Comparison of a face-mask and a mouthpiece for measuring resting energy expenditure with the 'Oxylog'.

Resting energy expenditure measurements were made simultaneously with an Oxylog and a ventilated hood indirect calorimeter, using (a) the Oxylog with the original oronasal face-mask and (b) the Oxylog with a mouthpiece and a noseclip, both in 15 subjects. The mean difference between Oxylog energy expenditure, when used with the mouthpiece, and ventilated hood measurements was very small: -0.4% (p less than 0.4; n = 15). When used with the face-mask, the Oxylog tended to underestimate energy expenditure relative to the ventilated hood, the mean difference being -2.4% (p = 0.054; n = 13). In 2 subjects, the Oxylog face-mask showed obvious leaks. The subjects did not find the mouthpiece less comfortable than the face-mask, and there was no evidence that the mouthpiece disturbed respiration more than the face-mask. The Oxylog is therefore suitable for measurements of resting energy expenditure, but the use of a mouthpiece and noseclip is preferable to the use of the original Oxylog face-mask.