Long term tidal breathing measurements in newborns: influence of a bias flow on ventilatory parameters

Long-term pneumotachographic measurements in newborns are hampered by the apparatus dead space, which can be functionally eliminated by a constant bias flow (flow-through technique). In a clinical study the influence of this bias flow was investigated. - In 80 infants (group 1:20 healthy controls, group 2: 30 infants recovering from a pulmonary disease, group 3: 30 pulmonary ill infants) with a body weight 1060-9000g and an age 1-402d respiratory frequency (f) was measured pneumotachographically (PN) as well as with breathing belts (BB) in four periods: before any alteration (period 1, only BB), during the first calm interval after attaching the face mask, in which lung function tests are usually performed (period 2, PN and BB), in a defined steady state giving longer time for adaptation (period 3, PN and BB) and after removing of the mask (period 4, only BB). During periods 2 and 3 tidal breathing parameters were measured from the air flow signal. - In period 1 there were significant differences between the groups in f (1/min) (42.1 +/- 8.3, 44.2 +/- 9.3, 54.2 +/- 16.5, p <0.05) which also existed in periods 3 and 4, but not in period 2 (47.6 +/- 14.9, 44.8 +/- 16.3, 51.2 +/- 15.8). In periods 3 and 4 in all groups, f returned to the initial values. In contrast to group 3 we found significant differences in most of the tidal breathing parameters between periods 2 and 3 in group 1 and 2. - It was concluded that the bias flow has a negligible influence on tidal breathing measurements and allows a sufficient adaptation period necessary to recognise small deviations.