A method for estimating respiratory muscle efficiency using an automated metabolic cart

We measured respiratory muscle efficiency (RME) in twelve healthy human subjects by dividing the added energy cost of breathing against a threshold resistance load by the associated increase in caloric expenditure. Caloric expenditure was calculated using steady-state measurements of oxygen consumption and carbon dioxide production during loaded and unloaded breathing. Calculated RME ranged from 1.7% to 5.5% (mean 3.5%). The coefficient of variation in six subjects averaged 13%. We compared these calculations with a previously described oxygen consumption-based method that did not incorporate carbon dioxide production measurements. We found that changes in the respiratory quotient during resistive breathing could cause significant errors in oxygen consumption-based calculations of RME. Limits of agreement of 95% suggest that the oxygen-consumption-based calculations could potentially overestimate efficiency by as much as 5.0% or underestimate by up to 3.4%. We recommend that carbon dioxide production be measured when this technique for estimation of RME is used. This can be easily accomplished through the use of an automated metabolic cart.