EFFECTS OF POSTURE ON RESPIRATORY MECHANICS IN OBESITY

Increased abdominal mass in obesity should enhance normal gravitationa l effects on supine respiratory mechanics. We have examined respirator y impedance (forced oscillation over 4-26 Hz applied at the mouth duri ng tidal breathing), maximum inspiratory and expiratory mouth pressure s (MIP and MEP), and maximum effort flow-volume curves seated and supi ne in seven obese subjects (O) (mean age 51 yr, body mass index 43.6 k g/m(2)) and seven control subjects (C) (mean age 50 yr, body mass inde x 21.8 kg/m(2)). Seated mean total lung capacity was smaller in O than in C (82 vs. 100% of predicted); ratio of functional residual capacit y (FRC) to total lung capacity averaged 43% in O and 61% in C (P < 0.0 1). Total respiratory resistance (Rrs) at 6 Hz seated was higher in O (4.6 cmH(2)O . 1(-1). s) than in C (2.2 cmH(2)O . 1(-1). s; P < 0.001) ; total respiratory reactance (Xrs) at 6 Hz was lower in O than in C. In C, on changing to the supine posture, mean Rrs at 6 Hz rose to 2.9 cmH(2)O . 1(-1). s, FRC fell by 0.68 liter, and Xrs at 6 Hz showed a s mall fall. In O, despite no further fall in FRC, supine Rrs at 6 Hz in creased to 7.3 cmH(2)O . 1(-1). s, and marked frequency dependency of Rrs and falls in Xrs developed. Seated, MIP and MEP in C and O were si milar; supine there were small falls in MEP and maximum expiratory flo w in O. The site and mechanism of the increase in supine Rrs and reduc tion in supine Xrs and the mechanism maintaining supine FRC in obesity all need further investigation.