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Ann Thorac Surg 2006;82:1267-1277
© 2006 The Society of Thoracic Surgeons

Original Articles: Cardiovascular

Impaired Power Output and Cardiac Index With Hypoplastic Left Heart Syndrome: A Magnetic Resonance Imaging Study

Kartik S. Sundareswaran, BSa, Kirk R. Kanter, MDb, Hiroumi D. Kitajima, MSa, Resmi Krishnankutty, BSa, Jennifer F. Sabatier, MSc, W. James Parks, MDd, Shiva Sharma, MDe, Ajit P. Yoganathan, PhDa,*, Mark Fogel, MDf

a Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology, Atlanta, Georgia
b Department of Surgery, Emory University, Atlanta, Georgia
c Biostatistics Consulting Center, Emory University, Atlanta, Georgia
d Sibley Heart Center and Children's Healthcare of Atlanta, Emory University, Atlanta, Georgia
e Pediatric Cardiology Services, Lawrenceville, Georgia
f Division of Cardiology, Children's Hospital of Philadelphia, Philadelphia, Pennsylvania

Accepted for publication May 5, 2006.

Abbreviations and Acronyms AL = anterior left; AR = anterior right; BSA = body surface area; CHOA = Children's Healthcare of Atlanta; CHOP = Children's Hospital of Philadelphia; HLHS = hypoplastic left heart syndrome; LPA = left pulmonary artery; PC MRI = phase contrast magnetic resonance imaging; PL = posterior left; PR = posterior right; SVRI = systemic vascular resistance index

* Address correspondence to Dr Yoganathan, Wallace H. Coulter School of Biomedical Engineering, Georgia Institute of Technology and Emory University, Room 2119, U.A. Whitaker Bldg, 313 Ferst Dr, Atlanta, GA 30332-0535 (Email: ajit.yoganathan{at}bme.gatech.edu).

Presented at the Forty-second Annual Meeting of The Society of Thoracic Surgeons, Chicago, IL, Jan 30–Feb 1, 2006.


   Abstract
Top
 Abstract
Introduction
 Material and Methods
 Results
 Comment
 Notice From the American...
 Discussion
 Acknowledgments
 References
 
BACKGROUND: Unfavorable cardiac mechanics in children with hypoplastic left heart syndrome (HLHS) when compared with other single-ventricle defects may affect long-term morbidity and outcome. Using noninvasive phase contrast magnetic resonance imaging (PC MRI), we examined cardiac mechanics in children with HLHS and compared the results to other single-ventricle defects.

METHODS: Eighteen children with HLHS and 18 children with other single-ventricle defects were studied after the Fontan operation. Phase contrast MRI scans were obtained perpendicular to the ascending aorta, and flow was quantified using an in-house segmentation and reconstruction scheme. The total power output was determined using the modified Bernoulli equation along with cardiac output and systemic vascular resistance index.

RESULTS: Compared with non-HLHS congenital heart defects, children with HLHS had significantly lower power output (1.40 ± 0.39 versus 1.78 ± 0.38 W/m2, p < 0.004) and cardiac index (3.15 ± 0.97 versus 4.09 ± 1.23 L · Min–1 · m–2, p < 0.009) with a concomitant higher systemic vascular resistance index (28.94 ± 11.5 versus 22.7 ± 8.53 WU, p < 0.03) despite generating similar systolic blood pressures (112.9 ± 22.4 versus 115.2 ± 23 mm Hg, p > 0.05).

CONCLUSIONS: Minimally invasive measurements with PC MRI in children with HLHS showed significantly lower power output and cardiac index when compared with other single-ventricle physiologies. Abnormal aortic flow patterns may contribute to power loss and may have long-term survival and morbidity implications associated with the Fontan procedure. Elevated systemic vascular resistance index despite similar blood pressure opens avenues for therapeutic intervention for afterload reduction.


   Introduction
Top
 Abstract
 Introduction
Material and Methods
 Results
 Comment
 Notice From the American...
 Discussion
 Acknowledgments
 References
 
A type of single-ventricle physiology is hypoplastic left heart syndrome (HLHS), which is a potentially fatal combination of congenital cardiovascular lesions with a high rate of mortality within the first year of life when untreated. One of the main differences between HLHS and other single-ventricle defects is the absence of a developed left ventricle. The single right ventricle is responsible for the entire workload, pumping blood through the systemic circulation and the passive flow to the pulmonary circulation through the total cavopulmonary connection. That may cause right ventricular failure in some patients as it is not morphologically designed to handle such heavy loads [1–3]. Recent studies indicate a postoperative survival rate of 78% [4] whereas an older study has indicated a 10-year survival rate of 39% [5]. Previous studies have attempted to provide an explanation for such low survival rates with the use of magnetic resonance imaging, cardiac catheterization, and echocardiography [1, 6–10]. Fogel and colleagues [8, 9] showed with the help of magnetic resonance tagging that there were marked differences in regional wall motion and strain in systemic right ventricles when compared with systemic left ventricles. Joshi and coworkers [11] looked into the differences in exercise tolerance between patients with HLHS and other Fontan patients and reported no significant differences, although this study was limited by a very small sample size (n = 7).

In this study, we have tried to evaluate several variables illustrating the different performances of single-right and other variations of single ventricles. The quantitative variables retained for comparison were derived from phase contrast magnetic resonance imaging (PC MRI) of the ascending aorta, so as to showcase the contrasting flow mechanisms arising out of the two types of ventricles. They included computing the total power, cardiac index, systemic vascular resistance index (SVRI), time of forward flow, mean velocities, quadrant flow, stroke volume, and time to maximum flow. Quadrant analysis was done in order to quantify velocity profile differences between the groups. Although the shape of the velocity profile is debatable, most investigators agree that flow in the ascending aorta is axisymmetric [12, 13]. This is not the case in patients born with HLHS since they have a hypoplastic aorta enlarged during the first stage of the surgery through the Norwood procedure. This geometric modification possibly results in a nonuniform flow profile that could potentially result in increased frictional losses near the wall. In this study, we have attempted to evaluate the effect of such flow asymmetries on output power and systemic vascular resistance for HLHS and other single-ventricle defects.


   Material and Methods
Top
 Abstract
 Introduction
 Material and Methods
Results
 Comment
 Notice From the American...
 Discussion
 Acknowledgments
 References
 
Patient Population
A multicenter database of PC MRI through the ascending aorta has been established for studying flow out of single ventricles. The images were taken as part of an ongoing Fontan flow study for understanding and improving flow dynamics in the total cavopulmonary connection. Informed consent was obtained, and all associated studies were approved by the Institutional Review Boards of Children's Hospital of Philadelphia (CHOP), Children's Healthcare of Atlanta (CHOA), University of North Carolina at Chapel Hill, and Georgia Institute of Technology. No sedatives or other medications were administered to patients from CHOP, while the patients from CHOA were sedated using propofol before being imaged.

The patient population was divided into two groups: the HLHS group consisted of 18 patients diagnosed with HLHS, and the non-HLHS group consisted of 18 patients with other congenital defects that required the Fontan procedure for correction. Of the 18 in HLHS, 14 had undergone a Fontan operation (11 intra-atrials and 3 extracardiacs), 2 a bidirectional Glenn procedure, 2 hemi-Fontans, and all of them underwent the stage 1 aortic reconstruction (Norwood procedure). The non-HLHS group consisted of 15 patients who had undergone the Fontan operation (9 intra-atrials, 5 extracardiacs, and 1 atriopulmonary), 3 who had a bidirectional Glenn procedure, and 3 who had an aortic reconstruction (Damus-Stansel-Kaye procedure) done. Twenty-eight of 36 patients were from CHOP and the rest were from CHOA. The patient distribution and their diagnoses are summarized in Tables 1 and 2. Go


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  		Table 1. Distribution of Patients With Hypoplastic Left Heart Syndrome (HLHS)
 

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  		Table 2. Distribution of Patients Without Hypoplastic Left Heart Syndrome (HLHS)
 
MRI Data Acquisition
The study utilized a Siemens 1.5 Tesla whole-body MRI scanner for patients from CHOP, and a GE 1.5 Tesla whole-body MRI scanner was utilized for patients from CHOA. Retrospectively triggered, through-plane, phase-encoded velocity mapping was performed on a plane perpendicular to the flow in the ascending aorta positioned approximately 2 to 4 mm above the aortic valve. The velocity encoding used was ± 150 cm/s. The effective repetition time was 12 to 25 ms, which yielded 15 to 35 phases, the echo time was 2.5 to 4 ms, and the image matrix size ranged from 128 x 128 pixels to 256 x 256 pixels. The field of view ranged from 180 to 400 mm depending upon the patient's size, and a rectangular field of view was used when appropriate. The slice thickness was 5 mm.

Image Processing and Analysis
An in-house program was written in Matlab (The MathWorks, Natick, Massachusetts) to read and process the PC MRI images. A simple gradient-based active contour algorithm was implemented for the semiautomatic segmentation of the ascending aorta throughout the entire cardiac cycle [14]. A contour was manually outlined around the vessel of interest. This contour would automatically evolve based on gradient based forces until it identified the vessel boundary for all the cardiac phases. The segmentation was visually inspected for accuracy, and incorrect contours were adjusted manually. The segmented pixel values were converted into velocity values, which were integrated over the entire vessel area for each cardiac phase to extract flow for that phase. The technique was previously validated on phantom models where a known flow rate was passed through the models and the computed flow-rate was compared with the flow-rate expected. Owing to the proximity of the ascending aorta to the lungs, some noise was incorporated into the velocity values that had to be removed using an in-house noise removal scheme. This process was followed for all 36 patients used in the study. Several measurements were computed from the PC MRI data, and their values were compared between the two groups of patients. To evaluate the output power and SVRI, pressure data were required. For this purpose, patient blood pressure was recorded before the study under similar conditions of MRI.

Cardiac Index
The cardiac output was evaluated by integrating the velocities over the vessel cross-sectional area for each cardiac phase. The mean cardiac output was obtained by integrating the area under the flow curve over the entire cardiac cycle and dividing it by the total number of phases. This was indexed to the body surface area (BSA) to obtain cardiac index.

Total Power
The output power was computed using the modified Bernoulli equation:


Formula

where {rho} = blood density (a constant value of 1060 kg/m3), v = mean systolic velocity, Pmean = mean static pressure, and Q = mean systolic flow rate. To evaluate this equation, the mean arterial pressure was used as the static pressure. The computed power was indexed using BSA for ease of comparison across all patient sizes.

Systemic Vascular Resistance Index
To compute the SVRI, pressure difference between the two ends of the vascular system was required. One end was taken to be the mean arterial pressure while the other end was measured using cardiac catheterization. Because each of these patients undergo cardiac catheterization routinely after surgery, the left pulmonary artery (LPA) pressure measured at this point was used as it was available for 34 of 36 patients. However, there was a significant time lag (5.35 ± 4.90 years) between cardiac catheterization and MRI acquisition, and for this reason no other parameters from catheterization were retained for this study. The resistance index was then computed by:


Formula

Quadrant Analysis
The vessel was divided into four quadrants, namely, anterior left (AL), anterior right (AR), posterior left (PL), and posterior right (PR), based upon the computed centroid of the vessel; and flow was calculated for each quadrant. Figure 1 shows the scheme followed for labeling the quadrants. The quadrant flows were divided by the total cardiac output for evaluating the flow percentage through each quadrant. A regression model was then setup to evaluate the impact of flow profile on output power.


Figure 1
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  		Fig 1. Location of the ascending aorta as seen from a sagittal perspective. The slice is chosen so that flow is perpendicular to the slice. The vessel was then divided into four quadrants: AR = anterior right; AL = anterior left; PR = posterior right; PL = posterior left.

 
Other velocity and flow measurements used in this study were as follows: mean velocity = average velocity over the entire cardiac cycle; max velocity = maximum systolic velocity over the entire cardiac cycle; time of forward flow = (percentage of cardiac cycle spent in forward flow) / heart rate; stroke index = heart rate / cardiac index; and time to maximum flow = time taken to reach maximum flow in one cardiac cycle.

Statistical Methods
The methods employed for comparisons include two-sampled two-tailed Student t tests with Bonferroni correction to test for group differences. Nonnormal variables were log transformed to increase normality. Total power, SVRI, and the effect of quadrant flow for differences between the groups were analyzed using a dummy variable regression model and were log transformed for normality. Body surface area had a strong correlation with both power and SVRI, and was used as a covariate in the model. The regression model was set up as follows:


Formula

where AL and AR were the flow percentages through the anterior left and anterior right quadrants, while ßo, ß1,and ß2 were the regression estimates. A similar scheme was adopted for SVRI as well. The descriptive statistics are presented as mean, median, minimum, and maximum values. For quadrant flow analysis, the log transformation was not effective for establishing normality, and hence an F-approximation to a nonparametric repeated measures analysis of variance was used to look for group differences in quadrant flow. The reported p values are two-sided unless otherwise stated, and a p value of 0.05 or less indicated statistical significance.


   Results
Top
 Abstract
 Introduction
 Material and Methods
 Results
Comment
 Notice From the American...
 Discussion
 Acknowledgments
 References
 
Patient Population
A summary of results is displayed as part of Table 3. The patient age and BSA were comparable in both the groups. The mean age of HLHS patients was 10.8 ± 6.14 years, whereas for the non-HLHS category it was 11.14 ± 6.6 years. There were no significant group differences in age (p = 0.88) and BSA (p = 0.99), indicating that the two groups of patients used in the study were similar in physical characteristics. Owing to the large variation in age and BSA of patients, the primary variables of interest were indexed to BSA for comparison.


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  		Table 3. Summary of Computed and Measured Variables
 
Power Output
There was a strong correlation of power with age (Fig 2A) and BSA (Fig 2B). For both the groups, power increased with increase in age and BSA, although for HLHS patients the slope was smaller compared with non-HLHS patients. When the output power was indexed to BSA, the results yielded significantly lower values for HLHS as compared with non-HLHS (p < 0.004), as depicted in Figure 3A. There were no significant differences in the pressures generated by the two groups (Fig 3D), although there were significant differences in the cardiac index of these patients (p < 0.009; Fig 3B). This finding indicated that patients with HLHS may be experiencing higher resistances, and for this reason SVRI was computed for each of the patients and group differences were evaluated.


Figure 2
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  		Fig 2. Variation of output power for hypoplastic left heart syndrome (HLHS) and non-HLHS with (A) age and (B) body surface area (BSA). (A) {diamondsuit} = output power vs age (HLHS); {blacksquare} = output power vs age (non-HLHS). Fit for HLHS: y = 0.0947x + 0.4296; R2 = 0.6475. Fit for non-HLHS: y = 0.1312x + 0.3788; R2 = 0.7379. (B) {diamondsuit} = output power vs BSA (HLHS); {blacksquare} = output power vs BSA (non-HLHS). Fit for HLHS: y = 1.4768x – 0.0796; R2 = 0.7937. Fit for non-HLHS: y = 1.9749x – 0.1884; R2 = 0.8546.

 

Figure 3
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  		Fig 3. Bar graphs showing differences between hypoplastic left heart syndrome (HLHS) group and non-HLHS group in (A) output power normalized by body surface area (BSA), (B) cardiac index, (C) systemic vascular resistance index (SVRI), and (D) systolic pressure. (W.U. = Wood Units.)

 
Systemic Vascular Resistance Index
The SVRI for the HLHS group was significantly higher compared with patients in the non-HLHS group (p < 0.03, Fig 3C). There were no significant differences in the LPA pressures between the two groups (Table 3). Owing to the time lag that existed between cardiac catheterization and MRI, SVRI was also computed without using the LPA pressures (mean arterial pressure / cardiac index), and the result indicated similar trends. The HLHS group (28.93 ± 11.46 WU) still had significantly higher values compared with non-HLHS (22.65 ± 8.53 WU, p < 0.03).

Quadrant Flow Analysis
Figure 4A depicts bar graphs of the quadrant flow ratios computed for each of the patient groups. There were clear group differences between HLHS and non-HLHS groups in AL and PR quadrants, respectively. Pairwise comparisons revealed group differences in AL flow ratio approached statistical significance (p < 0.07; Table 4). There were large variations observed in the HLHS group due to the presence of an outlier, and its removal reduced these variations (Fig 4B). Table 4 also shows flow differences between each of the quadrants. Statistically significant differences between the groups existed when AL-AR was compared, showing greater skewness toward AL in HLHS. For the HLHS group, AL-AR had a significant negative impact (ß1(estimate) = –0.15) on the output power (p < 0.04) for the HLHS group. The negative trend was expected because a higher skew toward the wall of the aorta could result in higher frictional losses near the wall, which could account for some of the reduced power, lower velocities, and consequently lower cardiac output. Other combinations of variables were also tried out as predictors, although they were not significant. Similar analyses were performed to establish a relationship between SVRI and quadrant flow. When AL-AR was used as a predictor, there was a trend toward significance (p < 0.1) in its effect on SVRI.


Figure 4
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  		Fig 4. Bar graphs showing differences in quadrant flow between hypoplastic left heart syndrome (HLHS) groups (black bars) and non-HLHS groups (gray bars) using (A) the entire dataset, and (B) without an outlier that caused large standard deviations. (AL = anterior left; AR = anterior right; PR = posterior right; PL = posterior left.)

 

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  		Table 4. Summary of Quadrant Flow Ratios Seen in the Ascending Aorta
 
Other Variables
Besides the variables mentioned above, other flow-based variables were also computed. The results and the group differences are displayed as part of Table 3. There were significant differences in the mean velocities generated by the two groups, although there were no significant differences in the maximum systolic flow measured. The time spent in forward flow was significantly higher for HLHS, but there were no significant differences in the time it took to reach maximum flow. The non-HLHS group seemed to have a higher stroke index compared with the HLHS group, although this was not statistically significant.


   Comment
Top
 Abstract
 Introduction
 Material and Methods
 Results
 Comment
Notice From the American...
 Discussion
 Acknowledgments
 References
 
Hypoplastic left heart syndrome is one of the most complex and lethal forms of single-ventricle diseases, and its management is critical at the neonatal stage [3, 5, 15–25]. With the introduction of the staged reconstruction process and improved diagnosis of HLHS by fetal echocardiography, the early survival rate has markedly improved [4, 24]. Even though right ventricle function has not directly been linked with operative survival in most of the studies, they do have an impact on the long-term survival of HLHS patients [3, 6, 15, 26]. Altman and colleagues [6] observed that qualitative assessment of right ventricle function was the best predictor for operative survival. They reported that the survival rate of patients with diminished qualitative right ventricle function was only 35% compared with 70% with normal or hyperdynamic function. Through our study, we have indirectly evaluated single-ventricle performance by computing its power capacity and have shown significant differences between HLHS and non-HLHS patients.

Various studies are being done [27–32] on flow dynamics in the total cavopulmonary connection, and one of the performance measures constantly used for evaluating its efficacy is power loss. However, it is important to know the input power to the system to evaluate the significance of power loss. In this study, a simple method for evaluating single-ventricle output power was proposed based upon classical fluid mechanics principles using noninvasive PC MRI through the aorta. Power losses through the total cavopulmonary connection and the ascending aorta can now be expressed as a percentage of the total power input into the system. This has the potential to be a useful diagnostic tool for clinical evaluation of Fontan patients. Power differences observed between HLHS and other variations of single ventricles may seem obvious, but does bring to the fore the importance of flow efficiencies for long-term performance of single right ventricles. Senzaki and colleagues [10] demonstrated that the hydraulic power cost per unit of forward flow was 40% less in a dual chamber circulation than in the single-ventricle circulation. A pressure overloaded right ventricle may not be able to handle the high power requirements during normal and elevated activity compared with single left ventricle. That may provide an explanation for the high rate of morbidity associated with HLHS patients in the first 10 years of life, and highlights the importance of designing efficient Fontan circuits, especially in the case of HLHS patients.

No significant differences in blood pressures were observed between the two groups despite the lower cardiac index of the HLHS patients, which suggested a significantly higher SVRI than for non-HLHS patients. When the effective SVRI was computed for the two groups, it was observed that HLHS patients had significantly higher resistances than non-HLHS patients. Studies have shown that single-ventricle patients have higher systemic vascular resistances compared with the normal circulation especially after stage 1 (Norwood) reconstruction [33, 34]. We speculate that nonuniform flow arising out of the reconstructed aortas could be one of the causes for decreased power and increased SVRI. Previous studies have shown marked differences in aortic flow profiles in patients who have reconstructed aortas compared with other single-ventricle patients with normal aortas [13]. Flow through the reconstructed aortas in this study was skewed anteriorly and the time to peak flow was longer than in patients without reconstructed aortas. Our results on quadrant flow analysis agreed with this study, although a larger standard deviation was observed within the hypoplastic group. Our analysis further indicated that skewed flow had a statistically significant effect on the output power of HLHS patients (p < 0.05). As there were no significant differences between the quadrants for non-HLHS patients (only 16 of 18 patients had a normal aorta), a strong regression relationship was absent (since AL-AR equals approximately 0). This suggests that reconstructed aortas could have a detrimental effect on single-ventricle performance.

In conclusion, this study showed marked cardiac performance differences between the two groups of patients during an attempt to provide an explanation for the poor long-term performance of patients with hypoplastic left heart syndrome. Myocardial preservation and protection can be done operatively to preserve energetics; however, lower energetic performance of the right ventricle under pressure-loaded conditions at present points toward two avenues where ventricular performance can be improved: the aortic reconstruction done during stage 1 and the total cavopulmonary connection during stage 3. The efficiency of each of these procedures is critical. Clinically, the increased calculated SVRI in the HLHS group compared with the non-HLHS group suggests that the introduction of pharmacologic afterload reduction might be beneficial.


   Notice From the American Board of Thoracic Surgery Regarding Trainees and Candidates for Certification Who Are Called to Military Service Related to the War on Terrorism
Top
 Abstract
 Introduction
 Material and Methods
 Results
 Comment
 Notice From the American...
Discussion
 Acknowledgments
 References
 
The Board appreciates the concern of those who have received emergency calls to military service. They may be assured that the Board will exercise the same sympathetic consideration as was given to candidates in recognition of their special contributions to their country during the Vietnam conflict and the Persian Gulf conflict with regard to applications, examinations, and interruption of training. If you have any questions about how this might affect you, please call the Board office at (312) 202-5900.

Carolyn E. Reed, MD

Chair

The American Board of Thoracic Surgery


   Discussion
Top
 Abstract
 Introduction
 Material and Methods
 Results
 Comment
 Notice From the American...
 Discussion
Acknowledgments
 References
 
DR THOMAS L. SPRAY (Philadelphia, PA): I think your summary is very nice, and it's a fascinating study, and obviously we're involved in looking at a lot of these things together. But you brought up, I think, a critical issue and, that is, really, if I look at the way you make these calculations, much of it is based upon velocity across the outflow tract and mean arterial blood pressure. And in essence, if you assume that the stroke volume is the same between your two groups of the hypoplast and nonhypoplast group, then having a dilated aortic root should affect the systolic velocity out the outflow tract, if I'm correct.

DR KANTER: Since you are integrating every data point for velocity at the aortic valve, you should therefore be able to get the entire cardiac output measured and not miss any. So I'm not sure that I would agree with that observation that you just made. It should be a real cardiac output that you're measuring, because you're measuring all the blood going out. If you know the velocity and you know the area—

DR SPRAY: But power loss is going to be related to the velocity and the mean pressure. So you would expect ejection of the same amount of blood into a dilated aortic root to result in power loss compared with a less dilated aortic root. So could you actually compare the nonhypoplastic left heart group in terms of aortic root diameter to the hypoplastic left heart syndrome group that had a more dilated aortic root and arch?

DR KANTER: Now, I see what you're saying. No, we did not. We compared them with body surface area and with age, and there is a direct linear correlation for both comparisons. We did not compare it against the aortic root diameter itself.

DR SPRAY: Because it would tend to create a lot of these changes just from that geometric abnormality. And you bring it up as certainly one of the limitations of the study, but I just wonder how much that would affect all of the various calculations that you make in the MRI scan. I think this is really fascinating information.

DR MARSHALL L. JACOBS (Philadelphia, PA): I echo Tom's compliments to you on a very nice presentation of fascinating material. I had a chance to collaborate with both Tom and with Mark Fogel on an earlier study where Mark used MRI measurements to demonstrate the variance from plug flow in the reconstructed aorta and it probably was one of the things that led to this study of energy losses. And my questions are the following, and it's really regarding the same issue Tom addressed.

In the nonhypoplastic left heart syndrome group, did all of the patients have an aorta arising from the systemic ventricle, or had any of them undergone Damus-type reconstructions, or surgical reconstructions, such that the anatomy, the plumbing, bore any similarity to the HLHS group? In other words, is the power loss a mysterious HLHS phenomenon, or is it, as Mark speculated 10 years ago, related to the method of aortic reconstruction? Question one is, were there any Damus connections in the non-HLHS group?

Second, when Roger Mee and Chuck Fraser and others introduced the direct connection of the pulmonary trunk to the underside of the arch, I went ahead and speculated that notwithstanding the abnormalities we'd seen in this type of arch reconstruction, that the power losses might be greater with that type of arch reconstruction, where instead of starting with a bulbous root that gradually tapers, you start with a trunk and things basically spread off in different directions from the truck. Were all of your HLHS patients constructed in one fashion with homograft patch augmentation, or did any of them have this direct pulmonary trunk to aortic arch anastomosis?

DR KANTER: In answer to your first question, about a quarter of the non-HLHS patients had some form of DSK or Norwood-type procedure with a dominant left ventricle.

DR JACOBS: And were they any different from the remainder of the non-HLHS group?

DR KANTER: We did not separate them out. In answer to your second question, the area of interest that we investigated was just above the aortic commissures. So unless there is a downstream effect of the anastomosis, we would not see it. But I don't think it's unreasonable to expect that there would be a downstream effect, because we see it with other lesions such as coarctation, in which you can see an abnormal flow pattern in the transverse aorta in the patients who had a repaired aorta.

The direct answer to your question, however, as far as I know, none of the patients had a direct anastomosis to the undersurface of the transverse aorta in this series.

DR JACOBS: I think, if I understand the physics of this correctly, if you go from a large trunk to a smaller arch, there has to be what you refer to as a downstream effect, at least in those measurements. Thank you.

DR KANTER: It's an upstream effect, Marshall.

DR JACOBS: Distal effect.

DR KANTER: No, proximal effect. In effect, upstream from where the obstruction is you can see that.

DR CHRISTIAN PIZARRO (Wilmington, DE): Regarding the presence of aortic atresia versus aortic hypoplasia, do you think the variability in the size of the ascending aorta, could have influenced your calculations? Additionally, did you have the opportunity to look at differences between patients with morphologic right versus a left ventricle and find out if the power loss observed relates in any way to the anatomic nature of that ventricle?

DR KANTER: No, we did not. For the hypoplastic left heart syndrome group, we did not separate out aortic atresia versus aortic stenosis. The numbers in the non-HLHS group were too small for us to separate out the 5 patients who had a dominant right ventricle compared with the 12 patients who had a dominant left ventricle, so we could not compare that and have any statistical relevance.

DR BRADLEY S. ALLEN (Houston, TX): I'm just wondering if some of this loss of power could be due to poor myocardial protection. The hypoplastic left hearts were all subjected to a period of myocardial arrest during aortic reconstruction. In contrast, other single-ventricle lesions may only require minor palliation without a period of myocardial ischemia. Did you compare the patients that did and did not undergo prolonged aortic clamping (a major procedure), to see if a lot of power loss could be secondary to poor myocardial protection during the time that the Norwood was done?

DR KANTER: No, we did not look at that. Remember, these studies were done at 10 years of age on average. But the effect of poor protection can last a lifetime, can't it?

DR JOHN E. MAYER, JR (Boston, MA): Is this parameter, power output, load dependent? In other words, it looked to me, from your slides, that the systemic vascular resistance was higher in the HLHS group than in the non-HLHS group. So the question is whether power output is load dependent?

DR KANTER: Yes, it has to be because the blood pressure is in the equation, so the higher the blood pressure—

DR MAYER: I mean, my recollection is that the only truly load-independent parameter is maximum rate of change of power per unit time, D power/DT. I think that's what I've learned from Steve Colan. So what I'm wondering is whether or not this technology allows you to calculate that. I would think that the MRI would allow you to do that. You would just have to have instantaneous arterial pressure, so you would need to have an intra-arterial line when you're doing the study. But I think if you had those parameters, then you could actually arrive at this calculation of D power/DT.

DR KANTER: The afterload was the same in the groups as very roughly measured by blood pressure.

DR MAYER: Right. But the SVR was higher in the HLHS group because they had a lower output.

DR KANTER: That's a calculated number.

DR MAYER: I understand.

DR CHARLES D. FRASER, JR (Houston, TX): I'm still struggling with the mathematics and the fluid dynamics. The group that underwent the Norwood palliation, all the aortic reconstructions were the same; is that correct?

DR KANTER: We didn't go into great detail, but they all had a Norwood procedure in the HLHS group.

DR FRASER: It seems to me there a lot of assumptions that are being made between the relationship between the power source and the conduit. And as we know, the normal human aorta is an active structure. We've talked about that in other sessions today. So if the reconstruction among the Norwood group is different in terms of what portion of the aorta remains pulsatile, then this is going to really confound the influence of this energy sink, or whatever the cause of the dissipation of power.

So again, I think it would be real important to look at what the nature of the conduit is along with the nature of the pump. We focus a lot on the morphology of the ventricle. But if the conduit is dissimilar, then that's going to also confound that variable.

DR KANTER: I think that is a very important point. In fact, we have studied that at our institution, not in this set of patients, but using echocardiography in patients with single-ventricle physiology and reconstructed aortas, compared with those with single-ventricle physiology without reconstructed aortas. The distensibility of the reconstructed aorta is far different and far inferior to the others, as you would think. That has to have some effect on power loss, and may be one of the factors involved in the differences we demonstrated in this study.

DR JOSEPH M. FORBESS (Dallas, TX): If one were to simplify this to a simple circuit, based on Ohm's law, the major energy loss has to be taking place out somewhere in the arterial or capillary bed. When one sees a patient who has low output and high SVR, I think heart failure. I am suspicious that these data are identifying a patient population that's in chronic heart failure. As Kirk is alluding to, ACE inhibition is one thing, but the adult cardiologists have shown us that that's probably not all we can do. I know there is a pediatric Carvedilol study wrapping up, and that the results have not been revealed. I just wanted to pose the question to the audience: how many groups are using beta-blockers routinely in high-risk Fontan patients like this, nowadays? [Show of hands.] A very low number—that's what I thought.


   Acknowledgments
Top
 Abstract
 Introduction
 Material and Methods
 Results
 Comment
 Notice From the American...
 Discussion
 Acknowledgments
References
 
This work was supported by a BRP Grant from the National Heart, Lung, and Blood Institute (HL67622).


   References
Top
 Abstract
 Introduction
 Material and Methods
 Results
 Comment
 Notice From the American...
 Discussion
 Acknowledgments
 References
 

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