Cerebral blood flow and resistances during hypotensive haemorrhage in the rabbit: transcranial Doppler and laser-Doppler flowmetry.

Transcranial Doppler (TCD) determined cerebral blood flow velocity and laser Doppler flowmetry (LDF) measured cortical perfusion were simultaneously assessed during hypotensive haemorrhage in 15 anaesthetized rabbits. Systolic (Fsys), diastolic (Fdia) and mean (Fmean) blood flow velocities were recorded into the intracranial internal carotid (ICA) and basilar artery (BA). Resistance (RI = Fsys-Fdia/Fsys) and pulsatility (PI = Fsys-Fdia/Fmean) indices were calculated. Step decreases of 10 mmHg of mean arterial pressure (MAP) from 80 to less than 30 mmHg provoked a fall of LDF signal below 50 mmHg. Blood velocities decreased into BA below 40 mmHg, and below 50 mmHg into ICA indicating regional differences in cerebral autoregulation. Cortical resistances (resLDF = MAP/LDF) fell below 60 mmHg whereas RI and PI increased when MAP decreased into BA below 40 mmHg and ICA below 50 mmHg. A weak correlation was found between Fmean and LDF (BA: r = 0.55, P < 0.01 and ICA = 0.46, P < 0.01). Both RI and PI were poorly correlated to resLDF into BA (RI-resLDF: r = -0.39, P < 0.01; PI-resLDF: r = -0.39, P < 0.01) and ICA (RI-resLDF: r = -0.18, ns; PI-resLDF: r = -0.22, ns). Pulse pressure (systolic-diastolic pressure) correlated with RI (ICA: r = -0.62, P < 0.001; BA: r = -0.61, P < 0.001) and PI (ICA: r = -0.61, P < 0.001; BA: r = -0.62, P < 0.001). In conclusion, during haemorrhagic shock, TCD correlates with LDF and indicates regional differences in autoregulatory settings. However, Doppler indices do not reflect the changes in cerebral resistances because they are influenced by the changes in pulsatile pressure.