Prof. S. Greenwald and Dr R. Pearse
There is much evidence that the ability of the vascular endothelium to respond to a reduction in flow by releasing vasodilators is impaired in a number of vascular pathologies. It has also been shown that this impairment may be detectable before obvious clinical symptoms become evident, suggesting that its quantitative assessment may provide a powerful screening tool for vascular disease. However, non invasive measurement of endothelial function requires sophisticated equipment such as MRI, echo-tracking or duplex ultrasound. We have developed an optical technique for the transcutaneous detection of the diameter pulse in superficial arteries such as the femoral, carotid and radial; and the device is currently used to measure vascular compliance. We are now investigating the ability of the device to detect changes in arterial pulse wave velocity (PWV, a measure of arterial compliance) in response to cuff induced ischaemia. We have carried out two experiments. In the first the PWV in the radial artery is measured under baseline conditions for one minute. Then a cuff around the upper arm is inflated to suprasystolic pressure, occluding the brachial artery. The occlusion is maintained for 5 minutes rendering the forearm and hand ischaemic. On releasing the cuff, flow in the brachial artery rapidly increases (reactive hyperaemia), causing an increase in mean shear stress which leads to the release of nitric oxide and the consequent relaxation of vascular smooth muscle: so called flow dependent vasodilatation. This causes the vessel to become more compliant leading to a reduction in PWV. By continuously monitoring the PWV, its gradual return to baseline values can be monitored. The magnitude of the change in PWV and the time it takes to return to baseline value may allow us to obtain quantitative data concerning endothelial function. The second experiment is similar except that we monitor the pulse arrival time at the index finger of each hand, one of which belongs to the occluded arm. Under baseline conditions the pulse arrives at the two fingers almost simultaneously. However, following release of the occlusion the pulse wave travels more slowly in the ischaemic arm, for the reasons explained above and the pulse arrives at the finger of the occluded arm later than that in the finger of the control arm. Although the second experiment does not provide an absolute value of PWV it is easier to perform and requires less intervention from the operators.
Last year we showed that the PWV and pulse arrival times do indeed change as expected following occlusion release. This year we plan to validate the technique against the gold standard of duplex ultrasound. We hypothesise that the magnitude and timing of the change in PWV and difference in pulse arrival time between the two fingers will be closely correlated with the change in radial artery diameter measured by duplex ultrasound.