Metabolic and functional response of neonatal pig hearts to the development of ischemic contracture: is recovery possible?

Myocardial histology and outcome after cardiopulmonary bypass of neonatal piglets

Background

Early after neonatal cardiac surgery hemodynamic dysfunction may be evident. However, still is not clear if dysfunction and outcome is related to visible myocardial alterations. The aim of the present study was the histological analysis of myocardial tissue of neonatal piglets after cardiopulmonary bypass (CPB) and cardioplegic arrest.

Methods

Neonatal piglets (younger than 7 days) were connected to CPB for 180 min, including 90 min of cardioplegic heart arrest at 32 °C. After termination of CPB the piglets were observed up to 6 h. During this observational period animals did not receive any inotropic support. Some piglets died within this period and formed the non-survivors group (CPB-NS group) and the remaining animals formed the CPB-6 h group. Myocardial biopsies (stained with H&E) were scored from 0 to 3 regarding histological alterations. Then, the histological data were evaluated and compared to the probes of animals handled comparable to previous piglets but without CPB (non-CPB group; n = 3) and to sibling piglets without specific treatment (control; n = 5).

Results

In the first hours after CPB six piglets out of 10 died (median 3.3 h). The animals of CPB-6 h group (n = 4) were sacrificed at the end of experiments (6 h after CPB). Although the myocardial histological score of CPB-6 h group and CPB-NS group were higher than non-CPB group (2.0 ± 0.8, 1.5 ± 0.9, and 0.8 ± 0.3 respectively), these differences were statistically not significant. But compared to control animals (score 0.3 ± 0.5) the scores of CPB-6 h and CPB-NS groups were significantly higher (p < 0.05). Between the left and the right ventricular tissue there were no significant differences.

Conclusions

Myocardial tissue alterations in newborn piglets are related to the surgical trauma and potentiated by cardiopulmonary bypass and ischemia. However, outcome is not related to the degree of tissue alteration.

Sex differences in newborn myocardial metabolism and response to ischemia

In children with congenital heart disease, female sex has been linked to greater in-hospital mortality associated with low cardiac output, yet the reasons for this are unclear. Therefore, we examined whether newborn sex differences in the heart's metabolic response to ischemia exist. Left ventricular (LV) in vivo and ischemic biopsies of newborn male and female piglets were compared. Tissue ATP, creatine phosphate (CP), glycogen, anaerobic end-products lactate and hydrogen ion (H), and key regulatory enzymes were measured. Compared with males, newborn females displayed 14% lower ATP, 22% lower CP, and 32% lower glycogen reserves (p < 0.05) at baseline. During ischemia, newborn females accumulated 17% greater lactate and 40% greater H accumulation (p < 0.02), which was associated with earlier cessation of glycolysis and lower ischemic ATP levels (p < 0.02) compared with males. Newborn females demonstrated a greater ability to use their glycogen reserves, resulting in significantly lower (p < 0.003) glycogen levels throughout the ischemic period. Thus, newborn females are at a metabolic disadvantage because they exhibited lower energy levels and greater tissue lactic acidosis, both linked to an increase susceptibility to ischemic injury and impair myocardial function on reperfusion.

Myocardial contractile function in survived neonatal piglets after cardiopulmonary bypass

Background

Hemodynamic function may be depressed in the early postoperative stages after cardiac surgery. The aim of this study was the analysis of the myocardial contractility in neonates after cardiopulmonary bypass (CPB) and mild hypothermia.

Methods

Three indices of left ventricular myocardial contractile function (dP/dt, (dP/dt)/P, and wall thickening) were studied up to 6 hours after CPB in neonatal piglets (CPB group; n = 4). The contractility data were analysed and then compared to the data of newborn piglets who also underwent median thoracotomy and instrumentation for the same time intervals but without CPB (non-CPB group; n = 3).

Results

Left ventricular dP/dt_max and (dP/dt_max)/P remained stable in CPB group, while dP/dt_max decreased in non-CPB group 5 hours postoperatively (1761 ± 205 mmHg/s at baseline vs. 1170 ± 205 mmHg/s after 5 h; p < 0.05). However, with regard to dP/dt_max and (dP/dt_max)/P there were no statistically significant differences between the two groups. Comparably, although myocardial thickening decreased in the non-CPB group the differences between the two groups were not statistically significant.

Conclusions

The myocardial contractile function in survived neonatal piglets remained stable 6 hours after cardiopulmonary bypass and mild hypothermia probably due to regional hypercontractility.

Ventricle-specific metabolic differences in the newborn piglet myocardium in vivo and during arrested global ischemia

Ventricular dysfunction is reported greater in the left (LV) versus right ventricle (RV) in infants following surgically induced ischemia. Ventricle-specific differences in baseline metabolism may alter response to ischemia thus affecting postischemic functional recovery. This study identifies ventricle-specific metabolic differences in the newborn (piglet) heart at baseline (working) and during ischemia (arrested). Baseline LV citrate synthase (CS) and hydroxyacyl-CoA dehydrogenase (HAD) activities were 15% and 18% lower (p < 0.02), whereas creatine kinase (CK) and phosphofructokinase (PFK) activities were 40% and 23% higher (p < 0.04) than the RV. Baseline LV glycogen reserves were also 55% higher (p = 0.004). By 15 min of ischemia, LV ATP was 20% lower (p < 0.05), lactate was 51% higher (p = 0.001), and hydrogen ions (H) were 43% higher (p = 0.03) compared with the RV. These differences persisted for the entire ischemic period (p < 0.02). After 45 min of ischemia, the LV used 58% less (p < 0.05) glycogen than the RV. These findings demonstrate that the enhanced glycolytic capacity of the newborn LV was accompanied by greater anaerobic end-product accumulation and lower energy levels during ischemia. This profile may offer one explanation for greater LV-dysfunction relative to the RV in children following ischemia.

Newborn hearts are at greater 'metabolic risk' during global ischemia--advantages of continuous coronary washout

BACKGROUND

Altered metabolic responses of the newborn heart to ischemia, which may increase irreversible injury, may at least partially explain the greater morbidity and mortality experienced by some children undergoing congenital cardiac repair. The present study compared newborn heart metabolic responses to global ischemia with those of adult, and evaluated whether continuous coronary artery washout in the newborn heart during 'ischemia' could favourably affect these responses.

METHODS

Adult (n=12) and newborn (n=12) pigs were anesthetized, and right ventricular biopsies were taken before global ischemia and at set intervals during ischemia. Another 12 newborns were subdivided into groups of nonperfused hearts and hearts receiving continuous perfusion. Time to onset of and time to peak of ischemic contracture were recorded. Biopsies were assayed for lactate, myocardial glycogen, glucose-6-phosphate and ATP.

RESULTS

Newborn hearts were more sensitive to global ischemia than adult hearts, based on shorter time to onset of and time to peak of ischemic contracture, and had a significantly greater rate of ATP decline (P<0.01). This was due in part to a more rapid accumulation of lactate (P<0.05) and only a 50% use of glycogen, compared with 93% by adult hearts. Continuous washout of newborn hearts prevented lactate accumulation, allowing a 90% use of glycogen and delaying time to ischemic contracture by twofold. This was accompanied by lower levels of glucose-6-phosphate accumulation (P<0.05) and a threefold reduction in the rate of ATP decline.

CONCLUSIONS

Significant differences in myocardial metabolism during ischemia in newborns compared with adults could predispose them to earlier ischemic injury, which can be eliminated by the removal of end products. Perfusion strategies taking these differences into account may further optimize pediatric myocardial protection and improve outcomes in newborn children undergoing cardiac procedures.

Modulation of ischaemic contracture in mouse hearts: a 'supraphysiological' response to adenosine

While inhibition of ischaemic contracture was one of the first documented cardioprotective actions of exogenously applied adenosine, it is not known whether this is a normal function of endogenous adenosine generated during ischaemic stress. Additionally, the relevance of delayed contracture to postischaemic outcome is unclear. We tested the ability of endogenous versus exogenous adenosine to modify contracture (and postischaemic outcomes) in C57/Bl6 mouse hearts. During ischaemia, untreated hearts developed peak contracture (PC) of 85 +/- 5 mmHg at 8.9 +/- 0.8 min, with time to reach 20 mmHg (time to onset of contracture; TOC) of 4.4 +/- 0.3 min. Adenosine (50 microm) delayed TOC to 6.7 +/- 0.6 min, as did pretreatment with 10 microm 2-chloroadenosine (7.2 +/- 0.5 min) or 50 nm of A(1) adenosine receptor (AR) agonist N(6)-cyclohexyladenosine (CHA) (6.7 +/- 0.3 min), but not A(2A)AR or A(3)AR agonists (20 nm 2-[4-(2-carboxyethyl) phenethylamino]-5' N-methylcarboxamidoadenosine (CGS21680) or 150 nm 2-chloro-N(6)-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (Cl-IB-MECA), respectively). Adenosinergic contracture inhibition was eliminated by A(1)AR gene knockout (KO), mimicked by A(1)AR overexpression, and was associated with preservation of myocardial [ATP]. This adenosine-mediated inhibition of contracture was, however, only evident after prolonged (10 or 15 min) and not brief (3 min) pretreatment. Ischaemic contracture was also insensitive to endogenously generated adenosine, since A(1)AR KO, and non-selective and A(1)AR-selective antagonists (50 microm 8-sulphophenyltheophylline and 150 nm 8-cyclopentyl-1, 3-dipropylxanthine (DPCPX), respectively), all failed to alter intrinsic contracture development. Finally, delayed contracture with A(1)AR agonism/overexpression or ischaemic 2,3-butanedione monoxime (BDM; 5 microm to target Ca(2+) cross-bridge formation) was linked to enhanced postischaemic outcomes. In summary, adenosinergic inhibition of contracture is solely A(1)AR mediated; the response is 'supraphysiological', evident only with significant periods of pre-ischaemic AR agonism (or increased A(1)AR density); and ischaemic contracture appears insensitive to locally generated adenosine, potentially owing to the rapidity of contracture development versus the finite time necessary for expression of AR-mediated cardioprotection.

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 (N = 36) 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, 56.4 +/- 2.5* vs 36.3 +/- 0.4%, (right ventricle/left ventricle) weight (dry) ratio, 0.50 +/- 0.04* vs 0.28 +/- 0.02. Baseline: developed pressure (DeltaP), 96 +/- 4 vs 93 +/- 5 mmHg; coronary flow, 90 +/- 10* vs 62 +/- 4 ml/min/gdry. No-flow ischemia: TOC, 12 +/- 1* vs 24 +/- 2 minutes. All hypoxic (no normoxic) hearts reached peak contracture. Reperfusion: Just after onset of contracture, DeltaP, 80 +/- 3* vs 67 +/- 4 mmHg; EDP, 5 +/- 1* vs 13 +/- 2 mmHg; after 30 minutes of no-flow ischemia, DeltaP, 58 +/- 5 vs 46 +/- 4 mmHg; EDP, 13 +/- 1* vs 24 +/- 3 mmHg; lactate release (LR), 0.15 +/- 0.01 vs 0.17 +/- 0.01 mmol/gdry, creatine kinase release (CKR), 46 +/- 8* vs 242 +/- 28 U/gdry. For hypoxic hearts reperfused after onset of contracture, LR was 0.11 +/- 0.03 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.

Hyperoxia causes oxygen free radical-mediated membrane injury and alters myocardial function and hemodynamics in the newborn

Newborn children can be exposed to high oxygen levels (hyperoxia) for hours to days during their medical and/or surgical management, and they also can have poor myocardial function and hemodynamics. Whether hyperoxia alone can compromise myocardial function and hemodynamics in the newborn and whether this is associated with oxygen free radical release that overwhelms naturally occurring antioxidant enzymes leading to myocardial membrane injury was the focus of this study. Yorkshire piglets were anesthetized with pentobarbital sodium (65 mg/kg), intubated, and ventilated to normoxia. Once normal blood gases were confirmed, animals were randomly allocated to either 5 h of normoxia [arterial Po(2) (Pa(O(2))) = 83 +/- 5 mmHg, n = 4] or hyperoxia (Pa(O(2)) = 422 +/- 33 mmHg, n = 6), and myocardial functional and hemodynamic assessments were made hourly. Left ventricular (LV) biopsies were taken for measurements of antioxidant enzyme activities [superoxide dismutase (SOD), glutathione peroxidase (GPx), and catalase (CAT)] and malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE) as an indicator of oxygen free radical-mediated membrane injury. Hyperoxic piglets suffered significant reductions in contractility (P < 0.05), systolic blood pressure (P < 0.03), and mean arterial blood pressure (P < 0.05). Significant increases were seen in heart rate (P < 0.05), whereas a significant 11% (P < 0.05) and 61% (P < 0.001) reduction was seen in LV SOD and GPx activities, respectively, after 5 h of hyperoxia. Finally, MDA and 4-HNE levels were significantly elevated by 45% and 38% (P < 0.001 and P = 0.02), respectively, in piglets exposed to hyperoxia. Thus, in the newborn, hyperoxia triggers oxygen free radical-mediated membrane injury together with an inability of the newborn heart to upregulate its antioxidant enzyme defenses while impairing myocardial function and hemodynamics.

Preischemic administration of ribose to delay the onset of irreversible ischemic injury and improve function: studies in normal and hypertrophied hearts

Compared with normal hearts, those with pathology (hypertrophy) are less tolerant of metabolic stresses such as ischemia. Pharmacologic intervention administered prior to such stress could provide significant protection. This study determined, firstly, whether the pentose sugar ribose, previously shown to improve postischemic recovery of energy stores and function, protects against ischemia when administered as a pretreatment. Secondly, the efficacy of this same pretreatment protocol was determined in hearts with pathology (hypertrophy). For study 1, Sprague-Dawley rats received equal volumes of either vehicle (bolus i.v. saline) or ribose (100 mg/kg) before global myocardial ischemia. In study 2, spontaneously hypertensive rats (SHR; blood pressure approximately 200/130) with myocardial hypertrophy underwent the same treatment protocol and assessments. In vivo left ventricular function was measured and myocardial metabolites and tolerance to ischemia were assessed. In normal hearts, ribose pretreatment significantly elevated the heart's energy stores (glycogen), and delayed the onset of irreversible ischemic injury by 25%. However, in vivo ventricular relaxation was reduced by 41% in the ribose group. In SHR, ribose pretreatment did not produce significant elevations in the heart's energy or improvements in tolerance to global ischemia, but significantly improved ventricular function (maximal rate of pressure rise (+dP/dt(max)), 25%; normalized contractility ((+dP/dt)/P), 13%) despite no change in hemodynamics. Thus, administration of ribose in advance of global myocardial ischemia does provide metabolic benefit in normal hearts. However, in hypertrophied hearts, ribose did not affect ischemic tolerance but improved ventricular function.

Postischemic functional recovery in immature hearts is influenced by performance index and assessment technique

In the in vivo immature heart, conflicting results are reported for postischemic functional recovery. This study determines whether interpretations of functional recovery are influenced by the contractile performance index (systolic pressure, developed pressure, and maximum rate of systolic pressure increase per unit time) reported or the assessment technique (isovolumetric and variable-volume) utilized. In neonatal pigs (n = 6) on cardiopulmonary bypass, each performance index was examined using both assessment techniques before myocardial ischemia and at 15, 30, and 60 min of reperfusion. With the use of the isovolumetric technique, all performance indexes had significantly different recovery. With the use of the variable-volume assessment technique, recovery of systolic pressure was significantly better than the other indexes. When recovery was compared between the two assessment techniques, systolic pressure recovered significantly better when assessed using the variable-volume technique. For each performance index, the correlation between isovolumetric and variable-volume techniques was positive before ischemia but negative during reperfusion, suggesting that the assessment techniques identified conflicting postischemic contractile performances. Thus both the contractile performance index reported and the assessment technique employed are ultimately important in interpreting postischemic functional recovery in the immature heart.

Effects of hyperoxia on neonatal myocardial energy status and response to global ischemia

BACKGROUND

This study examines the effect of neonatal exposure to clinically relevant hyperoxia levels on both in vivo myocardial metabolism and the subsequent metabolic response to global ischemia.

METHODS

Three-day-old pigs were ventilated to normoxia (80 mm Hg, 2 or 5 hours, n = 11), mild hyperoxia (250 mm Hg, 2 hours, n = 9), or severe hyperoxia (500 mm Hg, 5 hours, n = 14). Ventricular biopsies obtained at the end of the ventilation period, and at early and late ischemia were analyzed for ATP, ADP, AMP, creatine phosphate, glycogen, and lactate.

RESULTS

Hyperoxia did not significantly alter in vivo metabolism. During early ischemia, hearts exposed to severe hyperoxia had better ATP and glycogen preservation (p < 0.003). These hearts exhibited almost complete (92%) creatine phosphate depletion, in contrast to incomplete creatine phosphate use in all other neonatal hearts, even in the face of 30% ATP reductions. However, hearts exposed to severe hyperoxia also had a higher incidence of fibrillation during ischemia, which accelerated ATP and glycogen degradation.

CONCLUSIONS

Although severe hyperoxia provided an energy-sparing effect during early ischemia, it also increased the incidence of ventricular fibrillation, which negated this beneficial effect.