Interaction between hypoxia and hypercapnia in regulation of
arteriolar diameter in the canine diaphragm .
Ward, Michael E.
Division of Critical Care, Royal Victoria Hospital, McGill
University, Montr[acute]eal, Qu[acute]ebec, Canada
APStracts 2:0474A, 1995.
In [alpha]-chloralose anaesthetized mongrel dogs, the microvascular
responses to hypoxia and hypercapnia were studied in the vascularly
isolated, ex-vivo, left hemidiaphragm. The diaphragm was perfused
with arterial blood diverted from the femoral artery using a pump. A
series of membrane oxygenators was used to adjust the blood gas
composition of the blood perfusing the diaphragm. Arteriolar
diameters were measured by intravital microscopy during infusion of
sodium nitroprusside (SNP), moderate hypoxia (phrenic venous PO2=25
mmHg), severe hypoxia (phrenic venous PO2 = 13 mmHg), hypercapnia
(phrenic venous PCO2 = 100 mmHg) and during the simultaneous
presentation of hypoxia and hypercapnia. Recordings were made after
15 minutes under each condition when steady state had been
established for vessel diameter and flow. Pump speed was adjusted
manually under each condition in order to ensure that steady state
perfusion pressure was the same as that under the control condition.
Moderate hypoxia generally resulted in dilation, however,
vasoconstriction was seen in some arterioles. Severe hypoxia caused
vasodilation which was inversely related to baseline vessel diameter
and paralleled the response to SNP. Hypercapnia resulted in
vasodilation of the diaphragmatic circulation at values of phrenic
venous PCO2 above 80 mmHg. The arteriolar response to hypercapnia was
also inversely related to baseline vessel diameter. Hypoxia and
hypercapnia in combination exerted an additive effect on arteriolar
diameter but produced a greater than additive effect on blood flow.
Both PO2 and PCO2 may contribute to the local regulation of
diaphragmatic blood flow. The vasodilator effects of both hypoxia and
hypercapnia are greater in smaller than in larger arterioles. The
interaction between PO2 and PCO2 on arteriolar diameter is additive.
An apparent synergistic effect on blood flow results from the power
function relating diameter to flow.
Received 6 February 1995; accepted in final form 5 October 1995.
APS Manuscript Number A135-5.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 6 November 95