PHYSIOLOGY OF DUCTAL DEPENDENT SINGLE VENTRICLE DEFECTS
By Janet Simsic, M.D.
In ductal-dependent single ventricle defects, there is a single great artery that carries all of the blood flow out of the heart. This single great artery is either the pulmonary artery or the aorta. The ductus arteriosus is a vascular connection between the main pulmonary artery and the aorta in fetal life. It serves to shunt blood from the pulmonary bed, which is not ventilated in utero, into the aorta for systemic circulation. Normally, this ductus arteriosus closes shortly after birth.
The ductus arteriosus in a neonate with a ductal-dependent single ventricle defect serves as the life-giving connection, that supplies either all of the blood flow to the systemic circulation, or all of the blood flow to the pulmonary circulation, depending on the specific defect (Figure 1B and 1C). Without the patency of the ductus arteriosus, these types of single ventricle defects are nonviable heart defects. These neonatal patients are completely dependent on prostaglandin therapy to maintain ductal patency until surgical palliation is undertaken. Any compromise in ductal flow during this period can result in compromise of the cerebral, renal or gastrointestinal blood flow.
During the first hours to days after birth, pulmonary vascular resistance falls in response to oxygen content in the blood and arteriolar bed of the lung. With the decrease in pulmonary vascular resistance, the flow through the single great artery will preferentially be directed toward the lower resistance circulation, the pulmonary circulation. This may result in compromised systemic circulation. Secondary to this decreased systemic output, the distal organs, brain, kidneys and intestines are in danger of compromised perfusion with resultant ischemic injury to these organs.
Preoperative and operative management of the ductal-dependent single ventricle patient consists of measures to balance the pulmonary and systemic circulation, thus preventing pulmonary overcirculation and lessening the potential for distal organ compromise. Preoperative management strategies may include inotropic therapy to maximize cardiac output, sedation and deliberate hypoventilation, and decreased inspired oxygen content. These strategies are aimed at limiting pulmonary blood flow while maximizing systemic blood flow.
Surgical Palliation for Single Ventricle Anatomy
Staged reconstruction procedures are used for the separation of the systemic and pulmonary venous return in those patients with functional single ventricles (Figure 1B and 1C).1,2 The initial stage must establish sources of aortic and controlled pulmonary blood flow, typically through a combination of a systemic to pulmonary artery shunt (Figure 2A) and aortic reconstruction (Figure 2B) as needed. Aortic reconstruction must be performed under cardiopulmonary bypass with or without deep hypothermic circulatory arrest. Establishment of a systemic to pulmonary artery shunt alone, Blalock-Taussig shunt, may in certain cases, be performed without the use of cardiopulmonary bypass.
The second stage, bidirectional superior cavopulmonary connection (BSCC) (Figure 3), directs superior vena caval blood flow to the pulmonary arteries, while inferior vena caval flow continues to return directly to the systemic single ventricle.1, 3-5
In the final stage (Figure 4), the Fontan procedure, inferior vena caval flow is directed to the pulmonary arteries, completely separating the pulmonary and systemic circulations.1
Recent Modifications to the Norwood Procedure for HLHS
Recent modification to the systemic to pulmonary artery shunt portion of the Norwood procedure is called the Sano procedure (Figure 5), named after the Japanese surgeon Dr. Sano.6 The Sano procedure is the placement of a 5-6mm Gortex shunt between the right ventricle and the pulmonary arteries instead of the traditional Blalock-Taussig shunt between the inominate artery and the pulmonary artery. The proposed benefits of this modification are enhanced stability in the immediate postoperative period and higher diastolic blood pressures. This may result in decreased interstage mortality and improved single ventricular function.
Interstage Mortality
Survival after the Norwood procedure has significantly improved in the past 10 years.7-10 This improvement is due to advances in surgical strategy, myocardial protection, intraoperative anesthesia management, and pre and post operative care.7,8,10 A number of institutions have reported recent era operative survivals greater than 75 percent.8-13
However, discharged patients remain at risk for death prior to second stage palliation. Such interstage mortality has been reported in up to 24 percent of hospital survivors.11,13-16 Potential causes of interstage mortality include coronary obstruction,17 respiratory infection,7,9 sepsis,7,11 aortic arch obstruction,7,11,16 low cardiac output,7,11,16 arrhythmia,16 and sudden unexplained cardiac death.7,9,16 Several studies have documented the median number of days from hospital discharge following the Norwood procedure to interstage mortality as around five weeks.16, 20 This suggests that increased follow-up early after discharge could reduce the incidence of interstage mortality.
Arrhythmia in the postoperative period was found to be an independent predictor of interstage mortality.16,20 Blaufox and associates found a strong association between sinoatrial node reentrant tachycardia and structural heart disease in infants with clinical supraventricular tachycardia.18 The incidence of sinoatrial node reentrant tachycardia was significantly higher in patients who underwent a Norwood procedure than in patients who underwent other surgeries (19 percent vs 0.4 percent).18 Their study also demonstrated successful treatment with digoxin and lack of recurrence in short-term follow-up.18
Decreased ventricular function at hospital discharge was also found to be an independent predictor of interstage mortality.20 Two additional studies have examined the affect of preoperative ventricular function on interstage mortality. Altmann et al found that decreased right ventricular function at initial presentation, while not a risk factor for Norwood surgical mortality, was a risk factor for interstage mortality.15 Mahle et al also found that decreased ventricular function in the preoperative period was a risk factor for interstage mortality.16
Anatomic subtypes have also been implicated as risk factors for interstage mortality. Both Tweddell et al10 at the Children’s Hospital of Wisconsin and Forbess et al14 at Boston Children’s Hospital identified aortic atresia and smaller ascending aorta diameter as risk factors for interstage mortality. However, other studies were unable to identify anatomic subclass of hypoplastic left heart syndrome as a risk factor for interstage mortality.20, 16
The results of the studies referenced suggest several potential ways to decrease interstage mortality. More frequent follow-up, especially during the first two months following hospital discharge, may identify a potential problem and prompt earlier intervention thus reducing interstage mortality. Ghanayem and associates from the Children’s Hospital of Wisconsin have begun home monitoring with daily pulse oximetry and daily weights for all patients following Norwood procedure.19 They have demonstrated a preliminary reduction in interstage mortality since induction of this home monitoring.19
In conclusion, there remains a significant risk factor for interstage mortality among Norwood survivors. Patients with post operative arrhythmias and/or decreased ventricular function at hospital discharge are at increased risk for interstage death after Norwood procedure. The Shunt Dependent Protocol has been proposed and implicated as our prospective intervention to reduce the interstage mortality in this high-risk shunt dependent single ventricle patient population.
The Shunt Dependent Protocol consists of weekly cardiology outpatient visits for the first four weeks after discharge following initial surgery, then every other week until their pre-Glenn cardiac catheterization (approximately 3 to 4 months of age). At the clinic visits the following information will be obtained: pulse oximetry, weight check, blood pressure, history/physical, echocardiogram – done at each of the first four follow-up visits and then every other week throughout the follow-up period, chest X-ray – once a month, 12-Lead EKG, and feeding information – specifically inquiring into any vomiting, diarrhea or upper respiratory illnesses.
Labs are ordered as the physician deems necessary. Each shunt dependent patient will receive a wallet card to carry with them at all times when they are discharged from the hospital.
These patients are extremely tenuous, therefore heightened vigilance is warranted. If the patient has problems related to vomiting, diarrhea, fever or any symptoms on the shunt dependent card, they should be sent to the Emergency Department at Children’s Healthcare of Atlanta at Egleston. If the patient does not live in metro Atlanta they will be directed to the closest Emergency Department. Based on the findings for admission, as indicated on the wallet card, the Children’s Sibley Heart Center Cardiology and the local physician will make the decision regarding admission or transfer. If at all possible, the patient should be admitted to Children’s at Egleston.
If you have any questions about this article you may contact me or any member of the Children’s Sibley Heart Center Cardoiology team at 404-256-2593.
References:
1 Norwood WI, Jacobs ML. Fontan’s procedure in two stages. Am J Surg 1993;166:548-551.
2 De Leval M, Kilner P, Gewillig M, Bull C. Total cavopulmonary connection: a logical alternative to atriopulmonary connection for complex Fontan operations. J Thorac Cardiovasc Surg 1988;96:682-695.
3 Pridjian AK, Mendelsohn AM, Lupinetti FM, Beekman RH, Dick M, Serwer G, Bove EL. Usefulness of the bidirectional Glenn procedure as staged reconstruction for the functional single ventricle. Am J Cardiol 1993; 71: 959-962.
4 Bridges ND, Jonas RA, Mayer JE, Flanagan MF, Keane JF, Castaneda AR. Bidirectional cavopulmonary anastamosis as interim palliation for high-risk Fontan candidates. Circulation 1990; 82(suppl IV):IV-170-IV-176.
5 Douville EC, Sade RM, Fyfe DA. Hemi-Fontan operation in surgery for single ventricle: a preliminary report. Ann Thorac Surg 1991;51:893-900.
6 Sano S, Ishino K, Kawada M, Arai S, Kasahara S, Asai T, Masuda Z, Takeuchi M, Ohtsuki S. Right ventricle-pulmonary artery shunt in first-stage palliation of hypoplastic left heart syndrome. J Thorac Cardiovasc Surg. 2003 Aug;126(2):504-9; discussion 509-10.
7 Mahle WT, Spray TL, Wernovsky G, Gaynor JW, Clark BJ. Survival after reconstructive surgery for hypoplastic left heart syndrome: A 15-year experience from a single institution. Circulation 2000;120(suppl III):III-136-III141.
8 Bove EL, Lloyd TR. Staged reconstruction for hypoplastic left heart syndrome: Contemporary results. Ann Surg 1996;224(3):387-395.
9 Gaynor JW, Mahle WT, Cohen MI, Ittenbach RF, DeCampli WM, Stevens JM, Nicholson SC, Spray TL. Risk factors for mortality after the Norwood procedure. European Journal of Cardio-thoracic Surgery 2002;22:82-89.
10 Tweddell JS, Hoffman GM, Mussatto KA, Fedderly RT, Berger S, Jaquiss RD, Ghanayem NS, Frisbee SJ, Litwin SB. Improved survival of patients undergoing palliation of hypoplastic left heart syndrome: Lessons learned form 115 consecutive patients. Circulation 2002;106(12) suppl I:182-189.
11 Kern JH, Hayes CJ, Michler RE, Gersony WM, Quaegebeur JM. Survival and risk factor analysis for the Norwood procedure for hypoplastic left heart syndrome. Am J Cardiol 1997;80:170-174.
12 Daebritz SH, Nollert GD, Zurakowski D, Khaili PN, Lang P, del Nido PJ, Mayer JE Jr, Jonas RA. Results of Norwood stage 1 operation: Comparison of hypoplastic left heart syndrome with other malformations. J Thorac Cardiovasc Surg 2000;119(2):358-367.
13 Azakie T, Merklinger Sl, McCrindle BW, VanArsdell GS, Lee KJ, Benson LN, Coles JG, Williams WG. Evolving strategies and improving outcomes of the modified Norwood procedure: A 10-year single institution experience. Ann Thorac Surg 2001;72(4):1349-1353.
14 Forbess JM, Cook N, Roth SJ, Serraf A, Mayer JE, Jonas RA. Ten-year institutional experience with palliative surgery for hypoplastic left heart syndrome: Risk factors related to stage 1 mortality. Circulation 1995;92(9)Suppl II:II-262-II-266.
15 Altmann K, Printa BF, Solowiejczyk DE, Gersony WM, Quaegebeur J, Apfel HD. Two-dimensional echocardiographic assessment of right ventricular function as a predictor of outcome in hypoplastic left heart syndrome. Am J Cardiol 2000;86:964-968.
16 Mahle WT, Spray TL, Gaynor JW, Clark BJ. Unexpected death after reconstructive surgery for hypoplastic left heart syndrome. Ann Thorac Surg 2001;76:61-65.
17 Bartran U, Grunenfelder J, Van Praagh R. Causes of death after the modified Norwood procedure: A study of 122 postmortem cases. Ann Thorac Surg 1997;64(6):1795-1802.
18 Blaufox AD, Numan N, Knick BJ, Saul JP. Sinoatrial node reentrant tachycardia in infants with congenital heart disease. Am J Cardiol 2001;88(9):1050-1054.
19 Ghanayem NS, Hoffman GM, Mussatto KA, Cava JR, Frommelt PC, Rudd N, Steltzer M, Bevandic S, Jaquiss RD, Litwin SB, Tweddell JS. Home surveillance program prevents interstage mortality following the Norwood procedure. Presented at the 82nd AATS Annual Meeting, Washington DC, May 2002.
20 Simsic JM, Atz AM, Stroud MR, Bradley SM. Risk Factors for Interstage Death after Norwood Procedure. Pediatric Cardiology 2002;23(6):664.
JANET M. SIMSIC, M.D.
Currently Dr. Simsic is the co-director of the Children’s Healthcare of Atlanta Cardiac Intensive Care Unit (CICU) and teaches at Emory University School of Medicine as the Assistant Professor of Pediatrics.Prior to joining Children’s, Dr. Simsic served as the director of pediatric cardiac transplantation at the Medical University of South Carolina, in addition to her role as an attending physician for the CICU. She also participated in Heart Care International and Children’s Heartlink, which are organizations dedicated to delivering medical care to pediatric cardiac patients in developing countries. Dr. Simsic completed a senior fellowship in pediatric cardiac critical care at the Children’s Hospital of Philadelphia, a senior fellowship in cardiac intensive care at Children’s Hospital of Boston, a fellowship in pediatric cardiology at the Medical University of South Carolina, and a residency in pediatrics at the Carolinas Medical Center in Charlotte, N.C. She earned a medical degree from the Brody School of Medicine at East Carolina University and a bachelor of arts degree in chemistry from the University of North Carolina at Chapel Hill.
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