Pulmonary interstitial pressure in intact in situ lung: transition to interstitial edema

Abstract

In anesthetized rabbits (n = 25) subject to slow intravenous saline loading (0.4 ml.min-1.kg-1) for 3 h, we measured pulmonary interstitial pressure (Pip) in intact in situ lungs with glass micropipettes inserted directly into the lung parenchyma via a "pleural window." Measurements were done in apneic animals at the end-expiratory volume with O2 delivered in the trachea. Pip was -10 +/- 1.5 (SD) cmH2O in control and increased to 0.6 +/- 3.8 and 5.7 +/- 3.3 cmH2O at 66 and 180 min, respectively. The wet-to-dry weight ratio (W/D) of the lung was 5.04 +/- 0.2 in the control group and 5.34 +/- 0.7 at 180 min (+6%); the corresponding W/D for intercostal muscles were 3.25 +/- 0.03 and 4.19 +/- 0.5 (+28%). Pulmonary interstitial compliance was 0.47 ml.mmHg-1.100 g wet wt-1. Pulmonary arterial and left atrial pressures were 18.4 +/- 2 and 3 +/- 1 cmH2O in control and increased to 19.5 +/- 2.9 and 4.6 +/- 1.7 cmH2O at 180 min, respectively. Aortic flow (cardiac output) increased from 103 +/- 35 to 131 +/- 26 ml/min; pulmonary resistance fell from 0.17 +/- 0.06 to 0.14 +/- 0.05 cmH2O.min.ml-1 (-18%), suggesting that the increase in Pip did not limit blood flow. The pulmonary capillary-to-interstitium filtration pressure gradient decreased sharply from a control value of 10 cmH2O to 0 cmH2O within 60 min because of the increase in Pip and remained unchanged for < or = 180 min. Data suggest that the pulmonary interstitial matrix can withstand fluid pressures above atmospheric, preventing the development of pulmonary alveolar flooding.

Impeded Alveolar-Capillary Gas Transfer With Saline Infusion in Heart Failure

“…Several human and animal studies [11][12][13][14] have investigated the transition of pulmonary edema or cardiovascular adaptations during fluid or salt loading. In these studies, liquid volumes were large enough to induce obvious hemodynamic variations.…”

Hemorrhagic Shock and Resuscitation-Mediated Tissue Water Distribution is Normalized by Adjunctive Peritoneal Resuscitation

“…This pattern of lung water redistribution is related to the unique mechanisms that control lung fluid balance. [47][48][49] In particular, compliance of the lung interstitium is at least 20-fold lower compared with other tissues 47 ; implying that, in the early phase of interstitial edema, the associated increase in interstitial hydrostatic pressure is not necessarily accompanied by expansion of the IFV, as in other tissues with high compliance. This low interstitial lung compliance is attributed to the structure of the interstitial ground matrix, and represents an important "tissue safety factor" to counteract a rapid progression of pulmonary edema.…”

Physiology and pathophysiology of pleural fluid turnover

“…In the lung parenchyma, for example, a very tight control exists to guarantee a minimum amount of interstitial fluid [26,29] which is crucial to assure gas diffusion. The comparison between pleural space and lung interstitium offers, in fact, an important clue to the understanding of how a condition of minimum interstitial volume is achieved.…”

“…The comparison between pleural space and lung interstitium offers, in fact, an important clue to the understanding of how a condition of minimum interstitial volume is achieved. The pulmonary interstitium has a very low mechanical compliance; accordingly, when facing a condition of increased filtration, this leads to a marked increase in pulmonary interstitial pressure [29]. This represents the so-called "tissue safety factor" against the development of pulmonary oedema, as it opposes, based on the Starling balance of pressures, a further filtration.…”

“…In fact, it was also found that during lung interstitial oedema in rabbits (that have a thin visceral pleura), pulmonary interstitial pressure rose well above atmospheric [29], creating a gradient for liquid filtration from the lung parenchyma into the pleural space. However, this did not result (at least up to 3 h of observation) in any appreciable increase in pleural liquid volume, indicating that the permeability of the visceral pleura is physiologically fairly low.…”