Effect of outflow pressure upon lymph flow from dog lungs

Abstract

The pulmonary lymph flow rate (Q̇(L)) should be a function of the lymph vessels' resistance and the pressure gradient along the vessels. We attempted to study how these factors affect lymph flow. We assumed that the lymph system could be represented by a single pressure generated within the lung (P(L)) and a single resistance (R(L)). Thus, Q̇(L) should be a function of the lymph vessel outflow pressure (P(O)): Q̇(L) = (P(L) - P(O))/R(L). We cannulated tracheobronchial lymph vessels in eight anesthetized dogs and varied P(O) by raising the outflow end of the cannula. Q̇(L) decreased linearly when we increased P(O). We estimated R(L) as -ΔP(O)/ΔQ̇(L) and P(L) as the extrapolated P(O) at which Q̇(L) = 0. At baseline P(L) = 7.7 ± 2.7 (SD) cm H2O and R(L) = 0.36 ± 0.25 cm H2O.min/μl. After we increased capillary pressure to produce edema, P(L) and R(L) averaged 22.8 ± 8.8 and 0.14 ± 0.12, respectively. After we reduced the reduced the capillary pressure to baseline in the edematous lungs, P(L) and R(L) averaged 11.6 ± 2.8 and 0.08 ± 0.09, respectively. All changes in P(L) and R(L) were significant (P < 0.05). These results show that (1) lymph flow rate depends upon lymph vessel outflow pressure, and (2) the Q̇(L) vs. P(O) relationship is changed by edema. P(L) may be equal to the pressure causing lymph to flow and R(L) may equal the lymph vessel resistance.