CO2 REACTIVITY IN THE ISCHEMIC CORE, PENUMBRA, AND NORMAL TISSUE 6 HOURS AFTER ACUTE MCA-OCCLUSION IN PRIMATES

Testing vasoreactivity with CO2 or Diamox is a common diagnostic procedure for the study of haemodynamics in stroke patients. CO2 reactivity (CO2 R) was tested in 5 baboons six hours after permanent occlusion of the left middle cerebral artery (MCA) in order to attain new insights into interpretation of vasoreactivity tests. Using the microsphere method, cerebral blood flow (CBF) was determined in the various vascular territories as well as in the centre of the ischaemia, the penumbra and the remaining MCA-tissue. CBF dccreased significantly in the affected MCA in all animals and in addition in the contralateral cerebellum in one animal (p < 0.05). In addition, the left anterior cerebral artery (ACA) demonstrated a similiar decrease. During hypercapnia CBF increased in all areas with the exception of the left, occluded MCA territory. Thus CO2 enhanced the difference between ischaemic and non-ischaemic tissue (i.e., tissue with diaschisis). Mean CO2R was 3.37 ml/100 g/min/mmHg in the right MCA, 0.16 in the left. While the left ACA demonstrated a decreased perfusion during normocapnia in a similar range to the MCA territory, only CO2 R was able to identify precisely the territory of the occluded vessel. CO2 R was zero or negative in the ischaemic core, close to zero in the penumbra and profoundly decreased in the remaining MCA tissue. The overall CO2 R of the MCA was almost zero, suggesting vasoparalysis in response to hypercapnia in the core and penumbra and exhausted CO2 R even in non-infarcted, non-penumbral tissue. One animal displayed a negative CO2 R equivalent to an intracerebral steal-phenomenon. Thus the CO, challenge test helps to interpret CBF values by enhancing the differences between ischaemic and non-ischaemic tissue potentially distinguishing ischaemic tissue from low flow areas of different origin. The occluded vascular territory was identified correctly by CO2 R, not, however, by the normocapnic CBF measurement alone. Because of the risk of intracerebral steal phenomena it is recommended that the CO2 challenge test should not be performed during the acute stages of stroke.