The relationship of blood flow velocity fluctuations to intracranial pressure B waves

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✓ Intracranial pressure (ICP) and continuous transcranial Doppler ultrasound signals were monitored in 20 head-injured patients and simultaneous synchronous fluctuations of middle cerebral artery (MCA) velocity and B waves of the ICP were observed. Continuous simultaneous monitoring of MCA velocity, ICP, arterial blood pressure, and expired CO2 revealed that both velocity waves and B waves occurred despite a constant CO2 concentration in ventilated patients and were usually not accompanied by fluctuations in the arterial blood pressure. Additional recordings from the extracranial carotid artery during the ICP B waves revealed similar synchronous fluctuations in the velocity of this artery, strongly supporting the hypothesis that blood flow fluctuations produce the velocity waves. The ratio between ICP wave amplitude and velocity wave amplitude was highly correlated to the ICP (r = 0.81, p < 0.001). Velocity waves of similar characteristics and frequency, but usually of shorter duration, were observed in seven of 10 normal subjects in whom MCA velocity was recorded for 1 hour. The findings in this report strongly suggest that B waves in the ICP are a secondary effect of vasomotor waves, producing cerebral blood flow fluctuations that become amplified in the ICP tracing, in states of reduced intracranial compliance.

Article Information

Dr. Newell is a recipient of the William P. Van Wagenen fellowship awarded in 1989 by the American Association of Neurological Surgeons, and also the Allied Signal Corporation/National Stroke Association Research fellowship from 1990 to 1993.Address reprint requests to: David W. Newell, M.D., Department of Neurological Surgery, Harborview Medical Center, 325 9th Avenue, Seattle, Washington 98104.

© AANS, except where prohibited by US copyright law.

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Figures

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    Upper Left: Illustration of typical velocity fluctuations occurring in the presence of B waves in intracranial pressure (ICP) (in mm Hg) which were independent of changes in arterial blood pressure (ABP) and CO2 concentration levels (both in mm Hg). Upper Right: Fourier analysis of the frequency of repeating waves (W) in the recorded parameters showing the velocity and ICP waves at a frequency of 2 cylces per minute and no changes at this frequency in ABP and CO2 levels. The respirator frequency (R) is seen at between 16 and 17 cycles per minute with an effect on all four parameters. Lower: Continuous trend in a head-injured patient demonstrating the persistence of waves throughout the entire recording session (> 70 minutes) unaffected by a rapid decline in blood pressure (arrow) or hyperventilation (arrow). MCA = middle cerebral artery velocity (in cm/sec).

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    Left: Linear regression analysis between velocity wave amplitude and intracranial pressure (ICP) indicating a slight inverse relationship between the two variables) r = −0.39, p < 0.05). MCA = middle cerebral artery. Center: Exponential regression analysis between ICP wave amplitude and ICP in the presence of B waves, indicating that B waves are more prominent at higher ICP levels (r = 0.69, p < 0.001). Right: Exponential regression analysis between the ICP and the ratio between ICP and velocity wave amplitudes (r = 0.81, p < 0.001). This graph illustrates that increasing ICP induces a sharp increase in the ICP B wave amplitude relative to the velocity amplitude of the same wave.

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    Comparison of the effect of velocity fluctuations on intracranial pressure (ICP, in mm Hg) fluctuations at different ICP levels. ABP = arterial blood pressure (in mm Hg); CO2 = CO2 concentration (in mm Hg); MCAV = middle cerebral artery velocity (in cm/sec). Left: Illustration of marked ICP fluctuations at higher ICP levels (mean ICP 24 mm Hg). Lines illustrate the slower rate of increase than rate of decrease in the velocity waves. Right: Illustration showing minimal ICP fluctuations in response to similar velocity fluctuations at a lower ICP (mean ICP 5 mm Hg).

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    Illustration of simultaneous velocity recording from both middle cerebral arteries in the presence of B waves using a two-channel Doppler ultrasound device. The velocity fluctuations are synchronous with each other and in phase with the intracranial pressure (ICP) B waves. ABP = arterial blood pressure; RMCV = right middle cerebral artery velocity; LMCV = left middle cerebral artery velocity.

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    Illustration of simultaneous recording from the neck of arterial blood pressure (ABP, in mm Hg), intracranial pressure (ICP, in mm Hg), and extracranial internal carotid artery velocity (ICAV, in cm/sec) in the presence of B waves, strongly indicating that velocity fluctuations occur due to volume flow fluctuations.

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    Upper: Continuous recording of middle cerebral artery velocity (MCAV, in cm/sec) and CO2 (in mm Hg) in a normal subject demonstrating a brief episode of velocity fluctuations (arrow) similar in frequency and character to those seen in the presence of B waves in the intracranial pressure of head-injured patients. Lower: Enlarged detail of this interval illustrating small fluctuations in end-tidal CO2 during the waves; however, these fluctuations appeared to be insufficient to be causative.

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