(Circulation. 1997;95:1054-1061.)
© 1997 American Heart Association, Inc.
Articles |
Post–Cardiopulmonary Bypass Pulmonary Hypertension in Lambs With Increased Pulmonary Blood Flow
A Role for Endothelin 1
the Departments of Cardiothoracic Surgery (V.M.R., H.A.R., E.P., F.L.H.) and Pediatrics (J.R.F.), University of California, San Francisco, and the Department of Pediatrics (K.D.H.-M.), New York University.
Correspondence to Jeffrey R. Fineman, MD, University of California, San Francisco, 505 Parnassus Ave, Box 0106, M-680, San Francisco, CA 94143-0106.
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Background After cardiopulmonary bypass (CPB), pulmonary hypertension and its associated increased vascular reactivity are a major source of morbidity, particularly for children with increased pulmonary blood flow. Although post-CPB pulmonary hypertension is well described, its mechanisms remain incompletely understood. Plasma levels of endothelin 1, a potent vasoactive substance implicated in pulmonary hypertension, are increased after CPB. The purpose of the present study was threefold: to characterize the changes in pulmonary vascular resistance and vascular reactivity induced by hypothermic CPB; to investigate the effects of preexisting increased pulmonary blood flow on these changes; and to better define the role of endothelin 1 in the pathogenesis of post-CPB pulmonary hypertension.
Methods and Results Vascular pressures and blood flows were monitored in 14 1-month-old lambs with increased pulmonary blood flow (after in utero placement of an aortopulmonary shunt) and 6 age-matched control lambs. During the 2-hour study period after 105.3±20.6 minutes of hypothermic CPB, the increase in pulmonary vascular resistance was significantly augmented in lambs with increased pulmonary blood flow compared with control lambs (P<.05). Pretreatment with PD 145065 (a nonselective endothelin receptor blocker; 50 µg·kg-1·min-1) completely blocked this increase in pulmonary vascular resistance and blocked the increase in pulmonary vascular resistance in response to acute alveolar hypoxia after CPB (96.3±88.5% versus -9.7±16.4%; P<.05). Plasma endothelin 1 levels increased after CPB in all lambs.
Conclusions Preexisting increased pulmonary blood flow alters the response of the pulmonary circulation to hypothermic CPB; the increase in pulmonary vascular resistance induced by CPB is augmented in lambs with increased pulmonary blood flow. Pretreatment with endothelin 1 receptor blockers eliminated the increase in pulmonary vascular resistance and the pulmonary vasoconstricting response to alveolar hypoxia, suggesting a role for endothelin 1 in post-CPB pulmonary hypertension. Endothelin 1 receptor blockers may decrease morbidity in children at risk for pulmonary hypertension after surgical repair with CPB and warrants further study.
Key Words: hypertension, pulmonary • cardiopulmonary bypass • endothelin
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Introduction |
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The development of pulmonary hypertension and its associated increased vascular reactivity is a common accompaniment of congenital heart disease with increased pulmonary blood flow.1 Although early surgical repair of these congenital heart defects has decreased the incidence of irreversible pulmonary vascular disease, those children with reversible vascular changes suffer morbidity and mortality in the postoperative period secondary to elevations in pulmonary vascular resistance and increased pulmonary vascular reactivity immediately after cardiopulmonary bypass (CPB).2 Although increases in pulmonary vascular resistance and pulmonary vascular reactivity after CPB are well described, their mechanisms are incompletely understood.3 Recent evidence4 suggests that pulmonary vascular tone is regulated by a complex interaction of vasoactive substances that are locally produced by the vascular endothelium. Endothelial injury secondary to increased pulmonary blood flow disturbs its regulatory mechanisms and may contribute to the development of pulmonary hypertensive disorders.5 6 During CPB, further pulmonary vascular endothelial injury secondary to a variety of factors, including the disruption of normal pulmonary blood flow, complement activation, and neutrophil activation, may contribute to post-CPB pulmonary hypertension.3 7 8 9 10
Endothelin 1 (ET-1) is a 21–amino acid polypeptide produced by vascular endothelial cells, the potent vasoactive properties of which have been implicated in the pathophysiology of pulmonary hypertensive disorders.11 The vasoactive properties of ET-1 are complex, and studies have shown various hemodynamic effects on different vascular beds. The hemodynamic effects of ET-1 are mediated by at least two distinctive receptor populations, ETa and ETb, the densities of which are different depending on the vascular bed studied.12 The ETa receptors and a subpopulation of ETb receptors mediate vasoconstriction and are located on vascular smooth muscle cells. A second subpopulation of ETb receptors mediates vasodilatation and is located on vascular endothelial cells.12 In patients with congenital heart disease and pulmonary hypertension, plasma concentrations of ET-1 are increased immediately after CPB.13 In addition, in normal piglets, inhibition of endothelin-converting enzyme 1, the enzyme that converts proendothelin into its functional form, attenuates post-CPB pulmonary hypertension.14 Therefore, alterations in ET-1 induced during CPB may be responsible in part for the increased pulmonary vascular resistance and increased vascular reactivity noted in children immediately after open heart surgery.
Recently, we established a model of pulmonary hypertension with increased pulmonary blood flow in the lamb after in utero placement of an aorta-to-pulmonary vascular graft. At 1 month of age, these lambs (shunted lambs) have a pulmonary-to–systemic blood flow ratio of 
2:1, a mean pulmonary arterial pressure that is 75% of mean systemic arterial pressure, and pulmonary vascular remodeling characteristic of children with pulmonary hypertension and increased pulmonary blood flow.15 In addition, these lambs have alterations in endothelial function, which include increased plasma concentrations of ET-1 and increased ET-1–induced pulmonary vasoconstriction.16
The purposes of the present study were to characterize the changes in pulmonary vascular resistance and reactivity induced by hypothermic CPB and to investigate the following questions: (1) What is the effect of preexisting increased pulmonary blood flow on the pulmonary circulatory response to CPB? and (2) What is the role of ET-1 in the pulmonary circulatory response to CPB? To characterize the changes in pulmonary vascular resistance induced by CPB and the potential effects of preexisting increased pulmonary blood flow, we determined and compared the hemodynamic responses of the pulmonary circulation of 6 1-month-old normal lambs and 7 1-month-old shunted lambs to 90 minutes of hypothermic CPB. To characterize the changes in pulmonary vascular reactivity induced by CPB, we determined the effects of acute alveolar hypoxia before and after CPB in these shunted lambs. To assess the role of ET-1 in post-CPB pulmonary hypertension, we determined the effects of ET receptor blockade on the hemodynamic response of the pulmonary circulation to CPB in an additional 7 shunted lambs, determined the effects of ET receptor blockade on the hemodynamic response of the pulmonary circulation to acute alveolar hypoxia in these shunted lambs, and measured plasma levels of ET-1 before and after CPB in all 20 lambs.
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Surgical Preparations and Care
Ewes
Fourteen mixed-breed Western pregnant ewes (138.1±1.4 days' gestation, term=145 days) were operated on under sterile conditions as previously described.15 Through a left lateral fetal thoracotomy, an 8.0-mm expanded polytetrafluoroethylene vascular graft (
2 mm length) (WL Gore and Assoc) was anastomosed between the ascending aorta and main pulmonary artery of the fetus with 7.0 proline (Ethicon Inc) by use of a continuous suture technique as previously described.15 After recovery from anesthesia, the ewes were returned to the cages with free access to food and water. Antibiotics (2 million U of penicillin G potassium and 100 mg of gentamicin sulfate) were administered to the ewes during surgery and daily thereafter until 2 days after spontaneous delivery of the lambs.
Lambs
After spontaneous delivery, antibiotics (1 million U of penicillin G potassium and 25 mg of gentamicin sulfate IM) were administered for 2 days. The lambs were weighed daily, and the respiratory and heart rates were obtained. Furosemide (1 mg/kg IM) was administered daily. Elemental iron (50 mg IM) was given weekly.
At 1 month of age, 20 lambs (14 shunted and 6 age-matched controls) were anesthetized with ketamine hydrochloride (1 mg·kg-1·min-1) and mechanically ventilated as previously described.15 16 Through a midsternotomy incision, polyurethane catheters were inserted into the left and right atria and the main pulmonary artery distal to the vascular graft. Ultrasonic flow probes (Transonics Systems Inc) were placed around the left and right pulmonary arteries to measure pulmonary blood flow. After a 30-minute recovery, blood was obtained from the left and right atria, distal pulmonary artery, right ventricle, and descending aorta for hemoglobin and oxygen saturation determinations. In the 14 shunted lambs, a vascular clip was then placed on the graft to completely occlude it, and oxygen saturation determinations were obtained from the right ventricle and distal pulmonary artery to document shunt closure. The sternotomy incision was then temporarily closed in layers. An intravenous injection of 250 000 U of penicillin G potassium and 25 mg of gentamicin sulfate suspension was administered.
CPB
The bypass circuit is similar to the standard neonatal circuit.17 It consists of a membrane oxygenator (Minimax, Medtronic), an infant venous reservoir (Medtronic), an arterial filter (40 µm, Bentley), and a cardiotomy reservoir and suction. An ultrasonic flow probe (Transonics Systems) is incorporated into the circuit to monitor pump flows continuously. The circuit is primed with fresh heparinized sheep whole blood (400 mL), Normosol (600 mL; Abbott Labs), heparin (2500 U), sodium bicarbonate (10 mEq), Solu-medrol (30 mg/kg), and Kefzol (1 g/kg). The bypass methodology is similar to standard neonatal methods.17 Heparin (300 U/kg) was administered to the lamb into the right atrium. Bicaval venous cannulation was performed with the use of a 16F venous cannula (DLB Inc) for the superior vena cava and a 20F venous cannula for the inferior vena cava. The ascending aorta was cannulated with a 14F cannula (Electro-catheter Corp). The thoracotomy incision was then closed in layers. CPB was commenced, and surface and core cooling were initiated. Normothermic flows ranged from 150 to 200 mL/kg. The lambs were cooled to 25°C with the heart arrested at a flow rate of 100 mL/kg. After 60 minutes at 25°C, rewarming was started. Throughout the CPB period, an 
-stat blood gas strategy was maintained, whereby the temperature-uncorrected PaCO2 is maintained near 40 mm Hg (measured at 37°C) and the temperature-uncorrected pH is maintained near 7.40, irrespective of body temperature.18 Mannitol (0.5 gm/kg) and furosemide (0.5 mg/kg) were added to the prime at the onset of rewarming. After the core temperature reached 32°C, calcium gluconate (1 gm) was added to the prime. After the lambs were rewarmed, ventilation was resumed and the lambs were weaned off CPB. Heparin was completely reversed with protamine (3 mg/kg) given into the left atrium.
Measurements
Pulmonary and systemic arterial pressures and right and left atrial pressures were measured with the use of Statham P23Db pressure transducers. Mean pressures were obtained by electrical integration. Heart rate was measured by a cardiotachometer triggered from the phasic systemic arterial pressure pulse wave. Left and right pulmonary blood flows and bypass flows were measured on an ultrasonic flowmeter (Transonics Systems). All hemodynamic variables were recorded continuously on a Gould multichannel electrostatic recorder. Systemic arterial blood gases and pH were measured on a Corning 158 pH/blood gas analyzer (Corning Medical and Scientific). Hemoglobin concentration and oxygen saturation were measured by a hemoximeter (model OSM 2, Radiometer). The ratio of pulmonary to systemic blood flow (Qp/Qs) was calculated with the use of the Fick equation. Pulmonary and systemic vascular resistances were calculated with the use of standard formulas.
ET-1 Determinations
Four milliliters of systemic arterial blood was collected and placed in iced polypropylene tubes containing 330 µL aprotinin and 100 µL EDTA. The tubes were immediately centrifuged at 4000g for 20 minutes. Collected plasma was treated with equal volumes of 0.1% trifluoroacetic acid and stored at -70°C. The acidified supernatant was centrifuged at 1000g for 20 minutes and loaded on a 3x18 C18 Sep-Pak column (Peninsula Laboratories) equilibrated with 0.1% trifluoroacetic acid. The adsorbed material was eluted with 3 mL of 0.1% trifluoroacetic acid/60% acetonitrile. The eluent was dried in a Savant Speed Vac and stored at -70°C or assayed immediately for immunoreactive endothelin. ET-1 standard, 125I ET-1, anti-ET antibody, and secondary antibody were purchased from Peninsula Laboratories. Cross-reactivity for measured human and bovine ET-1 antiserum is 100% for human ET-1, 7% for human ET-2 and ET-3, and 0% for bovine ET-2 and ET-3. Interassay and intra-assay variabilities were 10% and 4%, respectively. Each sample was assayed in duplicate. This assay was modified from a previously published method.19
Experimental Protocol
Nonshunted Lambs
Sixty minutes after chest closure, an intravenous infusion of saline was begun in six 1-month-old control lambs and continued throughout the study period. Thirty minutes after initiation of the infusion, baseline measurements of the hemodynamic variables (pulmonary and systemic arterial pressures, heart rate, pulmonary blood flow, left and right atrial pressures) were obtained, and systemic arterial blood gases and pH were measured (pre-CPB). The hemodynamic variables were monitored continuously during and for 2 hours after CPB. Systemic arterial blood gases were determined intermittently, and ventilation was adjusted to achieve a PaCO2 between 35 and 45 mm Hg and a PaO2 >50 mm Hg. Sodium bicarbonate was administered to maintain a pH >7.30. Normal saline and red blood cells were administered to maintain atrial pressures and hemoglobin concentrations at pre-CPB levels. Blood from the femoral artery was obtained for ET-1 levels before and 30, 60, and 120 minutes after CPB.
Shunted Lambs
Sixty minutes after shunt and chest closure, baseline measurements of the hemodynamic variables and systemic arterial blood gases and pH were obtained in 14 1-month-old shunted lambs (prehypoxia). Acute alveolar hypoxia (10% oxygen) was then induced by the addition of nitrogen to the ventilation gas mixture. After 15 minutes (hypoxia), all measurements were obtained, and ventilation with 21% oxygen was resumed. After recovery, baseline measurements of the hemodynamic variables were repeated, and blood was obtained from the femoral artery for ET-1 determinations. Then, an infusion of PD 145065, a nonselective ET receptor blocker (50 µg·kg-1·min-1, synthesized by the Medicinal Chemistry Department, Parke-Davis Pharmaceutical Research), or drug vehicle (saline), randomly selected, was begun and continued during CPB and throughout the 2-hour study period after CPB.20 This dose of PD 145065 was chosen after several preliminary studies showed that a 30-minute infusion completely blocked the vasoconstricting effects of exogenous ET-1. Thirty minutes after initiation of the infusion, all measurements were repeated (pre-CPB) and CPB was begun. The hemodynamic variables were monitored continuously during and for 2 hours after CPB, and blood was obtained for ET-1 levels as described above. Two hours after CPB, acute alveolar hypoxia was again induced as described above. All lambs were killed with a lethal injection of sodium pentobarbital followed by bilateral thoracotomy as described in the NIH Guidelines for the Care and Use of Laboratory Animals. All protocols and procedures were approved by the Committee on Animal Research of the University of California, San Francisco.
Statistical Analysis
The mean±SD values were calculated for the baseline hemodynamic variables, systemic arterial blood gases and pH, and plasma ET-1 concentrations. The general hemodynamic variables, systemic arterial blood gases and pH, and ET-1 concentrations were compared over time within each study group by ANOVA for repeated measures with multiple comparison testing. The general hemodynamic variables, systemic arterial blood gases and pH, and ET-1 concentrations were compared between the three study groups by ANOVA for repeated measures with multiple comparison testing. The effects of acute alveolar hypoxia were compared with the previous steady-state condition by the paired t test, with the Bonferroni correction used when necessary. The percent changes in mean pulmonary arterial pressure and pulmonary vascular resistance induced by hypoxia were compared between the two groups of shunted lambs before and after CPB by ANOVA for repeated measures with multiple comparison testing. A linear regression analysis of the pulmonary vascular resistance and mean pulmonary arterial pressure with plasma ET-1 concentrations was performed to identify possible relevant relations. A value of P<.05 was considered statistically significant.
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During this study, the effects of hypothermic CPB were characterized and compared between three different groups of lambs: age-matched nonshunted lambs that were treated with saline before CPB (n=6), shunted lambs treated with saline before CPB (n=7), and shunted lambs treated with an infusion of PD 145065 (a nonselective ET receptor blocker) before CPB (n=7). To assess the effect of preexisting increased pulmonary blood flow on the pulmonary circulatory response to CPB, comparisons between saline-treated nonshunted and saline-treated shunted lambs were made. To assess the potential role of ET-1 on the pulmonary circulatory response to CPB, plasma ET-1 concentrations were determined, and comparisons between saline-treated shunted lambs and PD 145065–treated shunted lambs were made.
The general hemodynamics of the three groups before closure of the vascular graft or initiation of CPB are presented in Table 1
. All shunted lambs had an audible continuous murmur and an increase in oxygen saturation between the right ventricle and distal pulmonary artery. Shunted lambs had increased mean pulmonary arterial pressure, pulmonary blood flow, and left and right atrial pressures (P<.05). Pulmonary vascular resistance and body weight were decreased in shunted lambs (P<.05). Between the two groups of shunted lambs, there were no differences in general hemodynamics except for a lower mean systemic arterial pressure in the group that went on to be treated with PD 145065. There were no differences in systemic arterial blood gases and pH between the three groups, and all were within the normal range for the laboratory.
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Effect of Preexisting Increased Pulmonary Blood Flow on the Pulmonary Circulatory Response to CPB
In nonshunted lambs, pulmonary vascular resistance did not change during the 120-minute study period after CPB. Mean pulmonary arterial pressure and pulmonary blood flow increased (P<.05), while mean systemic arterial pressure was unchanged (Table 2). In saline-treated shunted lambs, both pulmonary vascular resistance and mean pulmonary arterial pressure increased during the 120-minute study period after CPB (P<.05). Pulmonary blood flow and mean systemic arterial pressure were unchanged (Table 3). After CPB, both pulmonary vascular resistance and mean pulmonary arterial pressure were greater in saline-treated shunted lambs than in nonshunted lambs (P<.05; Fig 1).
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P<.05 vs pre-CPB nonshunted lambs; 
P<.05 vs corresponding nonshunted lamb (ANOVA).
Role of ET-1 in the Pulmonary Circulatory Response to CPB
The intravenous infusion of PD 145065 did not change any of the hemodynamic variables. In PD 145065–treated shunted lambs, neither pulmonary vascular resistance or mean pulmonary arterial pressure increased during the 120-minute study period after CPB. In fact, pulmonary vascular resistance was decreased at 30 and 45 minutes, and mean pulmonary arterial pressure was decreased after 30 minutes (P<.05). Pulmonary blood flow was unchanged, and mean systemic arterial pressure decreased (P<.05) (Table 4). After CPB, both pulmonary vascular resistance and mean pulmonary arterial pressure were greater in saline-treated shunted lambs than in PD 145065–treated shunted lambs (P<.05) (Fig 2). Throughout the study period, the mean airway pressure, minute ventilation, and FIO2 required to maintain the predetermined ventilatory parameters were similar in saline-treated shunted lambs and in PD 145065–treated shunted lambs.
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Acute Alveolar Hypoxia
In saline-treated shunted lambs, the induction of alveolar hypoxia increased both pulmonary vascular resistance and mean pulmonary arterial pressure before and after CPB (P<.05). Pulmonary blood flow increased before CPB (P<.05) and was unchanged after CPB. The percent increase in pulmonary vascular resistance induced by hypoxia was augmented after CPB (P<.05) (Fig 3).
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In PD 145065–treated shunted lambs, the induction of alveolar hypoxia similarly increased pulmonary vascular resistance, mean pulmonary arterial pressure, and pulmonary blood flow before CPB (P<.05). However, after CPB, mean pulmonary arterial pressure and pulmonary blood flow increased (P<.05), but pulmonary vascular resistance was unchanged. In fact, the percent increase in pulmonary vascular resistance induced by hypoxia was completely blocked after CPB in PD 145065–treated shunted lambs (P<.05) (Fig 3
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ET-1 Concentrations
Before CPB, plasma ET-1 concentrations were similar in all three groups of lambs. After CPB, ET-1 concentrations increased in all groups of lambs (Fig 4). Thirty minutes after CPB, ET-1 concentrations were greater in PD 145065-treated shunted lambs than in saline-treated shunted lambs or in saline-treated nonshunted lambs (P<.05). There was no correlation between plasma ET-1 concentrations and pulmonary vascular resistance (r=.02, P=.56) or mean pulmonary arterial pressure (r=.07, P=.87).
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Discussion |
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The present study provides support for a role for ET-1 in the pathogenesis of post-CPB pulmonary hypertension. In 1-month-old lambs with preexisting increased pulmonary blood flow, the intravenous infusion of PD 145065, a nonselective ET receptor antagonist, completely blocked the increase in pulmonary vascular resistance and mean pulmonary arterial pressure after hypothermic CPB. In addition, PD 145065 completely blocked the pulmonary vasoconstricting response to acute alveolar hypoxia after CPB, suggesting a role for ET-1 in the increased pulmonary vascular reactivity associated with post-CPB pulmonary hypertension.
In normal 1-month-old lambs, pulmonary vascular resistance did not change and mean pulmonary arterial pressure increased moderately after hypothermic CPB. However, in lambs with preexisting increased pulmonary blood flow and pulmonary hypertension, the increases in both pulmonary vascular resistance and mean pulmonary arterial pressure were significantly augmented after CPB. These data correlate with clinical observations that children with preexisting pulmonary hypertension are at greater risk for acute increases in pulmonary vascular resistance after CPB.2 The exact mechanisms for these differences remain unclear, but potential explanations include structural abnormalities of the pulmonary circulation and preexisting endothelial dysfunction. For example, lambs with increased pulmonary blood flow have pulmonary vascular remodeling that is similar to children with increased pulmonary blood flow and pulmonary hypertension: increased medial thickness of the pulmonary arteries and abnormal extension of muscle in the walls of more peripheral arteries.1 15 In addition, these lambs, as well as children with increased pulmonary blood flow, display an impaired ability to generate agonist-induced nitric oxide.5 21 Because nitric oxide is an important mediator of the pulmonary vasculature and its response to pulmonary vasoconstricting stimuli, this preexisting endothelial dysfunction may contribute to their augmented pulmonary vascular response to CPB.4 22
Lambs with preexisting increased pulmonary blood flow also display alterations in the ET-1 cascade, which may contribute to their augmented pulmonary vascular response to CPB. Compared with age-matched controls, shunted lambs have increased plasma concentrations of ET-1 and an augmented pulmonary vasoconstricting response to exogenous ET-1.16 Preliminary investigations suggest that these alterations are associated with an increase in ET-1, endothelin-converting enzyme 1, and ETa receptor gene expression and a decrease in ETb receptor gene expression.23 An infusion of PD 145065, initiated 30 minutes before CPB, completely blocked the increase in pulmonary vascular resistance and mean pulmonary arterial pressure after CPB. These data strongly suggest that ET-1 mediates post-CPB pulmonary hypertension in shunted lambs, at least in part. Although endothelial injury during hypothermic CPB is well described, the aberrations induced in the ET-1 cascade during CPB are not well understood.3 10 24 Possible aberrations include alterations in receptor function, availability, and/or their secondary messengers and increased ET-1 release.
After CPB, plasma ET-1 concentrations increased. This has been noted previously in patients with pulmonary hypertension and congenital heart disease.13 The cause of increased ET-1 concentrations after CPB is unknown, but several factors associated with CPB, such as surgical stress, hypothermia, alveolar hypoxia, and cardiogenic shock are known to increase ET-1 concentrations.25 26 27 28 A limitation of the present study is that ET-1 concentrations were not determined in the pulmonary artery, negating the opportunity to determine net extraction or secretion of ET-1 across the lung. Because plasma concentrations of ET-1 increased similarly in shunted and nonshunted lambs, the plasma concentration of ET-1 did not correlate with the degree of post-CPB pulmonary hypertension. However, these data do not negate a role for ET-1 in the pathophysiology of post-CPB pulmonary hypertension. First, circulating ET-1 levels may be less important than the higher tissue concentrations achieved by local ET-1 release.29 Second, ET-1 has a different physiological effect on the remodeled pulmonary circulation.16 For example, 1-month-old shunted lambs have increased ET-1–induced pulmonary vasoconstriction, most likely secondary to increased ETa receptor densities and/or decreased ETb receptor densities.16 23 Therefore, significantly greater ET-1–induced pulmonary vasoconstriction may occur in shunted lambs even if plasma concentrations are similar. Thirty minutes after CPB, ET-1 concentrations were greater in PD 145065-treated shunted lambs than in nontreated shunted or nonshunted lambs. In humans, infusions of nonselective ET receptor antagonists have also demonstrated an increase in ET-1 plasma concentrations.30 The cause of this increase is currently unclear, but possible explanations include the displacement of ET-1 from its receptors, decreased clearance of ET-1, and the disturbance of a feedback mechanism secondary to ET-1 receptor blockade.
In shunted lambs, the pulmonary vasoconstricting response to acute alveolar hypoxia was augmented after CPB. This increased pulmonary vascular reactivity has been noted after CPB in previous experimental animal models as well as in children with congenital heart disease.2 3 31 32 Its cause remains unknown but may involve a disruption of the regulatory mechanisms of the pulmonary vascular endothelium secondary to CPB-induced endothelial injury.3 24 31 32 The potential role of ET-1 in the increased pulmonary vascular reactivity after CPB has not been investigated. There are conflicting animal data concerning the role of ET-1 in acute hypoxic pulmonary vasoconstriction of the normal pulmonary circulation.33 34 However, in the present study, the response to acute alveolar hypoxia was completely blocked in shunted lambs pretreated with PD 145065. These data strongly support a role for ET-1 in the augmented pulmonary vasoconstricting response to acute alveolar hypoxia after CPB and implicate a role for ET-1 in the global increased pulmonary vascular reactivity associated with post-CPB pulmonary hypertension.
In summary, we found that the pulmonary circulatory response to hypothermic CPB was dependent on the preexisting status of the circulation. Those lambs with preexisting increased pulmonary blood flow and endothelial dysfunction had an augmented increase in pulmonary vascular resistance after CPB compared with lambs with a normal pulmonary circulation. These data stress the importance of studying CPB with a clinically appropriate pathophysiological animal model. In addition, we found that the infusion of PD 145065, a nonselective ET receptor antagonist, completely blocked the increase in pulmonary vascular resistance and the pulmonary vasoconstricting response to acute alveolar hypoxia after hypothermic CPB. These data suggest an important role for ET-1 in the pathophysiology of post-CPB pulmonary hypertension and its associated increased pulmonary vascular reactivity. In addition to the alterations induced in the pulmonary circulation, ischemia to the brain and kidneys is a significant source of morbidity and mortality in children with congenital heart disease after hypothermic CPB.17 ET-1 produces potent vasoconstriction in both the cerebral and renal circulations, and recent data demonstrate that treatment with ETa receptor blockers decreases ischemic injury in animal models of cerebral and renal ischemia.35 36 37 Therefore, the potential use of ET receptor blockers during CPB may have significant clinical benefits and warrants further study.
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Acknowledgments |
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This research was supported by grant No. 94-212 from the American Heart Association, California Affiliate; by a Basil O'Connor Award from the March of Dimes; and by a grant from Parke-Davis Pharmaceutical Research. The authors thank Joan Keiser PhD, for ongoing pharmaceutical expertise; Roger Chang, Rene Garrets, and Michael Johengen for technical assistance; and Randy Kikukawa for editorial assistance.
Received July 22, 1996; revision received September 16, 1996; accepted September 30, 1996.
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