Acute severe asthma: performance of ventilator at simulated altitude

Introduction

Exacerbation of asthma can appear during air transport. Severe patients, not responding to conventional therapy can require ventilator support. We evaluated the performance of two transport ventilators, built with turbine technology - the T-bird VSO2 and the LTV 1000 - for use during aeromedical evacuation of acute severe asthma. We have assessed the ability of both ventilators to deliver to an acute severe asthma model a tidal volume (Vt) set at different simulated altitudes, by changing the ambient air pressure.

Methods

We used a decompression chamber to mimic the hypobaric environment at a range of simulated cabin altitudes of 1,500, 2,500 and 3,000 meters (4,000, 6,670, 8,000 feet). Ventilators were tested with realistic parameters. Vt was set at 700 ml and 400 ml in an acute severe asthma lung model. The protocol included three measurements for each simulated altitude. Comparisons of preset to actual measured values were accomplished using a t test for each altitude.

Results

Figure 1 summarizes the data. With altitude, the T-bird VSO2 showed a decrease in volume delivered. Comparisons of actual delivered Vt and set Vt demonstrated a significant difference starting at 1,500 m for a Vt set of 700 ml, at 2,500 m for Vt set of 400 ml, with a negative variation of more than 10% compared with tidal volume set at respectively 3,000 and 2,500 m. With decreasing barometric pressure, the LTV 1000 showed mostly an increase in volume delivered. Comparisons of actual delivered Vt and set Vt demonstrated a significant difference at 1,500 m for a Vt set of 700 ml, at 2,500 m for a V test of 400 ml. The delivered tidal volume remained within 10% of the set Vt.

Figure 1

Acute severe asthma, Vt delivered in altitude.

Conclusions

The T-bird VSO2 progressively delivered lower volumes as barometric pressure decreased; whereas the LTV 1000 showed a moderate increase in volume delivered for the acute severe asthma lung model, with increasing altitude, but maintained the delivered volume within 10% of the set Vt up to 3,000 m.