Transcranial Doppler sonography is frequently used to monitor cerebral blood flow during open heart surgery. However, there is little information about cerebral blood flow velocity in patients under hypothermia.
In this study, mean flow velocities (FVs) in the middle cerebral artery (MCA) during the rewarming period were studied in 67 patients. In all cases, cardiac operations were performed with mild hypothermia and non-pulsatile cardiopulmonary bypass. The pump flow was fixed at 2.6 L/m2/min in all patients. MCA blood flow velocity changes on TCD records were compared with nasopharyngeal temperature (NPT) during the rewarming period. At first the patients were divided into two groups; 1) those with occlusive cerebrovascular attacks (CVA) (53 patients), and 2) those without occlusive CVA (14 patients) . In the patients without occlusive CVA, linear regression analysis demonstrated a relationship between NPT and FV of MCA (mean FV=-1.78+1.23 (NPT), R2=0.278) . There were no significant correlations between NPT and mean FV in patients with occlusive CVA. The patients without occlusive CVA were subdivided into two groups according to age (cut-off value, 70 years) . In the patients aged under 70 years (40 patients), there was a close correlation between NPT and mean FV (mean FV=0.211+1.33 (NPT), R2 =0.278), whereas in the patients aged over 70 years (13 patients), there was a relationship between them (mean FV=10.80+0.649 (NPT), R2 =0.114) . In the patients under 70 years old, there were 21 patients who received coronary artery bypass grafts (CABG group) and 19 who underwent closure of septal defects or replacement of heart valves (non-CABG group) . As in the CABG group, there was a linear correlation between NPT and mean FV (mean FV=5.77+0.911 (NPT), R2 =0.290) . Non-CABG patients showed the strongest correlations between these parameters (mean FV= -2.73 + 1.38 (NPT), R2=0.357) .
It is concluded that flow velocity is reduced under hypothermia in proportion to the reduced NPT in patients aged under 70 years. TCD can provide an approximation of changes in cerebral circulation during mild hypothermia.
