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J Thorac Cardiovasc Surg 1994;107:1129-1135
© 1994 Mosby, Inc.
SURGERY FOR CONGENITAL HEART DISEASE |
Paris, France
Received for publication Feb. 22, 1993. Accepted for publication Oct. 5, 1993. Address for reprints: Didier Journois, MD, Hôpital Laennec, 42, rue de Sèvres, 75340 Paris, France.
Abstract
Seventeen infants were treated with inhaled nitric oxide for critical pulmonary artery hypertension after operations for congenital heart defects. In all 17 patients conventional medical therapy consisting of hyperventilation, deep sedation/analgesia, and correction of metabolic acidosis had failed. All children were monitored with a transthoracic pulmonary artery catheter inserted at operation. Pulmonary artery hypertension was defined as an acute rise in pulmonary pressure associated with a decrease in oxygen arterial or venous saturation. After failure of conventional medical therapy, 20 ppm of inhaled nitric oxide was administered to the patient. In all patients the pulmonary pressures decreased (mean pulmonary arterial pressure decreased by -34% ± 21%) without significant change in systemic arterial pressure, whereas the oxygen arterial saturation and oxygen venous saturation increased by 9.7% ± 12% and 37% ± 28%, respectively. Fifteen children were discharged from the intensive care unit at 10 ± 6 days (range 3 to 26 days) and two died. This study demonstrates that inhaled nitric oxide exerts a selective pulmonary vasodilation without decreasing systemic arterial pressure in children with congenital heart disease. The increased values of mixed venous oxygen saturation and urinary output suggest that this selective lowering of pulmonary vascular resistance improved the overall hemodynamics. The potential toxic effects of nitric oxide and nitrogen dioxide necessitate careful consideration of the risks and benefits of inhaled nitric oxide therapy. (J THORAC CARDIOVASC SURG 1994;107:1129-35)
After neonatal cardiac operations for certain congenital heart defects, a critical rise in pulmonary artery pressure (PAP) may occur.
1-3 Despite aggressive therapy with manual hyperventilation, deep levels of analgesia and sedation, muscle paralysis and vasodilator therapy, the mortality remains usually high when major hypertensive crises occur.
2,3 Furthermore, pulmonary artery vasodilator therapy is often nonselective and may produce systemic hypotension, partially contributing to the morbidity.
2-4
Recent reports have demonstrated that nitric oxide is a selective pulmonary vasodilator in adults
5,6 and in newborn infants.
7 The usefulness of nitric oxide has been suggested by several isolated case reports after pediatric cardiac operations.
8-10 We therefore systematically evaluated the hemodynamic changes during inhalation of low concentrations of nitric oxide in the treatment of postoperative pulmonary hypertensive crises after repair of congenital heart defects.
PATIENTS AND METHODS
A pulmonary arterial catheter was inserted transthoracically via the right ventricular myocardium in 42 children thought to be at risk for postoperative pulmonary hypertension.
1 Twenty of these catheters were 4F fiberoptic pulmonary artery catheters that allowed continuous mixed venous oxygen saturation measurement (SvO2; Oximetrix Lab., Abbott Laboratories, North Chicago, Ill.).
11 After operation, central venous and left atrial pressures, systemic arterial pressure, systemic arterial saturation by pulse oximetry, and body temperature were continuously monitored. Blood gas analysis was done at least every 6 hours. The arterial pulse oximetry was continuously monitored. The lungs were ventilated with a Siemens Servo 900C ventilator (Siemens-Elema, Division of Elema-Schönander, Inc., Solna, Sweden) with an inspired oxygen fraction (FiO2) of 1 and slightly hyperventilated to reach an arterial carbon dioxide tension of approximately 30 mm Hg. Continuous infusions of pancuronium and fentanyl were administered during the first 12 postoperative hours. Dobutamine or dopamine, or both, infused at rates less than or equal to 5 µg · kg-1 · min-1 were used as inotropic agents in several patients at the end of cardiopulmonary bypass.
Conventional management of acute pulmonary hypertension crisis was instituted if PAP exceeded 75% of systemic pressure in association with a decrease in SvO2 and/or arterial (SaO2) oxygen saturation. This therapy included 100% oxygen with manual ventilation, deepening the level of anesthesia, and correction of the metabolic acidosis with sodium bicarbonate administration. After 5 minutes of unsuccessful therapy, inhaled nitric oxide was administered.
The measured variables during nitric oxide administration were heart rate; systolic, diastolic, and mean systemic arterial pressures (AP); systolic, diastolic, and mean PAPs; central venous pressure; left atrial pressure; urinary output; and venous and arterial blood gases.
A complete set of measurements was recorded before (T0) and 20 minutes after administration of 20 ppm of inhaled nitric oxide (T1). Nitric oxide administration was continued until pulmonary artery hypertension was absent for 6 hours in the face of tracheal suctioning, progressive weaning of sedation, and progressive reduction of FiO2. Persistence of a low SvO2, a low SaO2 or a high mean PAP/mean AP ratio was treated by increasing the inspired nitric oxide fraction from 20 ppm to 80 ppm. Further sets of measurements were recorded every hour or in the event of paroxysmal pulmonary hypertension crisis. Methemoglobin concentration was measured twice daily.
A tank containing 225 ppm of nitric oxide in nitrogen (Compagnie Française des Produits Oxygénés, Paris, France) was connected to a specially designed low-flow blender. The mixture of nitric oxide and nitrogen was continuously delivered through a needle inserted at the end of the inspiratory limb into the breathing circuit. A 1.0 (16 cases) or a 1.5 mm (1 case) external diameter catheter was inserted into the tracheal tube and positioned 1 cm from its distal extremity. The catheter was connected to an oxygen/nitrogen analyzer (Servo gas monitor 120, Siemens) via an electrochemical nitric oxide/nitrogen dioxide analyzer (Polytron NO/NO2, Dräger, Antony, France), which allowed continuous measurements of FiO2, nitric oxide, nitrogen dioxide, and nitrogen. The gas aspiration rate for analysis of 105 ml · min-1 was compensated by an equivalent increase in ventilation. The flow of the nitric oxide nitrogen mixture was adjusted to achieve the desired inhaled nitric oxide concentration. After extubation, if PAP remained elevated, nitric oxide was given by means of a facial mask. Expired and analyzer's gases were scavanged.
This investigation was done with approval of the hospital review board and informed consent was obtained from each infant's family.
Data are expressed as mean ± standard deviation and were compared between T0 and T1 by Wilcoxon test for paired values. Further measurements were compared by analysis of variance for repeated measures followed by a two-tailed Dunnett's post-hoc test with T0 as control value. Comparisons between the two patient groups used Wilcoxon or Fisher's exact test. The relationship between nitric oxide and nitrogen dioxide concentrations was studied by linear regression. A p value less than 0.05 was considered statistically significant.
RESULTS
Seventeen children with pulmonary artery hypertension, from 5 days to 24 months of age (median 50 days), in whom conventional medical therapy had failed, were studied. Demographic data and description of congenital heart diseases are reported in
Table I.
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In 238 measurements near the tracheal outlet of the endotracheal tube, nitric oxide concentration was 22.2 ± 14 ppm (range 8.5 to 60) and nitric oxide 1.41 ± 1.8 ppm (range 0.1 to 8.2). Linear regression showed a strong relationship between nitric oxide and nitrogen dioxide concentrations (nitrogen dioxide = 0.079 x nitric oxide - 0.1; p = 0.0001). The highest methemoglobin concentration was 1.8%. No other side effects were observed.
DISCUSSION
This study confirms that inhaled nitric oxide exerts a selective pulmonary vasodilation without decreasing systemic AP even in neonates and infants with pulmonary artery hypertension after repair of congenital heart disease. The improvement in SvO2 and urinary output suggests that the selective lowering of pulmonary vascular resistance promoted a better hemodynamic condition. Because oxygen consumption and hemoglobin levels are likely to remain nearly constant, this SvO2 increase is probably caused by both increases in SaO2 and cardiac output. Although cardiac output was not measured, an improvement in the right ventricular function is probably responsible for this effect.
1 This phenomenon, in association with an increased SaO2, might improved arterial oxygen transport. Furthermore, inhaled nitric oxide may specifically dilate pulmonary vessels next to ventilated alveoli, enhancing ventilation-perfusion matching.
Prolonged postoperative anesthesia has been demonstrated to control episodes of pulmonary artery hypertension in response to tracheal stimulation.
12 This technique requires several days for pulmonary artery reactivity to decrease and may facilitate pulmonary infections. In our study, inhaled nitric oxide seemed to be effective as a sole therapy and allowed discontinuation of vasodilators, inotropic support, and lightening of anesthesia. We speculate that this therapy may reduce durations of ventilatory support and intensive care unit stay in patients at risk for pulmonary artery hypertension.
An important characteristic of pulmonary artery hypertension is its paroxysmal nature.
1 This may be due to the auto-intensification of pulmonary artery hypertension, mediated by hypoxemia, that is a major cause of pulmonary vasoconstriction in young children.
13 Pulmonary artery hypereactivity seems to persist several days after a complete surgical correction that should eliminate the cause of pulmonary hypertension.
3 This phenomenon suggests the coexistence of an abnormality in arterial reactivity induced by the congenital heart disease.
14 It seems to occur in association with an increase in medial thickness resulting from preexisting elevated pulmonary arterial pressure.
14,15 Furthermore, hypoxemia and increased pulmonary blood flow induce the persistence of fetal pulmonary vascular muscularization in neonates.
16 Abnormalities in endothelin-derived relaxing factor or in endothelin releases have also been suggested.
17,18 Most of these abnormalities usually do not persist more than a few days after surgical correction.
3,18 A therapeutic aim of inhaled nitric oxide administration, therefore, is stabilization until pulmonary arterial reactivity is reduced after operation.
1
Oxygen inhalation is an effective pulmonary vasodilator when pulmonary hypertension is due to a hypoxic stimulation of precapillary arteriolar vasculature.
4 The reduction in FiO2 that inhaled nitric oxide allows is of particular interest in neonates in whom high FiO2s are deleterious. Correction of acidosis is also necessary to repress the pulmonary arterial reactivity.
1 This is achieved by prevention of low cardiac output, which inhaled nitric oxide provides.
Many pharmacologic agents have been reported to have pulmonary vasodilating properties, including
-blockers,
19 tolazoline,
4 ß-agonists,
20 nitrates,
21,22 calcium channel blockers,
23 and prostaglandins.
24 All these agents induce tissue edema and a systemic vasodilation.
1,7,25 These deleterious effects can be partially counteracted by combining their infusion with norepinephrine.
24 Furthermore, the frequent association of atrial or ventricular right-to-left shunts during pulmonary hypertension crisis leads to shunting of increased levels of the administered vasodilators toward the systemic circulation.
1 This may lead to a predominant systemic vasodilation that is particularly undesirable inasmuch as left ventricular output is simultaneously impaired by the pulmonary hypertensioninduced preload reduction. Ultimately, when pulmonary hypertension crisis persists despite the optimal use of these pharmacologic agents, extracorporeal membrane oxygenator support has been proposed as a last resort to avoid the complications of these therapies.
1
Nitric oxide produces vasodilation by directly activating guanylate cyclase, which increases the intracellular cyclic guanosine 3,5-monophosphate levels in smooth muscle cells and produces vasodilation.
26 The rapid nitric oxide inactivation by thehemoglobin contained in the pulmonary blood vessels
27 explains its short-acting and local effects.
Inhaled nitric oxide doses in human beings appear safe.
5,7,8,25,28,29 Nevertheless, nitric oxide is known to be a toxic gas.
30 Major toxic effects have been described with high nitric oxide concentrations (50,000 ppm).
31 They involve bronchial and tracheal damage,
31 methemoglobinemia,
31 and nitrogen dioxide and peroxynitrite production.
32,33 Methemoglobinemia levels were low in our study. Because exposure to only 2 ppm of nitrogen dioxide during 24 hours induces ciliary depletion and bronchial dysplasia,
34 maximal continuous inhaled nitric oxide concentration should be limited to 20 ppm or strictly restricted to crisis treatment.
Administration of nitric oxide can be beneficial in treatment of patients with certain congenital heart defects. The small number of patients in this study does not permit any subgroup analysis to determine which congenital heart defects are appropriate for therapy with inhaled nitric oxide.
At low doses, inhaled nitric oxide is effective, simple to use, and had no serious adverse effect. Inhaled nitric oxide has therapeutic potential in postoperative pulmonary artery hypertension after operations for congenital heart disease. Nevertheless, risks and benefits of inhaled nitric oxide need careful consideration. Further tolerance studies are required.
Acknowledgments
We thank W. J. Greeley, MD, Associate Professor of Anesthesiology and Pediatrics, Duke University Medical Center, Durham, N.C., for his valuable advice and comments; and the staff and the nurses of our Cardiovascular Surgery Intensive Care Unit for their support.
Footnotes
From the Departments of Anesthesia and Intensive Care Medicine a and Cardiovascular Surgery, b Hôpital Laennec, Paris, France. ![]()
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