Number 1 in this SeriesA review on pleural space is certainly most welcome, for the simple reason that on opening a textbook of physiology the concepts concerning pleural fluid turnover date back to 1927 [1]. The same comment, in fact, applies in general to microvascular water and solute exchange. This appears particularly misleading, considering the major advances achieved in the field over the last 70 yrs. Around the turn of the 19th century, STARLING and TUBBY [2] interpreted microvascular fluid and solute exchange as resulting from the balance between hydraulic and colloidosmotic pressures. This concept is still valid today, but the complete formulation of transmicrovascular exchange has become much more complex because water crosses biological membranes more easily than large solutes (namely, plasma proteins) do [3]. In fact, different equations describe water and solute fluxes [4].As a result, the general model of transcapillary fluid exchange still taught to students, based on fluid filtration at the arteriolar end of the capillary bed and reabsorption at the venular end, appears rather simplistic. Due to the different permeability to water and solutes, one could predict on a mathematical basis that such a model would lead to a progressive increase in interstitial protein concentration over time [3,4], a condition that cannot be confirmed experimentally.Similarly, the old hypothesis claiming that pleural fluid filters at parietal level and is reabsorbed through the visceral pleura [1] would imply that pleural liquid protein concentration would keep increasing with age, but again there are no indications that this occurs. The existence of partial restriction to the movement of large solutes, compared to that of water molecules, posed scientists the major problem of explaining how interstitial volume and protein concentration are kept fairly steady. Ideologically, this led to re-evaluation of lymphatics as the major route for interstitial fluid drainage. In fact, since lymphatics do not sieve proteins, they leave the interstitial protein concentration unaltered the latter depending only upon the sieving properties of the filtering membrane. Accordingly, the description of interstitial fluid homeostasis under steady state conditions is now being explained as a balance between capillary filtration and lymphatic absorption. Major validation of this model has come from the accumulating experimental data over the past 30 yrs concerning interstitial tissues [4] and serous spaces [5]. Interestingly, since the pioneering, work of STARLING and TUBBY [2] the pleural space has been used as a useful experimental model to study the interaction between microvascular filtration and lymphatic drainage.This article presents an integrated view of pleural fluid turnover, based on data gathered from experimental studies on animals over the last 15 yrs. The situation in man is, unfortunately, still poorly defined; yet, where possible, Under physiological conditions, the lung interstitium and the pleural space behave as function...

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.

Work capacity and cardiopulmonary performance were studied in a group of 11 young obese subjects (BMI 39.9 kg/m²) and a group of 10 young normal subjects (BMI 22 kg/m²). First of all they underwent an incremental cycle ergometer test up to exhaustion. Subsequently, every subject of the two groups performed a constant work rate test at different work loads to estimate cardiac output (q̇) below anaerobic threshold (AT) by a 20-second CO₂ rebreathing method. Obese subjects had a significantly lower AT (79 vs. 109 W). The ratio between oxygen uptake and heart rate (v̇O₂/HR) (O₂ pulse) was higher in the obese group; nevertheless, this variable became significantly lower if we took into consideration the ratio between O₂ pulse and kilogram fat-free body mass or kilogram body weight. Both these observations suggest that their reduced work tolerance is linked with a reduced oxygen supply to the muscles in activity. q̇ increased in similar ways in obese and normal subjects at the preset work rates. The ratio q̇/body surface (cardiac index; CI) that we considered in order to try to minimize the differences in body sizes between the two groups, increased less in response to increasing work rates in our obese subjects than in normal subjects. As a whole, these data appear to be in line with a relatively less efficient cardiac performance during progressive work rates in obese subjects.

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We developed an experimental approach to measure the pulmonary interstitial pressure with the micropuncture technique in in situ lungs with an intact pleural space. Experiments were done in anesthetized paralyzed rabbits that were oxygenated via an endotracheal tube with 50% humidified oxygen and kept in either the supine or the lateral position. A small area of an intercostal space was cleared of the intercostal muscles down to the endothoracic fascia. Subsequently a "pleural window" was opened by stripping the endothoracic fascia over a 0.2-cm2 surface and leaving the parietal pleura (approximately 10 microns thick). Direct micropuncture through the pleural window was performed with 2- to 3-microns-tip pipettes connected to a servo-null pressure-measuring system. We recorded pleural liquid pressure and, after inserting the pipette tip into the lung, we recorded interstitial pressure from subpleural lung tissue. Depth of recording for interstitial pressure averaged 263 +/- 122 (SD) microns. We report data gathered at 26, 53, and 84% lung height (relative to the most dependent portion of the lung). For the three heights, interstitial pressure was -9.8 +/- 3, -10.1 +/- 1.6, and -12.5 +/- 3.7 cmH2O, respectively, whereas the corresponding pleural liquid pressure was -3.4 +/- 0.5, -4.4 +/- 1, and -5.2 +/- 0.3 cmH2O, respectively.