A Study of Cerebral Vasoreactivity: Middle Cerebral Artery (MCA) Versus Ophthalmic Artery (OPA)

Abstract

background: Cerebral vaso reactivity describes the ability of the cerebral circulationto respond to vasomotor stimuli which would reflect preserved auto regulatorymechanism. Recently, cerebral autoregulatory capacity in humans can be studiednoninvasively by TCD. The degree of cerebral vasodilatation can be measured by theincreased middle cerebral artery (MCA) blood flow in response to hypercapnea induced byeither administration of 5% CO2 or I.V acetazolamide; few studies available on breath holdinduced cerebral vasodilatation. Cerebral vasoconstriction can be measured by the decreasedMCA blood flow in response to hypocapnea induced by hyperventilation. However,conflicting reports exist on the modulatory effect of hypercapnea (induced by breath holding)or hypocapnea (Induced by hyperventilation) on the ophthalmic artery (OPA) blood flow.Objective: To demonstrate changes in MCA & OPA blood flow using physiological stimuli.Secondly, to investigate whether or not OPA would respond in a similar fashion to MCA, tothe fore mentioned stimuli.Methods: 30 healthy individuals were enrolled, using a TCCD sonography; the MCA andOPA were insonated utilizing transtemporal and transorbital windows respectively. A mean of10 cardiac cycles were used to estimate the base line control of Doppler derived spectral waveforms regarding mean flow velocities (MFV) and resistant indices (RI). The response ofchange of MCA flow or OPA flow to hypercapnea induced by breath hold (BH) wasmeasured during the last 5 seconds and that to hypocapnea induced by hyperventilation (HV)was measured during 1.5min.Results: In 30 Middle cerebral arteries examined, during breath hold, the MFV(mca) wassignificantly increased (p<0.001) from a mean of 41.15±2.00cm/sto a mean of 55.22±2.66cm/s. No significant increase of RI obtained (P>0.05); the breath hold index was of a mean of0.38±0.077. When performing hyperventilation, The MFV(mca) significantly decreased(p<0.001) from a mean of 41.15±2.00cm/s to a mean of 26.72 ±1.72cm/s; there wassignificant increase in RI (p<0.001) from a mean of 0.54±0.011to a mean of 0.63±0.015. Thecalculated MCA full range of vasodilatation was of a mean of 60℅±3.51. In 30 ophthalmicarteries examined, during breath hold, The MFV(opa) significantly decreased (p<0.001) from amean of 18.49±1.12cm/s to a mean of 14.55± 1.20cm/s; no statistical significant decrease ofRI during breath hold obtained. When performing hyperventilation, the MFV(opa) significantlyincreased (p<0.001) from a mean of 18.49±1.12cm/s to a mean of 24.09±1.27cm/s; therewas no statistical increase of RI (P: 0.05). The calculated OPA full range of vasodilatationwas of a mean of 57.03%±4.53.Conclusion: Ophthalmic artery flow behaves in a different and opposite manner to that ofMCA in response to Hypercapnea and hypocapnea