Long-term vascular volume expansion maintains elevated thoracic duct lymph flow.

OBJECTIVE

To investigate the mechanisms responsible for edema seen during pregnancy, we tested whether lymph vessels are able to pump high volumes over long periods of time.

STUDY DESIGN

In six chronically catheterized nonpregnant ewes, we examined left thoracic duct lymph flow rate and fluid balance responses to the administration of a balanced isotonic solution at a rate of 1 L/hr for 20 hours. Because estrogen administration decreases lymphatic contractility against outflow pressure, we also administered conjugated estrogens (Premarin) during the last 3 hours of the fluid infusion (experimental time 17 to 20 hours).

RESULTS

After volume loading for 16 hours, the mean +/- SEM lymph flow rate, blood volume, and arterial pressure rose 100% +/- 26%, 20% +/- 2.3%, and 16% +/- 8.1%, respectively. Vascular compliance decreased significantly and, as evidenced by a lack of body weight changes, interstitial fluid volume failed to change (p < 0.05, analysis of variance). The transcapillary oncotic pressure difference increased by 2 mm Hg; venous pressure increased by 5.2 mm Hg. These data suggest that transcapillary forces favored fluid movement into the interstitium. Lymph flow rate remains elevated after blood volume expansion to a level similar to that described during pregnancy in sheep. A transient decrease in urinary output (approximately 20%) occurred with no changes in lymph flow rate, arterial pressure, or blood volume.

CONCLUSION

Lymph flow rate is able to compensate for the increased capillary filtration observed during prolonged blood volume expansion.