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Mechanism of the tilt induced increase in collateral arterial resistance in patients with occlusion of the superficial femoral artery.

Resistance in collateral arteries bypassing an occlusion of the superficial femoral artery was studied in five patients before and during epidural blockade. The patients were placed on a tilt table and the measurements were performed with the patients both in supine and tilted position (40 degrees head up). Arterial blood pressure was measured directly in the brachial, common femoral and popliteal artery. Relative and absolute blood flows were calculated as the relative change in arteriovenous oxygen difference and by an indicator dilution technique. Tilting caused a decrease in relative leg blood flow of 28%. The relative pressure gradient between the femoral and popliteal artery did not change significantly. Calculated relative collateral resistance increased by 49%. During epidural blockade blood flow increased by 39% in supine position corresponding to a decrease in collateral resistance of 22%. During blockade tilting caused a decrease in relative blood flow of 22% compared with the value in supine position. The relative pressure gradient over the occluded superficial femoral artery decreased by 14%. The corresponding calculated relative collateral resistance increased by 14%. The results indicate that the tilt induced constriction of the collateral arteries is mainly neurogenically mediated via the lumbar sympathetic chain, whereas local mechanisms only seems to play a minor role.