Background and Purpose: Hypercapnia leads to increased cerebral blood flow and impaired autoregulation by changing the cerebrovascular resistance (CVR). We studied the effect of hypercapnia on critical closing pressure (CCP) and resistance area product (RAP) which effectively reflects the role of vascular tone and resistance respectively in altering cerebrovascular resistance and maintaining cerebral autoregulation during transient hypotension.
Methods: –26 healthy subjects had continuous cerebral blood flow velocity and arterial blood pressure recording. Transient hypotension was induced by sudden release of thigh cuff after suprasystolic inflation for 3 minutes. 3 trials of thigh cuff maneuver were performed with normal breathing, 3% CO2breathing and 5% CO2breathing. CO2 inhalation was given for last 45 seconds of thigh cuff maneuver and continued for 90 seconds after release. Autoregulatory Index (ARI) was calculated for each trial with Tieck’s model. CCP and RAP were computed for 10 seconds’ window each at 1 minute after inflation (Baseline), immediately before release (Prerelease) and immediately after release (Release) of inflated thigh cuff.
Results: –ARI values decreased significantly (p < 0.0001) from 5.46 ± 1.47 to 2.0 ± 1.35 and 1.63 ± 1.09 during 3% CO2and 5% CO2 trial respectively. No significant changes were observed in CCP value during thigh cuff release with all three trials. RAP significantly dropped from prerelease value after thigh cuff release in all 3 trials. Drop in RAP attenuated during hypercapnia trial and had significant (p < 0.0001) difference between RAPPrerelease – Release(dRAP) for Normal breathing (0.20 ± 0.12) and 3% and 5 % CO2 trials (0.089 ± 0.090, 0.081 ± 0.093). dRAP was significantly correlated (Spearmen r = 0.42, p = 0.0004) with the ARI values where higher dRAP had better autoregulation.
Conclusion: – Cerebral autoregulation during transient hypotension acts by changing vascular resistance i.e. RAP without any effect on vascular tone i.e. CCP and had been suggested to reflect myogenic component of autoregulation to blood pressure changes. We concluded that hypercapnia leads to impairment of myogenic component of autoregulation during transient arterial blood pressure changes as suggested by attenuated dRAP.