Effects of posture on respiratory mechanics in obesity.
Yap, J. C. H., R. A. Watson, S. Gilbey, N. B. Pride.
DEPARTMENT OF MEDICINE, ROYAL POSTGRADUATE MEDICAL SCHOOL,
HAMMERSMITH HOSPITAL, DUCANE ROAD, LONDON W12 ONN
APStracts 2:0222A, 1995.
Although the increased abdominal mass in obesity would be expected to
enhance normal gravitational effects on supine respiratory mechanics,
few studies are available because of the difficulty in obtaining
reliable measurements of oesophageal pressure. We have examined the
changes in respiratory mechanics (using forced oscillation over the
range 4-26Hz applied at the mouth), subdivisions of lung volumes,
maximum inspiratory and expiratory mouth pressures, maximum effort
flow-volume curves and oxygen saturation (SaO2) in the sitting and
supine positions in 7 obese(O) subjects (mean age 51 years, weight
120 kg, body mass index 43.6 kg.m-2) and 7 age-matched controls(C)
(mean age 50 years, weight 66 kg, body mass index 21.8 kg.m-2).
Seated, mean total lung capacity(TLC) was smaller in O than C (82% vs
100% of predicted); functional residual capacity(FRC)/TLC averaged
43% in O and 61% in C (p<0.01). Total respiratory resistance at 6
Hz (Rrs6) seated during tidal breathing was higher in O(4.6 cmH2O.l
-1.s) than in C(2.2 cmH2O.l-1.s)(p<0.001).; total respiratory
reactance at 6Hz(Xrs6) was lower in O (-1.20cmH2O.l-1.s) than in C(
-0.37cmH2O.l-1.s) and resonant frequency(fR) higher in O(17.0Hz) than
in C(7.8Hz) In C, on changing to the supine posture, Rrs rose to
2.9cmH2O.l-1.s, FRC fell by a mean of 0.68 l and there were small
falls in Xrs6 and increase in fR. In contrast in O, although there
was no further fall in FRC, Rrs6 increased to 7.3cmH2O.l-1.s, marked
frequency dependency of Rrs developed, there were striking falls in
Xrs and a further increase in fR. There were no significant
differences in maximum effort mouth pressures between C and O when
seated, but in O there was a small fall in maximum expiratory
pressure and reductions in maximum expiratory flow in the supine
posture. The results confirm considerable reduction in FRC/TLC in
seated obese subjects, but, in contrast to normal subjects, there was
no further fall in FRC in the supine posture. This may explain why
supine SaO2 was relatively well maintained (mean value 96.3% sitting,
94.7% supine). Despite the lack of change in FRC there were
considerable increases in respiratory impedance in the supine posture
due to increase in Rrs and fall in Xrs. The serial site of the
increase in supine Rrs, the cause of the reduction in supine Xrs and
the mechanism maintaining FRC, despite the increased mass of the
abdominal contents in obese subjects, all need further investigation.
Received 27 January 1995; accepted in final form 27 April 1995.
APS Manuscript Number A101-5.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 30 May 1995.