Journal of Applied Physiology, Vol 60, Issue 3 802-808, Copyright © 1986 by American Physiological Society
Hypoxemia lowers cerebrovascular resistance without changing brain and blood [H+]
S. Javaheri
We designed the present study to see whether, during acute moderate
isocapnic hypoxemia, changes in cerebral vascular resistance (CVR) and
brain extracellular fluid (ECF) [H+] can or cannot be dissociated from each
other. In seven anesthetized and paralyzed dogs we measured brain ECF pH
with surface electrodes (n = 4) or double-barreled microelectrodes (n = 3)
with tip diameters of less than 30 micron inserted 5 mm below the surface.
Cerebral blood flow (CBF) was measured by radioactive microspheres during
normoxemia and moderate hypoxemia, whereas brain ECF pH was measured
continuously. In six of the seven dogs brain pH did not change during
moderate hypoxemia of 4-20 min duration. In these six animals the mean
arterial O2 partial pressure decreased from 84.8 +/- 12.9 (SD) to 46.7 +/-
10.2 Torr during hypoxic gas breathing, resulting in a significant drop in
CVR from 3.88 +/- 1.88 to 3.27 +/- 1.97 Torr X ml-1 X min X 100 g and a
rise in CBF from 31.7 +/- 12.7 to 47.8 +/- 31.5 ml X min-1 X 100 g-1. The
mean brain ECF [H+] was 57.4 +/- 8.2 nmol/l (pH = 7.24) during normoxemia
and did not change significantly during hypoxic gas breathing [56.6 +/- 7.7
nmol/l (pH = 7.25)]. Furthermore, arterial and sagittal venous blood and
cisternal cerebrospinal fluid (CSF) pH did not change significantly during
hypoxic gas breathing. We conclude that during acute moderate hypoxemia
reduction in CVR can occur independently from increases in brain ECF,
cisternal CSF, and arterial and sagittal venous blood [H+] and PCO2.
