The effects of the rate of reoxygenation on the recovery of hypoxemic hearts

The therapeutic effect of controlled reoxygenation on chronic hypoxia-associated brain injury

Cardiopulmonary bypass (CPB) is the most general technique applied in congenital heart disease (CHD). However, standard CPB poses a specific pathologic condition for patients during surgery: exposure to reoxygenation. When surgery is performed on cyanotic infants, standard CPB is usually initiated at a high concentration of oxygen without consideration of cytotoxic effects. Controlled reoxygenation is defined as using normoxic CPB with a pump primed to the PO2 (oxygen tension in the blood), which is matched to the patient's preoperative saturation. The aim of this study was to determine whether controlled reoxygenation could avoid standard reoxygenation injury and also to clarify the molecular signaling pathways during hypoxia. We successfully reproduced the abnormal brain observed in mice with chronic hypoxia during early postnatal development – equivalent to the third trimester in human. Mice were treated with standard reoxygenation and controlled reoxygenation after hypoxia for 24 h. We then assessed the brain tissue of these mice. In standard reoxygenation-treated hypoxia mice, the caspase-3-dependent neuronal apoptosis was enhanced by increasing concentration of oxygen. Interestingly, controlled reoxygenation inhibited neuron and glial cell apoptosis through suppressing cleavage of caspase-3 and PARP. We also found that controlled reoxygenation suppressed LCN2 expression and inflammatory cytokine (including TNF-α, IL-6, and CXCL10) production, in which the JAK2/STAT3 signaling pathway might participate. In conclusion, our findings propose the novel therapeutic potential of controlled reoxygenation on CPB during CHD.

Summary: Controlled reoxygenation may provide an effective therapeutic strategy for hypoxia-induced tissue injury via regulation of the JAK2/STAT3 signaling pathway. It will help make better informed clinical treatment decisions for cyanotic infants.

Myocardial Damage Induced by Uncontrolled Reoxygenation

To evaluate myocardial impairment induced by uncontrolled reoxygenation, the effects of hypoxia-reoxygenation were compared with ischemia-reperfusion in isolated rat hearts. After stabilization, 2 groups (n = 8) of Langendorff-perfused rat hearts were exposed to 40 minutes of ischemia (10% of baseline flow) or hypoxia (10% of baseline oxygen content) followed by a sudden return to baseline conditions (reperfusion or reoxygenation). The O2 content was identical for the two groups during baseline conditions, O2 shortage, and O2 readmission. Metabolic (lactate production) and functional parameters (heart rate, peak systolic pressure, left ventricular developed pressure, maximal contraction and relaxation rates, end-diastolic pressure, coronary perfusion pressure) were recorded at the end of stabilization, after O2 deficiency, and after 2 minutes of reoxygenation. Systolic function was significantly depressed after ischemia (p < 0.0001) but completely recovered to baseline values after 2 minutes of reperfusion. In contrast, systolic function was less severely depressed after hypoxia but failed to return to baseline after 2 minutes of reoxygenation. Diastolic function, unchanged during ischemia-reperfusion, remained significantly impaired during hypoxia-reoxygenation.

Normothermia for pediatric and congenital heart surgery: an expanded horizon

Cardiopulmonary bypass (CPB) in pediatric cardiac surgery is generally performed with hypothermia, flow reduction and hemodilution. From October 2013 to December 2014, 55 patients, median age 6 years (range 2 months to 52 years), median weight 18.5 kg (range 3.2-57 kg), underwent surgery with normothermic high flow CPB in a new unit. There were no early or late deaths. Fifty patients (90.9%) were extubated within 3 h, 3 (5.5%) within 24 h, and 2 (3.6%) within 48 h. Twenty-four patients (43.6%) did not require inotropic support, 31 (56.4%) received dopamine or dobutamine: 21 ≤5 mcg/kg/min, 8 5-10 mcg/kg/min, and 2 >10 mcg/kg/min. Two patients (6.5%) required noradrenaline 0.05-0.1 mcg/kg/min. On arrival to ICU and after 3 and 6 h and 8:00 a.m. the next morning, mean lactate levels were 1.9 ± 09, 2.0 ± 1.2, 1.6 ± 0.8, and 1.4 ± 0.7 mmol/L (0.6-5.2 mmol/L), respectively. From arrival to ICU to 8:00 a.m. the next morning mean urine output was 3.8 ± 1.5 mL/kg/h (0.7-7.6 mL/kg/h), and mean chest drainage was 0.6 ± 0.5 mL/kg/h (0.1-2.3 mL/kg/h). Mean ICU and hospital stay were 2.7 ± 1.4 days (2-8 days) and 7.2 ± 2.2 days (4-15 days), respectively. In conclusion, normothermic high flow CPB allows pediatric and congenital heart surgery with favorable outcomes even in a new unit. The immediate post-operative period is characterized by low requirement for inotropic and respiratory support, low lactate production, adequate urine output, minimal drainage from the chest drains, short ICU, and hospital stay.

Chronic hypoxia: a model for cyanotic congenital heart defects

OBJECTIVE

The postoperative course of cyanotic children is generally more complicated than that of acyanotic children. A possible reason is reoxygenation injury at the beginning of cardiopulmonary bypass. In this study we tested the hypothesis that reoxygenation of chronically hypoxic hearts is worse than that of normoxic hearts.

METHODS

Two groups of rats (n = 9 each) were exposed to either room air (fraction of inspired oxygen, 0.21%) or chronic hypoxia (fraction of inspired oxygen, 0.10%) for 2 weeks. Hearts were then isolated and perfused for 30 minutes with hypoxic buffer (oxygen saturation, 10%), followed by 30 minutes of reoxygenation (oxygen saturation, 100%).

RESULTS

In hypoxic rats hematocrit values, hemoglobin concentrations, and red cells were higher (69% +/- 6% vs 40% +/- 6%, 219 +/- 14 vs 124 +/- 12 g/L, and 10.30 +/- 0.6 vs 6.32 +/- 0.5/microL/1000, respectively; P <.0001); the amount of ingested food was less (22.3 +/- 4.8 vs 30.7 +/- 3.9 g/d, P <.001), as was the amount of ingested water (21.0 +/- 3.1 vs 50.4 +/- 14.6 mL/d, P <.0001); and body weight was lower (182 +/- 14.2 vs 351 +/- 40.1 g, P <.0001), as was heart weight (1107 +/- 119 vs 1312 +/- 128 mg, P <.005). The heart weight/body weight ratio was higher (6.10 +/- 0.8 vs 3.74 +/- 0.1 mg/g, P <.0001). Systolic and diastolic functions, not different during the hypoxic baseline period, were more impaired in hypoxic than in normoxic hearts after the reoxygenation, whereas coronary resistance remained lower. During the hypoxic perfusion, the venous partial pressure of oxygen remained low in both groups, whereas during reoxygenation, partial pressure of oxygen was higher in hypoxic hearts, with a lower (P <.01) oxygen uptake. During hypoxic baseline adenosine triphosphate turnover, lactate production and lactate turnover were lower in hypoxic hearts (P <.005, P <.0001, and P <.0001, respectively).

CONCLUSIONS

Body and blood values are severely affected by chronic hypoxia, and the cardiac effects of uncontrolled reoxygenation after chronic hypoxia are more severe than after acute hypoxia.