Citations

In previous studies, we found that prolonged hypothermia (2 h or greater) followed by rewarming induces cardiac contractile dysfunction (rewarming shock) mimicking the effects of accidental hypothermia followed by rapid rewarming in trauma patients. We found that rewarming shock was also presented in isolated cardiomyocytes in which reduced contractility was attributed to PKA‐mediated TnI phosphorylation and reduced Ca 2+ sensitivity. During hypothermia rewarming in cardiomyocytes there is increased production of reactive oxidant species (ROS ‐ superoxide anion), which may trigger the intracellular mechanisms underlying reduced contractility. We hypothesized that in cardiomyocytes, hypothermia/rewarming‐induced ROS production is due to altered mitochondrial respiratory function. To test this hypothesis, isolated cardiomyocytes from 6 rats underwent the following hypothermia/rewarming protocol in which they were electrically stimulated every 2 s: experimental temperature was cooled from 35°C to 15°C during the initial 30 min, then maintained at 15°C for 2 h, followed by rewarming to 35°C in the final 30 min. Time‐matched control cardiomyocytes were also electrically stimulated every 2 s and maintained at 35°C for 3 h. After the 3‐h protocol, cardiomyocytes were placed into separate chambers of an Oroboros Oxygraph (high‐resolution respirometry) to measure O 2 consumption rate (OCR) under varying conditions. Initially, basal OCR was measured for 5 min followed by serial exposure to oligomycin (1 μM for 5 min) to block ATP synthase activity, FCCP (0.5 μM repeated 3 times for 5 min each) an H + ionophore to dissipate mitochondrial membrane potential; and antimycin‐A (0.5 μM for 5 min) to block the electron transport chain. This procedure allowed assessment of the key parameters of mitochondrial respiratory function: basal and maximal respiration (after FCCP exposure), ATP production and proton leak (after oligomycin), and reserve respiratory capacity (calculated after antimycin‐A). We found that compared to control cardiomyocytes, hypothermia/rewarming decreased key parameters of mitochondrial function, which suggests that altered mitochondrial function may have contributed to increased ROS production. In addition, decreased OCR following rewarming may contribute to contractile dysfunction by limiting ATP supply in response to metabolic demand, as well as by contributing to a decline in cell viability (e.g., triggering apoptosis). Support or Funding Information 5T32HL105355 (NSS); Norwegian Air Ambulance Foundation (YSH)

Hypothermia/Rewarming (H/R) induces myocardial dysfunction although the underlying intracellular mechanisms remain elusive. We hypothesized that in cardiomyocytes, H/R disrupts intracellular Ca 2+ ([Ca 2+ ] cyt ) homeostasis via an effect on both release and clearance of cytosolic Ca 2+ leading to a prolongation of evoked [Ca 2+ ] cyt transients associated with activation with downstream impact on contractility. To test this hypothesis, isolated rat cardiomyocytes (13–15 cells from 6 rats per group) were loaded with the Ca 2+ indicator, Fura‐2AM (0.5 μM), and exposed to H/R with or without electrical pacing at 0.5 Hz. In both groups, evoked [Ca 2+ ] cyt and contractile (sarcomere length (SL) shortening) responses were simultaneously measured at 35°C before and after hypothermia using an IonOptix system. H/R experiments were conducted as follows: experimental temperature was cooled from 35°C to 15°C in 30 min, maintained at 15°C for 2 h, and then rewarmed back to 35°C in 30 min. Time‐matched controls were implemented for 3 h at 35°C. With electrical pacing throughout hypothermia, basal [Ca 2+ ] cyt increased and the kinetics of the evoked [Ca 2+ ] cyt responses were prolonged. In addition, the extent and velocity of SL shortening was reduced in cardiomyocytes that were paced during hypothermia. In contrast, in cardiomyocytes that were not electrically paced during hypothermia, basal [Ca 2+ ] cyt was maintained and the kinetics of evoked [Ca 2+ ] cyt transients were unaffected. In addition, in the absence of pacing during hypothermia, contractility of cardiomyocytes was unaffected. These results suggest that H/R affects the release and clearance of evoked [Ca 2+ ] cyt transients (e.g., RyR mediated Ca 2+ release and SERCA mediated clearance) such that basal [Ca 2+ ] cyt increases. This effect of H/R on [Ca 2+ ] cyt homeostasis is associated with reduced cardiac contractility.

Hypothermia is defined as a core body temperature of <35°C, and as body temperature is reduced the impact on physiological processes can be beneficial or detrimental. The beneficial effect of hypothermia enables circulation of cooled experimental animals to be interrupted for 1–2 h without creating harmful effects, while tolerance of circulation arrest in normothermia is between 4 and 5 min. This striking difference has attracted so many investigators, experimental as well as clinical, to this field, and this discovery was fundamental for introducing therapeutic hypothermia in modern clinical medicine in the 1950s. Together with the introduction of cardiopulmonary bypass, therapeutic hypothermia has been the cornerstone in the development of modern cardiac surgery. Therapeutic hypothermia also has an undisputed role as a protective agent in organ transplantation and as a therapeutic adjuvant for cerebral protection in neonatal encephalopathy. However, the introduction of therapeutic hypothermia for organ protection during neurosurgical procedures or as a scavenger after brain and spinal trauma has been less successful. In general, the best neuroprotection seems to be obtained by avoiding hyperthermia in injured patients. Accidental hypothermia occurs when endogenous temperature control mechanisms are incapable of maintaining core body temperature within physiologic limits and core temperature becomes dependent on ambient temperature. During hypothermia spontaneous circulation is considerably reduced and with deep and/or prolonged cooling, circulatory failure may occur, which may limit safe survival of the cooled patient. Challenges that limit safe rewarming of accidental hypothermia patients include cardiac arrhythmias, uncontrolled bleeding, and “rewarming shock.”

Rewarming patients following profound hypothermia may result in acute heart failure and high mortality (50‐80%); however underlying pathophysiological mechanisms are largely unknown. We characterized myocardial dysfunction induced by hypothermia/rewarming (H/R) by measuring intracellular Ca 2+ concentration ([Ca 2+ ] i ) and twitch force in intact electrically stimulated left ventricular (LV) rat papillary muscles loaded with Fura‐2AM. In skinned papillary muscle fibers, we assessed maximal Ca 2+ ‐activated force (F max ) and Ca 2+ sensitivity of force. Following H/R, peak twitch force decreased by 30‐40%, but [Ca 2+ ] i was not significantly altered. In skinned preparations, maximal F max was decreased by ~30%, while the pCa required for 50% of F max (pCa 50 ) was reduced by ~0.14. Interestingly, in rewarmed papillary muscle, both total cTnI phosphorylation and protein kinase A (PKA)‐mediated cTnI phosphorylation at Ser23/24 were significantly increased compared to controls. We conclude that after H/R myocardial contractility is significantly reduced as evidenced by reduced twitch force and F max . Reduced myocardial contractility is attributed to decreased Ca 2+ sensitivity of force that likely results from increased cTnI phosphorylation.

Rewarming shock following accidental hypothermia is a potentially fatal scenario involving cardiac dysfunction. Previously our lab demonstrated that rewarming shock results in a decrease in Ca 2+ sensitivity of contraction in cardiac muscle due to an increase in cTnI phosphorylation. In this study, we hypothesized that rewarming shock initiates a cascade of intracellular events including reactive oxygen species (ROS) formation, endoplasmic reticulum (ER) stress, and mitochondrial disruption. Isolated cardiomyocytes from Sprague‐Dawley rats were exposed to a hypothermia/rewarming protocol (cooled for 1 h, held at 17 o C for 30 min, rewarmed for 1 h) that results in contractile dysfunction indicated by an increase in cytosolic Ca 2+ and reduced cell shortening measured using an IonOptix system. Rewarming shock increased ROS levels measured by an HPLC‐based dihydroethidium assay and by MitoSox fluorescence. Hypothermia/rewarming was also associated with ER stress indicated by increased expression of ER stress markers (Grp78, PERK, and CHOP) measured by Western blot. Finally, rewarming shock was associated with mitochondrial disruption evidenced by mitochondrial membrane depolarization (decreased in TMRM fluorescence) and by an increase in cytosolic cytochrome c (measured by Western blot). Our results support our overall hypothesis and suggest that these ER and mitochondrial intracellular events may underlie myocardial contractile dysfunction following hypothermia/rewarming. Supported by NIH Grant 5T32HL105355‐02 (GCS)

Diving under cold conditions may add an additional risk for bubble production and decompression sickness. Additionally, there is sparse information regarding hemodynamic changes occurring during a dive and its effects on the outcome of a dive. Thus, detailed recordings of cardiovascular changes at different core temperatures during diving were studied. Anesthetized rats were monitored with a pressure‐volume catheter in the left ventricle and the femoral artery. Rats were randomly assigned to be normothermic controls, to perform a dry hyberbaric dive at constant core temperature (37ºC) or a dry hyperbaric dive with a core temperature of 35ºC during decompression and the early post dive period. The rats were compressed to 600 kPa, maintained for 45 min breathing air, and decompressed linearly to the surface at a rate of 50 kPa·min −1 . Immediately after surfacing, right ventricle and pulmonary artery were insonated for bubble detection using ultrasound. Compression increased mean arterial pressure, pulse pressure, stroke volume, cardiac output, LV end‐systolic volume, LV end‐systolic pressure, LV dp/dt max, and total peripheral resistance in the rats. Most of these changes gradually declined and returned to baseline after decompression. However, rewarming to 37ºC after a 2ºC drop in core temperature caused a significant reduction in SV and CO, but this did not have an effect on bubble production and survival.

Recovery from accidental hypothermia followed by rewarming (H/R) is often unsuccessful due to heart failure. Although the causes for H/R‐induced heart failure remain elusive, studies at the cellular level suggest alterations in both cytoplasmic and mitochondrial Ca 2+ regulation. We hypothesize that H/R is associated with mitochondrial Ca 2+ overload, which leads to: 1) excessive reactive oxygen species (ROS) generation, 2) mitochondrial membrane depolarization, and 3) mitochondrial cytochrome c release. To test this hypothesis, isolated cardiomyocytes from adult male Sprague‐Dawley rats were subjected to an in vitro H/R protocol (1 h cooling to 15 o C held for 30 min followed by 1 h rewarming to 37 o C). Cytoplasmic (CM‐H 2 DCFDA fluorescence) and mitochondrial (MitoSOX fluorescence) ROS generation increased during cooling and remained elevated throughout the H/R protocol. Mitochondrial membrane potential (TMRM fluorescence) depolarized during rewarming suggesting mitochondrial permeability transition pore opening. After rewarming, cytochrome c content (Western blot) in cytoplasmic to mitochondrial fractions increased indicating release. These results suggest that Ca 2+ overload associated with H/R induces ROS generation leading to mitochondrial disruption. Grant Funding Source : Supported by NIH Grant 5T32HL105355‐02 (GCS)

New Findings What is the central question of this study? Detailed guidelines for volume replacement to counteract hypothermia‐induced intravascular fluid loss are lacking. Evidence suggests colloids might have beneficial effects compared to crystalloids. Are central haemodynamic function and level of hypothermia‐induced calcium overload, as a marker of cardiac injury, restored by fluid substitution during rewarming, and are colloids favourable to crystalloids? What is the main finding and its importance? Infusion with crystalloid or dextran during rewarming abolished post‐hypothermic cardiac dysfunction, and partially mitigated myocardial calcium overload. The effects of volume replacement to support haemodynamic function are comparable to those using potent cardio‐active drugs. These findings underline the importance of applying intravascular volume replacement to maintain euvolaemia during rewarming. AbstractPrevious research exploring pathophysiological mechanisms underlying circulatory collapse after rewarming victims of severe accidental hypothermia has documented post‐hypothermic cardiac dysfunction and hypothermia‐induced elevation of intracellular Ca2+ concentration ([Ca2+]i) in myocardial cells. The aim of the present study was to examine if maintaining euvolaemia during rewarming mitigates cardiac dysfunction and/or normalizes elevated myocardial [Ca2+]i. A total of 21 male Wistar rats (300 g) were surface cooled to 15°C, then maintained at 15°C for 4 h, and subsequently rewarmed to 37°C. The rats were randomly assigned to one of three groups: (1) non‐intervention control (n = 7), (2) dextran treated (i.v. 12 ml/kg dextran 70; n = 7), or (3) crystalloid treated (24 ml/kg 0.9% i.v. saline; n = 7). Infusions occurred during the first 30 min of rewarming. Arterial blood pressure, stroke volume (SV), cardiac output (CO), contractility (dP/dtmax) and blood gas changes were measured. Post‐hypothermic changes in [Ca2+]i were measured using the method of radiolabelled Ca2+ (45Ca2+). Untreated controls displayed post‐hypothermic cardiac dysfunction with significantly reduced CO, SV and dP/dtmax. In contrast, rats receiving crystalloid or dextran treatment showed a return to pre‐hypothermic control levels of CO and SV after rewarming, with the dextran group displaying significantly better amelioration of post‐hypothermic cardiac dysfunction than the crystalloid group. Compared to the post‐hypothermic increase in myocardial [Ca2+]i in non‐treated controls, [Ca2+]i values with crystalloid and dextran did not increase to the same extent after rewarming. Volume replacement with crystalloid or dextran during rewarming abolishes post‐hypothermic cardiac dysfunction, and partially mitigates the hypothermia‐induced elevation of [Ca2+]i.