CHANGES IN TRANSCRANIAL DOPPLER FLOW VELOCITY WAVE-FORM FOLLOWING INHIBITION OF NITRIC-OXIDE SYNTHESIS

Background: Analysis of the transcranial Doppler blood flow velocity ( FV) waveform is used clinically to detect changes in cerebral haemodyn amic profile. Such changes may be initiated both by alterations in mic rovascular resistance and in the tone of the cerebral arteries. Method s: The role of endothelial mechanisms was investigated using inhibitio n of NO synthesis by systemic administration of N-G-nitro-L-arginine m ethyl ester (L-NAME, 6 mg/kg) followed by simultaneous monitoring of b oth basilar artery FV and cerebrocortical microcirculation (laser Dopp ler flowmetry, LDF) in aneasthetised, ventilated rabbits over 60 minut es. Results: Arterial blood pressure (AP) increased significantly (p < 0.01) above baseline level in the second minute following L-NAME and r emained elevated until the end of experiment. Time average mean and sy stolic FV decreased immediately following L-NAME injection, with the s tatistically significant (p<0.01) decrease from the third minute. Dias tolic FV did not show such radical changes. LDF exhibited a slow decre ase with time becoming significantly lower than baseline (p <0.01) at 50 min. Conclusion: A gradual decrease in cortical microcirculation pr eceded by a rapid reaction recorded in the TCD waveform implies that a n increase in the tone of the great cerebral arteries is the predomina nt phenomenon seen during the acute phase of NO synthase inhibition.