ß-adrenergic stimulation after rewarming does not mitigate hypothermia-induced contractile dysfunction in rat cardiomyocytes.

Victims of severe accidental hypothermia are frequently treated with catecholamines to counteract the hemodynamic instability associated with hypothermia-induced cardiac contractile dysfunction. However, we previously reported that the inotropic effects of epinephrine are diminished after hypothermia and rewarming (H/R) in an intact animal model. Thus, the goal of this study was to investigate the effects of Epi treatment on excitation-contraction coupling in isolated rat cardiomyocytes after H/R. In adult male rats, cardiomyocytes isolated from the left ventricle were electrically stimulated at 0.5 Hz and evoked cytosolic [Ca2+] and contractile responses (sarcomere length shortening) were measured. In initial experiments, the effects of varying concentrations of epinephrine on evoked cytosolic [Ca2+] and contractile responses at 37 °C were measured. In a second series of experiments, cardiomyocytes were cooled from 37 °C to 15 °C, maintained at 15 °C for 2 h, then rewarmed to 37 °C (H/R protocol). Immediately after rewarming, the effects of epinephrine treatment on evoked cytosolic [Ca2+] and contractile responses of cardiomyocytes were determined. At 37 °C, epinephrine treatment increased both cytosolic [Ca2+] and contractile responses of cardiomyocytes in a concentration-dependent manner peaking at 25-50 nM. The evoked contractile response of cardiomyocytes after H/R was reduced while the cytosolic [Ca2+] response was slightly elevated. The diminished contractile response of cardiomyocytes after H/R was not mitigated by epinephrine (25 nM) and epinephrine treatment reduced the exponential time decay constant (Tau), but did not increase the cytosolic [Ca2+] response. We conclude that epinephrine treatment does not mitigate H/R-induced contractile dysfunction in cardiomyocytes.

Autoregulation of Cerebral Blood Flow During 3-h Continuous Cardiopulmonary Resuscitation at 27°C.

Introduction: Victims of accidental hypothermia in hypothermic cardiac arrest (HCA) may survive with favorable neurologic outcome if early and continuous prehospital cardiopulmonary resuscitation (CPR) is started and continued during evacuation and transport. The efficacy of cerebral autoregulation during hypothermic CPR is largely unknown and is aim of the present experiment. Methods: Anesthetized pigs (n = 8) were surface cooled to HCA at 27°C before 3 h continuous CPR. Central hemodynamics, cerebral O2 delivery (DO2) and uptake (VO2), cerebral blood flow (CBF), and cerebral perfusion pressure (CPP) were determined before cooling, at 32°C and at 27°C, then at 15 min after the start of CPR, and hourly thereafter. To estimate cerebral autoregulation, the static autoregulatory index (sARI), and the CBF/VO2 ratio were determined. Results: After the initial 15-min period of CPR at 27°C, cardiac output (CO) and mean arterial pressure (MAP) were reduced significantly when compared to corresponding values during spontaneous circulation at 27°C (-66.7% and -44.4%, respectively), and remained reduced during the subsequent 3-h period of CPR. During the first 2-h period of CPR at 27°C, blood flow in five different brain areas remained unchanged when compared to the level during spontaneous circulation at 27°C, but after 3 h of CPR blood flow in 2 of the 5 areas was significantly reduced. Cooling to 27°C reduced cerebral DO2 by 67.3% and VO2 by 84.4%. Cerebral VO2 was significantly reduced first after 3 h of CPR. Cerebral DO2 remained unaltered compared to corresponding levels measured during spontaneous circulation at 27°C. Cerebral autoregulation was preserved (sARI > 0.4), at least during the first 2 h of CPR. Interestingly, the CBF/VO2 ratio during spontaneous circulation at 27°C indicated the presence of an affluent cerebral DO2, whereas after CPR, the CBF/VO2 ratio returned to the level of spontaneous circulation at 38°C. Conclusion: Despite a reduced CO, continuous CPR for 3 h at 27°C provided sufficient cerebral DO2 to maintain aerobic metabolism and to preserve cerebral autoregulation during the first 2-h period of CPR. This new information supports early start and continued CPR in accidental hypothermia patients during rescue and transportation for in hospital rewarming.

Enhanced Blood Clotting After Rewarming From Experimental Hypothermia in an Intact Porcine Model.

Introduction: Due to functional alterations of blood platelets and coagulation enzymes at low temperatures, excessive bleeding is a well-recognized complication in victims of accidental hypothermia and may present a great clinical challenge. Still, it remains largely unknown if hemostatic function normalizes upon rewarming. The aim of this study was to investigate effects of hypothermia and rewarming on blood coagulation in an intact porcine model. Methods: The animals were randomized to cooling and rewarming (n = 10), or to serve as normothermic, time-matched controls (n = 3). Animals in the hypothermic group were immersion cooled in ice water to 25°C, maintained at 25°C for 1 h, and rewarmed to 38°C (normal temperature in pigs) using warm water. Clotting time was assessed indirectly at different temperatures during cooling and rewarming using a whole blood coagulometer, which measures clotting time at 38°C. Results: Cooling to 25°C led to a significant increase in hemoglobin, hematocrit and red blood cell count, which persisted throughout rewarming. Cooling also caused a transiently decreased white blood cell count that returned to baseline levels upon rewarming. After rewarming from hypothermia, clotting time was significantly shortened compared to pre-hypothermic baseline values. In addition, platelet count was significantly increased. Discussion/Conclusion: We found that clotting time was significantly reduced after rewarming from hypothermia. This may indicate that rewarming from severe hypothermia induces a hypercoagulable state, in which thrombus formation is more likely to occur.

Rewarming With Closed Thoracic Lavage Following 3-h CPR at 27°C Failed to Reestablish a Perfusing Rhythm.

Introduction: Previously, we showed that the cardiopulmonary resuscitation (CPR) for hypothermic cardiac arrest (HCA) maintained cardiac output (CO) and mean arterial pressure (MAP) to the same reduced level during normothermia (38°C) vs. hypothermia (27°C). In addition, at 27°C, the CPR for 3-h provided global O2 delivery (DO2) to support aerobic metabolism. The present study investigated if rewarming with closed thoracic lavage induces a perfusing rhythm after 3-h continuous CPR at 27°C. Materials and Methods: Eight male pigs were anesthetized, and immersion-cooled. At 27°C, HCA was electrically induced, CPR was started and continued for a 3-h period. Thereafter, the animals were rewarmed by combining closed thoracic lavage and continued CPR. Organ blood flow was measured using microspheres. Results: After cooling with spontaneous circulation to 27°C, MAP and CO were initially reduced by 37 and 58% from baseline, respectively. By 15 min after the onset of CPR, MAP, and CO were further reduced by 58 and 77% from baseline, respectively, which remained unchanged throughout the rest of the 3-h period of CPR. During CPR at 27°C, DO2 and O2 extraction rate (VO2) fell to critically low levels, but the simultaneous small increase in lactate and a modest reduction in pH, indicated the presence of maintained aerobic metabolism. During rewarming with closed thoracic lavage, all animals displayed ventricular fibrillation, but only one animal could be electro-converted to restore a short-lived perfusing rhythm. Rewarming ended in circulatory collapse in all the animals at 38°C. Conclusion: The CPR for 3-h at 27°C managed to sustain lower levels of CO and MAP sufficient to support global DO2. Rewarming accidental hypothermia patients following prolonged CPR for HCA with closed thoracic lavage is not an alternative to rewarming by extra-corporeal life support as these patients are often in need of massive cardio-pulmonary support during as well as after rewarming.

Effects of rewarming with extracorporeal membrane oxygenation to restore oxygen transport and organ blood flow after hypothermic cardiac arrest in a porcine model.

We recently documented that cardiopulmonary resuscitation (CPR) generates the same level of cardiac output (CO) and mean arterial pressure (MAP) during both normothermia (38 °C) and hypothermia (27 °C). Furthermore, continuous CPR at 27 °C provides O2 delivery (DO2) to support aerobic metabolism throughout a 3-h period. The aim of the present study was to investigate the effects of extracorporeal membrane oxygenation (ECMO) rewarming to restore DO2 and organ blood flow after prolonged hypothermic cardiac arrest. Eight male pigs were anesthetized and immersion cooled to 27 °C. After induction of hypothermic cardiac arrest, CPR was started and continued for a 3-h period. Thereafter, the animals were rewarmed with ECMO. Organ blood flow was measured using microspheres. After cooling with spontaneous circulation to 27 °C, MAP and CO were initially reduced to 66 and 44% of baseline, respectively. By 15 min after the onset of CPR, there was a further reduction in MAP and CO to 42 and 25% of baseline, respectively, which remained unchanged throughout the rest of 3-h CPR. During CPR, DO2 and O2 uptake (V̇O2) fell to critical low levels, but the simultaneous small increase in lactate and a modest reduction in pH, indicated the presence of maintained aerobic metabolism. Rewarming with ECMO restored MAP, CO, DO2, and blood flow to the heart and to parts of the brain, whereas flow to kidneys, stomach, liver and spleen remained significantly reduced. CPR for 3-h at 27 °C with sustained lower levels of CO and MAP maintained aerobic metabolism sufficient to support DO2. Rewarming with ECMO restores blood flow to the heart and brain, and creates a "shockable" cardiac rhythm. Thus, like continuous CPR, ECMO rewarming plays a crucial role in "the chain of survival" when resuscitating victims of hypothermic cardiac arrest.

Maintaining intravenous volume mitigates hypothermia-induced myocardial dysfunction and accumulation of intracellular Ca2.

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. ABSTRACT: Previous 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+ (45 Ca2+ ). 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 .

Study of the Effects of 3 h of Continuous Cardiopulmonary Resuscitation at 27°C on Global Oxygen Transport and Organ Blood Flow.

AIMS: Complete restitution of neurologic function after 6 h of pre-hospital resuscitation and in-hospital rewarming has been reported in accidental hypothermia patients with cardiac arrest (CA). However, the level of restitution of circulatory function during long-lasting hypothermic cardiopulmonary resuscitation (CPR) remains largely unknown. We compared the effects of CPR in replacing spontaneous circulation during 3 h at 27°C vs. 45 min at normothermia by determining hemodynamics, global oxygen transport (DO2), oxygen uptake (VO2), and organ blood flow. METHODS: Anesthetized pigs (n = 7) were immersion cooled to CA at 27°C. Predetermined variables were compared: (1) Before cooling, during cooling to 27°C with spontaneous circulation, after CA and subsequent continuous CPR (n = 7), vs. (2) before CA and during 45 min CPR in normothermic pigs (n = 4). RESULTS: When compared to corresponding values during spontaneous circulation at 38°C: (1) After 15 min of CPR at 27°C, cardiac output (CO) was reduced by 74%, mean arterial pressure (MAP) by 63%, DO2 by 47%, but organ blood flow was unaltered. Continuous CPR for 3 h maintained these variables largely unaltered except for significant reduction in blood flow to the heart and brain after 3 h, to the kidneys after 1 h, to the liver after 2 h, and to the stomach and small intestine after 3 h. (2) After normothermic CPR for 15 min, CO was reduced by 71%, MAP by 54%, and DO2 by 63%. After 45 min, hemodynamic function had deteriorated significantly, organ blood flow was undetectable, serum lactate increased by a factor of 12, and mixed venous O2 content was reduced to 18%. CONCLUSION: The level to which CPR can replace CO and MAP during spontaneous circulation at normothermia was not affected by reduction in core temperature in our setting. Compared to spontaneous circulation at normothermia, 3 h of continuous resuscitation at 27°C provided limited but sufficient O2 delivery to maintain aerobic metabolism. This fundamental new knowledge is important in that it encourages early and continuous CPR in accidental hypothermia victims during evacuation and transport.

Extracorporeal rewarming from experimental hypothermia: Effects of hydroxyethyl starch versus saline priming on fluid balance and blood flow distribution.

NEW FINDINGS: What is the central question of this study? Mortality in accidental hypothermia patients rewarmed by extracorporeal circulation remains high. Knowledge concerning optimal fluid additions for extracorporeal rewarming is lacking, with no apparent consensus. Does colloid versus crystalloid priming have different effects on fluid balance and blood flow distribution during extracorporeal rewarming? What is the main finding and its importance? In our rat model of extracorporeal rewarming from hypothermic cardiac arrest, hydroxyethyl starch generates less tissue oedema and increases circulating blood volume and organ blood flow, compared with saline. The composition of fluid additions appears to be important during extracorporeal rewarming from hypothermia. ABSTRACT: Rewarming by extracorporeal circulation (ECC) is the recommended treatment for accidental hypothermia patients with cardiac instability. Hypothermia, along with initiation of ECC, introduces major changes in fluid homeostasis and blood flow. Scientific data to recommend best practice use of ECC for rewarming these patients is lacking, and no current guidelines exist concerning the choice of priming fluid for the extracorporeal circuit. The primary aim of this study was to compare the effects of different fluid protocols on fluid balance and blood flow distribution during rewarming from deep hypothermic cardiac arrest. Sixteen anaesthetized rats were cooled to deep hypothermic cardiac arrest and rewarmed by ECC. During cooling, rats were equally randomized into two groups: an extracorporeal circuit primed with saline or primed with hydroxyethyl starch (HES). Calculations of plasma volume (PV), circulating blood volume (CBV), organ blood flow, total tissue water content, global O2 delivery and consumption were made. During and after rewarming, the pump flow rate, mean arterial pressure, PV and CBV were significantly higher in HES-treated compared with saline-treated rats. After rewarming, the HES group had significantly increased global O2 delivery and blood flow to the brain and kidneys compared with the saline group. Rats in the saline group demonstrated a significantly higher total tissue water content in the kidneys, skeletal muscle and lung. Compared with crystalloid priming, the use of an iso-oncotic colloid prime generates less tissue oedema and increases PV, CBV and organ blood flow during ECC rewarming. The composition of fluid additions appears to be an important factor during ECC rewarming from hypothermia.

Organ blood flow and O2 transport during hypothermia (27°C) and rewarming in a pig model.

NEW FINDINGS: What is the central question of this study? Absence of hypothermia-induced cardiac arrest is a strong predictor for a favourable outcome after rewarming. Nevertheless, detailed knowledge of preferences in organ blood flow during rewarming with spontaneous circulation is largely unknown. What is the main finding and its importance? In a porcine model of accidental hypothermia, we find, despite a significantly reduced cardiac output during rewarming, normal blood flow and O2 supply in vital organs owing to patency of adequate physiological compensatory responses. In critical care medicine, active rewarming must aim at supporting the spontaneous circulation and maintaining spontaneous autonomous vascular control. ABSTRACT: The absence of hypothermia-induced cardiac arrest is one of the strongest predictors for a favourable outcome after rewarming from accidental hypothermia. We studied temperature-dependent changes in organ blood flow and O2 delivery ( D O 2 ) in a porcine model with spontaneous circulation during 3 h of hypothermia at 27°C followed by rewarming. Anaesthetized pigs (n = 16, weighing 20-29 kg) were randomly assigned to one of two groups: (i) hypothermia/rewarming (n = 10), immersion cooled to 27°C and maintained for 3 h before being rewarmed by pleural lavage; and (ii) time-matched normothermic (38°C) control animals (n = 6), immersed for 6.5 h, the last 2 h with pleural lavage. Regional blood flow was measured using a neutron-labelled microsphere technique. Simultaneous measurements of D O 2 and O2 consumption ( V ̇ O 2 ) were made. During hypothermia, there was a reduction in organ blood flow, V ̇ O 2 and D O 2 . After rewarming, there was a 40% reduction in stroke volume and cardiac output, causing a global reduction in D O 2 ; nevertheless, blood flow to the brain, heart, stomach and small intestine returned to prehypothermic values. Blood flow in the liver and kidneys was significantly reduced. Cerebral D O 2 and V ̇ O 2 returned to control values. After hypothermia and rewarming there is a significant lowering of D O 2 owing to heart failure. However, compensatory mechanisms preserve O2 transport, blood flow and V ̇ O 2 in most organs. Nevertheless, these results indicate that hypothermia-induced heart failure requires therapeutic intervention.

Negative inotropic effects of epinephrine in the presence of increased ß-adrenoceptor sensitivity during hypothermia in a rat model.

BACKGROUND: Animal studies show reduced inotropic effects of cardiac ß-adrenoceptor agonists like epinephrine (Epi) during hypothermia and rewarming, while drugs targeting other pharmacological mechanisms have positive effects. This study therefore aimed to determine ß-adrenoceptor sensitivity in isolated cardiomyocytes and investigate hemodynamic effects of Epi and its ability to stimulate cardiac ß-adrenoceptors at different temperatures in vivo. METHODS: Isolated rat myocardial cells were incubated with the radioactive ß-adrenoceptor ligand [(3)H]-CGP12177 and propranolol, used as a displacer. Cells were subjected to normothermia (37 °C) or hypothermia (15 °C). After incubation, radioactivity was measured to estimate ß-adrenoceptor affinity for propranolol (IC50), as a measure of ß-adrenoceptor sensitivity. In separate in vivo experiments, Epi (1.25 µg/min) was administered the last 5min of experiments in normothermic (37 °C, 5h), hypothermic (4h at 15 °C) and rewarmed rats (4h at 15 °C, and subsequently rewarmed to 37 °C). Hemodynamic parameters were monitored during infusion. Hearts were thereafter freeze-clamped and tissue cAMP was measured. RESULTS: In vitro measurements of IC50 for propranolol showed a hypothermia-induced increase in ß-adrenoceptor sensitivity at 15 °C. Corresponding in vivo experiments at 15 °C showed decreased cardiac output and stroke volume, whereas total peripheral resistance (TPR) increased during Epi infusion, simultaneous with a 4-fold cAMP increase. CONCLUSIONS: This experiment shows a hypothermia-induced in vivo and in vitro increase of cardiac ß-adrenoceptor sensitivity, and simultaneous lack of inotropic effects of Epi in the presence of increased TPR. Our findings therefore indicate that hypothermia-induced reduction in inotropic effects of Epi is due to substantial elevation of TPR, rather than ß-adrenoceptor dysfunction.