Does the severity of acute hypoxia influence neonatal myocardial metabolism and sensitivity to ischemia?

Does young age really put the heart at risk?

Despite significant advances in the management and treatment of heart disease in children, there continue to be patients who have worse outcomes than might be expected. A number of risk factors that could be responsible have been identified. Evidence-based findings will be reviewed, including whether young age and (or) reduced body weight exacerbate these responses. For example, newborn children undergoing congenital cardiac surgery are known to have worse outcomes than older children. Evidence exists that newborn hearts do not tolerate ischemia as well as adult hearts, developing irreversible injury sooner and exhibiting at-risk metabolic profiles. As well, in response to the administration of heparin, elevations in free fatty acids occur during congenital heart surgery in children, which can have detrimental effects on the heart. Furthermore, myocardial energetic state has also been suggested to impact outcomes. Unfavourable energetic profiles were correlated to lower body weights in the same age healthy newborn piglet model. Newborn children suffering from congenital heart disease, with lower body weights, also had lower myocardial energetic state and this correlated with longer postoperative ventilatory support as well as a trend to longer intensive care unit stay. These findings imply that unfavourable myocardial metabolic profiles could contribute to postoperative complications.

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.

Increase of macrophage migration inhibitory factor (MIF) expression in cardiomyocytes during chronic hypoxia

BACKGROUND

Macrophage migration inhibitory factor (MIF) might play an important role in the myocardium during chronic hypoxia because MIF protects the heart during myocardial ischemia by activating 5'-adenosine monophosphate activated protein kinase (AMPK).

METHODS

We investigated 35 infants with cyanotic or acyanotic cardiac defects and H9c2 embryonic rat cardiomyocytes to examine the effect of chronic hypoxia on the expression of MIF in vivo and in vitro, respectively.

RESULTS

We found out an increase of endogenous cardiac MIF expression positively correlated with degree of hypoxia. Also, AMPK activation was elevated while MIF expression was increased in cells exposed to long periods of hypoxia in vitro. There was no significant difference in the growth ratio of cells cultivated in long periods of hypoxia and normoxia.

CONCLUSIONS

The expression of MIF is significantly increased in cardiomyocytes exposed to chronic hypoxia, and the activation of AMPK was increased accordingly.

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.

Physiopathology of the embryonic heart (with special emphasis on hypoxia and reoxygenation)

The adaptative response of the developing heart to adverse intrauterine environment such as reduced O2 delivery can result in alteration of gene expression with short- and long-term consequences including adult cardiovascular diseases. The tolerance of the developing heart of acute or chronic oxygen deprivation, its capacity to recover during reperfusion and the mechanisms involved in reoxygenation injury are still under debate. Indeed, the pattern of response of the immature myocardium to hypoxia-reoxygenation differs from that of the adult. This review deals with the structural and metabolic characteristics of the embryonic heart and the functional consequences of hypoxia and reoxygenation. The relative contribution of calcium and sodium overload, pH disturbances and oxidant stress to the hypoxia-induced cardiac dysfunction is examined, as well as various cellular signaling pathways (e.g. MAP kinases) involved in cell survival or death. In the context of the recent advances in developmental cardiology and fetal cardiac surgery, a better understanding of the physiopathology of the stressed developing heart is required.

A long-term stable normothermic cardiopulmonary bypass model in neonatal swine

BACKGROUND

An appropriate animal model of cardiopulmonary bypass (CPB) is critical in order to study the morbidity and mortality in newborn children undergoing long-term cardiac surgery. Since this has been reported to be technically difficult, this paper describes a neonatal porcine CPB model (3 days old, n = 18) for up to 8 h to study long-term bypass.

METHODS

After anesthesia, neonates had arterial/venous monitoring lines inserted, they were heparinized (300 IU/kg), the aorta was cannulated for arterial retroperfusion, and a two-stage venous drainage catheter was placed in the right atrium. A Medtronic Minimax Plus oxygenator and the bypass circuit were primed with donor blood and CPB was instituted.

RESULTS

Line and mean arterial pressures were kept at 147.7 +/- 73 and 62.7 +/- 9 mm Hg, respectively. Myocardial (38.1 +/- 1.0 degrees C) and rectal temperatures (37.7 +/- 1.0 degrees C) were maintained. Heart rate was 184.8 +/- 34.5 bpm. Hematocrits were 29.6 +/- 6.0%, activated clotting time was sustained above 400 s throughout bypass, blood gas parameters were maintained in the normal range (pH, 7.39 +/- 0.1; PO(2), 123.1 +/- 65.2 mm Hg; PCO(2), 37.2 +/- 8.5 mm Hg; and HCO(3)(-), 21.5 +/- 3.6 mmol/L). CPB was terminated after 8 h and no visceral edema or other imbalances normally associated with swine on bypass were observed.

CONCLUSIONS

Results demonstrate a model of stable long-term bypass in neonatal swine which can be used to study issues critical to children requiring surgical correction and CPB at a young age. Overall effects of surgery and bypass on these younger patients have yet to be explored and therefore a stable long-term normothermic model of CPB would allow the study of numerous parameters associated with this complicated procedure.

Does the degree of cyanosis affect myocardial adenosine triphosphate levels and function in children undergoing surgical procedures for congenital heart disease?

OBJECTIVE

The outcome of children with cyanosis after cardiac surgical procedures is inferior to that of children who are acyanotic. Animal studies indicated detrimental effects of chronic hypoxia on myocardial metabolism and function. We studied whether the presence or the degree of cyanosis adversely affected myocardial adenosine triphosphate, ventricular function, and clinical outcome in children.

METHODS

Forty-eight children who underwent repair of tetralogy of Fallot were divided according to their preoperative saturation: group I, 90% to 100% (n = 14 patients); group II, 80% to 89% (n = 16 patients); and group III, 65% to 79% (n = 18 patients). Adenosine triphosphate was measured from right ventricular biopsy specimens taken before ischemia, at 15 minutes of ischemia, at end-ischemia, and at 15 minutes of reperfusion. Ejection fraction was measured by echocardiography.

RESULTS

Even before surgical ischemia, compared with groups I and II, group III had lower preoperative ejection fraction (59% +/- 2.9% vs 67% +/- 1.7% and 68% +/- 1.0%; P <.01) and lower preischemic adenosine triphosphate levels (15.1 +/- 2.1 vs 19.1 +/- 1.9 and 21.4 +/- 1.5 micromol/g dry weight; P <.01). After 15 minutes of ischemia, group III had lower adenosine triphosphate levels (11.2 +/- 1.8 vs 14.77 +/- 2.3 and 17. 6 +/- 3.1 micromol/g dry weight; P <.01). With reperfusion, both cyanotic groups lost further adenosine triphosphate compared with partial recovery in the acyanotic group (-22% +/- 3.8%, -20% +/- 3. 1% vs +18% +/- 1.8%; P <.01). Children in group III had a more complicated postoperative course as evidenced by longer ventilatory support (85 +/- 25 hours vs 31 +/- 15 and 40 +/- 21 hours; P =.07), inotropic support (86 +/- 23 hours vs 38 +/- 12 and 36 +/- 4 hours; P <.01), and intensive care unit stay (160 +/- 35 hours vs 60 +/- 10 and 82 +/- 18 hours; P =.02).

CONCLUSIONS

The degree of cyanosis adversely affects myocardial adenosine triphosphate, function, and clinical outcome of children who undergo cardiac operation. Children with cyanosis should be identified as a higher risk group that could be targeted for supportive interventions.

Neonatal hemodynamic responses to extreme ranges of controlled graded hypoxia

OBJECTIVES

To determine the hemodynamic responses to a wide range of specific, controlled, graded levels of hypoxic hypoxia over 120 mins in a neonatal porcine model and to identify the PaO2 threshold for altered hemodynamic homeostasis.

DESIGN

Prospective, experimental, animal study.

SETTING

University cardiovascular research laboratory.

SUBJECTS

Three-day-old domestic swine.

INTERVENTIONS

Anesthetized, intubated, and ventilated 3-day-old pigs (n = 88) were assigned to one of five predetermined graded PaO2 groups: Group I (normoxia, PaO2 = 80 torr [10.7 kPa]); group II (PaO2 = 60 torr [8.0 kPa]); group III (PaO2 = 40 torr [5.3 kPa]); group IV (PaO2 = 30 torr [4.0 kPa]); or group V (PaO2 = 20 torr [2.7 kPa]).

MEASUREMENTS AND MAIN RESULTS

Hemodynamic parameters including heart rate, systolic blood pressure, diastolic blood pressure, mean arterial pressure (MAP), and pulse pressure were evaluated. Acid-base status (arterial pH and lactate) was monitored in each experimental group over the 120-min study period. Hemodynamic and acid-base parameters were unaltered in animals in groups I and II. In group III animals, blood pressure was maintained (partly by increased heart rate), and acid-base balance was unaltered. In contrast, group IV animals had a gradual and progressive decrease in systolic blood pressure, diastolic blood pressure, and MAP, and slightly decreased pulse pressure, despite sustained tachycardia. Group IV animals also developed mild lactic acidosis. Group V animals exhibited a biphasic hemodynamic response, while the heart rate response was characterized by tachycardia at the induction of hypoxia, which was reduced in magnitude by 120 mins. The biphasic hemodynamic response in this group of animals included an initial increase in systolic and pulse pressures, followed by a gradual and progressive decrease in systolic and diastolic blood pressures, MAP, and pulse pressure. In addition, group V animals also developed profound progressive lactic acidosis.

CONCLUSIONS

In anesthetized neonatal pigs, tachycardia occurred in response to a PaO2 of 40 torr (5.3 kPa), and thus marked the threshold for altered hemodynamic homeostasis. Beyond this threshold, both the 30 torr (4.0 kPa) and 20 torr (2.7 kPa) groups had a PaO2- dependent "late" hypotension, while only the 20 torr (2.7 kPa) group had a significant biphasic hemodynamic response characterized by "early" hypertension. The "late" hypotension which occurred in these two profound hypoxia groups indicates an inability to adequately adjust hemodynamics during prolonged hypoxic hypoxia.