H2O Positive End-expiratory Pressure Research Articles

Effect of positive end-expiratory pressure on left ventricular mechanics in patients with hypoxemic respiratory failure.

When positive end-expiratory pressure (PEEP) is added to intermittent positive pressure ventilation, cardiac output and stroke volume frequently fall despite unchanged or increased transmural left ventricular end-diastolic pressure. To determine whether a part of the fall in stroke volume with PEEP is explained by depressed left ventricular systolic function (increased end-systolic volume at a given end-systolic pressure on PEEP) the authors measured left ventricular end-diastolic volume (EDV), end-systolic volume (ESV), and the corresponding pressures in nine patients with acute hypoxemic respiratory failure. Measurements were made before and after 10 cm H2O PEEP was added to the ventilator. PEEP reduced mean stroke volume from 71 to 62 ml and this was explained entirely by a reduction in end-diastolic volume from 135 to 112 ml (P less than 0.005). Despite reduced EDV, pulmonary wedge pressure increased from 12 to 14 torr on PEEP, indicating reduced diastolic compliance or unstressed volume of the left ventricle in these patients similar to that reported in dogs. The authors conclude that PEEP reduces venous return and cardiac output without depressing left ventricular pumping function because end-systolic volume decreased from 64 to 49 ml on PEEP despite identical blood pressures (78 torr). They speculate that PEEP might improve ventricular performance by increasing intrathoracic pressure and left ventricular pressure relative to systemic blood pressure in extrathoracic vessels.

The effect of positive end-expiratory pressure ventilation (PEEP) on cerebral blood flow and cerebrospinal fluid pressure in goats.

The effect of cerebral blood flow (CBF) and cerebrospinal fluid pressure (CSFP) of mechanical ventilation with positive end-expiratory pressure (PEEP) at 5, 10, and 15 cm H2O was studied in 23 paralyzed, ventilated goats which were divided into three treatment groups. Group I received no volume expansion agent to counteract the hemodynamic effects of PEEP. Group II received normal saline to maintain a constant arterial blood pressure (BP), and Group III received a mannitol solution for BP maintenance. In all three groups there were similar increases in central venous pressure (CVP) of approximately 2.5 times the zero PEEP level at 15 cm H2O PEEP (P less than 0.001). In Group I, BP fell an average of 32 per cent, cardiac output fell 47 per cent and cisternal CSFP increased 40 per cent above zero PEEP levels at 15 cm H2O PEEP (P less than 0.01). CBF in this group was decreased 18 per cent when compared with the zero PEEP baseline at 15 cm H2O PEEP (P less than 0.008). In Group II animals there were no significant changes in BP or cardiac output (CO) at any of the PEEP levels. CSFP in this group, at 15 cm H2O PEEP, increased 84 per cent above the baseline zero level and CBF decreased 32 per cent at 15 cm H2O PEEP when compared to the zero PEEP level (P less than 0.008). In Group III there were no significant reductions in BP or CO. Unlike Groups I and II, no significant changes in CFSP were observed at any level of PEEP. In addition, CBF in this group did not change significantly from the zero-PEEP baseline level any level of PEEP. Thus, when PEEP therapy is associated with substantial decreases in BP and CO, CBF may decrease as well. Maintenance of BP and CO by volume expansion with a crystalloid solution resulted in a greater reduction in CBF than in the untreated group but maintenance of BP and CO by mannitol infusion resulted in maintenance of CBF at the baseline, pre-PEEP level. The authors conclude that brain interstitial fluid pressure is an important variable in the determination of cerebral blood flow during ventilation with PEEP.

The effect of unilateral PEEP on gas exchange and pulmonary perfusion in canine lobar pneumonia.

Positive end-expiratory pressure (PEEP) has little beneficial effect in improving gas exchange in canine left lower lobe (LLL) pneumonia. This is true because while PEEP improves lobar gas exchange, it also increases relative perfusion (QL) to the diseased lobe. The authors hypothesized that PEEP administered to only the diseased lung would avoid the increased QLLL. Six dogs with LLL pneumonia in which PEEP was applied only to the left lung were observed. The dogs were studied supine and each lung was ventilated separately with 100 per cent O2. Measurements of arterial oxygen tension (PaO2), shunt (Qs/Qt) and lobar distribution of pulmonary perfusion were made before, during, and after 12 cm H2O PEEP. Changes in QLLL expressed as per cent of cardiac output were determined using radiolabeled microspheres. PEEP improved PaO2 from 310 +/- 86 to 532 +/- 58 torr and Qs/Qt from 29 +/- 5 per cent to 12 +/- 5 per cent, returning after PEEP to 337 +/- 84 torr and 26 +/- 5 per cent, respectively. QLLL per cent did not increase during PEEP. These results suggest that unilateral PEEP improves regional gas exchange within the pneumonia lobe, probably by ventilating units which were previously perfused but not ventilated. Further, this improvement in regional gas exchange occurred without the diversion of blood flow towards consolidated lung that occurs with whole-lung PEEP, and so resulted in a substantial net improvement in overall gas exchange.

Postoperative atelectasis: Intermittent positive pressure breathing, incentive spirometry, and face-mask positive end-expiratory pressure.

Postoperative atelectasis has been treated with inspiratory maneuvers in an attempt to increase functional residual capacity. We compared the effect of intermittent positive pressure breathing (IPPB), incentive spirometry, and 5-cm H2O positive end-expiratory pressure (PEEP) applied with a face mask on the transpulmonary pressure (PL) at the end of expiration of eight patients 24 to 34 hours after aortocoronary bypass graft insertion. Intermittent positive pressure breathing and PEEP increased expiratory PL during therapy. After IPPB, expiratory PL fell below control values and then returned toward, but did not reach, control values After PEEP was discontinued, expiratory PL returned to control values within the next 30 minutes. The results suggest that face-mask PEEP will increase functional residual capacity, that incentive spirometry has little or effect, and that IPPB may decrease lung volume after treatment.

Time course and mechanisms of lung-volume increase with PEEP in acute pulmonary failure.

To determine the effects of a step change in end-expiratory pressure on functional residual capacity (FRC) and lung-thorax compliance (CLT), 10 cm H2O positive end-expiratory pressure (PEEP) was applied in eight patients who needed mechanical ventilation for acute pulmonary failure. Of the total change in FRC, 66 +/- 5.3 per cent (mean +/- SEM) was complete within the next breath, and 90 per cent change was achieved in 4.6 +/- 1.4 breaths (24 +/- 6.4 sec). There was no statistically significant difference between times to 90 per cent FRC change with application and with removal of PEEP. In another 13 patients, PEEP was increased in 5 cm H2O steps from 3 to 18 cm H2O. Mean FRC at 3 cm H2O PEEP was 1.51 +/- 0.20 1 (55 +/- 7.0 per cent predicted supine value). Mean CLT did not change significantly until 18 cm H2O PEEP was reached, at which point it decreased (P < 0.005). The static compliance derived from change in FRC (deltaFRC/deltaPEEP) increased with increments of PEEP (P < 0.05) compared with the initial level. At PEEP levels of 8 and 13 cm H2O, mean FRC was larger than would be predicted from mean CLT (P < 0.005), but it was not significantly different at 3 cm H2O PEEP. The lung component accounted for 62 +/- 3.7 per cent of the lung-thorax compliance difference. These data define a time-dependent increase in lung volume that resembles pressure-volume hysteresis in normal man. Possible mechanisms include surface tension changes, recruitment of nonventilated lung, and stress relaxation of lung and chest wall. This study may explain the greater efficiency of PEEP compared with large tidal-volume ventilation in increasing PaO2 in patients with acute pulmonary failure.

Use of PEEP in acute respiratory distress syndrome in dogs.

This study evaluates the effectiveness of combining mechanical ventilation and 5 cm H2O positive end-expiratory pressure (PEEP) at the onset of adult respiratory distress syndrome (ARDS) in dogs. Five cm H2O PEEP applied at the onset of ARDS in oleic acid injured dogs resulted in a decrease in cardiac output (CO). This decrease was accompanied by beneficial effects including a relatively stable SaO2 and PaO2. Control group dogs (receiving mechanical ventilation only) showed a less dramatic changing in CO, but demonstrated a dramatic drop in saturation, compromising oxygen transport of the tissues. Thus, despite decrease in CO experienced by the PEEP group, oxygen extraction at the tissue level remained high.

Role of Sinoaortic Baroreceptors in Initiating the Renal Response to Continuous Positive-pressure Ventilation in the Dog

Experiments were conducted to determine the role of cardiopulmonary receptors with vagal afferents and sinoaortic baroreceptors in initiating the reflex antidiuresis and antinatriuresis during continuous positive-pressure ventilation (CPPV). CPPV was applied to 18 dogs: seven control, five that underwent bilateral vagotomy; and six that underwent sinoaortic baroreceptor denervation. The dogs were anesthetized with pentobarbital, paralyzed with succinylcholine, and their lungs were mechanically ventilated with a volume ventilator. Renal function and systemic hemodynamics were monitored by clearance methods and pressures, respectively. After two 30-min control periods of intermittent positive-pressure ventilation (IPPV), CPPV using 10 cm H2O positive end-expiratory pressure was applied for two 30-min experimental periods. Three 30-min recovery periods of IPPV followed. In dogs of the control group and vagotomy group, CPPV caused statistically significant decreases from control levels in urinary flow, sodium excretion, and glomerular filtration rate. However, in the dogs of the sinoaortic baroreceptor denervation group, CPPV did not produce any significant change in these variables. Therefore, the results do not support the hypothesis that cardiopulmonary receptors mediate the renal response to CPPV. Rather, the data indicate that the aortic arch and carotid sinus baroreceptors participate in initiating the reflex antidiuresis and antinatriuresis during CPPV.

Continuous positive-pressure ventilation decreases right and left ventricular end-diastolic volumes in the dog.

We investigated the mechanism(s) responsible for the decreased cardiac output during continuous positive-pressure ventilation (CPPV). Seven dogs were anesthetized with chloralose-urethane, intubated, and ventilated using a volume ventilator. We measured heart rate, stroke volume, and the determinants of stroke volume: left and right ventricular end-diastolic volumes, isovolumic and ejection phase indices of myocardial contractility, and pulmonary and systemic arterial pressures. Myocardial blood flow was estimated using radioactive microspheres. Variables were measured during a control period of intermittent positive-pressure ventilation (IPPV), 8-20 minutes after the initiation of CPPV using 12 cm H2O positive end-expiratory pressure (PEEP), and 8-20 minutes after the removal of PEEP. CPPV decreased cardiac output but did not affect total or regional myocardial blood flow or the ratio of subendocardial to subepicardial blood flow. Isovolumic and ejection phase indices of myocardial cointractility, heart rate, and systemic arterial pressure did not change during CPPV. Right and left ventricular end-diastolic and end-systolic volumes decreased markedly during CPPV. We conclude that CPPV decreases cardiac output in accordance with Starling's law by decreasing preload.

Humorally Mediated Alterations in Cardiac Performance as a Consequence of Positive End-Expiratory Pressure

Two experimental designs were used to study the mechanism of the decreased cardiac output associated with the use of positive end-expiratory pressure (PEEP). In the first study of nine dogs the application of 15 cm H2O PEEP led to a decrease in cardiac output (CO) from 2.68 +/- 1.05 to 2.01 +/- 1.26 liters/min (+/- SD) (p less than 0.05) concomitant with an increase in transmural central venous pressure of 5.2 +/- 0.9 to 8.4 +/- 2.7 mm Hg (p less than 0.05) and a slight increase in transmural left atrial pressure of 6.8 +/- 3.3 to 7.3 +/- 3.6 mm Hg (p less than 0.1). These data are consistent with altered ventricular performance. In a second study nine pairs of dogs were cross-circulated. Application of 15 cm H2O PEEP to one member of the experimental pair led to a decrease in the CO of the other member from 2.71 +/- 0.98 to 2.21 +/- 0.81 liters/min (p less than 0.001). This decrease returned toward baseline with the removal of PEEP (p less than 0.02). Results indicate that one mechanism whereby PEEP reduces the cardiac output is through the action of a humoral agent.

Treatment of Cardiac and Renal Effects of PEEP with Dopamine in Patients with Acute Respiratory Failure

The hemodynamic and renal effects of mechanical ventilation with positive end-expiratory pressure (PEEP) were studied with and without continuous dopamine administration in ten patients who had acute pulmonary failure. The application of 20 cm H2O PEEP during mechanical ventilation resulted in improvements in arterial blood oxygen tension, from 63 +/- 6 to 81 +/- 12 torr (mean +/- SE), and intrapulmonary shunt fraction, from 29 +/- 3 to 21 +/- 3 per cent, whereas cardiac output, systemic oxygen transport and renal function were impaired by 20, 19 and 47 per cent, respectively. Dopamine infusion at a rate of 5 +/- 0.05 micrograms/kg/min reversed the deleterious effects of PEEP on cardiovascular and renal function: cardiac output increased from 4.5 +/- 0.3 to 6.0 +/- 0.51, urinary output from 1.0 +/- 0.3 to 1.7 +/- 0.4 ml/min, sodium excretion and creatinine clearance by 50 per cent. Systemic oxygen transport was improved from 680 +/- 44 to 925 +/- ml, arterial oxygen tension from 81 +/- 12 to 102 +/- 14 torr, and total deadspace to tidal volume ratio from 0.49 +/- 0.02 to 0.44 +/- 0.03 with dopamine. The authors conclude that the depression of cardiovascular and renal functions that may occur in patients who need high levels of PEEP for the treatment of acute pulmonary failure can be treated successfully with dopamine infusion. This represents a valuable alternative to expansion of blood volume for the improvement of systemic oxygen transport and arterial blood oxygen tension in critically ill patients.

EFFECTS OF POSITIVE END-EXPIRATORY PRESSURE ON GAS EXCHANGE IN DOGS WITH NORMAL AND EDEMATOUS LUNGS

The effects of positive end-expiratory pressure (PEEP) at 5, 10, 15, and 20 cm H2O on the distribution of ventilation-perfusion (VA/Q) ratios was determined in four normal dogs and in ten with oleic acid-induced acute hemorrhagic pulmonary edema. Tidal volume and frequency were held constant at all times with mechanical ventilation during intravenous pentobarbital and gallamine anesthesia. Normal dogs had little or no shunt, and no areas of low (less than 0.1) or high VA/Q (greater than 10.0) at zero end-expiratory pressure (intermittent positive-pressure breathing). In these animals increasing PEEP caused progressive depression of cardiac output, associated with an increase in ventilation to both high VA/Q and unperfused regions. PEEP greater than or equal to 10 cm H2O resulted in a reduction in Pao2 and an increase in PaCO2. In dogs with pulmonary edema, PEEP's of 5 and 10 cm H2O resulted in dramatic reductions in shunt, virtual obliteration of low VA/Q regions, and market improvement in Pao2. However, at 15 and 20 cm H2O PEEP's high VA/Q and dead space ventilation with CO2 retention again developed in all but the most severely affected (shunt greater than 40%) dogs.

The effect of Swan-Ganz catheter height on the wedge pressure-left atrial pressure relationships in edema during positive-pressure ventilation.

We have studied the effect of the ventrical height of the pulmonary wedge catheter in the lung on the pulmonary wedge pressure-left atrial relationship during positive end-expiratory pressure ventilation in oleic acid-induced pulmonary edema. Pulmonary wedge catheters were placed above and below the left atrium in normal dogs and in dogs with oleic acid-induced edema. Wedge pressure and left atrial pressure were measured simultaneously during positive end-expiratory pressure ventilation (range, 0 to 30 cm H2O positive end-expiratory pressure). Pulmonary wedge catheters below the left atrium correctly recorded left atrial pressure and change in left atrial pressure at all positive end-expiratory pressures studied. Pulmonary wedge catheters above the atrium consistently recorded pressures higher than the normal left atrial pressure. They did not correctly respond to increases in left atrial pressure until it was increased to a value higher than the initial upper pulmonary wedge pressure. Pulmonary arterial catheters, when properly placed, should be reliable indicators of left atrial pressure during positive-pressure ventilation in normal and edematous lungs.

Effects of Positive End-expiratory Pressure on Gas Exchange in Dogs with Normal and Edematous Lungs

The effects of positive end-expiratory pressure (PEEP) at 5, 10, 15, and 20 cm H2O on the distribution of ventilation-perfusion (VA/Q) ratios was determined in four normal dogs and in ten with oleic acid-induced acute hemorrhagic pulmonary edema. Tidal volume and frequency were held constant at all times with mechanical ventilation during intravenous pentobarbital and gallamine anesthesia. Normal dogs had little or no shunt, and no areas of low (less than 0.1) or high VA/Q (greater than 10.0) at zero end-expiratory pressure (intermittent positive-pressure breathing). In these animals increasing PEEP caused progressive depression of cardiac output, associated with an increase in ventilation to both high VA/Q and unperfused regions. PEEP greater than or equal to 10 cm H2O resulted in a reduction in Pao2 and an increase in PaCO2. In dogs with pulmonary edema, PEEP's of 5 and 10 cm H2O resulted in dramatic reductions in shunt, virtual obliteration of low VA/Q regions, and market improvement in Pao2. However, at 15 and 20 cm H2O PEEP's high VA/Q and dead space ventilation with CO2 retention again developed in all but the most severely affected (shunt greater than 40%) dogs.

Continuous positive airway pressure versus positive end-expiratory pressure in respiratory distress syndrome

The hemodynamic and respiratory effects of spontaneous ventilation with continuous positive airway pressure (CPAP) and mechanical ventilation with positive end-expiratory pressure (PEEP) were compared in nine patients who had adult respiratory distress syndrome. These patients were capable of maintaining spontaneous ventilation (tidal volume above 300 ml. and PaCO2 below 45 torr). Arterial and mixed venous blood gases, cardiac output, oxygen delivery and consumption, pulmonary artery pressure, and pulmonary wedge pressure were measured in 11 instances, with each patient on 5 or 10 cm. H2O CPAP or PEEP, and in nine instances, with each patient on the ventilator but without PEEP (O PEEP). During CPAP, when compared to PEEP at the same level of end-expiratory pressure, mean PaO2 increased significantly (p less than 0.05) and mean physiological shunt decreased (p less than 0.05). In nine of 11 instances, cardiac output was higher on CPAP than on a corresponding level of PEEP. Thus CPAP was more effective than the same amount of PEEP in improving arterial oxygenation by the lung without adversely affecting cardiac output.

Effect of positive end-expiratory pressure on lung mechanics during anaesthesia in dogs

We studied supine anaesthetized dogs to determine whether the use of positive end-expiratory pressure to increase functional residual capacity (FRC) could thereby prevent the small lung compliance values frequently reported during anaesthesia. We first made control measurements of FRC and of both static (CSTAT) and dynamic (CDYN) lung compliance. Continuous positive pressure ventilation (CPPV) with 5 cm H2O positive end-expiratory pressure was then commenced, and FRC and compliance measurements were repeated at 15-minute intervals during one hour of CPPV. We found that the positive end-expiratory pressure served to increase FRC to a level expected for unanaesthetized normally breathing dogs in the same posture (J. Appl. Physiol. 36: 93, 1974), and this FRC was well maintained during the one-hour observation period. Despite the volume preservation, we found a marked decline in lung compliance, equal in both CSTAT and CDYN; however, these changes were completely reversed to control values following hyperinflation of lung (artificial sighting). We conclude that diminution in lung compliance in our anaesthetized dogs was not dependent on low lung volume breathing but must have been due to another mechanism such as suppression of sighing or possibly regional hypoventilation. It might well be important to recognize that reversible reduction of compliance may occur despite increase of FRC in patients managed with positive end-expiratory pressure.

Positional hypoxaemia following post-traumatic pulmonary insufficiency

The effect of body position on ventilatory function was evaluated in a patient with unilateral lung disease. The patient's pulmonary dynamics were examined in the supine, right, and left decubitus positions under conditions of positive pressure ventilation with zero end-expiratory pressure (ZEEP) and 5 cm H2O (0.9 KPa) positive end expiratory pressure (PEEP). When the patient was positioned so that the "diseased" lung was dependent, there was a marked decrease in PaO2 and increase in venous admixture when compared to the values in the supine position. These changes were relatively greater in the ZEEP, than the PEEP situation. When the "diseased* lung was not dependent, there was an increase in PaO2 and a decrease in venous admixture. This was most pronounced when PEEP was applied. Changes in body position may result in clinically significant alterations in pulmonary gas exchange, especially in patients with pre-existing pulmonary dysfunction.

POSITIVE AIRWAY PRESSURE IN LAMBS SPONTANEOUSLY BREATHING AIR AND OXYGEN

Abstract The effects of 10 cm H2O positive endexpiratory pressure (PEEP) were studied in 26 unrestrained Hampshire and Corriedale lambs, breathing spontaneously air or pure oxygen. No changes in lung morphology, pulmonary mechanics or arterial blood gas tensions were detected after air breathing with 10 cm H2O PEEP for one week. No changes were found in mean pulmonary artery pressure, pulmonary capillary wedge pressure, pulmonary vascular resistance or cardiac output in oxygen breathing lambs until minutes before death. Addition of PEEP to oxygen breathing accelerated the development of respiratory acidemia and death (PEEP group mean lifespan 54 3 4 hr, without PEEP mean lifespan 71 3 4 hr). Morphologic changes of acute oxygen toxicity were present in oxygen breathing lambs and lung water was increased. Oxygen breathing lambs died from apnea with hypercarbia and a markedly increased (A-a) DO2. An increase in pulmonary vascular resistance or right ventricular failure do not appear to be major factors in the syndrome of fulminant acute respiratory insufficiency due to oxygen toxicity in lambs.

Ventilatory Pattern, Interpleural Pressure, and Cardiac Output

Continuous positive-pressure ventilation may decrease cardiac output. However, a few reports have separated the effects of positive and end-expiratory pressure (PEEP) from those of mechanical ventilation. Ten surgical patients requiring mechanical ventilatory support had catheters inserted for measurement of right atrial pressure (RAP), pulmonary artery occlusion pressure (PAOP), intrapleural, radial artery, airway, and atrial filling pressures, and cardiac output. All patients breathed spontaneously between mechanical breaths delivered every 30 seconds by intermittent mandatory ventilation (IMV). Measurements were made with 0, 5, and 10 cm H2O PEEP, and during intermittent positive-pressure ventilation (IPPV) with 12 breaths/min without PEEP. Airway pressure (Paw), intrapleural pressure, RAP, and PAOP were increased by PEEP and IPPV. Intrapleural pressure increased most during IPPV (p less than 0.001). Atrial filling pressures and cardiac output were unaffected by PEEP but decreased during IPPV (p less than 0.001). Patients receiving IMV maintained negative intrapleural pressure, atrial filling pressure, cardiac output and, therefore, O2 delivery, regardless of PEEP level. The authors conclude that patients requiring mechanical respiratory support, with or without PEEP, may maintain better cardiopulmonary function when allowed some spontaneous ventilatory activity.

Positive end expiratory pressure (PEEP) and right to left shunting in immature goats.

Immature fetal goats were exteriorized by caesarean section and catheters were placed in a pulmonary vein (PV), carotid artery (CA), femoral artery (FA), and the main pulmonary artery (PA). The goats were ventilated at 7.5 and 15 cm H20 positive end expiratory pressure (PEEP) between control periods at 0 PEEP. Right to left shunting (Qs/Qt) was calculated for three anatomic sites: lung, foramen ovale, and ductus arteriosus, and for two summation shunts: preductal and total. Total Qs/Qt increased in 8 of 14 animals at 7.5 cm H2O PEEP and in 7 of 12 animals at 15 cm H2O PEEP. Intrapulmonary shunting increased in only one experiment. The increase in total Qs/Qt resulted from elevated extrapulmonary shunting, always including the ductus arteriosus. The variation in response to PEEP was from animal to animal and not from one level to another within one animal. Changes in total Qs/Qt on PEEP did not correlate with the size of control Qs/Qt at any site.

Effects of Pattern of Ventilation on Pulmonary Metabolism and Mechanics

Effects of three patterns of mechanical ventilation on pulmonary mechanics, lung phospholipid and surface activity were studied in the normal closed-chest dog. The patterns were continuous mechanical ventilation with: 1) tidal volume (VT)=15 ml/kg; 2) VT=15 ml/kg with 10 cm H2O positive end-expiratory pressure (PEEP); 3) VT=50 ml/kg. THE DOGS IN EACH GROUP WERE VENTILATED FOR 24 HOURS, WITH CAREFUL ATTENTION PAID TO MAINTENANCE OF NORMAL BLOOD GASES, FLUID BALANCE, AND CARDIAC OUTPUT. The animals were sacrificed and the lungs studied to determine pressure-volume curves, dry lung weight/wet lung weight ratios, phospholpid contents and surface activities. The results were compared with control values in acutely sacrificed unventilated dogs. No significant change from controls was found with any pattern of ventilation employed with the exception of the tendency for lungs ventilated with PEEP to retain fluid (decreased dry lung weight/wet lung weight ratio).