AJP - Heart and Circulatory Physiology, Vol 261, Issue 2 575-H582, Copyright © 1991 by American Physiological Society
Efficacy of Cushing response during development in sheep
J. E. Backofen, R. C. Koehler, A. P. Harris, M. C. Rogers, R. J. Traystman and M. D. Jones Jr
Department of Anesthesiology/Critical Care Medicine, Johns Hopkins Medical Institutions, Baltimore, Maryland 21205.
Mean aortic pressure (MAP) increases (Cushing response) when intracranial
pressure (ICP) approaches MAP. We elevated ICP to levels equivalent to
normal baseline MAP with infusion of mock cerebrospinal fluid (CSF) into
the lateral cerebral ventricles and contrasted responses in near-term fetal
sheep, 1-wk-old lambs, and adult sheep anesthetized with pentobarbital
sodium. With CSF infusion 1-wk-old lambs and adults produced sustained
increases in MAP of 16 +/- 1 and 22 +/- 2 mmHg, respectively, over a 40-min
period. However, cerebral blood flow fell 66 and 57%, and cerebral O2
uptake fell 34 and 37%, respectively. In the near-term fetus, MAP increased
by 11 +/- 1 mmHg and cerebral blood fell 49% at 3 min of elevated ICP.
However, by 15 min MAP had increased further (+17 +/- 2 mmHg) and cerebral
blood flow was nearly restored. In contrast to postnatal sheep, cerebral O2
uptake was maintained throughout in the fetus. The mechanism of increased
MAP differed among groups. In adults total peripheral resistance fell
significantly, whereas in the fetus and lamb it remained constant. Cardiac
output increased in each group, but, because of the fall in peripheral
resistance, increased cardiac output was relatively more important to the
rise in MAP in adults. In addition, marked vasoconstriction occurred in
intestines and skin in the fetus. The Cushing response is well-developed in
near-term fetal sheep. After birth it may lose its effectiveness in
providing for the basal metabolic demand of the brain.
