Adaptive Support Ventilation: A Translational Study Evaluating the Size of Delivered Tidal Volumes

Abstract

Purpose

Adaptive support ventilation (ASV) is a microprocessor-controlled, closed-loop mode of mechanical ventilation that adapts respiratory rates and tidal volumes (V_Ts) based on the Otis least work of breathing formula. We studied calculated V_Ts in a computer simulation model, and V_Ts 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_Ts 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/cmH₂O, resistance 20 cmH₂0 l/s), “ALI using an open lung approach” (compliance 50 ml/cmH₂O, resistance 20 cmH₂0 l/s), “healthy lungs” (compliance 65 ml/cmH₂O, resistance 20 cmH₂0 l/s) and “chronic obstructive pulmonary disease (COPD)” (compliance 80 ml/cmH₂O, resistance 50 cmH₂O l/s). In a test setting using a human ventilator connected to a test lung set to mimic similar pulmonary conditions, V_Ts delivered by the ASV were studied. In a series of stable intensive care unit patients after cardiothoracic surgery, the delivered V_Ts were collected and analyzed.

Results

V_Ts with the Otis formula resembled those in the test setting. With ALI, the ventilator delivered V_Ts between 6 and 8 ml/kg. With ALI using an open lung approach and with healthy lungs, the ventilator delivered V_Ts between 8 and 10 ml/kg. With COPD, all V_Ts were above 10 ml/kg. In patients after coronary artery bypass surgery ASV delivered V_Ts between 7 and 9 ml/kg and V_Ts never exceeded 10 ml/kg.

Discussion

The ASV performed as intended, bearing in mind that the ASV algorithm was originally designed to provide V_Ts between 8 and 12 ml/kg. However, the V_Ts 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.