Ventilation-perfusion mismatch after lung ischemia-reperfusion. Protective effect of nitric oxide

Inhaled Pulmonary Vasodilators and Thoracic Organ Transplantation: Does Evidence Support Its Use and Cost Benefit?

In heart transplantation, pulmonary hypertension and increased pulmonary vascular resistance followed by donor right ventricular dysfunction remain a major cause of perioperative morbidity and mortality. In lung transplantation, primary graft dysfunction remains a major obstacle because it can cause bronchiolitis obliterans and mortality. Pulmonary vasodilators have been used as an adjunct therapy for heart or lung transplantation, mainly to treat pulmonary hypertension, right ventricular failure, and associated refractory hypoxemia. Among pulmonary vasodilators, inhaled nitric oxide is unique in that it is selective in pulmonary circulation and causes fewer systemic complications such as hypotension, flushing, or coagulopathy. Nitric oxide is expected to prevent or attenuate primary graft dysfunction by decreasing ischemia-reperfusion injury in lung transplantation. However, when considering the long-term benefit of these medications, little evidence supports their use in heart or lung transplantation. Current guidelines endorse inhaled vasodilators for managing immediate postoperative right ventricular failure in lung or heart transplantation, but no guidance is offered regarding agent selection, dosing, or administration. This review presents the current evidence of inhaled nitric oxide in lung or heart transplantation as well as comparisons with other pulmonary vasodilators including cost differences in consideration of economic pressures to contain rising pharmacy costs.

Interplay of endothelial and inducible nitric oxide synthases modulates the vascular response to ischaemia-reperfusion in the rabbit lung

AIM

Lung ischaemia-reperfusion induces nitric oxide synthesis and reactive nitrogen species, decreasing nitric oxide bioavailability. We hypothesized that in the ventilated lung, this process begins during ischaemia and intensifies with reperfusion, contributing to ischaemia-reperfusion-induced pulmonary vasoconstriction. The aim was to determine whether ischaemia-reperfusion alters inducible and endothelial nitric oxide synthase expression/activity, reactive nitrogen species generation, and nitric oxide bioavailability, potentially affecting pulmonary perfusion.

METHODS

Ischaemia-reperfusion was induced for various times in anesthetized rabbits with ventilated lungs by reversibly occluding the right pulmonary artery and initiating reperfusion. Nitric oxide synthase activity/expression and phosphorylation, reactive nitrogen species generation and total nitrate/nitrite were determined in lung tissue.

RESULTS

Inducible nitric oxide synthase expression and activity, and reactive nitrogen species formation coincided with increased pulmonary vascular resistance during reperfusion and increased with ischaemia duration, further increasing after 2-h reperfusion. Total nitrate/nitrite also increased with ischaemia but decreased after 2-h reperfusion. Pre-treatment with an inducible nitric oxide synthase inhibitor (1400W; Cayman Chemical Company, Ann Arbor, MI, USA) attenuated inducible nitric oxide synthase activity, reactive nitrogen species generation and pulmonary vascular resistance, but did not affect total nitrate/nitrite. Endothelial nitric oxide synthase expression was unchanged by ischaemia-reperfusion; however, its phosphorylation on serine 1177 and dephosphorylation on threonine 495 was uncoupled, suggesting decreased endothelial nitric oxide synthase activity. 1400W prevented uncoupling of endothelial nitric oxide synthase phosphorylation, maintaining its activity during reperfusion.

CONCLUSION

Ischaemia-reperfusion up-regulates inducible nitric oxide synthesis and/activity, which coincides with reduced endothelial nitric oxide synthase activity as suggested by its uncoupling and may contribute to ischaemia-reperfusion-induced pulmonary vasoconstriction.

The effects of anesthetic preconditioning with sevoflurane in an experimental lung autotransplant model in pigs

BACKGROUND

Ischemia-reperfusion lung injury is doubly important in thoracic surgery because of the associated ventilation damage to 1 lung. In this study we evaluated the cytoprotective effects of sevoflurane in a pulmonary autotransplant model in pigs.

METHODS

Twenty Large White pigs undergoing pneumonectomy plus lung autotransplant were divided into 2 10-member groups on the basis of the anesthetic received (propofol or sevoflurane). Proinflammatory mediators, oxidative stress, nitric oxide metabolism, and hemodynamic and blood variables were measured at 5 different time points.

RESULTS

There was an increase of oxidative stress markers and proinflammatory mediators in the propofol group, whereas the hemodynamic variables were similar in both groups.

CONCLUSIONS

We demonstrated that sevoflurane decreased the inflammatory response and oxidative stress in a live ischemia-reperfusion lung model.

Hypoxia-induced pulmonary hypertension: Different impact of iloprost, sildenafil, and nitric oxide

OBJECTIVES

Chronic alveolar hypoxia induces pulmonary hypertension, evident from elevated pulmonary artery pressure (PAP), pulmonary vascular resistance, right ventricular hypertrophy (RVH), and increased muscularization of the pulmonary vasculature. Additionally, the vasoconstrictor response to acute hypoxia (HPV) may be reduced in the remodeled vasculature. However, no direct comparison of different treatments on the various parameters characterizing pulmonary hypertension has been performed yet. Against this background, we compared the effects of inhaled NO, infused iloprost, a stable prostacyclin analogue, and oral sildenafil, a phosphodiesterase 5 inhibitor, on hypoxia-induced pulmonary hypertension.

METHODS

Exposure of rabbits to chronic hypoxia (FiO(2)=0.10) for 42 days. Treatment with infused iloprost, oral sildenafil, and inhaled nitric oxide.

RESULTS

We quantified PAP, pulmonary vascular resistance, RVH, vascular remodeling, vasoreactivity, and the strength of HPV. Chronic hypoxia resulted in an increase in (a) the right ventricle/(left ventricle+septum) ratio from 0.26+/-0.01 to 0.44+/-0.01, (b) PAP, and (c) the degree of muscularization from 14.0+/-4.0% to 43.5+/-5.3%. Treatment with iloprost and sildenafil, but not with NO, prevented the increase in muscularization. In contrast, RVH was strongly inhibited by sildenafil, whereas NO had some minor, and iloprost had no effect. Only iloprost reduced PAP compared to NO and sildenafil. The downregulation of HPV was abrogated only by NO.

CONCLUSION

We demonstrated (a) that the parameters characterizing hypoxia-induced pulmonary hypertension are not functionally linked, (b) that the downregulation of HPV under chronic hypoxia can be prevented by inhaled NO but not by sildenafil and iloprost, and (c) that iloprost is particularly effective in preventing vascular remodeling and sildenafil in preventing RVH.

Ventilation-perfusion relationships in pulmonary arterial hypertension: effect of intravenous and inhaled prostacyclin treatment

In seven patients with idiopathic or secondary pulmonary arterial hypertension (PAH), ventilation-perfusion (V (A)/Q ) relationships were measured during a right heart catheterization using the multiple inert-gas elimination technique before and during intravenous infusion with epoprostenol (EPO), and following 5 months of 20 microg inhaled iloprost taken three times daily (ILO). Pre-treatment pulmonary vascular resistance (PVR) was 9.3+/-5.0 mmHg/l/min and the dispersion of perfusion and ventilation for V (A)/Q -ratios was increased. EPO reduced PVR by 20%, and increased cardiac output, shunt, and mixed venous oxygenation (SV(O2)). The arterial oxygen tension (Pa(O2)) remained unchanged. Basal central haemodynamics did not change after 5 months of ILO. Fifteen minutes after ILO, PVR decreased by 20%, and the shunt, SV(O2), and Pa(O2) remained unaltered.

CONCLUSIONS

In secondary PAH with normal lung volumes, significant V (A)/Q mismatching occurred. The PVR was reduced to a similar degree during EPO and after ILO, but only EPO increased the shunt and SV(O2). EPO and ILO did not significantly affect the Pa(O2).

Toll-like receptor 4 mediates lung ischemia-reperfusion injury

BACKGROUND

We have previously reported that nuclear factor (NF)-kappaB activation and inflammatory cytokine expression were involved in the development of lung ischemia-reperfusion injury (LIRI). Because Toll-like receptor 4 (TLR4) activates NF-kappaB-dependent transcription of inflammatory cytokine genes during myocardial ischemia-reperfusion injury, we examined whether absence of TLR4 in TLR4-deficient mice protects against LIRI.

METHODS

Left lungs of wild-type (C57BL/6J) mice or TLR4-null (TLR4-/-) mice were made ischemic for 60 minutes and then reperfused for 180 minutes. Response to injury was quantified by tissue myeloperoxidase activity, vascular permeability ([125I]-bovine serum albumin extravasation), and leukocyte and inflammatory mediator accumulation in bronchoalveolar lavage expression. Lung homogenates were also analyzed for activation of mitogen-activated protein kinases and nuclear translocation of the transcription factors NF-kappaB and activator protein-1.

RESULTS

After LIRI, lungs from TLR4-/- mice demonstrated a 52.4% reduction in vascular permeability (p = 0.001), a 52.6% reduction in lung myeloperoxidase activity (p = 0.006), and a marked reduction in bronchoalveolar lavage leukocyte accumulation when compared with lungs from wild-type mice. The TLR4-/- mice lungs, subjected to LIRI, also demonstrated marked reductions in amounts of several proinflammatory cytokines/chemokines in bronchoalveolar lavage samples. Phosphorylation of c-Jun NH2-terminal kinase, and activation of NF-kappaB and activator protein-1 were also significantly reduced in homogenates of lungs from TLR4-/- mice injured by ischemia and reperfusion (p < 0.05).

CONCLUSIONS

These data suggest that TLR4 plays a role in LIRI. Thus, TLR4 may be a potential therapeutic target to minimize ischemic-reperfusion-induced tissue damage and organ dysfunction.

Therapeutic Hypercapnia and Ventilation-Perfusion Matching in Acute Lung Injury

STUDY OBJECTIVES

Hypercapnic acidosis has antiinflammatory effects in animal models of acute lung injury (ALI) and improves ventilation-perfusion (V/Q) matching in normal lungs. The effect of hypercapnia on V/Q matching in ALI is conflicting. Hypercapnic acidosis produced by reduced tidal volumes (Vts) was associated with an increased shunt fraction (QS/QT) in patients with ALI compared with control subjects. Vt differences between groups make the assessment of hypercapnic acidosis on V/Q matching difficult. Adding CO2 to the inhaled gas allows the comparison of gas exchange under identical Vt conditions. We hypothesized the presence of hypercapnic acidosis from inspired carbon dioxide (ICD) would improve gas exchange in ALI and would be superior to that of low minute ventilation (LVe) produced by reduced respiratory rate, rather than Vt.

DESIGN

University laboratory study of anesthetized New Zealand White rabbits.

INTERVENTIONS

Assessment of V/Q relationships using the multiple inert gas elimination technique was performed in 10 saline solution-lavaged animals, which were ventilated with 6 mL/kg Vts and a positive end-expiratory pressure of 8 cm H2O. Each rabbit was studied while it was in eucapnia, followed by hypercapnia (Pa(CO2), 95 to 100 mm Hg) induced by LVe from decreased respiratory rate and by 10% ICD, in random order.

MEASUREMENTS AND RESULTS

The Pa(O2) was greater in ICD and LVe compared to eucapnia, but no significant differences in alveolar-arterial oxygen pressure difference or Pa(O2)/fraction of inspired oxygen ratio occurred. LVe statistically reduced the mean V/Q distributions compared with ICD and eucapnia. Log SDs of ventilation and combined retention and excretion curves of the dispersion index were both increased during LVe, indicating the presence of unfavorable changes in ventilation distribution. Neither LVe nor ICD altered the QS/QT.

CONCLUSIONS

LVe slightly impairs overall gas exchange and ventilation distribution, but does not increase QS/QT compared with eucapnia and ICD. While ICD does not significantly improve gas exchange, it may be superior to LVe in achieving the antiinflammatory effects of "therapeutic" hypercapnia, since it does not adversely alter gas exchange and has the potential to make the lung more uniformly acidotic.

Inhaled Nitric Oxide Does Not Prevent Pulmonary Edema After Lung Transplantation Measured By Lung Water Content

STUDY OBJECTIVE

In order to assess the effects of inhaled nitric oxide (iNO) in preventing early-onset lung edema from occurring after lung transplantation, we measured extravascular lung water (EVLW) in a group of lung transplant recipients who were at high risk for developing ischemia-reperfusion-induced lung injury.

DESIGN

Prospective, randomized study.

SETTINGS

Surgical ICU in a teaching hospital.

PATIENTS

Thirty double-lung transplant recipients.

INTERVENTIONS

Patients were randomized to receive or not receive 20 ppm iNO at the time of reperfusion (ie, before any occurrence of lung edema). In the NO group, iNO was then administered for a 12-h period. A double-dilution technique was used for the serial assessment of EVLW, intrathoracic blood volume, and cardiac index. Standard hemodynamic and pulmonary parameters were also recorded during the first 3 postoperative days.

MEASUREMENTS AND RESULTS

Patients who received iNO did not have a different lung water content compared to control subjects (p = 0.61 [by analysis of variance (ANOVA)]). Blood oxygenation (ie, Pao(2)/fraction of inspired oxygen [Fio(2)] ratio) did not differ between the two groups (p = 0.61 [by ANOVA]). In both groups, EVLW and Pao(2)/Fio(2) ratio dropped significantly over time, regardless of the use of iNO (p < 0.01 [by ANOVA]).

CONCLUSIONS

In the population studied, prophylactic iNO that was administered at 20 ppm had no effect on pulmonary edema formation and resolution following lung transplantation.

Ultrastructural evaluation of the protective effect of nitroglycerin in preservation-reperfusion injury of rat lungs

AIM OF STUDY

Nitric oxide (NO) has been reported as a favorable protective supplement in donor lung preservation, but related ultrastructural studies are rare in the literature. This study was performed to assess the ultrastructural changes and to evaluate the protective effect of NO as donor nitroglycerin (NTG) treatment of ischemia-reperfusion injury in rat lungs.

MATERIALS AND METHODS

Fifteen Sprague-Dawley rats weighing 300 to 350 g were used in this study. The NTG group (n = 5) used intravenous administration followed by mixture in the University of Wisconsin (UW) solution. For the non-NTG group (n = 5), we injected the same amount of normal saline intravenously followed by admixture in the UW solution. The heart-lung blocks were removed, weighed, and kept in UW solution for 24 hours at 10 degrees C. Reperfusion using human blood diluted in Krebs-Hensleit solution was done for 60 minutes. For the control group (n = 5), we injected the same amount of normal saline intravenously, and removed the lungs with no preservation and reperfusion procedures.

RESULTS

The non-NTG group showed multiple patchy areas of alveolar collapse with marked swelling and destruction of type I epithelial cells, loss of type II cell surfactant granules, endothelial swelling and papillary projection, interstitial edema, and alveolar macrophages with active phagocytosis of the destroyed materials. The NTG group showed similar ultrastructural changes, but in a lesser severity compared with the non-NTG group.

CONCLUSION

Administration of the NTG reduced the ischemia-reperfusion injury in the rat donor lungs. Ultrastructural examination was an effective tool to evaluate the protective effect of NTG in ischemia-reperfusion procedures of donor lungs.

Nitric oxide post-transcriptionally up-regulates LPS-induced IL-8 expression through p38 MAPK activation

Nitric oxide (NO(.-)) contributes to vascular collapse in septic shock and regulates inflammation. Here, we demonstrate in lipopolysaccharide (LPS)-stimulated human THP-1 cells and monocytes that NO(.-) regulates interleukin (IL)-8 and tumor necrosis factor alpha (TNF-alpha) by distinct mechanisms. Dibutyryl-cyclic guanosine 5'-monophosphate (cGMP) failed to simulate NO(.-)-induced increases in TNF-alpha or IL-8 production. In contrast, dibutyryl-cyclic adenosine monophosphate blocked NO(.-)-induced production of TNF-alpha (P=0.009) but not IL-8. NO(.-) increased IL-8 (5.7-fold at 4 h; P=0.04) and TNF-alpha mRNA levels (2.2-fold at 4 h; P=0.037). However, nuclear run-on assays demonstrated that IL-8 transcription was slightly decreased by NO(.-) (P=0.08), and TNF-alpha was increased (P=0.012). Likewise, NO(.-) had no effect on IL-8 promoter activity (P=0.84) as measured by reporter gene assay. In THP-1 cells and human primary monocytes treated with actinomycin D, NO(.-) had no effect on TNF-alpha mRNA stability (P>0.3 for both cell types) but significantly stabilized IL-8 mRNA (P=0.001 for both cell types). Because of its role in mRNA stabilization, the p38 mitogen-activated protein kinase (MAPK) pathway was examined and found to be activated by NO(.-) in LPS-treated THP-1 cells and human monocytes. Further, SB202190, a p38 MAPK inhibitor, was shown to block NO(.-)-induced stabilization of IL-8 mRNA (P<0.02 for both cell types). Thus, NO(.-) regulates IL-8 but not TNF-alpha post-transcriptionally. IL-8 mRNA stabilization by NO(.-) is independent of cGMP and at least partially dependent on p38 MAPK activation.

Attenuation of Reperfusion-Induced Systemic Inflammation by Preconditioning With Nitric Oxide in an In Situ Porcine Model of Normothermic Lung Ischemia

STUDY OBJECTIVES

Inhalation of nitric oxide (NO) can ameliorate pulmonary ischemia/reperfusion (I/R) injury of the lung in several experimental models, but toxic effects of NO were also reported. Here we investigate whether NO inhalation for a short period prior to surgery is sufficient to prevent symptoms of lung I/R injury, especially the inflammatory response.

DESIGN

Using an in situ porcine lung model, normothermic left lung ischemia was maintained for 90 min, followed by a 5-h reperfusion period (group 1, n = 7). In group 2 (n = 6), I/R was preceded by inhalation of NO (10 min, 15 ppm). Animals in group 3 (n = 7) underwent sham surgery without NO inhalation or ischemia.

MEASUREMENTS

Oxygenation and hemodynamic parameters were measured as indicators of lung functional impairment. Plasma levels of interleukin (IL)-1beta, IL-6, and transforming growth factor (TGF)-beta1 were determined throughout the I/R maneuver. In addition, tissue macrophages were analyzed by lectin binding.

RESULTS

Symptoms of I/R injury (pulmonary hypertension and decreased oxygenation) in group 1 animals were attenuated by NO inhalation. The reperfusion-induced increases of the levels of IL-1beta and IL-6 in plasma were reduced by NO pretreatment. A peak of TGF-beta1 immediately after NO administration was observed in group 2, but not in groups 1 and 3. There was no significant effect of NO on tissue macrophages.

CONCLUSION

NO inhalation for a short period prior to lung I/R is sufficient to protect against pulmonary hypertension, impaired oxygenation, and the inflammatory response of pulmonary I/R injury.

Ischemia-reperfusion-induced lung injury

Ischemia-reperfusion-induced lung injury is characterized by nonspecific alveolar damage, lung edema, and hypoxemia occurring within 72 hours after lung transplantation. The most severe form may lead to primary graft failure and remains a significant cause of morbidity and mortality after lung transplantation. Over the past decade, better understanding of the mechanisms of ischemia-reperfusion injury, improvements in the technique of lung preservation, and the development of a new preservation solution specifically for the lung have been associated with a reduction in the incidence of primary graft failure from approximately 30 to 15% or less. Several strategies have also been introduced into clinical practice for the prevention and treatment of ischemia-reperfusion-induced lung injury with various degrees of success. However, only three randomized, double-blinded, placebo-controlled trials on ischemia-reperfusion-induced lung injury have been reported in the literature. In the future, the development of new agents and their application in prospective clinical trials are to be expected to prevent the occurrence of this potentially devastating complication and to further improve the success of lung transplantation.

Effects of nitrogen monoxide and carbon monoxide on molecular and cellular iron metabolism: mirror-image effector molecules that target iron

Many effector functions of nitrogen monoxide (NO) and carbon monoxide (CO) are mediated through their high-affinity for iron (Fe). In this review, the roles of NO and CO are examined in terms of their effects on the molecular and cellular mechanisms involved in Fe metabolism. Both NO and CO avidly form complexes with a plethora of Fe-containing molecules. The generation of NO and CO is mediated by the nitric oxide synthase and haem oxygenase (HO) families of enzymes respectively. The effects of NO on Fe metabolism have been well characterized, whereas knowledge of the effects of CO remains within its infancy. In terms of the role of NO in Fe metabolism, one of the best characterized interactions includes its effect on the iron regulatory proteins. These molecules are mRNA-binding proteins that control the expression of the transferrin receptor 1 and ferritin, molecules that are involved in Fe uptake and storage respectively. Apart from this, activated macrophages impart their cytotoxic activity by generating NO, which results in marked Fe mobilization from tumour-cell targets. This deprives the cell of the Fe that is required for DNA synthesis and energy production. Considering that HO degrades haem, resulting in the release of CO, Fe(II) and biliverdin, it is suggested that a CO-Fe complex will form. This may account for the rapid Fe mobilization observed from macrophages after haemoglobin catabolism. Intriguingly, overexpression of HO results in cellular Fe mobilization, suggesting that CO has a similar effect to NO on Fe trafficking. Preliminary evidence suggests that, like NO, CO plays important roles in Fe metabolism.

Nitric oxide mediates lung injury induced by ischemia-reperfusion in rats

Nitric oxide (NO) has been reported to play a role in lung injury (LI) induced by ischemia-reperfusion (I/R). However, controversy exists as to the potential beneficial or detrimental effect of NO. In the present study, an in situ, perfused rat lung model was used to study the possible role of NO in the LI induced by I/R. The filtration coefficient (Kfc), lung weight gain (LWG), protein concentration in the bronchoalveolar lavage (PCBAL), and pulmonary arterial pressure (PAP) were measured to evaluate the degree of pulmonary hypertension and LI. I/R resulted in increased Kfc, LWG, and PCBAL. These changes were exacerbated by inhalation of NO (20-30 ppm) or 4 mM L-arginine, an NO precursor. The permeability increase and LI caused by I/R could be blocked by exposure to 5 mM N omega-nitro-L-arginine methyl ester (L-NAME; a nonspecific NO synthase inhibitor), and this protective effect of L-NAME was reversed with NO inhalation. Inhaled NO prevented the increase in PAP caused by I/R, while L-arginine had no such effect. L-NAME tended to diminish the I/R-induced elevation in PAP, but the suppression was not statistically significant when compared to the values in the I/R group. These results indicate that I/R increases Kfc and promotes alveolar edema by stimulating endogenous NO synthesis. Exogenous NO, either generated from L-arginine or delivered into the airway, is apparently also injurious to the lung following I/R.

Apical, but not basolateral, endotoxin preincubation protects alveolar epithelial cells against hydrogen peroxide-induced loss of barrier function: the role of nitric oxide synthesis

The influence of LPS preincubation on hydrogen peroxide (H(2)O(2))-induced loss of epithelial barrier function was investigated in rat alveolar epithelial type II cells (ATII). Both apical and basolateral H(2)O(2) administration caused a manyfold increase in transepithelial [(3)H]mannitol passage. Apical but not basolateral preincubation of ATII with LPS did not influence control barrier properties but fully abrogated the H(2)O(2)-induced leakage response. The effect of apical LPS was CD14 dependent and was accompanied by a strong up-regulation of NO synthase II mRNA and protein and NO release. Inhibition of NO by N(G)-monomethyl-L-arginine suppressed the LPS effect, whereas it was reproduced by exogenous application of gaseous NO or NO donor agents. Manipulation of the glutathione homeostasis (buthionine-(S,R)-sulfoximine) and the cGMP pathway (1H-(1,2,4)oxadiazolo[4,3-alpha]quinoxaline-1-one; zaprinast) did not interfere with the protective effect of LPS. Superoxide (O*(-)(2)) generation by ATII cells was reduced by exogenous NO and LPS preincubation. O*(-)(2) scavenging with exogenous superoxide dismutase, the intracellular superoxide dismutase analog Mn(III)tetrakis(4-benzoic acid) porphyrin, and the superoxide scavenger nitroblue tetrazolium and, in particular, hydroxyl radical scavenging with hydroxyl radical scavenger 1,3-dimethyl-thiourea inhibited the H(2)O(2)-induced epithelial leakage response. In conclusion, apical but not basolateral LPS preincubation of ATII cells provides strong protection against H(2)O(2)-induced transepithelial leakage, attributable to an up-regulation of epithelial NO synthesis. It is suggested that the LPS-induced NO formation is effective via interaction with reactive oxygen species, including superoxide and hydroxyl radicals. The polarized epithelial response to LPS may be part of the lung innate immune system, activated by inhaled endotoxin or under conditions of pneumonia.

Intratidal compliance-volume curve as an alternative basis to adjust positive end-expiratory pressure: a study in isolated perfused rabbit lungs

OBJECTIVE

Repeated collapse and reopening of alveoli have been shown to aggravate lung injury, which could be prevented by positive end-expiratory pressure (PEEP). Yet, how to adjust optimum PEEP is a matter of debate. We suggest a new strategy to adjust PEEP, which is based on the analysis of the intratidal compliance-volume curve. This approach was compared with a strategy based on the static pressure-volume curve. Furthermore, two other ventilator settings were investigated. One served as a negative control likely to provoke atelectasis, and the other was expected to induce overdistension.

DESIGN

Prospective, randomized block design.

SETTING

Laboratory.

SUBJECTS

Isolated, perfused, and ventilated rabbit lungs.

INTERVENTIONS

Tidal volumes of 8 mL/kg of body weight were used throughout. After stabilization, the lungs were randomized to one of four protocols (lasting 120 mins; n = 6 per group). Group 1 was ventilated at zero end-expiratory pressure. In group 2, PEEP was set above the lower inflection point of the static pressure-volume curve. In group 3, adjustment of PEEP was based on the intratidal compliance-volume curve, as determined by the slice method. In group 4, increasing PEEP levels ensured a plateau airway pressure of 20-25 cm H2O likely to provoke overdistension.

MEASUREMENTS AND MAIN RESULTS

The ventilation/perfusion (VA/Q) distribution was analyzed by the multiple inert gas elimination technique. Alveolar derecruitment was indicated by shunt and low VA/Q areas as observed in group 1. In groups 2 and 3, VA/Q data initially indicated full recruitment. In contrast to group 3, shunt increased in group 2 near completion of the experiments. Group 4 showed complete recruitment, but the VA/Q distribution included high VA/Q areas.

CONCLUSIONS

The intratidal compliance-volume curve represents a rational basis for adjusting PEEP in the isolated lung model. Because this strategy does not require invasive measures and facilitates continuous assessment of ventilator settings, it may be of clinical interest.

Different strategies to keep the lung open: a study in isolated perfused rabbit lungs

OBJECTIVE

Atelectatic alveoli can be recruited or kept open either by sustained inflation maneuvers or by positive end-expiratory pressure (PEEP). Little is known about potential interactions between both approaches. Especially, it is not known whether the recruiting effect of sustained inflation maneuvers is maintained in combination with a low PEEP, as suggested recently. In an attempt to answer this question, we combined sustained inflation maneuvers with either high or low PEEP. Both approaches were compared with a strategy likely to result in alveolar atelectasis and with another ensuring adequate alveolar recruitment by adjustment of PEEP alone.

DESIGN

Randomized block design.

SETTING

Laboratory.

SUBJECTS

Isolated perfused rabbit lungs (n = 28).

INTERVENTIONS

The lungs were ventilated with a tidal volume of 8 mL/kg. After stabilization, the lungs were randomized to one of four ventilatory strategies, which then were followed for 120 mins: a) PEEP 1 cm H2O (PEEP1, negative control); b) PEEP 1 cm H2O and 30 sec-sustained inflations (20 cm H2O) every 30 mins (SI-1); c) PEEP 3 cm H2O combined with sustained inflations (SI-3); and d) PEEP repeatedly adjusted following a previously established strategy ensuring full alveolar recruitment (DYN, positive control).

MEASUREMENTS AND MAIN RESULTS

Distribution of ventilation and perfusion (Va/Q distribution) was analyzed by the multiple inert gas elimination technique. Volume-dependent compliance within the tidal volume was determined by using the slice method. Shunt and Va/Q mismatch significantly differed between SI-1 and SI-3, indicating full alveolar recruitment only in the latter. Data of SI-1 did not differ substantially from those of PEEP1, and data obtained in SI-3 were similar to those of DYN.

CONCLUSIONS

First, enduring alveolar recruitment by sustained inflation maneuvers is only possible when the alveoli are stabilized thereafter by sufficient PEEP. Second, a ventilation strategy that uses repeated sustained inflations on a comparably high PEEP may not be superior to adequate adjustment of PEEP alone.

Inhaled NO and prostacyclin during porcine single lung transplantation

BACKGROUND

Increased pulmonary vascular resistance (PVR) and decreased arterial oxygenation frequently complicate lung transplantation. Inhaled nitric oxide (NO) and aerosolized prostacyclin (PGI2) both dilate the pulmonary vasculature and improve oxygenation in adult respiratory distress syndrome. We investigated whether similar effects would occur during early reperfusion of a lung graft.

METHODS

Eighteen pigs underwent left lung transplantation. We measured blood flow distribution, mean pulmonary artery pressure, PVR, and gas exchange in each lung separately. Animals were randomized into three groups to receive NO (10 ppm/30 minutes, 40 ppm/30 minutes), nebulized PGI2 (25 microg/mL/30 minutes, 50 microg/mL/30 minutes), or no drugs (control).

RESULTS

In the transplanted lung, PVR was significantly higher than in the native lung. Pulmonary vascular resistance of the transplanted lung was lower in the NO and PGI2 groups in comparison with the control group. During the first hour of inhalation, NO decreased PVR more than PGI2. Neither drug improved oxygenation in the graft.

CONCLUSIONS

Nitric oxide and PGI2 decreased PVR of the transplanted lung slightly, but the effect did not produce a normal pressure in pulmonary vasculature.

Short-term "preconditioning" with inhaled nitric oxide protects rabbit lungs against ischemia-reperfusion injury

BACKGROUND

Pulmonary edema, owing to an impairment of microvascular barrier function, is an important feature in lung ischemia/reperfusion (IR) injury. Inhalation of nitric oxide (NO) during the period of reperfusion has previously been shown to reduce this leakage response.

METHODS

We investigated the impact of short-term (30 min) low-dose (10 ppm) pre-ischemic NO inhalation on IR injury in buffer-perfused rabbit lungs, subsequently undergoing 210 min of warm, anoxic-ventilated ischemia.

RESULTS

Far-reaching suppression of the leakage response, reflected by manifold increased capillary filtration coefficients and edema formation, was noted in lungs with pre-ischemic NO administration, corresponding to the beneficial effect of NO inhalation during reperfusion. The effect of NO pre-exposure was not related to vasodilation, because microvascular pressures were unchanged, and was mimicked by pre-ischemic intravascular administration of sodium nitroprusside with subsequent washout of this agent. NO inhalation during reperfusion, but not pre-ischemic, short-term NO administration, provoked a manifold increase in the accumulation of guanosine 3',5'-cyclic monophosphate (cGMP) in the perfusate. The cGMP-analogue, 8-Br-cGMP, mimicked the anti-edematous effect of NO when present during reperfusion, but pre-ischemic, short-term administration of 8-Br-cGMP provided only limited protection. The guanylate cyclase-inhibitor, 1H-[1, 2, 4]-Oxadiazolo-[4,3-a]-quinoxalin-1-one (ODQ), largely antagonized the beneficial effects of NO inhalation during reperfusion but had only minor influence on the effect of NO pre-exposure.

CONCLUSIONS

"Preconditioning" of the lung vasculature with short-term NO administration maintains endothelial integrity in a subsequent ischemia/reperfusion maneuver, with nonvasodilatory and non-cGMP-related mechanisms suggested to be largely responsible. This finding may offer interesting perspectives for donor management in clinical lung transplantation.

Endogenous nitric oxide synthesis and vascular leakage in ischemic-reperfused rabbit lungs

Pulmonary edema formation resulting from loss of capillary barrier properties is a prominent finding in lung ischemia/reperfusion (I/R) injury. The role of endogenous nitric oxide (NO) in this process is unresolved. We exposed buffer-perfused rabbit lungs to warm I/R and measured air space NO liberation and intravascular accumulation of NO degradation products. In lungs undergoing 210 min of ischemia with normoxic ventilation, with maintenance of positive intravascular pressure to avoid vascular collapse, NO synthesis was moderately reduced during ischemia but was fully restored upon reperfusion, and a moderate leakage response occurred during reperfusion. Pretreatment with the NO synthase inhibitor N(G)-monomethyl-L-arginine (L-NMMA) suppressed NO synthesis but did not affect the leakage. During ischemia with anoxic ventilation, NO synthesis was fully abrogated, but again promptly reappeared upon reperfusion and entrance of oxygen into the system. It was with this protocol that the most severe vascular leakage was encountered, which was markedly reduced in the presence of L-NMMA or superoxide dismutase. We conclude that endogenous NO does not play a major role in the induction or mitigation of I/R injury under conditions of normoxic ischemia, but that return of endogenous NO synthesis upon reperfusion after anoxic ischemia contributes substantially to the triggering of vascular leakage, possibly via interaction with superoxide.

Nitroglycerin alters alveolar type II cell ultrastructure after ischemia and reperfusion

BACKGROUND

Although administration of nitric oxide (NO) has been suggested to reduce pulmonary reimplantation response, concerns remain about cytotoxic side effects.

METHODS

Using light and electron microscopy, we examined the effects of the NO donor nitroglycerin (NTG) (0.1 mg/ml) as a supplement to the preservation solution Celsior on the structural integrity of rat lungs after extracorporeal ischemia (4 hours at 10 degrees C) and reperfusion (50 minutes) (IR). We performed evaluation in comparison with Celsior alone after IR using either standard antegrade perfusion through the pulmonary artery or retrograde perfusion through the left atrium as an alternative way to improve the preservation quality. Untreated, non-ischemic lungs served as controls (n = 5 per group). We recorded respiratory and hemodynamic parameters during reperfusion. Tissue collection using systematic uniform random sampling was representative for the whole organ and allowed stereologic quantification of structures.

RESULTS

After IR, histochemistry revealed no breaks in the alveolo-capillary barrier and we detected no alveolar flooding. Edema formed in the peribronchovascular cuffs, of which the volume fraction was increased (p =.008). Vasoconstriction of the smaller arteries accompanied antegrade flush, which occurred neither after administration of NTG nor after retrograde flush, as shown by immunostaining for alpha-smooth muscle actin. Treatment with NTG was associated with focal disintegration of Type II cells, which displayed edematous swelling of distinct cell compartments and lysis of mitochondria and cells. Nitroglycerin prevented alveolar collapse, which was increased in the other IR groups (p = 0.013). We observed alterations in intra-alveolar surfactant components.

CONCLUSION

These findings indicate pathologic effects of NTG treatment on alveolar epithelial integrity. Therefore, we suggest further critical evaluation of NTG/NO for therapeutic use in lung transplantation.

Aerosolized PGE1, PGI2 and nitroprusside protect against vascular leakage in lung ischaemia-reperfusion

High permeability oedema is an important feature in lung injury secondary to ischaemia-reperfusion. This study investigated the influence of aerosolized prostaglandin E1 (PGE1), prostaglandin I2 (PCI2) and the nitric oxide (NO)-donor, sodium nitroprusside (SNP) on microvascular barrier function in pulmonary ischaemia-reperfusion. Buffer-perfused rabbit lungs were exposed to 180 or 210 min of warm ischaemia while maintaining anoxic ventilation and a positive intravascular pressure. Reperfusion provoked a transient, mostly precapillary elevation of vascular resistance, followed by a severe increase of the capillary filtration coefficient (Kfc) versus nonischaemic controls (3.17+/-0.34 versus 0.85+/-0.05 cm3 x s(-1) cmH2O(-1) x g(-1) x 10(-4) after 30 min of reperfusion), and progressive oedema formation. Short-term aerosolization of SNP, PGE1 or PGI2 at the beginning of ischaemia largely suppressed the Kfc increase (1.36+/-0.22, 1.32+/-0.23 and 1.32+/-0.22 cm3 x s(-1) x cmH2O(-1) x g(-1) x 10(-4), respectively) and oedema formation. In contrast, application prior to reperfusion was much less effective, with some reduction of Kfc increase by PGI2 and SNP and no effect of PGE, (1.79+/-0.31, 2.2+/-0.53 and 3.2+/-0.05 cm3 x s(-1) x cmH2O(-1) x g(-1) x 10(-4), respectively). Haemodynamics, including microvascular pressure, were only marginally affected by the chosen doses of aerosolized vasodilators. It is concluded that short-term aerosolization of prostaglandin E1, prostaglandin I2 and sodium nitroprusside at the onset of ischaemia is highly effective in maintaining endothelial barrier properties in pulmonary ischaemia-reperfusion. This effect is apparently attributable to nonvasodilatory mechanisms exerted by these agents. Alveolar deposition of prostaglandins and/or nitric oxide donors by the aerosol technique may offer pulmonary protection in ischaemia-reperfusion injury.

Preventive effect of inhaled nitric oxide and pentoxifylline on ischemia/reperfusion injury after lung transplantation

BACKGROUND

The preventive effect of inhaled nitric oxide (NO) and pentoxifylline (PTX) administered during reperfusion has been demonstrated on experimental models of lung ischemia/reperfusion (I/R) injury but this strategy is not validated in clinical lung transplantation. The aim of this study was to assess retrospectively the protective effect of inhaled NO and PTX after lung transplantation.

METHODS

Twenty-three consecutive patients who received inhaled NO (10 ppm) and PTX (NO-PTX group) at the time of reperfusion were compared retrospectively with (1) 23 consecutive patients transplanted just before the use of NO-PTX (control group 23); (2) 95 patients representing all the patients of the series who did not receive NO-PTX (control group 95), with respect to I/R injury related complications. In particular, the incidence of pulmonary reimplantation edema and early hemodynamic failure, the PaO2/FIO2 ratio as well as the duration of mechanical ventilation and the 2-month mortality rates were compared.

RESULTS

Reimplantation edema was observed in 6/23 patients (26%) in the NO-PTX group vs. 13/23 patients (56%) in the control group 23 (P=0.035) and 48/95 patients (50%) in the control group 95 (P=0.035). The worst PaO2/FIO2 ratio during the first three postoperative days was 240-102 mmHg in the NO-PTX group vs. 162+/-88 mmHg (P=0.01) and 176+/-107 mmHg (P=0.01) in the control group 23 and the control group 95, respectively. The duration of mechanical ventilation was 2.1+/-2.4 days in the NO-PTX group vs. 7+/-9 days in the control group 23 (P=0.02) and 6+/-7 days in the control group 95 (P=0.01). The 2-month mortality rate was 4.3% in the NO-PTX group vs. 26% (P=0.04) and 21% (P=0.07) in the control group 23 and the control group 95, respectively.

CONCLUSIONS

The marked decrease in the incidence of allograft dysfunction compared with two historical control groups suggests that PTX and inhaled NO given before and throughout reperfusion are protective against I/R injury in the setting of clinical transplantation.

Protection against gas exchange abnormalities by pre-aerosolized PGE1, iloprost and nitroprusside in lung ischemia-reperfusion

BACKGROUND

Development of severe gas exchange abnormalities and respiratory failure is a major threat in lung transplantation.

METHODS

We used a model of ischemia-reperfusion injury in buffer-perfused rabbit lungs, with gas exchange conditions being analyzed in detail by the multiple inert gas elimination technique. A total of 150 min of warm ischemia was performed, and anoxic ventilation and a positive intravascular pressure were maintained throughout the ischemic period.

RESULTS

Reperfusion provoked a transient, mostly precapillary pulmonary artery pressure elevation and progressive lung edema formation attributable to increased capillary permeability. Severe ventilation-perfusion mismatch with predominance of shunt flow became apparent within minutes after onset of reperfusion. 5 min-aerosolization maneuvers for alveolar deposition of prostaglandin E1, the long-acting prostacyclin analogue iloprost or the nitric oxide donor agent sodium nitroprusside were undertaken at the onset of ischemia. All preaerosolized vasodilator agents markedly reduced the pulmonary artery pressure elevation and the leakage response upon reperfusion. Most impressively, maintenance of physiological ventilation-perfusion matching was achieved by these maneuvers, and the development of shunt flow was largely suppressed.

CONCLUSIONS

Preischemic alveolar deposition of PGE1, iloprost, and sodium nitroprusside by aerosol technique is highly effective in conserving normal pulmonary hemodynamics, microvascular integrity, and physiological gas exchange conditions upon reperfusion. This approach may offer as new strategy for maintenace of pulmonary function in lung transplantation.

Volume-dependent compliance and ventilation-perfusion mismatch in surfactant-depleted isolated rabbit lungs

OBJECTIVE

Volume-dependent alterations of lung compliance are usually studied over a very large volume range. However, the course of compliance within the comparably small tidal volume (intratidal compliance-volume curve) may also provide relevant information about the impact of mechanical ventilation on pulmonary gas exchange. Consequently, we determined the association of the distribution of ventilation and perfusion with the intratidal compliance-volume curve after modification of positive end-expiratory pressure (PEEP).

DESIGN

Repeated measurements in randomized order.

SETTING

An animal laboratory.

SUBJECTS

Isolated perfused rabbit lungs (n = 14).

INTERVENTIONS

Surfactant was removed by bronchoalveolar lavage. The lungs were ventilated thereafter with a constant tidal volume (10 mL/kg body weight). Five levels of PEEP (0-4 cm H2O) were applied in random order for 20 mins each.

MEASUREMENTS AND MAIN RESULTS

The intratidal compliance-volume curve was determined with the slice method for each PEEP level. Concurrently, pulmonary gas exchange was assessed by the multiple inert gas elimination technique. At a PEEP of 0-1 cm H2O, the intratidal compliance-volume curve was formed a bow with downward concavity. At a PEEP of 2 cm H2O, concavity was minimal or compliance was almost constant, whereas higher PEEP levels (3-4 cm H2O) resulted in a decrease of compliance within tidal inflation. Pulmonary gas exchange did not differ between PEEP levels of of 0, 1, and 2 cm H2O. Pulmonary shunt was lowest and perfusion of alveoli with a normal ventilation-perfusion was highest at a PEEP of 3-4 cm H2O. Deadspace ventilation did not change significantly but tended to increase with PEEP.

CONCLUSIONS

An increase of compliance at the very beginning of tidal inflation was associated with impaired pulmonary gas exchange, indicating insufficient alveolar recruitment by the PEEP level. Consequently, the lowest PEEP level preventing alveolar atelectasis could be detected by analyzing the course of compliance within tidal volume without the need for total lung inflation.

The PDE inhibitor zaprinast enhances NO-mediated protection against vascular leakage in reperfused lungs

Disruption of endothelial barrier properties with development of noncardiogenic pulmonary edema is a major threat in lung ischemia-reperfusion (I/R) injury that occurs under conditions of lung transplantation. Inhaled nitric oxide (NO) reduced vascular leakage in lung I/R models, but the efficacy of this agent may be limited. We coadministered NO and zaprinast, a cGMP-specific phosphodiesterase inhibitor, to further augment the NO-cGMP axis. Isolated, buffer-perfused rabbit lungs were exposed to 4.5 h of warm ischemia. Reperfusion provoked a transient elevation in pulmonary arterial pressure and a negligible rise in microvascular pressure followed by a massive increase in the capillary filtration coefficient and severe lung edema formation. Inhalation of 10 parts/million of NO or intravascular application of 100 microM zaprinast on reperfusion both reduced pressor response and moderately attenuated vascular leakage. Combined administration of both agents induced no additional vasodilation at constant microvascular pressures, but additively protected against capillary leakage paralleled by a severalfold increase in perfusate cGMP levels. In conclusion, combining low-dose NO inhalation and phosphodiesterase inhibition may be suitable for the maintenance of graft function in lung transplantation by amplifying the beneficial effect of the NO-cGMP axis and avoiding toxic effects of high NO doses.