An evaluation of the Deltatrac indirect calorimeter by gravimetric injection and alcohol burning.

We evaluated a 'state of the art' open hood type of indirect calorimeter (Deltatrac, Datex) to determine its stability and precision, and related these to in vivo measurements made with the same type of calorimeter. Three Deltatrac Metabolic Monitors were investigated at two centres over a period of twelve months by two methods of recovery: (i) gravimetric injection of a nitrogen/CO2 gas mix and (ii) ethanol burning using the manufacturers approved apparatus. We compared machine variation with in vivo variation in measurements of oxygen uptake and carbon dioxide production at rest in healthy subjects. The Deltatrac recovery of both oxygen and carbon dioxide was on average 4.8% and 4.7% higher respectively for the alcohol burning method. We suspected that this apparent over-recovery was because the alcohol burner produced a resistance to the airflow generated across the canopy. By gravimetrically injecting the gas mix into the canopy hose whilst the alcohol burner was attached we were able to confirm this error. There was no significant change in recovery for any of the Deltatracs by either method over the year of evaluation. The between-study variation for the machine and in vivo measurements were less than 4%, and the within-study variations were less than 6% and less than 7%, respectively. These studies re-emphasise that careful evaluation of commercially produced indirect calorimeters is essential before clinical measurements are made.