Lung transplantation with cardiopulmonary bypass exaggerates pulmonary vasomotor dysfunction in the transplanted lung

Intraoperative Extracorporeal Support during Lung Transplantation: Not Just for the High-Risk Patient

The use of intraoperative mechanical support during lung transplantation has traditionally been a controversial topic. Trends for intraoperative mechanical support strategies swing like a pendulum. Historically, cardiopulmonary bypass (CPB) was the modality of choice during transplantation. It provides full hemodynamic support including oxygenation and decarboxylation. Surgical exposure is improved by permitting the drainage of the heart and provides more permissive retraction. CPBs contain drainage reservoirs with hand-held pump suction catheters promoting blood conservation through collection and re-circulation. But CPB has its disadvantages. It is known to cause systemic inflammation and coagulopathy. CPB requires high doses of heparinization, which increases bleeding risks. As transplantation progressed, off-pump transplantation began to trend as a preferable option. ECMO, however, has many of the benefits of CPB with less of the risk. Outcomes were improved with ECMO compared to CPB. CPB has a higher blood transfusion requirement, a higher need for post-operative ECMO support, a higher re-intubation rate, high rates of kidney injury and need for hemodialysis, longer ICU stays, higher incidences of PGD grade 3, as well as overall in-hospital mortality when compared with ECMO use. The focus now shifts to using intraoperative mechanical support to protect the graft, helping to reduce ischemia-reperfusion injury and allowing for lung protective ventilator settings. Studies show that the routine use of ECMO during transplantation decreases the rate of primary graft dysfunction and many adverse outcomes including ventilator time, need for tracheostomy, renal failure, post-operative ECMO requirements, and others. As intraoperative planned ECMO is considered a safe and effective approach, with improved survival and better overall outcomes compared to both unplanned ECMO implementation and off-pump transplantation, its routine use should be taken into consideration as standard protocol.

Aspects of Anesthesia for Lung Transplantation

Anesthesia for lung transplantation is both a demand ing and rewarding experience. Success requires team- work, experience, knowledge of cardiorespiratory patho physiology and its anesthetic implications, appropriate use of noninvasive and invasive monitoring, and the ability to respond quickly and effectively to life- threatening perioperative events. Specific issues in clude management of a patient with end-stage lung and heart disease, lung isolation and one-lung ventilation, perioperative respiratory failure, pulmonary hyperten sion, and acute right ventricular failure. Recent ad vances include greater understanding of dynamic hyper inflation ("gas-trapping") during mechanical ventilation, perioperative use of inhaled nitric oxide and treatment of acute right ventricular failure. Successful anesthetic management leads to greater hemodynamic stability, improvement in gas exchange and a reduction in need for cardiopulmonary bypass, all of which should lead to improved patient outcome.

Mechanical circulatory support in lung transplantation: Cardiopulmonary bypass, extracorporeal life support, and ex-vivo lung perfusion

Lung transplant is the standard of care for patients with end-stage lung disease refractory to medical management. There is currently a critical organ shortage for lung transplantation with only 17% of offered organs being transplanted. Of those patients receiving a lung transplant, up to 25% will develop primary graft dysfunction, which is associated with an 8-fold increase in 30-d mortality. There are numerous mechanical lung assistance modalities that may be employed to help combat these challenges. We will discuss the use of mechanical lung assistance during lung transplantation, as a bridge to transplant, as a treatment for primary graft dysfunction, and finally as a means to remodel and evaluate organs deemed unsuitable for transplant, thus increasing the donor pool, improving survival to transplant, and improving overall patient survival.

Ultrafiltration attenuates cardiopulmonary bypass-induced acute lung injury in a canine model of single-lung transplantation

OBJECTIVE

The purpose of this study was to investigate the effects of cardiopulmonary bypass and ultrafiltration on graft function in a canine single-lung transplantation model.

METHODS

Fifteen left single-lung transplantations were done in weight-mismatched canine pairs. The animals were divided into 3 groups: group 1, in which transplantation was done without cardiopulmonary bypass; group 2, in which transplantation was done with cardiopulmonary bypass and in which the cardiopulmonary bypass flow was decreased slowly with controlled pulmonary artery pressure; and group 3, in which transplantation was done with cardiopulmonary bypass and ultrafiltration. Hemodynamic parameters and lung function were monitored for 6 hours after reperfusion. The grafts were harvested for histologic studies, myeloperoxidase assay, and real-time quantitative reverse transcription-polymerase chain reaction of mRNA encoding interleukin 6.

RESULTS

The hemodynamic parameters were similar among the 3 groups. In group 1 PaO2 and alveolar to arterial gradient for O2 levels were excellent throughout the 6-hour observation period, but in group 2 they progressively deteriorated. However, ultrafiltration significantly (P = .02) improved the PaO2 level in group 3. On histology, interstitial edema and polynuclear cell infiltration were most marked in group 2 and significantly worse than in groups 1 and 3. Myeloperoxidase assay and real-time quantitative reverse transcription-polymerase chain reaction showed increased myeloperoxidase activity and interleukin 6 gene expression in group 2 grafts compared with group 1 grafts. Myeloperoxidase activity and interleukin 6 gene expression were suppressed with ultrafiltration.

CONCLUSIONS

Cardiopulmonary bypass had negative effects on the graft, but ultrafiltration attenuated acute lung dysfunction by reducing the inflammatory response.

Use of an apical heart suction device for exposure in lung transplantation

Cardiopulmonary bypass is not necessary for the performance of bilateral sequential lung transplantation in most circumstances. Inadequate exposure to the left hilum is occasionally the sole indication for institution of cardiopulmonary bypass. We report the use of a suction heart positioning device to allow lifting of the heart for improvement of left hilar exposure. This technique has decreased the need for cardiopulmonary bypass when bypass is indicated due to difficult operative exposure.

Non-immune acute graft injury after lung transplantation and the risk of subsequent bronchiolitis obliterans syndrome (BOS)

BACKGROUND

Primary graft dysfunction remains a major cause of early morbidity and mortality after lung transplantation. Evidence from animal models shows acute non-immune lung injury increases organ immunogenicity by enhancing MHC Class II expression. We hypothesized that acute non-immune injury in the lung allograft may impact, not only on early survival, but also on longer term survival by increasing the incidence of bronchiolitis obliterans syndrome (BOS).

METHODS

A single-center, retrospective, observational study in a population of over 320 lung transplant recipients was undertaken. The histologic diagnosis of diffuse alveolar damage (DAD) in an early graft biopsy was used to define those recipients at risk. Serial measurements of forced expiratory volume in 1 second (FEV(1)) in long-term follow-up defined the incidence of BOS.

RESULTS

Early graft biopsy was available in 291 of the recipients following transplantation. DAD was confirmed in 55 (19%); their 30-day survival (62.5%) was significantly worse than in recipients without DAD (87.5%; p < 0.0001, chi-square test). When 30-day deaths were excluded, however, there was no difference in survival between recipients with and without DAD (hazards ratio 0.69 [0.37 to 1.3]; p = 0.25, Wilcoxon's survival analysis). The incidence of subsequent BOS over the follow-up period was not significantly different in those with and without DAD on early biopsy at 46% and 59%, respectively (hazards ratio 0.88 [0.48 to 1.62]; p = 0.22, chi-square test). BOS did not occur any earlier in the DAD group (median 953 days, range 152 to 1,393) days compared with the non-DAD group (median 665 days, range 52 to 4,299) (p = 0.48, Fisher's exact test).

CONCLUSIONS

The development of severe non-immune acute graft injury after lung transplantation has a poor early prognosis. However, recipients with non-immune acute graft injury who survive >30 days show no significant difference in long-term survival or BOS-free time compared with recipients without early injury.

Cardiopulmonary bypass for bilateral sequential lung transplantation in patients with chronic obstructive pulmonary disease without adverse effect on lung function or clinical outcome

OBJECTIVE

The use of cardiopulmonary bypass in lung transplantation remains controversial. Previous studies have concluded that cardiopulmonary bypass is deleterious, but these studies were confounded by the inclusion of patients with different diagnoses undergoing single- and double-lung transplantation with elective or emergency use of bypass. The goal of this study was to determine whether cardiopulmonary bypass has deleterious effects on lung function or clinical outcome by analyzing the cases of patients with a single disease entity and elective use of bypass for bilateral sequential lung transplantation.

METHODS

A retrospective review of 50 patients with chronic obstructive pulmonary disease who underwent bilateral sequential lung transplantation was performed. Fourteen patients who underwent elective cardiopulmonary bypass for 218.3 +/- 75.4 minutes were compared to 36 control patients.

RESULTS

After the operation, the bypass and nonbypass groups were not significantly different with respect to median duration of mechanical ventilation (1 day vs 1 day, P =.76), median stay in the intensive care unit (4 days vs 4 days, P =.44), median hospital stay (15.5 days vs 16 days, P =.74), mean increase in serum creatinine level (1.4 +/- 1.9 mg/dL vs 0.9 +/- 1.0 mg/dL, P =.33), and mean ratio of Pao(2) to fraction of inspired oxygen at 1 hour (376.6 +/- 123 vs 357.0 +/- 218, P =.75), at 24 hours (309.9 +/- 92 vs 350.6 +/- 122, P =.26), and at 48 hours (335.0 +/- 144 vs 316.2 +/- 120, P =.64). Late outcome markers compared between the bypass and nonbypass groups were the following: 1-year percentage predicted forced expiratory volume in 1 second (76.1% +/- 17.0% vs 85.3% +/- 21.7%, P =.24), 30-day mortality (7.1% vs 8.3%, P >.999), 1-year survival (85.7% vs 80.1%, P =.66), 3-year survival (64.3% vs 58.3%, P =.70), and the prevalence of bronchiolitis obliterans syndrome (0% vs 36.1%, P =.01).

CONCLUSION

Cardiopulmonary bypass appears to have no deleterious effect on early lung function or clinical outcome. We hope that this pilot study removes some of the unwarranted fear of the use of bypass in lung transplantation for chronic obstructive pulmonary disease.

Experimental study on size matching in a canine living-donor lobar lung transplant model

OBJECTIVE

In an effort to address the donor-shortage issue, living-donor lobar lung transplants have been performed with satisfactory results. The use of small grafts is potentially problematic because it could cause high pulmonary artery pressure, resulting in lung edema. The purpose of this study was to evaluate the effect of small grafts on early pulmonary function and to identify the predictor for acceptable size discrepancy.

METHODS

Seventeen pairs of mongrel dogs with various donor-recipient sizes were used. Body weight, height, chest circumference, and lung weight were measured as size parameters. Right middle, lower, and cardiac lobes were implanted as a right lung of the recipient and the left lower lobe was implanted as a left lung without use of cardiopulmonary bypass. Pulmonary function of the recipient was measured for 6 hours after reperfusion.

RESULTS

A significant negative correlation was found between donor/recipient body-weight ratio and mean pulmonary artery pressure at 1 hour (r = -0.594, P =.025). A significant correlation was found between donor/recipient body-weight ratio and PaO(2) at 6 hours (r = 0.704; P =.007). There was no significant correlation between the other 3 size parameters and postoperative pulmonary function. All 8 recipients with a donor/recipient body-weight ratio of 1.2 or greater survived the 6-hour assessment period, and their PaO(2) exceeded 500 mm Hg at 6 hours.

CONCLUSIONS

Donor/recipient weight ratio is an important predictor of early pulmonary function in a canine living-donor lobar lung transplant model.

Endothelins and the lung

Since endothelins were discovered by Yanasigawa in 1988 it has been recognised that they may have an important role in lung pathophysiology. Despite their biological importance as vasoconstrictors the physiological role of endothelin has not yet been defined within the lungs. This review explores their role in acute and chronic disease. During acute inflammation and ischaemia-reperfusion injury cytokines may induce release of endothelin. This is important in the realm of acute lung injury and during surgical procedures such as cardiopulmonary operations including lung resections and transplantation. Complications of surgery including primary organ failure resulting in poor gas exchange as well as increased pulmonary vascular resistance have been linked to the presence of excessive endothelin. Endothelin may have an important role in transplantation biology. The complex process leading to successful lung transplantation includes optimising the donor with brain death, harvesting the lungs, managing acute and chronic rejection, and protecting the vital organs from toxic effects of immunosuppressants. During chronic disease processes, the mitotic action of endothelin may be important in vascular and airway remodelling by means of smooth muscle cell proliferation. We also explore recent advances in drug development, animal models and future directions for research.

Combined lung and liver transplantation in a girl with cystic fibrosis

PURPOSE

To describe the anesthetic considerations of a combined lung and liver transplant in a 14-yr-old girl with cystic fibrosis.

CLINICAL FEATURES

A 14 yr-old girl with cystic fibrosis presented for combined liver and lung transplantation. Anesthetic management was complex in that the pulmonary, hemodynamic, and hematological changes after cardiopulmonary bypass and lung transplantation made the management of the subsequent liver transplant unique. We used a moderate dose fentanyl and isoflurane anesthetic with invasive monitoring including a pulmonary artery catheter. Upon reperfusion of the new liver our patient exhibited severe pulmonary hypertension that was associated with a decrease in cardiac output and systemic hypotension. Utilizing a pulmonary artery catheter, this episode was treated with an increase of prostaglandin E1 (PGE1) infusion to 0.025 microg x kg(-1) x min(-1) and the initiation of 3 microg x kg(-1) x min(-1) dobutamine. The pulmonary hypertension resolved and the cardiac output and blood pressure returned to baseline levels.

CONCLUSION

The anesthetic considerations for a combined lung and liver transplant are complex because of the interactions and alterations in cardiovascular, pulmonary and hemostatic systems. The use of a pulmonary artery catheter was critical to the management of our patient because it allowed us to accurately treat an episode of hypotension occurring during liver transplantation. This episode was secondary to acute pulmonary hypertension which is common after pulmonary transplantation but unusual during liver transplantation. It is also critical that a team approach is used to consider all of the concerns of the multiple services managing these complex patients.

Endothelial defect mediates attenuated vasorelaxant response to isoproterenol after lung transplantation

We have previously demonstrated that pulmonary vasodilation in response to isoproterenol is attenuated in conscious dogs after left lung autotransplantation (LLA). Our present goal was to identify the cellular mechanism responsible for this dysfunction. Size- and position-matched pulmonary arterial rings were isolated from the right (control) and left (LLA) lungs of 23 dogs 1-14 mo post-LLA. The rings were suspended for isometric tension recording and precontracted, and the vasorelaxant responses to activators of the beta-adrenoreceptor signaling pathway were examined. With the endothelium intact the maximal pulmonary vasorelaxant response to isoproterenol was reduced (P < 0.02) to 57 +/- 9% in LLA rings, compared with 87 +/- 3% in control rings. Responses to the Gs protein activator cholera toxin were also attenuated post-LLA, with the concentration-effect curve shifted to the right (P < 0.01) and no change in the maximal response. In contrast, the vasorelaxant responses to forskolin (adenylyl cyclase activator) or dibutyryl cAMP were similar in endothelium-intact control and LLA rings. In endothelium-denuded rings the maximal vasorelaxant responses to isoproterenol were reduced (P < 0.01) to approximately 25% in both control and LLA rings. In denuded rings cholera toxin, forskolin, and dibutyryl cAMP caused 100% vasorelaxation, and the IC50 values for these agonists were similar in control and LLA rings. Isoproterenol increased (P < 0.05) tissue cAMP to the same extent in control and LLA rings with or without endothelium. In contrast, isoproterenol increased (P < 0.05) tissue cGMP only in endothelium-intact rings, and this effect was reduced (P < 0.05) approximately 50% in LLA rings compared with control. Oxypurinol (endothelial xanthine oxidase inhibitor) restored the pulmonary vasorelaxant response to isoproterenol in endothelium-intact LLA rings. Our results provide the first evidence that activation of the beta-adrenoreceptor signaling pathway in endothelium-intact pulmonary arterial rings results in an increase in cGMP. Moreover, the attenuation in beta-adrenoreceptor-mediated pulmonary vasorelaxation post-LLA is due to inactivation of nitric oxide by endothelium-derived superoxide anion.

Acute and chronic effects of bilateral lung transplantation without cardiopulmonary bypass on the first transplanted lung

BACKGROUND

Bilateral lung transplantation (BLT) without cardiopulmonary bypass (CPB) may exacerbate reperfusion injury to the initially engrafted lung because of increases in pulmonary flow during implantation of the second graft.

METHODS

In a retrospective review of 23 BLT patients, we hypothesized that BLT without CPB injures the first transplanted lung measured by acute and late graft dysfunction compared to the second transplanted lung. Of the 23 BLT, 19 underwent transplantation without CPB while 4 patients were placed on CPB secondary to hemodynamic instability.

RESULTS

Acute graft function was assessed by radiographic scoring of lung quadrants (blinded radiologist; 0 = no infiltrate; 1 = infiltrate; maximum = 2 per lung) and by arterial/alveolar oxygen tension ratios (PaO2/ FiO2) ratios. Late graft function was evaluated by quantitative perfusion scan. Lung perfusion was graded as abnormal if less than 50% on the right or less than 45% on the left (Fisher's exact). Radiographic scores were not different between first and second implanted lungs at 1 and 24 hours, PaO2/FiO2 ratios at 1 and 24 hours were 273+/-26 and 312+/-23, respectively, and perfusion scans at 3 and 12 months revealed normal differential blood flow.

CONCLUSIONS

These findings suggest no acute or chronic differences occur between the first or second transplanted lung completed without CPB.

Perioperative management of patients undergoing lung transplantation

This review focuses on recent developments in the perioperative management of patients undergoing lung transplantation. Relevant current literature and the experience of the Munich Lung Transplant Group were taken into consideration. Recent advances include the use of inhalational nitric oxide for the treatment of early graft dysfunction and the use of aerosolized cyclosporine for the treatment of recurrent and steroid-resistant acute rejection. Opportunistic infections remain a major source of morbidity and mortality in lung transplant recipients.

Perioperative anaesthetic considerations for patients undergoing lung transplantation

PURPOSE

Five thousand, two hundred and eight lung transplants were performed worldwide before April, 1996. This review will discuss lung transplantation from an historical perspective, its indications, donor and recipient selection criteria, donor lung preparation, surgical considerations, perioperative anaesthetic management, and associated morbidity and mortality.

SOURCE

Recent literature on perioperative anaesthetic management of lung transplantation and experience from international centres including the Toronto Lung Transplant Group and the St. Louis Lung Transplant Group.

PRINCIPAL FINDINGS

Lung transplantation comprises of a family of operations, including single lung transplant, bilateral single lung transplant, lobar transplant and block heart-lung transplant. Improved donor lung preservation techniques have increased the duration of cold ischaemic time. The advent of bilateral single lung transplant has decreased the requirement for cardiopulmonary bypass, and airway complications have been reduced by adoption of the telescoping bronchial anastomoses. Advances in perioperative monitoring (including transoesophageal echocardiography), pulmonary vasodilators (e.g., nitric oxide and prostaglandin E1), cardiopulmonary bypass and ventilatory management, and a better understanding of the pathophysiological processes during the procedure have improved perioperative anaesthetic management. Also, advances in broad spectrum antibiotics and immunosuppressant drugs have improved the outcome by better management of the complications of infection and rejection.

CONCLUSION

Lung transplantation improves the quality of life with marginal improvement in life expectancy of the recipients. It is an expensive procedure requiring continued resources for long term management of these patients.

Inhaled nitric oxide: therapeutic applications in cardiothoracic surgery

Hypoxemia and increased pulmonary vascular resistance can greatly complicate the management of cardiothoracic surgical patients. These complications are commonly found in the setting of thoracic organ transplantation, adult and pediatric cardiac surgical procedures, and general thoracic surgical procedures. Inhaled nitric oxide is a new therapy that promises to be extremely valuable to the cardiothoracic surgeon. It has been shown to improve oxygenation in the setting of acute lung injury and to selectively lower pulmonary vascular resistance, without producing unwanted systemic vasodilation. The purpose of this review is to discuss the biochemistry, toxicity, experimental studies, and therapeutic applications of inhaled nitric oxide administration in cardiothoracic surgical patients.

Abnormal in vivo response to sodium nitroprusside after porcine single lung transplantation

The chronic increase of pulmonary vascular resistance after lung transplantation is only partly due to an active increase in baseline vasomotor tone, but the nature of the acute pulmonary hypertension after ischemia and reperfusion is not known. We studied the effects of sodium nitroprusside on pulmonary hemodynamics during reperfusion in porcine left lung allotransplants. In twelve pigs (weight: 18 to 24 kg) pulmonary arteries of the native and the transplanted lung were cannulated for right-heart bypass. The total blood flow was 2 L/min. Flow distribution between the lungs was measured at equal mean pulmonary artery pressure, and pulmonary vascular resistance at equal and constant flow-i.e., 1 L/min to each lung. After baseline measurements sodium nitroprusside (1, 3, and 9 microg/kg/min) was administered to six animals (SNP group). The control group (n=6) received an equal amount of the vehicle. After 30 min of discontinuation of the drug infusion, the schedule was repeated. In the transplanted lung, pulmonary vascular resistance decreased in all animals during the first hour of reperfusion. During the second drug infusion pulmonary vascular resistance was significantly lower in the SNP group compared with the control group only at the highest infusion rate of the drug (9 microg/kg/min), which also induced a 44% decrease in systemic vascular resistance. Arterial oxygen tension remained comparable in the two groups throughout the study. Our data suggest that other factors besides active vasoconstriction may contribute to the acute increase of pulmonary vascular resistance after lung transplantation.

Inhaled nitric oxide reduces human lung allograft dysfunction

OBJECTIVE

Early severe graft dysfunction, as manifested by hypoxia and pulmonary hypertension, occurs in 10% to 20% of lung transplant recipients. We retrospectively investigated whether inhaled nitric oxide would reduce human lung allograft dysfunction by comparing postoperative hemodynamic data, gas exchange, and outcome in lung transplant recipients with early graft dysfunction treated with or without nitric oxide.

METHOD

Among 243 adult lung transplant procedures, there were 32 patients (13.2%) in whom immediate severe allograft dysfunction developed (arterial oxygen tension/inspired oxygen concentration ratio <150). Group 1 (n = 17) included patients who underwent transplantation before nitric oxide became available in our center and were treated conventionally. Group 2 (n = 15) included those treated with nitric oxide as soon as severe allograft dysfunction was diagnosed. Duration of nitric oxide therapy (20 to 60 ppm) was 15 to 217 hours (average 84 hours).

RESULTS

In group 2, nitric oxide lowered mean pulmonary artery pressure from 30 +/- 2 to 26 +/- 2 mm Hg (p < 0.05), improved the ratio of arterial oxygen tension to inspired oxygen fraction from 88 +/- 10 to 153 +/- 30 (p < 0.05) within 1 hour, and caused a sustained improvement in these parameters during extended therapy. Mean arterial pressure and cardiac index were unchanged during nitric oxide therapy. Transient methemoglobinemia (>6%) developed in two patients. However, no complications were associated with nitric oxide use. Duration of mechanical ventilation was 17 +/- 5 days in group 1 and 12 +/- 3 days in group 2. Four patients had airway complications in group 1, whereas no airway complication was encountered in group 2. Mortality was 24% (4/17) in group 1 and 7% (1/15) in group 2.

CONCLUSION

Nitric oxide improves oxygenation and decreases pulmonary artery pressure without systemic circulatory effects in patients with severe allograft dysfunction. Furthermore, in these patients, nitric oxide may shorten postoperative mechanical ventilation time and reduce airway complications and mortality.

Pulmonary vasomotor dysfunction is produced with chronically high pulmonary blood flow

This study examined the hypothesis that chronic high pulmonary blood flow produces dysfunction of the mechanisms of pulmonary vasorelaxation. A 3:1 left-to-right shunt was created in dogs by bilateral femoral artery-femoral vein shunts with use of 6 mm polytetrafluoroethylene grafts. Isolated pulmonary artery rings were studied at the following times: 3 days (n = 2), 2 weeks (n = 4), and 5 months (n = 6). Control animals had no shunt. The following mechanisms of pulmonary vasorelaxation were studied in isolated pulmonary artery rings (4 rings from each dog): (1) endothelium-dependent cyclic guanosine monophosphate-mediated relaxation (response to acetylcholine), (2) endothelium-independent cyclic guanosine monophosphate-mediated relaxation (response to sodium nitroprusside), and (3) beta-adrenergic cyclic adenosine monophosphate-mediated relaxation (response to isoproterenol). Statistical analysis was done by analysis of variance. This model of high pulmonary flow did not produce an increase in pulmonary arterial pressure or transpulmonary gradient. However, chronic high pulmonary flow produced progressive dysfunction of all three of these mechanisms of pulmonary vasorelaxation. By 5 months of high pulmonary flow, acetylcholine produced only 36% +/- 6% relaxation versus 95% +/- 5% in control animals (p < 0.05). Likewise, sodium nitroprusside produced only 69% +/- 6% relaxation versus 100% in control animals (p < 0.05). Finally, isoproterenol produced only 55% +/- 5% relaxation versus 94% +/- 6% in control animals (p < 0.05). We conclude that dysfunction of the mechanisms of pulmonary vasorelaxation may contribute to exaggerated perioperative pulmonary vasoconstriction in the setting of chronic high pulmonary blood flow.