs" (compliance 65 ml/cmH(2)O, resistance 20 cmH(2)0 l/s) and "chronic obstructive pulmonary disease (COPD)" (compliance 80 ml/cmH(2)O, resistance 50 cmH(2)O l/s). In a test setting using a human ventilator connected to a test lung set to mimic similar pulmonary conditions, V(T)s delivered by the ASV were studied. In a series of stable intensive care unit patients after cardiothoracic surgery, the delivered V(T)s were collected and analyzed.
Results: V(T)s with the Otis formula resembled those in the test setting. With ALI, the ventilator delivered V(T)s between 6 and 8 ml/kg. With ALI using an open lung approach and with healthy lungs, the ventilator delivered V(T)s between 8 and 10 ml/kg. With COPD, all V(T)s were above 10 ml/kg. In patients after coronary artery bypass surgery ASV delivered V(T)s between 7 and 9 ml/kg and V(T)s never exceeded 10 ml/kg.
Discussion: The ASV performed as intended, bearing in mind that the ASV algorithm was originally designed to provide V(T)s between 8 and 12 ml/kg. However, the V(T)s that were calculated and delivered were frequently higher than those presently recommended in the guidelines. Considering the size of V(T) delivered in the setting of ALI using an open lung approach as well as in the setting of COPD, we feel caution should be taken when applying ASV in patients with these conditions.