Practical limitations of the Deltatrac indirect calorimeter.

Indirect calorimetry is used to assess energy requirements. The Deltatrac Metabolic Monitor is a relatively inexpensive indirect calorimeter which uses a 'fixed' flow of ambient air to collect expired air. Only oxygen and carbon dioxide concentrations are measured and the 'fixed flow' is assumed in the calculation of oxygen consumption ((.)VO(2)) and carbon dioxide production ((.)VCO(2)). Using inert gas dilution we have studied the effect on (.)VO(2), and on the variability in (.)VO(2), of changing and lengthening the 1.77 m length of 35 mm tubing supplied with the instrument to collect expired air, and of using a mask to collect expired air instead of the manufacturer's hood. One would anticipate that changing the tubing could cause a change in resistance to gas flow and thus affect the true flow rate. This would alter the gas concentrations seen by the analysers, but the 'fixed flow' would still be assumed so the results would be in error. Adding extra lengths of manufacturers tubing caused an apparent rise in (.)VO(2) of 0.36%/m of tubing added, and using 22 mm tubing instead of the manufacturer's 35 mm tubing increased (.)VO(2) by 0.42% for each 10 cm of tubing added. Using the mask to collect expired air instead of the canopy (.)VO(2) was higher, possibly due to the energy cost of holding the mask, and was more variable, probably because of poorer mixing of the expired air. To measure (.)VO(2) using a mask with the same precision as a 10 min measurement made with the hood would entail measuring (.)VO(2) for 14.5 min. The methods used to collect expired air (mask or canopy, length and type of tubing) when measuring metabolic rate with the Deltatrac do affect the results obtained but these effects are small and predictable.