Evaluation of the cerebral hemodynamic response to rhythmic handgrip.

The response of the cerebral circulation to exercise has been studied with transcranial Doppler ultrasound (TCD) because this modality provides continuous measurements of blood velocity and is well suited for the exercise environment. The use of TCD as an index of cerebral blood flow, however, requires the assumption that the diameter of the insonated vessel is constant. Here, we examine this assumption for rhythmic handgrip using a spectral index designed to measure trends in vessel flow. Nineteen normal subjects were studied during 5 min of volitional maximum rhythmic right handgrip at 1 Hz. TCD velocities from both middle arteries (left and right), blood pressure, and end-tidal PCO(2) were recorded every 10 s. A spectral weighted sum was also calculated as a flow index (FI). Averages were computed from the last 2 min of handgrip. Relative changes in velocity, FI, and pressure were calculated. The validity of FI was tested by comparing the change in diameter derived from equations relating flow and diameter. Mean blood pressure increased 23.8 +/- 17.8% (SD), and velocity increased 13.3 +/- 9.8% (left) and 9.6 +/- 8.3% (right). Although the mean change in FI was small [2.0 +/- 18. 2% (left) and 4.7 +/- 29.7% (right)], the variation was high: some subjects showed a significant increase in FI and others a significant decrease. Diameter estimates from two equations relating flow and luminal area were not significantly different. Decreases in FI were associated with estimated diameter decreases of 10%. Our data suggest that the cerebral blood flow (CBF) response to rhythmic handgrip is heterogeneous and that middle cerebral artery flow can decrease in some subjects, in agreement with prior studies using the Kety-Schmidt technique. We speculate that the velocity increase is due to sympathetically mediated vasoconstriction rather than a ubiquitous flow increase. Our data suggest that the use of ordinary TCD velocities to interpret the CBF response during exercise may be invalid.

Oscillations in cerebral blood flow detected with a transcranial Doppler index.

Although transcranial Doppler ultrasound (TCD) has been used to detect oscillations in CBF, interpretation is severely limited, since only blood velocity and not flow is measured. Oscillations in vessel diameter could, therefore, mask or alter the detection of those in flow by TCD velocities. In this report, the authors use a TCD-derived index of flow to detect and quantify oscillations of CBF in humans at rest. A flow index (FI) was calculated from TCD spectra by averaging the intensity weighted mean in a beat-by-beat manner over 10 seconds. Both FI and TCD velocity were measured in 16 studies of eight normal subjects at rest every 10 seconds for 20 minutes. End tidal CO2 and blood pressure were obtained simultaneously in six of these studies. The TCD probe position was meticulously held constant. An index of vessel area was calculated by dividing FI by velocity. Spectral estimations were obtained using the Welch method. Spectral peaks were defined as peaks greater than 2 dB above background. The frequencies and magnitudes of spectral peaks of FI, velocity, blood pressure, and CO2 were compared with t tests. The Kolmogorov-Smirnov test was used to further confirm that the data were not white noise. In most cases, three spectral peaks (a, b, c) could be identified, corresponding to periods of 208+/-93, 59+/-31, and 28+/-4 (SD) seconds for FI, and 196+/-83, 57+/-20, and 28+/-6, (SD) seconds for velocity. The magnitudes of the spectral peaks for FI were significantly greater (P<0.02) than those for velocity. These magnitudes corresponded to variations of at least 15.6%, 9.8%, and 6.8% for FI, and 4.8%, 4.2%, and 2.8% for velocity. The frequencies of the spectral peaks of CO2 were similar to those of FI with periods of 213+/-100, 60+/-46, and 28+/-3.6 (SD) seconds. However, the CO2 spectral peak magnitudes were small, with an estimated maximal effect on CBF of (+/-) 2.5+/-0.98, 1.5+/-0.54, and 1.1+/-0.31 (SD) percent. The frequencies of the blood pressure spectral peaks also were similar, with periods of 173+/-81, 44+/-8, and 26+/-2.5 (SD) seconds. Their magnitudes were small, corresponding to variations in blood pressure of (+/-) 2.1+/-0.55, 0.97+/-0.25, and 0.72+/-0.19 (SD) percent. Furthermore, coherence analysis showed no correlation between CO2 and FI, and only weak correlations at isolated frequencies between CO2 and velocity, blood pressure and velocity, or blood pressure and FI. The Kolmogorov-Smirnov test distinguished our data from white noise in most cases. Oscillations in vessel flow occur with significant magnitude at three distinct frequencies in normal subjects at rest and can be detected with a TCD-derived index. The presence of oscillations in blood velocity at similar frequencies but at lower magnitudes suggests that the vessel diameters oscillate in synchrony with flow. Observed variations in CO2 and blood pressure do not explain the flow oscillations. Ordinary TCD velocities severely underestimate these oscillations and so are not appropriate when small changes in flow are to be measured.

Distribution of hematocrit values after aneurysmal subarachnoid hemorrhage.

Low hematocrit values are common after subarachnoid hemorrhage and may be associated with elevated cerebral blood flow and transcranial Doppler ultrasound (TCD) velocities, which may confound the interpretation of velocity as an indicator of vasospasm. The exact distribution of hematocrit among a neurosurgical population would be useful in assessing the magnitude of this difficulty but has not been previously reported. A database containing hematocrit values recorded at TCD examinations over a period of 7 years was reviewed. Two thousand four hundred thirteen hematocrit values were recorded for 575 patients. The distribution of hematocrit values was recorded among patients and among TCD studies. Eighteen percent of the patient population achieved a hematocrit of 26% or less at some point in their hospital stay, whereas 57% of patients achieved a hematocrit of 30% or less. Six percent of the TCD studies were associated with a hematocrit of 26% or less, while 33% of the studies were associated with a hematocrit of 30% or less.

Estimation of vessel flow and diameter during cerebral vasospasm using transcranial Doppler indices.

OBJECTIVE: An important limitation of transcranial Doppler (TCD) ultrasonography is its inability to directly measure blood flow or vessel diameter. To extend the ability of TCD ultrasonography, indices were derived from an intensity-weighted mean of the entire Doppler spectrum. The objective of this article is to test the behavior of these indices under conditions of diameter constancy (hyper- and hypoventilation) and when vessel diameter decreases (vasospasm). METHODS: A flow index (FI) was calculated by averaging several heartbeats of spectral data and calculating the first spectral moment. An area index (AI) was defined as the FI divided by the mean velocity, motivated by the knowledge that vessel flow is the product of vessel diameter and mean velocity. To test the FI and the AI under conditions of diameter constancy, middle cerebral artery Doppler signals were obtained from 20 patients during conditions of hypercarbia, hypocarbia, and normocarbia. To test the ability of these indices to evaluate a decrease in vessel diameter, signals from 41 sites on 23 arteries were obtained from patients who underwent both TCD and angiographic studies on two separate occasions after the occurrence of subarachnoid hemorrhage. The changes in the AI were compared with the arterial diameters measured from angiograms. RESULTS: The FI was proportional to the mean velocity in the cohort of healthy patients (r=0.97). The AI changed by less than 3% in the same cohort. The AI predicted the direction of the diameter change in all vessels showing angiographic changes in area. Changes in the AI and the measured angiographic changes in cross-sectional areas were correlated (overall, r=0.90; with two outlines removed, r=0.86). CONCLUSION: This variant of the intensity-weighted mean predicts changes in vessel cross-sectional area under conditions of changes in CO2 and cerebral vasospasm. This preliminary study suggests that careful use of this tool may provide accurate evaluation of cerebral blood flow through the large vessels and quantitative changes in diameter, which occur frequently after subarachnoid hemorrhage.

Detection of emboli after surgery for intracerebral aneurysms.

BACKGROUND: Neurological change after surgery for cerebral aneurysm caused by embolic events is commonly suspected, but direct detection of emboli has not been possible in the past. Transcranial Doppler ultrasound (TCD) is able to detect emboli, and large numbers of emboli detected in TCD studies have been associated with radiological changes and clinical deterioration. METHODS: During a 2-year period, 11 patients were observed to have emboli during routine TCD studies after aneurysm surgery. The computed tomographic (CT) scans of these patients were reviewed for low-density areas, suggesting ischemia. All patients studied during a 1-year interval (July 1995-July 1996) served as a control group and were reviewed for similar CT findings, and the two groups were compared using Fisher's exact test. RESULTS: Nine of the 11 patients (82%) observed to have emboli developed low-density areas on their CT scans, whereas 30 of the 123 (24%) patients without emboli developed low-density areas on their CT scans. The difference was significant (P < 0.001, Fisher's exact test). Credible sources for emboli were readily identified in each of the 11 patients. CONCLUSION: TCD allows detection of emboli after aneurysm surgery, and this detection is strongly associated with CT evidence of ischemia. Although detection of emboli was relatively rare in this study, rates of emboli occurrence may increase if systematic monitoring is used.

A new method for fixation of probes for transcranial Doppler ultrasound.

Although monitoring of blood velocity in the cerebral arteries with transcranial Doppler ultrasound is standard practice, methods for fixation of the ultrasound probe to the skull continue to impose a technical challenge. This report describes a novel method of probe fixation in which an inexpensive polymer block is custom-made for each patient to hold the probe at a fixed angle. In addition to comfort and durability, its major advantage is that the polymer block prevents dislodgment of the probe from the intended angle by forces of gravity or by patient movement. Thirty-one temporal windows were monitored with this technique, with stable signals obtained in all subjects despite aggressive movements. Disadvantages include the inability to insonate more than one vessel and the cost for each block. However, the method is quick, is relatively inexpensive, and provides significant advantages of probe stability.

An unusual transcranial Doppler waveform associated with vessel distortion in giant intracranial aneurysms.

Although the shapes of velocity waveforms obtained with transcranial Doppler examination can indicate such abnormalities as increased intracranial pressure and proximal arterial compromise, the significance of unusual waveform morphologies is often obscure. In this report, we describe four cases in which an unusual waveform morphology was obtained from vessels distorted and narrowed by intracranial masses. The appearance of this unusual morphology as an isolated signal within a transcranial Doppler examination should, therefore, suggest a structural deformation of the insonated vessel to those who interpret transcranial Doppler studies.