Venous collapse and the respiratory variability in systemic venous return.

OBJECTIVE Venous collapse limits systemic venous return, but its effects on beat to beat respiratory venous return variations are less well known. The aim of this study was to investigate the effects of venous collapse on respiratory variations in venous return. METHODS A model of venous collapse which included both an increase in haemodynamic resistance to flow and an increase in vessel compliance was incorporated in a previously described cardiovascular model. Respiration was simulated by 5 mm Hg swings of intrathoracic pressure (PTH) at different mean pressures such that the abdominal vena cava and jugular vein were either fully collapsed (mean PTH -11 mm Hg), in the transition zone between collapse and distension (mean PTH -6 mm Hg), or fully distended (mean PTH 9 mm Hg). The mean and standard deviations over each respiratory cycle of the venous return volume (flow integral over heart cycle) and the abdominal vena caval volume were recorded. RESULTS Different venous return volume variabilities in the three operating zones of the vena cava were identified: (1) reduced variability in the collapsed zone associated with the increased haemodynamic resistance [venous return 93(SD 6) ml, abdominal vena caval volume 30(3) ml. absolute right atrial pressure -6.3(1.1) mm Hg]; (2) increased variability in the transition zone [venous return 86(24) ml, abdominal vena caval volume 81(15) ml, right atrial pressure -2.2(0.8) mm Hg]; (3) low variability in the distended zone [venous return 42(11) ml, abdominal vena caval volume 120(2) ml, right atrial pressure 10.1(1.1) mm Hg]. The greater the change in compliance with collapse the greater the increase in flow variability in the transition zone; with no change in compliance there was no increased flow variability in the transition zone. CONCLUSIONS The results suggest that venous collapse increases the respiratory variations in venous return in the transition zone. As venous return variations contribute to arterial pressure variations, the collapsible nature of the great veins may influence respiratory variations in systemic arterial pressure.