Habituation of the cold pressor response in normo- and hypertensive human subjects

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Abstract
The changes in cardiovascular response to repeated cold-pressor test were studied in young normotensive and in young hypertensive subjects. The cold stimulus consisted of immersing one foot in cold water (4‡ C) for 60 s. Non-invasive methods were used to record the cardiovascular responses: blood flow of the calf was measured using venous occlusion plethysmography, arterial blood pressure with sphygmomanometery, heart rate with electrocardiography. The vascular conductance level in the calf was higher in hypertensive subjects than in normotensives. The difference remained throughout the series of 6 daily experiments. In both hypertensive and normotensive groups of subjects some individuals responded to the cold stimulus with vasodilatation in the calf muscles, others with vasoconstriction. In the hypertensives blood flow increased more and habituation was only transient with a strong tendency for the vasodilatory response to recover, while in normotensives habituation was rapid and complete. Vasoconstrictor responses showed no signs of reduction. The blood pressure increases were larger in hypertensives and remained unaltered within the period of repeated tests (6 days). There was not significant difference between the heart rate changes in the two groups of subjects. It is concluded that the vasculature of the calf shows lower tone and is more labile during the early stage of hypertension. On the basis of evidence found in the literature it is suggested that persistent renal vasoconstriction is the main contributing factor to blood pressure increases in the cold-pressor test. Young hypertensives are unable to habituate this response.