Inhalational administration of nitric oxide (NO) has been

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1 Regulation of Pulmonary Vascular Resistance by Endogenous and Exogenous Nitric Oxide Joseph R. Van Camp, MD, Christopher Yian, BS, and Flavian M. Lupinetti, MD Section of Thoracic Surgery, Department of Surgery, University of Michigan School of Medicine, Ann Arbor, Michigan Inhaled nitric oxide (NO) causes selective pulmonary vasodilatation under conditions of hypoxia or pulmonary vascular dysfunction. We have observed that NO affects canine pulmonary vascular resistance minimally under normal conditions. We hypothesized that endogenous NO is partly responsible for pulmonary vasomotor regulation in normoxic and hypoxic states. Dogs were studied before and after pulmonary endothelial injury with monocrotaline and N-omega-nitro-L-arginine (LNNA). Systemic vascular resistance was unaffected by NO. Under normal conditions, exogenous NO had little effect on pulmonary vascular resistance. After monocrotaline administration, baseline pulmonary vascular resistance was unchanged but decreased further in response to NO. After LNNA administration, pulmonary vascular resistance increased and there was an exaggerated increase with hypoxia that was reduced by NO. The effect of monocrotaline on in vitro endothelial function was evaluated with isolated pulmonary arteries, which showed a decreased relaxation response to bradykinin (an endothelial-dependent vasodilator) and a normal response to nitroprusside (an endothelial-independent vasodilator). These results support the hypothesis that endogenous NO is an important regulator of pulmonary vasomotor tone and is of even greater importance during hypoxia. (Ann Thorne Surg ) Inhalational administration of nitric oxide (NO) has been proposed as a selective pulmonary vasodilator for use in critical care units to improve ventilation-to-perfusion matching by dilating the ventilated pulmonary vasculature. Inhaled NO has been used successfully in the treatment of patients with congenital heart defects [1], mitral valve disease [2], primary pulmonary hypertension [3], persistent pulmonary hypertension of the newborn [4], and the adult respiratory distress syndrome [5]. The use of inhaled NO has evolved based on its pharmacologic properties, which are identical to those of the endotheliumderived relaxing factor [6]. Although inhaled NO was reported to be effective in these clinical studies, animal tests of this substance have been less uniform [7-11]. Preliminary experiments on normal dogs in this laboratory failed to demonstrate consistently any effects of inhaled NO that would correspond to its clinical usefulness. This lack of NO effect, however, may have been caused by the use of normal animals. In the setting of normal pulmonary vascular resistance and with a capacity for endogenous NO production in response to alveolar hypoxia, exogenous NO may have little or no demonstrable benefit on pulmonary vascular tone. We hypothesized that endogenous NO is necessary for maintenance of basal pulmonary vascular resistance but is more important in the autoregulation of pulmonary vascular resistance in response to hypoxic stress. To test this hypothesis, canine response to hypoxia and inhaled NO was established. Next, a model of mild, chronic pulmonary endothelial injury was used to study Presented at the Thirtieth Annual Meeting of The Society of Thoracic Surgeons, New Orleans, LA. Jan 31-Feb 2, Address reprint requests to Dr Lupinetti, Division of Cardiac Surgery, Children's Hospital and Medical Center, CM-03, 4800 Sand Point Way NE, Seattle, WA by The Society of Thoracic Surgeons the effects of inhaled NO on pulmonary and systemic hemodynamics under normal and hypoxic conditions. Finally, these results were compared with those found with administration of a NO synthase inhibitor, N-omeganitro-L-arginine (LNNA). Material and Methods Instrumentation Adult-conditioned mongrel dogs (body weight, 23 to 28 kg) were studied. All animals were cared for in accordance with the "Guide for the Care and Use of Laboratory Animals" published by the National Institutes of Health (NIH publication 85-23, revised 1985). Each dog was subjected to general anesthesia with thiamylal administered as a 20-mg/kg intravenous bolus and a continuous infusion of 0.1 to 0.2 mg' kg- 1 min- 1. Endotracheal intubation was performed and the animals were ventilated mechanically with a volume-controlled ventilator at a tidal volume of 10 ml/kg body weight and a ventilatory rate of 12 breaths/min with no positive end-expiratory pressure. Invasive hemodynamic monitoring was established with a fluid-filled aortic catheter and a Swan-Ganz catheter advanced into the pulmonary artery, both inserted percutaneously through femoral vessels. Recordings and Calculations Systemic and pulmonary arterial pressures were continuously recorded. After each intervention, right atrial pressure and pulmonary capillary wedge pressure were recorded, and thermodilution cardiac output was measured in triplicate. Arterial blood gas measurements were also obtained with each change in inhaled gases. Calculations of systemic (SVR) and pulmonary vascular resistance /94/$7.00

2 1026 VAN CAMP ET AL Ann Thorac Surg (PVR) were made according to the following formulas and expressed in Wood units: PVR = (mean pulmonary artery pressure - pulmonary capillary wedge pressure)i cardiac output SVR = (mean aortic pressure - mean right atrial pressure)i cardiac output Experimental Protocol Initial studies were performed in 10 animals that were allowed to stabilize for 15 minutes after instrumentation was complete. Hemodynamic variables were recorded, and a blood gas measurement was obtained. Nitric oxide, supplied at a standard concentration of 239 ppm in nitrogen with an NO x concentration of 243 ppm (Airco; BOC Group, Murray Hill, NT), was administered through the endotracheal tube using the minimum possible length of tubing to avoid NO degradation. Gas concentrations were adjusted with flowmeters (Foregger Co, Smithtown, NY) to deliver oxygen at an inspired oxygen fraction (FiOz) of 0.21 diluted with nitrogen. The NO then was administered at increasing doses of 0, 30, 60, and 120 ppm, with the gas flow rates adjusted to maintain the FiOz at Variables were recorded at each setting after allowing 5 minutes for equilibration. Administration of NO was discontinued for 15 minutes. The FiOz then was decreased to 0.10 and variables were recorded again. The NO then was administered at increasing doses of 0, 30, 60, and 120 ppm, with the gas flow rates adjusted to maintain the FiOz at The observations then were terminated, and the animals were ventilated with room air until awake and allowed to recover. Monocrotaline Administration Five of these animals and 4 others not previously tested were used for creation of a chronic pulmonary endothelial injury model using monocrotaline (MCT). The MCT (Sigma Chemical, St. Louis, MO) was dissolved in 1 N HCl and diluted with sterile distilled water. The ph was normalized with 0.1 N KOH. The solution was finally diluted to a 20% concentration with sterile distilled water. After recovery from anesthesia, the MCT was injected intraperitoneally at a dose of 90 mg/kg body weight in equally divided doses over 3 days. The animals were again studied 28 days after the initial MCT injection, using the same experimental protocol described. Nitric Oxide Synthesis Inhibition Five animals that had not recovered from anesthesia were used immediately for evaluation of NO synthesis inhibition. LNNA (Sigma, St. Louis, MO) was used as an inhibitor of NO synthesis and administered as a constant infusion of 20 mglmin for 60 minutes, or a total dose of 120 mglanimal. Constant infusion during the period of observation was used to ensure total NO synthesis inhibition at the time of all data collection and has been shown to block the pulmonary vasodilation in response to bradykinin in other canine models [12]. The experimental protocol was repeated as described. Previously, LNNA has been shown to be a potent inhibitor of NO synthesis [13, 14]. Vessel Loop Studies Additional animals were sacrificed by induction of general anesthesia and exsanguination either without MCT administration (n = 4) or 28 days after MCT (n = 4). Pulmonary arteries were removed rapidly and placed in oxygenated Kreb's solution. Vessel loops 3 mm in length taken from intraparenchymal pulmonary arteries then were suspended on a force transducer (model FT03; Grass Instrument Co, Quincy, MA) and set to standardized wall tension to approximate an intraluminal pressure of 40 mm Hg. This technique is similar to that described by Dignan and colleagues [15]. This was necessary to achieve a standard preload tension in vessels of variable diameter. The vessel loop was allowed to rest in oxygenated Kreb's solution for 2 hours. A maximal contraction was induced by 40 meq/l potassium chloride, followed by a rinse and return to baseline. Dose-response relaxation curves to bradykinin and nitroprusside then were performed and recorded on an AID converter for graphic analysis. Data Analysis The PVR, SVR, oxygen tension, and cardiac output (CO) were analyzed using a paired t test to assess significant differences within groups before and after experimental interventions. Statistical analysis was performed on a Macintosh Quadra 700 computer (Apple Computer Inc, Cupertino, CA) with the Statview 4.0 statistical program package (Abacus Concepts, Berkeley, CA). Results are reported as group means ± standard error of the mean. Significant differences are reported at a p value of less than Vessel dose-response curves are reported as percent maximal relaxation versus inverse logarithmic concentration. Results Results of hemodynamic measurements are summarized in Table 1. In the normal control animals, NO administered at 30 to 120 ppm did not alter significantly any of the hemodynamic variables. Hypoxia was obtained by decreasing the FiOz to 0.10, which caused a decline in arterial oxygen tension from 82 to 29 mm Hg and an increase in PVR from 1.8 to 2.2 Wood units. This also was associated with an increased SVR but without a change in CO. Even at hypoxia with increased PVR, there were no significant changes at increasing concentrations of NO. The MCT-induced injury increased PVR slightly during both normoxic and hypoxic states. However, this difference was not statistically significant. Unlike the control animals, during normoxic ventilation, inhaled NO produced a significantly lower PVR compared with baseline at NO doses of 60 and 120 ppm. The SVR was not changed significantly by NO administration. When the MCTtreated animals were made hypoxic, PVR and SVR both increased significantly. At increasing concentrations of inhaled NO, a significant reduction in PVR was seen without a change in SVR.

3 Ann Thorac Surg VAN CAMP ET AL 1027 Table 1. Hemodynamic Response to Nitric Oxide, Monocroialine, and LNNA PVR NO CO SVR MPAP PCWP (Wood no, (ppm) (L/min) (Woods) (mm Hg) (mm Hg) units) poz Initial observations before monocrotaline or LNNA :!:: :!:: :!:: 2.9 1O.6:!:: :!:: :!:: :!:: :!:: :!:: :!:: :!:: :!:: :!:: :!:: :!:: 2.9 1O.6:!:: :!:: :!:: :!:: :!:: :!:: 2.8 1O.2:!:: :!:: :!:: :!:: :!:: :!:: :!:: :!:: :!:: :!:: :!:: :!:: :!:: :!:: :!:: :!:: :!:: :!:: :!:: :!:: :!:: :!:: :!:: :!:: :!:: :!:: :!:: 4 Results after monocrotaline administration :!:: :!:: 5 20 :!:: :!:: :!:: :!:: :!:: :!:: 5 19 :!:: :!:: :!:: :!:: :!:: :!:: 5 19:!:: :!:: :!:: :!:: :!:: :!:: 5 19:!:: :!:: :!:: o.i- 110 :!:: 5 a :!:: :!:: 5 29:!:: :!:: :!:: :!:: :!:: :!:: 5 24 :!:: 2.7 a 7.0:!:: :!:: :!:: :!:: :!:: 5 22 :!:: 2.8 a 7.2:!:: :!:: 0.2 a 34:!:: :!:: :!:: 5 20 :!:: 3.1 a 6.8:!:: :!:: 0.2 a 35:!:: 3 Results after LNNA administration :!:: 0.2 b 47:!:: 4 b 21.7 :!:: :!:: :!:: 0.3 b 86:!:: :!:: 0.2 b 69 :!:: 11 b 23.0 :!:: :!:: :!:: LOb 107:!:: :!:: 0.2 b 64:!:: 8 b 21.3 :!:: :!:: :!:: 0.7 b 115:!:: 14 a :!:: 0.2 b 60:!:: 7 b 20.0 :!:: :!:: :!:: 0.9 b 124:!:: 12 a,b :!:: 0.3 b 55 :!:: 9 b 33.3 :!:: :!:: :!:: 1.0 b 33 :!:: 1 a p < 0.05 versus 0 ppm at same FiO,; :!:: 0.2 b 63:!:: lob 32.7:!:: :!:: :!:: 1.0 b 33:!:: :!:: O.1 b 68 :!:: 5 b 26.0:!:: 1.3 a 12.0:!:: :!:: i.o-> 36:!:: :!:: 0.2 b 60:!:: 7 b 25.0:!:: IS 13.0:!:: :!:: i.o-> 36:!:: 1 b p < 0.05 versus same FiO z and NO in initial observations. CO = cardiac output; FiOz = inspired fraction oxygen; LNNA = N-omega-nitro-L-arginine; MCT = monocrotaiine; MPAP = mean pulmonary artery pressure; NO = nitric oxide; PCWP = pulmonary capillary wedge pressure; PVR = pulmonry vascular resistance; SVR = systemic vascular resistance. The isolated vessel loops from animals treated with MCT showed a decrease in their ability to relax in response to bradykinin but no change in their response to nitroprusside. The dose-response curves are depicted in Figures 1 and 2. Infusion of LNNA caused several hemodynamic changes, shown in Table 1. The CO decreased from 6.0 to 2.4 L/min, a significant decrease from control levels. The PVR increased significantly from 2.3 to 4.5 Wood units and was unaffected by inhaled NO under normoxic conditions. When the FiOz was decreased to 10%, the PVR rose significantly. Under these hypoxic conditions, NO partly reversed this pulmonary vasoconstriction, and PVR fell to a level significantly lower than without NO. The SVR was elevated significantly with the LNNA and was unchanged with increasing levels of NO during normoxia and hypoxia. Comment This investigation showed that inhaled NO had no effect on the PVR, SVR, CO, or oxygen tension in the normal dog in either the normal or the hypoxic state. After pulmonary injury was induced with MCT, NO was found to have a significant effect on the PVR and oxygen tension without causing a change in the SVR or CO. The minimal effects of NO in the uninjured pulmonary vasculature differ from the results of other studies using rabbits and lambs [7-9]. These latter studies observed a decrease in the PVR caused by inhaled NO during both hypoxic and normoxic conditions. This may suggest an interspecies difference in the pulmonary vasomotor activity related to endogenous production of NO by endothelial cells. In the present study, there are two features of MCT injury that make it useful for investigating the effect of inhaled NO. First, the resulting pattern of injury appears to affect only the endothelial cell function and not to alter the ability of the pulmonary vasculature to react to other stimuli. This produces a stable model of pulmonary vascular injury. The endothelial injury altered baseline vasomotor tone slightly, but influenced more profoundly the response to exogenous NO. In this canine model, this change in vascular reactivity is not accompanied by the

4 1028 VAN CAMP ET AL Ann Thorae Surg Z 0 o rw a:: 30..J 40 <C.. ::E o o ::E 70 I- ffioo 90 a:: W e, i I I CONTROL m MCT \ I '\ \ T -. \J '\ r ""-..T I"" T f. <, I' Fig 1. Logarithmic dose-response curves demonstrating the percent maximal relaxation of isolated vessel loops versus the inverse log of the concentration of bradykinin. A distinct alteration in the response curve to bradykinin after monocrotaline (MCT) injury is demonstrated. 0{ LOG[BRADYKININ] dramatic elevations in pulmonary artery pressures seen in pulmonary hypertension in humans. Monocrotaline is a pyrrolizidine alkaloid derived from certain plant seeds and leaves of the Croialaria genus. This compound has been demonstrated to cause injury to the pulmonary vasculature in animals after conversion in the liver to a second, short-lived toxin [16, 17]. The injury produced by MCT has been described as endothelial cell vacuolization and cell death associated with pulmonary edema, inflammation, platelet sequestration, large arterial remodeling, and eventual pulmonary hypertension [16 19]. These changes have been studied mostly in bovine, porcine, and rodent models. In dogs, MCT has induced pulmonary hypertension and right ventricular hypertrophy when administered by intraperitoneal injection [17]. Z 0 : w <C.. ::E 6 0 ::E 70 I Zoo W 90 a:: W 100 "'-I I I CONTROL m MCT I f\.t "- "ị T Fig 2. Logarithmic dose-response curves demonstrating the percent maximal relaxation of isolated vessel loops versus the inverse log of the concentration of nitroprusside. No alteration in the response curve to nitroprusside is observed in the arteries subjected to monocrotaline (MCT) injury. Other studies have used MCT to produce a model of pulmonary hypertension in rats to investigate the response to single-lung transplantation [20]. Clinical trials have used inhalational NO successfully for the treatment of elevated pulmonary vascular resistance. Inhalational administration of NO has been used in persistent pulmonary hypertension of the newborn, resulting in improved oxygenation without causing systemic hypotension [4]. Inspired NO has been used for adult respiratory distress syndrome as well, with better oxygenation and lower pulmonary arterial pressure [5]. Inhaled NO has been used for patients undergoing mitral valve replacement who exhibited mild pulmonary hypertension in the perioperative period; NO resulted in reduced pulmonary arterial pressure and improved hemodynamic stability [2]. In patients undergoing operations for congenital heart defects, NO has similar beneficial responses [1]. Compared with prostacyclin, inspired NO has been shown to be an effective pulmonary vasodilator. Unlike prostacyclin, however, NO does not reduce systemic vascular resistance [3]. It is noteworthy that all of these successful clinical applications of inspired NO have been performed in patients with primary or secondary elevations of pulmonary arterial pressure. Inhaled NO in such acute settings may be beneficial by reducing right ventricle afterload without causing systemic hypotension and possibly jeopardizing coronary perfusion. Decreasing PVR also may improve ventilation/perfusion matching in the lung. One possible explanation for the minimal effects of inhaled NO before MCT treatment is a species-related insensitivity. This is contrary to other evidence that release of endogenous NO in the dog plays an important role in controlling arterial pressure and flow [10, 12, 14, 21]. The present study suggests a possible explanation for what may be interpreted as an inconsistent response to inspired NO. Normal lungs of numerous species, including humans, produce NO endogenously, which can be detected in exhaled gases. This endogenous NO is produced by NO synthase, as evidenced by its inhibition by LNNA and LNMA [12-14,21,22]. The absence of a response to NO in normal lungs may reflect the presence of endogenous NO production. Sufficient production of endogenous NO may exceed exogenous administration disproportionately. These experiments have shown that NO has minimal effects on the normal pulmonary vasculature. In the injured state, as evidenced by the MCT model of pulmonary endothelial injury, exogenous NO has a much more pronounced effect. Pulmonary vascular injury reduces the capacity for endogenous NO production, leading to an increased level of constriction in vascular smooth muscle, thereby allowing any effect of exogenous NO to be more clearly demonstrated. Thus, NO appears to be necessary for regulation of basal pulmonary vascular tone and is even more important in the autoregulation in the lung under hypoxic conditions. These findings may be helpful in selecting patients likely to respond favorably to inhaled NO, and may predict which patients will fail to respond. When pulmonary hypertension results from a fixed anatomic defect or when pulmonary arterial smooth muscle is severely damaged, inhaled NO may be ineffective. Addi-

5 Ann Thorae Surg VAN CAMP ET AL 1029 tional clinical and experimental investigations are necessary to define the patient populations that will benefit most from inhaled NO. References 1. Roberts JD [r, Lang P, Bigatello LM, Vlahakes GJ, Zapol WM. Inhaled nitric oxide in congenital heart disease. Circulation 1993;87: Girard C, Lehot JJ, Pannetier jc. Filley S, French P, Estanove S. Inhaled nitric oxide after mitral valve replacement in patients with chronic pulmonary artery hypertension. Anesthesiology 1992;77: Pepke-Zaba J, Higenbottam TW, Dinh-Xuan AT, Stone 0, Wallwork J. Inhaled nitric oxide as a cause of selective pulmonary vasodilation in pulmonary hypertension. Lancet 1991;338: Roberts JD [r, Polaner OM, Lang P, Zapol WM. Inhaled nitric oxide in persistent pulmonary hypertension of the newborn. Lancet 1992;340: Rossaint R, Falke KJ, Lopez F, Slama K, Pison U, Zapol WM. Inhaled nitric oxide for the adult respiratory distress syndrome. N Engl J Med 1993;328: Ignarro LJ. Biological actions and properties of endotheliumderived nitric oxide formed and released from artery and vein. Circ Res 1989;65: Frostell c. Fratacci MD, Wain rc. Jones R, Zapol WM. Inhaled nitric oxide. A selective pulmonary vasodilator reversing hypoxic pulmonary vasoconstriction. Circulation 1991;83: Wiklund NP, Persson MG, Gustafsson LE, Moncada S, Hedqvist P. Modulatory role of endogenous nitric oxide in pulmonary circulation in vivo. Eur J Pharmacol 1990;185: Persson MG, Gustafsson LE, Wiklund NP, Moncada S, Hedqvist P. Endogenous nitric oxide as a probable modulator of pulmonary circulation and hypoxic pressor response in vivo. Acta Physiol Scand 1990;140: Elsner 0, Muntze A, Kromer EP, Riegger GAJ. Inhibition of synthesis of endothelium-derived nitric oxide in conscious dogs. Hemodynamic, renal, and hormonal effects. Am J Hypertens 1992;5: Archer SL, Rist K, Nelson DP, DeMaster EG, Cowan N, Weir EK. Comparison of the hemodynamic effects of nitric oxide and endothelium-dependent vasodilators in intact lungs. J Appl Physiol 1990;68: Nishiwaki K, Nyhan DP, Rock P, et al. N-omega-nitro-Larginine and pulmonary vascular pressure-flow relationship in conscious dogs. Am J Physiol 1992;262:H Ishii K, Chang B, Kerwin JF, et al. - n i t r o - L -a apotent r g i n i n e : inhibitor of endothelium-derived relaxing factor formation. Eur J Pharmacol 1990;176: Elsner 0, Muntze A, Kromer EP, Riegger GAJ. Inhibition of synthesis of endothelium derived nitric oxide in conscious dogs hemodynamic, renal, and hormonal effects. Am J Hypertens 1992;5: Dignan RJ, Yeh T, Dyke CM, Lutz HA, Wechsler AS. The influence of age and sex on human internal mammary artery size and reactivity. Ann Thorac Surg 1992;53: Reindel JF, Hoorn CM, Wagner JG, Roth RA. Comparison of response of bovine and porcine pulmonary arterial endothelial cells to monocrotaline pyrrole. Am J Physiol 1991;261:L Larson OF, Womble JR, Copeland JG, Russell DH. Concurrent left and right ventricular hypertrophy in dog models of right ventricular overload. J Thorac Cardiovasc Surg 1982;84: Reindel JF, Ganey PE, Wagner JG, Slocombe RF, Roth RA. Development of morphologic, hemodynamic, and biochemical changes in lungs of rats given monocrotaline pyrrole. Toxicol Appl Pharmacol 1990;106: Werchan PM, Summer WR, Gerdes AM, McDonough KH. Right ventricular performance after monocrotaline-induced pulmonary hypertension. Am J Physiol 1989;256:H Kawaguchi AT, Mizuta T, Matsuda H, et al. Single lung transplantation in rats with chemically induced pulmonary hypertension. J Thorac Cardiovasc Surg 1992;103: Perrella MA, Edell ES, Krowka MJ, Cortese DA, Burnett jc. Endothelium-derived relaxing factor in pulmonary and renal circulations during hypoxia. Am J Physiol 1992;263:R Gustafsson LE, Leone AM, Persson MG, Wiklund NP, Moncada S. Endogenous nitric oxide is present in the exhaled air of rabbits, guinea pigs and humans. Biochem Biophys Res Commun 1991;181: DISCUSSION DR JOHN C. WAIN (Boston, MA): This is very interesting work, studying the molecular mechanism for the efficacy of nitric oxide, which is being used clinically with greater frequency at the present time. I had two questions. Did you do any of your studies with the LNNA before monocrotaline administration? And secondarily, mechanistically, what do you think is going on when the exogenous nitric oxide seems to reverse hypoxic pulmonary vasoconstriction after monocrotaline injury, but does not appear to reverse the pulmonary vasoconstriction seen with monocrotaline in normoxic conditions? DR VAN CAMP: I think the answer to the first question is yes. You were asking whether we controlled the LNNA group before LNNA administration. The nitric oxide synthase inhibitor was administered acutely to animals that had been studied previously for a control state. They were injected with saline solution 28 days before, so in fact their results were entered into the control group. Therefore, there was no significant difference between those two control groups. Under normoxic conditions, increasing doses of nitric oxide did not change the pulmonary vascular resistance. I think that this probably indicates that under normal, uninjured and normoxic conditions, the baseline pulmonary vasomotor tone is at a dilated state, and that they have minimal reserve dilatational control as opposed to more constrictive control, so that in the normoxic normal state, they are already maximally dilated. DR CONSTANTINE MAVROUDIS (Chicago, IL): I enjoyed that very much. I have some preliminary data, which I would like to share with you, and then maybe ask you what your interpretation is of those data, because this was done in humans, and it was done in a series of patients with congenital heart disease who had repair. I was interested to see that one of your parameters was measuring cardiac output after administration of inhaled nitric oxide. We found the same thing, that when you close the ventricular septal defect, or whatever you are doing, and measure cardiac output, even doing ventricular function curves with inflow occlusion, the curves were exactly the same. I do not know why we thought we might find something different, but we did not, and that sort of confirms or at least is associated with what you found as well. But we did find something very interesting, and I think maybe

6 1030 VAN CAMP ET AL Ann ThoraeSurg 1994;58: one of the reasons why you did not show it is because of the mechanism by which you make pulmonary hypertension. These children already had pulmonary hypertension, but they had it on the basis of inflow, if you will, with a large left-to-right shunt or outflow with pulmonary vascular resistance. And it seems that the ones who benefited, and there was a clear benefit in reduction of pulmonary artery pressure, were those patients who had the obstruction on the other side of the pulmonary arterioles, that is to say, total anomalous pulmonary venous return, cor triatriatum, and mitral valve disease, mitral ring, and so forth. That is the preliminary thing that we are finding and we do not have enough patients; we have about 15 or 20 patients to say this. What do you think about that? Because one of your projections was to find which patients are going to benefit from this, as we are as well, I would be interested to hear what your comments are on anything that I have said and anything that you want to say further. Thank you. This was a very nice paper. DR VAN CAMP: We similarly tried inhaled nitric oxide anecdotally in a couple of patients who had single ventricle physiologies, and we showed no decrease in pulmonary vascular resistance in the intensive care unit setting. We assumed that this was because they had normal lungs and the endothelium worked correctly. It may be that in congenital heart disease where there is disease on the left side or in the venous side, there may be some underlying intrinsic injury to the endothelium that makes that particular patient population responsive to exogenous nitric oxide. It certainly is an interesting question, and I do not think this study answers that question for you. Bound volumes available to subscribers Bound volumes of the 1993 issues of The Annals of Thoracic Surgery are available only to subscribers from the Publisher. The cost is $96.00 (outside US add $20.00 for postage) for volumes 55 and 56. Each bound volume contains a subject and author index, and all advertising is removed. The binding is durable buckram with the name of the journal, volume number, and year stamped on the spine. Payment must accompany all orders. Contact Elsevier Science Inc, 655 Avenue of the Americas, New York, NY 10010; or telephone (212) (facsimile: (212) ).