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J Thorac Cardiovasc Surg 1999;117:529-542
© 1999 Mosby, Inc.
SURGERY FOR CONGENITAL HEART DISEASE |
From the Departments of Cardiac Surgery,a Cardiology,b and Surgery,c Children's Hospital, Boston, Mass.
Received for publication Dec 29, 1997. Revisions requested April 9, 1998; revisions received June 8, 1998. Accepted for publication Oct 7, 1998. Address for reprints: Jay M. Wilson, MD, Department of Surgery, Children's Hospital, 300 Longwood Ave, Boston, MA 02115.
| Abstract |
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| Introduction |
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Extracorporeal membrane oxygenation (ECMO) has been used less frequently for support in adult patients with cardiac disease. In pediatric patients with cardiac disease, however, ECMO has been the most commonly used form of mechanical circulatory support. A major reason for this is the ready availability of ECMO for the treatment of neonatal respiratory failure at pediatric centers. We have successfully used both ECMO and VAD systems, depending on the clinical situation, to support pediatric patients with cardiac disease. This report describes our experience with ECMO and VAD systems to provide mechanical circulatory support in children with cardiac disease.
| Patients and methods |
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Multiple clinical variables were analyzed for their impact on survival to hospital discharge by means of a statistical program (JMP Software, SAS Institute, Inc, Cary, NC). Two sets of variables were evaluated depending on their temporal relationship to the initiation of circulatory support with either ECMO or VAD (Appendix: Pre-support and post-support variables). These clinical variables included demographic factors, details of cardiopulmonary bypass, hemodynamics before and after support, ventilatory parameters before and after support, and details regarding the conduct of support including complications. The impact of continuous variables on the survival of patients was then assessed with parametric (Student t test) or nonparametric (Wilcoxon rank sum test) tests of statistical significance. Categoric variables were assessed with Pearson's
2 test. Pre-support factors with an influence on survival at a significance level of 0.10 or less were then subjected to multivariate analysis by means of a stepwise logistic regression model. Comparisons of outcomes in terms of cardiac diagnoses and indications for support for ECMO versus VAD were performed by Pearson's
2 test. Comparisons of complication rates between ECMO and VAD and the influence of neck cannulation or carotid reconstruction on the incidence of neurologic complications in ECMO-supported patients were also performed with Pearson's
2 test.
Components of the ECMO circuit
Techniques that we used for management of the ECMO circuit have been previously reported for neonatal and pediatric respiratory support.
1,2 In brief, we used a servoregulated flow system driven by a roller pump with a membrane oxygenator (Avecor ECMO Membrane Oxygenator; Avecor Cardiovascular, Inc, Plymouth, Minn). Pre-membrane and post-membrane in-line pressure monitors were used. A disposable heat exchanger was used to maintain constant temperature of blood in the circuit. The surface area of the oxygenator, based on the patient's size, determined priming volumes of the circuit and ranged from 350 mL (neonates) to 2.5 L (adults). Activated clotting times were maintained within 180 to 220 seconds by a continuous heparin infusion and were maintained at lower levels if significant bleeding occurred on support. Antifibrinolytic therapy with aminocaproic acid (Amicar; Lederle Parenterals, Carolina, Puerto Rico) was used in essentially all of these patients after 1990.
1 Aminocaproic acid was initiated as an intravenous bolus of 100 mg/kg, maintained as a continuous intravenous infusion of 30 mg/kg per hour for the initial 48 hours of support, and then discontinued.
Cannulation for ECMO
The site of cannulation for ECMO was based on the discretion of the surgeon, but in general, patients requiring support in the immediate postoperative period had direct transthoracic cannulation of the aorta and the right atrial appendage. Transthoracic cannulation was especially useful to expedite the institution of support in patients who had cardiac arrest in the postoperative period. Peripheral cannulation via the neck or femoral vessels was generally performed in patients who had not had cardiac surgery or in patients who required ECMO later in their postoperative course because of concerns about mediastinal adhesions. Neck cannulation was performed in infants and young children, whereas older children and young adults were often cannulated via the femoral vessels. Carotid reconstruction was attempted in all cases after neck cannulation; however, this was not always possible in cases in which extensive endothelial or full-thickness injury of the artery had occurred.
Components of the VAD circuit
Our entire experience with VADs used a centrifugal pump system (Bio-Pump; Medtronic Bio-Medicus, Minneapolis, Minn). For infants and children less than 10 kg the 50-mL Bio-Pump was used. For patients above 10 kg the 80-mL Bio-Pump was used. The priming volume for the 50-mL Bio-Pump and 1/4-inch tubing to complete the circuit was approximately 180 mL. For the 80-mL Bio-Pump and 3/8-inch tubing the priming volume was approximately 350 mL. After September of 1994 we used cannulas and polyvinylchloride tubing coated with the Carmeda BioActive Surface (Medtronic Corporation, Minneapolis, Minn) in VAD circuits. The Carmeda BioActive Surface was not used for ECMO tubing and cannulas. The activated clotting time was maintained at 180 to 200 seconds, and lower activated clotting times (160-180 seconds) were maintained if tubing coated with the Carmeda BioActive Surface was used.
| Results |
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The proportions of these patients successfully weaned from support (ECMO 45/67 [67.2%]; VAD 19/29 [65.5%]) and those who survived to hospital discharge (ECMO 27/67 [40.3%]; VAD 12/29 [41.4%]) were nearly identical for the 2 modes of support (Fig. 1). The median age of the ECMO-supported patients (2.6 months [range 1 day243 months]) was considerably younger than that of the VAD-supported patients (20.2 months [range 2 days280 months]). This was also reflected in the median weights for the 2 groups (ECMO 4.3 kg [range 2.4-82 kg] vs VAD 9.0 kg [range 2.7-71 kg]). Fig. 2 demonstrates the survival by age group for ECMO and VAD. The median duration of support was longer for the ECMO-supported patients (4.8 days [range 0-29 days]) than for the VAD-supported patients (1.8 days [range 0-7.8 days]).
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Cardiac diagnoses and indications for support
The survival according to cardiac diagnoses (Table I) and the indications for support (Table II) are as listed. The number of runs in Table II
exceeds the number of patients in Table I
because of the multiple periods of support in the 3 ECMO- and 2 VAD-supported children discussed earlier. As seen in Table II
, a significant number of ECMO-supported patients required support for hypoxemia (25/70 [35.7%]), which included a large number of children requiring support for pulmonary disease (pneumonia, pulmonary hemorrhage) irrespective of their cardiac disease. The indication for support was for cardiac arrest in 17 of 70 (24.3%) ECMO runs and 5 of 32 (15.6%) VAD runs. The survival for this critically ill subset of patients (7/17 [41%] ECMO survivors; 2/5 [40%] VAD survivors) was equivalent to the survival for all other indications for each modality (ECMO 20/53 [38%], VAD 10/27 [37%]). Three of the 7 ECMO-treated patients and 1 of the 2 VAD-treated patients surviving after pre-support cardiac arrest had cardiopulmonary resuscitation for more than 1 hour before the institution of support. Statistical analysis comparing outcomes in terms of underlying cardiac diagnosis or indications for support was performed for ECMO versus VAD. There was no difference in outcome for ECMO- or VAD-supported patients when analyzed for a given diagnostic group or specific indication for support (data not shown).
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Multivariate analysis of survival factors
Pre-support factors that had a .10 or smaller P value for all patients supported with ECMO or VAD were then analyzed with a multivariate model by means of stepwise logistic regression. Multivariate analysis was also performed on pre-support factors with a .10 level of statistical significance by univariate analysis for patients supported with either ECMO or VAD in the perioperative period. No combination of factors attained a .05 significance level with these analyses.
Transplantation experience
The patterns of use of mechanical circulatory support and outcomes for patients undergoing cardiac transplantation are listed in Table IV. Five patients were successfully supported with ECMO (2 patients) or VAD (3 patients) as a bridge to transplantation. Four of these patients survived to hospital discharge, whereas a single VAD-supported patient died after cardiac transplantation. In 2 patients attempted bridging to transplantation was unsuccessful (1 ECMO, 1 VAD), that is, support was initiated in anticipation of transplantation but was discontinued because of complications before a donor organ became available.
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| Discussion |
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Cardiac diagnoses
Examining the underlying diagnoses of these patients demonstrates that the appropriate use of ECMO or VAD is based on considerations regarding the specific anatomy and physiology present in a given case. Cyanotic heart disease was present in more than half of the children who were supported with ECMO. The most common diagnoses in the VAD-supported patients were predominant univentricular failure such as ALCAPA or cardiomyopathy. We believe that ECMO is superior to VAD for the support of most children with complex congenital heart disease such as cyanotic lesions in which hypoxia, pulmonary hypertension, or biventricular failure contributes to the pathophysiology necessitating mechanical circulatory support. ECMO provides greater flexibility than VAD in these instances. In lesions in which univentricular failure predominates, such as ALCAPA, VAD provides an effective approach.
11 These considerations are generally true, but we and others have successfully used VAD to support newborn infants with complex cyanotic lesions when univentricular failure predominates.
9,10
Indications for support
Examination of the indications for support demonstrates the high incidence of hypoxia and pulmonary hypertension in the ECMO-treated patients. ECMO was required in several of these patients despite the use of high-frequency ventilation, nitric oxide, and liquid ventilation. These measures have been reported to decrease the need for ECMO when used to treat neonatal and pediatric respiratory failure.
12,13 Our experience indicates that these adjunctive respiratory measures may fail when severe hypoxia occurs in the setting of congenital heart disease, with many of these children still ultimately requiring ECMO support.
The survival for patients who required ECMO or VAD support after having a cardiac arrest was nearly identical to the survival for all other indications. These results are in agreement with earlier reports that demonstrate survival of 25% to 65% after cardiac arrest in these patients with good long-term functional status.
14,15 In the surviving patients rescued with ECMO, there was no higher complication rate for any organ system. Importantly, the incidence of neurologic complications was no different from that in other survivors (data not shown).
Contraindications for support
We have not developed rigid contraindications for mechanical support but evaluate each case individually. Severe dysfunction of other organ systems (especially central nervous system) before initiation of support and extreme prematurity represent situations in which support would not be offered. We have successfully supported patients with a shunted single ventricle, patients with pre-support cardiac arrest, patients undergoing palliative cardiac operations, and patients with coexisting congenital diaphragmatic hernia. None of these represent absolute contraindications for support in our experience.
Determinants of survival
Determinants of the adequacy of perfusion within the first 24 hours of support as determined by the pH, serum bicarbonate, and urine output had prognostic significance in the ECMO group. We had no survivors with a pH below 7.38 or a serum bicarbonate below 22 mmol/dL at 24 hours of support. Inasmuch as cardiac output was presumably normalized by the institution of support, abnormal values for these variables measured at this early point probably reflected the impact of significant hypoperfusion in the period before the institution of ECMO, rather than ongoing systemic hypoperfusion. The requirement of significant pressor support, with dopamine or epinephrine, to maintain perfusion during perioperative ECMO carried a poor prognosis.
Patients requiring ECMO support for failure to wean from cardiopulmonary bypass had a 90% mortality rate. Although not a statistically significant improvement in outcome, VAD-supported patients who could not be weaned from cardiopulmonary bypass fared somewhat better, with 7 survivors of 19 total patients (37%). A major difference in these 2 groups was due to the influence of the LVAD-supported patients with ALCAPA, many of whom cannot be weaned from cardiopulmonary bypass, yet have an overall good outlook with mechanical circulatory support.
11 The predominance of younger patients with complex cyanotic lesions was largely responsible for the poorer outcome of patients with ECMO support instituted in the operating room. Failure to wean from cardiopulmonary bypass is not a contraindication for ECMO support, but its use in younger patients with complex anatomy should be selective in this setting because of an appreciation of the poor outlook for this group.
Conclusions were more difficult to draw in the VAD-supported group because of their smaller numbers; however, right-sided filling pressures on support, the use of circulatory arrest during the cardiac operation, and the curative or palliative nature of the cardiac repair were important clinical parameters for outcome in patients supported in the perioperative period. Right ventricular failure manifest by an elevated central venous pressure in LVAD-supported patients should suggest the need for ECMO or BVAD. The need for hypothermic circulatory arrest for operative repair implies a complex procedure on a newborn patient. As previously discussed, newborn patients with complex heart disease are often best supported with ECMO.
Complications
Although hemorrhagic complications were the most common complication in both groups, excessive blood loss was a statistically significant risk factor for death only in the ECMO-supported patients. Risk factors for excessive bleeding in our patients included chest cannulation and the need for support in the operating room (data not shown). We have previously described the use of aminocaproic acid to control bleeding while on ECMO,
1 which we also use for VAD-supported patients. We have used aprotinin and tranexamic acid only sporadically.
Considered by organ system, only renal failure in ECMO-supported patients had a significant negative impact on survival. Neurologic complications were more common during ECMO support than during VAD support. Neurologic complications were not associated with higher mortality in these patients but obviously represent a source of great morbidity. Much of the increased incidence of neurologic complications in the ECMO-supported patients can be attributed to higher rates of intracranial hemorrhage. This higher incidence of intracranial bleeding reflects the younger age of the ECMO-supported patients, including a large number of newborn infants in addition to the higher levels of anticoagulation required for the ECMO circuit. The higher incidence of central nervous system complications was independent of carotid cannulation or reconstruction.
The relatively higher rate of mechanical complications in the ECMO group is due to the increased complexity of the circuit. The presence of the oxygenator itself is a significant source of morbidity resulting in trauma to blood elements and activation of systemic inflammatory and coagulation cascades. In addition, multiple connector sites required in the ECMO circuit increase the risk of air and particulate embolism, and oxygenator failure requires interruption of flow and replacement. Complications related to the circuit were not associated with a significantly higher mortality.
Survival and length of time until return of ventricular function
We observed that ventricular function returned early after the institution of support in survivors who did not require transplantation. Lack of return of ventricular function within 48 to 72 hours was an ominous sign in our experience. We have used these results as additional prognostic data for these children. Patients without return of ventricular function within 48 to 72 hours of support are currently considered for transplantation or termination of support if there are contraindications to transplantation. Delaying this decision while awaiting return of ventricular function beyond the first 48 to 72 hours of support is not justified according to our data. Because of the scarcity of organ donors in the pediatric population, early consideration for transplantation optimizes the chances of successful organ procurement. Over this 10-year period only seven patients were supported with ECMO or VAD in an attempt to bridge them to transplantation, with five patients having successful transplantation. Our current practice of listing these children for transplantation when ventricular function has not returned after 48 to 72 hours may increase the number of patients who are successfully bridged to transplantation.
Cardiac failure and multiple system organ failure were the major causes of death in both ECMO- and VAD-supported patients. For both of these conditions, early and aggressive consideration for transplantation may lead to higher salvage rates. We have maintained patients for as long as 4 weeks with mechanical circulatory support; however, the development of infectious complications and multiple system organ failure eventually supervenes. The use of pulsatile circulatory perfusion devices for long-term support in children may delay the development of this process. Although no pulsatile flow systems are currently available in the United States for chronic pediatric circulatory support, the development of such devices will be a welcome addition to the therapeutic options for these critically ill children.
Selection of the appropriate modality of support: ECMO versus VAD.
Table IX is a "scorecard" reflecting the relative advantages and disadvantages of ECMO and VAD in the treatment of pediatric patients with cardiac disease. These general points may aid in selecting the appropriate mode of support in a given clinical setting. Pediatric centers have extensive experience with ECMO for the treatment of neonatal respiratory failure where expansion of its use to include patients with cardiac disease is often easily accomplished. Because of the absence of an oxygenator, VAD circuits are simpler, require less anticoagulation, and result in less blood trauma. ECMO may be instituted peripherally with neck or groin cannulation, whereas VAD requires a sternotomy. Left ventricular decompression is effectively performed by LVAD or BVAD; in patients supported with ECMO, the occurrence of left ventricular distention requires aggressive monitoring and prompt treatment with left atrial venting or balloon atrial septostomy. ALCAPA is the paradigm lesion successfully supported with LVAD, whereas ECMO provides greater flexibility in dealing with some forms of complex congenital heart disease in which pulmonary hypertension and hypoxia contribute significantly to the pathophysiology. Finally, biventricular support is easier to institute with ECMO, which requires only 2 cannulation sites compared with 4 cannulation sites required for BVAD. Because of size constraints, this is an important consideration when biventricular support is required in neonates.
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| Summary |
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| Appendix: Clinical parameters evaluated for impact on survival |
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| Acknowledgments |
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| References |
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