Effect of PEEP on lung water content in experimental noncardiogenic pulmonary edema

Transpulmonary thermodilution detects rapid and reversible increases in lung water induced by positive end-expiratory pressure in acute respiratory distress syndrome

PURPOSE

It has been suggested that, by recruiting lung regions and enlarging the distribution volume of the cold indicator, increasing the positive end-expiratory pressure (PEEP) may lead to an artefactual overestimation of extravascular lung water (EVLW) by transpulmonary thermodilution (TPTD).

METHODS

In 60 ARDS patients, we measured EVLW (PiCCO2 device) at a PEEP level set to reach a plateau pressure of 30 cmH₂O (HighPEEP_start) and 15 and 45 min after decreasing PEEP to 5 cmH₂O (LowPEEP_15' and LowPEEP_45', respectively). Then, we increased PEEP back to the baseline level (HighPEEP_end). Between HighPEEP_start and LowPEEP_15', we estimated the degree of lung derecruitment either by measuring changes in the compliance of the respiratory system (Crs) in the whole population, or by measuring the lung derecruited volume in 30 patients. We defined patients with a large derecruitment from the other ones as patients in whom the Crs changes and the measured derecruited volume were larger than the median of these variables observed in the whole population.

RESULTS

Reducing PEEP from HighPEEP_start (14 ± 2 cmH₂O) to LowPEEP_15' significantly decreased EVLW from 20 ± 4 to 18 ± 4 mL/kg, central venous pressure (CVP) from 15 ± 4 to 12 ± 4 mmHg, the arterial oxygen tension over inspired oxygen fraction (PaO₂/FiO₂) ratio from 184 ± 76 to 150 ± 69 mmHg and lung volume by 144 [68-420] mL. The EVLW decrease was similar in "large derecruiters" and the other patients. When PEEP was re-increased to HighPEEP_end, CVP, PaO₂/FiO₂ and EVLW significantly re-increased. At linear mixed effect model, EVLW changes were significantly determined only by changes in PEEP and CVP (p < 0.001 and p = 0.03, respectively, n = 60). When the same analysis was performed by estimating recruitment according to lung volume changes (n = 30), CVP remained significantly associated to the changes in EVLW (p < 0.001).

CONCLUSIONS

In ARDS patients, changing the PEEP level induced parallel, small and reversible changes in EVLW. These changes were not due to an artefact of the TPTD technique and were likely due to the PEEP-induced changes in CVP, which is the backward pressure of the lung lymphatic drainage. Trial registration ID RCB: 2015-A01654-45. Registered 23 October 2015.

Positive End-expiratory Pressure Ventilation Increases Extravascular Lung Water Due to a Decrease in Lung Lymph Flow

Positive end-expiratory pressure (PEEP) is used to improve gas exchange, increase functional residual capacity, recruit air spaces, and decrease pulmonary shunt in patients suffering from respiratory failure. The effect of PEEP on extravascular lung water (EVLW), however, is still not fully understood. This study was designed as a prospective laboratory experiment to evaluate the effects of PEEP on EVLW and pulmonary lymph flow (QL) under physiologic conditions.

Twelve adult sheep were operatively prepared to measure haemodynamics of the systemic and pulmonary circulation, and to assess EVLW. In addition, the lung lymphatic duct was cannulated and a tracheostomy performed. The animals were then mechanically ventilated in the awake-state without end-expiratory pressure (PEEP 0). After a two-hour baseline period, PEEP was increased to 10 cmH2O for the duration of two hours, and then reduced back to 0 cmH2O. Cardiopulmonary variables, QL, and arterial blood gases were recorded intermittently; EVLW was determined two hours after each change in PEEP.

The increase in PEEP resulted in a decrease in QL (7±1 vs 5±1 ml/h) and an increase in EVLW (498±40 vs 630±58 ml; P<0.05 each) without affecting cardiac output. As PEEP was decreased back to baseline, QL increased significantly (5±1 vs 10±2 ml/h), whereas EVLW returned back to baseline.

This study suggests that institution of PEEP produces a reversible increase in EVLW that is linked to a decrease in QL.

Chronic pulmonary hypertension--the monocrotaline model and involvement of the hemostatic system

Monocrotaline (MCT) is a toxic pyrrolizidine alkaloid of plant origin. Administration of small doses of MCT or its active metabolite, monocrotaline pyrrole (MCTP), to rats causes delayed and progressive lung injury characterized by pulmonary vascular remodeling, pulmonary hypertension, and compensatory right heart hypertrophy. The lesions induced by MCT(P) administration in rats are similar to those observed in certain chronic pulmonary vascular diseases of people. This review begins with a synopsis of the hemostatic system, emphasizing the role of endothelium since endothelial cell dysfunction likely underlies the pathogenesis of MCT(P)-induced pneumotoxicity. MCT toxicology is discussed, focusing on morphologic, pulmonary mechanical, hemodynamic, and biochemical and molecular alterations that occur after toxicant exposure. Fibrin and platelet thrombosis of the pulmonary microvasculature occurs after administration of MCT(P) to rats, and several investigators have hypothesized that thrombi contribute to the lung injury and pulmonary hypertension. The evidence for involvement of the various components of the hemostatic system in MCT(P)-induced vascular injury and remodeling is reviewed. Current evidence is consistent with involvement of platelets and an altered fibrinolytic system, yet much remains to be learned about specific events and signals in the vascular pathogenesis.

Continuous negative extrathoracic pressure ventilation, lung water volume, and central blood volume. Studies in dogs with pulmonary edema induced by oleic acid

The effect of continuous positive-pressure ventilation (CPPV) on extravascular lung water volume has been investigated, but there is only one report which studied the effect of continuous negative extrathoracic pressure ventilation (CNETPV). The effect of CNETPV on central blood volume (CBV) has not been studied. Changes in intrathoracic pressure by CNETPV may alter lung water volume and CBV. In this study the effects of CNETPV on lung water volume and CBV were compared with those of intermittent positive-pressure ventilation (IPPV) and CPPV in dogs with pulmonary edema induced by oleic acid. Nine mongrel dogs were anesthetized and given oleic acid at 0.06 ml/kg intravenously to induce pulmonary edema; CNETPV was applied with a cuirass and a negative thoracic pressure ventilator (Kimura OKT-100) for 1 h. Extravascular lung water volume (as extravascular thermal volume [EVTV]) and CBV were estimated with the double-indicator dilution method using thermal-sodium; PEEP and continuous negative extrathoracic pressure were matched to produce the same increments in FRC. The EVTV increased during CNETPV but did not change during CPPV. The CBV decreased during CPPV but did not change during CNETPV. An increase of transmural pulmonary microvascular pressure was thought to be one of the reasons for the increase in EVTV with CNETPV.

Lung extravascular volume during venovenous bypass with extracorporeal CO2-removal in dogs

Extracorporeal circulation can cause lung damage, which would be especially counterproductive during extracorporeal gas exchange for the treatment of acute respiratory failure. To test the hypothesis that partial venovenous bypass with extracorporeal CO2-removal combined with low-frequency positive pressure ventilation (ECCO2R-LFPPV) can adversely affect lung fluid balance, extravascular thermal lung volume (ETV) and hemodynamics were assessed before, during and after ECCO2R-LFPPV in normal closed chest dogs. In series I dogs (n = 6) subjected to 10 h of ECCO2R-LFPPV, ETV did not change significantly from control (7.1 ml/kg +/- 0.99 SE) during or after bypass. Gravimetric extravascular lung water and lung histology after bypass were found to be normal. In series II dogs (n = 5), subjected to shorter periods of ECCO2R-LFPPV, ETV also remained unchanged. In contrast to previous reports using sheep, pulmonary arterial hypertension during bypass was not observed. Thus, ECCO2R-LFPPV was not associated with increased lung water, pulmonary hypertension or morphological lung changes under the conditions studied and does not seem to cause lung damage in normal lungs.

Thoracic duct lymph and PEEP studies in anaesthetized dogs. II. Effect of a thoracic duct fistula on the development of a hyponcotic-hydrostatic pulmonary oedema

PEEP impedes thoracic duct drainage (LF). This can be counteracted by a thoracic duct fistula. Consequently, lung oedema (LOE) should develop during PEEP more slowly with LF at atmospheric pressure (LFAP) than with LF against jugular venous pressure (LFJVP). In 12 anaesthetized dogs LOE was produced by Ringer's solution i.v. (2.5 ml/min per kg) for 6 h during PEEP (10 mmHg) with either LFAP or LFJVP. Ringer's + PEEP greatly increased aortic, pulmonary artery and wedge pressures, JVP, and cardiac output. Colloid osmotic pressures in plasma and lymph were drastically reduced, pulmonary effective filtration pressure (EFP) rose by about 20 mmHg. LFJVP increased 7-fold, LFAP about 19-fold, the respective loss of plasma proteins was 1.83 and 1.06 g/kg during 6 h. Thermal-dye extravascular lung water showed an increment of 68 with LFJVP versus 43 microliter/h/g per mmHg with LFAP. Final lung water content was at any delta EFP (12.8-31.9 mmHg) lower with LFAP than with LFJVP amounting 512 with LFJVP versus 377 microliters/g/per mmHg with LFAP. LFAP decreased the development of LOE during PEEP by bypassing the PEEP-induced high JVP and thus facilitating the removal of interstitial fluid. It is hypothesized that a thoracic duct fistula might aid the treatment of patients with LOE due to ARDS and therefore requiring high levels of PEEP.

Alterations in pulmonary fluid balance induced by positive end-expiratory pressure

We examined the consequences of positive end-expiratory pressure ventilation (PEEP) on pulmonary lymph flow in normal lungs and in lungs with alloxan-induced vascular injury. Studies were made in anesthetized sheep prepared with a lung lymph fistula. Prior to lung injury, the sheep were subjected to a protocol consisting of ventilation with zero end-expiratory pressure (ZEEP) for 1 h followed by ventilation with 20 cm H2O PEEP for 1 h. Subsequently, each animal received an infusion of alloxan (0.75 mg/kg i.v.) during ZEEP ventilation to induce lung vascular injury. ZEEP ventilation was maintained for 1 h and was followed by 1 h of PEEP ventilation. Alloxan infusion more than doubled lymph flow and lymph protein flux from baseline. PEEP ventilation did not affect lymph flow and lymph protein flux of normal lungs. However, in the alloxan-injured lung, PEEP ventilation caused significant decreases in lymph flow and lymph protein flux. Although PEEP ventilation redistributed regional pulmonary blood flow, as determined by radiolabelled microspheres, changes were identical in the normal and the alloxan-injured lung. The results indicate that PEEP ventilation does not alter fluid balance in the normal lung. However, PEEP ventilation depresses lymphatic flow in the injured lung, which may increase extravascular lung water content.

The early use of positive end expiratory pressure (PEEP) ventilation in emergency medicine, and some experiments on pigs

The effects on near drowning of young pigs of positive end expiratory pressure and zero end expiratory pressure were studied. The arterial PO2 and blood gases were examined during recovery from near drowning and after haemorrhagic shock. The cardiovascular parameters were measured after shock. The findings were considered in relation to the use of positive and expiratory pressure in patients. It was recommended that it be only administered by experienced staff in hospitals, and should not exceed 10 mbars, while 5 mbar was adequate and avoided complications in most cases.