Noninvasive markers for acute heart transplant rejection in children with the use of automatic border detection

Current status of heart transplantation in children: update 2003

Heart transplant is an effective therapy for children with end-stage heart disease. Success of this treatment depends on coordination and careful communication among the family, primary care physician, and transplant team. Primary care physicians play an essential role in the monitoring and management of the medical, nutritional, developmental, and psychosocial issues of pediatric heart transplant patients and their families (Box 3). Ongoing assessment of the child and parent's progress in adapting to transplant is crucial in order for appropriate referrals to occur. Relationships with the primary care team can improve medical outcomes for this complex group of patients and provide a framework for improved adherence to care.

The current state of, and future prospects for, cardiac transplantation in children

During the last two decades, several advances have resulted in marked improvement in medium-term survival, with excellent quality of life, in children undergoing cardiac transplantation. Improved outcomes reflect better selection of donors and recipients, increased surgical experience in transplantation for complex congenital heart disease, development of effective surveillance for rejection, and wider choice of immunosuppressive medications. Despite all of these advances, recipients continue to suffer from the adverse effects of non-specific immunosupression, including infections, induction of lymphoproliferative disorders and other malignancies, renal dysfunction, and other important end-organ toxicities. Furthermore, newer immunosuppressive regimes, thus far, appear to have had relatively little impact on the incidence of chronic rejection. Progress in our understanding of the immunologic mechanisms of rejection and graft acceptance should lead to more targeted immunosuppressive therapy and avoidance of non-specific immunosupression. The ultimate goal is to induce a state of tolerance, wherein the recipient will accept the allograft indefinitely, without the need for long-term immunusupression, and yet remain immuno-competent to all non-donor antigens. This quest is currently being realized in many animal models of solid organ transplantation, and offers great hope for the future.

An improved echocardiographic rejection-surveillance strategy following pediatric heart transplantation

BACKGROUND

The unique demands of cardiac transplantation in infancy have led to non-invasive rejection-surveillance strategies. ECHO-A is a multiparametric, two-dimensionally guided, M-mode analysis algorithm that assigns an empirically derived score for deviations of recipient parameters to age-adjusted, population-based normal values. A cumulative ECHO-A score > or =4 is highly predictive of endomyocardial biopsy Grade > or =3 and of cellular rejection.

METHODS

This study determined whether modifying ECHO-A to score for deviations of recipient parameters from the recipient's baseline would improve the predictive power of ECHO-A. We reanalyzed 701 consecutive echocardiograms of 18 pediatric cardiac transplant recipients (median age at transplantation, 142 days) and based scoring on significant (Z score > or =1) deviation from the patients' baseline means (ECHO-B).

RESULTS

Eight episodes of treated rejection occurred during the first year after transplantation (median, 1.4 years). Approximately 10% (72) of the analyses had ECHO-A scores > or =4 that were not associated with treated rejection and were considered false positives. We identified parameters that contributed to the false-positive evaluations and calculated patient-specific baseline mean +/- standard deviation. The ECHO-B, in comparison with ECHO-A, decreased the number of false positives from 72 to 10, increased specificity from 90% to 99%, and increased the positive predictive value about 4-fold (10% to 44%). With treated rejection episodes, ECHO-B increased ECHO-A scores in 7 of 8 recipients and increased the mean score from 6 to 8.

CONCLUSIONS

analysis algorithm based on change from baseline improved the positive predictive power without reducing the negative predictive value of multiparametric quantitative analyses of echocardiograms following pediatric heart transplantation.

Unsolved problems in diastole

It is now recognized that a sizable portion of patients who exhibit symptoms of congestive heart failure have relatively well-preserved systolic function, but have significantly elevated LV filling pressures. This syndrome, termed "diastolic heart failure," is associated with various conditions such as aging, anatomic abnormalities, hypertension, ischemic disease, tachycardia, and atrial fibrillation. Advances in the proper medical and surgical management of these patients will depend on the continued delineation of the basic physiologic mechanisms that account for normal and pathologic cardiac diastolic function. This goal can only be achieved by the integration of information acquired from basic science investigations conducted in vitro and in vivo, mathematic modeling simulation studies, and prospective, community-based investigations that characterize the incidence, prevalence, and natural history of the disease. In addition, randomized clinical trials will be needed to determine the optimal treatment strategies for this group of patients--strategy choices undoubtably complicated by a disease whose treatment is influenced to a large extent by its origin. The future therapies evaluated in these randomized clinical trials will most likely range from medical therapies that target either the heart directly or the peripheral vascular system, to surgical interventions such as direct myocardial revascularization, to gene therapy. Finally, it is worth mentioning one more unresolved issue that is of general practical concern not only to the physiologist studying diastolic function, but also to the clinician: whether or not it is even feasible to develop a single, sensitive, specific, clinically relevant index of diastolic function that is free from the contaminating influences of rate, contractility, and load. As observed by Glantz 20 years ago, developing indexes with the hope that one might fully delineate the left ventricle's diastolic properties, rather than concentrating on discovering the physiologic significance of such indexes, is probably counterproductive. More recently, in a related article, Slinker implied that an operational definition of any aspect of cardiac function must allow for the measurement of that function over an adequate range of essential variables. Therefore, as previously mentioned, the physiologist studying cardiac function has the daunting task of trying to understand, in a precise way, how the processes and mechanisms of the various phases of the cardiac cycle couple together to produce either a normal or abnormal functioning heart. It seems clear that because of the complex weave of factors that control overall cardiac diastolic function, the derivation of any single index that adequately describes LV diastolic function in vivo may not be possible.

Noninvasive imaging in congenital heart disease

Imaging algorithms in congenital heart disease, as in the patient with acquired heart diseases continue to evolve, with more and more information gleaned noninvasively. The emphasis will be on the newer aspects of imaging, not cross sectional echocardiography with color Doppler.

Evaluation of Left Ventricular Diastolic Performance Using Automated Border Detection

Acoustic quantification (AQ) is a noninvasive technique which provides online left ventricular (LV) area/volume waveforms. The filling portion of the AQ waveform can be used to assess LV diastolic properties. Analysis of signal-averaged AQ curves enhances the waveforms and allows reliable, quantitative, and automated analysis. From signal-averaged AQ LV waveforms, the phases of diastole can be easily detected and several parameters of diastolic performance calculated. Analysis of signal-averaged LV waveforms is complementary to that of LV AQ analysis. AQ has been used to identify diastolic dysfunction in patients with LV hypertrophy and systemic hypertension. Normal values of these parameters are age dependent and reference values will soon be available.

Online Quantification of Cardiovascular Function in Children

Echocardiography is the most valuable diagnostic tool in evaluating congenital anatomic pathology. Improvements in transducer resolution and development of color Doppler echocardiography have resulted in the ability to delineate cardiac anatomy in detail. Online quantification of cardiovascular function represents a similar advance in terms of evaluating physiology. Performing an online quantification examination requires attention to technical factors such as avoiding chamber foreshortening, optimizing endocardial tracking, ensuring patient cooperation, and accounting for the effects of respiration. A typical examination consists of performing acoustic quantification on the left ventricle in both short-axis and apical four-chamber views and on the left atrium from the apical four-chamber view. From these views, ventricular systolic and diastolic function, as well as atrial systolic function, can be assessed. As in adult patients, acoustic quantification indices in children correlate well with similar indices measured off line. Acoustic quantification data in children show that, unlike adults, the left ventricular filling is almost exclusively dependent on passive filling, with minimal atrial contribution. Acoustic quantification indices have also been useful in monitoring left ventricular function in intensive care and emergency department settings to evaluate the acute effects of a clinical manipulation. Acoustic quantification can also be used to assess the function of ventricles of unusual geometry such as in hypoplastic left and right heart, double-inlet single ventricle, and unbalanced atrial ventricular septal defect. In these types of ventricles, systolic and diastolic systemic ventricular function has been shown to be markedly altered compared with a normal left ventricle. Acoustic quantification can also be used to assess diastolic function in a variety of pediatric diseases, such as transplant rejection and insulin-dependent diabetes mellitus. In summary, online quantification of cardiovascular function represents a significant advance in the ability to understand cardiovascular physiology in children.