Mechanism of the postural vasoconstrictor response in the human foot

Abstract

1. The mechanism of postural vasoconstriction in the skin of the foot was examined in 102 healthy subjects by using laser Doppler flowmetry. 2. In 45 subjects, when one foot was lowered 50 cm below heart level and the other foot kept horizontal, blood flow was progressively reduced in the dependent foot (by 79%) with a concomitant, but less pronounced, reduction in flow in the horizontal foot (by 18%), indicating that a central mechanism is involved. After lumbar sympathetic blockade (in 10 patients with epidural anaesthesia), the flow in the horizontal foot remained virtually constant, indicating that the central component is mainly mediated via efferent sympathetic nerves, whereas the postural fall in flow in the dependent foot, though partially attenuated, was preserved, indicating that a local mechanism is mainly involved. 3. On lowering one foot below heart level in 12 subjects, there was a small but significant reduction in systolic and mean arterial pressures during the first minute of dependency. During the fourth minute, systolic pressure decreased, diastolic pressure and heart rate increased, but the mean arterial pressure was maintained. 4. In 19 subjects postural vasoconstriction was nearly abolished during local nervous blockade (lignocaine 3.7 x 10(-4)-7.4 x 10(-2) mol/l), indicating that the local mechanism mediating the vasoconstriction is mainly neurogenic in nature. However, there was still a small fall (19%) in flow in the dependent foot during blockade, probably indicating a minor contribution of a local myogenic mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of ulnar nerve blockade on skin microvascular blood flow*

“…After lumbar sympathetic blockade, the flow in the horizontal foot remains virtually constant, indicating that the central component is mainly mediated via efferent sympa thetic nerves. The present study once more shows that the vasoconstrictor response during tilting is mainly mediated by local neurogenic and/or myogenic mechanisms, partially supplemented by a central component [36].…”

Evidence of a myogenic response in vasomotor control of forearm and palm cutaneous microcirculations

“…This response has been identified in skin, subcutaneous tissue, and muscle (3,8,10,11,13,23). The venoarteriolar response is generally preserved in individuals with a neuronal lesion proximal to the site of measurement (23), but it is absent during application of local anesthesia at the site of blood flow measurement (3,7,10,23). Importantly, this response is independent of perfusion pressure because similar vasoconstrictor responses are observed during limb suspension, when venous pressure is elevated but perfusion pressure is unaffected, relative to venous occlusion in which venous pressure is elevated and perfusion pressure is reduced.…”

“…However, the stimulus for this response is a change in venous pressure regardless of whether perfusion pressure changes or remains constant. This response also requires an intact local neural network (3,7,10,13,23), which is in contrast to traditional myogenic vascular autoregulation. Thus findings pertaining to the cutaneous venoarteriolar response shed little insight regarding whether cutaneous tissue is capable of modulating vascular tone specifically due to changes in perfusion pressure.…”

“…Given this, the requirement to autoregulate blood flow through this vascular bed during relatively small changes in perfusion pressure would be less critical relative to other vascular beds. Such a response is likely advantageous given that precise autoregulation of cutaneous vasculature would oppose thermally induced cutaneous vasoconstriction and vasodilation, which are necessary to maintain temperature homeostasis (21).The venoarteriolar response is an example of a vascular autoregulatory mechanism that has been identified and widely investigated in human skin (3,7,9,13,23). However, the stimulus for this response is a change in venous pressure regardless of whether perfusion pressure changes or remains constant.…”

“…investigated in human skin (3,7,9,13,23). However, the stimulus for this response is a change in venous pressure regardless of whether perfusion pressure changes or remains constant.…”