The purpose of this study was to relate baroreflex sensitivity to age and arterial pressure in 61 male and 20 female untreated subjects, aged 19-66 years, whose mean arterial pressures ranged from 70 to 150 mm Hg. In this selected group of subjects there was no correlation between age and arterial pressure. The index of sensitivity used was the increase in pulse interval which occurs reflexly in response to a rise in systolic pressure induced by the intravenous injection of phenylephrine and is measured as the increase in pulse interval in milliseconds per mm Hg rise in systolic blood pressure. It ranged from 1.9 to 48.9 msec/mm Hg. Increasing age and arterial pressure act independently to reduce baroreflex sensitivity. Eight subjects who had normal blood pressure at the time of testing but whose pressure had been elevated in the past, had reflex sensitivities significantly less than expected in persons of the same age and mean arterial pressure. The heart rate in these subjects was not significantly different from that in the controls; the heart rate of the 12 hypertensive subjects aged under 40 years was significantly faster than that of age-matched normotensive subjects but not that of older hypertensive subjects.

The reflex bradycardia produced by a transient phenylephrine-induced rise of arterial pressure was investigated in man during rest and supine exercise, before and after autonomic blockade of the heart. Reflex bradycardia diminished proportionally to the tachycardia of exercise. Propranolol slowed the heart at rest and during exercise, but increased the reflex response only at rest, having no effect during exercise. Atropine, or atropine with propranolol, blocked the reflex during rest and exercise. The tachycardia following hypotension induced by amyl nitrite was similarly affected by the two drugs. Tachycardia induced by standing up and by isoprenaline also diminished the reflex bradycardia. It is concluded that reflex heart rate changes following sudden changes of arterial pressure are predominantly parasympathetic, and diminish during exercise in parallel with the decrease of parasympathetic tone. The reflex response is determined partly by the interaction of parasympathetic and sympathetic impulses at the sinoatrial node, shown by the effects of peripheral sympathetic stimulation and blockade at rest. During exercise central depression of the reflex may also occur. KEY WORDSphenylephrine atropine propranolol amyl nitrite posture parasympathetic nerves sympathetic nerves

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The use of arterial pulse wave velocity (PWV) as a continuous measure of blood pressure changes is outlined. Theoretical considerations indicate that changes in PWV reflect changes in blood pressure, and an experiment was carried out to assess this relationship. PWV along an arm artery was monitored in 26 subjects at a time when the arterial distending pressure of the limb was altered over a wide range by means of externally applied positive and negative pressures. The results show that changes in PWV reliably follow changes in blood pressure. This method can be considered suitable for studies requiring changes rather than absolute values of blood pressure.

An adaptation of the technique for measuring pulse wave velocity is described in which the interval between the R wave of the ECG and the radial pressure pulse is monitored. The resulting transit time (TT) is suitable for use as an indirect measure of blood pressure change. The reliability of the measure was assessed in 5 subjects who volunteered for arterial cannulation on 2 occasions. Simultaneous recordings of intra‐arterial pressure and TT were taken during a variety of maneouvres, including mental arithmetic, isometric exercise, and amyl nitrite inhalation. The dependence of changes in TT on arterial pressure was confirmed for all procedures except amyl nitrite inhalation. Linear correlations between TT and mean arterial pressure were high, varying between −.913 and −.98. The regression coefficients were reproducible and similar in all individuals. The reasons for the anomalous amyl nitrite response are discussed, together with the limits of application of the measure.