Effect of Graded Hyperventilation on Cerebral Metabolism in a Cisterna Magna Blood Injection Model of Subarachnoid Hemorrhage in Rats

Abstract

In subarachnoid hemorrhage (SAH) with cerebrovascular instability, hyperventilation may induce a risk of inducing or aggravating cerebral ischemia. We measured cerebral blood flow (CBF) and cerebral metabolic rates of oxygen (CMRO2), glucose (CMRglc), and lactate (CMRlac) at different PaCO2 levels after experimental SAH in rats (injection of 0.07 mL of autologous blood into the cisterna magna). Four groups of Sprague-Dawley male rats were studied at predetermined PaCO2 levels: group A: normocapnia (5.01-5.66 kPa [38.0-42.0 mm Hg]); group B: slight hyperventilation (4.34-5.00 kPa [32.5-37.5 mm Hg]); group C: moderate hyperventilation (3.67-4.33 kPa [27.5-32.4 mm Hg]); group D: profound hyperventilation (3.00-3.66 kPa [22.5-27.4 mm Hg]). Each of the four groups included eight rats with SAH and eight sham-operated controls. CBF was determined by the intracarotid Xe method; CMRo2, CMRglc, and CMRlac were obtained by cerebral arteriovenous differences. In both SAH rats and controls, hyperventilation decreased CBF in proportion to the decrement in PaCO2 without affecting either CMRO2, CMRglc, or CMRlac. In groups C and D, CBF decreased by 20%-35%, but CMRs were maintained by a compensatory increase in oxygen extraction fraction (OEF). The results show that even profound hyperventilation in this model of SAH is associated with an adequate increase in OEF so that CMRs of oxygen, glucose, and lactate remain similar to levels observed in normocapnic conditions.

Brain Changes to Hypocapnia Using Rapidly Interleaved Phosphorus-Proton Magnetic Resonance Spectroscopy at 4 T

“…In that study, the cerebral metabolic rate of oxygen, as measured by the gold-standard of PET imaging, was unchanged during HV, supporting the conclusion that tissue metabolic needs are being fully met despite CBF reduction. A similar conclusion was reached in a recent study investigating HV-effects in rats having undergone subarachnoid hemorrhage (Ma et al , 2006). Despite reductions in CBF (20% to 35%), cerebral metabolic rates were maintained during HV (22 to 27 mm Hg) by increasing cerebral oxygen extraction (Ma et al , 2006).…”

“…A similar conclusion was reached in a recent study investigating HV-effects in rats having undergone subarachnoid hemorrhage (Ma et al , 2006). Despite reductions in CBF (20% to 35%), cerebral metabolic rates were maintained during HV (22 to 27 mm Hg) by increasing cerebral oxygen extraction (Ma et al , 2006). Nonetheless, physiologic measures continue to be used to support the potential deleterious consequences of HV, with the noted problem that they fail to add quantitative information to the scientific discussion (Cruz, 2002).…”