Vasomotor Reactivity Testing

The main reason for evaluating vasomotor reactivity (VMR) of brain vessels is to identify patients at higher risk for stroke by challenging the ability of brain vessels to dilate and, thus, recruit more collateral flow. VMR has been most extensively studied in the setting of carotid artery stenosis or occlusion. A variety of tests were introduced to evaluate intracranial hemodynamics using the phenomenon of VMR (60-65), including CO2-reactivity with TCD, acetazolamide testing with TCD, and the breath-holding index (BHI). The latter is the simplest way to challenge VMR if the patient is compliant and capable of a 30-second breath-hold (65). This index is calculated using the mean flow velocities obtained by TCD before breath-holding (baseline) and at the end of four seconds of breathing after 30 seconds of breath-holding, such that:

MFVbaseline seconds of breath-holding

The patient should be able to hold breath voluntarily for at least 24 seconds and, preferably, 30 seconds. To help the patient complete this task, the practitioner should explain the procedure in detail and demonstrate that no major chest excursions should be made at the beginning and end of breath-holding. Major chest volume changes associated with forced breathing change intrathoracic pressure and may affect velocity and flow pulsatility. The practitioner should then announce the duration of breath-hold to the patient at 10-seconds intervals after breath-holding has started; this helps the patient to be more confident that he or she can complete the task. Use envelope or average mean velocities beginning four seconds after the patient resumes breathing (i.e., the optimized signals from the entire display if the sweep speed was set at 4 to 5 seconds).

BHI values of less than 0.69 are predictive of risk of stroke in patients with asymptomatic, severe ICA stenosis and symptomatic occlusion (66,67). If a patient has greater than 70% proximal ICA stenosis or occlusion and a BHI <0.69, risk of subsequent stroke is at least 3 times greater than a similar patient with normal VMR. This information, together with other findings, such as blunting of the MCA waveform (68), plaque morphology, and emboli as described above, can identify patients at particularly high risk for stroke progression or recurrence and who are, therefore, more suitable for surgery, noninvasive flow augmentation, or ventilatory correction. The hypothesis that acute and subacute stroke patients with persistent proximal arterial occlusions and impaired VMR could benefit from external enhanced counterpulsation or positive airway pressure correction is being tested in prospective trials (A.W. Wojner-Alexandrov, unpublished pilot data, 2005).

Breath-holding with TCD does not require any gas-monitoring equipment or IV injections. Although the subjectivity of patient effort and the unknowns of blood gas concentration potentially limit reliability, BHI has been prospectively validated to predict clinical outcomes in steno-occlusive ICA disease, and BHI represents a simple screening test that can be used at the bedside to identify patients with impaired VMR.

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