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Ann Thorac Surg 2004;78:933-941
© 2004 The Society of Thoracic Surgeons
a Division of Cardiothoracic Surgery, Medical University of South Carolina, Charleston, South Carolina, USA
b Division of Pediatric Cardiology, Medical University of South Carolina, Charleston, South Carolina, USA
c Division of Pediatric Critical Care, Medical University of South Carolina, Charleston, South Carolina, USA
Accepted for publication April 5, 2004.
* Address reprint requests to Dr Bradley, Division of Cardiothoracic Surgery, Medical University of South Carolina, 96 Jonathan Lucas St, Charleston, SC 29425, USA.
bradlesm{at}musc.edu
Presented at the Fortieth Annual Meeting of The Society of Thoracic Surgeons, San Antonio, TX, Jan 26–28, 2004.
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Abstract |
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METHODS: From June 2000 to November 2003, 44 consecutive neonates with hypoplastic left heart syndrome underwent a Norwood procedure. The first 25 patients received an mBT shunt; the subsequent 19 an RV-PA shunt. Hemodynamic data, including mixed venous oxygen saturation, was determined during the first 48 hours after surgery.
RESULTS: The mBT and RV-PA shunt patients had no significant differences in systemic oxygen saturation, mixed venous oxygen saturation, arteriovenous oxygen saturation difference, or oxygen excess factor during the first 48 hours. Mixed venous saturation declined to a nadir in both groups at 6 to 12 hours. The RV-PA patients had significantly higher diastolic and mean blood pressures, and lower systolic blood pressure. Mean heart rate, common atrial pressure, and inotrope score did not differ between the two groups. The RV-PA patients received higher fraction of inspired oxygen and minute ventilation to achieve partial pressures of arterial oxygen and carbon dioxide, and pH, similar to mBT patients. Durations of mechanical ventilation, intensive care unit stay, and hospital stay did not differ between mBT and RV-PA patients. Operative survival in the mBT versus RV-PA group was 20 of 25 (80%) versus 17 of 19 (89%; p = 0.7).
CONCLUSIONS: Indicators of postoperative systemic oxygen delivery are equivalent in neonates who have undergone a Norwood procedure with an mBT or RV-PA shunt. Both mBT and RV-PA patients undergo similar declines in hemodynamic status 6 to 12 hours after surgery. Any advantages of one approach over the other lie in areas other than systemic oxygen delivery, such as resistance to physiologic insults, or preservation of ventricular function.
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Introduction |
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TopAbstractIntroductionPatients and methodsResultsCommentDiscussionReferences |
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The physiologic goal of postoperative management after a Norwood procedure is to optimize systemic oxygen delivery, as indicated by mixed venous oxygen saturation, and arteriovenous oxygen saturation difference. Monitoring mixed venous oxygen saturation has been an important addition to postoperative management [11–13]. The mixed venous saturation provides an accurate indicator of postoperative hemodynamic status. Although the RV-PA shunt is purported to improve postoperative hemodynamic status, its effects on mixed venous oxygen saturation and systemic oxygen delivery in the early postoperative period have not been reported. The aim of this study is to compare hemodynamic status, in particular systemic oxygen delivery, in patients undergoing a Norwood procedure with an RV-PA versus an mBT shunt.
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Patients and methods |
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TopAbstractIntroductionPatients and methodsResultsCommentDiscussionReferences |
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The operation included pulmonary homograft reconstruction of the neoaorta in all cases. All shunts were nonvalved, polytetrafluoroethylene material (Gore-Tex, W.L. Gore & Associates, Inc, Flagstaff, AZ). Modified Blalock-Taussig shunts were placed from the innominate artery to the central pulmonary arteries; RV-PA shunts were placed from the right ventricular infundibulum to the central pulmonary artery confluence. In general, patients weighing less than 3.0 kg received a 3.5-mm mBT or a 5.0-mm RV-PA shunt; those weighing more than 3.0 kg received a 4.0-mm mBT or 6.0-mm RV-PA shunt (Table 1). Regional low flow perfusion through the innominate artery was used during aortic arch reconstruction in 42 of 44 patients. All patients received aprotinin during operation and underwent modified ultrafiltration. Operations were performed by a single surgeon (S.M.B.).
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Arterial blood gases and systemic blood pressure were determined from umbilical or femoral arterial catheters. Systemic arterial oxygen saturation (SaO2) was measured by extremity pulse oximetry (Hewlett-Packard), verified by cooximetry. Common atrial pressure was measured by means of transthoracic catheters placed in the operating room. Mixed venous oxygen saturation (SvO2) was determined by cooximetry from samples drawn from transthoracic catheters in the superior vena cava. These catheters were placed through the right atrial free wall into the superior vena cava; position was verified by postoperative chest roentgenogram. Arteriovenous oxygen saturation (AVO2) difference was calculated as SaO2 minus SvO2. Oxygen excess factor [14] was calculated as systemic oxygen delivery/systemic oxygen consumption, or CaO2 x Qs/(CaO2 – CvO2) x Qs = SaO2/(SaO2 – SvO2); where CaO2 is the oxygen content of systemic arterial blood, CvO2 is the oxygen content of systemic venous blood, and Qs is the systemic blood flow. If oxygen consumption remains constant, then the oxygen excess factor is directly proportional to systemic oxygen delivery [14]. Pulmonary venous saturation was not measured in our patients. Calculations of Qp/Qs were therefore avoided because of the realization that small errors in assumed pulmonary venous saturation result in relatively large errors in the calculated Qp/Qs [14, 15]. Inotropic score was calculated as dose of (dopamine) + (milrinone x 10) + (epinephrine x 100) [16].
Analysis
Hemodynamic data were determined during the first 48 hours after surgery. The analysis excluded data from 1 patient in the mBT shunt group, who was placed on extracorporeal membrane oxygenation during the first 48 hours, and from 2 patients in the RV-PA group, who underwent takedown to mBT shunts (see Results). Operative mortality was defined as death within 30 days of surgery, or before hospital discharge. Data are shown as mean ± standard error of the mean or median (range), as noted. Data from the mBT and RV-PA groups were compared by unpaired Student's t test, Mann-Whitney U test, Fisher's exact test, or 
2 test, as appropriate. Postoperative hemodynamic data determined during the first 48 hours were analyzed by repeated-measures analysis of variance. Statistical significance was defined as p less than 0.05.
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Results |
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TopAbstractIntroductionPatients and methodsResultsCommentDiscussionReferences |
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Hemodynamic data
The mBT and RV-PA patients had no significant differences in mean systemic oxygen saturation, mixed venous oxygen saturation, or arteriovenous oxygen saturation difference during the first 48 hours (Fig 1A–1C). During this period, systemic oxygen saturation was fairly constant in both groups (Fig 1A). In contrast, mixed venous oxygen saturation declined to a nadir at 6 hours, before rising to a plateau at 18 to 24 hours (Fig 1B). Arteriovenous oxygen saturation difference demonstrated a corresponding peak at 6 hours in both groups (Fig 1C). The oxygen excess factor, reflecting systemic oxygen delivery, had a nadir at 6 hours after surgery, which was also independent of the type of shunt (Fig 1D).
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Resource utilization did not differ between the mBT and RV-PA groups. The median duration of mechanical ventilation was 6 days (range, 4 to 29 days) in mBT and 5 days (range, 3 to 22 days) in RV-PA patients (p = 0.7). Median intensive care unit stay was 14 days (range, 9 to 34 days) in mBT and 12 days (range, 7 to 30 days) in RV-PA patients (p = 0.3). Median hospital stay was 22 days (range, 13 to 66 days) in mBT and 24 days (range, 11 to 55 days) in RV-PA patients (p = 0.7). Interstage mortality among the patients discharged from the hospital with an mBT shunt was 4 of 22 (18%). No patient discharged with an RV-PA shunt has died: 12 have completed, and 3 await second-stage palliation (p = 0.1 versus mBT patients).
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Comment |
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TopAbstractIntroductionPatients and methodsResultsCommentDiscussionReferences |
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The use of an RV-PA shunt to provide pulmonary blood flow in palliative surgery for hypoplastic left heart syndrome was initially explored by Norwood in the early 1980s [17]. However, this approach was soon abandoned in favor of a systemic-to-pulmonary shunt. In the interim, major advances were made in the surgical management and perioperative care of neonates with hypoplastic left heart syndrome. Despite these advances, the success of the Norwood procedure has remained at a level below that achieved for complex, biventricular repairs in neonates. Operative mortality in several recent series falls between 15% and 30% [1, 2, 8, 18–22]. Furthermore, interstage death between hospital discharge and second-stage palliation has been reported in an additional 12% to 24% of patients [22–24]. The systemic-to-pulmonary shunt produces diastolic runoff from the aorta, with reduced coronary perfusion pressure. Reduced myocardial oxygen delivery has been demonstrated in survivors of a Norwood procedure [25] and is one of the potential causes of both operative and interstage mortality. In an effort to improve postoperative hemodynamic status, several groups have revisited the use of an RV-PA instead of an mBT [3–10].
The physiologic goal of postoperative management after a Norwood procedure is to optimize systemic oxygen delivery, as indicated by mixed venous oxygen saturation and arteriovenous oxygen saturation difference. Monitoring mixed venous oxygen saturation has been an important addition to postoperative management [11–13]. The mixed venous saturation can identify patients at risk for early mortality, and has proven to be a useful and accurate indicator of postoperative hemodynamic status [11–13]. It is therefore of interest to define the relative effects of the mBT and RV-PA shunt on mixed venous oxygen saturation and systemic oxygen delivery in the early postoperative period. By increasing coronary perfusion pressure, the RV-PA shunt might be expected to improve myocardial performance and cardiac output, and thus increase systemic oxygen delivery. On the other hand, the effects of a ventriculotomy might harm myocardial performance and decrease systemic oxygen delivery. The overall effect on oxygen delivery of the RV-PA compared to the mBT shunt has not been previously studied.
This study suggests that during the first 48 hours, systemic oxygen delivery is equivalent in patients with either an mBT or an RV-PA shunt. Several indicators of systemic oxygen delivery, including mixed venous saturation, arteriovenous saturation difference, and oxygen excess factor, were examined. Not only the absolute value but also the pattern of each of these indicators was similar in the two groups. The mean systemic oxygen saturation in both groups was fairly constant, at approximately 78%. In contrast, mean mixed venous saturation declined at 6 to 12 hours. A similar decline in mixed venous saturation early after a Norwood procedure with an mBT shunt has been observed by other groups [11, 26, 27]. This decline is similar to that in patients who have undergone an arterial switch procedure [28] and may reflect a period of depressed myocardial function. Arteriovenous oxygen saturation difference showed a corresponding increase at 6 to 12 hours, which was also similar in mBT and RV-PA patients.
The oxygen excess factor is defined as the ratio of systemic oxygen delivery to oxygen consumption [14, 27]. If oxygen consumption remains constant, the oxygen excess factor is directly proportional to systemic oxygen delivery. The oxygen excess factor has been used as a measure of oxygen delivery in a previous computer model of the single ventricle circulation [14], as well as studies of both preoperative and postoperative Norwood patients [27, 29]. In the current study, oxygen excess factor reflected mixed venous saturation, with a nadir at 6 to 12 hours, and no difference between mBT and RV-PA patients. These results suggest that the mBT and RV-PA approaches result in equivalent oxygen delivery in the early postoperative period. Pulmonary venous oxygen saturation was not measured in this study, so we did not calculate the ratio of pulmonary to systemic blood flow (Qp/Qs) [14, 15]. However, the similarity of systemic and mixed venous saturations in the mBT and RV-PA patients suggests that the two approaches also result in equivalent Qp/Qs.
In the current study, the only hemodynamic measurement that differed significantly between mBT and RV-PA patients was systemic blood pressure. A higher diastolic pressure and narrower pulse pressure are the expected results of a shunt that avoids diastolic runoff from the aorta. These differences between mBT and RV-PA patients are in agreement with several previous reports [6, 8–10]. Other hemodynamic variables, including heart rate and common atrial pressure, did not differ between our two patient groups. The similar values of atrial pressure in the mBT and RV-PA patients further supports the suggestion that the two approaches result in equivalent Qp/Qs and ventricular volume load, at least in the early postoperative period.
The level of inotropic support during the first 48 hours tended to be lower in RV-PA patients, although this difference did not achieve statistical significance. The lower inotropic score may reflect the comfort of caregivers with the higher diastolic and mean blood pressure in RV-PA patients. In both mBT and RV-PA patients, inotropic support peaked at 12 to 18 hours. This finding further highlights this period as one of compromised hemodynamic status.
Compared with mBT patients, the RV-PA patients received higher levels of ventilatory support to achieve equivalent arterial blood gas values. During the first 48 hours, the mean fraction of inspired oxygen, ventilator rate, and minute ventilation were higher in RV-PA patients, whereas there were no differences in partial pressure of arterial oxygen, partial pressure of arterial carbon dioxide, and pH. The reasons for the higher ventilatory support are unclear. In a recent prospective, patient-controlled study, we reported that a higher fraction of inspired oxygen can improve oxygen delivery, and that higher levels of ventilation are not harmful in postoperative Norwood patients [30]. These findings may have led to a decreased tendency to wean from ventilatory support in our more recent experience, rather than an absolute need for higher support in the RV-PA patients. Nonetheless, our findings do not support the hypothesis that mBT and RV-PA patients require markedly different approaches to postoperative ventilatory management.
Several groups have reported that use of the RV-PA instead of the mBT shunt significantly decreased operative mortality [5, 6, 8, 10], as well as interstage mortality [10]. We also observed lower operative and interstage mortality among RV-PA patients, although neither of these differences was statistically significant given the relatively small number of patients. Our findings indicate that improved survival in RV-PA patients does not appear to be caused by higher postoperative systemic oxygen delivery. Other possibilities include a greater resistance to physiologic insults and hemodynamic perturbations, perhaps related to higher coronary perfusion pressure. Another possibility is that despite involving a ventriculotomy, the RV-PA shunt may result in improved ventricular function as a result of improved coronary flow during the period between first-stage and second-stage palliation. These issues require further study.
This study is limited by its retrospective, nonrandomized design and the fact that the mBT and RV-PA patients were not operated on concurrently. Results in the RV-PA patients may have been influenced by a learning curve, with 2 patients undergoing takedowns to mBT shunts. The use of mixed venous oxygen saturation to guide postoperative management may have compensated for potential differences between the two groups that might be observed under a different management strategy. The results in both groups reflect the approach taken to shunt size selection. Our use of 6-mm RV-PA shunts in larger patients is similar to that of some groups [3, 4; Hanley FL. Presented at the Congenital Heart Disease Symposium of the 83rd annual meeting of the American Association for Thoracic Surgery, Boston, MA, May 2003 (unpublished data)]. Consistent use of a 5-mm RV-PA shunt [5–8, 10] may result in lower pulmonary blood flow and systemic oxygen saturation. Our RV-PA group had slightly higher hematocrits, which would be expected to result in slightly higher systemic oxygen content and delivery for equivalent systemic oxygen saturations.
In conclusion, indicators of postoperative systemic oxygen delivery are equivalent in neonates who have undergone a Norwood procedure with an mBT or RV-PA shunt. Hemodynamic differences between the two approaches are limited to lower systolic and higher diastolic and mean systemic blood pressures in the RV-PA group. Both mBT and RV-PA patients undergo similar declines in hemodynamic status 6 to 12 hours after surgery, marked by decreased mixed venous saturation, increased arteriovenous saturation difference, and increased need for inotropic support. In terms of oxygen delivery, the potential negative effects of a ventriculotomy in RV-PA patients appear to be offset by the potential positive effects of higher diastolic and coronary perfusion pressure. These findings indicate that any advantages of one approach over the other lie in areas other than systemic oxygen delivery, such as resistance to physiologic insults or preservation of ventricular function.
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Discussion |
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DR BRADLEY: We did specifically look at that and all of the information is included in the paper. What we found was that the ventilatory settings in the right ventricle–to–pulmonary artery shunt group were somewhat higher during the first 48 hours. Specifically, they had a higher fraction of inspired oxygen and higher ventilator rate and minute ventilation, resulting in equivalent blood gases, partial pressure of arterial oxygen, pH, and partial pressure of arterial carbon dioxide.
I think our overall impression is that there is not really a very marked difference in approach to the ventilatory support that is needed in the two groups. Some of the previous conclusions about the need for more ventilatory support in modified Blalock-Taussig shunt patients have been based on studies in which inspired gases, either carbon dioxide or nitrogen, have been used postoperatively; and we did not use either of those in any patient in either of these two groups.
DR CHRISTIAN PIZARRO (Wilmington, DE): I enjoyed your paper, Scott. The data are great.
I am not sure that in our practice we are seeing the same patient condition that you have related here. I find it highly unusual that the patients are ventilated for longer than 1 or 2 days, and in the intensive care unit for more than a week. Is it possible that your management strategy either during surgery or the perfusion, or some other factor, could have blunted the potential difference that you might see between these two sources of pulmonary blood flow?
DR BRADLEY: I would have to say that when we went into this study, it was our subjective impression that the right ventricle–to–pulmonary artery shunt patients are bit more stable in the intensive care unit, but we were not able to find any actual quantitative difference in any of the variables we looked at, other than the obvious differences in blood pressure.
I do believe that right ventricle–to–pulmonary artery shunt patients are more resistant to hemodynamic pertubations, and tend to spiral downhill less frequently than the modified Blalock-Taussig shunt patients.
In terms of time on the ventilator and in the intensive care unit, those things are obviously influenced by a large number of variables which vary from institution to institution. So I do not think I can meaningfully compare our experience directly with yours. In the course of this study, we did not deliberately change very much in the management of these patients, and there were no differences in resource utilization between the two groups.
DR CHRISTIAN BRIZARD (Melbourne, Australia): Congratulations for your study.
In Melbourne we have, as you did, compared our last 6 patients with a conventional Norwood and the first 6 conduits. And as you did, we did not find any metabolic differences in oxygen consumption and so on.
We also compared our patients using tissue Doppler imaging and a relatively sophisticated index. And that demonstrated a striking difference between the two groups, with a much better systolic function in the right ventricle–to–pulmonary artery conduit group.
Also, there was significant discrepancy between the value of the index in the conventional Norwood. And the one who did the worst on the clinical point of view, the 2 who died in that group, at late interstage mortality, had the worst index as well.
DR BRADLEY: So you found that the right ventricle–to–pulmonary artery shunt patients had better systolic function postoperatively?
DR BRIZARD: Yes. The index is a bit difficult to interpret. As it is in clinical settings, there are numerous factors that can influence that index. In particular, there is the lower ejection resistance afterload in the right ventricle–to–pulmonary artery conduit that could increase the ejection fraction and that index. But the gross result is that the index is strikingly better and very significantly better in the right ventricle–to–pulmonary artery group.
DR BRADLEY: I think that is very interesting and I would certainly like to see the information, because I think right now we do not have very good information on ventricular function in these two groups of patients.
DR BRIZARD: This paper has just been published in Chest.
DR JOHN E. MAYER (Boston, MA): It seems like we clearly are still in a learning curve phase with this operation. And I notice that you had some 5-mm and some 6-mm shunts, which raises two questions.
Number one, what was your standard management protocol for the timing of the second stage? Were they brought back at 3 months, 6 months, or whenever their saturations fell to a certain level?
The second related question is, did the patients who had the right ventricle–to–pulmonary artery type of pulmonary blood flow source develop more episodes of hypoxia or have a higher incidence of hypoxia requiring an earlier second stage then planned? That requirement for an earlier second-stage procedure has been reported by others who have used this procedure previously.
Finally, I would be interested in your comments on what you think the relationship is between an earlier second-stage procedure and what appeared to be a lower interstage mortality in the right ventricle–to–pulmonary artery conduit group. Do you think that lower mortality is related to just doing the second stage earlier?
DR BRADLEY: Part of my approach to shunt size selection was based on what had been written in a couple of the early studies by Kishimoto and Imoto. It is also very much along the lines of what was reported by Frank Hanley at least year's American Association of Thoracic Surgeons meeting.
Given this approach to shunt sizing, we have really not observed this phenomenon of the right ventricle–to–pulmonary artery shunt patients becoming hypoxic much earlier. Our general approach is to catheterize all the post-Norwood patients at 4 to 5 months of age and do their second stage between 4 and 6 months of age. Thus far, we have not altered that for the right ventricle–to–pulmonary artery shunt patients. I do realize that a number of groups, who I think have consistently used 5-mm shunts, have ended up moving to their second-stage surgery earlier.
In terms of interstage mortality, our numbers are small. The difference that I showed is interesting, but not yet statistically significant. I think Dr. Norwood's group is going to address this topic more extensively in the next talk.
DR DAVID OVERMAN (Minneapolis, MN): A nice study, Scott.
As you know, the problem with the modified Blalock-Taussig shunt in the perioperative period historically has been these episodic events for patients resulting in sudden death. And I noticed in your slides that in the modified Blalock-Taussig shunt group, there were several early, or what might be termed perioperative, deaths, as opposed to the right ventricle–to–pulmonary artery conduit group in which the first death was 4 weeks out from surgery. And I was wondering, in looking at the oxygen delivery, whether there may have been a greater "oscillation" of that delivery in the modified Blalock-Taussig shunt group, or perhaps an increased frequency of brief episodes of low oxygen delivery that were not present in the right ventricle–to–pulmonary artery conduit group. That might explain the latter group's resistance, as you say, to physiologic insult.
DR BRADLEY: Just to clarify, in the modified Blalock-Taussig shunt group, the earliest death was a week after surgery. As you point out, the deaths in the right ventricle–to–pulmonary artery shunt group did occur somewhat later.
Your suggestion is a good one. The monitoring that we use does give a continuous readout of superior vena cava saturation, but more rapid vacillations in mixed venous oxygen saturation are not something that we have observed, so I do not think I can answer your question well. I do think you are right that the right ventricle–to–pulmonary artery shunt patients are more resistant to hemodynamic pertubation.
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J. M. Simsic, A. Cuadrado, P. M. Kirshbom, K. R. Kanter, D. Ramaswamy, M. Clabby, and J. M. Forbess Novel management strategy for severe cyanosis after Sano modification of the Norwood procedure J. Thorac. Cardiovasc. Surg., June 1, 2005; 129(6): 1450 - 1451. [Full Text] [PDF]
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; p = 0.6), and (D) oxygen excess factor (p = 0.5) during the first 48 hours after Norwood procedure.
