Lymph flow in sheep with rapid cardiac ventricular pacing

Increases in systemic venous pressure (Pv) associated with heart failure cause an increase in microvascular fluid filtration into the tissue spaces. By removing this excess filtrate from the tissues, lymphatic vessels help to prevent edema. However, the lymphatics drain into systemic veins and an increase in Pv may interfere with lymphatic flow. To test this, we cannulated caudal mediastinal node efferent lymphatics in sheep. We used rapid cardiac ventricular pacing (240-275 beats/min) to cause heart failure for 4-7 days. Each day we determined the lymph flow rate two ways. First, we adjusted the lymph cannula height so that the pressure at the outflow end of the lymphatic was zero. After we determined the lymph flow with zero outflow pressure, we raised the cannula so that outflow pressure was equal to the actual venous pressure. We quantitated the effect of venous pressure on lymph flow rate by comparing the flow rate with outflow pressure = Pv to the flow rate with zero out low pressure. At baseline, Pv = 5.0 +/- 2.5 (SD) cmH2O and we found no difference in the two lymph flow rates. Pacing caused Pv and both lymph flow rates to increase significantly. However for Pv < 15 cmH2O, we found little difference in the two lymph flow rates. Thus increases in Pv to 15 cmH2O at the outflow to the lymphatics had little effect on lymph flow. By comparison, Pv > 15 cmH2O slowed lymph flow by 55 +/- 29% relative to the lymph flow rate with zero outflow pressure. Thus Pv values > 15 cmH2O interfere with lymph flow from the sheep caudal mediastinal lymph node.