THORACIC-DUCT LYMPH-FLOW IN FETAL SHEEP WITH INCREASED VENOUS-PRESSURE FROM ELECTRICALLY-INDUCED TACHYCARDIA

The intent of this study was to investigate thoracic duct lymph flow, as it is related to the development of hydrops fetalis during rapid at rial pacing. We studied 6 fetal sheep at 128 +/- 6 days of gestation w ho had chronically placed thoracic duct catheters, aortic and superior vena cava catheters, and atrial pacing electrodes. Atrial pacing at 3 17 beats/min caused an elevation in central venous pressure from a bas eline value of 3 Torr to 7 Torr without affecting pH, arterial blood g as tensions, aortic blood pressure, total protein concentration, or co lloid osmotic pressure, although there was a small rise in hematocrit. The thoracic duct lymph flow rate at baseline was 41 +/- 6 ml/h. Afte r atriaI pacing for 6 h, the lymph flow rate as measured over at least three consecutive 10-min intervals, and presumably the transvascular fluid filtration rate, increased to 67 +/- 7 ml/h if it was collected at an outflow pressure of 3 Torr, equal to the venous pressure prior t o the onset of atrial pacing. However, if the lymph was collected inst ead at an outflow pressure of 7 Torr, equal to the actual venous press ure measured with rapid atrial pacing, then the lymph flow rate dimini shed to 48 +/- 5 ml/h This difference in lymph flow secondary to the i ncrease in venous pressure could account for a maximum of 19 ml/h of e dema that accumulates in fetal interstitium and body cavities with atr ial pacing. We conclude that the increased venous pressure measured af ter rapid atrial pacing for 6 h not only increases the transvascular f luid filtration rate, but it also impairs the return of lymph flow to the circulation.