The superior cavopulmonary (SCP, “Glenn procedure”) shunt is a palliative procedure predominantly used in patients with functionally univentricular hearts and occasionally to create a 1½ ventricle circulation. The outcomes of the SCP are improving, but high pulmonary vascular resistance (PVR) is still a significant risk factor.1 Inhaled nitric oxide (NO) therapy has been reported as safe and is widely used to treat postoperative pulmonary hypertension of cardiac surgery including congenital heart surgeries.2 It is believed that NO inhalation increases cardiac output (CO) by decreasing PVR that leads to increased preload of the left ventricle. However, there is little evidence about any beneficial effect of NO inhalation therapy in the SCP circulation, let alone in the 1½ ventricle circulation.
The purpose of this study was to evaluate the hemodynamic changes with inhaled NO therapy in lambs with 1½ ventricle circulation.
Materials and Methods
We used ten Suffolk lambs (4 to 8 weeks old; mean age: 6.0 ± 1.5 weeks). Because it has been reported that inhaled NO therapy after cavopulmonary shunt is used in patients with elevated pulmonary artery pressure (PAP) or PVR, we used young lambs with relatively high PVR.3 All animals received humane care in compliance with the Guide for the Care and Use of Laboratory Animals (National Academy Press, Washington DC, 1996). All conditions for animal surgery and care were approved by the Institutional Animal Care and Use Committee of Stanford University.
After induction with intramuscular ketamine (10 mg/kg), valium (0.5 mg/kg), and morphine (0.5 mg/kg), the animals were intubated and placed on isoflurane maintenance anesthesia. After placing a venous line on an ear, right carotid arterial line and right jugular venous line were inserted by cut-down. Arterial pressure, central venous pressure (CVP), percutaneous oxygen saturation, end-tidal carbon dioxide, electrocardiogram, and rectal temperature were monitored continuously.
A median sternotomy was performed, and the pericardium was opened. Heparin (100 IU/kg) was given, and a bypass between superior vena cava (SVC) and main pulmonary artery (PA) was constructed using a ringed expanded polytetrafluoroethylene graft between 8 and 12 mm diameter with running sutures under side clamping. A left atrial line and a PA line were placed. The azygous vein was ligated, and flow probes were placed on the ascending aorta and the SVC. Then, the SVC was clamped at the right atrial junction, and the graft was unclamped to create a SCP shunt, establishing a 1½ ventricle circulation (Figure 1). No blood transfusions or inotropes were used during the surgery.
When the surgical procedure was completed, we added inhaled NO to the ventilatory circuit with an INOvent drug delivery system (Mallinckrodt Pharmaceuticals, Chesterfield, UK) at a dose of 20 parts per million (ppm). All animals were mechanically ventilated with volume control mode. Because hypercapnia affects flow of the SCP shunt and systemic oxygenation,4,5 tidal volume and respiratory rate were controlled to keep end-tidal carbon dioxide levels stable. The fraction of inspiratory oxygen (FIO2) was kept at 50%.
We measured heart rate (HR), mean arterial blood pressure (ABP), left atrial pressure (LAP), mean PAP, CVP, SVC flow, ascending aortic flow as a surrogate for CO, and arterial blood gases before and 20 minutes after NO inhalation. Transpulmonary gradient (TPG) was calculated with (PAP−LAP), and PVR was calculated with (TPG/ascending aortic flow).
All results are expressed as means ± standard deviation. Statistical analysis was performed using commercially available software (JMP pro 12; SAS Institute, Cary, North Carolina). Paired t-test was used to compare the data before and after inhalation of NO for 20 minutes. A p value of <0.05 was regarded as statistically significant.
Hemodynamic Changes After Inhaled NO Therapy
There were no surgical complications during the study. HR (127.7 ± 13.9 to 123.4 ± 15.9 bpm; p = 0.082), mean ABP (46.6 ± 5.4 to 44.6 ± 5.9 mm Hg; p = 0.06), and LAP (4.0 ± 2.5 to 4.0 ± 2.3 mm Hg; p = 1.0) did not change significantly under NO inhalation. Mean PAP (13.6 ± 2.4 to 11.7 ± 2.9 mm Hg; p = 0.006), TPG (9.6 ± 2.5 to 7.9 ± 2.6 mm Hg; p = 0.019), and PVR (5.47 ± 2.9 to 4.54 ± 2.6 Wood; p = 0.037) decreased significantly. SVC flow (24.8 ± 11.3 to 22.0 ± 9.7 ml/min/kg; p = 0.09) did not change, and CO decreased (140.2 ± 37.2 to 132.1 ± 39.2 ml/min/kg; p = 0.033) (Figure 2).
Arterial Blood Gas Changes After Inhaled NO Therapy
Arterial oxygen tension (PaO2), arterial carbon dioxide tension (PaCO2), pH, base excess (BE), and oxygenation index (OI; mean ABP × FIO2/PaO2) were compared before and during NO inhalation. The results are shown in Figure 3. PaO2 increased significantly (103.72 ± 29.30 to 132.43 ± 47.02 mm Hg; p = 0.007), and OI decreased significantly (24.99 ± 9.38 to 19.57 ± 8.41; p = 0.0003). There were no changes in PaCO2 (43.25 ± 7.55 to 43.52 ± 7.16 mm Hg; p = 0.847), pH (7.41 ± 0.06 to 7.40 ± 0.06; p = 0.886), and BE (1.82 ± 2.28 to 1.87 ± 2.09 mmol/l; p = 0.847).
Because the cavopulmonary shunt is critically dependent on low PVR,6 NO inhalation therapy is used in patients with relatively high PVR after Glenn or Fontan surgery. There is some evidence that NO inhalation increases CO7 and systemic ABP8 after Fontan surgery. However, the effect of NO inhalation on CO or ABP in the Glenn circulation including a 1½ ventricle circulation remains unclear. Previous reports show that NO inhalation therapy reduced CVP, PAP, or TPG in patients with Glenn circulation.3,9–11 Agarwal et al. reported that they observed significant reductions of Glenn pressures, inotrope score, and fluid volume support after inhaled NO administration in patients who underwent Glenn operation with elevated PAP.9 They suggested that the decrease in Glenn pressures after NO inhalation therapy facilitated forward flow from the SVC to the pulmonary circulation and improved the stroke volume of the single ventricle and systemic perfusion. On the other hand, Gamillscheg et al. reported that NO inhalation therapy decreased CVP and TPG, but LAP and systemic ABP did not change in patients with critical pulmonary perfusion (CVP > 20 mm Hg or TPG > 10 mm Hg) after bidirectional Glenn anastomosis.10 They concluded that improvement of pulmonary perfusion by inhaled NO therapy might only partially influence LAP and systemic ventricular preload because blood from the inferior vena cava (IVC) reenters the systemic circulation directly without passing the lungs. The results of our study also showed decreased PVR, mean PAP, TPG, OI, and increased PaO2 with inhaled NO therapy, which suggests that pulmonary perfusion had improved. However, CO had decreased. We suggest three hypotheses to explain this phenomenon. First, in this animal model, blood flow from the IVC also enters the PA via the right ventricle. Because inhaled NO is a selective pulmonary vasodilator,12 more blood volume may pool in the pulmonary capacitance blood vessels resulting in decreased left ventricular preload unless additional blood volume is given. The increase of pulmonary capacitance with vasodilation has previously been reported by Newman et al.13 Second, although it was not significantly different, the trend toward a decreased HR may have led to the reduction of CO. A third hypothesis is that CO could have decreased because of improved oxygen delivery, given that PaO2 increased with NO. Regardless, inhaled NO did not improve CO in this specific animal model. The fact that SVC flow, LAP, and ABP did not change suggests that improvement in pulmonary perfusion did not have enough influence to increase left ventricular preload, which resulted in decrease of CO.
Although we used young lambs with relatively high PAP and PVR, PVR was still lower than that of patients who received NO inhalation in these other studies.3,9–11 In addition, the animals had completely normal left and right ventricular function. The response of CO to inhaled NO might be different in patients with elevated PAP or ventricular dysfunction because of pulmonary hypertension. Therefore, further investigation is needed with regard to whether NO has the same effects in patients with elevated PAP.
There is also controversy about the benefits of NO inhalation therapy to oxygenation. Most studies show that inhaled NO therapy improves oxygen saturation,3 OI,9 or arterial oxygen saturation10 in patients after bidirectional Glenn. Improvement of pulmonary perfusion or intrapulmonary distribution of blood flow and ventilation (V/Q distribution) was considered as contributing to improved oxygenation. Furthermore, it has been reported that NO inhalation improves oxygenation in patients after fenestrated Fontan operation because it reduces right-to-left shunt through the fenestration by reducing PVR.8,14 This might also contribute to the improvement of oxygenation after Glenn because patients receiving a Glenn procedure have cardiac shunts. Because our animal model did not have any cardiac shunts and no hypoxia, we think that improvement of V/Q distribution led to improved oxygenation. In contrast, Adatia et al. reported that inhaled NO did not improve systemic oxygenation in patients with bidirectional Glenn.11 They retrospectively analyzed the effect of inhaled NO in 26 patients and observed a significant decrease in CVP, but systemic oxygenation remained unchanged. They concluded that hypercapnia, which increases cerebral blood flow and consequently increases SVC flow, is more important than NO inhalation to improve oxygenation. They quoted a report from Bradley et al. that hypercapnia produced by hypoventilation significantly increased PaO2 and systemic oxygen saturation in 15 patients after bidirectional Glenn.4 However, the mean PAP (14 ± 1 mm Hg) of the patients in this study was lower than that of NO inhalation therapy studies mentioned above. Mean PAP of patients receiving NO inhalation was 18.1 ± 2.3 mm Hg,3 22.4 ± 3.9 mm Hg,9 and 24.5 ± 1.2 mm Hg,10 respectively. Because hypercapnia is well known to increase PVR, the effect of hypercapnia on the patients who already have elevated PAP might be different. Therefore, it is unclear that hypercapnia is effective for patients with elevated Glenn pressure. Our results support the evidence that inhaled NO improves oxygenation in patients with 1½ ventricle circulation.
A limitation of our animal model is the fact that it establishes a 1½ ventricle circulation with a biventricular heart without cardiac shunts or hypoxia. This model cannot reflect the effect of NO in the more common form of SCP circulation in the setting of a single ventricle. The only single ventricle model in animals that we are aware of is a complex model used by Riordan et al. that involves the creation of an ASD and tricuspid insufficiency by using catheter intervention with a balloon,15 which we were not aware of at the time of our experiments. We also did not think we would safely be able to reproduce this model in our laboratory. We may, however, attempt this in the future, and it may lead to different results.
However, we still believe that the results of this animal model may be helpful to improve the understanding of cavopulmonary circulation.
Inhaled NO significantly improved pulmonary perfusion and systemic oxygenation but decreased CO in this specific animal model of a 1½ ventricle circulation with mild pulmonary hypertension and normal ventricular function. These results suggest that inhaled NO therapy has beneficial effects to pulmonary perfusion and systemic oxygenation after bidirectional Glenn surgery. However, further investigation is needed about whether inhaled NO has the same effects in patients with elevated PAP or ventricular dysfunction in the setting of single ventricle anatomy.
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