Mechanisms underlying hypothermia-induced cardiac contractile dysfunction

β-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.

The effects of nickel chloride on papillary muscle contractility under normothermic and hypothermic conditions: Comparison of active and hibernating ground squirrels (Urocitellus undulatus) with Wistar rats

Extracellular Ca2+ plays a pivotal role in the regulation of cardiac contractility under normal and extreme conditions. Here, by using nickel chloride (NiCl2), a non-specific blocker of extracellular Ca2+ influx, we studied the input of extracellular Ca2+ on the regulation of papillary muscle (PM) contractility under normal and hypothermic conditions in ground squirrels (GS), and rats. By measuring isometric force of contraction, we studied how NiCl2 affects force-frequency relationship and the rest effect in PM of these species at 30 °C and 10 °C. We found that at 30 °C 1.5 mM NiCl2 significantly reduced force of contraction across entire frequency range in active GS and rats, whereas in hibernating GS force of contraction was reduced at low and high frequency range. Additionally, NiCl2 evoked spontaneous contractility in rats but not GS PM. The rest effect was significantly reduced by NiCl2 for active GS and rats but not hibernating GS. At 10 °C, NiCl2 fully reduced contractility in active GS and, to a lesser extent, in rats, whereas in hibernating GS it was significant only at 0.3 Hz. The rest effect was significantly reduced by NiCl2 in both active and hibernating GS, whereas it was unmasked in rats that had high contractility under hypothermic conditions in control. Our results show a significant contribution of extracellular Ca2+ to myocardial contractility in GS not only in active but also in hibernating states, especially under hypothermic conditions, whereas limitation of extracellular Ca2+ influx in rats under hypothermia can play protective role for myocardial contractility.

Moderate but not severe hypothermia increases intracellular cyclic AMP through preserved production and reduced elimination

Rewarming from accidental hypothermia could be complicated by acute cardiac dysfunction but providing supportive pharmacotherapy at low core temperatures is challenging. Several pharmacological strategies aim to improve cardiovascular function by increasing cAMP in cardiomyocytes as well as cAMP and cGMP levels in vascular smooth muscle, but it is not clear what effects temperature has on cellular elimination of cAMP and cGMP. We therefore studied the effects of differential temperatures from normothermia to deep hypothermia (37 °C-20 °C) on cAMP levels in embryonic H9c2 cardiac cells and elimination of cAMP and cGMP by PDE-enzymes and ABC-transporter proteins. Our experiments showed significant elevation of intracellular cAMP in H9c2-cells at 30 °C but not 20 °C. Elimination of both cAMP and cGMP through ABC transport-proteins and PDE-enzymes showed a temperature dependent reduction. Accordingly, the increased cardiomyocyte cAMP-levels during moderate hypothermia appears an effect of preserved production and reduced elimination at 30 °C. This correlates with earlier in vivo findings of a positive inotropic effect of moderate hypothermia.

Functional recovery after accidental deep hypothermic cardiac arrest: Comparison of different cardiopulmonary bypass rewarming strategies

Introduction: Using a porcine model of accidental immersion hypothermia and hypothermic cardiac arrest (HCA), the aim of the present study was to compare effects of different rewarming strategies on CPB on need for vascular fluid supply, level of cardiac restitution, and cerebral metabolism and pressures.

Materials and Methods: Totally sixteen healthy, anesthetized castrated male pigs were immersion cooled to 20°C to induce HCA, maintained for 75 min and then randomized into two groups: 1) animals receiving CPB rewarming to 30°C followed by immersion rewarming to 36°C (CPB₃₀, n = 8), or 2) animals receiving CPB rewarming to 36°C (CPB₃₆, n = 8). Measurements of cerebral metabolism were collected using a microdialysis catheter. After rewarming to 36°C, surviving animals in both groups were further warmed by immersion to 38°C and observed for 2 h.

Results: Survival rate at 2 h after rewarming was 5 out of 8 animals in the CPB₃₀ group, and 8 out of 8 in the CPB₃₆ group. All surviving animals displayed significant acute cardiac dysfunction irrespective of rewarming method. Differences between groups in CPB exposure time or rewarming rate created no differences in need for vascular volume supply, in variables of cerebral metabolism, or in cerebral pressures and blood flow.

Conclusion: As 3 out of 8 animals did not survive weaning from CPB at 30°C, early weaning gave no advantages over weaning at 36°C. Further, in surviving animals, the results showed no differences between groups in the need for vascular volume replacement, nor any differences in cerebral blood flow or pressures. Most prominent, after weaning from CPB, was the existence of acute cardiac failure which was responsible for the inability to create an adequate perfusion irrespective of rewarming strategy.

Treatment of Cardiovascular Dysfunction with PDE3-Inhibitors in Moderate and Severe Hypothermia—Effects on Cellular Elimination of Cyclic Adenosine Monophosphate and Cyclic Guanosine Monophosphate

Introduction: Rewarming from accidental hypothermia is often complicated by hypothermia-induced cardiovascular dysfunction, which could lead to shock. Current guidelines do not recommend any pharmacological treatment at core temperatures below 30°C, due to lack of knowledge. However, previous in vivo studies have shown promising results when using phosphodiesterase 3 (PDE3) inhibitors, which possess the combined effects of supporting cardiac function and alleviating the peripheral vascular resistance through changes in cyclic nucleotide levels. This study therefore aims to investigate whether PDE3 inhibitors milrinone, amrinone, and levosimendan are able to modulate cyclic nucleotide regulation in hypothermic settings.

Materials and methods: The effect of PDE3 inhibitors were studied by using recombinant phosphodiesterase enzymes and inverted erythrocyte membranes at six different temperatures—37°C, 34°C, 32°C, 28°C, 24°C, and 20°C- in order to evaluate the degree of enzymatic degradation, as well as measuring cellular efflux of both cAMP and cGMP. The resulting dose-response curves at every temperature were used to calculate IC₅₀ and Ki values.

Results: Milrinone IC₅₀ and Ki values for cGMP efflux were significantly lower at 24°C (IC₅₀: 8.62 ± 2.69 µM) and 20°C (IC₅₀: 7.35 ± 3.51 µM), compared to 37°C (IC₅₀: 22.84 ± 1.52 µM). There were no significant changes in IC₅₀ and Ki values for enzymatic breakdown of cAMP and cGMP.

Conclusion: Milrinone, amrinone and levosimendan, were all able to suppress enzymatic degradation and inhibit extrusion of cGMP and cAMP below 30°C. Our results show that these drugs have preserved effect on their target molecules during hypothermia, indicating that they could provide an important treatment option for hypothermia-induced cardiac dysfunction.

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.

Cooling to Hypothermic Circulatory Arrest by Immersion vs. Cardiopulmonary Bypass (CPB): Worse Outcome After Rewarming in Immersion Cooled Pigs

Introduction

Cooling by cardiopulmonary bypass (CPB) to deep hypothermic cardiac arrest (HCA) for cardiac surgical interventions, followed by CPB-rewarming is performed on a routine basis with relatively low mortality. In contrast, victims of deep accidental hypothermia rewarmed with CPB generally have a much worse prognosis. Thus, we have developed an intact pig model to compare effects on perfusion pressures and global oxygen delivery (DO₂) during immersion cooling versus cooling by CPB. Further, we compared the effects of CPB-rewarming between groups, to restitute cardiovascular function, brain blood flow, and brain metabolism.

Materials and Methods

Total sixteen healthy, anesthetized juvenile (2–3 months) castrated male pigs were randomized in a prospective, open placebo-controlled experimental study to immersion cooling (IMM_c, n = 8), or cooling by CPB (CPB_c, n = 8). After 75 minutes of deep HCA in both groups, pigs were rewarmed by CPB. After weaning from CPB surviving animals were observed for 2 h before euthanasia.

Results

Survival rates at 2 h after completed rewarming were 4 out of 8 in the IMM_c group, and 8 out of 8 in the CPB_c group. Compared with the CPB_c-group, IMM_c animals showed significant reduction in DO₂, mean arterial pressure (MAP), cerebral perfusion pressure, and blood flow during cooling below 25°C as well as after weaning from CPB after rewarming. After rewarming, brain blood flow returned to control in CPB_c animals only, and brain micro dialysate-data showed a significantly increase in the lactate/pyruvate ratio in IMM_c vs. CPB_c animals.

Conclusion

Our data indicate that, although global O₂ consumption was independent of DO₂, regional ischemic damage may have taken place during cooling in the brain of IMM_c animals below 25°C. The need for prolonged extracorporeal membrane oxygenation (ECMO) should be considered in all victims of accidental hypothermic arrest that cannot be weaned from CPB immediately after rewarming.

Physiological Changes in Subjects Exposed to Accidental Hypothermia: An Update

Background

Accidental hypothermia (AH) is an unintended decrease in body core temperature (BCT) to below 35°C. We present an update on physiological/pathophysiological changes associated with AH and rewarming from hypothermic cardiac arrest (HCA).

Temperature Regulation and Metabolism

Triggered by falling skin temperature, Thyrotropin-Releasing Hormone (TRH) from hypothalamus induces release of Thyroid-Stimulating Hormone (TSH) and Prolactin from pituitary gland anterior lobe that stimulate thyroid generation of triiodothyronine and thyroxine (T4). The latter act together with noradrenaline to induce heat production by binding to adrenergic β3-receptors in fat cells. Exposed to cold, noradrenaline prompts degradation of triglycerides from brown adipose tissue (BAT) into free fatty acids that uncouple metabolism to heat production, rather than generating adenosine triphosphate. If BAT is lacking, AH occurs more readily.

Cardiac Output

Assuming a 7% drop in metabolism per °C, a BCT decrease of 10°C can reduce metabolism by 70% paralleled by a corresponding decline in CO. Consequently, it is possible to maintain adequate oxygen delivery provided correctly performed cardiopulmonary resuscitation (CPR), which might result in approximately 30% of CO generated at normal BCT.

Liver and Coagulation

AH promotes coagulation disturbances following trauma and acidosis by reducing coagulation and platelet functions. Mean prothrombin and partial thromboplastin times might increase by 40–60% in moderate hypothermia. Rewarming might release tissue factor from damaged tissues, that triggers disseminated intravascular coagulation. Hypothermia might inhibit platelet aggregation and coagulation.

Kidneys

Renal blood flow decreases due to vasoconstriction of afferent arterioles, electrolyte and fluid disturbances and increasing blood viscosity. Severely deranged renal function occurs particularly in the presence of rhabdomyolysis induced by severe AH combined with trauma.

Conclusion

Metabolism drops 7% per °C fall in BCT, reducing CO correspondingly. Therefore, it is possible to maintain adequate oxygen delivery after 10°C drop in BCT provided correctly performed CPR. Hypothermia may facilitate rhabdomyolysis in traumatized patients. Victims suspected of HCA should be rewarmed before being pronounced dead. Rewarming avalanche victims of HCA with serum potassium > 12 mmol/L and a burial time >30 min with no air pocket, most probably be futile.

Physiological Impact of Hypothermia: The Good, the Bad and the Ugly

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 1950's. 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".

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 (ḊO₂) 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 ḊO₂ 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, ḊO₂ 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, ḊO₂, 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 ḊO₂. 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.

Effects of TNFα on Dynamic Cytosolic Ca2 + and Force Responses to Muscarinic Stimulation in Airway Smooth Muscle

Previously, we reported that in airway smooth muscle (ASM), the cytosolic Ca²⁺ ([Ca²⁺]_cyt) and force response induced by acetyl choline (ACh) are increased by exposure to the pro-inflammatory cytokine tumor necrosis factor α (TNFα). The increase in ASM force induced by TNFα was not associated with an increase in regulatory myosin light chain (rMLC₂₀) phosphorylation but was associated with an increase in contractile protein (actin and myosin) concentration and an enhancement of Ca²⁺ dependent actin polymerization. The sensitivity of ASM force generation to elevated [Ca²⁺]_cyt (Ca²⁺ sensitivity) is dynamic involving both the shorter-term canonical calmodulin-myosin light chain kinase (MLCK) signaling cascade that regulates rMLC₂₀ phosphorylation and cross-bridge recruitment as well as the longer-term regulation of actin polymerization that regulates contractile unit recruitment and actin tethering to the cortical cytoskeleton. In this study, we simultaneously measured [Ca²⁺]_cyt and force responses to ACh and explored the impact of 24-h TNFα on the dynamic relationship between [Ca²⁺]_cyt and force responses. The temporal delay between the onset of [Ca²⁺]_cyt and force responses was not affected by TNFα. Similarly, the rates of rise of [Ca²⁺]_cyt and force responses were not affected by TNFα. The absence of an impact of TNFα on the short delay relationships between [Ca²⁺]_cyt and force was consistent with the absence of an effect of [Ca²⁺]_cyt and force on rMLC₂₀ phosphorylation. However, the integral of the phase-loop plot of [Ca²⁺]_cyt and force increased with TNFα, consistent with an impact on actin polymerization and, contractile unit recruitment and actin tethering to the cortical cytoskeleton.

Dynamic cytosolic Ca2+ and force responses to muscarinic stimulation in airway smooth muscle

During agonist stimulation of airway smooth muscle (ASM), agonists such as ACh induce a transient increase in cytosolic Ca²⁺ concentration ([Ca²⁺]_cyt), which leads to a contractile response [excitation-contraction (E-C) coupling]. Previously, the sensitivity of the contractile response of ASM to elevated [Ca²⁺]_cyt (Ca²⁺ sensitivity) was assessed as the ratio of maximum force to maximum [Ca²⁺]_cyt. However, this static assessment of Ca²⁺ sensitivity overlooks the dynamic nature of E-C coupling in ASM. In this study, we simultaneously measured [Ca²⁺]_cyt and isometric force responses to three concentrations of ACh (1, 2.6, and 10 μM). Both maximum [Ca²⁺]_cyt and maximum force responses were ACh concentration dependent, but force increased disproportionately, thereby increasing static Ca²⁺ sensitivity. The dynamic properties of E-C coupling were assessed in several ways. The temporal delay between the onset of ACh-induced [Ca²⁺]_cyt and onset force responses was not affected by ACh concentration. The rates of rise of the ACh-induced [Ca²⁺]_cyt and force responses increased with increasing ACh concentration. The integral of the phase-loop plot of [Ca²⁺]_cyt and force from onset to steady state also increased with increasing ACh concentration, whereas the rate of relaxation remained unchanged. Although these results suggest an ACh concentration-dependent increase in the rate of cross-bridge recruitment and in the rate of rise of [Ca²⁺]_cyt, the extent of regulatory myosin light-chain (rMLC₂₀) phosphorylation was not dependent on ACh concentration. We conclude that the dynamic properties of [Ca²⁺]_cyt and force responses in ASM are dependent on ACh concentration but reflect more than changes in the extent of rMLC₂₀ phosphorylation.

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/dt_max ) 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/dt_max . 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 .

Severe Aortic Thrombosis and Profound Hypothermia: A Case Report

Background

Blood clot formation is a multifactorial process and has been related many times in intensive care units. Here is presented a multiple thrombosis formation in a rewarming patient.

Case description

A 68-year-old patient was admitted to our intensive care unit after lying on the floor for an unknown time. She presented a severe hypothermia at 26° and a severe cardiogenic shock. Because she was confused and was hypoxemic, she had been intubated at her admission. After intravascular warming, we could stop sedative medications. She presented a right hemiparesis and acute left leg ischemia. Computed tomography (CT) scan revealed a constituted left Sylvian stroke and a massive clot along the aorta. She required a surgical embolectomy and fasciotomy. She died after she presented a severe bowel ischemia on the third day after her admission.

Conclusion

Relevant hypothesis for blood clot formation in this patient may include prolonged lying position or blood temperature variation. Hypothermia and rewarming responsibilities may explain multiple thrombosis development.

How to cite this article

Schmitt J, Esnault P, Sartre M, Cungi PJ, Meaudre E. Severe Aortic Thrombosis and Profound Hypothermia: A Case Report. Indian J Crit Care Med 2021;25(5):588-589.

Regional hypothermia improves gastric microcirculatory oxygenation during hemorrhage in dogs

Mild systemic hypothermia increases gastric mucosal oxygenation (μHbO₂) during hemorrhagic shock in dogs. In the context of critical blood loss hypothermia might be fatal due to adverse side effects. Selective regional hypothermia might overcome these limitations. The aim of our study was to analyze the effects of regional gastric and oral mucosal hypothermia on μHbO₂ and perfusion (μflow). In a cross-over study six anesthetized dogs were subjected to local oral and gastric mucosal hypothermia (34°C), or maintenance of local normothermia during normovolemia and hemorrhage (-20% blood volume). Macro- and microcirculatory variables were recorded continuously. During normovolemia, local hypothermia increased gastric microcirculatory flow (μflow) without affecting oxygenation (μHbO₂) or oral microcirculation. During mild hemorrhagic shock gastric μHbO₂ decreased from 72±2% to 38±3% in the normothermic group. This was attenuated by local hypothermia, where μHbO₂ was reduced from 74±3% to 52±4%. Local perfusion, oral microcirculation and macrocirculatory variables were not affected. Selective local hypothermia improves gastric μHbO₂ during hemorrhagic shock without relevant side effects. In contrast to systemic hypothermia, regional mucosal hypothermia did not affect perfusion and oxygen supply during hemorrhage. Thus, the increased μHbO₂ during local hypothermia rather indicates reduced mucosal oxygen demand.

Protocol for a multicentre randomised controlled trial evaluating the effects of moderate hypothermia versus normothermia on mortality in patients with refractory cardiogenic shock rescued by venoarterial extracorporeal membrane oxygenation (VA-ECMO) (HYPO-ECMO study)

INTRODUCTION

Venoarterial extracorporeal membrane oxygenation (VA-ECMO) is widely used to support the most severe forms of cardiogenic shock (CS). Nevertheless, despite extracorporeal membrane oxygenation (ECMO) use, mortality still remains high (50%). Moderate hypothermia (MH) (33°C-34°C) may improve cardiac performance and decrease ischaemia-reperfusion injuries. The use of MH during VA-ECMO is strongly supported by experimental and preliminary clinical data.

METHODS AND ANALYSIS

The Hypothermia-Extracorporeal Membrane Oxygenation (HYPO-ECMO) study is a multicentre, prospective, controlled randomised trial between an MH group (33°C≤T°C≤34°C) and normothermia group (36°C≤T°C≤37°C). The primary endpoint is all-cause mortality at day 30 following randomisation. The study will also assess as secondary endpoints the effects of targeted temperature management strategies on (1) mortality rate at different time points, (2) organ failure and supportive treatment use and (3) safety. All intubated adults with refractory CS supported with VA-ECMO will be screened. Exclusion criteria are patients having undergone cardiac surgery for heart transplantation or left or biventricular assist device implantation, acute poisoning with cardiotoxic drugs, pregnancy, uncontrolled bleeding and refractory cardiac arrest.Three-hundred and thirty-four patients will be randomised and followed up to 6 months to detect a 15% difference in mortality. Data analysis will be intention to treat. The differences between the two study groups in the risk of all-cause mortality at day 30 following randomisation will be studied using logistic regression analysis adjusted for postcardiotomy setting, prior cardiac arrest, prior myocardial infarction, age, vasopressor dose, Sepsis-related Organ Failure Assessment (SOFA) score and lactate at randomisation.

ETHICS AND DISSEMINATION

Ethics approval has been granted by the Comité de Protection des Personnes Est III Ethics Committee. The trial has been approved by the French Health Authorities (Agence Nationale de la Sécurité du Médicament et des Produits de Santé). Dissemination of results will be performed via journal articles and presentations at national and international conferences. Since this study is also the first step in the constitution of an 'ECMO Trials Group', its results will also be disseminated by the aforementioned group.

TRIAL REGISTRATION NUMBER

NCT02754193.

Cardiac troponin-I phosphorylation underlies myocardial contractile dysfunction induced by hypothermia rewarming

Rewarming the intact heart after a period of hypothermia is associated with reduced myocardial contractility, decreased Ca²⁺ sensitivity, and increased cardiac troponin-I (cTnI) phosphorylation. We hypothesized that hypothermia/rewarming (H/R) induces left ventricular (LV) contractile dysfunction due to phosphorylation of cTnI at Ser^23/24. To test this hypothesis, the response of wild-type mice (n = 7) to H/R was compared with transgenic (TG) mice expressing slow skeletal TnI (TG-ssTnI; n = 7) that lacks the Ser^23/24 phosphorylation sites. Hypothermia was induced by surface cooling and maintained at 23-25°C for 3 h. Subsequently, the animals were rewarmed to 37°C. LV systolic and diastolic function was assessed using a 1.4 F pressure-volume Millar catheter introduced via the right carotid artery. At baseline conditions, there were no significant differences in LV systolic function between wild-type and TG-ssTnI mice, whereas measurements of diastolic function [isovolumic relaxation constant (τ) and end-diastolic pressure-volume relationship (EDPVR)] were significantly (P < 0.05) reduced in TG-ssTnI animals. Immediately after rewarming, significant differences between groups were found in cardiac output (CO; wild-type 6.6 ± 0.7 vs. TG-ssTnI 8.8 ± 0.7 mL/min), stroke work (SW; wild-type 796 ± 112 vs. TG-ssTnI 1208 ± 67 mmHg/μL), and the preload recruited stroke work (PRSW; wild-type 38.3 ± 4.9 vs. TG-ssTnI 68.8 ± 8.2 mmHg). However, EDPVR and τ returned to control levels within 1 h in both groups. We conclude that H/R-induced LV systolic dysfunction results from phosphorylation of cTnI at Ser^23/24.NEW & NOTEWORTHY Rewarming following a period of accidental hypothermia leads to a form of acute cardiac failure (rewarming shock), which is in part due to reduced sensitivity to Ca²⁺ activation of myocardial contraction. The results of the present study support the hypothesis that rewarming shock is due to phosphorylation of cardiac troponin I.

Accidental Hypothermia Associated with Intracardiac Thrombi

Accidental hypothermia and thrombosis are rarely associated and encountered. A 66-year-old male and 62-year-old male were both admitted with accidental hypothermia. Patient 1 had a rectal temperature of 28.5 °Celcius (C). After 1 day of hospitalization, he developed worsening shortness of breath due to worsening pulmonary edema. Further investigation with echocardiogram showed large left ventricular thrombi as well and global hypokinesis and apical akinesis. Patient 2 had a rectal temperature of 28.5 °C, he was also discovered to have a multifactorial shock. Echocardiogram for shock evaluation showed small apical thrombus as well as global hypokinesis. Hypothermia has been associated with hypocoagulability rather than hypercoagulability secondary to platelet dysfunction and clotting factor enzyme derangements. Moreover, hypothermia has also been associated with myocardial dysfunction that could have predisposed the development of intracardiac thrombi. Further research needs to be done to help better understand these possible association.

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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>D</mml:mi> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:msub> </mml:math> ) 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>D</mml:mi> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:msub> </mml:math> and O₂ consumption ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub><mml:mover><mml:mi>V</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:msub> </mml:math> ) were made. During hypothermia, there was a reduction in organ blood flow, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub><mml:mover><mml:mi>V</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:msub> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>D</mml:mi> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:msub> </mml:math> . After rewarming, there was a 40% reduction in stroke volume and cardiac output, causing a global reduction in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>D</mml:mi> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:msub> </mml:math> ; 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>D</mml:mi> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:msub> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub><mml:mover><mml:mi>V</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:msub> </mml:math> returned to control values. After hypothermia and rewarming there is a significant lowering of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>D</mml:mi> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:msub> </mml:math> owing to heart failure. However, compensatory mechanisms preserve O₂ transport, blood flow and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub><mml:mover><mml:mi>V</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:msub> </mml:math> in most organs. Nevertheless, these results indicate that hypothermia-induced heart failure requires therapeutic intervention.

Discontinued stimulation of cardiomyocytes provides protection against hypothermia-rewarming-induced disruption of excitation-contraction coupling

NEW FINDINGS

What is the central question of this study? Will discontinued stimulation of isolated cardiomyocytes (asystole) during hypothermia mitigate hypothermia-rewarming-induced cytosolic Ca²⁺ overload? What is the main finding and its importance? Mimicking asystole or hypothermic cardiac arrest by discontinued stimulation of cardiomyocytes during hypothermia resulted in normal contractile function after rewarming. This result suggests that asystole during severe hypothermia provides protection from hypothermia-rewarming-induced contractile dysfunction in cardiomyocytes.

ABSTRACT

After exposure of spontaneously beating hearts or electrically stimulated isolated cardiomyocytes to hypothermia-rewarming (H/R), cardiac dysfunction or alteration in excitation-contraction coupling, respectively, is a consequence. In contrast, hypothermic cardiac arrest, as routinely applied during cardiac surgery, will not impose any hazard to cardiac function after rewarming. We hypothesize that by maintaining asystole during H/R, cardiomyocytes will avoid Ca²⁺ overload attributable to the transient stimulation-evoked elevation of [Ca²⁺ ]_i and thus, H/R-induced elevation of phosphorylated cardiac troponin I and reduced Ca²⁺ sensitivity after rewarming. To test this hypothesis, the aim of the study was to determine whether discontinued electrical stimulation (to imitate hypothermic cardiac arrest) versus stimulation during 3 h of H/R prevents disruption of excitation-contraction coupling in our established cardiomyocyte H/R model. Cytosolic Ca²⁺ and the contractile response (sarcomere length shortening) were measured using an IonOptix system, and the dynamic assessment of Ca²⁺ sensitivity of contraction was conducted using a phase-loop plot. Cardiomyocytes were divided into three groups. Group 1 (time-matched control) was continuously stimulated at 0.5 Hz for 3 h at 35°C. Group 2 was continuously stimulated during H/R at 0.5 Hz, whereas in group 3 stimulation was discontinued during H/R and thus the cells remained quiescent until the resumption of stimulation after rewarming. The results demonstrate that discontinued stimulation of cardiomyocytes during H/R, imitating hypothermic cardiac arrest during cardiac surgery, provides protection against H/R-induced disruption of excitation-contraction coupling. We suggest that protective effects are caused by preventing the protein kinase A-induced elevation of phosphorylated cardiac troponin I, which is a key mechanism to reduce myofilament Ca²⁺ sensitivity of contraction.

Role of superoxide ion formation in hypothermia/rewarming induced contractile dysfunction in cardiomyocytes

Rewarming following accidental hypothermia is associated with circulatory collapse due primarily to impaired cardiac contractile (systolic) function. Previously, we found that reduced myofilament Ca²⁺ sensitivity underlies hypothermia/rewarming (H/R)-induced cardiac contractile dysfunction. This reduced Ca²⁺ sensitivity is associated with troponin I (cTnI) phosphorylation. We hypothesize that H/R induces reactive oxygen species (ROS) formation in cardiomyocytes, which leads to cTnI phosphorylation and reduced myofilament Ca²⁺ sensitivity. To test this hypothesis, we exposed isolated rat cardiomyocytes to a 2-h period of severe hypothermia (15 °C) followed by rewarming (35 °C) with and without antioxidant (TEMPOL) treatment. Simultaneous measurements of cytosolic Ca²⁺ ([Ca²⁺]_cyto) and contractile (sarcomere shortening) responses indicated that H/R-induced contractile dysfunction and reduced Ca²⁺ sensitivity was prevented in cardiomyocytes treated with TEMPOL. In addition, TEMPOL treatment blunted H/R-induced cTnI phosphorylation. These results support our overall hypothesis and suggest that H/R disrupts excitation-contraction coupling of the myocardium through a cascade of event triggered by excessive ROS formation during hypothermia. Antioxidant treatment may improve successful rescue of accidental hypothermia victims.

Effects of hypothermia and rewarming on cardiovascular autonomic control in vivo

Rewarming from accidental hypothermia is associated with cardiovascular dysfunction that complicates rewarming and contributes to a high mortality rate. We investigated autonomic cardiovascular control, as well as the separate effects of cooling, hypothermia, and rewarming on hemodynamic function, aiming to provide knowledge of the pathophysiology causing such complications in these patients. A rat model designed for circulatory studies during cooling, hypothermia (15°C), and rewarming was used. Spectral analysis of diastolic arterial pressure and heart rate allowed assessment of the autonomic nervous system. Hemodynamic variables were monitored using a conductance catheter in the left ventricle and a pressure transducer connected to the left femoral artery. Sympathetic cardiovascular control was reduced after rewarming. Stroke volume increased during cooling but decreased during stable hypothermia and did not normalize during rewarming. Despite autonomic dysfunction, total peripheral resistance increased during cooling and did not normalize after rewarming. The present data show that sympathetic cardiovascular control is reduced by hypothermia and rewarming. A simultaneous systolic dysfunction is seen in rewarmed animals, caused by reduced filling of the left ventricle and impaired contractile function, in the presence of normal diastolic function. These findings show that dysfunction of the efferent sympathetic nervous system could be instrumental in development of rewarming shock. NEW & NOTEWORTHY The present study shows impaired autonomic control of cardiovascular function after rewarming from severe hypothermia. In victims of accidental hypothermia, rewarming shock is a much feared and lethal complication. The pathophysiology causing such cardiovascular collapse appears complex. Our findings indicate that dysfunction of the autonomic nervous system is an important part of the pathophysiology. Thus the present study gives novel information, important for further development of treatment strategies in this patient group.

The beneficial hemodynamic effects of afterload reduction by sodium nitroprusside during rewarming from experimental hypothermia

BACKGROUND

Rewarming from hypothermia is associated with depressed cardiac function, known as hypothermia-induced cardiac dysfunction (HCD), and increased systemic vascular resistance (SVR). Previous studies on pharmacological treatment of HCD have demonstrated beneficial effects when using drugs with the combined effects; cardiac inotropic support and peripheral vasodilation. The presented study aims to investigate the isolated effects of arterial dilatation on cardiac functional variables during rewarming from hypothermia using sodium nitroprusside (SNP).

METHODS

We utilized a rat model designed to induce HCD following 4 h at 15 °C and rewarming. To study effects on left ventricular (LV) functional variables in response to afterload reduction by SNP during rewarming a conductance catheter was used. Index of LV contractility, preload recruitable stroke work (PRSW), was obtained with inferior vena cava occlusions at 37 °C before and after hypothermia. Pressure signals from a catheter in the left femoral artery was used to pharmacologically adjust SVR.

RESULTS

After rewarming both animal groups showed significant reduction in both SV and CO as a manifestation of HCD. However, compared to saline controls, SV and CO in SNP-treated animals increased significantly during rewarming in response to afterload reduction displayed as reduced SVR, mean arterial- and end-systolic pressures. The cardiac contractility variable PRSW was equally reduced after rewarming in both groups.

CONCLUSION

When rewarming the present model of HCD a significant increase in SVR takes place. In this context, pharmacologic intervention aimed at reducing SVR show clear positive results on CO and SV. However, a reduction in SVR alone is not sufficient to fully alleviate CO during HCD, and indicate the need of additional inotropic support.

Altered pharmacological effects of adrenergic agonists during hypothermia

Rewarming from accidental hypothermia is often complicated by hypothermia-induced cardiac dysfunction, calling for immediate pharmacologic intervention. Studies show that although cardiac pharmacologic support is applied when rewarming these patients, a lack of updated treatment recommendations exist. Mainly due to lack of clinical and experimental data, neither of the international guidelines includes information about pharmacologic cardiac support at temperatures below 30 °C. However, core temperature of accidental hypothermia patients is often reduced below 30 °C. Few human studies exploring effects of adrenergic drugs during hypothermia have been published, and therefore prevailing information is collected from pre-clinical studies. The most prominent finding in these studies is an apparent depressive effect of adrenaline on cardiac function when used in doses which elevate cardiac output during normothermia. Also noradrenaline and isoprenaline largely lacked positive cardiac effects during hypothermia, while dopamine is a more promising drug for supporting cardiac function during rewarming. Data and information from these studies are in support of the prevailing notion; not to use adrenergic drugs at core temperatures below 30 °C.

Hypothermia/rewarming disrupts excitation-contraction coupling in cardiomyocytes

Hypothermia/rewarming (H/R) is poorly tolerated by the myocardium; however, the underlying intracellular basis of H/R-induced cardiac dysfunction remains elusive. We hypothesized that in cardiomyocytes, H/R disrupts excitation-contraction coupling by reducing myofilament Ca(2+) sensitivity due to an increase in cardiac troponin I (cTnI) phosphorylation. To test this hypothesis, isolated rat cardiomyocytes (13-15 cells from 6 rats per group) were electrically stimulated to evoke both cytosolic Ca(2+) ([Ca(2+)]cyto) and contractile (sarcomere shortening) responses that were simultaneously measured using an IonOptix system. Cardiomyocytes were divided into two groups: 1) those exposed to hypothermia (15°C for 2 h) followed by rewarming (35°C; H/R); or 2) time-matched normothermic (35°C) controls (CTL). Contractile dysfunction after H/R was indicated by reduced velocity and extent of sarcomere length (SL) shortening compared with time-matched controls. Throughout hypothermia, basal [Ca(2+)]cyto increased and the duration of evoked [Ca(2+)]cyto transients was prolonged. Phase-loop plots of [Ca(2+)]cyto vs. contraction were shifted rightward in cardiomyocytes during hypothermia compared with CTL, indicating a decrease in Ca(2+) sensitivity. Using Western blot, we found that H/R increases cTnI phosphorylation. These results support our overall hypothesis and suggest that H/R disrupts excitation-contraction coupling of cardiomyocytes due to increased cTnI phosphorylation and reduced Ca(2+) sensitivity.

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

BACKGROUND

Animal studies show reduced inotropic effects of cardiac β-adrenoceptor agonists like epinephrine (Epi) during hypothermia and rewarming, while drugs targeting other pharmacological mechanisms have positive effects. This study therefore aimed to determine β-adrenoceptor sensitivity in isolated cardiomyocytes and investigate hemodynamic effects of Epi and its ability to stimulate cardiac β-adrenoceptors at different temperatures in vivo.

METHODS

Isolated rat myocardial cells were incubated with the radioactive β-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 β-adrenoceptor affinity for propranolol (IC50), as a measure of β-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 β-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 β-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 β-adrenoceptor dysfunction.

Cardiovascular effects of levosimendan during rewarming from hypothermia in rat

BACKGROUND

Previous research aimed at ameliorating hypothermia-induced cardiac dysfunction has shown that inotropic drugs, that stimulate the cAMP, - PKA pathway via the sarcolemmal β-receptor, have a decreased inotropic effect during hypothermia. We therefore wanted to test whether levosimendan, a calcium sensitizer and dose-dependent phosphodiesterase 3 (PDE3) inhibitor, is able to elevate stroke volume during rewarming from experimental hypothermia.

METHODS

A rat model designed for circulatory studies during experimental hypothermia (4h at 15°C) and rewarming was used. The following three groups were included: (1) A normothermic group receiving levosimendan, (2) a hypothermic group receiving levosimendan the last hour of stable hypothermia and during rewarming, and (3) a hypothermic placebo control group. Hemodynamic variables were monitored using a Millar conductance catheter in the left ventricle (LV), and a pressure transducer connected to the left femoral artery. In order to investigate the level of PKA stimulation by PDE3 inhibition, myocardial Ser23/24-cTnI phosphorylation was measured using Western-blot.

RESULTS

After rewarming, stroke volume (SV), cardiac output (CO) and preload recruitable stroke work (PRSW) were restored to within pre-hypothermic values in the levosimendan-treated animals. Compared to the placebo group after rewarming, SV, CO, PRSW, as well as levels of Ser23/24-cTnI phosphorylation, were significantly higher in the levosimendan-treated animals.

CONCLUSION

The present data shows that levosimendan ameliorates hypothermia-induced systolic dysfunction by elevating SV during rewarming from 15°C. Inotropic treatment during rewarming from hypothermia in the present rat model is therefore better achieved through calcium sensitizing and PDE3 inhibition, than β-receptor stimulation.

The effect of deep hypothermia on the human pulmonary circulation

OBJECTIVE

Acute rises in pulmonary artery pressures following complex cardiac surgery are associated with high morbidity and mortality. We hypothesised that periods of deep hypothermia predispose to elevated pulmonary pressures upon rewarming. We investigated the effect of this hypothermic preconditioning on isolated human pulmonary arteries and isolated perfused lungs.

METHODS

Isometric tension was measured in human pulmonary artery rings (n=24). We assessed the constriction and dilation of these arteries at 37 °C and 17 °C. Isolated perfused human lung models consisted of lobes ventilated via a bronchial cannula and perfused with Krebs via a pulmonary artery cannula. Bronchial and pulmonary artery pressures were recorded. We investigated the effect of temperature using a heat exchanger.

RESULTS

Rewarming from 17 °C to 37 °C caused a 1.3 fold increase in resting tension (p<0.05). Arteries constricted 8.6 times greater to 30 nM KCl, constricted 17 times greater to 1 nM Endothelin-1 and dilated 30.3 times greater to 100 μM SNP at 37 °C than at 17 °C (p<0.005). No difference was observed in the responses of arteries originally maintained at 37 °C compared to those arteries maintained at 17 °C and rewarmed to 37 °C. Hypothermia blunted the increase in pulmonary artery pressures to stimulants such as potassium chloride as well as to H-R but did not precondition arteries to higher pulmonary artery pressures upon re-warming.

CONCLUSIONS

Deep hypothermia reduces the responsiveness of human pulmonary arteries but does not, however, precondition an exaggerated response to vasoactive agents upon re-warming.

Role of calcium desensitization in the treatment of myocardial dysfunction after deep hypothermic circulatory arrest

Introduction

Rewarming from deep hypothermic circulatory arrest (DHCA) produces calcium desensitization by troponin I (cTnI) phosphorylation which results in myocardial dysfunction. This study investigated the acute overall hemodynamic and metabolic effects of epinephrine and levosimendan, a calcium sensitizer, on myocardial function after rewarming from DHCA.

Methods

Forty male Wistar rats (400 to 500 g) underwent cardiopulmonary bypass (CPB) through central cannulation and were cooled to a core temperature of 13°C to 15°C within 30 minutes. After DHCA (20 minutes) and CPB-assisted rewarming (60 minutes) rats were randomly assigned to 60 minute intravenous infusion with levosimendan (0.2 μg/kg/min; n = 15), epinephrine (0.1 μg/kg/min; n = 15) or saline (control; n = 10). Systolic and diastolic functions were evaluated at different preloads with a conductance catheter.

Results

The slope of left ventricular end-systolic pressure volume relationship (Ees) and preload recruitable stroke work (PRSW) recovered significantly better with levosimendan compared to epinephrine (Ees: 85 ± 9% vs 51 ± 11%, P<0.003 and PRSW: 78 ± 5% vs 48 ± 8%, P<0.005; baseline: 100%). Levosimendan but not epinephrine reduced left ventricular stiffness shown by the end-diastolic pressure-volume relationship and improved ventricular relaxation (Tau). Levosimendan preserved ATP myocardial content as well as energy charge and reduced plasma lactate concentrations. In normothermia experiments epinephrine in contrast to Levosimendan increased cTnI phosphorylation 3.5-fold. After rewarming from DHCA, cTnI phosphorylation increased 4.5-fold in the saline and epinephrine group compared to normothermia but remained unchanged with levosimendan.

Conclusions

Levosimendan due to prevention of calcium desensitization by cTnI phosphorylation is more effective than epinephrine for treatment of myocardial dysfunction after rewarming from DHCA.

Evaluation of a novel cryoablation system: in vitro testing of heat capacity and freezing temperatures

OBJECTIVE

Cryoablation has been used to ablate cardiac tissue for decades and has been shown to be able to replace incisions in the surgical treatment of atrial fibrillation. This in vitro study evaluates the performance of a novel cryoprobe and compares it with existing commercially available devices.

METHODS

A new malleable 10-cm aluminum cryoprobe was compared with a rigid 3.5-cm copper linear probe using in vitro testing to evaluate performances under different thermal loads and with different tissue thicknesses. Radial dimensions of ice formation were measured in each water bath by a high-precision laser 2 minutes after the onset of cooling. Probe-surface temperatures were recorded by thermocouples. Tissue temperature was measured at depths of 4 mm and 5 mm from the probe-tissue interface. Time to reach a tissue temperature of -20°C was recorded.

RESULTS

Ice formation increased significantly with lower water-bath temperatures (P < 0.001). Width and depth of ice formation were significantly less for the rigid linear probe (P < 0.012 and P < 0.001, respectively). There was no difference between the probes in the maximal negative temperature reached under different thermal loads or at different tissue depths. The malleable probe achieved significantly lower temperatures at the proximal compared with the distal end (-61.7°C vs -55.0°C, respectively; P < 0.001). A tissue temperature of -20°C was reached earlier at 4 mm than at 5 mm (P < 0.001) and was achieved significantly faster with the 3011 Maze Linear probe (P < 0.021).

CONCLUSIONS

The new malleable probe achieved rapid freezing to clinically relevant levels in up to 5-mm-thick tissue. Both probes maintained their performance under a wide range of thermal loads.

Evaluation of a Novel Cryoablation System: In vitro Testing of Heat Capacity and Freezing Temperatures

Objective

Cryoablation has been used to ablate cardiac tissue for decades and has been shown to be able to replace incisions in the surgical treatment of atrial fibrillation. This in vitro study evaluates the performance of a novel cryoprobe and compares it with existing commercially available devices.

Methods

A new malleable 10-cm aluminum cryoprobe was compared with a rigid 3.5-cm copper linear probe using in vitro testing to evaluate performances under different thermal loads and with different tissue thicknesses. Radial dimensions of ice formation were measured in each water bath by a high-precision laser 2 minutes after the onset of cooling. Probe-surface temperatures were recorded by thermocouples. Tissue temperature was measured at depths of 4 mm and 5 mm from the probe-tissue interface. Time to reach a tissue temperature of −20°C was recorded.

Results

Ice formation increased significantly with lower water-bath temperatures (P < 0.001). Width and depth of ice formation were significantly less for the rigid linear probe (P < 0.012 and P < 0.001, respectively). There was no difference between the probes in the maximal negative temperature reached under different thermal loads or at different tissue depths. The malleable probe achieved significantly lower temperatures at the proximal compared with the distal end (–61.7°C vs −55.0°C, respectively; P < 0.001). A tissue temperature of −20°C was reached earlier at 4 mm than at 5 mm (P < 0.001) and was achieved significantly faster with the 3011 Maze Linear probe (P < 0.021).

Conclusions

The new malleable probe achieved rapid freezing to clinically relevant levels in up to 5-mm–thick tissue. Both probes maintained their performance under a wide range of thermal loads.

Force relaxation and thin filament protein phosphorylation during acute myocardial ischemia

Ischemia impairs myocardial function and may contribute to the progression of heart failure. In this study, rats subjected to acute ischemia demonstrated reduced Ca(2+) -activated force as well as a decrease in myosin-binding protein-C, titin, and Ser23/24 phosphorylation of troponin I (TnI). All three proteins have been demonstrated to be downstream targets of β-adrenergic receptor activation (β-AR), leading to the hypothesis that decreased β-AR signaling during ischemia leads to reduced protein phosphorylation and reduced rate constants of force relaxation. To test this hypothesis, force relaxation transients were recorded from permeabilized perfused and ischemic rat heart fibers following photolysis of the caged chelator diazo-2. Relaxation transients were best fit by double exponential functions whereby the majority (>70%) of the force decline was described by the fast rate constant, which was ∼5 times faster than the slow rate constant. However, rate constants of relaxation between perfused and ischemic fibers were not different, despite significant decreases in sarcomeric protein phosphorylation in ischemic fibers. Treatment of perfused fibers with a cAMP analog increased Ser23/24 phosphorylation of TnI, yet the rate constants of relaxation remained unchanged. Interestingly, similar treatment of ischemic fibers did not impact TnI phosphorylation or force relaxation transients. Therefore, acute ischemia does not influence the rate constants of relaxation of permeabilized fibers. These results also suggest that the physiological level of sarcomeric protein phosphorylation is unlikely to be the primary driver of relaxation kinetics in permeabilized cardiac muscle fibers.