Maturation and Cardiac Contractility

Effective dose of ephedrine for treatment of hypotension after induction of general anaesthesia in neonates and infants less than 6 months of age: a multicentre randomised, controlled, open label, dose escalation trial

BACKGROUND

The recommended dose of ephedrine in adults (0.1 mg kg^-1) frequently fails to treat hypotension after induction of general anaesthesia in neonates and infants less than 6 months of age. The aim of this study was to determine the optimal dose of ephedrine in this population for the treatment of hypotension after induction of general anaesthesia with sevoflurane.

METHODS

We conducted a multicentre, prospective, randomised, open-label, controlled, dose-escalation trial. Subjects were randomised if presenting a >20% change from baseline in MAP. Six cohorts of 20 subjects each were enrolled. Ten subjects in the first cohort received 0.1 mg kg^-1 i. v. (reference dose). For each subsequent cohort, 10 subjects were assigned to the next higher dose (consecutively 0.6, 0.8, 1, 1.2, and 1.4 mg kg^-1 i. v.), and the other subjects were assigned to one or more doses already investigated in previous cohorts. The primary outcome was the return of MAP to >80% of baseline at least once within 10 min after ephedrine administration.

RESULTS

A total of 119 infants (25% females), with a mean age (standard deviation) of 2.7 (1.3) months, received their allocated dose of ephedrine. The optimal dose of ephedrine was 1.2 mg kg^-1, with a percentage of success of 65.5% (95% confidence interval, 35.6-86.4). The doses of ephedrine investigated did not induce adverse events.

CONCLUSIONS

Doses of ephedrine much higher (∼10-fold) than those used in adults are necessary in neonates and infants for the treatment of hypotension after induction of general anaesthesia with sevoflurane.

CLINICAL TRIAL REGISTRATION

NCT02384876.

Hemodynamic considerations in preterm infants born at less than 25 weeks gestation

As survival rates continue to improve for infants born at less than 25 weeks gestation, delineating normal cardiovascular physiology from pathophysiology becomes much more challenging. With a paucity of 'normative' data for such infants, an over-reliance on studies at older gestations can result in a 'best guess' approach. Here we offer a pragmatic approach to these diagnostic challenges from a cardiovascular viewpoint. An appreciation of the unique physiology, from the immature myocardium and altered vascular tone to an innately large patent ductus arteriosus is essential, as is a thorough history for case specific contributing factors. We explore the additional difficulties in achieving a balance between minimal handling at the bedside and delineating important objective markers of perfusion. Finally, we discuss treatment approaches including inotrope therapy and patent ductus treatment, acknowledging the limited data available to guide these decisions.

Neonatal Rabbit Model for Pressure-Overloaded Heart Failure and Preliminary Exploration of Mechanism

BACKGROUND

This study aimed to establish a model of pediatric heart failure (PHF) with concomitant left ventricle pressure overload by transverse aortic constriction (TAC) and study the PHF mechanism primarily at the gene transcription level.

METHODS

Twenty-four neonatal rabbits within 7 days after birth were randomly divided into sham (n = 8), moderate TAC (50% constriction, n = 8) and severe TAC (sTAC; 75% constriction, n = 8) groups. After the procedure transthoracic echocardiography was performed at 2, 4, and 6 weeks to measure left ventricle structure and function. Histologic staining and gene sequencing of left ventricle myocardial tissue were performed at 6 weeks.

RESULTS

Six weeks after procedure transthoracic echocardiography showed that the pressure at the ligation of the aorta was 12.13 ± 0.95 mm Hg in the sTAC group, which was 26 times more than that of the sham group (P < .05), and left ventricular dilatation began to appear in the sTAC group. Gene sequencing showed significantly different microRNA expression between the sTAC and sham groups. Bioinformatics analysis among the 3 groups showed that the expression of ocu-miR-411-5p, ocu-miR-214-3p, and ocu-miR-432-5p was decreased in the sTAC group compared with the sham group (P < .05) and that the focal adhesion, insulin, and PI3K-Akt signaling pathways were also affected.

CONCLUSIONS

Aortic constriction of 75% was optimal for the establishment of the PHF model. The expression of ocu-miR-411-5p, ocu-miR-214-3p, and ocu-miR-432-5p was significantly decreased, and the focal adhesion, insulin, and PI3K/AKT pathways may play significant roles in PHF progression.

Clinical Trials in Hemodynamic Support: Past, Present, and Future

Managing low blood flow states in the preterm population remains a challenge in neonatal clinical care. The heterogeneity of the trials to date and the relatively low number of infants enrolled, in addition to a desire to oversimplify the underlying pathophysiology, have contributed to an inability to draw meaningful conclusions to direct clinical care. This article reviews the current literature on this topic in the preterm population and outlines the challenges that have been encountered in performing such trials. Alternative studies are proposed, based on the lessons learned over the past number of years.

Drug-induced cardiac abnormalities in premature infants and neonates

The Cardiac Safety Research Consortium (CSRC) is a transparent, public-private partnership that was established in 2005 as a Critical Path Program and formalized in 2006 under a Memorandum of Understanding between the United States Food and Drug Administration and Duke University. Our continuing goal is to advance paradigms for more efficient regulatory science related to the cardiovascular safety of new therapeutics, both in the United States and globally, particularly where such safety questions add burden to innovative research and development. This White Paper provides a summary of discussions by a cardiovascular committee cosponsored by the CSRC and the US Food and Drug Administration (FDA) that initially met in December 2014, and periodically convened at FDA's White Oak headquarters from March 2015 to September 2016. The committee focused on the lack of information concerning the cardiac effects of medications in the premature infant and neonate population compared with that of the older pediatric and adult populations. Key objectives of this paper are as follows: Provide an overview of human developmental cardiac electrophysiology, as well as the electrophysiology of premature infants and neonates; summarize all published juvenile animal models relevant to drug-induced cardiac toxicity; provide a consolidated source for all reported drug-induced cardiac toxicities by therapeutic area as a resource for neonatologists; present drugs that have a known cardiac effect in an adult population, but no reported toxicity in the premature infant and neonate populations; and summarize what is not currently known about drug-induced cardiac toxicity in premature infants and neonates, and what could be done to address this lack of knowledge. This paper presents the views of the authors and should not be construed to represent the views or policies of the FDA or Health Canada.

Heart Failure in Pediatric Patients With Congenital Heart Disease

Heart failure (HF) is a complex clinical syndrome resulting from diverse primary and secondary causes and shared pathways of disease progression, correlating with substantial mortality, morbidity, and cost. HF in children is most commonly attributable to coexistent congenital heart disease, with different risks depending on the specific type of malformation. Current management and therapy for HF in children are extrapolated from treatment approaches in adults. This review discusses the causes, epidemiology, and manifestations of HF in children with congenital heart disease and presents the clinical, genetic, and molecular characteristics that are similar or distinct from adult HF. The objective of this review is to provide a framework for understanding rapidly increasing genetic and molecular information in the challenging context of detailed phenotyping. We review clinical and translational research studies of HF in congenital heart disease including at the genome, transcriptome, and epigenetic levels. Unresolved issues and directions for future study are presented.

Hypotension and shock in the preterm infant

Between 16% and 98% of extremely preterm infants receive treatment for hypotension in the first few days of life. This enormous variation has arisen because of a lack of reliable information to create an evidence base for intervention. This review article provides the unique characteristics of the neonatal cardiovascular system, and addresses the definitions of hypotension and shock in the preterm infant, the indications for treatment and appropriate therapies in individual cases. The treatment of shock and hypotension in the preterm infant may be the area of neonatology where there is the greatest 'intervention/data imbalance'; more babies receive more treatments with less supportive evidence than in virtually any other domain. Treatment of hypotension in infants with good perfusion is probably unnecessary and may be harmful, but the assessment of adequate perfusion remains problematic. Infants with inadequate oxygen delivery to the tissues may benefit from treatment, but which treatments are effective are unknown. It is essential that better evidence be available to create a rational basis for intervention.

Functional arginine vasopressin system in early heart maturation

Since the neurohypophyseal hormone 8-arginine vasopressin (AVP) is involved in cardiovascular tissue hypertrophy and myocyte differentiation, it is possible that local AVP plays a role in heart maturation. AVP-specific RIA, RT-PCR, and immunoblot measurement of AVP receptors (VR) were used to investigate heart tissues from newborn and adult rats. To test AVP's role in differentiation and specialization into ventricle-like cardiomyocytes, we studied GFP-P19Cl6 stem cells, which express green fluorescence protein (GFP) reporter under transcriptional control of the myosin light chain-2v promoter. VR(1) transcripts and proteins were higher in adult than in newborn rat hearts. In contrast, VR(2) increased from postnatal day 1 to 5 and was barely detected in the adult rat heart. In cardiomyocytes expressing troponin C, immunofluorescence revealed VR(2) and VR(1). Intracellular cAMP increased 6.5- and 8.9-fold in response to the selective VR(2) agonist 1-desamino-8-D-AVP (DDAVP) after 1 and 24 h, respectively. Cardiac AVP was high in 1- and 5-day-old (330 +/- 26 and 276 +/- 53 pg/mg protein, respectively) but low in 66-day-old (98 +/- 15 pg/mg protein) rats. AVP immunostaining was detected in the tunica adventitia and endothelium of the coronary vessels. The possible role of AVP in cardiomyogenesis was indicated by DDAVP-AVP-dependent differentiation of GFP-P19Cl6 stem cells into contracting cells displaying GATA-4, a cardiac-specific marker, and ventricle-specific myosin light chain. Together, it is suggested that the AVP system is implicated in postnatal cardiac maturation.

Maturation enhances fluid shear-induced activation of eNOS in perfused ovine carotid arteries

The present study tests the hypothesis that age-dependent increases in endothelial vasodilator capacity are due to maturational increases in endothelial nitric oxide (NO) synthesis and release. Intact 4-cm carotid artery segments taken from term fetal lambs and nonpregnant adult sheep were perfused by using a closed system that enabled independent control of flow and inflow pressure and facilitated complete recovery of all NO released. Fluid shear stress induced a graded release of NO (in nmol NO x min x cm(-2) of luminal surface area) that was significantly greater in adult (890 +/- 140) than in fetal (300 +/- 40) carotid arteries at corresponding values of shear stress (5.9 +/- 0.3 dyn/cm2) but was independent of inflow pressure in both age groups. These age-related differences in NO release were not attributable to corresponding differences in endothelial NO synthase (eNOS) abundance, as eNOS protein levels (in ng of eNOS/cm2 of luminal surface area) were similar in adult (14 +/- 2) and fetal (12 +/- 1) arteries. Adult (80 +/- 15) and fetal (89 +/- 32) levels of eNOS mRNA (in 10(6) copies/cm2 of luminal surface area) were also similar. However, when NO release was normalized relative to the associated mass of eNOS protein to estimate eNOS-specific activity in situ, this value (in nmol NO x microg of eNOS(-1) x min(-1)) was significantly greater in adult (177 +/- 44) than in fetal (97 +/- 36) arteries when the endothelium was maximally activated by A-23187. Similarly, the slope of the relation between fluid shear stress and estimated eNOS-specific activity (in nmol NO x microg of eNOS(-1) x min(-1) per dyn/cm2) was also significantly greater in adult (6.8 +/- 0.1) than in fetal (2.9 +/- 0.1) arteries, which suggests that eNOS may be more sensitive to or more efficiently coupled to activating stimuli in adult compared with fetal arteries. We conclude that maturational increases in endothelial vasodilator capacity are attributable to age-dependent increases in NO release secondary to elevated eNOS-specific activity and involve more efficient coupling between endothelial activation and enhancement of eNOS activity in adult compared with fetal arteries.

Comparison of phenoxybenzamine to sodium nitroprusside in infants undergoing surgery

OBJECTIVES

The purpose of this study was to compare the effects of a direct-acting arterial dilator, sodium nitroprusside, to an alpha-adrenergic receptor blocker, phenoxybenzamine, in infants with congenital heart defects undergoing cardiac repairs on cardiopulmonary bypass.

DESIGN

A prospective, multicenter, observational study.

SETTING

Tertiary care center.

PARTICIPANTS

Sixty infants scheduled for elective congenital cardiac surgery repair requiring cardiopulmonary bypass.

INTERVENTIONS

Patients received either sodium nitroprusside 2 to 5 microg/kg/min infusion intraoperatively and in the intensive care unit (n=30 patients) or received phenoxybenzamine 1 mg/kg slowly intravenously at the onset of cardiopulmonary bypass (n=30 patients).

MEASUREMENT AND MAIN RESULTS

Despite similar mean arterial pressures during cardiopulmonary bypass in both groups, infants who received phenoxybenzamine had a significantly higher flow compared with those who received sodium nitroprusside (180+/-4.8 v 73+/-5.12 mL/kg/min, p<0.0001). Base deficit was significantly larger in the sodium nitroprusside group compared with the phenoxybenzamine group intraoperatively and postoperatively (3.4+/-0.5 v 1.3+/-0.5 mEq/L, p<0.05). The core-to-peripheral temperature gradient was significantly larger in the sodium nitroprusside group compared with the phenoxybenzamine group intra- and postoperatively at all points studied. In the intensive care unit, the left atrial pressure was significantly higher in the sodium nitroprusside group compared with the phenoxybenzamine group (9+/-0.4 v 7+/-0.4 mmHg, p<or=0.0005).

CONCLUSION

The use of phenoxybenzamine can maintain organ perfusion on cardiopulmonary bypass and improve peripheral circulation as shown by less base deficit and smaller temperature gradients intraoperatively and in the intensive care unit better than nitroprusside.

Expression of Ca(2+) channel subunits during cardiac ontogeny in mice and rats: identification of fetal alpha(1C) and beta subunit isoforms

Functional cardiac L-type calcium channels are composed of the pore-forming alpha(1C) subunit and the regulatory beta(2) and alpha(2)/delta subunits. To investigate possible developmental changes in calcium channel composition, we examined the temporal expression pattern of alpha(1C) and beta(2) subunits during cardiac ontogeny in mice and rats, using sequence-specific antibodies. Fetal and neonatal hearts showed two size forms of alpha(1C) with 250 and 220 kDa. Quantitative immunoblotting revealed that the rat cardiac 250-kDa alpha(1C) subunit increased about 10-fold from fetal days 12-20 and declined during postnatal maturation, while the 220-kDa alpha(1C) decreased to undetectable levels. The expression profile of the 85-kDa beta(2) subunit was completely different: beta(2) was not detected at fetal day 12, rose in the neonatal stage, and persisted during maturation. Additional beta(2)-stained bands of 100 and 90 kDa were detected in fetal and newborn hearts, suggesting the transient expression of beta(2) subunit variants. Furthermore, two fetal proteins with beta(4) immunoreactivity were identified in rat hearts that declined during prenatal development. In the fetal rat heart, beta(4) gene expression was confirmed by RT-PCR. Cardiac and brain beta(4) mRNA shared the 3 prime region, predicting identical primary sequences between amino acid residues 62-519, diverging however, at the 5 prime portion. The data indicate differential developmental changes in the expression of Ca(2+) channel subunits and suggest a role of fetal alpha(1C) and beta isoforms in the assembly of Ca(2+) channels in immature cardiomyocytes.

Caffeine-induced contractions in developing rabbit heart

Mature myocardium utilizes calcium released by the sarcoplasmic reticulum (SR) for cell contraction. Transient exposure of mature myocytes to caffeine is known to directly trigger Ca2+ release from the SR. In contrast, neonatal rabbit heart cells rely on transsarcolemmal Ca2+ influx for tension generation. SR function is decreased in immature heart and appears to play a minimal role as a calcium source. Accordingly, we hypothesized that neonatal rabbit myocytes would not respond to a caffeine pulse. Isolated neonatal and adult myocytes were paced to load the SR with calcium and then exposed to a 1-s pulse of 10 mM caffeine. As previously described, adult myocytes exhibited a brisk contraction in response to caffeine. Unexpectedly, neonatal myocytes also exhibited a similar, brisk response. These caffeine-induced contractions were not dependent on extracellular Ca2+ but were dependent upon the loading of SR Ca2+ stores. When SR Ca2+ stores were depleted by exposure to caffeine, mature myocytes exhibited only small, slow contractions in response to electrical field stimulation. Replenishing the SR Ca2+ stores resulted in normal, brisk contractions. In contrast, electrically stimulated contractions in immature myocytes were largely unaffected by caffeine-induced SR depletion. Thus, although neonatal myocytes are capable of loading and releasing calcium from the SR, such SR calcium release is not normally required for contraction in the developing heart. The minor role of SR Ca2+ release in immature rabbit heart may not result from immaturity of the SR, but rather from an inadequate mechanism to trigger SR calcium release.

Age dependence of tolerance to anoxia and changes in cytosolic calcium in rabbit renal proximal tubules

Calcium(Ca2+)-dependent processes mediate, in part, anoxic cell injury. These may account for the difference in sensitivity to anoxia between certain immature and mature renal cells. To address this question, we studied the effects of anoxia on cytosolic free Ca2+ concentration ([Ca2+]i), cell integrity, and transport functions in microdissected proximal convoluted tubules (PCT) of < 3-week-old (newborn) and > 12-week-old (adult) rabbits. Tubules were loaded with 10 microM fura-2 AM by incubation for 60 min at 37 degrees C, and then superfused with isosmotic saline solution gassed with either 95%O2-5%CO2 (control group) or 95%N2-5%CO2 (anoxia group) for 30 min. [Ca2+]i was measured ratiometrically; cell damage was assessed by nuclear binding of propidium iodide (PI). Anoxia resulted in a fourfold increase in [Ca2+]i in adult tubules (from resting values of 245 +/- 10 to 975 +/- 100 nM, P < 0.001), whereas in newborn tubules the rise was significantly less (from resting values of 137 +/- 5 to 165 +/- 5 nM, P < 0.001 between anoxic groups). Transient exposure to 100 mM potassium chloride, which depolarizes the PCT cells, induced increases in [Ca2+]i from baseline, to 920 +/- 90 nM in tubules from adult and to 396 +/- 16 nM in those from newborn rabbits (P < 0.001 between age groups). After exposure to ligands such as parathyroid hormone (PTH) and ATP, [Ca2+]i increased in both newborn and adult tubules, but to lower levels in newborn tubules. The response to PTH and ATP was transient in both age groups, [Ca2+]i returning to baseline levels after 2 min. Following anoxia, tubules from adult animals exhibited staining of all cell nuclei by 1 min exposure to PI, indicative of gross permeabilization of the cells. Nuclei of anoxic immatures tubules did not stain with PI. The sodium-dependent uptakes of a glucose analogue (14C-alpha-methyl-glucopyranoside) and phosphate (32Pi) were preserved in agarose-filled tubules of newborns after anoxia, whereas in those of adults recovery from anoxia was associated with drastic reduction in the uptake of these solutes. Overall, our results suggest that: (1) during anoxia, cell Ca2+ rises to critical levels in PCTs of adults compared with those of < 3-week-old animals, (2) Ca2+ influx occurs via a pathway activated by exposure to high [K+]o, presumably voltage-sensitive Ca2+ channels or reversal of Na(+)-Ca2+ exchange, (3) these pathways are either less active or less abundant in proximal tubules of newborn compared with adult rabbits, and (4) secondary active transport activity and cellular integrity are well preserved after anoxia in PCT cells of newborn but not of adult rabbits.

Ca2+ channel kinetics in acutely isolated fetal, neonatal, and adult rabbit cardiac myocytes

Measurement of transsarcolemmal voltage-gated Ca2+ current (ICa) in myocytes isolated from immature rabbit heart has demonstrated an unexpectedly low level of Ca2+ channel activity. We have characterized the kinetic properties of ICa in acutely isolated 21-day fetal, 1-5-day-old neonatal, and adult cardiac myocytes by the whole-cell voltage-clamp technique. The membrane potential for half-maximal steady-state inactivation became less negative with maturation (-24 +/- 3 [mean +/- SEM] mV, n = 5; -19 +/- 2 mV, n = 5; and -11 +/- 2 mV, n = 6 for fetal, neonatal, and adult myocytes, respectively; p < 0.005). In contrast, the membrane potential for half-maximal steady-state activation was not statistically different among the age groups studied. These parameters accurately predicted the voltage dependence of the sustained ICa present at the end of a 400-msec depolarization. This "window" current was significantly smaller in immature cells than in adult cells and occurred at a more negative membrane potential in the younger age groups. The time course of inactivation of ICa was not significantly different between age groups. However, ICa was inhibited by increasing the frequency of stimulation. This effect was most prominent in immature cells, particularly at more positive holding potentials. This developmental alteration in the frequency dependence of ICa was due in part to a prolonged time constant of recovery from inactivation in the younger age groups. In summary, the kinetic properties of ICa in immature cardiac cells place them at a relative disadvantage in terms of the total Ca2+ influx during a depolarization. Thus, the role of ICa in the control of cell contraction may change with development.