Cardiac performance after deep hypothermic circulatory arrest in chronically cyanotic neonatal lambs

Porcine model of congenital heart defect with decreased pulmonary blood flow

A porcine model was produced to study the pathophysiology of congenital heart defect (CHD) with decreased pulmonary blood flow. Twenty piglets (1-2 months) were randomly divided as mild to moderate stenosis/T(1) group (n = 7) in which artificial atrial septal defect (ASD) with pulmonary artery banding generated a systolic pressure gradient of 20-30 mmHg; severe stenosis/T group (n = 7) group with a systolic pressure gradient of ≥30-50 mmHg; and controls/C group (n = 6) underwent sham surgery. At 1 month postoperatively, 64-slice computed tomography (CT) was performed. At 2 months, left-chest surgery was performed to measure ASD diameter, arterial blood gas, hemoglobin, and Trans-PABP. Our data show successful establishment of porcine CHD model. ASDs in T(1)/T(2) groups were 8.0 ± 0.5/8.9 ± 1.4 mm, respectively. Trans-PABP showed that the pressure increase in T(2) was higher (P < 0.01) than in T(1) group. Arterial blood PaO(2) and SaO(2) of T(1/)T(2) groups were significantly lower than controls. AoD was significantly lower in T(1) than in C group. Balloon dilation was significantly lower than AoD in T(1)/T(2) groups. Besides, one animal in T(1), two animals in T(2) ,and one animal in C group died. We conclude that ASD with pulmonary artery banding is a successful intervention to establish such a model, and our findings may influence the treatment of patients with similar heart disease.

Chronic hypoxemia increases ventricular brain natriuretic peptide precursors in neonatal swine

BACKGROUND

Circulating levels of atrial natriuretic peptide and brain natriuretic peptide (BNP) are elevated in patients with cyanotic congenital heart disease and associated with the severity of ventricular dysfunction. We evaluated the effect of chronic hypoxemia on left ventricle pro-atrial natriuretic peptide and pro-BNP, the cytoplasmic precursors of the plasma hormones.

METHODS

Forty newborn piglets were randomized to placement of a pulmonary artery to left atrium shunt to create hypoxemia or sham thoracotomy. Animals were studied at 1 or 2 weeks after the procedure (four groups, n = 10 per group). Arterial oxygen tension and hematocrit were obtained. Left ventricular shortening fraction was measured by echocardiography. Left ventricular tissue was harvested and cytoplasm was extracted. Pro-BNP levels were determined by Western blot analysis. Pro-atrial natriuretic peptide levels were determined using enzyme-linked immunosorbent assay.

RESULTS

Significant differences among treatment groups were observed for arterial oxygen tension (p < 0.001) and hematocrit (p < 0.001). Pairwise comparisons indicated lower arterial oxygen tension and higher hematocrit for hypoxemic piglets compared with control piglets at 1 and 2 weeks. Left ventricular shortening fraction was not decreased in the hypoxemic animals at any time (p = 0.638). Left ventricular pro-atrial natriuretic peptide decreased in hypoxemic piglets (p = 0.029), whereas left ventricular pro-BNP increased in hypoxemic piglets at 2 weeks (p = 0.002).

CONCLUSIONS

Chronic hypoxemia alone, even in the absence of cardiac dysfunction, is sufficient to increase ventricular levels of pro-BNP. This finding may have implications for the interpretation of BNP levels in the clinical management of patients with cyanotic congenital heart disease.

Cobalt Chloride Pretreatment Attenuates Myocardial Apoptosis After Hypothermic Circulatory Arrest

BACKGROUND

Deep hypothermic circulatory arrest (DHCA) causes myocyte injury as a consequence of ischemia and reperfusion. Previous studies have shown that hypoxia or hypoxia-mimetic agents (cobalt chloride [CoCl2] or deferoxamine [DFX]) limit myocyte necrosis by upregulating the transcription factor hypoxia-inducible factor. However, it remains unknown whether these agents attenuate myocardial apoptosis after DHCA. This study tested the hypotheses (1) that hypoxia, DFX, or CoCl2 preconditioning attenuates myocardial apoptosis during DHCA; and (2) that the protective mechanism involves the altered expression of apoptosis regulatory proteins pAkt (antiapoptotic), Bcl-2 (antiapoptotic), and Bax (proapoptotic).

METHODS

Anesthetized neonatal piglets were randomly assigned to four groups (n = 6 in a group): control (NaCl injection); hypoxia (pO2 of 30 to 40 mm Hg for 3 hours); DFX injection; or CoCl2 injection. Twenty-four hours later, the animals underwent cardiopulmonary bypass (CPB) and 110 minutes of DHCA. One week after CPB, percentage of apoptotic myocytes (terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling [TUNEL] assay) and expression of the pAKT, Bcl-2, and Bax were assessed by Western blot.

RESULTS

Although preconditioning with hypoxia and DFX failed to show a protective benefit, CoCl2 pretreatment significantly attenuated myocardial apoptosis (9.3% +/- 4.1%) versus controls (33.8% +/- 9.7%, p = 0.042). That was associated with increased myocardial pAkt expression (0.19 +/- 0.006 in CoCl2 versus 0.12 +/- 0.008 in control, p < 0.001). The expression of Bcl-2 was also significantly higher in the CoCl2 group (0.15 +/- 0.02) versus control (0.11 +/- 0.01, p = 0.007), whereas Bax expression was lower (0.34 +/- 0.04 versus 0.54 +/- 0.03 for control, p < 0.001).

CONCLUSIONS

Preconditioning with CoCl2 before prolonged DHCA in neonatal piglets attenuates myocardial apoptosis by mechanisms involving phosphorylation of Akt, upregulation of the antiapoptotic protein Bcl-2, and decreased expression of the proapoptotic protein Bax.

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.

Chronic hypoxemia increases myocardial cytochrome oxidase

OBJECTIVE

Cyanotic patients have potentially decreased tissue oxygen tension. Cytochrome oxidase catalyzes the reduction of oxygen and is integral to adenosine triphosphate production. Cytochrome oxidase subunit I, the active site, is encoded by mitochondrial DNA. Using a newborn swine model of chronic hypoxemia, we evaluated ventricular cytochrome oxidase subunit I mRNA and protein expression and assessed cytochrome oxidase activity.

METHODS

Thirty-two newborn piglets underwent thoracotomy and placement of a pulmonary artery-to-left atrium shunt or sham operation. Two weeks later, partial pressure of arterial oxygen, hematocrit, and left ventricular shortening fraction values were compared with baseline values. Northern blot hybridization and protein immunoblotting for ventricular cytochrome oxidase subunit I were performed. Cytochrome oxidase kinetic activity was measured. Heme a,a3 content and turnover number were determined. Significance was assessed with a t test.

RESULTS

Baseline partial pressure of arterial oxygen and hematocrit values were similar. Hypoxemic piglets had a lower partial pressure of arterial oxygen of 38 +/- 10 mm Hg (P < .001) and higher hematocrit value of 31.4% +/- 2.9% (P < .001) compared with a partial pressure of arterial oxygen of 140 +/- 47 mm Hg and hematocrit value of 24.6% +/- 3.9% after the sham operation. Baseline and postprocedure left ventricular shortening fraction were similar within and between groups. Chronic hypoxemia increased right ventricular and left ventricular cytochrome oxidase I mRNA and protein by more than 1.4-fold. Cytochrome oxidase activity increased significantly in hypoxemia by 2.5-fold compared with that seen after the sham operation. Heme a,a3 content and turnover number increased by 1.5-fold during hypoxemia.

CONCLUSIONS

Chronic hypoxemia increases cytochrome oxidase I message, protein expression, and activity. The increase in kinetics was due to increased enzyme content and catalytic activity. This is a possible adaptive mechanism that might preserve organ function during chronic hypoxemia.

Performance of the chronically hypoxic young rabbit heart

Hearts isolated from 30 rabbits, raised from birth to approximately 5 weeks of age under either hypoxic (FIO2, 0.10) or normoxic (FIO2, 0.21) conditions, underwent retrograde aortic perfusion using a non-recirculating, well-oxygenated crystalloid solution. The left ventricular end diastolic pressure was initially set at approximately 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/dt(max), 2294+/-125* vs 1549+/-144 mmHg/sec; relaxation time constant (Tau), 26.9+/-1.1* vs 41.6+/-4.8 msec; (-) dP/dt(max), 1422+/-43* vs 1001+/-63 mmHg/sec; coronary flow, 86.3+/-4.2* vs 59.9+/-2.9 ml/min/g(dry); glucose oxidation, 3511+/-118* vs 2979+/-233 nmol/min/g(dry); oxygen consumption, 28.2+/-1.4* vs 22.7+/-1.4 micromol/min/g(dry); 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.