EFFECT OF SURFACE TENSION ON CIRCULATION IN THE EXCISED LUNGS OF DOGS

Pulmonary surfactant surface tension influences alveolar capillary shape and oxygenation

Alveolar capillaries are located in close proximity to the alveolar epithelium and beneath the surfactant film. We hypothesized that the shape of alveolar capillaries and accompanying oxygenation are influenced by surfactant surface tension in the alveolus. To prove our hypothesis, surfactant surface tension was regulated by conditional expression of surfactant protein (SP)-B in Sftpb(-/-) mice, thereby inhibiting surface tension-lowering properties of surfactant in vivo within 24 hours after depletion of Sftpb. Minimum surface tension of isolated surfactant was increased and oxygen saturation was significantly reduced after 2 days of SP-B deficiency in association with deformation of alveolar capillaries. Intravascularly injected 3.2-mum-diameter microbeads through jugular vein were retained within narrowed pulmonary capillaries after reduction of SP-B. Ultrastructure studies demonstrated that the capillary protrusion typical of the normal alveolar-capillary unit was reduced in size, consistent with altered pulmonary blood flow. Pulmonary hypertension and intrapulmonary shunting are commonly associated with surfactant deficiency and dysfunction in neonates and adults with respiratory distress syndromes. Increased surfactant surface tension caused by reduction in SP-B induced narrowing of alveolar capillaries and oxygen desaturation, demonstrating an important role of surface tension-lowering properties of surfactant in the regulation of pulmonary vascular perfusion.

Left atrial pressure can be accurately transmitted to the pulmonary artery despite zone 1 conditions

Pulmonary arterial occlusion pressure is not thought to reflect left atrial pressure (Pla) when alveolar pressure (PA) exceeds pulmonary venous pressure because alveolar capillaries collapse and the required continuous fluid column between the pulmonary artery and left atrium is interrupted. However, arterial-to-venous flow can occur when PA exceeds both the pulmonary arterial pressure (Ppa) and pulmonary venous pressure (i.e., in Zone 1 conditions), indicating the existence of a continuous patent vascular channel. Accordingly, Ppa should reflect Pla under these conditions. To investigate this connection cannulas were placed in the pulmonary arteries and left atria of eight excised rabbit lungs. Ppa and Pla were set 5 cm H2O above PA, which ranged from 0 to 25 cm H2O. Pla was then reduced in 2 to 4 cm H2O decrements while recording Ppa when arterial-to-venous flow ceased. At all PAs greater than 0 cm H2O, Pla was accurately reflected by the Ppa when both were exceeded by PA. The greater the PA, the lower the Ppa could track Pla below PA. Pla can be accurately measured by a pulmonary arterial catheter under Zone 1 conditions.

A theoretical study of chemical delivery within the lung using exogenous surfactant

A mathematical model is developed for lung injury treatments involving the delivery of therapeutic chemicals, such as drugs and gene vectors, into the lung using simultaneous tracheal instillation of exogenous pulmonary surfactant. The influence of exogenous surfactant dose, flow rate, bulk liquid viscosity, pulmonary absorption rate and chemical molecular diffusivity on the chemical delivery to the lung is investigated. Our results reveal that different pulmonary absorption rates lead to significantly different distribution patterns and change the time taken for the total amount of chemical to be absorbed along the airways. The various factors can also influence where the majority of the chemical is placed within the lung and this is relevant to the targeting of drugs to particular lung generations.

Increased surface tension decreases pulmonary capillary volume and compliance

Increased surface tension is an important component of several respiratory diseases, but its effects on pulmonary capillary mechanics are incompletely understood. We measured capillary volume and specific compliance before and after increasing surface tension with nebulized siloxane in excised dog lungs. The change in surface tension was sufficient to increase lung recoil 5 cm H(2)O at 50% total lung capacity. Increased surface tension decreased both capillary volume and specific compliance. The changes in capillary volume and compliance were greatest at the lung volumes at which the surface tension change was greatest. Near functional residual capacity, capillary volume postsiloxane was approximately 30% of control. Presiloxane capillary specific compliance was approximately 7%/cm H(2)O near functional residual capacity and approximately 2.5%/cm H(2)O near total lung capacity. Postsiloxane capillary-specific compliance was 3%/cm H(2)O, and was independent of lung volume. We conclude that in addition to their well-known effects on lung mechanics, changes in surface tension also have important effects on capillary mechanics. We speculate that these changes may in turn affect ventilation and perfusion, worsen gas exchange, and alter leukocyte sequestration.

Effect of zonal conditions and posture on pulmonary blood flow distribution to subpleural and interior lung

Observations made on vessels seen directly beneath the pleura may not accurately reflect what occurs in vessels located deeper in the interior of the lung. We quantified flow to subpleural and deeper, interior regions under zone 1 or 2 conditions in excised (n = 5) and in vivo (n = 6) rabbit lungs, in the head-up or inverted position. After infusion of radiolabeled microspheres, lungs were dried at alveolar pressure of 25 cmH(2)O and sliced in 1-cm sections along the gravitational plane and in three planes in the dorsal-ventral axis. Regions located <1 mm from the pleural surface were dissected away from the remaining tissue. In both zonal conditions, 1) weight-normalized flow to the interior exceeded that found in subpleural regions; and 2) flow followed the gravitational gradient, with the correlation varying with the scale of measurement. We conclude that flow through subpleural vessels is less than that which occurs deeper in the interior, but the regional distributions of flow and the effects of zonal conditions are similar in the two regions.

Ventilation above closing volume reduces pulmonary vascular resistance hysteresis

The aim of this study was to determine the relationship of pulmonary vascular resistance (PVR) hysteresis and lung volume, with special attention to the effects of ventilation around closing volume (CV). Isolated, blood-perfused canine left lower lung lobes (LLL) were incrementally inflated and deflated. Airway and pulmonary artery pressures (PAP) were recorded after each stepwise volume change. Constant blood flow was provided (600 ml/min) and the pulmonary vein pressure (PVP) was held constant at 5 cm H2O. PAP changes, therefore, were a direct index of PVR changes. Group 1 lobes underwent a full inflation from complete collapse to total lobe capacity (TLC) followed by a full deflation. Group 2 lobes underwent two deflation/inflation cycles, after an initial full inflation. These cycles, both beginning at TLC, had deflation end above and below CV, respectively. Significant PVR hysteresis was noted when the first inflation and deflation were compared. The maximum difference in PAP on deflation was 3.3 cm H2O or 11%. The mean decrease was 2.7 cm H2O for 18 lobes (p < 0.0001). The PAPs on all subsequent inflations or deflations that began above CV remained 9% lower than the initial inflation (n = 9, p < 0.0001), but were not different from each other. However, the final inflation which began from below CV resulted in a 30% return of PVR hysteresis (mean increase in PAP of 0.8 cm H2O, n = 7, p < 0.004). We conclude that there is hysteresis in the PVR response during ventilation, with decreased PVR during deflation relative to the initial inflation, that this hysteresis is absent when lung volume is maintained greater than CV, and that hysteresis returns when inflation occurs after deflation below CV.

Irreversibility of birth-related changes in the pulmonary circulation

We hypothesized that establishing conditions of hypoxia and fluid filling of the airways in lungs of newborns would reproduce the high levels of pulmonary vascular resistance (PVR) observed in the fetal state. We assessed the hemodynamics of the left pulmonary circulation of 1- to 3-day-old lambs during a variety of airway states while attempting to reestablish fetal conditions. Eleven animals were studied during both normoxemia and hypoxemia in a baseline airway state with a positive end-expiratory pressure (PEEP) of 4 cm H2O, and in experimental airway states, of atelectasis, and fluid filling to 15 and 30 mL/kg and with PEEP of 12 cm H2O. PVR increased while pulmonary blood flow decreased with all airway state changes as compared to baseline, suggesting a passive mechanism for these changes. With the addition of hypoxemia there was a further increase in PVR in all states accompanied by an increase in pulmonary blood flow, indicating that active vasoconstriction was responsible for the increase in PVR. The combined effects of hypoxemia and fluid filling, designed to approximate the fetal state, increased PVR to only 20-30% of fetal values. Thus, additional factors appear to be important in maintaining the high PVR of the fetal state. We speculate that ventilation of the lungs at birth irreversibly alters these factors.

Alveolar liquid pressures in nonedematous and kerosene-washed rabbit lung by micropuncture

We studied the relationship between alveolar interfacial pressure and lung volume in kerosene-filled lungs, nonedematous air-filled lungs on lung deflation and inflation, and air-filled lungs after washing with kerosene or the Dow Corning oil, 0.65 cs dimethyl siloxane (DC200). We used the micropipet-servonulling technique to measure alveolar liquid pressure (Pliq) in the alveolar liquid layer of isolated rabbit lungs at different airway pressures (Palv). It was not possible to measure pressure in kerosene or in DC200 by micropuncture because of its low electrical conductivity. We used the Laplace law for a spherical membrane to estimate alveolar surface tension (T). In the kerosene-filled lung, the pressure drop (delta P = Pliq - Palv) across the alveolar surfactant-kerosene interface was 1.1 cm H2O at TLC and decreased to 0.5 cm H2O at 71% TLC. These values corresponded to T values of 2.2 and 0.9 dyne/cm at TLC and 71% TLC, which were in agreement with in vitro measurements using the captive bubble technique. In the air-filled lung on inflation, delta P values were 12.7 and 15.7 cm H2O at 48% and 76% TLC. Corresponding T values were 14 and 21 dyne/cm. Thus, alveolar surface tension on lung inflation is surface area dependent. In the kerosene-washed and DC200-washed lungs, delta P values were 16 and 14.5 cm H2O at TLC and decreased to 9 and 8 cm H2O at 50-56% TLC. These values indicated a reduction of 40-60% in alveolar surface tension with lung deflation from TLC to 50% TLC. The results indicate that alveolar surface tension in both kerosene-filled and kerosene-washed air-filled lungs is surface area dependent. This is due to a surfactant-kerosene interface in the kerosene-filled lung and a surfactant-kerosene-air interface in the kerosene-washed lung.

Alveolar liquid pressure measured by micropuncture in isolated lungs of mature and immature fetal rabbits

Increased alveolar surface tension due to surfactant deficiency is thought to result in a negative pressure surrounding pulmonary capillaries and to promote fluid filtration. To test this hypothesis, alveolar liquid pressure (Pliquid) was measured by micropuncture in isolated lungs of mature and immature fetal rabbits (with and without surfactant replacement) at different air inflation pressures (Pairway). Lung maturity was assessed by air pressure-volume (P-V) curves. Pliquid was correlated with surfactant content in the lungs and with alveolar size. Pliquid was lower in immature (2.3 +/- 0.7 cmH2O) than in mature (8.4 +/- 1.0 cmH2O) lungs at comparable Pairway (25 cmH2O) (P less than 0.01). The mean linear intercept, a measure of airspace dimensions was similar in all lungs (42.1 +/- 2.0 micron), but alveolar wash phospholipid/g wet lung was lower in immature than in mature lungs (0.05 +/- 0.01 vs. 0.49 +/- 0.30 mg) (P less than 0.01). Surfactant replacement in immature lungs resulted in P-V curves and Pliquid similar to those of mature lungs. If pericapillary interstitial liquid pressure approximates Pliquid, surfactant deficiency will predispose preterm infants to pulmonary edema.

Lung liquid and protein exchange: the four inhomogeneities

William of Ockham, 14th-century scholastic philosopher at Oxford and Munich, emphasized the principle of economy, "pleurality is not to be supposed without necessity" (Ockham's razor). Necessity is the key word. In the modeling of steady-state lung liquid and protein exchange, the desire for simplicity has sometimes outweighed good judgment. In fact, we and others have shown that simple models do not work. It is necessary to include several forms of inhomogeneity. The air-filled lung shows regional (top to bottom) variations of mass, microvascular pressure, and perimicrovascular protein concentration. Normally, the small longitudinal (arterioles to venules) gradient of microvascular and perimicrovascular pressures is not a major concern, but in nonuniform disease processes, such as microembolism, longitudinal inhomogeneity, and parallel inhomogeneity are dominant. Multiple pores should also be considered a form of inhomogeneity. The effect on liquid and protein exchange, when plasma protein concentration or microvascular pressure change, can be readily explained using pore heterogeneity. The model I am currently using consists of a large number of discrete compartments (18), rather than a continuous distribution. We have recently identified a fifth inhomogeneity, which is that lung lymph flow might not always represent steady-state transvascular filtration because interstitial liquid may leak through the pleura or along the bronchovascular liquid cuffs into the mediastinum.

Alveolar liquid pressure measured using micropipets in isolated rabbit lungs

The relationships among alveolar liquid pressure (Pliq), transpulmonary pressure (Ptp), and alveolar edema were studied in isolated rabbit lungs. Different amounts of edema were induced by instilling measured amounts of normal saline via the trachea. Micropipets were used in conjunction with a servonulling pressure measuring system to measure Pliq. Pliq was -1 cm H2O relative to alveolar air pressure (Palv) at Ptp of 3 cm H2O and decreased to -16 cm H2O at Ptp of 25 cm H2O. Pliq increased slightly as wet-to-dry weight ratio (W/D) of the lungs increased from 5.8 to 16. Alveolar surface tension estimated from these values of Pliq - Palv using an alveolar radius of curvature of 40 micron at Ptp of 25 cm H2O was consistent with the direct measurements of Schürch. In lungs made edematous by treating with oleic acid, values of Pliq were not different from those measured in lungs made edematous by normal saline instillation (control). Pressure-volume behaviour of the oleic acid treated lungs showed a marked reduction in lung volume at each Ptp value compared to the behavior in the control lungs. Most of this reduction in lung volume was removed by washing the airways with the bronchodilator, isoproterenol. We conclude that alveolar surface tension was not increased in the oleic acid treated lungs. The apparent increase in lung static recoil was most likely caused by the constriction of small airways.

High surface tension pulmonary edema

Dogs were anesthetized with pentobarbital and placed on a piston ventilator with room air. Ten animals received an endobronchial lavage of normal saline (3 mg/kg). Ten other animals received an endobronchial lavage of the same volume of a nonionic detergent, Tween 20, 5% in saline. Detergent lavage was shown by Wilhelmy balance to increase surface tension of lung extracts. Saline lavage did not alter the surface tension of lung extracts. No significant differences between the groups were noted in cardiac output, left ventricular and diastolic pressure, mean pulmonary artery pressure, or colloid oncotic pressure. Static compliance and arterial PO2 were decreased following detergent lavage. Animals were sacrificed 2 hr after lavage and pulmonary extravascular water volume (PEWV) was measured gravimetrically. Saline-lavaged lungs with normal surface tension had a PEWV of 4.3 ml/g dry lung. Tween-lavaged lungs with increased surface tension had a PEWV of 5.3 ml/g dry lung (P less than 0.005). When the estimated volume of residual lavage solution remaining in the lung parenchyma was subtracted from the total wet lung wt, the corrected PEWV was 3.62 +/- 0.12 ml/g dry lung for saline-lavaged lung and 4.76 +/- 0.19 ml/g dry lung for Tween-lavaged lung. PEWV for 11 control animals ventilated 2 hr without lavage was 3.61 +/- 0.13 ml/g dry lung. It is concluded that, experimentally, high alveolar surface tension can induce pulmonary edema even when pulmonary microvascular hydrostatic and colloid oncotic pressures are normal.

The effect of smoke inhalation on pulmonary surfactant

This paper details efforts to define the primary pathophysiology of acute smoke inhalation without the variables of infection, burns, or fluid resuscitation. A standard dose of smoke (wood and kerosene) was delivered at 37 C to mongrel dogs. The parameters studied included blood gases, carboxyhemoglobin, pulmonary and systemic hemodynamics, respiratory mechanics, surface tension area curves as an indication of surfactant activity, and in vivo photomicroscopy. The FiO2 of the smoke was 17 volumes per cent; the carbon monoxide 17,000 ppm. Immediately following smoke exposure, dense, nonsegmental atelectasis developed. Hemodynamic changes were insignificant, but the PaO2 fell to 49 mmHg; the right to left shunt rose from 5 to 41%. Surfactant reduction was significant: enough to cause an increase in the minimum surface tension from 7 to 22 dynes/cm. This surfactant loss may explain the atelectasis seen and the marked instability of subpleural alveolar walls. The data collected are consistent and support the acute inactivation of surfactant as one of the primary pathophysiologic events in smoke inhalation. The clinical correlation is good; surfactant loss may explain why victims of smoke inhalation are so vulnerable to fluid administration if they have thermal burns as well effectiveness of medical devices.

Increased surface tension favors pulmonary edema formation in anesthetized dogs' lungs

The possibility that surface tension may affect the hydrostatic transmural pressure of pulmonary vessels and the development of pulmonary edema was studied in anesthetized, open-chested dogs. Isogravimetric pressure (the static intravascular pressure at which transmural osmotic and hydrostatic pressures are balanced such that net fluid flux is zero and lung weight is constant) was measured in nine animals under three conditions: (a) control, normal surface tension, at an alveolar pressure of 30 cm H2O with the apenic lung at room temperature; (b) after increasing surface tension by cooling and ventilating at a low functional residual capacity, at an alveolar pressure sufficient to produce the same lung volume present during control measurements; and (c) after restoring surface tension by rewarming while holding the lung at a high inflation volume, again at the control lung volume. Lung volumes were established from external dimensions and confirmed +/- 10% by deflation spirometry. The isogravimetric pressure (relative to alveolar pressure) was significantly less with increased surface tension than during either the initial control condition (P less than 0.01), or when the surface tension has been restored (P less than 0.01). Similar changes occurred in each of three additional studies performed with control alveolar pressures of 10 cm H2O. Thus, increased surface tension favors fluid leakage presumably because it increases the microvascular transmural pressure.

Interstitial pressure of the lung

The effects of alveolar and pleural pressures on pulmonary interstitial pressure were studied in 36 anesthetized dogs by application of Starling's law of transcapillary exchange. Fluid accumulation in the lung was produced by increasing left atrial pressure to levels always higher than alveolar pressure and by hemodilution with saline. Using a lung divider, a difference in alveolar pressure of from 5 to 14 mm Hg was achieved between the two sides in 24 dogs. Increased alveolar pressure did not reduce the rate of fluid accumulation, indicating its lack of effect on interstitial pressure. A relationship between the rate of fluid accumulation and the forces in the Starling equation was demonstrated when pleural pressure was included as an index of interstitial pressure. The rate of fluid accumulation increased markedly when interstitial pressure exceeded atmospheric. Fluid accumulation was considerably less in lobes statically inflated with plasma than in contralateral lobes ventilated with air (6 dogs); this difference could not be attributed to static inflation as opposed to ventilation (6 dogs). These findings suggest that surface tension opposes the transmission of alveolar pressure to the interstitial space. The interstitial pressure, as measured by application of Starling's law, acts on the small vessels within the alveolar-capillary membrane.

The application of Starling's law of capillary exchange to the lungs

The forces governing the movement of water across the pulmonary capillaries were studied in 39 intact, spontaneously breathing dogs. A situation favoring the net movement of water out of the pulmonary capillaries was created by means of partial pulmonary venous obstruction (left atrial balloon catheter) followed by rapid saline hemodilution. A predetermined difference between pulmonary capillary and plasma colloid osmotic pressures was maintained for periods of 1 to 2 hours. Left atrial (P(LA)) and plasma colloid osmotic pressures (pi(pl)) were measured directly. The water content of the lungs was measured serially by an indicator-dilution technique, and at autopsy by drying the lungs. The rate of accumulation of lung water was measured in four groups of animals: in three of the groups, the capillary hydrostatic and colloid osmotic pressures were varied; in the fourth group, the right lymphatic duct was obstructed in addition. The average rate of water accumulation in the lungs varied in a nonlinear way with the level of the capillary hydrostatic-plasma colloid osmotic pressure difference and was unaffected by the level of the capillary hydrostatic pressure. At low levels of P(LA) - pi(pl), water accumulated in the lung at an average rate of 0.09 g per g dry lung per hour per mm Hg pressure difference. At higher levels of P(LA) - pi(pl) the average rate of accumulation was 0.22 g per g per hour per mm Hg DeltaP; in most of the experiments in this group water accumulated in the lungs slowly during the first 30 minutes of the test period and more rapidly as the period was extended. Obstruction of right lymphatic duct outflow did not alter the rate of water accumulation. Based on the control data of the present experiments, the pericapillary pressure in normal lungs is estimated to be of the order of - 9 mm Hg in the normal dog lung. The filtration coefficient for the pulmonary capillaries is estimated to be of the order of one-tenth to one-twentieth of that for canine muscle capillaries. The data of the present study indicate that edema formation in lung tissue cannot be defined solely in terms of intravascular forces, but may be governed to a significant degree by changes in pericapillary forces in the pulmonary interstitium.