Critical closing pressure and vasomotor tone in the hind leg and the kidney of the cat

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Abstract
The isolated vascular beds of the hind leg and kidney in cats have been studied by analysis of the pressure-time curves obtained in the distal segment of the artery following occlusion of the arterial blood supply. Arterial flow after 2–3 minutes of occlusion becomes ‘zero,’ according to the criterion used (less than 0.0067 ml/min.), at an arteriovenous pressure difference of 7.8 ± 4.5 (S.D.) mm Hg in the leg and 7.0 ± 3.6 mm Hg in the kidney. Increased resistance to flow has been produced in both the leg and kidney by electrical stimulation of vasomotor nerves, intra-arterial injection of noradrenaline, lowering of the temperature of the local arterial blood and by increasing tissue pressure. The pressure-time curves show a slow but continuous drop and zero arterial flow, i.e. a ‘critical closure,’ is not obtained within the occlusion period. Decreased resistance to flow has been produced in the leg by denervation and by infusion of a ganglionic-blocking agent. Then, the pressure-time curves, as under normal conditions, show a final horizontal level of pressure (zero arterial flow) and the pressure at this point is lower than in the control tests. It is concluded that the level of arterial pressure at zero arterial flow (if obtained) is determined largely by the vasomotor tone and the tissue pressure, when the venous pressure is below 10 mm Hg. Considerable changes in resistance to flow occur with very small changes in the ‘minimum arteriovenous pressure difference’ required for flow. That ‘zero’ arterial flow is obtained only under special conditions would seem to limit the practicability of using critical closing pressure as a measure of vasomotor tone.

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