The study by Aaslid et al. investigates the response of cerebral blood flow (CBF) to acute step decreases in arterial blood pressure (ABP) in 10 healthy volunteers during normocapnia, hypocapnia, and hypercapnia. The ABP decrease was induced by rapidly deflating thigh blood pressure cuffs after a 2-minute inflation. Instantaneous ABP and CBF changes were measured using a servo-cuff method and transcranial Doppler ultrasound, respectively. The results show that in hypocapnia, CBF returned to pre-test levels within 4.1 seconds, while the response was slower in normocapnia and hypercapnia. The rate of cerebrovascular resistance (CVR) regulation, defined as the normalized change in CVR per second, was 0.38, 0.20, and 0.11/sec in hypocapnia, normocapnia, and hypercapnia, respectively. There was a significant inverse relationship between the rate of regulation and PaCO2, indicating that cerebral autoregulation is highly dependent on vascular tone. The study suggests that the rapid response of cerebral autoregulation in normal humans is mediated by a metabolic mechanism, and that this response is significantly faster than the baroreceptor reflex regulating ABP. The findings have implications for clinical applications, such as monitoring autoregulatory effectiveness in patients with cerebrovascular diseases.The study by Aaslid et al. investigates the response of cerebral blood flow (CBF) to acute step decreases in arterial blood pressure (ABP) in 10 healthy volunteers during normocapnia, hypocapnia, and hypercapnia. The ABP decrease was induced by rapidly deflating thigh blood pressure cuffs after a 2-minute inflation. Instantaneous ABP and CBF changes were measured using a servo-cuff method and transcranial Doppler ultrasound, respectively. The results show that in hypocapnia, CBF returned to pre-test levels within 4.1 seconds, while the response was slower in normocapnia and hypercapnia. The rate of cerebrovascular resistance (CVR) regulation, defined as the normalized change in CVR per second, was 0.38, 0.20, and 0.11/sec in hypocapnia, normocapnia, and hypercapnia, respectively. There was a significant inverse relationship between the rate of regulation and PaCO2, indicating that cerebral autoregulation is highly dependent on vascular tone. The study suggests that the rapid response of cerebral autoregulation in normal humans is mediated by a metabolic mechanism, and that this response is significantly faster than the baroreceptor reflex regulating ABP. The findings have implications for clinical applications, such as monitoring autoregulatory effectiveness in patients with cerebrovascular diseases.