Respiratory control during volume cycled ventilation in normal
humans.
Puddy, Andrew, Ward Patrick, Kimberly Webster, Magdy Younes.
Respiratory Investigation Unit, University of Manitoba, Winnipeg,
Canada R3A 1R8
APStracts 2:0556A, 1995.
The purpose of this study was to evaluate the extent of inhibition to
respiratory rhythm associated with high volumes of ventilation during
volume-cycled mechanical ventilation (neuromechanical inhibition).
Two approaches were used. 1) in 18 normal awake subjects ventilator
tidal volume in the assist mode (A/C) was increased in steps from the
minimum tolerable level (VTmin) up to 80% of the subject's
inspiratory capacity or ventilator's maximum VT (VTmax). We looked
for appearance of intermittent apnea or a reduction in spontaneous
rate (F). 2) In another 18 normal awake subjects, subjects were
placed on controlled ventilation (CMV). When apnea was established we
abruptly terminated CMV and measured the time before the appearance
of the next spontaneous effort. In the assist mode (protocol 1) we
did not observe intermittent apnea as VT was increased from (mean +/-
SD) 944 +/- 198 ml to 1867 +/- 277 ml and there was only a modest
reduction in F (14.1 +/- 3.9 to 12.4 +/- 4.0 min.-1 ). PETco2
decreased precipitously as VT was increased. In protocol 2, we did
not observe apnea after discontinuation of CMV in any subject. TTOT
of the first breath after discontinuation did not differ
significantly from TTOT during (A/C) in the same subjects (4.84 +/-
2.2 vs 5.2 +/- 2.0 sec). This similarly applied regardless of route
of breathing (nose vs mouth) or PETco2 level at time of
discontinuation. We conclude that neuromechanical inhibition is quite
weak and provides very little negative feedback that may help control
Pco2 in the face of excessive VT and F demand of the subject.
Received 28 June 1995; accepted in final form 12 December 1995.
APS Manuscript Number A696-5.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 23 December 95