Venous Flow Restriction: The Role of Vein Wall Motion in Venous Admixture

(a) Schematic diagram of venous flow model used in the experiments. Two ‘venous’ reservoirs (#1 and 2) of identical elevation confluence through a ‘Y’ connector (‘node’) to drain into an ‘atrial’ reservoir of same elevation as #1 and 2 through ‘calf’ and ‘abdominal’ pumps mounted in series. Energy for flow through the system is provided by additional graduated pressure head into reservoirs #1 and 2 from a pressurized air tank. The flow pattern when the pressure head to the two reservoirs are identical (‘base pressure head’) is different when reservoir #2 receives a differentially higher pressure head than reservoir #1 as shown by the heavier arrow. The flow pattern when the two pumps are full (‘steady state flow’) is also different from the pattern observed when the pumps are empty and collapsed (‘calf pump refill’ and ‘abdominal pump flow’). (b) Detailed diagram of venous flow model used in the experiments. (a) Mercury manometer-controlled regulators to set pressure head into the two reservoirs. (b) and (c) Reservoirs #1 and #2. (d) Shut off solenoid valves. (e) Pressurization valve (pressurized tank, not shown) for calf pump sleeve. (f) Flow meters. (g) Manometer regulator to control abdominal pump compression. (h) ‘Atrial’ reservoir. (k) Dump lines for abdominal and calf pump sleeves. P, pressure transducers; CPA, calf pump assembly; APA, abdominal pump assembly; CV, check valve.

Basal flow. Flow curves for identical base pressure head setting of 15 mmHg (zero pressure differential) at the reservoirs with abdominal compression of +5 mmHg. Note huge augmentation of flow rate (velocity) during calf pump refill increasing several folds from the steady state flow. Calf pump pressure (yellow line, right scale) falls to near zero.

Venous flow restriction flow recording for the two reservoirs with input pressures of +15 and +25 mmHg, respectively, (differential pressure +10 mmHg) at abdominal compression of +5 mmHg. There is no flow from reservoir #1 during steady state; flow resumes during calf pump refill with peak flow nearly the same as from reservoir #2 and similar to calf pump refill-flows shown in Fig. 1. There is a small insignificant (±3 ml) ‘bump’ in restricted flow from reservoir #1 during abdominal pump action. Note venous flow restriction is not detectable in the pressure curves (right scale) which are virtually identical to Fig. 1. See text.

Network diagram of the circulation. The left and right atria (LA and RA) are shown as reservoirs, the latter larger as the systemic circuit contains more blood volume than does the pulmonary circuit. The ventricles (LV and RV) are shown as pumps (P). Major branches of the aorta (A, artery) and major named veins (V, vein) are shown with intervening resistance depicted as pressure reduction valves (PRV). The renal and hepatic circuits contain two separate vascular beds and are each depicted with two PRVs. The pressurized abdominal cavity, the narrow diaphragmatic hiatus and the first rib produce back pressure to lower limb flows entering the abdomen, abdominal vena cava flow entering the chest, and upper limb flow entering thorax, respectively. This is represented as back pressure valves (BPV) at the respective sites. The various venous pumps function as booster pumps and reduce the pressure head at their respective sites. They are depicted as differential head devices or ΔH. Significant reversal of flow does not occur in vascular beds during venous flow restriction as the capillary in effect functions as a back pressure valve (BPV).

Flow dynamics between three interconnected reservoirs. Left: Classic three-reservoir problem with rigid conduits. EGL relationships at the node determine whether reservoir B will flow outward or receive inflow from reservoir A. Right: Modification of flow with collapsible conduits: The presence of a collapsible tube in conduit c′ (calf pump in the model) allows for dynamic change in EGL relationships at the node influencing flow behaviour of reservoir B′. See text.