Redistribution of regional blood flow and oxygen delivery in experimental cyanotic heart disease in newborn lambs

Hyperoxemia and hypoxemia impair cellular oxygenation: a study in healthy volunteers

INTRODUCTION

Administration of oxygen therapy is common, yet there is a lack of knowledge on its ability to prevent cellular hypoxia as well as on its potential toxicity. Consequently, the optimal oxygenation targets in clinical practice remain unresolved. The novel PpIX technique measures the mitochondrial oxygen tension in the skin (mitoPO2) which allows for non-invasive investigation on the effect of hypoxemia and hyperoxemia on cellular oxygen availability.

RESULTS

During hypoxemia, SpO2 was 80 (77-83)% and PaO2 45(38-50) mmHg for 15 min. MitoPO2 decreased from 42(35-51) at baseline to 6(4.3-9)mmHg (p < 0.001), despite 16(12-16)% increase in cardiac output which maintained global oxygen delivery (DO2). During hyperoxic breathing, an FiO2 of 40% decreased mitoPO2 to 20 (9-27) mmHg. Cardiac output was unaltered during hyperoxia, but perfused De Backer density was reduced by one-third (p < 0.01). A PaO2 < 100 mmHg and > 200 mmHg were both associated with a reduction in mitoPO2.

CONCLUSIONS

Hypoxemia decreases mitoPO2 profoundly, despite complete compensation of global oxygen delivery. In addition, hyperoxemia also decreases mitoPO2, accompanied by a reduction in microcirculatory perfusion. These results suggest that mitoPO2 can be used to titrate oxygen support.

Validation of transpulmonary thermodilution cardiac output measurement in a pediatric animal model

OBJECTIVE

This study was undertaken to validate the transpulmonary thermodilution cardiac output measurement (CO(TPTD)) in a controlled newborn animal model under various hemodynamic conditions with special emphasis on low cardiac output.

DESIGN

Prospective, experimental, pediatric animal study.

SETTING

Animal laboratory of a university hospital.

SUBJECTS

Twelve lambs.

INTERVENTIONS

We studied 12 lambs under various hemodynamic conditions. Cardiac output was measured using the transpulmonary thermodilution technique with central venous injections of ice-cold saline. An ultrasound transit time perivascular flow probe around the main pulmonary artery served as the standard reference measurement (CO(UFP)). During the experiment, animals were resuscitated from hemodynamic shock using fluid boluses. Cardiac output measurements were performed throughout the experiment.

MEASUREMENTS AND MAIN RESULTS

The correlation coefficient between CO(TPTD) and CO(UFP)was .97 (95% confidence interval .94-.98, p < .0001). Bland-Altman analysis showed a mean bias of 0.19 L/min with limits of agreement of -0.04 and 0.43 L/min (12.0% and +/-14.7%, respectively). The correlation coefficient between changes in CO(TPTD) and CO(UFP) during volume loading was .95 (95% confidence interval .91-.96, p < .0001). There was a significant correlation between changes in global end-diastolic volume and changes in stroke volume (r = .59) but not between changes in central venous pressure and changes in stroke volume (r = .03).

CONCLUSIONS

The transpulmonary thermodilution technique is a reliable method of measuring cardiac output in newborn animals. It is also capable of tracking changes in cardiac output.

Impaired transient elevation of blood hemoglobin in response to acute hypoxia in neonates with asplenia

BACKGROUND

It has been shown that acute hypoxia induces the transient elevation of blood hemoglobin concentration ([Hb]) as a consequence of sympathetic-mediated splenic contraction to maintain the supply of oxygen, and splenectomy abolishes this phenomenon. The purpose of the present paper was to determine, retrospectively, whether the ability of transient elevation of [Hb] against acute hypoxia would be impaired in neonates with asplenia.

METHOD

Eleven neonates who underwent surgery from 1998 to 2003 were enrolled in this retrospective study. They were divided into two groups: (i) five patients with asplenia syndrome with cyanotic congenital heart disease (asplenia group); and (ii) six patients with hypoplastic left heart syndrome who needed nitrogen gas inhalation therapy (HLHS group). In the asplenia group simultaneous data of arterial oxygen saturation (Sao(2)) and [Hb] were obtained before and after the temporary unexpected decrease of percutaneous arterial oxygen saturation. In the HLHS group they were obtained before and after nitrogen gas administration therapy. The arterial oxygen content (Cao(2)) changes and the ratio of Cao(2) change (Cao(2) after hypoxia divided by Cao(2) before hypoxia) were also calculated.

RESULTS

In the asplenia group [Hb] was unchanged (12.9 +/- 1.6 g/dL to 12.8 +/- 1.4, n.s.) and Cao(2) was decreased (14.5 +/- 1.6 mL/dL to 11.9 +/- 1.1, P = 0.018). In the HLHS group [Hb] was increased (14.6 +/- 1.3 g/dL to 15.4 +/- 1.5, P = 0.028), but Cao(2) was changed (18.2 +/- 2.2 mL/dL to 16.7 +/- 3.0, P = 0.043). The ratio of Cao(2) change for the HLHS group was significantly different from that of the asplenia group (0.92 +/- 0.10, 0.83 +/- 0.10, respectively, P = 0.02).

CONCLUSIONS

Patients with asplenia syndrome have some disadvantage regarding this protective mechanism against acute hypoxia.

Down regulation of IGF-I and IGF-IR gene expression in right atria tissue of ventricular septal defect infants with right atria hypoxemia

BACKGROUND

Our previous studies showed serum insulin-like growth factor-I (IGF-I) concentrations significantly decreased in infants with congenital ventricular septal defect (VSD) and that they were associated with increased concentrations of growth hormone. In order to confirm the relationship between IGF-I axis and VSD, we further compared the IGF-I and insulin-like growth factor-I receptor(IGF-IR) gene expressions in the cardiac tissue of VSD infants.

METHODS

Right atrium biopsies of 27 infants were studied. Five infants not having VSD were classified as controls (Group I). Twenty VSD patients were then divided into 2 groups according to their shunting magnitude index (level of pulmonary vascular resistance compared with systemic vascular resistance, Qp/Qs). VSD patients with minor shunts (Qp/Qs<1.7) were classified as Group II; VSD patients with larger shunts (Qp/Qs> or =2) as Group III. Besides, seven tetralogy of fallot (TOF) with shunt (Qp/Qs>4) infants were classified as the Group IV. A non-radioactive DIG-RNA probe detection system, western blotting and immunohistochemistry were used to detect the gene expression levels and protein products of IGF-I and IGF-IR in the right atrium samples of VSD infants.

RESULTS

The relative protein levels of IGF-I were 0.96+/-0.05, 0.43+/-0.03, 0.15+/-0.04, 0.12+/-0.03 and IGF-IR were 0.80+/-0.08, 0.57+/-0.03, 0.38+/-0.02, 0.24+/-0.04 in the right atrium of 4 group patients. The relative mRNA levels of IGF-I were 0.95+/-0.01, 0.41+/-0.03, 0.29+/-0.05, 0.15+/-0.01 and IGF-IR were 0.85+/-0.05, 0.56+/-0.03, 0.17+/-0.01, 0.18+/-0.01, respectively. There was a significantly greater but more gradual decrease in protein levels and in mRNA levels of IGF-I and IGF-IR in Group II (p<0.05), Group III and IV (p<0.01) than in Group I. The results of immunohistochemistry also demonstrated a similar decrease in VSD patients. In addition, the decrease of mRNA and protein levels in IGF-I/IGF-IR of VSD patients show related to the saturation of oxygen in the right atrium and the ratio of systolic right ventricular pressure to left ventricular pressure.

CONCLUSION

We further confirmed the down regulation of IGF-I/IGF-IR in cardiac tissue of VSD infants and the decrease to be associated with shunt magnitude and the severity of hypoxemia in the cardiac chamber of VSD.

Hypoxic regulation of the fetal cerebral circulation

Fetal cerebrovascular responses to acute hypoxia are fundamentally different from those observed in the adult cerebral circulation. The magnitude of hypoxic vasodilatation in the fetal brain increases with postnatal age although fetal cerebrovascular responses to acute hypoxia can be complicated by age-dependent depressions of blood pressure and ventilation. Acute hypoxia promotes adenosine release, which depresses fetal cerebral oxygen consumption through action of adenosine on neuronal A1 receptors and vasodilatation through activation of A2 receptors on cerebral arteries. The vascular effect of adenosine can account for approximately half the vasodilatation observed in response to hypoxia. Hypoxia-induced release of nitric oxide and opioids can account for much of the adenosine-independent cerebral vasodilatation observed in response to hypoxia in the fetus. Direct effects of hypoxia on cerebral arteries account for the remaining fraction, although the vascular endothelium contributes relatively little to hypoxic vasodilatation in the immature cerebral circulation. In contrast to acute hypoxia, fetal cerebral blood flow tends to normalize during acclimatization to chronic hypoxia even though cardiac output is depressed. However, uncompensated chronic hypoxia in the fetus can produce significant changes in brain structure and function, alteration of respiratory drive and fluid balance, and increased incidence of intracranial hemorrhage and periventricular leukomalacia. At the level of the fetal cerebral arteries, chronic hypoxia increases protein content and depresses norepinephrine release, contractility, and receptor densities associated with contraction but also attenuates endothelial vasodilator capacity and decreases the ability of ATP-sensitive and calcium-sensitive potassium channels to promote vasorelaxation. Overall, fetal cerebrovascular adaptations to chronic hypoxia appear prioritized to conserve energy while preserving basic contractility. Many gaps remain in our understanding of how the effects of acute and chronic hypoxia are mediated in fetal cerebral arteries, but studies of adult cerebral arteries have produced many powerful pharmacological and molecular tools that are simply awaiting application in studies of fetal cerebral artery responses to hypoxia.

Neonatal tolerance to hypoxia: a comparative-physiological approach

Newborn mammals exhibit a number of physiological reactions which differ from normal adult physiology and are often regarded as signs of immaturity. However, when looked upon from a comparative point of view, it becomes obvious that some of these 'physiological peculiarities' bear striking similarity to adaptation mechanisms known from hypoxia-tolerant animals and may thus contribute to the well-established, yet poorly understood, phenomenon of neonatal hypoxia tolerance. As the mammalian fetus lives at oxygen partial pressures corresponding to 8000 m altitude, the first line of perinatal hypoxia defense consists of long-term adaptations to limited intrauterine oxygen supply: (1) improved O2 transport by fetal acclimatization to high altitude, (2) reduced metabolic rate by hibernation-like deviation from metabolic size allometry, (3) diminished cerebral vulnerability by functional analogies to diving turtle brain, and (4) enhanced metabolic flexibility by optional repartitioning of energy supply from growth to maintenance metabolism. In the case of birth asphyxia, these background mechanisms are complemented by short-term responses to acute oxygen lack: (1) reduction of body temperature as in natural torpor, (2) reduction of heart rate and redistribution of circulation as in diving mammals, (3) reduction of respiration rate typical of 'hypoxic hypometabolism', and (4) reduction of blood pH according to the concept of 'acidotic torpidity'. Although anaerobic metabolism is improved in neonatal mammals by increased glycogen stores, reduced metabolic demands, and sustained wash-out of acid metabolites, neonatal hypoxia tolerance seems to be primarily based on the ability to maintain tissue aerobiosis as long as possible. This is even reflected by isoenzyme patterns which do not consistently favour anaerobic glycolysis and, thus, are reminiscent of the 'lactate paradox' found in high altitude adaptation. Altogether, from a biological point of view, the perinatal period appears as a source of adaptive mechanisms that can be refound, in varying combinations, in many survival strategies. From a clinical point of view, the interplay of long- and short-term mechanisms offers a novel approach to estimation of the newborn's ability to withstand temporary oxygen lack. However, most of these mechanisms are not unambiguous and, above all, not unlimited in their protective effect so that they do not release obstetricians or neonatologists from their obligation to counteract fetal or neonatal hypoxia without delay.

Treatment of children with congenital heart disease and growth retardation with recombinant human growth hormone

Seven prepubertal short children with congenital heart disease were treated with recombinant human growth hormone (GH). Although complete surgical correction was performed for their heart disease at least 2 years before the start of GH therapy, improvement in growth was less than expected in these children. They received 0.5 IU kg-1 week-1 of GH daily for 2 years or more. The growth rate increased from a mean of 4.3 cm year-1 before treatment to a mean of 7.8 cm year-1 in the first year and to a mean of 6.3 cm year-1 in the second year of treatment. Their mean standardized height improved from -3.41 +/- 0.78 to -2.54 +/- 0.62 after 2 years. The mean height age difference minus the bone age difference became positive in these children. We conclude that recombinant GH increases the growth rate in children with congenital heart disease and prepubertal growth retardation.

Direct effects of halothane and isoflurane in infant rabbit hearts with right ventricular hypertrophy secondary to chronic hypoxemia

In this study we compared the direct myocardial effects of halothane and isoflurane between chronically hypoxemic and normoxemic infant hearts with an isolated, perfused, nonworking rabbit heart model. Anesthetic effects were measured on heart rate, atrioventricular time, coronary flow, O2 consumption and extraction, and left and right ventricular peak systolic and end-diastolic pressures, nd -dP/dtmax; and tau, the time constant of isovolumic relaxation. Control values were similar between chronically hypoxemic and normoxemic groups except for variables affected by right ventricular (RV) hypertrophy in chronically hypoxemic hearts. For these variables values were significantly larger in the chronically hypoxemic group (P < or = 0.05): coronary flow (12.3 +/- 0.4 vs 10.3 +/- 0.3 mL.min-1.g-1), RV peak systolic pressure (68 +/- 3 vs 53 +/- 4 mm Hg), RV + dP/dtmax (1.42 +/- 0.07 vs 1.03 +/- 0.08 mm Hg/ms), and RV -dP/dtmax (-0.99 +/- 0.04 vs -0.78 +/- 0.08 mm Hg/ms). (Values are mean +/- SEM.) With anesthesia, values were similar between chronically hypoxemic and normoxemic groups except for coronary flow, which was significantly greater in chronically hypoxemic hearts for both anesthetics (14.1 +/- 0.9 vs 11.3 +/- 0.7 mL.min-1.g-1 for halothane and 15.4 +/- 1.1 vs 12.2 +/- 0.6 mL.min-1.g-1 for isoflurane). The degree of depression of RV peak systolic pressure and RV + dP/dtmax by both anesthetics, and of RV - dP/dtmax by isoflurane, was significantly larger in chronically hypoxemic hearts because these hearts had greater control values but similar anesthetic values to normoxemic hearts.(ABSTRACT TRUNCATED AT 250 WORDS)

Decreased serum insulin-like growth factor-I associated with growth failure in newborn lambs with experimental cyanotic heart disease

To determine whether chronic hypoxemia results in alterations in endocrine function that may contribute to growth failure, we measured growth hormone (GH), somatomedins (insulin-like growth factors I and II, IGF-I and IGF-2), hepatic growth hormone receptors, and circulating IGF-binding proteins IGFBP-3 and IGFBP-2 in 12 newborn lambs with surgically created pulmonic stenosis and atrial septal defect, and in 10 controls. During chronic hypoxemia (oxygen saturation of 60-74% for 2 wk), weight gain was 60% of control (hypoxemic, 135 +/- 20 vs. control, 216 +/- 26 g/d, P less than 0.02). IGF-I was decreased by 43% (hypoxemic 253.6 +/- 29.3 SE vs. control 448.0 +/- 75.5 ng/ml, P = 0.01), whereas GH was unchanged (19.9 +/- 5.1 vs. 11.9 +/- 3.0 ng/ml, NS). The increase in IGF-1 was associated with a decrease in IGFBP-3 (hypoxemic, 5.09 +/- 1.25 vs. control, 11.2 +/- 1.08 arbitrary absorbency units per mm (Au.mm), P less than 0.01), and increase in IGFBP-2 (0.47 +/- 0.03 vs. 0.19 +/- 0.13 Au.mm, P less than 0.05), but no significant downregulation of hepatic GH receptors (hypoxemic, 106.1 +/- 20.1 vs. control, 147.3 +/- 25.9 fmol/mg, NS). Thus, chronic hypoxemia in the newborn is associated with a decrease in IGF-I and IGFBP-3 in the face of normal GH. This suggests peripheral GH unresponsiveness, similar to protein-calorie malnutrition or GH receptor deficiency dwarfism, but mediated at a level distal to the hepatic GH receptor.

Differential regulation of right and left ventricular beta-adrenergic receptors in newborn lambs with experimental cyanotic heart disease

To determine whether chronic hypoxemia secondary to an intracardiac right-to-left shunt alters regulation of the myocardial beta-adrenergic receptor/adenylate cyclase system, we produced chronic hypoxemia in nine newborn lambs by creating right ventricular outflow obstruction and an atrial septal defect. Oxygen saturation was reduced to 65-74% for 2 wk. Eight lambs served as normoxemic controls. beta-receptor density (Bmax) and ligand affinity (KD) were determined with the radio-ligand [125I]iodocyanopindolol and adenylate cyclase activity determined during stimulation with isoproterenol, sodium fluoride (NaF), and forskolin. During chronic hypoxemia, Bmax decreased 45% (hypoxemic, 180.6 +/- 31.5 vs. control, 330.5 +/- 60.1 fmol/mg) in the left ventricle (exposed to hypoxemia alone) but was unchanged in the right ventricle (exposed to hypoxemia and pressure overload). KD was not different from control in either ventricle. Left ventricular isoproterenol-stimulated adenylate cyclase activity was decreased by 39% (30.0 +/- 4.3% increase vs. 44.1 +/- 9.5% increase) whereas right ventricular adenylate cyclase activity was unchanged. Stimulation of adenylate cyclase with NaF or forskolin was not different from control in either ventricle. Circulating epinephrine was increased fourfold whereas circulating and myocardial norepinephrine were unchanged. These data demonstrate a down-regulation of the left ventricular beta-adrenergic receptor/adenylate cyclase system during chronic hypoxemia secondary to an intracardiac right-to-left shunt.