N.T. Ross-Ascuitto

Substrate metabolism in the developing heart.

The developing heart undergoes a remarkable metabolic transformation as it adjusts to the higher-oxygen, extrauterine environment. During gestation, glycolysis and lactate oxidation constitute the major sources of adenosine triphosphate (ATP) for the fetal heart. After birth, however, there is a rapid shift from carbohydrate to fatty acid utilization. Despite the transition to primarily aerobic metabolism, the neonatal heart retains an enhanced capacity for anaerobic energy production. This unique metabolic adaptation is important when assessing the immature hearts responses to states of oxygen insufficiency, such as ischemia, hypoxia, and tachycardia. This article reviews the dramatic changes in enzyme activities, mitochondrial morphology and function, and substrate availability that underlie this change in metabolism in the maturing heart.

Systematic-to-Pulmonary Collaterals: A Source of Flow Energy Loss in Fontan Physiology

Patients with Fontan-modified, single-ventricle heart frequently have systemic collaterals that increase pulmonary blood flow. Competitive flow from these auxiliary vessels can also elevate pulmonary artery pressure, a process leading to erosion of flow’s mechanical energy. An analogous analytical description of mixing fluid streams was used to provide insight into flow energetics associated with systemic-to-pulmonary collaterals in Fontan-type circulation. We find that theoretical pressure increases and flow energy losses due to mixing vary quadratically as the velocity differences of the interacting fluid streams. Moreover, the predicted flow energy loss is shown to depend directly on the resultant pressure increase. Based on studies of aortopulmonary collaterals in patients with Fontan anatomy, we provide an estimate of pulmonary artery pressure elevation and flow energy loss, factors that are of considerable clinical importance.

Pressure Loss from Flow Energy Dissipation: Relevance to Fontan-Type Modifications

Abstract. Pressure loss from flow energy dissipation may impair cardiac performance when a heart with a single ventricle must support the circulation. Therefore, the goal of this study was to use a simple description of fluid motion to provide insight into flow energetics relevant to Fontan-type procedures. Our findings indicate that when either the cross-sectional area or the axial direction of flow changes ``abruptly, disturbances are set up within the fluid that lead to dissipation of available energy. The theoretical pressure losses associated with these flow disturbances were described by relating the initial and final average velocities of the streams to anatomical features within the fluids connection pathway causing obstruction to flow, e.g., the ratio of diameters characterizing a change in the cross-sectional area and/or the angle governing an alteration in the axial direction of the flow. Significant pressure losses were found in situations in which the ``magnitude of the fluids velocity is suddenly changed as flow enters or leaves a large chamber or when the ``direction of the fluids velocity is acutely altered as flow negotiates a sharp bend in a vessel or conduit. We found the Bernoulli equation to be inaccurate when predicting the corresponding changes in pressure under these conditions. In view of these findings, we discuss operative strategies aimed at avoiding pressure losses, thus aiding univentricular heart function by conserving flow energy.

Contractile and coronary vascular effects of pituitary adenylate cyclase activating polypeptide in neonatal pig hearts

OBJECTIVEnThe purpose was to investigate the influence of the 38-amino-acid neuropeptide, pituitary adenylate cyclase activating polypeptide (PACAP38), on contractile function and coronary vascular tone in neonatal hearts.nnnMETHODSnIsolated, paced (150 bpm), isovolumically-beating, piglet hearts (n = 19) underwent retrograde aortic perfusion at constant coronary flow (approximately 2.5 ml/min/gwet) with an erythrocyte-enriched (Hct 15-20%) solution (37 degrees C). Agonists were injected into the aortic root of hearts, and the changes in +dP/dtmax and -dP/dtmax (reflecting contractility), and coronary perfusion pressure (reflecting vascular tone) were determined. Responses to PAPCAP38 were compared to isoproterenol, and to the truncated peptide PACAP6-38.nnnRESULTSnPACAP38 (0.1 and 0.5 nmol) increased +dP/dtmax from 1387.4 +/- 134.6 to 1619.0 +/- 118.7, and from 1296.2 +/- 93.4 to 1872.2 +/- 111.4 mmHg/s (P < 0.05); changed -dP/dtmax from -1087.6 +/- 107.5 to -1206.6 +/- 93.6, and from -1025.0 +/- 46.8 to -1375.4 +/- 80.9 mmHg/s (P < 0.05) and decreased coronary perfusion pressure from 61.8 +/- 2.5 to 51.0 +/- 3.8, and from 62.5 +/- 1.0 to 45.3 +/- 3.3 mmHg (P < 0.005), respectively. In comparison, isoproterenol (0.1 nmol) increased +dP/dtmax from 1313.6 +/- 62.8 to 1679.0 +/- 74.4 (P < 0.05), and -dP/dtmax from -1026.4 +/- 54.1 to -1222.6 +/- 57.4 mmHg/s (P < 0.05). PACAP6-38 reduced PACAP38s coronary vasodilatory, but not its contractile, effect. When compared to our previous studies of the 27-amino-acid neuropeptide PACAP27, PACAP38 had less potent contractile, but similar vasodilatory effects.nnnCONCLUSIONSnPACAP38 enhanced contractility and produced coronary vasodilation in piglet hearts, which may make PACAP38 a promising cardiotonic agent for the treatment of neonates with heart failure.

The effects of milrinone in the neonatal pig heart.

SummaryMilrinone, a selective inhibitor of phosphodiesterase (PDE), was examined in neonatal hearts and in ventricular myocytes. Isolated, paced (180 beats/min), isovolumically beating hearts from pigs, <3 days of age, were perfused with an erythrocyte-enriched solution. In one group (control, n=6), milrinone was studied at perfusate concentrations of 1, 10, and 100 μg/ml. In a second group (postischemia, n=10), hearts were subjected to 30 minutes of no-flow ischemic arrest, prior to the addition of milrinone. Left ventricular peak systolic pressure (PSP) and end-diastolic pressure, coronary flow (CF), heart rate (HR), and myocardial oxygen consumption (MVO2) were measured. The PSP averaged ∼100 mmHg during the baseline periods for both groups and decreased to ∼85 mmHg in those hearts subjected to ischemic arrest. In both groups, PSP increased ∼14% at the 1μg/ml concentration of milrinone. No additional increases in PSP were observed in the control group at the higher concentrations. However, PSP increased 28% and 41% (p<0.05), in the postischemia group at the 10 and 100 μg/ml concentrations, respectively. The CF averaged ∼3 ml/min/g during the baseline periods of both groups and increased significantly at each milrinone concentration. The HR in both groups increased to ∼200 and ∼250 beats/min at the 10 and 100 μg/ml concentrations, respectively. Additionally, milrinones effects in intact hearts were found to be comparable to those of isobutylmethyl xanthine (IBMX), a nonspecific PDE inhibitor. In isolated myocytes, however, milrinone produced only modest increases in cAMP levels, compared to IBMX. We conclude that milrinone has positive inotropic, coronary vasodilatory, and chronotropic effects in the neonatal pig heart. In particular, milrinone also was capable of reversing the contractile dysfunction that resulted when these immature hearts were subjected to 30 minutes of normothermic, no-flow ischemic arrest. Thus, milrinone may be a useful agent in the treatment of neonates with contractile dysfunction.

A Direct Comparison of Right and Left Ventricular Performance in the Isolated Neonatal Pig Heart

Abstract. A comparison is presented between the performance of the right ventricle (RV) and the left ventricle (LV) in neonatal hearts studied under conditions of volume loading and tachycardia. Isolated, atrially paced (150 or 300 bpm), isovolumically beating pig hearts (1–3 days of age) underwent retrograde aortic perfusion with a nonrecirculating, crystalloid solution. Ventricular pressure was assessed with saline-filled balloon catheters, which allowed separate loading of the RV or LV. Both ventricles showed an initial increase followed by a leveling off, but no decline, in peak systolic pressure (PSP) and +dP/dtmax with volume loading up to an end-diastolic pressure (EDP) of 18 mmHg. The LV generated a higher PSP and +dP/dtmax compared to the RV at equivalent pressure or volume preloads. However, the maximal systolic elastance (Emax) was comparable for both ventricles. Although the RV demonstrated a greater compliance than the LV, the myocardial relaxation time constant (τ) was similar for both chambers at equivalent volume preloads (sarcomere stretch). Positive dP/dtmax correlated closely and in the same linear fashion with −dP/dtmax for both ventricles, indicating that the RV and LV exhibited similar contraction–relaxation coupling. Increasing the heart rate to 300 bpm decreased PSP, +dP/dtmax, and −dP/dtmax and increased EDP for both ventricles, whereas Emax and τ were not significantly altered. Thus, although there are differences between the functional properties of the neonatal RV and LV, there are also important similarities, especially with regard to myocardial relaxation.

Streamlining fluid pathways lessens flow energy dissipation: relevance to atriocavopulmonary connections.

Flow energy dissipation reduces cardiac efficiency, particularly in the Fontan-modified, single-ventricle heart. To provide insight into flow energetics relevant to Fontan-type anatomy, a simple, analytical description of fluid motion was employed. Mechanical energy balance and the force-momentum relationship were used to describe theoretical pressure changes and flow energy losses. When either flows cross-sectional area or direction changes abruptly, fluid disturbances ensue that can lead to erosion of a streams available energy. Conversely, passages anatomically streamlined to favor gradual alterations in these flow characteristics lessen such fluid dynamic effects. We show by analogy that flow energy dissipation becomes clinically important in Fontan circuits when the magnitude and/or direction of the average velocity of flow suddenly changes as fluid enters or leaves a large chamber or negotiates a sharp bend in its connection pathway.

Performance of the Chronically Hypoxic Young Rabbit Heart

Hearts isolated from 30 rabbits, raised from birth to ~5 weeks of age under either hypoxic (FIO2, 0.10) or normoxic (FIO2, 0.21) conditions, underwent retrograde aortic perfusion using a nonrecirculating, well-oxygenated crystalloid solution. The left ventricular end diastolic pressure was initially set at ~5 mmHg. Aerobic performance was studied by measuring peak systolic pressure (PSP), coronary flow, glucose oxidation, and oxygen consumption. Anaerobic function was assessed by determining time for the onset of contracture (TOC) in the presence of zero coronary flow. Hypoxic vs normoxic hearts (mean ± SEM): heart rate, 197 ± 6 vs 190 ± 5 beats per minute; PSP, 136 ± 4* vs 108 ± 4 mmHg; (+) dP/dtmax, 2294 ± 125* vs 1549 ± 144 mmHg/sec; relaxation time constant (Tau), 26.9 ± 1.1* vs 41.6 ± 4.8 msec; (−) dP/dtmax, 1422 ± 43* vs 1001 ± 63 mmHg/sec; coronary flow, 86.3 ± 4.2* vs 59.9 ± 2.9 ml/min/gdry; glucose oxidation, 3511 ± 118* vs 2979 ± 233 nmol/min/gdry; oxygen consumption, 28.2 ± 1.4* vs 22.7 ± 1.4 µmol/min/gdry; and TOC, 11.8 ± 1.2* vs 22.9 ± 2.2 min (*p < 0.05). Hearts isolated from young rabbits, exposed to hypoxia from birth, exhibited enhanced ventricular systolic and diastolic mechanical function, elevated coronary flow, retained capacity for aerobic metabolism, and a shorter TOC compared to their normoxic counterparts.

Recovery of the chronically hypoxic young rabbit heart reperfused following no-flow ischemia.

The objective of this study was to test whether chronically hypoxic immature hearts exhibit greater tolerance to no-flow ischemia than normoxic hearts. Rabbits (Nu2009=u200936) were raised from birth to 5 weeks of age in either hypoxic (10% O2/90% N2) or normoxic (room air) environment. Isolated, isovolumically beating hearts, with a fluid-filled balloon catheter in the left ventricular chamber, were perfused with a well-oxygenated buffer and studied during baseline [30 minutes; perfusion pressure, 60 mmHg; end diastolic pressure (EDP), 5 mmHg], no-flow ischemia (until onset of contracture or for 30 minutes), and Reperfusion (30 minutes; perfusion pressure, 60 mmHg). Time for onset of contracture (TOC) was defined by an increase in balloon pressure of 5 mmHg. The results were as follows: hypoxic vs normoxic: Hct, 56.4u2009±u20092.5* vs 36.3u2009±u20090.4%, (right ventricle/left ventricle) weight (dry) ratio, 0.50u2009±u20090.04* vs 0.28u2009±u20090.02. Baseline: developed pressure (ΔP), 96u2009±u20094 vs 93u2009±u20095 mmHg; coronary flow, 90u2009±u200910* vs 62u2009±u20094 ml/min/gdry. No-flow ischemia: TOC, 12u2009±u20091* vs 24u2009±u20092 minutes. All hypoxic (no normoxic) hearts reached peak contracture. Reperfusion: Just after onset of contracture, ΔP, 80u2009±u20093* vs 67u2009±u20094 mmHg; EDP, 5u2009±u20091* vs 13u2009±u20092 mmHg; after 30 minutes of no-flow ischemia, ΔP, 58u2009±u20095 vs 46u2009±u20094 mmHg; EDP, 13u2009±u20091* vs 24u2009±u20093 mmHg; lactate release (LR), 0.15u2009±u20090.01 vs 0.17u2009±u20090.01 mmol/gdry, creatine kinase release (CKR), 46u2009±u20098* vs 242u2009±u200928 U/gdry. For hypoxic hearts reperfused after onset of contracture, LR was 0.11u2009±u20090.03 mmol/gdry, comparable to that following 30 minutes of no-flow ischemia (*p < 0.05). Rabbit hearts subjected to hypoxia from birth developed ischemic contracture earlier and reached peak contracture, showed no significant increase in LR after onset of contracture, exhibited better recovery of EDP, and had markedly reduced CKR compared to normoxic controls.

Performance of the Neonatal Pig Heart Subjected to Oxygen Insufficiency

Isolated, paced, isovolumically beating, neonatal pig (n = 32) hearts underwent retrograde aortic perfusion with a solution containing insulin (100 µU/ml), glucose (5.5 mM), and palmitate (0.55 mM). Glycolysis, lactate release, glucose oxidation, palmitate oxidation, and oxygen consumption were assessed. The hearts were perfused during three periods: (1) baseline, pO2 ≈ 500 mm Hg, heart rate 150 bpm; (2) hypoxia, pO2 ≈ 60–80 mm Hg, heart rate 150 bpm, or tachycardia, pO2 ≈ 500 mm Hg, heart rate 300 bpm, and (3) recovery, return to baseline conditions. For hypoxia and tachycardia, the oxygen supply-demand ratio was comparable (≈1 nmol O2/mm Hg/gdry). During baseline, the left ventricular peak systolic pressure (PSP) averaged 126 ± 6 mm Hg, the end-diastolic pressure (EDP) 5 mm Hg, and the relaxation time constant (Tau) 34 ± 2 ms; the coronary flow was 36 ± 2 ml/min/gdry. During hypoxia, the PSP decreased to 70 ± 2 mm Hg, while EDP, Tau, and coronary flow increased to 26 ± 2 mm Hg, 104 ± 14 ms, and 70 ± 2 ml/min/gdry, respectively; palmitate oxidation and oxygen consumption decreased well below baseline. During tachycardia, the PSP decreased to 88 ± 1 mm Hg, and the EDP increased to 11 ± 1 mm Hg, while Tau and coronary flow did not change significantly; palmitate oxidation and oxygen consumption increased above baseline. For both stressors, the predicted lactate release underestimated the measured values by a factor of ≈2, but were comparable during baseline and recovery. Upon recovery, PSP returned to ≈80% of baseline, while EDP remained elevated, for both stressors. Glucose oxidation returned to baseline, but palmitate oxidation became accelerated. We conclude for neonatal pig hearts subjected to oxygen insufficiency: (1) that PSP decreases and (2) that EDP and Tau increase with hypoxia, whereas EDP increases, while Tau remains unchanged with tachycardia. Following both stressors, palmitate oxidation becomes enhanced and dissociated from mechanical activity.