Non-invasive baroreflex testing should be used to assess prognosis

This issue includes an important methodological paper comparing ‘non-invasive’ measurement of baroreflex sensitivity (i.e. Finapres beat-to-beat arterial pressure) to the original ‘invasive’ method, (i.e. using intra-arterial pressure measurement) developed in Oxford more than 30 years ago. Both methods are invasive in the sense that they use intravenous injections of a vasoconstrictor agent phenylephrine. The importance of the paper is twofold. First it represents by far the largest comparative study of the two methods, and second, baroreflex sensitivity measurement has recently been shown to give prognostic information for a range of cardiovascular problems notably post myocardial infarction, and heart failure/left ventricular dysfunction, which is additional to, and of equal power to, established markers such as ejection fraction. The phenylephrine test measures the beat-to-beat lengthening in RR interval during the resulting ramp increase in arterial pressure. It is a measure of vagal tone, and of the increase in vagal tone resulting from the baroreflex response to a rise in arterial pressure. The methodology has been validated in numerous studies, but its use has hitherto been limited to research projects rather than clinical practice. This demonstration that baroreflex sensitivity determined by finger cuff pressure measurement in patients can give very similar information to the original method using direct intra-arterial pressure will therefore increase its routine applicability considerably. One limitation to the test is that there is considerable intra-individual variability in the measurement, so most workers use an average of two to three measures in the same steady state. Baroreflex sensitivity is rapidly decreased by arousal, or by exercise. We do not fully understand why this measurement of autonomic function is so predictive. Vagal tone has been shown to raise the threshold for ventricular fibrillation in a dog model of ischaemic heart disease. Baroreflex sensitivity may also be a marker for neurohumoral abnormalities, since baroreflex sensitivity is reduced by angiotensin (at the reflex centres in the medulla oblongata), and by sympathetic arousal. Another limitation is that the discriminatory ability of the test is reduced in groups where baroreflex sensitivity is low (e.g. in elderly subjects). Other measures of autonomic control (e.g. heart rate variability, or power spectrum analysis) may be more useful. Again the mechanisms by which these latter tests can discriminate are at present poorly understood. Low heart rate variability reflects many things — poor modulation of autonomic tone, but also it may be due to lack of ability to exercise, and so be related to severe cardiac damage, rather than autonomic factors. In time these methods may be widely used clinically, but first we need to understand much more of the physiology and pathophysiology underlying the different tests. I must declare an interest in that I have worked on and off with the group at Montescano for 8 years or more. They combine a large service load associated with the big cardiac transplantation service in Pavia, with a large bioengineering group who have developed sophisticated programmes to compare many of the tests of autonomic function used in patients before or after transplantation, or post-myocardial infarction. They are to be complimented on this useful validation of this easier methodology. P. SLEIGHT John Radcliffe Hospital, Oxford, U.K.