Introduction two minutes. To record instantaneous values of arterial blood pressure they used a servo-controlled cuff method 3. Averaged recordings with deflating leg cuffs are shown in figure 2. Experiments have been performed at normocapnia, hypocapnia through voluntary hyperven- tilation and hypercapnia through breathing of 5% CO2 in air. At each of three levels of paCO2 a same drop of approximately 20% in ABP has been achieved through leg cuff deflation. Approximately 10 seconds after the drop ABP has recovered to normal levels. Dynamic cerebral autoregulation has caused CBF to increase before ABP recovery. This increase is caused by adaptation of vessel resistance. According to Ohm’s law cerebrovascular resistance (CVR) can be defined as ABP/CBF. During the ABP drop clearly CVR adapts to achieve fast recovery of CBF. With hypocapnia CBF recovery is faster and with hypercapnia slowed compared to normocapnic conditions. Figure 2Averaged recordings from leg cuff tests at three different levels of paCO2 (from Aaslid, 1989). Tieckset al 27 described a model to quantify this autoregulatory response to an ABP transient. Based on a set of model parameters ten responses to an ABP step decrease were constructed as model output. The number of each of these ten responses ranging from 0 to 9 indicate the cerebral autoregulatory index (ARI) and so quantify dynamic cerebral autoregulation (dCA) . An autoregulatory index of 0 represents no cerebral autoregulation, i.e. CBFV passively follows ABP. An ARI of 9 represents fast CBFV recovery after a step change in ABP. Normal autoregulation is represented by an ARI of 5±1. 11
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