Relationship between pulsatility indices of Doppler flow signals and CO2-reactivity within the middle cerebral artery in extracranial occlusive disease

CO2-dependent vasomotor reactivity (VMR) of the middle cerebral artery (MCA) distribution has been shown to be a reliable predictor of the patient's risk for low-flow strokes. In order to clarify whether various Pulsatility Indices (PIs) of MCA flow signals would be useful to estimate VMR, 222 subjects with various degrees of stenosing internal carotid artery lesions were studied. VMR of the MCA territory was expressed as the percentage changes in mean blood flow velocity during inhalation of CO2 and hyperventilation. Although the correlation coefficients between VMR and PIs were significantly different from zero, these relationships were not strong enough for PI values to accurately predict the individual values of VMR. Furthermore, VMR could convincingly discriminate subgroups of patients with different severities of extracranial occlusive disease, whereas PIs discriminated only weakly or not at all. PIs may give some vague idea of the vasomotor tone, but cannot predict precisely enough whether a patient is at risk for low-flow infarctions.