Hypothermia increases the threshold for ischemic preconditioning

Cardioprotective effect of combination therapy by mild hypothermia and local or remote ischemic preconditioning in isolated rat hearts

A multitargeted strategy to treat the consequences of ischemia and reperfusion (IR) injury in acute myocardial infarction may add cardioprotection beyond reperfusion therapy alone. We investigated the cardioprotective effect of mild hypothermia combined with local ischemic preconditioning (IPC) or remote ischemic conditioning (RIC) on IR injury in isolated rat hearts. Moreover, we aimed to define the optimum timing of initiating hypothermia and evaluate underlying cardioprotective mechanisms. Compared to infarct size in normothermic controls (56 ± 4%), mild hypothermia during the entire or final 20 min of the ischemic period reduced infarct size (34 ± 2%, p < 0.01; 35 ± 5%, p < 0.01, respectively), while no reduction was seen when hypothermia was initiated at reperfusion (51 ± 4%, p = 0.90). In all groups with effect of mild hypothermia, IPC further reduced infarct size. In contrast, we found no additive effect on infarct size between hypothermic controls (20 ± 3%) and the combination of mild hypothermia and RIC (33 ± 4%, p = 0.09). Differences in temporal lactate dehydrogenase release patterns suggested an anti-ischemic effect by mild hypothermia, while IPC and RIC preferentially targeted reperfusion injury. In conclusion, additive underlying mechanisms seem to provide an additive effect of mild hypothermia and IPC, whereas the more clinically applicable RIC does not add cardioprotection beyond mild hypothermia.

Mild Hypothermia as a Cardioprotective Approach for Acute Myocardial Infarction: Laboratory to Clinical Application

In many animal models, mild therapeutic hypothermia is a powerful intervention, reducing myocardial infarct size, reducing the no-reflow phenomenon, and improving healing after infarction. Cooling in these models has been produced by various means including whole-body hypothermia, synchronized hypothermic coronary venous retro-perfusion, heat exchangers, and regional hypothermia targeting the heart alone. However, in humans, the most widely used techniques are surface cooling and cooling by endovascular heat-exchange catheters. The reduction in temperature necessary to produce cardioprotection is mild (32-34°C), appears to have no detrimental effects on left ventricular function or regional myocardial blood flow, and may improve microvascular reflow to previously ischemic heart tissue. It has been shown in experimental and clinical studies that for therapeutic hypothermia to be effective it must be (1) initiated as early as possible after the onset of ischemia and (2) initiated before reperfusion. This may require initiation of hypothermia in the ambulance, well before mechanical reperfusion occurs. The mechanisms of protection produced by hypothermia have yet to be conclusively determined but may include a decrease in tissue metabolic rate, preservation of high energy phosphates, a reduction in tissue apoptosis or induction of heat shock proteins.

Sevoflurane- and desflurane-induced human myocardial post-conditioning through Phosphatidylinositol-3-kinase/Akt signalling

BACKGROUND

The role of phosphatidylinositol-3-kinase (PI3K) in sevoflurane- and desflurane-induced myocardial post-conditioning remains unknown.

METHODS

We recorded isometric contraction of isolated human right atrial trabeculae (oxygenated Tyrode's at 34 degrees C, stimulation frequency 1 Hz). In all groups, a 30-min hypoxic period was followed by a 60-min reoxygenation period. At the onset of reoxygenation, muscles were exposed to 5 min of sevoflurane 1%, 2%, and 3%, and desflurane 3%, 6%, and 9%. In separate groups, sevoflurane 2% and desflurane 6% were administered in the presence of 100 nM wortmannin, a PI3K inhibitor. Recovery of force after the 60-min reoxygenation period was compared between groups (mean +/- SD).

RESULT

As compared with the Control group (49 +/- 7% of baseline) PostC by sevoflurane 1%, 2%, and 3% (78 +/- 4%, 79 +/- 5%, and 85 +/- 4% of baseline, respectively) and desflurane 3%, 6%, and 9% (74 +/- 5%, 84 +/- 4%, and 86 +/- 11% of baseline, respectively) enhanced the recovery of force. This effect was abolished in the presence of wortmannin (56 +/- 5% of baseline for sevoflurane 2%+wortmannin; 56 +/- 3% of baseline for desflurane 6%+wortmannin). Wortmannin alone had no effect on the recovery of force (57 +/- 7% of baseline).

CONCLUSION

In vitro, sevoflurane and desflurane post-conditioned human myocardium against hypoxia through activation of phosphatidylinositol-3-kinase.

Etomidate has no effect on hypoxia reoxygenation and hypoxic preconditioning in isolated human right atrial myocardium

BACKGROUND

We examined the effects of etomidate on recovery of contractile function after hypoxia reoxygenation and hypoxic preconditioning in vitro using isolated human myocardium.

METHODS

Human right atrial myocardium were obtained at the time of cardiac surgery from 38 adults patients. We recorded isometric force of contraction (FoC) of atrial trabeculae suspended in an oxygenated Tyrode's solution (34 degrees C, stimulation frequency 1 Hz). In all groups, a 30-min hypoxic period was followed by 60 min of reoxygenation (HR). In separate groups, muscles were exposed to etomidate (10(-7), 10(-6), 10(-5) M) 10 min before and throughout the HR periods. Hypoxic preconditioning was induced by 4-min hypoxia followed by 7-min reoxygenation applied before HR periods. Etomidate 10(-5) M was administered before, throughout, and after the hypoxic preconditioning stimulus. Recovery of FoC (expressed as % of baseline value) at the end of HR was compared among groups.

RESULTS

Compared with the control group (FoC: 52%+/-10%), etomidate 10(-7) M (FoC: 57%+/-9%; P=0.24), 10(-6) M (FoC: 61%+/-11%; P=0.10), and 10(-5) M (FoC: 54%+/-9%; P=0.29) did not modify the recovery of FoC after HR. Hypoxic preconditioning-induced increase in the recovery of FoC (87%+/-5%; P<0.001 vs control group) was not modified in the presence of etomidate 10(-5) M (FoC: 86%+/-7%; P=0.74 vs hypoxic preconditioning group).

CONCLUSIONS

Etomidate did not modify the in vitro FoC of human myocardium exposed to HR. Furthermore, etomidate did not modify the protective effect of hypoxic preconditioning.

Postoperative stiffness and adhesion formation around repaired and immobilized Achilles tenotomies are prevented using a model of heat shock protein induction

BACKGROUND

Tendon repair and subsequent immobilization is frequently complicated by postoperative stiffness secondary to inflammation and peritendinous adhesions. Thermal preconditioning is known to reduce inflammation by inducing formation of cytoprotective heat shock proteins. This study evaluates the role of thermal preconditioning following complete division and repair of the Achilles tendon, with subsequent immobilization, mimicking the typical clinical scenario.

MATERIALS AND METHODS

Twenty-four New Zealand White rabbits were used in the study. The treatment group underwent thermal preconditioning, by elevating their core temperature to 41.5 degrees C for 20 minutes. The Achilles tendon of the hindlimb was divided and repaired 18 hours following thermal preconditioning. The animals were sacrificed following 3 weeks of immobilization. Range of movement of the ankle, tendon gliding, quantity of adhesions, and weight of repaired tendons were assessed.

RESULTS

Loss of range of movement at the ankle was significantly less in the treatment group versus controls (P = 0.02). The quantity of adhesions and weight of the repaired tendons were significantly reduced in the treatment group (P = <0.001 and P = 0.005, respectively). Tendon gliding relative to the surrounding soft tissue was also significantly improved in the treatment group (P = 0.002).

CONCLUSION

Preconditioned animals demonstrated a significantly better range of ankle movement, decreases in adhesion formation and in the gliding, and dimensions of tendons. Thermal preconditioning therefore has the potential to improve clinical results in tendon surgery following repair and immobilization.

Deep hypothermia during ischemia improves functional recovery and reduces free-radical generation in isolated reperfused rat heart

BACKGROUND

We investigated the influence of deep hypothermia (4 degrees C) during ischemia-reperfusion in the isolated rat heart model.

METHODS

Isolated, perfused rat hearts underwent either 30 minutes of normothermic ischemia (control group) or 30 minutes of hypothermic ischemia (hypothermia-treated group), followed by 30 minutes of reperfusion in both groups. We recorded functional parameters and used electron spin resonance (ESR) spectroscopy to detect ascorbyl radicals, as markers of free-radical production, in samples of coronary effluents.

RESULTS

Functional parameters were stable in the 2 groups during pre-ischemic and ischemic periods. During reperfusion, coronary flow, left diastolic ventricular pressure, left ventricular developed pressure, and heart rate more rapidly recovered to values close to those obtained during the pre-ischemic period in the hypothermia-treated group than in the control group. Moreover, the post-ischemic contracture observed in the control group did not appear in the hypothermia-treated group. Finally, ESR analysis showed that the post-ischemic release of ascorbyl radicals decreased in the hypothermia-treated group.

CONCLUSIONS

These results demonstrate that the protective effect of hypothermia against functional injury caused by ischemia-reperfusion may decrease the free-radical burst at reperfusion.

Clinically applicable thermal preconditioning attenuates leukocyte-endothelial interactions

BACKGROUND

We have previously demonstrated that clinically applicable thermal preconditioning induces heat shock protein 72 (HSP72) and protects against a subsequent ischemia-reperfusion (I/R) injury in an animal model. A core component of I/R injuries is the interaction between activated leukocytes and endothelial cells. We hypothesized that the effects of clinically applicable thermal preconditioning are mediated through attenuation of this leukocyte-endothelial (L-E) interaction.

STUDY DESIGN

Twenty-one male Sprague Dawley rats were divided into control, I/R, and preconditioning plus I/R groups. Preconditioning was done under general anesthesia and the animals' temperature raised by 1 degrees C for 15 minutes in a water bath. This was repeated once a day for 5 successive days. I/R injury was caused by occlusion of the superior mesenteric artery for 10 minutes followed by 1 hour of reperfusion. L-E interactions were analyzed using intravital microscopy of a mesenteric vessel in vivo. L-E interactions were determined using leukocyte velocity (which decreases as cells interact), and number of adherent and migrated leukocytes. HSP72 was assessed by Western blot.

RESULTS

Ischemia-reperfusion caused a decrease in leukocyte rolling velocity at all timepoints (p < 0.05 versus controls). Preconditioning attenuated the effects of I/R, and leukocyte rolling velocity was significantly improved versus I/R (p < 0.05) to levels similar to those in controls. Similarly, the number of adherent and migrating leukocytes increased significantly (p < 0.05) after I/R versus control at all time points, and preconditioning attenuated these to control levels, (p < 0.05 versus I/R) at both the 30- and 60-minute postischemia time points. Upregulation of HSP72 was demonstrated on Western blot.

CONCLUSIONS

These results demonstrate that the benefit of clinically applicable thermal preconditioning is at least partially because of an immunomodulatory role in attenuating leukocyte-endothelial interactions associated with an increased expression of HSP 72.

Clinically relevant thermal preconditioning attenuates ischemia-reperfusion injury

INTRODUCTION

Thermal preconditioning has previously been shown to attenuate ischemia-reperfusion induced injuries, possible due to increased expression of heat shock proteins (HSP). The model of thermal preconditioning used, however, was not clinically relevant as preconditioning was to 41 degrees C, leading to cellular damage. Our aim was thus to establish a novel and clinically applicable method of preconditioning.

MATERIALS AND METHODS

Twenty-six male Sprague-Dawley rats were split into three groups (nine control, nine ischemia-reperfusion, and eight preconditioned followed by ischemia-reperfusion). To precondition the animals, they were anesthetized and, using a water bath, their core temperature was raised by 1 degrees C for 15 min once a day for five successive days. I/R injury consisted of 30 min of aortic cross-clamping followed by 120 min of reperfusion; control animals had a laparotomy only. Indicators of lung injury were tissue myeloperoxidase, broncho-alveolar lavage protein concentration, and tissue edema. Tissue heat shock protein expression was detected by Western blot analysis.

RESULTS

Lower torso ischemia-reperfusion causes significant lung injury versus control, with raised levels of myeloperoxidase 4.53 iu/g to 7.88 iu/g (P < 0.05), raised B.A.L. protein concentration 419 microg/ml to 684 microg/ml (P < 0.05) and altered wet dry ratio 4.63 to 5.50. Clinically relevant thermal preconditioning attenuates all of these parameters back to control levels: myeloperoxidase 3.87 iu/g (P < 0.05 vs I/R), B.A.L. to 284 microg/ml (P < 0.01 vs I/R) and wet dry ratio to 4.44 (P < 0.05 vs I/R). Western blot demonstrated increased expression of H.S.P. 72 in the preconditioned group versus control and I/R alone. Western blot demonstrated increased expression of HSP72 in the preconditioned group vs control and I/R alone.

CONCLUSIONS

We conclude that clinically applicable thermal preconditioning can attenuate ischemia-reperfusion induced lung injury, possibly through increased expression of HSP72.

Combination therapy for maximal myocardial infarct size reduction

The aim of this study was to test whether myocardial infarct size reduction would be optimized by combining three known effective therapies: cariporide, regional hypothermia, and ischemic preconditioning (CHIP). Before coronary artery occlusion (CAO), treated rabbits (CHIP, n = 7) received cariporide (0.3 mg/kg), ischemic preconditioning (7 minutes ischemia and 5 minutes reperfusion), then 20 minutes of mild regional hypothermia (34 degrees C). Control rabbits (n = 7) received saline and a 34-minute waiting period. All received 30 minutes of CAO and reperfusion. In another study, rabbits (n = 8 in each group) received 90 minutes of CAO. In the 30-minute protocol, the authors found that hearts in both groups were equally ischemic during CAO. Mean ischemic risk zones were similar in both groups; however, in CHIP hearts, infarct size was 4 +/- 1% of risk zone, a reduction of 91% compared with control rabbits (44 +/- 7% of the risk zone, P = 0.001). In the 90-minute protocol, risk zone size was similar in both groups, but infarct size in control hearts was 76 +/- 3% of the risk zone compared with 34 +/- 7% in CHIP treated hearts (P = 0.0003). In summary, the combined treatment provided extraordinary protection. Infarct comprised only 4% of the risk region after 30-minute ischemia-a far greater reduction than was previously observed in the same laboratory using any single intervention. After 90 minutes of ischemia, infarct was 55% lower in CHIP hearts, suggesting that this therapeutic approach dramatically reduces ischemia/reperfusion cell death, even during long occlusions.

Backtable heat-enhanced preconditioning: a simple and effective means of improving function of heart transplants

BACKGROUND

Cardiac harvest teams are usually committed to immediately transfer the explanted donor heart into its cold storage solution. We tested the opposite hypothesis that a brief prestorage episode of heat-enhanced ischemic preconditioning could be protective.

METHODS

Fifty-three isolated isovolumic rat hearts underwent 4 hours of cold (4 degrees C) storage in the Celsior preservation solution and 2 hours of reperfusion. Control hearts were immediately immersed after arrest. In the 3 treated groups, 2 customized thermal probes were first applied onto the left ventricular free wall of the explanted heart at 22 degrees C, 37 degrees C or 42.5 degrees C for 15 minutes before immersion. Each of the selected temperatures were monitored at the probe-tissue interface by a thermocouple.

RESULTS

Whereas base line end-diastolic pressure was set at = 8 mm Hg in all groups, it increased during reperfusion (mean +/- SEM) to 28+/-3, 27+/-3, 17+/-1, and 18+/-2 mm Hg in control, 22 degrees C, 37 degrees C and 42.5 degrees C-heated hearts, respectively (37 degrees C and 42.5 degrees C: p < 0.05 versus controls and 22 degrees C). Slopes of pressure-volume curves featured similar patterns. Likewise, reperfusion dP/dT (mm Hg/s(-1)) was significantly lower in control and 22 degrees C hearts (1,119+/-114 and 1,076+/-125, respectively) than in those undergoing prestorage heating to 37 degrees C and 42.5 degrees C (1,545+/-109 and 1,719+/-111, p < 0.05 and p < 0.01 versus controls and 22 degrees C, respectively). Western blot analysis of LV samples did not demonstrate any upregulation of HSP 72 in either group. Conversely, the involvement of preconditioning was evidenced by the loss of protection in the 42.5 degrees C-heated hearts when, in 2 additional groups, the storage solution was supplemented with either the protein kinase C and tyrosine kinase inhibitors chelerythrine (5 micromol/L) and genistein (50 micromol/L) or the mitochondrial K(ATP) channel inhibitor 5-hydroxydecanoate (200 micromol/L).

CONCLUSIONS

A brief period of postexplant ischemia with enhancement by topical heating ("backtable preconditioning") could be a simple and effective means of improving the functional recovery of heart transplants.

Preconditioning the Brain and Heart: Implications for Cardiac Surgery

Despite many recent advances in emboli detection, aortic imaging, myocardial preservation, and perfusion equipment, ischemic injury to the heart and brain remains a serious complications after cardiac surgery. Hypoperfusion (particularly in the heart) and microem boli (particularly in the brain) during cardiopulmonary bypass constitute the etiology of ischemia. Although hypothermia has traditionally been the mainstay for systemic protection from transient ischemia, there has been a general trend to accept warmer heart and core temperatures during bypass, which increases the poten tial for ischemic injury to various organs. This article discusses recent advances in the understanding of myocardial and brain preconditioning and their poten tial role to provide additional protection during cardiac surgery.

Ischemic preconditioning and myocardial hypothermia in rabbits with prolonged coronary artery occlusion

This study tests whether combining regional hypothermia and ischemic preconditioning (IP) provides greater myocardial protection during prolonged coronary artery occlusion (CAO) than either intervention alone, and whether increasing the duration of IP from 5 to 7 min extends the window of protection to include a 2-h CAO. Anesthetized rabbits were randomized to four groups (n = 8 rabbits/group): control (C), hypothermia alone (H), IP alone for two 7-min episodes (IP7), and IP plus hypothermia (H + IP7). To compare differences in IP for 5 versus 7 min, additional rabbits (n = 6) received one 5-min episode of ischemia (IP5). All rabbits got 2 h of CAO and 3 h of reperfusion. In comparison with the infarct size in the control group (72 +/- 4% of the risk zone), infarct size was significantly reduced in H (50 +/- 7%), IP7 (49 +/- 5%), and H + IP7 (42 +/- 6%) (all P < 0.05 vs. control group). IP5 failed to confer protection (67 +/- 5% of the risk zone). Therefore, IP can protect against a 2-h CAO if the IP regimen is increased from 5 to 7 min. The combination therapy significantly improved regional myocardial blood flow in the previously ischemic region to a greater extent than either treatment alone.

Pretreatment with a potassium-channel opener before prolonged cardiac storage: an evaluation in an experimental brain death model

BACKGROUND

Pretreatment with a potassium-channel opener has been shown to improve functional recovery after long-term cardioplegic arrest. We evaluated whether pretreatment with the potassium-channel opener cromakalim is beneficial in a more clinically relevant experimental model of brain death in the rabbit.

METHODS

Four groups of rabbits were studied in a 2 x 2 factorial experiment (n = 8 per group). Rabbits were subjected to a sham operation or 90 minutes of brain death induced by inflating a subdurally placed balloon. Thirty minutes before heart explantation, rabbits received either no pretreatment or an intravenous injection of cromakalim, 30 microg/kg. Hearts then received 5 hours' hypothermic storage in St. Thomas' Hospital solution and were assessed on a buffer-perfused isolated heart preparation. Hemodynamic recovery, coronary flow, and creatine kinase release were determined after 60 minutes of reperfusion.

RESULTS

Systolic function and diastolic function were significantly altered in hearts explanted from brain-dead rabbits compared with hearts from rabbits having a sham operation. Cromakalim pretreatment had no significant effect on poststorage systolic or diastolic function of hearts explanted from brain-dead or sham-operation rabbits. Further, cromakalim pretreatment did not affect coronary flow or overall creatine kinase release during reperfusion. CONCLUSIONS; In vivo pretreatment of brain-dead rabbits or anesthetized rabbits with an intravenous injection of cromakalim had no significant effect on functional recovery of or enzymatic release from explanted hearts after 5 hours' hypothermic storage and 60 minutes' reperfusion. These findings underscore the importance of clinically relevant experimental models.