Effects of hypothermia on myocardial substrate selection

Hypothermic, oxygenated perfusion (HOPE) provides cardioprotection via succinate oxidation prior to normothermic perfusion in a rat model of donation after circulatory death (DCD)

In donation after circulatory death (DCD), cardiac grafts are subjected to warm ischemia in situ, prior to a brief period of cold, static storage (CSS) at procurement, and ex situ, normothermic, machine perfusion (NMP) for transport and graft evaluation. Cold ischemia and normothermic reoxygenation during NMP could aggravate graft injury through continued accumulation and oxidation, respectively, of mitochondrial succinate, and the resultant oxidative stress. We hypothesized that replacing CSS with hypothermic, oxygenated perfusion (HOPE) could provide cardioprotection by reducing cardiac succinate levels before NMP. DCD was simulated in male Wistar rats. Following 21 minutes in situ ischemia, explanted hearts underwent 30 minutes hypothermic storage with 1 of the following: (1) CSS, (2) HOPE, (3) hypothermic deoxygenated perfusion (HNPE), or (4) HOPE + AA5 (succinate dehydrogenase inhibitor) followed by normothermic reperfusion to measure cardiac and metabolic recovery. After hypothermic storage, tissue ATP/ADP levels were higher and succinate concentration was lower in HOPE vs CSS, HNPE, and HOPE + AA5 hearts. After 60 minutes reperfusion, cardiac function was increased and cellular injury was decreased in HOPE compared with CSS, HNPE, and HOPE + AA5 hearts. HOPE provides improved cardioprotection via succinate oxidation prior to normothermic reperfusion compared with CSS, and therefore is a promising strategy for preservation of cardiac grafts obtained with DCD.

Mild Hypothermia May Offer Some Improvement to Patients with MODS after CPB Surgery

OBJECTIVE:

To summarize the effect of mild hypothermia on function of the organs in patients with multiple organ dysfunction syndrome after cardiopulmonary bypass surgery.

METHODS:

The patients were randomly divided into two groups, northermia group (n=71) and hypothermia group (n=89). We immediately began cooling the hypothermia group when test results showed multiple organ dysfunction syndrome, meanwhile all patients of two groups were drawn blood to test blood gas, liver and kidney function, blood coagulation function, and evaluated the cardiac function using echocardiography from 12 to 36 hours. We compared the difference of intra-aortic balloon pump, extracorporeal membrane oxygenation rate and mortality within one month after intensive care unit admission.

RESULTS:

Among the 160 patients, 36 died, 10 (11.24%) patients were from the hypothermia group and 26 (36.6%) from the northermia group (P <0.05). In northermia group, 45 (63.38%) patients used intra-aortic balloon pump and 4 (5.63%), extracorporeal membrane oxygenation; in hypothermia group, 35 (39.32%) patients used intra-aortic balloon pump and 2 (2.25%), extracorporeal membrane oxygenation（ P <0.05). The patients' heart rate decreased significantly in the hypothermia group. The heart rate of hypothermia group is significantly slower than the northermia group at the 36^th hour (P <0.05). But the mean arterial pressure of hypothermia group is significantly higher than the northermia group at the 36^th hour (P <0.05). In hypothermia group, PO₂, SvO₂ and lactate were improved significantly compared to pre-cooling (P <0.05), and they were significantly better than the northermia group at the 36^th hour (P <0.05%). Prothrombin time and activated partial thromboplastin time have no significantly difference between the two groups (P >0.05). But the platelet count has significantly difference between the two groups at the 36^th hour (P <0.05). The aspartate transaminase, alanine transaminase and creatinine were improved significantly in the hypothermia group, and they were significantly better than the northermia group (P <0.05).

CONCLUSION:

Mild hypothermia is feasible and safe for patients with multiple organ dysfunction syndrome after cardiopulmonary bypass surgery.

Enhanced pyruvate dehydrogenase activity improves cardiac outcomes in a murine model of cardiac arrest

Rationale

Post-ischemic changes in cellular metabolism alter myocardial and neurological function. Pyruvate dehydrogenase (PDH), the limiting step in mitochondrial glucose oxidation, is inhibited by increased expression of PDH kinase (PDK) during ischemia/reperfusion injury. This results in decreased utilization of glucose to generate cellular ATP. Post-cardiac arrest (CA) hypothermia improves outcomes and alters metabolism, but its influence on PDH and PDK activity following CA are unknown. We hypothesized that therapeutic hypothermia (TH) following CA is associated with the inhibition of PDK activity and increased PDH activity. We further hypothesized that an inhibitor of PDK activity, dichloroacetate (DCA), would improve PDH activity and post-CA outcomes.

Methods and results

Anesthetized and ventilated adult female C57BL/6 wild-type mice underwent a 12-minute KCl-induced CA followed by cardiopulmonary resuscitation. Compared to normothermic (37°C) CA controls, administering TH (30°C) improved overall survival (72-hour survival rate: 62.5% vs. 28.6%, P<0.001), post-resuscitation myocardial function (ejection fraction: 50.9±3.1% vs. 27.2±2.0%, P<0.001; aorta systolic pressure: 132.7±7.3 vs. 72.3±3.0 mmHg, P<0.001), and neurological scores at 72-hour post CA (9.5±1.3 vs. 5.4±1.3, P<0.05). In both heart and brain, CA increased lactate concentrations (1.9-fold and 3.1-fold increase, respectively, P<0.01), decreased PDH enzyme activity (24% and 50% reduction, respectively, P<0.01), and increased PDK protein expressions (1.2-fold and 1.9-fold, respectively, P<0.01). In contrast, post-CA treatment with TH normalized lactate concentrations (P<0.01 and P<0.05) and PDK expressions (P<0.001 and P<0.05), while increasing PDH activity (P<0.01 and P<0.01) in both the heart and brain. Additionally, treatment with DCA (0.2 mg/g body weight) 30 min prior to CA improved both myocardial hemodynamics 2 hours post-CA (aortic systolic pressure: 123±3 vs. 96±4 mmHg, P<0.001) and 72-hour survival rates (50% vs. 19%, P<0.05) in normothermic animals.

Conclusions

Enhanced PDH activity in the setting of TH or DCA administration is associated with improved post-CA resuscitation outcomes. PDH is a promising therapeutic target for improving post-CA outcomes.

(13)C glucose labelling studies using 2D NMR are a useful tool for determining ex vivo whole organ metabolism during hypothermic machine perfusion of kidneys

Background

The aim of this study is to determine the feasibility of using nuclear magnetic resonance (NMR) tracer studies (¹³C-enriched glucose) to detect ex vivo de novo metabolism in the perfusion fluid and cortical tissue of porcine kidneys during hypothermic machine perfusion (HMP).

Methods

Porcine kidneys (n = 6) were subjected to 24 h of HMP using the Organ Recovery Systems LifePort Kidney perfusion device. Glucose, uniformly enriched with the stable isotope ¹³C ([U-¹³C] glucose), was incorporated into KPS-1-like perfusion fluid at a concentration of 10 mM. Analysis of perfusate was performed using both 1D ¹H and 2D ¹H,¹³C heteronuclear single quantum coherence (HSQC) NMR spectroscopy. The metabolic activity was then studied by quantifying the proportion of key metabolites containing ¹³C in both perfusate and tissue samples.

Results

There was significant enrichment of ¹³C in a number of central metabolites present in both the perfusate and tissue extracts and was most pronounced for lactate and alanine. The total amount of enriched lactate (per sample) in perfusion fluid increased during HMP (31.1 ± 12.2 nmol at 6 h vs 93.4 ± 25.6 nmol at 24 h p < 0.01). The total amount of enriched alanine increased in a similar fashion (1.73 ± 0.89 nmol at 6 h vs 6.80 ± 2.56 nmol at 24 h p < 0.05). In addition, small amounts of enriched acetate and glutamic acid were evident in some samples.

Conclusions

This study conclusively demonstrates that de novo metabolism occurs during HMP and highlights active metabolic pathways in this hypothermic, hypoxic environment. Whilst the majority of the ¹³C-enriched glucose is metabolised into glycolytic endpoint metabolites such as lactate, the presence of non-glycolytic pathway derivatives suggests that metabolism during HMP is more complex than previously thought. Isotopic labelled ex vivo organ perfusion studies using 2D NMR are feasible and informative.

Ischemia hypothermia improved contractility under normothermia reperfusion in the model of cultured cardiomyocyte

Though mild hypothermia displays an optimistic alleviation of contractive failure in the ischemia/reperfusion myocardium, we still lacked answers to many questions about its potential mechanisms. Our hypothesis is that hypothermia (32°C) induced in ischemia can ease mitochondrial injury resulting in improvement of myocardial contractility even under the condition of a normothermic reperfusion. Fifty newly born 1-2 d Sprague-Dawley rats were executed and the primary cardiomyocytes were obtained and cultivated in vitro. Myocytes were randomized into three groups and then subjected to ischemia either at 32°C or 37°C, both prior to undergoing reperfusion at 37°C. Contractility was presented as frequency and velocity. Ultrastructural alterations of cardiomyocytes and mitochondrion underwent semi-quantitative analysis with transmission electron microscopy and respiratory function of mitochondria was further assessed simultaneously. During cooling ischemia and following reperfusion, cardiomyocytes acquired a more immediate restoration to baseline level and had a significant difference as compared with those in normothermia (P < 0.05). Furthermore, hypothermia preserved the ultrastructure of myocytes and mitochondrion after ischemia. However, measurement on Heart Injury Score and form factor revealed no differences after 2-h reperfusion either in hypothermia or normothermia. On the contrary, the surface area and respiratory function of mitochondrion in reperfusion differed significantly in both groups (P < 0.05) which had an accordance with the variation on contractile performance. Hypothermia only induced in ischemia can bring contractility benefit even under a normothermia reperfusion in cultured cardiomyocytes.

Hypothermia improves ventricular myocyte contractility under conditions of normal perfusion and after an interval of ischemia

AIM

Recent investigations have reported improved myocardial function during hypothermia following resuscitation from cardiac arrest. The effects of hypothermia on myocyte contractility were investigated under conditions of normal perfusion and after a 10min interval of ischemia.

METHODS

Ventricular myocytes were obtained from 10 male Sprague-Dawley rats weighing 400+/-50g. The myocytes were randomized to be perfused at: 37 degrees C, 34 degrees C, 32 degrees C, or 30 degrees C. A subsequent set of myocytes was subjected to 10min of ischemia at 37 degrees C, prior to being randomized to reperfusion at: 37 degrees C, 34 degrees C, 32 degrees C or 30 degrees C. Myocyte contractility was expressed as length-shortening percentage. Intracellular Ca(2+) transients were assessed in a separate group of myocytes preloaded with Fura-2/AM. Sensitivity to Ca(2+) was tested by increasing perfusate Ca(2+) content, i.e. 0.5mM, 1mM and 2mM.

RESULTS

During normal perfusion and following reperfusion after 10min of ischemia, myocyte contractility increased at 34 degrees C compared to 37 degrees C (P<0.01). When the perfusion temperature was decreased to 32 degrees C and 30 degrees C, contractility further increased (P<0.001). Intracellular Ca(2+) transients were greater during perfusion at 34 degrees C compared to those at 37 degrees C (P<0.001) and further increased at 30 degrees C (P<0.001). Increases in extracellular Ca(2+) concentration from 0.5mM to 2mM resulted in greater myocyte contractility during perfusion at 30 degrees C compared to that observed at 37 degrees C (P<0.001). Effects of hypothermia on intracellular Ca(2+) transients and sensitivity to Ca(2+) persisted after ischemia.

CONCLUSIONS

Hypothermia improved myocyte contractility, intracellular Ca(2+) transients and sensitivity to Ca(2+) under conditions of normal perfusion and following reperfusion after 10min of ischemia.

Cardiac and metabolic effects of hypothermia and inhaled hydrogen sulfide in anesthetized and ventilated mice

OBJECTIVE

To test the hypothesis whether inhaled hydrogen sulfide amplifies the effects of deliberate hypothermia during anesthesia and mechanical ventilation as hypothermia is used to provide organ protection after brain trauma or circulatory arrest. Awake mice inhaling hydrogen sulfide exhibit reduced energy expenditure, hypothermia, and bradycardia despite unchanged systolic heart function. In rodents, anesthesia alone causes decreased metabolic rate and thus hypothermia and bradycardia.

DESIGN

Prospective, controlled, randomized study.

SETTING

University animal research laboratory.

SUBJECTS

Male C57/B6 mice.

INTERVENTIONS

After surgical instrumentation (central venous, left ventricular pressure-conductance catheters, ultrasound flow probes on the portal vein and superior mesenteric artery), normo- or hypothermic animals (core temperature = 38 degrees C and 27 degrees C) received either 100 ppm hydrogen sulfide or vehicle over 5 hrs (3 hrs hydrogen sulfide during normothermia).

MEASUREMENTS AND MAIN RESULTS

During normothermia, hydrogen sulfide had no hemodynamic or metabolic effect. With or without hydrogen sulfide, hypothermia decreased blood pressure, heart rate, and cardiac output, whereas stroke volume, ejection fraction, and end-diastolic pressure remained unaffected. Myocardial and hepatic oxidative deoxyribonucleic acid damage (comet assay) and endogenous glucose production (rate of appearance of 1,2,3,4,5,6-13C6-glucose) were similar in all groups. Hypothermia comparably decreased CO2 production with or without inhaled hydrogen sulfide. During hypothermia, inhaled hydrogen sulfide increased the glucose oxidation rate (derived from the expiratory 13CO2/12CO2 ratio). This shift toward preferential carbohydrate utilization coincided with a significantly attenuated responsiveness of hepatic mitochondrial respiration to stimulation with exogenous cytochrome-c-oxidase (high-resolution respirometry).

CONCLUSIONS

In anesthetized and mechanically ventilated mice, inhaled hydrogen sulfide did not amplify the systemic hemodynamic and cardiac effects of hypothermia alone. The increased aerobic glucose oxidation together with the reduced responsiveness of cellular respiration to exogenous cytochrome-c stimulation suggest that, during hypothermia, inhaled hydrogen sulfide improved the yield of mitochondrial respiration, possibly via the maintenance of mitochondrial integrity. Hence, inhaled hydrogen sulfide may offer metabolic benefit during therapeutic hypothermia.

Effect of hypothermia on transthoracic defibrillation in a swine model

BACKGROUND

Induced hypothermia (H) appears a promising intervention to protect the heart and brain after resuscitation from cardiac arrest. However, the influence of H on transthoracic defibrillation energy requirements is not well documented.

METHODS

In 39 swine (21.4+/-1.3(S.E.) kg) hypothermia was induced by surrounding the head, thorax and abdomen with ice. The swine were divided into four groups: (1) normothermia (N) followed by severe H (30 degrees C) (n=10), (2) severe H followed by N (n=10), (3) N followed by moderate H (33 degrees C) (n=10) and (4) moderate H followed by N (n=9). After 30s of electrically induced ventricular fibrillation (VF), the swine were defibrillated (biphasic waveform) at energies of 20J, 30J, 50J and 100J in random order in both N and H conditions.

RESULTS

For pigs in Group 1 (N followed by severe H), shock success in terminating VF was higher during hypothermia (odds ratio 4.09 (95% CI: 2.21, 5.58; p<0.0001), despite the fact that impedance rose from 39+/-3Omega (N) to 42+/-3Omega (H) (p<0.001) and current fell from 22+/-8 (N) to 21+/-7A (H) (p<0.001). There were no significant differences in the shock success between N and H for the other groups. Post-defibrillation ventricular asystole occurred less often during hypothermia compared to normothermia (p=0.0002).

CONCLUSION

Severe H facilitated transthoracic defibrillation in this swine model. Since impedance rose and current fell during H, the improved shock success must be due to a hypothermia-induced change in the mechanical or electrophysiologic properties of the myocardium. Moderate hypothermia did not alter the energy requirement for defibrillation.

Hypothermia Improves Defibrillation Success and Resuscitation Outcomes From Ventricular Fibrillation

Background— Induced hypothermia is recommended to improve neurological outcomes in unconscious survivors of out-of-hospital ventricular fibrillation (VF) cardiac arrest. Patients resuscitated from a VF arrest are at risk of refibrillation, but there are few data on the effects of already existing hypothermia on defibrillation and resuscitation.

Methods and Results— Thirty-two swine (mean±SE weight, 23.0±0.6 kg) were divided into 4 groups: normothermia (n=8), mild hypothermia (35°C) (n=8), moderate hypothermia (33°C) (n=8), and severe hypothermia (30°C) (n=8). Hypothermia was induced by surrounding the animal with ice, and VF was electrically induced. After 8 minutes of unsupported VF (no CPR), the swine were defibrillated (biphasic waveform) with successive shocks as needed and underwent CPR until resumption of spontaneous circulation or no response (≥10 minutes). First-shock defibrillation success was higher in the moderate hypothermia group (6 of 8 hypothermia versus 1 of 8 normothermia; P =0.04). The number of shocks needed for late defibrillation (≥1 minute after initial shock) was less in all 3 hypothermia groups compared with normothermia (all P <0.05). None of the 8 animals in the normothermia group achieved resumption of spontaneous circulation compared with 3 of 8 mild hypothermia ( P =NS), 7 of 8 moderate hypothermia ( P =0.001), and 5 of 8 severe hypothermia ( P =0.03) animals. Coronary perfusion pressure during CPR was not different between the groups.

Conclusions— When VF was induced in the setting of moderate or severe hypothermia, resuscitative measures were facilitated with significantly improved defibrillation success and resuscitation outcome. The beneficial effect of hypothermia was not due to alteration of coronary perfusion pressure, which suggests that changes in the mechanical, metabolic, or electrophysiological properties of the myocardium may be responsible.

Perioperative blood glucose control during adult coronary artery bypass surgery

Coronary artery bypass graft (CABG) procedures are among the most frequently performed surgical procedures in the United States. People with cardiovascular disease who also have diabetes have a greater risk of poor outcomes after CABG procedures than patients who do not have diabetes. This literature review examines current information regarding perioperative blood glucose (BG) control. It emphasizes BG control in adults during the hypothermic period of cardiopulmonary bypass. Hyperglycemia, not the diagnosis of diabetes, significantly increases the risk of adverse clinical outcomes, longer hospitalizations, and increased health care costs.

Glucose-insulin-potassium solution improves left ventricular energetics in chronic ovine diabetes

BACKGROUND

Therapeutic modulation of myocardial metabolism improves outcomes in diabetic patients following myocardial infarction and coronary artery surgery. However, the mechanism of this beneficial effect has not been fully elucidated. This study evaluated the effect of glucose-insulin-potassium solution (GIK) on left ventricular (LV) energetics and oxygen utilization efficiency in a chronic ovine model of diabetes.

METHODS

Diabetes was induced in sheep with streptozotocin. Experiments were performed following 12 months untreated diabetes (n = 6) and in controls (n = 6). Open-chest anesthetized sheep were instrumented to determine the LV pressure-volume relationship, oxygen consumption, and free fatty acid uptake. Glucose-insulin-potassium was infused at 1.5 mL x kg(-1) x h(-1) for 60 minutes and assessment repeated.

RESULTS

Glucose-insulin-potassium decreased LV free fatty acid uptake in control: 0.090 +/- 0.047 microg/beat/100 g to 0.024 +/- 0.022 microg/beat/100 g, p = 0.02 and diabetes: 0.33 +/- 0.32 microg/beat/100 g to 0.11 +/- 0.13 microg/beat/100 g, p = 0.04. Similarly, GIK decreased unloaded left ventricular oxygen consumption (LVVO(2)) in both control (0.42 +/- 0.05 to 0.37 +/- 0.13J/beat/100 g, p < 0.001) and diabetic sheep (0.40 +/- 0.24 to 0.23 +/- 0.23J/beat/100 g, p < 0.001). The slope of the LVVO(2)-pressure-volume area relation (contractile efficiency) was unchanged in either group. Glucose-insulin-potassium improved LV contractility 58% +/- 37% (p = 0.005) and stroke work efficiency 18% +/- 10% (p = 0.009) in diabetic animals but not controls. Therefore, oxygen utilization efficiency (stroke work-LVVO(2)) increased only in diabetic animals (16.6% +/- 4.8% to 26.9% +/- 3.6%, p = 0.002) following GIK.

CONCLUSIONS

This study provides in vivo evidence that GIK improves LV energetics in diabetes. Oxygen utilization efficiency is improved as a result of improved stroke work efficiency and decreased unloaded LVVO(2). Improved efficiency of oxygen utilization provides a physiologic rationale for the beneficial effect of GIK in diabetic patients.