Beneficial Effect of Calcium Treatment for Hyperkalemia Is Not Due to "Membrane Stabilization".

OBJECTIVES: Hyperkalemia is a common life-threatening condition causing severe electrophysiologic derangements and arrhythmias. The beneficial effects of calcium (Ca2+) treatment for hyperkalemia have been attributed to "membrane stabilization," by restoration of resting membrane potential (RMP). However, the underlying mechanisms remain poorly understood. Our objective was to investigate the mechanisms underlying adverse electrophysiologic effects of hyperkalemia and the therapeutic effects of Ca2+ treatment. DESIGN: Controlled experimental trial. SETTING: Laboratory investigation. SUBJECTS: Canine myocytes and tissue preparations. INTERVENTIONS AND MEASUREMENTS: Optical action potentials and volume averaged electrocardiograms were recorded from the transmural wall of ventricular wedge preparations (n = 7) at baseline (4 mM potassium), hyperkalemia (8-12 mM), and hyperkalemia + Ca2+ (3.6 mM). Isolated myocytes were studied during hyperkalemia (8 mM) and after Ca2+ treatment (6 mM) to determine cellular RMP. MAIN RESULTS: Hyperkalemia markedly slowed conduction velocity (CV, by 67% ± 7%; p < 0.001) and homogeneously shortened action potential duration (APD, by 20% ± 10%; p < 0.002). In all preparations, this resulted in QRS widening and the "sine wave" pattern observed in severe hyperkalemia. Ca2+ treatment restored CV (increase by 44% ± 18%; p < 0.02), resulting in narrowing of the QRS and normalization of the electrocardiogram, but did not restore APD. RMP was significantly elevated by hyperkalemia; however, it was not restored with Ca2+ treatment suggesting a mechanism unrelated to "membrane stabilization." In addition, the effect of Ca2+ was attenuated during L-type Ca2+ channel blockade, suggesting a mechanism related to Ca2+-dependent (rather than normally sodium-dependent) conduction. CONCLUSIONS: These data suggest that Ca2+ treatment for hyperkalemia restores conduction through Ca2+-dependent propagation, rather than restoration of membrane potential or "membrane stabilization." Our findings provide a mechanistic rationale for Ca2+ treatment when hyperkalemia produces abnormalities of conduction (i.e., QRS prolongation).

Post-ROSC Atrial fibrillation is not associated with rearrest but is associated with stroke and mortality following out of hospital cardiac arrest.

BACKGROUND: Atrial fibrillation (AF) in patients resuscitated from cardiac arrest (CA) is associated with increased short-term mortality. However, whether this is because AF adversely affects early resuscitation success, causes post-resuscitation morbidity, or because it is a marker for patient co-morbidities, remains unclear. We aimed to determine the prevalence of AF in patients with ROSC to test the hypothesis that AF is associated with increased risk of rearrest and to determine its impact on mortality and stroke risk. METHODS: We performed a retrospective study of emergency medical services patients with OHCA and ROSC. To examine long-term morbidity and mortality due to AF, an additional observational cohort analysis was performed using a large electronic health record (EHR) database. RESULTS: One hundred nineteen patients with ROSC prior to ED arrival were identified. AF was observed in 39 (33%) of patients. Rearrest was not different between AF and no AF groups (44% vs. 41%, p = 0.94). In the EHR analysis, mortality at one year in patients who developed AF was 59% vs. 39% in no AF patients. Odds of stroke was 5x greater in AF patients (p < 0.001), with the majority not anticoagulated (93%, p < 0.001) and comorbidities were greater p < 0.001). CONCLUSIONS: AF was common following ROSC and not associated with rearrest. AF after CA was associated with increased mortality and stroke risk. These data suggest rhythm control for AF in the immediate post-ROSC period is not warranted; however, vigilance is required for patients who develop persistent AF, particularly with regards to stroke risk and prevention.

Spontaneous Repolarization Alternans Causes VT/VF Rearrest That Is Suppressed by Preserving Gap Junctions.

BACKGROUND: Ventricular tachycardia (VT)/ventricular fibrillation (VF) rearrest after successful resuscitation is common, and survival is poor. A mechanism of VT/VF, as demonstrated in ex vivo studies, is when repolarization alternans becomes spatially discordant (DIS ALT), which can be enhanced by impaired gap junctions (GJs). However, in vivo spontaneous DIS ALT-induced VT/VF has never been demonstrated, and the effects of GJ on DIS ALT and VT/VF rearrest are unknown. OBJECTIVES: This study aimed to determine whether spontaneous VT/VF rearrest induced by DIS ALT occurs in vivo, and if it can be suppressed by preserving Cx43-mediated GJ coupling and/or connectivity. METHODS: We used an in vivo porcine model of resuscitation from ischemia-induced cardiac arrest combined with ex vivo optical mapping in porcine left ventricular wedge preparations. RESULTS: In vivo, DIS ALT frequently preceded VT/VF and paralleled its incidence at normal (37°C, n = 9) and mild hypothermia (33°C, n = 8) temperatures. Maintaining GJs in vivo with rotigaptide (n = 10) reduced DIS ALT and VT/VF incidence, especially during mild hypothermia, by 90% and 60%, respectively (P < 0.001; P < 0.013). Ex vivo, both rotigaptide (n = 5) and αCT11 (n = 7), a Cx43 mimetic peptide that promotes GJ connectivity, significantly reduced DIS ALT by 60% and 100%, respectively (P < 0.05; P < 0.005), and this reduction was associated with reduced intrinsic heterogeneities of action potential duration rather than changes in conduction velocity restitution. CONCLUSIONS: These results provide the strongest in vivo evidence to date suggesting a causal relationship between spontaneous DIS ALT and VT/VF in a clinically realistic scenario. Furthermore, our results suggest that preserving GJs during resuscitation can suppress VT/VF rearrest.

Effect of Amiodarone and Hypothermia on Arrhythmia Substrates During Resuscitation.

Background Amiodarone is administered during resuscitation, but its antiarrhythmic effects during targeted temperature management are unknown. The purpose of this study was to determine the effect of both therapeutic hypothermia and amiodarone on arrhythmia substrates during resuscitation from cardiac arrest. Methods and Results We utilized 2 complementary models: (1) In vitro no-flow global ischemia canine left ventricular transmural wedge preparation. Wedges at different temperatures (36°C or 32°C) were given 5 µmol/L amiodarone (36-Amio or 32-Amio, each n=8) and subsequently underwent ischemia and reperfusion. Results were compared with previous controls. Optical mapping was used to measure action potential duration, dispersion of repolarization (DOR), and conduction velocity (CV). (2) In vivo pig model of resuscitation. Pigs (control or targeted temperature management, 32-34°C) underwent ischemic cardiac arrest and were administered amiodarone (or not) after 8 minutes of ventricular fibrillation. In vitro: therapeutic hypothermia but not amiodarone prolonged action potential duration. During ischemia, DOR increased in the 32-Amio group versus 32-Alone (84±7 ms versus 40±7 ms, P<0.05) while CV slowed in the 32-Amio group. Amiodarone did not affect CV, DOR, or action potential duration during ischemia at 36°C. Conduction block was only observed at 36°C (5/8 36-Amio versus 6/7 36-Alone, 0/8 32-Amio, versus 0/7 32-Alone). In vivo: QTc decreased upon reperfusion from ischemia that was ameliorated by targeted temperature management. Amiodarone did not worsen DOR or CV. Amiodarone suppressed rearrest caused by ventricular fibrillation (7/8 without amiodarone, 2/7 with amiodarone, P=0.041), but not pulseless electrical activity (2/8 without amiodarone, 5/7 with amiodarone, P=0.13). Conclusions Although amiodarone abolishes a beneficial effect of therapeutic hypothermia on ischemia-induced DOR and CV, it did not worsen susceptibility to ventricular tachycardia/ventricular fibrillation during resuscitation.

Translational Models of Arrhythmia Mechanisms and Susceptibility: Success and Challenges of Modeling Human Disease.

We discuss large animal translational models of arrhythmia susceptibility and sudden cardiac death, focusing on important considerations when interpreting the data derived before applying them to human trials. The utility of large animal models of arrhythmia and the pros and cons of specific translational large animals used will be discussed, including the necessary tradeoffs between models designed to derive mechanisms vs. those to test therapies. Recent technical advancements which can be applied to large animal models of arrhythmias to better elucidate mechanistic insights will be introduced. Finally, some specific examples of past successes and challenges in translating the results of large animal models of arrhythmias to clinical trials and practice will be examined, and common themes regarding the success and failure of translating studies to therapy in man will be discussed.

Arrhythmogenic cardiac alternans in heart failure is suppressed by late sodium current blockade by ranolazine.

BACKGROUND: Cardiac alternans is promoted by heart failure (HF)-induced calcium (Ca2+) cycling abnormalities. Late sodium current (INa,L) is enhanced in HF and promotes Ca2+ overload; however, mechanisms underlying an antiarrhythmic effect of INa,L blockade in HF remain unclear. OBJECTIVE: The purpose of this study was to determine whether ranolazine suppresses cardiac alternans in HF by normalizing Ca2+ cycling. METHODS: Transmural dual optical mapping of Ca2+ transients and action potentials was performed in wedge preparations from 8 HF and 8 control (normal) dogs. Susceptibility to action potential duration alternans (APD-ALT) and Ca2+ transient alternans (Ca-ALT) was compared at baseline and with ranolazine (5-10 µM). RESULTS: HF increased APD- and Ca-ALT compared to normal (both P <.05), and ranolazine suppressed APD- and Ca-ALT in both groups (P <.05). The incidence of spatially discordant alternans (DIS-ALT) was increased by HF (8/8) compared to normal (4/8; P <.05), and ranolazine decreased DIS-ALT in HF (4/8; P <.05).Not only did ranolazine mitigate HF-induced Ca2+ overload, it also attenuated APD-ALT to Ca-ALT gain (amount of APD-ALT produced by Ca-ALT). In HF, APD-ALT to Ca-ALT gain was significantly increased (0.55 ± 0.02) compared to normal (0.44 ± 0.02; P <.05) and was normalized by ranolazine (0.36 ± 0.05; P <.05), representing a complementary mechanism by which INa,L blockade suppressed cardiac alternans. CONCLUSION: Ranolazine attenuated arrhythmogenic cardiac alternans in HF, both by suppressing Ca-ALT and decreasing the coupling gain of APD-ALT to Ca-ALT. Blockade of INa,L may reverse impaired Ca2+ cycling to mitigate cardiac alternans, representing a mechanism underlying the antiarrhythmic benefit of INa,L blockade in HF.

Hypothermia Modulates Arrhythmia Substrates During Different Phases of Resuscitation From Ischemic Cardiac Arrest.

BACKGROUND: We designed an innovative porcine model of ischemia-induced arrest to determine dynamic arrhythmia substrates during focal infarct, global ischemia from ventricular tachycardia or fibrillation (VT/VF) and then reperfusion to determine the effect of therapeutic hypothermia (TH) on dynamic arrhythmia substrates and resuscitation outcomes. METHODS AND RESULTS: Anesthetized adult pigs underwent thoracotomy and regional plunge electrode placement in the left ventricle. Subjects were then maintained at either control (CT; 37°C, n=9) or TH (33°C, n=8). The left anterior descending artery (LAD) was occluded and ventricular fibrillation occurred spontaneously or was induced after 30 minutes. Advanced cardiac life support was started after 8 minutes, and LAD reperfusion occurred 60 minutes after occlusion. Incidences of VF/VT and survival were compared with ventricular ectopy, cardiac alternans, global dispersion of repolarization during LAD occlusion, and LAD reperfusion. There was no difference in incidence of VT/VF between groups during LAD occlusion (44% in CT versus 50% in TH; P=1s). During LAD occlusion, ectopy was increased in CT and suppressed in TH (33±11 ventricular ectopic beats/min versus 4±6 ventricular ectopic beats/min; P=0.009). Global dispersion of repolarization and cardiac alternans were similar between groups. During LAD reperfusion, TH doubled the incidence of cardiac alternans compared with CT, with a marked increase in VF/VT (100% in TH versus 17% in CT; P=0.004). Ectopy and global dispersion of repolarization were similar between groups during LAD reperfusion. CONCLUSIONS: TH alters arrhythmia substrates in a porcine translational model of resuscitation from ischemic cardiac arrest during the complex phases of resuscitation. TH worsens cardiac alternans, which was associated with an increase in spontaneous VT/VF during reperfusion.

Arrhythmogenic Delayed Afterdepolarizations Are Promoted by Severe Hypothermia But Not Therapeutic Hypothermia.

BACKGROUND: Severe hypothermia (SH) is known to be arrhythmogenic, but the effect of therapeutic hypothermia (TH) on arrhythmias is unclear. It is hypothesized that susceptibility to Ca-mediated arrhythmia triggers would be increased only by SH.Methods and Results:Spontaneous Ca release (SCR) and resultant delayed afterdepolarizations (DADs) were evaluated by optical mapping in canine wedge preparations during normothermia (N, 36℃), TH (32℃) or SH (28℃; n=8 each). The slope (amplitude/rise time) of multicellular SCR (mSCR) events, a determinant of triggered activity, was suppressed in TH (24.4±3.4%/s vs. N: 41.5±6.0%/s), but significantly higher in SH (96.3±8.1%/s) producing higher amplitude DADs in SH (35.7±1.6%) and smaller in TH (5.3±1.0% vs. N: 10.0±1.1%, all P<0.05). Triggered activity was only observed in SH. In isolated myocytes, sarcoplasmic reticulum (SR) Ca release kinetics slowed in a temperature-dependent manner, prolonging Ca transient rise time [33±3 (N) vs. 50±6 (TH) vs. 88±12 ms (SH), P<0.05], which can explain the decreased mSCR slope and DAD amplitude in TH. Although the SR Ca content was similar in TH and SH, Ca spark frequency was markedly increased only in SH, suggesting that increased ryanodine receptor open probability could explain the increased triggered activity during SH. CONCLUSIONS: Temperature dependence of Ca release can explain susceptibility to Ca-mediated arrhythmia triggers in SH. This may therefore explain the increased risk of lethal arrhythmia in SH, but not during TH.

Mild hypothermia preserves myocardial conduction during ischemia by maintaining gap junction intracellular communication and Na+ channel function.

Acute cardiac ischemia induces conduction velocity (CV) slowing and conduction block, promoting reentrant arrhythmias leading to sudden cardiac arrest. Previously, we found that mild hypothermia (MH; 32°C) attenuates ischemia-induced conduction block and CV slowing in a canine model of early global ischemia. Acute ischemia impairs cellular excitability and the gap junction (GJ) protein connexin (Cx)43. We hypothesized that MH prevented ischemia-induced conduction block and CV slowing by preserving GJ expression and localization. Canine left ventricular preparations at control (36°C) or MH (32°C) were subjected to no-flow prolonged (30 min) ischemia. Optical action potentials were recorded from the transmural left ventricular wall, and CV was measured throughout ischemia. Cx43 and Na+ channel (NaCh) remodeling was assessed using both confocal immunofluorescence (IF) and/or Western blot analysis. Cellular excitability was determined by microelectrode recordings of action potential upstroke velocity (dV/dtmax) and resting membrane potential (RMP). NaCh current was measured in isolated canine myocytes at 36 and 32°C. As expected, MH prevented conduction block and mitigated ischemia-induced CV slowing during 30 min of ischemia. MH maintained Cx43 at the intercalated disk (ID) and attenuated ischemia-induced Cx43 degradation by both IF and Western blot analysis. MH also preserved dV/dtmax and NaCh function without affecting RMP. No difference in NaCh expression was seen at the ID by IF or Western blot analysis. In conclusion, MH preserves myocardial conduction during prolonged ischemia by maintaining Cx43 expression at the ID and maintaining NaCh function. Hypothermic preservation of GJ coupling and NaCh may be novel antiarrhythmic strategies during resuscitation.NEW & NOTEWORTHY Therapeutic hypothermia is now a class I recommendation for resuscitation from cardiac arrest. This study determined that hypothermia preserves gap junction coupling as well as Na+ channel function during acute cardiac ischemia, attenuating conduction slowing and preventing conduction block, suggesting that induced hypothermia may be a novel antiarrhythmic strategy in resuscitation.

Effects of ethanol on systemic hemodynamics in a porcine model of accidental hypothermia.

OBJECTIVES: Accidental hypothermia is frequently associated with ethanol intoxication. Each has independent effects on systemic hemodynamics, but their combined effects are poorly understood. We aimed to describe the hemodynamic effects of ethanol intoxication in a model of severe hypothermia and rewarming. METHODS: Anesthetized pigs was assigned to control (n=8) or ethanol groups (ETOH) (n=7, 3 mg/kg of ethanol via an orogastric tube). Subjects were cooled to 25°C using ice packs and then warmed to baseline core temperature with passive external and active core rewarming. RESULTS: In the ETOH group, peak serum ethanol concentration was 202 mg/dL at 25°C. Ethanol had no effect on time of cooling or rewarming. In both the control and ETOH, there were similar maximal decreases in mean arterial pressure (from 94±24 to 50±15 mm Hg and 100±27 to 31±12 mm Hg, respectively), ventricular contractility (rate of maximal left ventricular pressure rise from 5731±1462 to 2610±596 mm Hg/s and 6832±1384 to 1937±437 mm Hg/s, respectively), and cardiac output (from 2.14±0.8 to 0.53±0.3 L/min and 2.93±0.9, to 0.44±0.2 L/min, respectively; all P<.001). After rewarming, only in the ETOH group were persistent decreases in mean arterial pressure (59±14 mm Hg), contractility (3982±1573 mm Hg/s), and cardiac output (1.6±0.9 L/min, all P<.03) observed. CONCLUSIONS: Hypothermia caused significant adverse effects on cardiac function and systemic hemodynamics, which returned to baseline with rewarming. Ethanol intoxication had no additional effects on systemic hemodynamics during cooling; however, it caused more prolonged depression of cardiac function and adverse effects on systemic hemodynamics during rewarming. These data may have implications for resuscitation of ethanol-intoxicated victims of accidental hypothermia.

Ionic bases for electrical remodeling of the canine cardiac ventricle.

Emerging evidence suggests that ventricular electrical remodeling (VER) is triggered by regional myocardial strain via mechanoelectrical feedback mechanisms; however, the ionic mechanisms underlying strain-induced VER are poorly understood. To determine its ionic basis, VER induced by altered electrical activation in dogs undergoing left ventricular pacing (n = 6) were compared with unpaced controls (n = 4). Action potential (AP) durations (APDs), ionic currents, and Ca(2+) transients were measured from canine epicardial myocytes isolated from early-activated (low strain) and late-activated (high strain) left ventricular regions. VER in the early-activated region was characterized by minimal APD prolongation, but marked attenuation of the AP phase 1 notch attributed to reduced transient outward K(+) current. In contrast, VER in the late-activated region was characterized by significant APD prolongation. Despite marked APD prolongation, there was surprisingly minimal change in ion channel densities but a twofold increase in diastolic Ca(2+). Computer simulations demonstrated that changes in sarcolemmal ion channel density could only account for attenuation of the AP notch observed in the early-activated region but failed to account for APD remodeling in the late-activated region. Furthermore, these simulations identified that cytosolic Ca(2+) accounted for APD prolongation in the late-activated region by enhancing forward-mode Na(+)/Ca(2+) exchanger activity, corroborated by increased Na(+)/Ca(2+) exchanger protein expression. Finally, assessment of skinned fibers after VER identified altered myofilament Ca(2+) sensitivity in late-activated regions to be associated with increased diastolic levels of Ca(2+). In conclusion, we identified two distinct ionic mechanisms that underlie VER: 1) strain-independent changes in early-activated regions due to remodeling of sarcolemmal ion channels with no changes in Ca(2+) handling and 2) a novel and unexpected mechanism for strain-induced VER in late-activated regions in the canine arising from remodeling of sarcomeric Ca(2+) handling rather than sarcolemmal ion channels.

Mild hypothermia decreases arrhythmia susceptibility in a canine model of global myocardial ischemia*.

OBJECTIVES: Although the majority of sudden cardiac arrests occur in patients with ischemic heart disease, the effect of therapeutic hypothermia on arrhythmia susceptibility during acute global ischemia is not well understood. While both ischemia and severe hypothermia are arrhythmogenic, patients undergoing therapeutic hypothermia do not have an increase in arrhythmias, despite the fact that most sudden cardiac arrest occur in the setting of ischemia. We hypothesized that mild hypothermia induced prior to myocardial ischemia and reperfusion will have a beneficial effect on ischemia-related arrhythmia substrates. DESIGN: We developed a model of global ischemia and reperfusion in the canine wedge preparation to study the transmural electrophysiologic effects of ischemia at different temperatures. SETTING: Animal study. SUBJECTS: Male mongrel dogs. INTERVENTIONS: Canine left ventricle wedge preparations at 1) control (36°C) or 2) mild hypothermia, to simulate temperatures used in therapeutic hypothermia (32°C), were subjected to 15 mins of no-flow ischemia and subsequently reperfused. MEASUREMENTS AND MAIN RESULTS: Optical action potentials were recorded spanning the transmural wall of left ventricle. Action potential duration for epicardial, mid-myocardial, and epicardial cells was measured. Transmural dispersion of repolarization and conduction velocity were measured at baseline, during ischemia, and during reperfusion. No difference was seen at baseline for conduction velocity or dispersion of repolarization between groups. Conduction velocity decreased from 0.46 ± 0.02 m/sec to 0.23 ± 0.07 m/sec, and dispersion of repolarization increased from 30 ± 5 msecs to 57 ± 4 msecs in the control group at 15 mins of ischemia. Mild hypothermia attenuated both the ischemia-induced conduction velocity slowing (decreasing from 0.44 ± 0.02 m/sec to 0.35 ± 0.03 m/sec; p = .019) and the ischemia-induced increase in dispersion of repolarization (25 ± 3 msecs to 37 ± 7 msecs; p = .037). Epicardial conduction block was observed in six of seven preparations of the control group, but no preparations in the mild hypothermia group developed conduction block (0/6). CONCLUSIONS: Mild hypothermia attenuated ischemia-induced increase in dispersion of repolarization, conduction slowing, and block, which are known mechanisms of arrhythmogenesis in ischemia. These data suggest that therapeutic hypothermia may decrease arrhythmogenesis during myocardial ischemia.

Transmural dispersion of repolarization as a preclinical marker of drug-induced proarrhythmia.

Torsade de Pointes (TdP) proarrhythmia is a major complication of therapeutic drugs that block the delayed rectifier current. QT interval prolongation, the principal marker used to screen drugs for proarrhythmia, is both insensitive and nonspecific. Consequently, better screening methods are needed. Drug-induced transmural dispersion of repolarization (TDR) is mechanistically linked to TdP. Therefore, we hypothesized that drug-induced enhancement of TDR is more predictive of proarrhythmia than QT interval. High-resolution transmural optical action potential mapping was performed in canine wedge preparations (n = 19) at baseline and after perfusion with 4 different QT prolonging drugs at clinically relevant concentrations. Two proarrhythmic drugs in patients (bepridil and E4031) were compared with 2 nonproarrhythmic drugs (risperidone and verapamil). Both groups prolonged the QT (all P < 0.02), least with the proarrhythmic drug bepridil, reaffirming that QT is a poor predictor of TdP. In contrast, TDR was enhanced only by proarrhythmic drugs (P < 0.03). Increased TDR was due to a preferential prolongation of midmyocardial cell, relative to epicardial cell, APD, whereas nonproarrhythmic drugs similarly prolonged both cell types. In contrast to QT prolongation, augmentation of TDR was induced by proarrhythmic but not nonproarrhythmic drugs, suggesting TDR is a superior preclinical marker of proarrhythmic risk during drug development.

Enhanced dispersion of repolarization explains increased arrhythmogenesis in severe versus therapeutic hypothermia.

BACKGROUND: Hypothermia is proarrhythmic, and, as the use of therapeutic hypothermia (TH) increases, it is critically important to understand the electrophysiological effects of hypothermia on cardiac myocytes and arrhythmia substrates. We tested the hypothesis that hypothermia-enhanced transmural dispersion of repolarization (DOR) is a mechanism of arrhythmogenesis in hypothermia. In addition, we investigated whether the degree of hypothermia, the rate of temperature change, and cooling versus rewarming would alter hypothermia-induced arrhythmia substrates. METHODS AND RESULTS: Optical action potentials were recorded from cells spanning the transmural wall of canine left ventricular wedge preparations at baseline (36°C), during cooling and during rewarming. Electrophysiological parameters were examined while varying the depth of hypothermia. On cooling to 26°C, DOR increased from 26±4 ms to 93±18 ms (P=0.021); conduction velocity decreased from 35±5 cm/s to 22±5 cm/s (P=0.010). On rewarming to 36°C, DOR remained prolonged, whereas conduction velocity returned to baseline. Conduction block and reentry was observed in all severe hypothermia preparations. Ventricular fibrillation/ventricular tachycardia was seen more during rewarming (4/5) versus cooling (2/6). In TH (n=7), cooling to 32°C mildly increased DOR (31±6 to 50±9, P=0.012), with return to baseline on rewarming and was associated with decreased arrhythmia susceptibility. Increased rate of cooling did not further enhance DOR or arrhythmogenesis. CONCLUSIONS: Hypothermia amplifies DOR and is a mechanism for arrhythmogenesis. DOR is directly dependent on the depth of cooling and rewarming. This provides insight into the clinical observation of a low incidence of arrhythmias in TH and has implications for protocols for the clinical application of TH.

Spontaneous calcium release in tissue from the failing canine heart.

Abnormalities in calcium handling have been implicated as a significant source of electrical instability in heart failure (HF). While these abnormalities have been investigated extensively in isolated myocytes, how they manifest at the tissue level and trigger arrhythmias is not clear. We hypothesize that in HF, triggered activity (TA) is due to spontaneous calcium release from the sarcoplasmic reticulum that occurs in an aggregate of myocardial cells (an SRC) and that peak SCR amplitude is what determines whether TA will occur. Calcium and voltage optical mapping was performed in ventricular wedge preparations from canines with and without tachycardia-induced HF. In HF, steady-state calcium transients have reduced amplitude [135 vs. 170 ratiometric units (RU), P < 0.05] and increased duration (252 vs. 229 s, P < 0.05) compared with those of normal. Under control conditions and during beta-adrenergic stimulation, TA was more frequent in HF (53% and 93%, respectively) compared with normal (0% and 55%, respectively, P < 0.025). The mechanism of arrhythmias was SCRs, leading to delayed afterdepolarization-mediated triggered beats. Interestingly, the rate of SCR rise was greater for events that triggered a beat (0.41 RU/ms) compared with those that did not (0.18 RU/ms, P < 0.001). In contrast, there was no difference in SCR amplitude between the two groups. In conclusion, TA in HF tissue is associated with abnormal calcium regulation and mediated by the spontaneous release of calcium from the sarcoplasmic reticulum in aggregates of myocardial cells (i.e., an SCR), but importantly, it is the rate of SCR rise rather than amplitude that was associated with TA.