Cardiac troponin T in serum as marker for myocardial injury in newborns.

In neonates, acute perinatal asphyxia may lead to ischemic myocardial damage [1]. In some cases, subendocardial infarction has been documented [2]. Generally, diagnosis of the myocardial injury (MI) is based on clinical findings, suggestive electrocardiographic and echocardiographic patterns [3], decrease in myocardial uptake of thallium [4], and classical creatine kinase (CK)-MB isoenzyme measurement [5]. However, CK-MB in serum cannot be regarded as a cardiac-specific marker in the neonate, and extreme caution should be used in the interpretation of increased CK-MB activity during this period [6]. Cardiac troponin T (cTnT), the structural protein that binds the troponin complex to the tropomyosin molecular strand, has recently been proposed as a more specific biochemical marker for diagnosis of myocardial infarction in the adult population [7]. Here we evaluated the use of cTnT measurement in serum in the diagnosis of MI in newborns, as well as that of the determination of CK-MB mass concentration by a sensitive and specific monoclonal anti-CK-MB antibody-based immunoassay. Three groups of infants were studied. Group I consisted of 27 preterm infants (gestational age ranging from 28 to 36 weeks) without major respiratory and cardiovascular dysfunctions. Group II was 27 healthy full-term newborns (15 born by vaginal delivery and 12 by cesarean section) with a mean gestational age of 39.7 weeks. Group III was composed of seven infants (four preterm and three term) who demonstrated, during the first 3 days after birth, clinical, electrocardiographic, and echocardiographic signs of MI. In particular, in electrocardiogram (ECG) evaluation, MI was considered to be present when inversion of T waves or ST-segment depression $1 mm in more than two precordial leads was noted. Groups I and II underwent a clinical examination, ECG, echocardiogram, and blood collection for the measurement of total CK, CK-MB, and cTnT on day 2 after birth. Group III was evaluated with ECG and echocardiography at presentation and every 24 h until clinical recovery or death. Total CK, CK-MB, and cTnT were measured 12 and 48 h after presentation and, when possible, after 45 days. The protocol of the study was approved by the local Ethical Committee, and parental consent was obtained. An ELISA (Boehringer Mannheim, Mannheim, Germany) was used to determine cTnT in serum (detection limit, 0.02 mg/L). Total CK activity was measured at 37 °C by the method recommended by the IFCC. CK-MB mass concentrations were determined with the Magic Lite CK-MB assay (Ciba Corning Diagnostic Corp., Medfield, MA). A relative index (RI), i.e., (CK-MB, mg/L/total CK, U/L) 3 100, was also calculated. In adults, the upper reference limit (URL) is 0.10 mg/L for cTnT, 175 U/L for total CK, 6.0 mg/L for CK-MB, and 3% for RI. M-mode, two-dimensional, and pulsed Doppler echocardiographic examinations were performed with an Acuson echocardiograph (Acuson, Mountain View, CA) with a 7.5-MHz transducer. Each patient was examined according to the standards of the American Society of Echocardiography [8], and echocardiograms from MI infants were compared with those of infants without MI of similar gestational age. Left ventricular fractional shortening was calculated by the following formula: [left ventricular-end diastolic diameter (LVDD) 2 left ventricular-end systolic diameter 3 100]/LVDD. Cardiac output was measured by multiplying the aortic flow integral by the area of the aortic orifice. Mitral and tricuspid incompetence were graded as trivial, mild, moderate, and severe. In group I, four infants were small for gestational age, but none had positive cardiovascular examination and ECG. At echocardiography, LVDD (12.6 6 1.6 mm) was within the previously reported age-related limits [9] and was positively related to weight (r 5 0.71, P ,0.001) and gestational age (r 5 0.65, P ,0.001). Fractional shortening (40.8% 6 7.2%) and cardiac output (0.81 6 0.32 L/min) were also within normal limits, and no signs of pulmonary hypertension were pointed out (mean right ventricular pressure 6 SD, 23.9 6 9.4 mmHg). Trivial mitral incompetence was present in two infants, both born at 28 weeks’ gestation. The incidence of tricuspid incompetence (no more than mild), present in 20 infants, decreased with increase in gestational age. All healthy full-term newborns (group II) also had negative clinical examination results and normal ECGs and echocardiograms. Furthermore, the method of delivery did not affect the concentrations of biochemical markers studied. Total CK and CK-MB concentrations were significantly (P ,0.001) lower in group I than in group II (mean CK 6 SE, 246 6 57 U/L vs 751 6 102 U/L; mean CK-MB 6 SE, 9.8 6 1.5 mg/L vs 29.1 6 3.5 mg/L, respectively), even if RI did not change between the two groups (5.1% 6 0.5% vs 4.3% 6 0.3%, P 5 0.34), suggesting that the difference in enzyme concentration reflected a difference in skeletal muscle mass. Indeed, total CK and CK-MB values were significantly correlated to birth weight (r 5 0.50, P ,0.001 for total CK, and r 5 0.56, P ,0.001 for CK-MB) and to gestational age (r 5 0.56, P ,0.001 for total CK, and r 5 0.60, P ,0.001 for CK-MB). cTnT did not change with gestational age (r 5 0.20, P 5 0.15), and no correlation was found between values for serum cTnT and total CK activity (r 5 0.24, P 5 0.08) or between cTnT and CK-MB mass concentration (r 5 0.21, P 5 0.12). However, virtually all healthy children (group I and II) had detectable cTnT (mean, 0.16 mg/L; range, 0.04–0.43 mg/L). The 95th percentile values for these biochemical markers in healthy newborns were: total CK (preterm), 635 U/L; total CK (term), 2126 U/L; CK-MB (preterm), 26 mg/L; CK-MB (term), 72 mg/L; RI, 8.8%; cTnT, 0.33 mg/L. Clinical, echocardiographic, electrocardiographic, and Technical Briefs