Relationship between transcranial Doppler-determined pulsatility index and cerebrovascular resistance: an experimental study

Abstract

Clinical studies with transcranial Doppler suggest that the pulsatility of the flow velocity (FV) waveform increases when the distal cerebrovascular resistance (CVR) increases. To clarify this relationship, the authors studied animal models in which the resistance may be decreased in a controlled manner by an increase in arterial CO2 tension, or by a decrease in cerebral perfusion pressure (CPP) in autoregulating animals. Twelve New Zealand white rabbits were anesthetized, paralyzed, and ventilated. Transcranial Doppler basilar artery FV, laser Doppler cortical blood flow, arterial pressure, intracranial pressure, and end-tidal CO2 concentration were measured continuously. Cerebrovascular resistance (CPP divided by laser Doppler cortical flux) and Gosling Pulsatility Index (PI, defined as an FV pulse amplitude divided by a timed average FV) were calculated as time-dependent variables for each animal. Four groups of animals undergoing controlled manipulations of CVR were analyzed. In Group I, arterial CO2 concentration was changed gradually from hypocapnia to hypercapnia. In Group II, gradual hemorrhagic hypotension was used to reduce CPP. In Group III, the short-acting ganglion blocking drug trimetaphan was injected intravenously to induce transient hypotension. Intracranial hypertension was produced by subarachnoid saline infusion in Group IV. During the hypercapnic challenge the correlation between the cortical resistance and Doppler flow pulsatility was positive (r = 0.77, p<0.001). In all three groups in which cerebral perfusion pressure was reduced a negative correlation between pulsatility index and cerebrovascular resistance was found (r = -0.84, p<0.001). The authors conclude that PI cannot be interpreted simply as an index of CVR in all circumstances.

Neurovascular Deficits in Cocaine Abusers

“…Slowed cerebral blood flow velocity with increased pulsatility in certain patient populations indicate increased cerebrovascular resistance was due to vasoconstriction of small cortical vessels (Martin et al 1994;Biedert et al 1995;Cho et al 1997;Schondorf et al 1977;Grubb et al 1998). Such an interpretation of pulsatility does not hold in situations where blood pressure differed among groups (Czosnyka et al 1996). Because the cocaine abusers had similar blood pressure as the control subjects in our study, it is possible that these blood flow velocity changes indicate increased cerebrovascular resistance in the cocaine abusers.…”

“…A critical difference between TCD and other methodologies is that TCD measures blood flow velocity in large arteries rather than cerebral perfusion in small vessels. Pulsatility which is derived from velocity is also measured by TCD (Arnolds and von Reutern 1986;Czosnyka et al 1996;Cho et al 1997). The TCD assessment is noninvasive, quick, and economical.…”

The Effect of Body Posture on Brain Glymphatic Transport

“…We did not observe any differences in respiratory rates between the groups of rats, but blood gases were not measured so it is not possible to elaborate further. However, we speculate that lower respiratory effort could reduce vascular pulsatility and vascular resistance (Czosnyka et al, 1996). More studies will be needed to characterize physiological and glymphatic transport relationships during sleep.…”

Critical Closing Pressure Determined with a Model of Cerebrovascular Impedance

“…Recordings from 29 experiments of male New Zealand White rabbits (weighting from 2.7 to 3.7 kg) were performed 15,19,20 including high-resolution sampling of ABP, ICP, and basilar artery FV. The experimental protocols have been described in detail [19][20][21][22] and are briefly recapitulated here. Rabbits were produced using methods described previously.…”