Journal of Applied Physiology, Vol 61, Issue 6 2238-2242, Copyright © 1986 by American Physiological Society
Constant-flow ventilation in pigs
P. Webster, A. S. Menon and A. S. Slutsky
Constant-flow ventilation (CFV) is a ventilatory technique in which
physiological blood gases can be maintained in dogs by a constant flow of
fresh gas introduced via two catheters placed in the main-stem bronchi (J.
Appl. Physiol. 53: 483-489, 1982). High-velocity gas exiting from the
catheters can create uneven pressure differences in adjacent lung segments,
and these pressure differences could lead to gas flow through collateral
channels. To examine this hypothesis, we studied CFV in pigs, animals known
to have a high resistance to collateral ventilation. In three pigs we
examined steady-state gas exchange, and in six others we studied unsteady
gas exchange at three flow rates (20, 35, and 50 l/min) and three catheter
positions (0.5, 1.5, and 2.5 cm distal to the tracheal carina). During
steady-state runs we were unable to attain normocapnia; the arterial CO2
partial pressure (PaCO2) was approximately 300 Torr at all flow rates and
all catheter positions, compared with 20-50 Torr at similar flows and
positions in dogs studied previously. The initial unsteady gas-exchange
experiments indicated no consistent effect of catheter position or flow
rate on the rate of rise of PaCO2. In three other pigs, the rates of rise
of PaCO2 were compared with the rates observed with apneic oxygenation
(AO). At the maximum flow and deepest position, the rate of rise of PaCO2
was lower during CFV than during AO. These data suggest that flow through
collateral channels might be important in producing adequate gas transport
during CFV; however, other factors such as airway morphometry and the
effects of cardiogenic oscillations may explain the differences between the
results in pigs and dogs.
