Effect of outflow pressure on liver lymph flow in unanesthetized sheep.

We used lymph flow rate (QL) to lymphatic vessel outflow pressure (Po) relationships to analyze lymphatic flow in five unanesthetized sheep with liver lymphatic cannulas. The olecranon was the zero reference level for pressures. Increases in Po did not change QL until Po exceeded 19 +/- 4 (SD) cmH2O. However, for Po greater than 19 +/- 4 cmH2O, QL decreased linearly with increases in Po. We fit regression lines to the QL vs. Po data for Po greater than 19 cmH2O and estimated the effective lymphatic resistance (RL) as -delta Po/delta QL. The effective pressure driving lymph (PL) was the Po at which QL = 0. At baseline, RL = 0.18 +/- 0.10 cmH2O.min.microliter-1 and PL = 29.6 +/- 3.4 cmH2O. When we increased hepatic vein pressure by 5.7 +/- 1.7 cmH2O, QL increased to 6.2 +/- 3.2 times baseline, RL decreased to 0.050 +/- 0.015 cmH2O.min.microliter-1, and PL increased to 37.1 +/- 3.5 cmH2O (P less than 0.05). Thus 1) liver lymph flow is very sensitive to increases in hepatic vein pressure, 2) there is a substantial QL vs. Po plateau for liver lymphatics, and 3) after hepatic venous pressure elevations, liver lymph flow increases as if it were driven by a higher pressure through a lower resistance.