1. A method has been developed for estimating resistance to blood flow in the collateral arteries around the elbow. Arterial pressure is recorded continuously from the radial artery and blood flow in the forearm and hand is measured by venous occlusion plethysmography. The brachial artery is occluded for short periods, and the pressure drop across the collaterals and the flow through them are determined. From these observations an index of resistance can be calculated.

2. During 2 min occlusions of the brachial artery, collateral arterial resistance fell progressively to reach a level that was on average 45% lower than the initial resistance (P < 0.01).

3. There was an inverse relation between distending pressure in the collateral arteries and calculated resistance.

4. Ergotamine tartrate (0.25 mg intravenously) increased collateral resistance by an average of 135%. Glyceryl trinitrate (0.5 mg sublingually) reduced collateral resistance by an average of 45%. Hydrallazine and isoprenaline had an inconsistent dilator effect; the direct action of these drugs may have been offset by the reduction in distending pressure which they induced.

5. The elbow collateral arteries provide a useful model for studying physiological and pharmacological responses of small limb arteries in man.

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