AJP - Heart and Circulatory Physiology, Vol 266, Issue 6 2423-H2429, Copyright © 1994 by American Physiological Society
Changes in the neural drive to abdominal expiratory muscles in hemorrhagic hypotension
R. F. Fregosi
Department of Exercise and Sport Sciences, University of Arizona Health Sciences Center, Tucson 85721.
The purpose of this study was to test the hypothesis that
hemorrhage-induced hypotension increases the neural drive to the abdominal
expiratory muscles in chloralose-urethan-anesthetized cats that are studied
under conditions of constant arterial PCO2 (PaCO2) and hyperoxia. A
secondary aim was to describe in detail the concomitant changes in inspired
pulmonary ventilation (VI) and the pattern of breathing under these
conditions. The rectified and integrated electromyogram (EMG) of the
external oblique and rectus abdominis muscles and VI were recorded in
moderate and severe hemorrhagic hypotension, leading to reductions in mean
blood pressure of approximately 30 and 60%, respectively. The PaCO2 was
prevented from falling, and the arterial PO2 was maintained at a hyperoxic
level (> 200 mmHg) by adding CO2 and O2 to the inspired gas mixture. VI
increased by 2.5- and 5-fold in moderate and severe hypotension (P <
0.05). The changes in VI were mediated exclusively by changes in tidal
volume, indicating that the reflex did not alter the activity of
respiratory rhythm-generating structures. The EMG of external oblique
muscles averaged 2, 44, and 100% in control conditions and in moderate and
severe hypotension, respectively; corresponding values in rectus abdominis
muscles were 10, 28, and 100% (P < 0.05 for both muscles). Bilateral
cervical vagotomy caused a one- to three-fold decrease in the ventilatory
response to hemorrhage and abolished the increase in abdominal muscle EMG
activities. In conclusion, hemorrhagic hypotension reflexly increases
pulmonary ventilation and the neural drive to the abdominal muscles. The
reflex is vagally mediated, but the location of the receptors was not
identified.
