Current practice of exercise stress testing among pediatric cardiology and pulmonology centers in the United States.

The objective of this study was to characterize current practice patterns for clinical exercise stress testing (EST) in children in the United States. We conducted a survey of 109 pediatric cardiology programs and 91 pediatric pulmonology programs at children's hospitals or university hospitals in the United States. A total of 115 programs from 88 hospitals responded (response rate, 58%). A higher percentage of cardiology programs (98.7%) have exercise laboratories compared with pulmonology programs (77.5%). Sixty-three percent of respondents have both a treadmill and a cycle ergometer. A larger proportion of respondents (76%) rely primarily or exclusively on treadmill, whereas a smaller number use cycle ergometer (24%). Sixty-seven percent of respondents reported that they include metabolic measurements in EST protocols. Respondents have varying minimum age criteria for EST, with 9% reporting < or = 4 years, 25% reporting 5 years, 31% reporting 6 years, 16% reporting 7 years, and 20% reporting > or =8 years. Programs using cycle ergometers tend to test children at a younger age and to measure metabolic parameters. Seventy-nine percent of respondents use Bruce and modified Bruce protocols. Institutional protocols are used by 14%. Ninety percent of respondents use technicians to perform EST and 8% use nurses, but 76% require physician presence during testing. The majority of respondents (57%) perform < 100 pediatric tests per year. There are wide variations in the current practice of EST among pediatric subspecialty programs in the United States. Treadmills are used more frequently than cycle ergometers, and Bruce and modified Bruce protocols are commonly used. Most survey respondents measure metabolic parameters during EST.

Inhibition of L-type Ca2+ channel current in Xenopus oocytes by amiodarone.

BACKGROUND: Although amiodarone has been referred to as a class III antiarrhythmic agent, it also possesses electrophysiologic characteristics of the three other classes (classes I and IV and minor class II effects). Previous studies have demonstrated that amiodarone inhibits Ca2+ channel current in intact cardiac myocytes. However, it is not clear whether this response reflects a pure class IV effect (direct Ca2+ channel inhibition) or a class II effect (beta-adrenergic receptor blockade) of amiodarone. METHODS: In the current study, the effects of amiodarone on Ca2+ current were studied in the absence of sympathetic regulation using a Xenopus oocyte expression system. The L-type Ca2+ channel alpha1C subunit was coexpressed with the alpha2delta and beta2a subunits in enzymatically digested Xenopus oocytes. Ca2+ currents were recorded using the cut-open oocyte preparation. RESULTS: We found that perfusion of 10 microM isoproterenol produced no significant change in peak Ca2+ current (from 223+/-33 to 210+/-29 nA, mean+/-SEM, n=5, P=not significant), indicating the absence of a functional stimulatory sympathetic signal pathway in these oocytes. After 10 minutes of exposure to 10 microM amiodarone, Ca2+ current amplitude was significantly decreased from 174+/-33 to 100+/-26 nA (n=8, P<0.01; control group: 220+/-33 to 212+/-29 nA, n=5, P=not significant). These effects were similar to those of 10 microM nifedipine (201+/-48 to 108+/-48 nA, n=6, P<0.05), a typical Ca2+ channel blocker. On the other hand, neither amiodarone nor nifedipine significantly altered the Ca2+ current activation or inactivation kinetics. CONCLUSIONS: These results demonstrate that amiodarone inhibits Ca2+ current in the absence of a functional intrinsic beta-adrenergic stimulatory system and, therefore, represents a true class IV effect.

Echocardiographic findings of hypertrophic cardiomyopathy in children after orthotopic liver transplantation.

This study was carried out to compare echocardiographic findings of children taking tacrolimus and cyclosporin A (CsA) after orthotopic liver transplantation (OLT). Echocardiograms of 19 children were reviewed during hospitalizations after OLT, and echocardiograms were performed on 23 children who returned to the clinic for a routine follow-up visit after OLT. Measurements were made of the left ventricle (LV) end-diastolic dimension, and of the thickness of the LV free wall (LVFW) and the inter-ventricular septum (IVS). From these measurements, the LV mass was calculated. LV outflow gradient was measured by using Doppler interrogation. Comparisons were made between patients on CsA and patients on tacrolimus. Children with hypertrophic cardiomyopathy (HCM) were identified. Two patients from the in-patient tacrolimus group were found to have HCM. These two patients had asymmetric septal hypertrophy with dynamic LV outflow obstruction and were successfully treated with propranolol, with or without discontinuing tacrolimus. In the out-patient studies, there was no difference in LVFW and IVS thickness, or LV mass index, between children on CsA and children on tacrolimus. Hence, tacrolimus is associated with the development of HCM in children. The effect of tacrolimus on HCM development may be acute and temporary. More data are needed to determine the incidence of HCM in children on tacrolimus therapy and to establish guidelines for clinicians who follow-up these children.

Factors associated with age at operation for children with congenital heart disease.

OBJECTIVE: Previous studies have shown that children with congenital heart disease (CHD) who live in nonurban areas or who do not have private insurance are at risk for delayed referral to a pediatric cardiologist. However, the effect of these factors on the age at which cardiac surgery is performed has not been evaluated. This study is designed to evaluate the factors that influence the age at which definitive surgical repair is performed. METHODS: Data on hospital discharges for 1995 and 1996 in California were obtained from the Office of Statewide Health Planning and Development database. Children <18 years who underwent surgical repair for atrial septal defect (ASD), ventricular septal defect (VSD), tetralogy of Fallot (TOF), or atrioventricular canal (AVC) were included in the study. Age at surgery was evaluated using type of CHD, gender, race, type of insurance, surgical centers, urban or rural home location, and distance between home and surgical center as independent variables. RESULTS: In 1995-1996, 666 children underwent ASD closure (mean age: 5.1 years; median: 4.0 years), 582 VSD closure (mean age: 2.8; median: 1.1 years), 394 TOF repair (mean age: 1.7; median:.9 years), and 177 AVC repair (mean age: 1.1; median:.6 years). Comparing median and mean age at surgery, we found: AVC

Infection of human fetal cardiac myocytes by a human immunodeficiency virus-1-derived vector.

Cardiomyopathy associated with HIV-1 infection is a well-recognized complication. However, it is unknown whether direct cardiomyocyte infection is involved in the pathogenesis of the cardiomyopathy. An HIV-1-based lentiviral vector and wild-type HIV-1 were used to infect human fetal cardiac myocytes in a primary culture. Quantitative polymerase chain reaction, viral p24 antigen determination, and immunofluorescence were used to detect the synthesis of HIV-1 DNA and proteins after the infection. High-efficiency infection occurred using the HIV-1-based lentiviral vector, although no infection occurred with the wild-type HIV-1 strain. Dual-labeling immunofluorescence for HIV-1 proteins and myosin confirmed that cardiomyocytes were infected. This in vitro analysis suggests that direct myocyte infection with wild-type HIV-1 may not be involved in the pathogenesis of HIV-1 cardiomyopathy. However, HIV-1-based vectors may prove useful for ex vivo cardiovascular gene therapy.

The relationship between temperature and calcium in acute cell damage after exposure to radiofrequency or thermal energy in isolated neonatal and adult rabbit cardiac myocytes.

Radiofrequency (RF) ablation is a nonsurgical technique using catheter-directed RF energy for treating cardiac arrhythmias in children and adults. Previous reports have suggested that sequestration of calcium (Ca2+) by the sarcoplasmic reticulum may partially protect mature cardiac myocytes from the effects of RF energy. The purposes of this study were to determine whether differences exist between neonatal and adult myocyte responses to RF energy and if myocyte damage is a Ca2+-dependent process. Because immature myocardium is functionally deficient in sarcoplasmic reticulum, we hypothesized that immature myocytes would be more susceptible to damage induced by RF energy. Isolated ventricular myocytes were obtained from neonatal and adult New Zealand White rabbits by enzymatic dissociation, then placed in a perfusion chamber designed to deliver RF energy or a heated perfusate solution. Measurements of bath temperature, cell morphology, and contractile response to electrical stimuli were recorded. RF energy application associated with increased perfusate temperature resulted in cell death, but not when the temperature rise was inhibited. Thus, the acute damage to cells exposed to RF energy appears to be mediated by thermal energy. After exposure to thermal energy, neonatal cells underwent contracture at lower temperatures than did adult cells. Perfusion with solutions containing low Ca2+ concentrations, comparable to intracellular diastolic Ca2+ levels, had a protective effect for both neonatal and adult myocytes. These findings indicate that acute cell damage after exposure to RF energy is mediated by a Ca2+-dependent process. Furthermore, immature myocardium is particularly susceptible to RF-mediated cell damage, possibly secondary to reduced Ca2+ sequestration by the sarcoplasmic reticulum.

Anatomic and electrophysiologic relation between the coronary sinus and mitral annulus: implications for ablation of left-sided accessory pathways.

To determine whether precise left-sided accessory pathway localization is possible from the coronary sinus, electrocardiogram (ECG) characteristics from the coronary sinus pair demonstrating earliest activation via the accessory pathway were compared to simultaneous mitral annular ablation catheter ECGs at successful ablation sites in 48 patients. To define the coronary sinus-mitral annular relation, the coronary sinus to mitral annulus distance (D) was measured at sequential distances from the coronary sinus os in 10 cadaver hearts. Mitral annular ECGs demonstrated earliest activation via the accessory pathway more frequently than the earliest coronary sinus pair (p < 0.001), more frequent continuous electrical activity (p < 0.001), and more frequent accessory pathway potentials (p < 0.01). D was >10 mm at 20, 40, and 60 mm, respectively, from the coronary sinus os. Coronary sinus ECGs do not precisely localize left-sided accessory pathways, which may be due in part to an average anatomic separation of more than 10 mm between the coronary sinus and accessory pathways bridging the mitral annulus.

Mechanisms of amiodarone-induced inhibition of Ca2+ current in isolated neonatal rabbit ventricular myocytes.

BACKGROUND: Amiodarone is an effective antiarrhythmic drug used to treat a wide variety of ventricular and supraventricular tachyarrhythmias. Recent voltage clamp studies indicate that amiodarone may possess a variety of antiarrhythmic effects. METHODS: The tight-seal, whole-cell voltage clamp technique was used to investigate the acute effects of amiodarone on L-type Ca2+ channel kinetics in isolated neonatal ventricular myocytes. RESULTS: We found that acute perfusion with 1 mumol/L amiodarone inhibited peak inward Ca2+ current by 39.1% (4.85 +/- 0.42 to 2.95 +/- 0.6 pA/pF, n = 10, p < 0.001) without changing the shape of the current-voltage relation. In addition, amiodarone shifted Ca2+ channel steady-state inactivation to more negative membrane potentials. In the absence of amiodarone, half inactivation of the Ca2+ current occurred at a membrane potential of -23.8 +/- 0.2 mV compared to -34.2 +/- 0.6 mV after addition of amiodarone (n = 11, p < 0.01). Furthermore, amiodarone significantly delayed Ca2+ current recovery from previous inactivation. CONCLUSIONS: These results provide evidence that amiodarone blocks voltage-dependent Ca2+ current in isolated neonatal rabbit ventricular myocytes by a variety of different mechanisms. The inhibitory effect of amiodarone on L-type Ca2+ current may represent an important facet of amiodarone's acute antiarrhythmic activity in the immature heart.

Effects of isoflurane on electrophysiological measurements in children with the Wolff-Parkinson-White syndrome.

This study was designed to assess the effects of isoflurane (ISO) on the electrophysiological properties of the accessory pathway, atrium, ventricle, and AV node in children with the Wolff-Parkinson-White (WPW) syndrome. The results of programmed electrical stimulation were analyzed in 51 patients (4 months to 17 years of age) with WPW. The study population was divided into two groups. Twenty-seven patients received local anesthesia and intramuscular injection of meperidine, promethazine, and chlorpromazine (MPC group). Twenty-four patients received general anesthesia with ISO inhalation (ISO group). We compared the antegrade effective refractory period of the accessory pathway (antegrade APERP), ventricular effective refractory period (VERP), atrial effective refractory period (AERP), AH interval, and cycle length of circus movement tachycardia (CMT-CL) in 12 pairs of age and sex matched patients selected from the MPC and ISO groups. Of the 12 pairs of age and sex matched patients, antegrade APERP in patients who received ISO (299 +/- 17 ms, mean +/- SEM) was significantly longer as compared with matched patients in the MPC group (262 +/- 5 ms, P < 0.025). The VERP and AERP in patients from the ISO group were significantly prolonged compared with the MPC patients (239 +/- 7 vs 210 +/- 8 ms, P < 0.025, and 228 +/- 11 vs 180 +/- 6 ms, P < 0.01, respectively). There was no significant difference in the AH interval or CMT-CL between the two subgroups. Thus, ISO prolongs the antegrade APERPs as well as the effective refractory periods of atrial and ventricular muscle in children with WPW, while the AH interval and CMT-CL appear to be unaffected. Care must be taken in interpreting measurements of the antegrade APERP made in patients under general anesthesia for RF ablation of accessory pathways.

Developmental changes in the effects of pH on contraction and Ca2+ current in rabbit heart.

Protons inhibit Ca2+ current and contraction in heart muscle. The present study compares the effects of lowering the pH of the bath solution on single-cell contraction, action potential configuration and Ca2+ currents between neonatal and adult rabbit hearts. We found that a reduction of extracellular pH from 7.3 to 6.3 decreased cell contraction amplitude to 84.3% of control in isolated neonatal myocytes. A comparable change in extracellular pH resulted in a decrease in cell contraction to 56.2% of control in adult cells. Similarly, tension generation in intact neonatal papillary muscles was less sensitive to a decrease in external pH as compared to papillary muscles from adult animals. In addition, acidosis caused a less pronounced inhibition of Ca2+ current in neonatal cells than in adult cells (85 +/- 4% nu 62 +/- 4% of control, pH = 6.3, P < 0.001; 63 +/- 5% nu 32 +/- 5% of control, pH = 5.8, P < 0.001). Thus, the effect of external acidosis on myocardial contractility is commensurate with the effect on trans-sarcolemmal Ca2+ current. The membrane potential at which peak Ca2+ current occurred was not altered by low pH in neonatal cells but was shifted toward positive potentials by 17.7 mV in adult myocytes. Further, low external pH solution reduced Ca2+ current conductance more in adult cells than in neonatal cells. Moreover, action potential configuration in neonatal cells was altered less by acidosis as compared with adult cells. These findings may help explain the greater resistance of neonatal hearts to extracellular acidosis.

Developmental cardiac electrophysiology recent advances in cellular physiology.

This overview of cardiac ion channel development does not suggest any particular theme underlying the expression or regulation of all channel subtypes. Calcium and potassium channels generally exhibit increased expression in more mature hearts. However, this increase in channel number or activity as determined under voltage clamp conditions may not be translated into increased activity in vivo. Concomitant changes in other physiological factors such as local intracellular Ca2+ accumulation, increased resting membrane potential and decreased heart rate in mature heart may inhibit or augment channel activity. Na(+)-Ca2+ exchange activity appears to decrease with development, possibly reflecting its decreasing role in both systolic and diastolic Ca2+ regulation. Na+ channel activity follows a middle course, exhibiting little change in channel conductance. The reported shift in the voltage dependence of channel inactivation toward more negative membrane potentials may reflect a concomitant shift in the resting membrane potential in mature heart. However, this change is in the direction opposite to that reported for L-type Ca2+ channel inactivation, suggesting that the regulation of these channels is not modulated by a common factor such as membrane surface charge. A detailed characterization of multiple channel subtypes in mature myocardium has resulted in significant advances in models of the cardiac action potential and excitation-contraction coupling. Recently, developmental changes in ion channel physiology have been described, setting the stage for a comparable elucidation of the ontogeny of the cardiac action potential. Ca2+ and K+ channel currents generally become more prominent with development. In contrast, developmental changes in Na+ currents are less dramatic and Na(+)-Ca2+ exchange currents appear to decrease with age. These changes may, in part, be reflected by the increasingly important role of transsarcolemmal Ca2+ influx in triggering Ca2+ release from the SR in mature heart as compared to its direct role of providing Ca2+ for cell contraction in immature heart. These developmental changes in ion channel expression and function are likely to have a significant effect on the generation of the action potential in individual myocytes. Developmental changes in the characteristics of the action potential may then have a major influence on the initiation, propagation and termination of autonomic, triggered, and re-entrant arrhythmias. Progress in this area provides an essential foundation for the evolution of a systematic approach to pediatric arrhythmias comparable to that under development for mature heart [3].

MR imaging of surgical complications of systemic-to-pulmonary artery shunts.

Patients with a systemic-to-pulmonary artery shunt and positive findings on traditional imaging modalities such as chest X-ray, echocardiography, or cardiac angiography often can benefit from additional noninvasive imaging with magnetic resonance imaging (MRI). Diagnostic dilemmas encountered include: pseudoaneurysms, contained fluid collection (seroma) surrounding a shunt, and stenosis of the shunt anastomoses. MRI studies using traditional cardiac-triggered spin-echo (SE) imaging and the newer breathhold MRI studies with k-space segmented gradient-recalled echo (GRE) imaging can greatly help resolve diagnostic dilemmas. By combining different MR imaging techniques it becomes possible to clearly distinguish between pseudoaneurysms and seroma, to exclude an active leak and to sometimes visualize the distal anastomosis with more precision than conventional angiography. MRI is often able to add information needed for clinical decision making prior to surgical repair.

Steady-state mRNA levels of G protein subunits in developing rabbit myocardium.

Cardiac responsiveness to beta-adrenergic stimulation changes with age. Developmental changes in expression of guanine nucleotide-binding coupling protein (G protein) subunits may account for these physiologic changes. We measured steady-state levels of mRNA encoding the alpha-subunit of the specific G protein that stimulates adenylyl cyclase (Gs alpha) and three isoforms of beta-subunit of G proteins (G beta) in developing myocardium. Total RNA prepared from the right and left ventricles of fetal, neonatal, juvenile, and adult rabbits was size-fractionated, blotted, and probed with 32P-labeled cDNAs encoding rat Gs alpha, bovine G beta-1, human G beta-2, and human G beta-3. For standardization, these blots were subsequently hybridized with a 32P-labeled cDNA encoding glyceraldehyde 3-phosphate dehydrogenase (GAPD). Two-dimensional densitometric analysis of autoradiographs was used to quantify relative hybridization intensities. An age-dependent decrease in mRNAs encoding Gs alpha, G beta-1, and G beta-2 relative to mRNA encoding GAPD was observed in both ventricles, while G beta-3 mRNA was not detected. At all ages studied, levels of Gs alpha and G beta-1 mRNA were similar in the two ventricles. However, G beta-2 mRNA declined more in the left ventricle than in the right ventricle during maturation. Our results demonstrate developmental control in heart for mRNAs encoding several G protein subunits. In addition, differential declines in G beta-1 and G beta-2 mRNA in the right ventricle suggest that these G beta isoforms are regulated uniquely and may reflect functional roles for these G beta isoforms in different signaling cascades.

Adenylyl cyclase integrates multiple G protein signals to modulate calcium currents in neonatal rabbit heart.

We investigated the effects of added beta gamma subunits of G proteins (G beta gamma) on beta-adrenergic responsiveness of transmembrane Ca2+ currents (ICa) in ventricular myocytes from neonatal rabbits. G beta 1 gamma 1 purified from retinal rods was dialyzed into cells via the voltage clamp micro-electrode. Stimulation of ICa by isoproterenol was not affected by added intracellular G beta 1 gamma 1 or by carbachol alone but was completely blocked by combined G beta 1 gamma 1 and carbachol. Pretreatment of cells with pertussis toxin or temporal separation of carbachol and isoproterenol allowed stimulation of ICa by isoproterenol in cells dialyzed with G beta 1 gamma 1. Carbachol and G beta 1 gamma 1 together also did not prevent stimulation of ICa by dibutyryl-cyclic AMP. Thus, rather than simply inactivating Gs alpha by mass action, G beta 1 gamma 1 acts in concert with carbachol to inhibit isoproterenol stimulation of ICa.

Age- and anesthesia-related changes in accessory pathway conduction in children with Wolff-Parkinson-White syndrome.

The anterograde effective refractory period of the accessory pathway is age-dependent in pediatric patients with the WPW syndrome. Thus, age should be considered when developing electrophysiologic criteria for the risk of hypotensive arrhythmias in these patients. In addition, general anesthesia must also be considered in interpreting age-related changes in the anterograde APERP, especially in children.

Distribution of the Na+/Ca2+ exchange protein in developing rabbit myocytes.

The Na+/Ca2+ exchanger is a major pathway for transmembrane flux of Ca2+ in cardiac cells. Immunolabeling in adult rabbit myocytes showed localization of the Na+/Ca2+ exchanger to the peripheral sarcolemma and especially in the T tubules. Previous studies have also demonstrated higher Na+/Ca2+ exchanger activity in fetal and newborn rabbit hearts in which the T tubular system is not completely developed. Indirect immunofluorescent studies were performed to localize the Na+/Ca2+ exchanger in isolated myocytes from immature (5, 11, 17, and 30 days) and adult rabbits. Cells were incubated with a monoclonal antibody to the exchanger followed by fluorescein-labeled goat anti-mouse antibody. It is found that at 5 days of age the immunofluorescent labeling was very intense and confined to the peripheral sarcolemma. After 11 days of age, localization of labeling followed the development of the T tubules. The exchanger appeared in the T tubules as soon as they were formed. The Na+/Ca2+ exchange protein is abundantly localized to the peripheral sarcolemma before and during the development of T tubule system.

Na+/Ca2+ exchange and cell contraction in isolated neonatal and adult rabbit cardiac myocytes.

Recent studies have demonstrated a relative deficiency in voltage-gated Ca2+ currents (ICa) in immature myocardium. We hypothesized that contraction in developing heart results in part from Ca2+ influx via "reverse" Na+/Ca2+ exchange current (INa/Ca). Accordingly, INa/Ca and cell contraction amplitude were measured in single neonatal and adult rabbit ventricular myocytes. INa/Ca was dependent on Ca2+ concentration, Na+ concentration, and membrane potential and was blocked by 5 mM Ni2+ but not by the Ca(2+)-channel blocker nifedipine. In neonatal cells, contraction amplitude reached a plateau for depolarizations positive to 0 mV. In adult myocytes, contraction amplitude was maximal at 0 mV and decreased at positive membrane potentials. Inhibition of ICa with nifedipine did not affect maximal contraction amplitude in neonatal myocytes but almost completely suppressed contraction of adult cells. These data suggest that Ca2+ influx via ICa is not required for contraction of neonatal rabbit cardiac myocytes. Moreover, Ca2+ influx via reversal of the Na+/Ca2+ exchange mechanism may provide a significant portion of the Ca2+ regulating cell contraction, especially during depolarization to positive membrane potentials.