Effect of ventilator-induced lung injury on the development of reperfusion injury in a rat lung transplant model

Mechanical Power Density Predicts Prolonged Ventilation Following Double Lung Transplantation

Prolonged mechanical ventilation (PMV) after lung transplantation poses several risks, including higher tracheostomy rates and increased in-hospital mortality. Mechanical power (MP) of artificial ventilation unifies the ventilatory variables that determine gas exchange and may be related to allograft function following transplant, affecting ventilator weaning. We retrospectively analyzed consecutive double lung transplant recipients at a national transplant center, ventilated through endotracheal tubes upon ICU admission, excluding those receiving extracorporeal support. MP and derived indexes assessed up to 36 h after transplant were correlated with invasive ventilation duration using Spearman's coefficient, and we conducted receiver operating characteristic (ROC) curve analysis to evaluate the accuracy in predicting PMV (>72 h), expressed as area under the ROC curve (AUROC). PMV occurred in 82 (35%) out of 237 cases. MP was significantly correlated with invasive ventilation duration (Spearman's ρ = 0.252 [95% CI 0.129-0.369], p < 0.01), with power density (MP normalized to lung-thorax compliance) demonstrating the strongest correlation (ρ = 0.452 [0.345-0.548], p < 0.01) and enhancing PMV prediction (AUROC 0.78 [95% CI 0.72-0.83], p < 0.01) compared to MP (AUROC 0.66 [0.60-0.72], p < 0.01). Mechanical power density may help identify patients at risk for PMV after double lung transplantation.

Critical Care of the Lung Transplant Patient

Lung transplantation is a therapeutic option for end-stage lung disease that improves survival and quality of life. Prelung transplant admission to the intensive care unit (ICU) for bridge to transplant with mechanical ventilation and extracorporeal membrane oxygenation (ECMO) is common. Primary graft dysfunction is an important immediate complication of lung transplantation with short- and long-term morbidity and mortality. Later transplant-related causes of respiratory failure necessitating ICU admission include acute cellular rejection, atypical infections, and chronic lung allograft dysfunction. Lung transplantation for COVID-19-related ARDS is increasingly common..

Critical Care Management Following Lung Transplantation

Postoperative critical care management for lung transplant recipients in the intensive care unit (ICU) has expanded in recent years due to its complexity and impact on clinical outcomes. The practical aspects of post-transplant critical care management, especially regarding ventilation and hemodynamic management during the early postoperative period in the ICU, are discussed in this brief review. Monitoring in the ICU provides information on the patient’s clinical status, diagnostic assessment of complications, and future management plans since lung transplantation involves unique pathophysiological conditions and risk factors for complications. After lung transplantation, the grafts should be appropriately ventilated with lung protective strategies to prevent ventilator-induced lung injury, as well as to promote graft function and maintain adequate gas exchange. Hypotension and varying degrees of pulmonary edema are common in the immediate postoperative lung transplantation setting. Ventricular dysfunction in lung transplant recipients should also be considered. Therefore, adequate volume and hemodynamic management with vasoactive agents based on their physiological effects and patient response are critical in the early postoperative lung transplantation period. Integrated management provided by a professional multidisciplinary team is essential for the critical care management of lung transplant recipients in the ICU.

Postoperative Management of Lung Transplant Recipients in the Intensive Care Unit

The number of lung transplantations is progressively increasing worldwide, providing new challenges to interprofessional teams and the intensive care units. The outcome of lung transplantation recipients is critically affected by a complex interplay of particular pathophysiologic conditions and risk factors, knowledge of which is fundamental to appropriately manage these patients during the early postoperative course. As high-grade evidence-based guidelines are not available, the authors aimed to provide an updated review of the postoperative management of lung transplantation recipients in the intensive care unit, which addresses six main areas: (1) management of mechanical ventilation, (2) fluid and hemodynamic management, (3) immunosuppressive therapies, (4) prevention and management of neurologic complications, (5) antimicrobial therapy, and (6) management of nutritional support and abdominal complications. The integrated care provided by a dedicated multidisciplinary team is key to optimize the complex postoperative management of lung transplantation recipients in the intensive care unit.

Lung Transplantation, Pulmonary Endothelial Inflammation, and Ex-Situ Lung Perfusion: A Review

Lung transplantation (LTx) is the gold standard treatment for end-stage lung disease; however, waitlist mortality remains high due to a shortage of suitable donor lungs. Organ quality can be compromised by lung ischemic reperfusion injury (LIRI). LIRI causes pulmonary endothelial inflammation and may lead to primary graft dysfunction (PGD). PGD is a significant cause of morbidity and mortality post-LTx. Research into preservation strategies that decrease the risk of LIRI and PGD is needed, and ex-situ lung perfusion (ESLP) is the foremost technological advancement in this field. This review addresses three major topics in the field of LTx: first, we review the clinical manifestation of LIRI post-LTx; second, we discuss the pathophysiology of LIRI that leads to pulmonary endothelial inflammation and PGD; and third, we present the role of ESLP as a therapeutic vehicle to mitigate this physiologic insult, increase the rates of donor organ utilization, and improve patient outcomes.

Analysis of different levels of positive end-expiratory pressure during lung retrieval for transplantation: an experimental study

Atelectasis and inadequate oxygenation in lung donors is a common problem during the retrieval of these organs. Nevertheless, the use of high positive end-expiratory pressure (PEEP) is not habitual during procedures of lung retrieval. Twenty-one Sprague-Dawley male consanguineous rats were used in the study. The animals were divided into 3 groups according to the level of PEEP used: low (2 cmH2O), moderate (5 cmH2O), and high (10 cmH2O). Animals were ventilated with a tidal volume of 6 mL/kg. Before lung removal, the lungs were inspected for the presence of atelectasis. When atelectasis was detected, alveolar recruitment maneuvers were performed. Blood gasometric analysis was performed immediately. Finally, the lungs were retrieved, weighed, and submitted to histological analysis. The animals submitted to higher PEEP showed higher levels of oxygenation with the same tidal volumes PO2=262.14 (PEEP 2), 382.4 (PEEP 5), and 477.0 (PEEP 10). The occurrence of atelectasis was rare in animals with a PEEP of 10 cmH2O, which therefore required less frequent recruitment maneuvers (need for recruitment: PEEP 2=100%, PEEP 5 =100%, and PEEP 10=14.3%). There was no change in hemodynamic stability, occurrence of pulmonary edema, or other histological injuries with the use of high PEEP. The use of high PEEP (10 cmH2O) was feasible and probably a beneficial strategy for the prevention of atelectasis and the optimization of oxygenation during lung retrieval. Clinical studies should be performed to confirm this hypothesis.

Perioperative Management of the Lung Graft Following Lung Transplantation

Perioperative management of patients undergoing lung transplantation is one of the most complex in cardiothoracic surgery. Certain perioperative interventions, such as mechanical ventilation, fluid management and blood transfusions, use of extracorporeal mechanical support, and pain management, may have significant impact on the lung graft function and clinical outcome. This article provides a review of perioperative interventions that have been shown to impact the perioperative course after lung transplantation.

Alveolar liquid clearance in lung injury: Evaluation of the impairment of the β2-adrenergic agonist response in an ischemia-reperfusion lung injury model

While alveolar liquid clearance (ALC) mediated by the β₂-adrenergic receptor (β₂-AR) plays an important role in lung edema resolution in certain models of lung injury, in more severe lung injury models, this response might disappear. Indeed, we have shown that in an ischemia-reperfusion-induced lung injury model, β₂-agonists do not enhance ALC. The objective of this study was to determine if downregulation of the β₂-AR could explain the lack of response to β₂-agonists in this lung injury model. In an in vivo canine model of lung transplantation, we observed no change in β₂-AR concentration or affinity in the injured transplanted lungs compared to the native lungs. Furthermore, we could not enhance ALC in transplanted lungs with dcAMP + aminophylline, a treatment that bypasses the β₂-adrenergic receptor and is known to stimulate ALC in normal lungs. However, transplantation decreased αENaC expression in the lungs by 50%. We conclude that the lack of response to β₂-agonists in ischemia-reperfusion-induced lung injury is not associated with significant downregulation of the β₂-adrenergic receptors but is attributable to decreased expression of the ENaC channel, which is essential for sodium transport and alveolar liquid clearance in the lung.

Performance and applications of bedside visual inspection of airway pressure-time curve profiles for estimating stress index in patients with acute respiratory distress syndrome

To determine the performance of bedside visual inspection of airway pressure-time (Paw-t) curve profiles (VI) for estimating stress index (SI) in patients with acute respiratory distress syndrome (ARDS). A prospective study in 30 subjects with ARDS receiving mechanical ventilation at two peak inspiratory flow (PIF) settings: 60 or 40 L/min. For each study session, two physicians inspected real-time Paw-t waveforms from mechanical ventilator's monitoring screens at bedside for 30 s and interpreted which of the three patterns (tidal recruitment, noninjurious ventilation or tidal overdistension) the Paw-t curve profile was compatible with. Subsequently, the study was repeated again at the second PIF setting. SI was derived from a standardized dedicated software program and categorized into three groups: SI < 0.9, or tidal recruitment; SI = 0.9-1.05, or noninjurious ventilation; and SI > 1.05, or tidal overdistension. The lower PIF setting increased the sensitivity of VI to correctly estimate SI (75% vs. 50%; p = 0.005). At PIF 40 L/min, the likelihood ratio of a positive test was 3.6, 5.4 or 7 if the Paw-t curve profile was interpreted as noninjurious ventilation, tidal recruitment or tidal overdistension, respectively. The likelihood ratio of a negative test ranged from 0.55 for tidal recruitment to 0.32 and 0.19 for noninjurious ventilation and tidal overdistension, respectively. Experience in mechanical ventilation did not influence the accuracy. Bedside VI is moderately accurate for estimating SI and may be used to monitor injurious ventilation in patients with ARDS, in addition to plateau airway pressure.

Ventilatory Management During Normothermic Ex Vivo Lung Perfusion: Effects on Clinical Outcomes

BACKGROUND

During ex vivo lung perfusion (EVLP), fixed ventilator settings and monitoring of compliance are used to prevent ventilator-induced lung injury (VILI). Analysis of the airway pressure-time curve (stress index) has been proposed to assess the presence of VILI. We tested whether currently proposed ventilator settings expose lungs to VILI during EVLP and whether the stress index could identify VILI better than compliance.

METHODS

Flow, volume, and airway opening pressure were collected continuously during EVLP. Durations of mechanical ventilation, intensive care unit (ICU) and hospital lengths of stay were recorded in lung recipients.

RESULTS

Fourteen lungs underwent EVLP and were transplanted. In 5 lungs, 95 ± 2% of the stress index values were within the 0.95 to 1.05 range (protected); in the remaining nine lungs, 69 ± 1% of the values were greater than 1.05 and 15 ± 3% were less than 0.95 (nonprotected). There was a significant (P < 0.05) increase in cytokine concentrations after 4 hours of EVLP in the nonprotected lungs. Durations of mechanical ventilation, ICU, and hospital lengths of stay were shorter in recipients of protected than that of nonprotected lungs (P < 0.05). There was no correlation between compliance during EVLP and duration of mechanical ventilation or ICU and hospital lengths of stay in recipients, but the stress index during EVLP was significantly correlated with the duration of mechanical ventilation and with ICU and hospital lengths of stay (P < 0.05).

CONCLUSIONS

This small, preliminary study shows that ventilator settings currently proposed for EVLP may expose lungs to VILI. Use of the stress index to personalize ventilator settings needs to be tested in further clinical studies.

How to minimise ventilator-induced lung injury in transplanted lungs: The role of protective ventilation and other strategies

Lung transplantation is the treatment of choice for end-stage pulmonary diseases. In order to avoid or reduce pulmonary and systemic complications, mechanical ventilator settings have an important role in each stage of lung transplantation. In this respect, the use of mechanical ventilation with a tidal volume of 6 to 8 ml  kg(-1) predicted body weight, positive end-expiratory pressure of 6 to 8 cmH2O and a plateau pressure lower than 30 cmH2O has been suggested for the donor during surgery, and for the recipient both during and after surgery. For the present review, we systematically searched the PubMed database for articles published from 2000 to 2014 using the following keywords: lung transplantation, protective mechanical ventilation, lung donor, extracorporeal membrane oxygenation, recruitment manoeuvres, extracorporeal CO2 removal and noninvasive ventilation.

Reliability of transpulmonary pressure-time curve profile to identify tidal recruitment/hyperinflation in experimental unilateral pleural effusion

The stress index (SI) is a parameter that characterizes the shape of the airway pressure-time profile (P/t). It indicates the slope progression of the curve, reflecting both lung and chest wall properties. The presence of pleural effusion alters the mechanical properties of the respiratory system decreasing transpulmonary pressure (Ptp). We investigated whether the SI computed using Ptp tracing would provide reliable insight into tidal recruitment/overdistention during the tidal cycle in the presence of unilateral effusion. Unilateral pleural effusion was simulated in anesthetized, mechanically ventilated pigs. Respiratory system mechanics and thoracic computed tomography (CT) were studied to assess P/t curve shape and changes in global lung aeration. SI derived from airway pressure (Paw) was compared with that calculated by Ptp under the same conditions. These results were themselves compared with quantitative CT analysis as a gold standard for tidal recruitment/hyperinflation. Despite marked changes in tidal recruitment, mean values of SI computed either from Paw or Ptp were remarkably insensitive to variations of PEEP or condition. After the instillation of effusion, SI indicates a preponderant over-distension effect, not detected by CT. After the increment in PEEP level, the extent of CT-determined tidal recruitment suggest a huge recruitment effect of PEEP as reflected by lung compliance. Both SI in this case were unaffected. We showed that the ability of SI to predict tidal recruitment and overdistension was significantly reduced in a model of altered chest wall-lung relationship, even if the parameter was computed from the Ptp curve profile.

Expanding the lung donor pool: advancements and emerging pathways

PURPOSE OF REVIEW

The number of patients listed for lung transplantation largely exceeds the number of available transplantable organs because of a shortage of organ donors and a low utilization rate of lungs from those donors who are available. In recent years, novel strategies have been developed to increase the donor lung pool: improved donor management, the use of lungs from donations after cardiac death (DCD), the use of lobar lung living-donors (LLLD) and the use of ex-vivo lung perfusion (EVLP) to assess and repair injured donor lungs.

RECENT FINDINGS

An adapted donor management strategy could expand the donor pool up to 20%. DCD lung transplant is an increasing part of the donor pool expansion. Outcomes after controlled DCD seem to be similar to donation after brain death. LLLD transplantation has excellent results for small and critically ill patients. EVLP treatment allows for a significant increase in the rate of suitable lungs and represents an optimal platform for lung reconditioning and specific lung therapies.

SUMMARY

A significant increase in the number of available lungs for transplantation is expected in the future because of the wider use of lungs from controlled or uncontrolled DCD and LLLD lungs, and with organ-specific EVLP treatment strategies.

Ex vivo lung perfusion

Lung transplantation is an established life-saving therapy for patients with end-stage lung disease. Unfortunately, greater success in lung transplantation is hindered by a shortage of lung donors and the relatively poor early-, mid-, and long-term outcomes associated with severe primary graft dysfunction. Ex vivo lung perfusion has emerged as a modern preservation technique that allows for a more accurate lung assessment and improvement in lung quality. This review outlines the: (i) rationale behind the method; (ii) techniques and protocols; (iii) Toronto ex vivo lung perfusion method; (iv) devices available; and (v) clinical experience worldwide. We also highlight the potential of ex vivo lung perfusion in leading a new era of lung preservation.

Primary Graft Dysfunction after Lung Transplantation

Primary graft dysfunction (PGD) is a severe form of acute lung injury that is a major cause of early morbidity and mortality encountered after lung transplantation. PGD is diagnosed by pulmonary oedema with diffuse alveolar damage that manifests clinically as progressive hypoxemia with radiographic pulmonary infiltrates. Inflammatory and immunological response caused by ischaemia and reperfusion is important with regard to pathophysiology. PGD affects short- and long-term outcomes, the donor organ is the leading factor affecting these adverse ramifications. To minimize the risk of PGD, reduction of lung ischaemia time, reperfusion optimisation, prostaglandin level regulation, haemodynamic control, hormone replacement therapy, ventilator management are carried out; for research regarding donor lung preparation strategies, certain procedures are recommended. In this review, recent updates in epidemiology, pathophysiology, molecular and genetic biomarkers and technical developments affecting PGD are described.

MECHANICAL VENTILATION FOR THE LUNG TRANSPLANT RECIPIENT

Mechanical ventilation (MV) is an important aspect in the intraoperative and early postoperative management of lung transplant (LTx)-recipients. There are no randomized-controlled trials of LTx-recipient MV strategies; however there are LTx center experiences and international survey studies reported. The main early complication of LTx is primary graft dysfunction (PGD), which is similar to the adult respiratory distress syndrome (ARDS). We aim to summarize information pertinent to LTx-MV, as well as PGD, ARDS, and intraoperative MV and to synthesize these available data into recommendations. Based on the available evidence, we recommend lung-protective MV with low-tidal-volumes (≤6 mL/kg predicted body weight [PBW]) and positive end-expiratory pressure for the LTx-recipient. In our opinion, the MV strategy should be based on donor characteristics (donor PBW as a parameter of actual allograft size), rather than based on recipient characteristics; however this donor-characteristics-based protective MV is based on indirect evidence and requires validation in prospective clinical studies.

Mechanical ventilation after lung transplantation. An international survey of practices and preferences

RATIONALE

Between 10% and 57% of lung transplant (LTx) recipients develop primary graft dysfunction (PGD) within 72 hours of LTx. PGD is clinically and histologically analogous to the acute respiratory distress syndrome. In patients at risk for or with acute respiratory distress syndrome, lung-protective ventilation strategies (low tidal volume and positive end-expiratory pressure) improve outcomes. There is, however, little information available on mechanical ventilation strategies after LTx.

OBJECTIVES

Our aim in this international survey was to describe the current practices of mechanical ventilation immediately after LTx.

METHODS

An electronic survey was sent to the medical and surgical directors of U.S. LTx programs (n = 111) and to members of the Pulmonary Council of the International Society for Heart and Lung Transplantation (n = 470).

RESULTS

A total of 149 individuals from 18 countries responded to the questionnaire. The most common modes of ventilation were pressure assist/control (37%) and volume assist/control (35%). Tidal volumes were most often determined by recipient characteristics. Donor characteristics were rarely considered (35%) and were infrequently known by the team managing the ventilator (42%). When presented with a choice of ideal tidal volumes, a majority of respondents selected 6 ml/kg recipient predicted body weight (58%), fewer selected 10 ml/kg (21%), and none selected 15 ml/kg. A majority preferred limiting the fraction of inspired oxygen rather than positive end-expiratory pressure (PEEP) (69% versus 31%, P = 0.006). The median minimum PEEP was 5 cm H2O, and the median maximum PEEP was 11.5 cm H2O. The presence of PGD increased the perceived importance of monitoring plateau pressure to adjust tidal volumes. The median plateau pressure limit perceived as a threshold triggering reduction in tidal volume was 30 cm H2O.

CONCLUSIONS

Most respondents reported using lung-protective approaches to mechanical ventilation after lung transplantation. Low tidal volumes based on recipient characteristics were frequently chosen. Donor characteristics often were not considered and frequently were not known by the team managing mechanical ventilation after LTx.

Accuracy of plateau pressure and stress index to identify injurious ventilation in patients with acute respiratory distress syndrome

BACKGROUND

Guidelines suggest a plateau pressure (PPLAT) of 30 cm H(2)O or less for patients with acute respiratory distress syndrome, but ventilation may still be injurious despite adhering to this guideline. The shape of the curve plotting airway pressure versus time (STRESS INDEX) may identify injurious ventilation. The authors assessed accuracy of PPLAT and STRESS INDEX to identify morphological indexes of injurious ventilation.

METHODS

Indexes of lung aeration (computerized tomography) associated with injurious ventilation were used as a "reference standard." Threshold values of PPLAT and STRESS INDEX were determined assessing the receiver-operating characteristics ("training set," N = 30). Accuracy of these values was assessed in a second group of patients ("validation set," N = 20). PPLAT and STRESS INDEX were partitioned between respiratory system (Pplat,Rs and STRESS INDEX,RS) and lung (PPLAT,L and STRESS INDEX,L; esophageal pressure; "physiological set," N = 50).

RESULTS

Sensitivity and specificity of PPLAT of greater than 30 cm H(2)O were 0.06 (95% CI, 0.002-0.30) and 1.0 (95% CI, 0.87-1.00). PPLAT of greater than 25 cm H(2)O and a STRESS INDEX of greater than 1.05 best identified morphological markers of injurious ventilation. Sensitivity and specificity of these values were 0.75 (95% CI, 0.35-0.97) and 0.75 (95% CI, 0.43-0.95) for PPLAT greater than 25 cm H(2)O versus 0.88 (95% CI, 0.47-1.00) and 0.50 (95% CI, 0.21-0.79) for STRESS INDEX greater than 1.05. Pplat,Rs did not correlate with PPLAT,L (R(2) = 0.0099); STRESS INDEX,RS and STRESS INDEX,L were correlated (R(2) = 0.762).

CONCLUSIONS

The best threshold values for discriminating morphological indexes associated with injurious ventilation were Pplat,Rs greater than 25 cm H(2)O and STRESS INDEX,RS greater than 1.05. Although a substantial discrepancy between Pplat,Rs and PPLAT,L occurs, STRESS INDEX,RS reflects STRESS INDEX,L.

Advances in lung preservation

After a brief review of conventional lung preservation, this article discusses the rationale behind ex vivo lung perfusion and how it has shifted the paradigm of organ preservation from conventional static cold ischemia to the utilization of functional normothermia, restoring the lung's own metabolism and its reparative processes. Technical aspects and previous clinical experience as well as opportunities to address specific donor organ injuries in a personalized medicine approach are also reviewed.

Effect of positive end-expiratory pressure after porcine unilateral left lung transplant

OBJECTIVES

To evaluate the effects of 2 different levels of positive end-expiratory pressure on pigs who had unilateral lung transplants.

MATERIALS AND METHODS

A left lung transplant was performed in 12 pigs. The animals were randomized into 2 groups based on positive end-expiratory pressure: group 1 (5 cm H(2)O) and group 2 (10 cm H(2)O). Hemodynamics, gas exchange, and respiratory mechanics were measured before and after surgery. Cytokines, oxidative stress, and histologic scores were assessed in the lung tissue of each pig.

RESULTS

Pigs in group 2 exhibited a significantly higher mean heart rate (P = .006), static compliance (P = .001), lower mean arterial pressure (P = .003), and airway resistance (P = .001) than did pigs in group 1. There were no postoperative differences between the groups in concentrations of thiobarbituric acid reactive substances, superoxide dismutase, and interleukin 8. At the end of the observation period, pigs in group 2 had higher levels of thiobarbituric acid reactive substances (P = .001) and interleukin 8 (P = .05), and pigs in group 1 had higher levels of superoxide dismutase (P = .05) than they did at baseline.

CONCLUSIONS

After unilateral lung transplant, higher positive end-expiratory pressure was associated with improved respiratory mechanics, a negative effect on hemodynamics, a stronger inflammatory response, and increased production of reactive oxygen species, but no effect on gas exchange.

[Positive end-expiratory pressure : adjustment in acute lung injury]

Treatment of patients suffering from acute lung injury is a challenge for the treating physician. In recent years ventilation of patients with acute hypoxic lung injury has changed fundamentally. Besides the use of low tidal volumes, the most beneficial setting of positive end-expiratory pressure (PEEP) has been in the focus of researchers. The findings allow adaption of treatment to milder forms of acute lung injury and severe forms. Additionally computed tomography techniques to assess the pulmonary situation and recruitment potential as well as bed-side techniques to adjust PEEP on the ward have been modified and improved. This review gives an outline of recent developments in PEEP adjustment for patients suffering from acute hypoxic and hypercapnic lung injury and explains the fundamental pathophysiology necessary as a basis for correct treatment.

Dynamic instability of central airways and peripheral airspace in rat lungs perfused with cold preservation solutions

BACKGROUND/AIMS

For lung preservation, one of two types of solutions is commonly employed: Euro-Collins (EC) or low potassium dextran glucose (LPDG). These two solutions have been compared regarding biological, morphometrical and physiological outcomes in many experiments. However, the dynamic mechanics of perfused lung are not well understood because the dynamic characteristics cannot be assessed under static conditions; hence, the primary goal of the present study was to assess this in perfused rat lungs during the preservation period, comparing EC with LPDG at 0 or 9 h at 4°C.

METHODS

Lung impedance was measured using a forced oscillation technique. Lung resistance and elastance values were obtained by the fast Fourier transform algorithm. The instability of central airways and heterogeneity of ventilation were estimated.

RESULTS

In the EC group, airway resistance and instability were high after perfusion, and the lung elastance was high and more heterogeneous after cold storage. In contrast, those parameters were stable in the LPDG group during cold storage.

CONCLUSION

Such dynamic stability might facilitate the handling of lung grafts and eliminate injurious cyclic ventilation stress after reperfusion. Thus, we conclude that the impedance frequency characteristic represents a novel informative parameter for investigating lung preservation techniques.

Ventilation strategy and its influence on the functional performance of lung grafts in an experimental model of single lung transplantation using non-heart-beating donors

OBJECTIVE

To compare the influence of two different ventilation strategies-volume-controlled ventilation (VCV) and pressure-controlled ventilation (PCV)-on the functional performance of lung grafts in a canine model of unilateral left lung transplantation using donor lungs harvested after three hours of normothermic cardiocirculatory arrest under mechanical ventilation.

METHODS

The study comprised 40 mongrel dogs, randomized into two groups: VCV and PCV. Of the 20 recipients, 5 did not survive the transplant, and 5 died before the end of the post-transplant assessment period. The remaining 10 survivors (5 in each group) were evaluated for 360 min after lung transplantation. The functional performance of the grafts was evaluated regarding respiratory mechanics, gas exchange, and lung graft histology.

RESULTS

There were no significant differences between the groups regarding respiratory mechanics (peak inspiratory pressure, plateau pressure, mean airway pressure, dynamic compliance, and static compliance) or gas exchange variables (PaO2, venous oxygen tension, PaCO2, venous carbon dioxide tension, and the arterial-venous oxygen content difference). The histopathological findings were consistent with nonspecific acute lung injury and did not differ between the groups.

CONCLUSIONS

This model of lung transplantation showed that the functional performance of lung grafts was not influenced by the ventilation strategy employed during the first six hours after reperfusion.

Open lung approach associated with high-frequency oscillatory or low tidal volume mechanical ventilation improves respiratory function and minimizes lung injury in healthy and injured rats

INTRODUCTION

To test the hypothesis that open lung (OL) ventilatory strategies using high-frequency oscillatory ventilation (HFOV) or controlled mechanical ventilation (CMV) compared to CMV with lower positive end-expiratory pressure (PEEP) improve respiratory function while minimizing lung injury as well as systemic inflammation, a prospective randomized study was performed at a university animal laboratory using three different lung conditions.

METHODS

Seventy-eight adult male Wistar rats were randomly assigned to three groups: (1) uninjured (UI), (2) saline washout (SW), and (3) intraperitoneal/intravenous Escherichia coli lipopolysaccharide (LPS)-induced lung injury. Within each group, animals were further randomized to (1) OL with HFOV, (2) OL with CMV with "best" PEEP set according to the minimal static elastance of the respiratory system (BP-CMV), and (3) CMV with low PEEP (LP-CMV). They were then ventilated for 6 hours. HFOV was set with mean airway pressure (PmeanHFOV) at 2 cm H2O above the mean airway pressure recorded at BP-CMV (PmeanBP-CMV) following a recruitment manoeuvre. Six animals served as unventilated controls (C). Gas-exchange, respiratory system mechanics, lung histology, plasma cytokines, as well as cytokines and types I and III procollagen (PCI and PCIII) mRNA expression in lung tissue were measured.

RESULTS

We found that (1) in both SW and LPS, HFOV and BP-CMV improved gas exchange and mechanics with lower lung injury compared to LP-CMV, (2) in SW; HFOV yielded better oxygenation than BP-CMV; (3) in SW, interleukin (IL)-6 mRNA expression was lower during BP-CMV and HFOV compared to LP-CMV, while in LPS inflammatory response was independent of the ventilatory mode; and (4) PCIII mRNA expression decreased in all groups and ventilatory modes, with the decrease being highest in LPS.

CONCLUSIONS

Open lung ventilatory strategies associated with HFOV or BP-CMV improved respiratory function and minimized lung injury compared to LP-CMV. Therefore, HFOV with PmeanHFOV set 2 cm H2O above the PmeanBP-CMV following a recruitment manoeuvre is as beneficial as BP-CMV.

Modified low-flow ultrafiltration ameliorates hemodynamics and early graft function and reduces blood loss in living-donor lobar lung transplantation

BACKGROUND

This study analyzed the clinical application of modified low-flow ultrafiltration (MUF) to minimize cardiopulmonary bypass (CPB)-related adverse effects in patients undergoing living-donor lobar lung transplantation (LDLLT).

METHOD

The study enrolled 33 consecutive patients who underwent LDLLT from 1999 to 2004: 8 patients underwent conventional CPB without MUF (control group), and 15 underwent arteriovenous MUF (MUF-treated group). Hemodynamics, graft function, blood loss and blood transfusion requirements, and clinical outcomes were analyzed.

RESULTS

There was a significant increase in systolic blood pressure and a decrease in pulmonary to systemic pressure ratio in the MUF-treated group (p < 0.05). No hemodynamic changes occurred in the control group. MUF resulted in significant improvements in arterial oxygen tension/fraction of inspired oxygen ratio (PaO(2)/FiO(2;) 411 +/- 107 vs 272 +/- 107 mm Hg, p < 0.05) and the alveolar-arterial oxygen difference (a-aDO(2); 158 +/- 84 vs 315 +/- 127 mm Hg, p < 0.05) at 15 minutes after CPB. There were no differences in PaO(2)/FiO(2) and A-aDO(2) between the groups beyond 6 hours post-operatively. Post-operative blood loss and blood transfusion requirements were lower in the MUF-treated group than in the control group (p < 0.05). There were no differences in survival, duration of ventilation, intensive care unit stay, and hospital stay between the groups.

CONCLUSIONS

The low-flow MUF brought improved hemodynamics and gas exchange capacity of transplanted grafts and lowered post-operative blood loss and blood transfusion requirement. This strategy may minimize CPB-related adverse effects in patients undergoing LDLLT.

Pulmonary atelectasis during low stretch ventilation: "open lung" versus "lung rest" strategy

OBJECTIVE

Limiting tidal volume (VT) may minimize ventilator-induced lung injury (VILI). However, atelectasis induced by low VT ventilation may cause ultrastructural evidence of cell disruption. Apoptosis seems to be involved as protective mechanisms from VILI through the involvement of mitogen-activated protein kinases (MAPKs). We examined the hypothesis that atelectasis may influence the response to protective ventilation through MAPKs.

DESIGN

Prospective randomized study.

SETTING

University animal laboratory.

SUBJECTS

Adult male 129/Sv mice.

INTERVENTIONS

Isolated, nonperfused lungs were randomized to VILI: VT of 20 mL/kg and positive end-expiratory pressure (PEEP) zero; low stretch/lung rest: VT of 6 mL/kg and 8-10 cm H2O of PEEP; low stretch/open lung: VT of 6 mL/kg, two recruitment maneuvers and 14-16 cm H2O of PEEP. Ventilator settings were adjusted using the stress index.

MEASUREMENT AND MAIN RESULT

Both low stretch strategies equally blunted the VILI-induced derangement of respiratory mechanics (static volume-pressure curve), lung histology (hematoxylin and eosin), and inflammatory mediators (interleukin-6, macrophage inflammatory protein-2 [enzyme-linked immunosorbent assay], and inhibitor of nuclear factor-kB[Western blot]). VILI caused nuclear swelling and membrane disruption of pulmonary cells (electron microscopy). Few pulmonary cells with chromatin condensation and fragmentation were seen during both low stretch strategies. However, although cell thickness during low stretch/open lung was uniform, low stretch/lung rest demonstrated thickening of epithelial cells and plasma membrane bleb formation. Compared with the low stretch/open lung, low stretch/lung rest caused a significant decrease in apoptotic cells (terminal deoxynucleotidyl transferase mediated deoxyuridine-triphosphatase nick end-labeling) and tissue expression of caspase-3 (Western blot). Both low stretch strategies attenuated the activation of MAPKs. Such reduction was larger during low stretch/open lung than during low stretch/lung rest (p < 0.001).

CONCLUSION

Low stretch strategies provide similar attenuation of VILI. However, low stretch/lung rest strategy is associated to less apoptosis and more ultrastructural evidence of cell damage possibly through MAPKs-mediated pathway.

Pathogenesis, management, and consequences of primary graft dysfunction

Primary graft dysfunction continues to be a major contributing factor to morbidity and mortality after lung transplantation. This condition is presumed to be the result of ischemia-reperfusion injury, which is associated with the release of endogenous substances that can activate the innate immune system. Primary graft dysfunction has been shown to be an independent risk factor for the development of bronchiolitis obliterans syndrome indicating that it can shape alloimmune responses. In this review, we focus on the classification, pathogenesis, possible prevention strategies, management and consequences of primary graft dysfunction.

ARDSnet ventilatory protocol and alveolar hyperinflation: role of positive end-expiratory pressure

RATIONALE

In patients with acute respiratory distress syndrome (ARDS), a focal distribution of loss of aeration in lung computed tomography predicts low potential for alveolar recruitment and susceptibility to alveolar hyperinflation with high levels of positive end-expiratory pressure (PEEP).

OBJECTIVES

We tested the hypothesis that, in this cohort of patients, the table-based PEEP setting criteria of the National Heart, Lung, and Blood Institute's ARDS Network (ARDSnet) low tidal volume ventilatory protocol could induce tidal alveolar hyperinflation.

METHODS

In 15 patients, physiologic parameters and plasma inflammatory mediators were measured during two ventilatory strategies, applied randomly: the ARDSnet and the stress index strategy. The latter used the same ARDSnet ventilatory pattern except for the PEEP level, which was adjusted based on the stress index, a monitoring tool intended to quantify tidal alveolar hyperinflation and/or recruiting/derecruiting that occurs during constant-flow ventilation, on a breath-by-breath basis.

MEASUREMENTS AND MAIN RESULTS

In all patients, the stress index revealed alveolar hyperinflation during application of the ARDSnet strategy, and consequently, PEEP was significantly decreased (P < 0.01) to normalize the stress index value. Static lung elastance (P = 0.01), plasma concentrations of interleukin-6 (P < 0.01), interleukin-8 (P = 0.031), and soluble tumor necrosis factor receptor I (P = 0.013) were significantly lower during the stress index as compared with the ARDSnet strategy-guided ventilation.

CONCLUSIONS

Alveolar hyperinflation in patients with focal ARDS ventilated with the ARDSnet protocol is attenuated by a physiologic approach to PEEP setting based on the stress index measurement.

Dynamic versus static respiratory mechanics in acute lung injury and acute respiratory distress syndrome

OBJECTIVES

It is not clear whether the mechanical properties of the respiratory system assessed under the dynamic condition of mechanical ventilation are equivalent to those assessed under static conditions. We hypothesized that the analyses of dynamic and static respiratory mechanics provide different information in acute respiratory failure.

DESIGN

Prospective multiple-center study.

SETTING

Intensive care units of eight German university hospitals.

PATIENTS

A total of 28 patients with acute lung injury and acute respiratory distress syndrome.

INTERVENTIONS

None.

MEASUREMENTS

Dynamic respiratory mechanics were determined during ongoing mechanical ventilation with an incremental positive end-expiratory pressure (PEEP) protocol with PEEP steps of 2 cm H2O every ten breaths. Static respiratory mechanics were determined using a low-flow inflation.

MAIN RESULTS

The dynamic compliance was lower than the static compliance. The difference between dynamic and static compliance was dependent on alveolar pressure. At an alveolar pressure of 25 cm H2O, dynamic compliance was 29.8 (17.1) mL/cm H2O and static compliance was 59.6 (39.8) mL/cm H2O (median [interquartile range], p < .05). End-inspiratory volumes during the incremental PEEP trial coincided with the static pressure-volume curve, whereas end-expiratory volumes significantly exceeded the static pressure-volume curve. The differences could be attributed to PEEP-related recruitment, accounting for 40.8% (10.3%) of the total volume gain of 1964 (1449) mL during the incremental PEEP trial. Recruited volume per PEEP step increased from 6.4 (46) mL at zero end-expiratory pressure to 145 (91) mL at a PEEP of 20 cm H2O (p < .001). Dynamic compliance decreased at low alveolar pressure while recruitment simultaneously increased. Static mechanics did not allow this differentiation. The decrease in static compliance occurred at higher alveolar pressures compared with the dynamic analysis.

CONCLUSIONS

Exploiting dynamic respiratory mechanics during incremental PEEP, both compliance and recruitment can be assessed simultaneously. Based on these findings, application of dynamic respiratory mechanics as a diagnostic tool in ventilated patients should be more appropriate than using static pressure-volume curves.

Independent ventilation of the graft and native lungs in vivo after rat lung transplantation

Rat lung transplantation is a proven experimental technique for the study of lung injury following lung transplantation. We have modified the surgical and ventilatory techniques to allow for independent ventilation in vivo of the transplanted graft and native lungs. This will provide additional data on the physiology and function of the transplanted graft and ameliorate the problem of progressive graft lung collapse and thereby allow for an improved model of ischemia-reperfusion injury and ventilator-induced lung injury in the setting of lung transplantation.

Update of early respiratory failure in the lung transplant recipient

PURPOSE OF REVIEW

Respiratory failure remains the most common complication in the perioperative period after lung transplantation. Consequently it is important to develop an approach to diagnosis and the treatment of respiratory failure in this population. This review highlights the advances made in the understanding and treatment of lung transplant patients in the early postoperative phase. Owing to its relative importance, advances in the understanding and treatment of ischaemia-reperfusion injury are highlighted.

RECENT FINDINGS

The causes of respiratory failure and the complications seen after transplantation are time dependent, with ischaemia-reperfusion, infection, technical problems and acute rejection being the most common in the early perioperative phase, and obliterative bronchiolitis, rejection, and infections secondary to bacteria, fungi, and viruses becoming more prevalent after 3 months. The advances in lung preservation and postoperative care may be overshadowed by an increase in the complexity of the recipients and the use of more marginal organs. An improved mechanistic understanding of ischaemia-reperfusion injury has translated into potential therapeutic targets. The development of prospective clinical trials, however, is hampered by a relatively small sample of patients and a significant degree of heterogeneity in the lung transplant population.

SUMMARY

Many advances have been made in the understanding of ischaemia-reperfusion injury. Owing to the acute and long-term implications of this complication, interventions that reduce the risk of developing ischaemia-reperfusion need to be evaluated in prospective clinical trials.

Lung transplantation: donor and recipient critical care aspects

PURPOSE OF REVIEW

The purpose of this paper is to highlight new developments in donor and recipient lung transplant issues for the critical care physician.

RECENT FINDINGS

A shortage of suitable lung donors has led to the use of extended donors and the development of novel techniques such as live-donor lung transplantation and the use of non-heart-beating donors. The increased experience and success with lung transplantation has also resulted in the extension of this therapy to patients previously considered unsuitable for transplantation. Postoperative outcomes can be affected by many of these recent donor and recipient changes. Improved preservation solutions and techniques to reduce reperfusion injury may be able to ameliorate some of the new perioperative graft dysfunction, but morbidity is still potentially significant, and extraordinary interventions such as extracorporeal membrane oxygenation may be required in selected cases.

SUMMARY

Patients undergoing lung transplantation continue to be very challenging in the intensive care unit. A multidisciplinary approach to care, and early recognition of serious problems, will help improve outcomes.

Massive brain injury enhances lung damage in an isolated lung model of ventilator-induced lung injury

OBJECTIVE

To assess the influence of massive brain injury on pulmonary susceptibility to injury attending subsequent mechanical or ischemia/reperfusion stress.

DESIGN

Prospective experimental study.

SETTING

Animal research laboratory.

SUBJECTS

Twenty-four anesthetized New Zealand White rabbits randomized to control (n = 12) or induced brain injury (n = 12) group.

INTERVENTIONS

After randomization, brain injury was induced by inflation of an intracranial balloon-tipped catheter, and animals were ventilated with a tidal volume of 10 mL/kg and zero end-expiratory pressure for 120 mins. Following heart-lung block extraction, isolated and perfused lungs were subjected to injurious ventilation with peak airway pressure 30 cm H2O and positive end-expiratory pressure 5 cm H2O for 30 mins.

MEASUREMENTS AND MAIN RESULTS

No difference was observed between groups in gas exchange, lung mechanics, or hemodynamics during the 2-hr in vivo period following induction of brain injury. However, after 30 mins of ex vivo injurious mechanical ventilation, lungs from the brain injury group showed greater change in ultrafiltration coefficient, weight gain, and alveolar hemorrhage (all p < .05).

CONCLUSIONS

Massive brain injury might increase lung vulnerability to subsequent injurious mechanical or ischemia-reperfusion insults, thereby increasing the risk of clinical posttransplant graft failure.

Lung preservation

Better understanding of the mechanisms of ischemia-reperfusion injury, improvement in the technique of lung preservation, and the recent introduction of a new preservation solution specifically developed for the lungs have helped to reduce the incidence of primary graft dysfunction after lung transplantation. Currently, the limitation in extending the ischemic time is more often related to the increasing use of non-ideal lung donors rather than to poor lung preservation. In this review, we have focused our attention on the experimental and clinical work performed to optimize the methods of lung preservation from the time of retrieval to the period of reperfusion after graft implantation.

Airway pressure-time curve profile (stress index) detects tidal recruitment/hyperinflation in experimental acute lung injury

OBJECTIVE

To evaluate whether the shape of the airway pressure-time (Paw-t) curve during constant flow inflation corresponds to radiologic evidence of tidal recruitment or tidal hyperinflation in an experimental model of acute lung injury.

DESIGN

Prospective randomized laboratory animal investigation.

SETTING

Department of Clinical Physiology, University of Uppsala, Sweden.

SUBJECTS

Anesthetized, paralyzed, and mechanically ventilated pigs.

INTERVENTIONS

Acute lung injury was induced by lung lavage. During constant inspiratory flow, the Paw-t curve was fitted to a power equation: airway pressure =a x time + c, where coefficient b (stress index) describes the shape of the curve:b = 1, straight curve; b < 1, progressive increase in slope; and b > 1, progressive decrease in slope. Tidal volume (Vt) was 6 mL/kg, and positive end-expiratory pressure was set to obtain a b value between 0.9 and 1.1 before (b = 1) and after (b = 1 after recruiting maneuver) application of a recruiting maneuver. Positive end-expiratory pressure was decreased and Vt increased to obtain 0.9 >b > 0.8 and 0.8 >b > 0.6, whereas positive end-expiratory pressure and Vt were both increased to obtain 1.3 >b > 1.1 and 1.5 >b > 1.3. Experimental conditions sequence was random.

MEASUREMENTS AND MAIN RESULTS

Pulmonary computed tomography was obtained during end-expiratory and end-inspiratory occlusions. Tidal recruitment was quantified as nonaerated (between -100 and +100 Hounsfield units) lung area at end-expiration minus end-inspiration. Tidal hyperinflation was quantified as hyperinflated (between -900 and -1000 Hounsfield units) lung area at end-inspiration minus end-expiration. Computed tomography images showed that tidal recruitment and tidal hyperinflation corresponded to b < 1 and b > 1, respectively. Stress index values and tidal recruitment and tidal hyperinflation values were significantly correlated (R =.917 and R =.911, p <.0001, respectively).

CONCLUSIONS

Shape of the Paw-t curve detects tidal recruitment and tidal hyperinflation.

Recipient T cells mediate reperfusion injury after lung transplantation in the rat

Leukocytes have been implicated in ischemia-reperfusion (IR) injury of the lung, but the individual role of T cells has not been explored. Recent evidence in mice suggests that T cells may play a role in IR injury. Using a syngeneic (Lewis to Lewis) rat lung transplant model, we observed that recipient CD4(+) T cells infiltrated lung grafts within 1 h of reperfusion and up-regulated the expression of CD25 over the ensuing 12 h. Nude rats (rnu/rnu) and heterozygous rats (rnu/+) were used to determine the role of T cells in IR injury. No significant difference in lung function was observed between nude and heterozygous recipient rats after 2 h of reperfusion. However, after 12 h of reperfusion, recipient nude rats had significantly higher oxygenation and lower peak airway pressure than recipient heterozygous rats. This was associated with significantly lower levels of IFN-gamma in transplanted lung tissue of recipient nude rats. Reconstitution of recipient nude rats with T cells from heterozygous rats restored IR injury after 12 h of reperfusion. The effect of T cells was independent of neutrophil recruitment and activation in the transplanted lung. These results demonstrate that recipient T cells are activated and mediate IR injury during lung transplantation in rats.

How long can we preserve the pulmonary graft inside the nonheart-beating donor?

BACKGROUND

The use of lungs from nonheart-beating donors (NHBD) might significantly alleviate the organ shortage. Extending the preharvest interval in NHBD would facilitate distant organ retrieval. We hypothesized that prolonged topical cooling inside NHBD after 60 minutes of initial warm ischemia would not affect the pulmonary graft.

METHODS

Domestic pigs were anesthetized and divided into three groups (n = 6 in each group). In the control group (HBD), lungs were flushed, explanted, and further stored in low potassium dextran solution (4 degrees C) for 4 hours. In the two study groups pigs were sacrificed by myocardial fibrillation and left untouched for 1 hour. Chest drains were then inserted for topical lung cooling (6 degrees C) for 3 hours (NHBD-TC3) or 6 hours (NHBD-TC6). The left lung in all groups was then prepared for evaluation. In an isolated circuit lungs were ventilated and reperfused through the pulmonary artery. Hemodynamic, aerodynamic, and oxygenation variables were measured 35 minutes after onset of controlled reperfusion. Wet-to-dry weight ratio was calculated.

RESULTS

No significant differences were observed among the three groups in pulmonary vascular resistance (p = 0.38), mean airway pressure (p = 0.39), oxygenation index (p = 0.62), and wet-to-dry weight ratio (p = 0.09).

CONCLUSIONS

These data confirm that 1 hour of warm ischemia does not affect the pulmonary graft from NHBD compared with HBD. The preharvest interval can be safely extended up to 7 hours postmortem by additional topical cooling of the graft inside the cadaver. This technique may facilitate distant organ retrieval in NHBD.