Lung Fluid Dynamics in Awake Newborn Lambs

Abstract

A B S T R A C T We measured steady-state lung lymph flow, lymph protein flow, and simultaneous pulmonary vascular pressures in 12 1-wk-old unanesthetized lambs and compared these measurements to those of previous studies, performed under similar conditions, on nine awake adult sheep. The purpose of these experiments was to compare newborn and adult sheep with respect to transvascular filtration of fluid and microvascular permeability to plasma proteins. We prepared the lambs surgically to isolate and collect lung lymph and measure average pulmonary arterial and left atrial pressures, allowing at least 2 days for the lambs to recover from surgery before studies began. Lambs had higher pulmonary arterial and left atrial pressures, lower lymph and plasma protein concentrations, and 57% more lymph flow per gram of dry bloodless lung than sheep; the difference in protein flow between lambs and sheep was not significant. Protein concentration in lymph relative to that in plasma was significantly lower in lambs than in sheep; but the ratio of albumin concentration to globulin concentration in both lymph and plasma was almost identical in the two groups of animals. Extravascular lung water per gram of dry bloodless lung was greater in lambs (4.82+0.11 g) than in sheep (4.45+0.08 g), but there was no histologic evidence of pulmonary edema in either group of animals. These findings suggest that lambs have more transvascular filtration of fluid per unit lung mass than sheep, but that microvascular sites for protein exchange do not differ appreciably in lambs and sheep. To test this conclusion, we measured steady-state lymph flow in three lambs before and after raising pulmonary microvascular pressure by rapid intravenous infusion of saline. Lymph flow increased as a function ofthe net transvascular driving pressure (hydraulic pressure gradient-protein osmotic pressure gradient). This response was almost identical to that of four sheep with pulmonary microvascular pressure augmented by inflation of a balloon in the left atrium. In eight lambs we measured the time for intravenously injected 1251_ albumin to equilibrate in lymph at half the specific activity of plasma: the protein tag equilibrated faster than in sheep. This difference could be explained partly by the higher pulmonary arterial and left atrial pressures oflambs than sheep, and possibly by the presence of more microvascular sites for protein exchange relative to the volume of distribution of protein in the lung of the younger animals.

Lung Fluid Balance in Hypoxic Lambs

“…If we assume that the concentration of protein in lymph approximates that in the pulmonary interstitium, as others have shown (30,44), then measurement of lung lymph flow and the concentration of protein in lymph provides a sensitive index of net transvascular movement of fluid and protein in the lung (4,8,39,40). Our results, therefore, suggest that alveolar hypoxia increases net transvascular filtration of fluid into the lungs of lambs.…”

“…In the three lambs killed after 12 h of hypoxia, extravascular lung water content per gram of dry lung tissue was not significantly different from that of control lambs (4.65 + 0.28 versus 4.72 + 0.14) (4 …”

“…Surgical preparation of the lambs is described in detail elsewhere (4,40). Briefly, we operated on each lamb twice, first within 48 h of birth and again 7-10 d later.…”

Lung Fluid Balance in Awake Newborn Lambs with Pulmonary Edema from Rapid Intravenous Infusion of Isotonic Saline

“…Using methods previously described (2,22), we surgically prepared nine newborn l a m b s~o collect lung _lymph and measure average pulmonary arterial (Ppa), left atrial (Pla), and aortic blood pressures. The lambs (average birth weight 3.9 kg., range 2.3-5.2 kg) had two thoracotomies, the first during the initial week of life, and the second 4-7 days later.…”

“…This was done with five lambs immediately after a 3-hr infusion of saline, on a day subsequent to the aforementioned three-part experiments in which lymph was collected. We compared the results to measurements of extravascular lung water content previously done with normal control lambs (2).…”