Ambulatory venous pressure: new concepts

Abstract

Background: The importance of the calf pump (the ‘peripheral heart’) in the lower limb venous circulation is well known. The ambulatory venous pressure (AMVP) is generally considered the quintessential functional test of calf pump function. However, much controversy exists on the basic hemodynamics of AMVP as well as its measurement. Recent work has helped to revise/clarify many of these controversies. Results from experimental simulations are used to illustrate key hemodynamic concepts. A multicameral model of calf pump Arnoldi popularized the notion that deep venous pressures can be monitored by inserting a needle in the dorsal foot vein (unicameral model). It has been shown recently that ambulatory venous pressures in the deep system is different from that in the dorsal foot vein and also the saphenous vein. AMVP profile in the three valved systems are different from each other (multicameral model). AMVP is traditionally monitored via % drop and also Venous refill time (VFT). Analysis of a large cohort of patients shows that VFT is more sensitive. % drop can be omitted as it is rare for it to be abnormal without concurrent abnormal VFT. AMVP is normal in venous obstruction, contradicting common belief. Ambulatory venous hypertension is a specific property of reflux, not obstruction. Supine venous pressure is elevated in obstruction but not reflux despite the suspected role of microvascular hypertension in reflux pathology. Role of calf capacitance & compliance: While severe reflux can shorten VFT, reduced calf capacitance and compliance are more important as can be shown in experimental set ups and clinical analysis. Calf Pump failure: Like the heart, the calf pump can eject all the inflow presented to it (up to 3X normal). Thus the popular concept of ‘calf pump failure’ from reflux overload has little concrete evidence to support it. Column segmentation: It is commonly assumed that valve closure results in column segmentation. It can be shown in experimental settings that collapse of the venous segment below the valve closure is necessary for column segmentation. Furthermore a reconstruction of the events surrounding column restoration makes it clear that a closed valve above the calf pump cannot reopen with the hydrostatic pressure of the restored column height below the closed valve alone. Much higher pressures generated by inflow interacting with wall tension of the infra-valvular segment is necessary to reopen the closed valve and restore flow. AMVP does not reach resting levels in experimental models till wall tension is restored to resting levels. A full blown reflux through an open valve will not transmit column pressure when the calf pump is partially collapsed. A non-invasive replacement for AMVP: Prevailing clinical practice and recent guidelines emphasize duration of reflux at the proximal saphenous, femoral and popliteal valves for assessment of reflux severity. It has been shown that these proximal valves play no significant role in column segmentation. A group of valves in the posterior tibial vein and the great saphenous vein near the ankle are the critical players in column interruption duration (CID). CID can be measured non-invasively by duplex after calf ejection by pressurized cuffs. Conclusion: AMVP has declined in clinical use as duplex identification of reflux in proximal valves has become common practice. More useful information can be obtained by using Duplex to measure CID.