The intent of this study was to investigate thoracic duct lymph flow, as it is related to the development of hydrops fetalis during rapid atrial pacing. We studied 6 fetal sheep at 128 +/- 6 days of gestation who had chronically placed thoracic duct catheters, aortic and superior vena cava catheters, and atrial pacing electrodes. Atrial pacing at 317 beats/min caused an elevation in central venous pressure from a baseline value of 3 Torr to 7 Torr without affecting pH, arterial blood gas tensions, aortic blood pressure, total protein concentration, or colloid osmotic pressure, although there was a small rise in hematocrit. The thoracic duct lymph flow rate at baseline was 41 +/- 6 ml/h. After 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 to the onset of atrial pacing. However, if the lymph was collected instead at an outflow pressure of 7 Torr, equal to the actual venous pressure measured with rapid atrial pacing, then the lymph flow rate diminished to 48 +/- 5 ml/h This difference in lymph flow secondary to the increase in venous pressure could account for a maximum of 19 ml/h of edema that accumulates in fetal interstitium and body cavities with atrial pacing. We conclude that the increased venous pressure measured after rapid atrial pacing for 6 h not only increases the transvascular fluid filtration rate, but it also impairs the return of lymph flow to the circulation.
