Children with single ventricle (SV) physiology have increased ventricular work and are at risk of heart failure (HF). However, a HF diagnosis is especially difficult, because few objective measures of HF have been validated in this cohort. We have previously shown that plasma B-type natriuretic peptide (BNP) levels are sensitive and specific for detecting HF in a small, heterogeneous SV cohort. The aim of the present study was to define the effect of SV morphology and stage of palliation on the correlation between BNP and HF. We also examined the utility of N-terminal pro-BNP (NT-proBNP), a more stable product of pre-BNP processing, as a biomarker of HF in these patients. A cross-sectional observational study of SV children aged 1 month to 7 years was conducted. The presence of HF was defined as a Ross score >2. The association of BNP or NT-proBNP with HF was assessed using logistic regression analysis and receiver operating characteristic curves. Of the 71 included children, 22 (31%) had clinical HF. A doubling of BNP was associated with an odds ratio for HF of 2.20 (95% confidence interval 1.36 to 3.55, p = 0.001) with a c-statistic >75%, yielding a detection threshold of ≥45 pg/ml. This threshold was preserved when patients were stratified by the right ventricular morphology or stage of surgical palliation. Similarly, a doubling of NT-proBNP was associated with an odds ratio for HF of 1.92 (95% confidence interval 1.17 to 3.14, p = 0.009). In contrast to BNP, the threshold value of NT-proBNP for predicting HF decreased with the stage of palliation. In conclusion, plasma BNP and NT-proBNP are reliable tests for clinical HF in young children with SV physiology, specifically those with right ventricular morphology, regardless of the stage of palliation.
Toward establishing B-type natriuretic protein (BNP) as a potential biomarker of heart failure (HF) in children with single ventricle (SV) physiology, we previously demonstrated its utility in a small cohort of these patients. We demonstrated in a sample of 29 children that plasma BNP was sensitive and specific for detecting mild HF, with a cutpoint value of ≥30 pg/ml. This threshold value was markedly lower than the value reported for HF in the adult population (>100 pg/ml). In addition, a recent post hoc analysis of patients in the Pediatric Carvedilol Trial suggested a threshold of >140 pg/ml for predicting adverse outcomes in children with known HF; however, that study did not distinguish between those with SV versus biventricular physiology. To determine the reliability of BNP as a predictor of HF in children with SV heart disease, we studied a larger cohort of patients and analyzed HF versus BNP according to ventricular morphology and the stage of surgical palliation. Because BNP has a relatively short half-life in plasma, we also compared BNP and N-terminal pro-B-type natriuretic peptide (NT-proBNP) in this same population to determine whether a related biomarker with a longer half-life would prove more useful.
Methods
A single-site cross-sectional observational study using a secondary study base was conducted. All children aged 1 month to 7 years with SV physiology presenting to the University of California San Francisco Pediatric Heart Center from February 2007 to December 2010 were eligible for the present study. The patients were excluded if they had renal failure, trisomy 21, an acute intercurrent illness, a congenital defect that interfered with feeding (e.g., cleft palate, esophageal atresia), or had participated in an investigational drug or device study in the previous 6 months. In addition, patients for whom a valid Ross score could not be determined (e.g., those receiving nasograstric tube feedings, those with postoperative diaphragmatic paralysis) were excluded. The present study was approved by the University of California San Francisco institutional review board, and written consent was obtained from the guardians of all subjects.
Each child was assigned a Ross score to determine the presence of clinical HF immediately before phlebotomy ( Table 1 ). For the present study, HF scoring was recorded, independent of the presumed mechanism, because the goal was to evaluate biomarkers in patients with a SV that would be clinically useful across the etiologies. The predictor measurement occurred after outcome determination, allowing assessors to be unaware of the plasma protein levels.
| Variable | Score | ||
|---|---|---|---|
| 0 | 1 | 2 | |
| Infants | |||
| Feeding history | |||
| Volume/feed (oz) | >3.5 | 2.5–3.5 | <2.5 |
| Time/feed (min) | <40 | >40 | |
| Physical examination | |||
| Respiration rate (breaths/min) | <50 | 50–60 | >60 |
| Heart rate (beats/min) | <160 | 160–170 | >170 |
| Respiration | Normal | Abnormal | |
| Perfusion | Normal | Decreased | |
| S3 | Absent | Present | |
| Liver size (cm) | <2 | 2–3 | >3 |
| Children | |||
| Feeding history | |||
| Meals | Normal | Variable | Less |
| Physical examination | |||
| Respiration rate (breaths/min) | <20 | 20–30 | >30 |
| Heart rate (beats/min) | <110 | 111–130 | >130 |
| Respiration | Dyspnea on exertion | Marked dyspnea on exertion | Dyspnea at rest |
| Perfusion | Normal | Decreased | |
| S3 | Absent | Present | |
| Liver size | Above costal margin | At costal margin | Below costal margin |
At cardiac catheterization, the preoperative evaluation for cardiac surgery, or medical admission, 6 ml of whole blood was obtained. The plasma BNP and NT-proBNP were measured after assignment of the Ross score. For BNP and NT-proBNP, the plasma was collected from whole blood within 2 hours and stored at −80°C before assay. BNP was assayed using the Biosite Triage kit (Biosite Diagnostic, San Diego, California), as previously described. NT-proBNP was assayed using the Elecsys proBNP kit (Roche Diagnostics, Pleasanton, California) according to the manufacturer’s instructions. Each sample was assayed in duplicate.
The primary outcome was HF (Ross score ≥3) versus no HF (Ross score ≤2). The relation between the raw Ross score and plasma protein levels was summarized using scatter plots and analyzed using both linear and logistic regression analysis. Univariate logistic regression analysis was used to assess the crude association between each of the cardiac-related plasma proteins and clinical HF. p Values <0.05 were considered statistically significant. Receiver operating characteristic (ROC) curves were generated to evaluate proteins as biomarkers in the entire cohort and stratified groups. The assays were deemed useful if they carried a c-statistic of ≥0.75 (graph area encompassed by the curve). For a useful test, a clinically relevant cutpoint was identified that would distinguish SV children in HF from those free of HF at the evaluation. All statistical analyses were performed using Stata, version 10 (StataCorp, College Station, Texas).
Results
We approached 91 patients with a SV who met the inclusion criteria and had presented to the University of California San Francisco Pediatric Heart Center from February 2007 to December 2010 for enrollment. Of the 91 patients, 14 (15%) declined. Of the 77 remaining subjects, 6 (8%) were subsequently excluded because of missing data. Thus, 71 children were studied ( Table 2 ). Of the patients with a single right ventricle, 28 had a hypoplastic left heart (10 with mitral/aortic atresia, 8 with mitral stenosis and aortic atresia, and 10 with mitral/aortic stenosis), 13 had a right dominant atrioventricular canal defect, 6 had double-outlet right ventricle, 2 had an interrupted aortic arch with ventricular septal defect and small left ventricle, and 1 each had aortic stenosis and severe left ventricular dysfunction or L-transposition of the great arteries with severe pulmonary stenosis and a small left ventricle. Of the patients with a single left ventricle, 10 had tricuspid atresia, 3 had pulmonary atresia with an intact ventricular septum, 2 had double-inlet left ventricle, 2 had D-transposition of the great arteries with a straddling mitral valve and small right ventricle, and 1 had critical pulmonary stenosis with a small right ventricle. The patients with indeterminate ventricular morphology had primitive ventricles with heterotaxy syndrome.
| Ventricular Morphology | Stage ⁎ | Patients (n) | Females/Males (n) | Age † (mo) | Ross score ≤2 (n) | Ross score >2 (n) | Median Ross Score (Range) |
|---|---|---|---|---|---|---|---|
| Right | Total | 51 | 22/29 | 16 (2–80) | 36 | 15 | 2 (0–9) |
| Stage I | 19 | 9/10 | 4.4 (2–8) | 13 | 6 | 2 (0–6) | |
| Stage II | 24 | 11/13 | 35 (5–72) | 17 | 7 | 2 (0–9) | |
| Stage III | 8 | 2/6 | 66 (34–80) | 6 | 2 | 1.5 (0–4) | |
| Left | Total | 18 | 4/14 | 44 (3–78) | 11 | 7 | 1.5 (0–8) |
| Stage I | 4 | 1/3 | 5 (3–11) | 4 | 0 | 0 | |
| Stage II | 11 | 4/7 | 45 (17–69) | 5 | 6 | 3 (1–8) | |
| Stage III | 3 | 0/3 | 73 (69–78) | 2 | 1 | 1 (0–3) | |
| Indeterminate | Total | 2 | 1/1 | 60 (56–66) | 2 | 0 | 1.5 (1–2) |
| Stage I | 0 | 0 | 0 | 0 | 0 | — | |
| Stage II | 2 | 1/1 | 60 (56–66) | 2 | 0 | 1.5 (1–2) | |
| Stage III | 0 | 0 | 0 | 0 | 0 | — |
⁎ Stage I, stabilization of aortic and pulmonary blood flow (e.g., Norwood, Sano, pulmonary artery band, Blalock-Taussig shunt, central shunt); stage II, establishment of partial cavopulmonary circulation between superior vena cava and pulmonary arteries (e.g., Glenn shunt, Kawashima); and stage III, completion of cavopulmonary circulation (e.g., Fontan).
Patients were assigned a Ross score as a measure of clinical HF, as previously described ( Table 1 ), and the surgical stage of palliation was noted ( Table 2 ). Of the 49 children free of clinical HF, the median Ross score was 1 ± 0.8. For the 22 children with clinical HF, the median Ross score was 4 ± 1.7. The overall prevalence of HF in the included sample was 31%, with morphology-specific prevalence of 30% in those with a single right ventricle and 39% in those with a single left ventricle.
A scatter plot of BNP versus the Ross score was used to assess the relation between the biomarker and clinical status. After log 2 transformation, BNP demonstrated a roughly linear relation with the Ross score ( Figure 1 ). A doubling of BNP was associated with an odds ratio for HF of 2.20 (95% confidence interval [CI] 1.36 to 3.55, p = 0.001) when the side and stage were held constant. Using univariate logistic regression analysis, and stratifying the subjects by morphology and stage, a statistically significant correlation was found between BNP and HF in the single right ventricle group, with an odds ratio for HF of 2.03 (95% CI 1.24 to 3.31, p = 0.005), and in all patients at stage II with an odds ratio of 1.5 (95% CI 1.03 to 2.16, p = 0.03). In the patients with single ventricle at stage I, the correlation between BNP and HF approached statistical significance, with an odds ratio of 2.92 (95% CI 0.90 to 9.51, p = 0.08), as did the correlation between BNP and HF among patients at stage III (odds ratio 3.51, 95% CI 0.75 to 16.5, p = 0.11). We could not preclude the possibility that our failure to detect a significant association in these 2 groups might have resulted from insufficient power. No statistically significant correlation was found between BNP and HF in the single left ventricle group (odds ratio 1.94, 95% CI 0.89 to 4.22, p = 0.095), although this too was suggestive of an association.
The ROC curve analysis of BNP data for the entire cohort met our prespecified threshold of ≥75% area contained by the curve ( Figure 1 ). The ROC analysis stratified by ventricular morphology demonstrated that the right SV group exceeded the threshold of ≥75% (c-statistic 81%); however, patients with a left SV did not (c-statistic 69%; Figure 2 ). ROC analysis of patients with a right SV stratified by stage of surgical palliation demonstrated that patients at all 3 stages of palliation exceeded the usefulness threshold (c-statistic of 82%, 77%, and 92% for stage I, II, and III, respectively; Figure 2 ). Together, logistic regression analysis and ROC curve analysis supported the conclusion that BNP is a useful biomarker of HF (Ross score ≥3) for patients with a single right ventricle at all stages of palliation.
