Hypoxia and hypothermia enhance spatial heterogeneities of repolarization in guinea pig hearts: analysis of spatial autocorrelation of optically recorded action potential durations

Sodium current preserves electrical excitability in the heart of hibernating ground squirrel (Citellus undulatus)

Hibernating mammals are capable of maintaining normal cardiac function at low temperatures. Excitability of cardiac myocytes crucially depends on the fast sodium current (I_Na), which is decreased in hypothermia due to both depolarization of resting membrane potential and direct negative effect of low temperature. Therefore, I_Na in hibernating mammals should have specific features allowing to maintain excitability of myocardium at low temperatures. The current-voltage dependence of I_Na, its steady-state inactivation and activation and recovery from inactivation were studied in winter hibernating (WH) and summer active (SA) ground squirrels and in rats using whole-cell patch clamp at 10 °C and 20 °C. I_Na peak amplitude and the parameters of steady-state activation and inactivation curves did not differ between SA and WH ground squirrels at both temperatures. However, at both temperatures strong positive shift of activation and inactivation curves by 5-12 mV was observed in both WH and SA ground squirrels if compared to rats. This peculiarity of cardiac I_Na in ground squirrels helps to maintain excitability in conditions of depolarized resting membrane potential. The time course of I_Na recovery from inactivation at 10 °C was faster in WH than in SA ground squirrels, which could ensure normal activation of myocardium during hibernation.

Hypothermia-induced accelerated idioventricular rhythm after cardiac surgery; a case report

BACKGROUND

Accelerated idioventricular rhythm (AIVR) is a slow ventricular arrhythmia, commonly due to myocardial ischemia in coronary artery disease. It is a transitory rhythm that rarely causes hemodynamic instability or necessitates any specific therapy. Besides, the common predisposing factors for ventricular arrhythmias after open-heart surgery are hemodynamic instability, electrolyte imbalances, hypoxia, hypovolemia, myocardial ischemia and infarction, acute graft closure, reperfusion injury, and administration of inotropes and antiarrhythmic drugs. Here we report a case of AIVR after cardiac surgery, mostly due to hypothermia that to our knowledge, it is the first report.

CASE PRESENTATION

We describe a 76-year-old man presenting with typical chest pain. Following routine investigations, the patient underwent coronary artery bypass grafting. Postoperatively, he was transferred to the intensive care unit with good hemodynamic status. However, about 3 h later, he developed rhythm disturbances, leading to hemodynamic instability without response to volume replacement or inotropic support. His rhythm was AIVR, although, at first glance, it resembled the left bundle branch block. Given his unstable hemodynamic status, he was emergently transferred to the operating room. Cardiopulmonary bypass (CPB) was resumed for hemodynamic support. After the patient was rewarmed to about 35 ºC, AIVR returned to normal. He was weaned from CPB successfully and with an uneventful hospital course.

CONCLUSIONS

Hypothermia is a potential cause of rhythm disturbance. Preventing the causes of arrhythmias, including hypothermia, is the best strategy.

Moderate but not severe hypothermia causes pro-arrhythmic changes in cardiac electrophysiology

Aims

Treatment of arrhythmias evoked by hypothermia/rewarming remains challenging, and the underlying mechanisms are unclear. This in vitro experimental study assessed cardiac electrophysiology in isolated rabbit hearts at temperatures occurring in therapeutic and accidental hypothermia.

Methods and results

Detailed ECG, surface electrogram, and panoramic optical mapping were performed in isolated rabbit hearts cooled to moderate (31°C) and severe (17°C) hypothermia. Ventricular activation was unchanged at 31°C while action potential duration (APD) was significantly prolonged (176.9 ± 4.2 ms vs. 241.0 ± 2.9 ms, P < 0.05), as was ventricular repolarization. At 17°C, there were proportionally similar delays in both activation and repolarization. These changes were reflected in the QRS and QT intervals of ECG recordings. Ventricular fibrillation threshold was significantly reduced at 31°C (16.3 ± 3.1 vs. 35 ± 3.5 mA, P < 0.05) but increased at 17°C (64.2 ± 9.9, P < 0.05). At 31°C, transverse conduction was relatively unchanged by cooling compared to longitudinal conduction, but at 17°C both transverse and longitudinal conduction were proportionately reduced to a similar extent. The gap junction uncoupler heptanol had a larger relative effect on transverse than longitudinal conduction and was able to restore the transverse/longitudinal conduction ratio, returning ventricular fibrillation threshold to baseline values (16.3 ± 3.1 vs. 36.3 ± 4.3 mA, P < 0.05) at 31°C. Rewarming to 37°C restored the majority of the electrophysiological parameters.

Conclusions

Moderate hypothermia does not significantly change ventricular conduction time but prolongs repolarization and is pro-arrhythmic. Further cooling to severe hypothermia causes parallel changes in ventricular activation and repolarization, changes which are anti-arrhythmic. Therefore, relative changes in QRS and QT intervals (QR/QTc) emerge as an ECG-biomarker of pro-arrhythmic activity. Risk for ventricular fibrillation appears to be linked to the relatively low temperature sensitivity of ventricular transmural conduction, a conclusion supported by the anti-arrhythmic effect of heptanol at 31°C.

Dispersion of ventricular repolarization: Temporal and spatial

Repolarization heterogeneity (RH) is an intrinsic property of ventricular myocardium and the reason for T-wave formation on electrocardiogram (ECG). Exceeding the physiologically based RH level is associated with appearance of life-threatening ventricular arrhythmias and sudden cardiac death. In this regard, an accurate and comprehensive evaluation of the degree of RH parameters is of importance for assessment of heart state and arrhythmic risk. This review is devoted to comprehensive consideration of RH phenomena in terms of electrophysiological processes underlying RH, cardiac electric field formation during ventricular repolarization, as well as clinical significance of RH and its reflection on ECG parameters. The formation of transmural, apicobasal, left-to-right and anterior-posterior gradients of action potential durations and end of repolarization times resulting from the heterogenous distribution of repolarizing ion currents and action potential morphology throughout the heart ventricles, and the different sensitivity of myocardial cells in different ventricular regions to the action of pharmacological agents, temperature, frequency of stimulation, etc., are being discussed. The review is focused on the fact that RH has different aspects – temporal and spatial, global and local; ECG reflection of various RH aspects and their clinical significance are being discussed. Strategies for comprehensive assessment of ventricular RH using different ECG indices reflecting various RH aspects are presented.

Hypothermia and cardiac electrophysiology: a systematic review of clinical and experimental data

Moderate therapeutic hypothermia procedures are used in post-cardiac arrest care, while in surgical procedures, lower core temperatures are often utilized to provide cerebral protection. Involuntary reduction of core body temperature takes place in accidental hypothermia and ventricular arrhythmias are recognized as a principal cause for a high mortality rate in these patients. We assessed both clinical and experimental literature through a systematic literature search in the PubMed database, to review the effect of hypothermia on cardiac electrophysiology. From included studies, there is common experimental and clinical evidence that progressive cooling will induce changes in cardiac electrophysiology. The QT interval is prolonged and appears more sensitive to decreases in temperature than the QRS interval. Severe hypothermia is associated with more pronounced changes, some of which are proarrhythmic. This is supported clinically where severe accidental hypothermia is commonly associated with ventricular fibrillation or asystole. J-waves in human electrocardiogram recordings are regularly but not always observed in hypothermia. Its relation to ventricular repolarization and arrhythmias is not obvious. Little clinical data exist on efficacy of anti-arrhythmic drugs in hypothermia, while experimental data show the potential of some agents, such as the class III antiarrhythmic bretylium. It is apparent that QT-prolonging drugs should be avoided.

Determinants of cardiac repolarization and risk for ventricular arrhythmias during mild therapeutic hypothermia

PURPOSE

We aimed to investigate the factors that modulate the extent of QTc prolongation and potential arrhythmogenic consequences during mild therapeutic hypothermia (MTH).

METHODS

We studied 205 patients after out-of-hospital cardiac arrest (131 underwent MTH). QTc was measured at baseline, 3h, 6h, 12h, 24h (end of hypothermia), 48h and 72h, and ventricular arrhythmias quantified.

RESULTS

During MTH, the QTc interval increased progressively peaking at 12h (mean increase 42ms, 95% CI 30-55). There was a strong gender effect (P<0.001) and a significant gender-by-MTH interaction (P=0.004). At 12h, the QTc interval was markedly longer in women as compared with men (mean difference 50ms [95% CI 27-73]. Anoxic brain injury (P=0.002) was also positively associated with QTc prolongation. The risk for ventricular arrhythmic events was not higher with MTH compared with no hypothermia (incidence rate ratio 0.57, 95% CI 0.32-1.02, P=0.06). However, typical cases of Torsade de pointes occurred in association with AV block and LQT2.

CONCLUSION

QTc prolongation during MTH is strongly affected by female gender and moderately by concomitant anoxic brain injury. Although the overall risk for ventricular arrhythmias is not greater with MTH, Torsade de pointes may develop when other contributing factors coexist.

Effect of action potential duration on Tpeak-Tend interval, T-wave area and T-wave amplitude as indices of dispersion of repolarization: Theoretical and simulation study in the rabbit heart

BACKGROUND

The aim of the study was to differentiate the effect of dispersion of repolarization (DOR) and action potential duration (APD) on T-wave parameters being considered as indices of DOR, namely, Tpeak-Tend interval, T-wave amplitude and T-wave area.

METHODS

T-wave was simulated in a wide physiological range of DOR and APD using a realistic rabbit model based on experimental data. A simplified mathematical formulation of T-wave formation was conducted.

RESULTS

Both the simulations and the mathematical formulation showed that Tpeak-Tend interval and T-wave area are linearly proportional to DOR irrespectively of APD range, while T-wave amplitude is non-linearly proportional to DOR and inversely proportional to the minimal repolarization time, or minimal APD value.

CONCLUSION

Tpeak-Tend interval and T-wave area are the most accurate DOR indices independent of APD. T-wave amplitude can be considered as an index of DOR when the level of APD is taken into account.

Role of abnormal repolarization in the mechanism of cardiac arrhythmia

In cardiac patients, life-threatening tachyarrhythmia is often precipitated by abnormal changes in ventricular repolarization and refractoriness. Repolarization abnormalities typically evolve as a consequence of impaired function of outward K⁺ currents in cardiac myocytes, which may be caused by genetic defects or result from various acquired pathophysiological conditions, including electrical remodelling in cardiac disease, ion channel modulation by clinically used pharmacological agents, and systemic electrolyte disorders seen in heart failure, such as hypokalaemia. Cardiac electrical instability attributed to abnormal repolarization relies on the complex interplay between a provocative arrhythmic trigger and vulnerable arrhythmic substrate, with a central role played by the excessive prolongation of ventricular action potential duration, impaired intracellular Ca²⁺ handling, and slowed impulse conduction. This review outlines the electrical activity of ventricular myocytes in normal conditions and cardiac disease, describes classical electrophysiological mechanisms of cardiac arrhythmia, and provides an update on repolarization-related surrogates currently used to assess arrhythmic propensity, including spatial dispersion of repolarization, activation-repolarization coupling, electrical restitution, TRIaD (triangulation, reverse use dependence, instability, and dispersion), and the electromechanical window. This is followed by a discussion of the mechanisms that account for the dependence of arrhythmic vulnerability on the location of the ventricular pacing site. Finally, the review clarifies the electrophysiological basis for cardiac arrhythmia produced by hypokalaemia, and gives insight into the clinical importance and pathophysiology of drug-induced arrhythmia, with particular focus on class Ia (quinidine, procainamide) and Ic (flecainide) Na⁺ channel blockers, and class III antiarrhythmic agents that block the delayed rectifier K⁺ channel (dofetilide).

Detachable glass microelectrodes for recording action potentials in active moving organs

Here, we describe new detachable floating glass micropipette electrode devices that provide targeted action potential recordings in active moving organs without requiring constant mechanical constraint or pharmacological inhibition of tissue motion. The technology is based on the concept of a glass micropipette electrode that is held firmly during cell targeting and intracellular insertion, after which a 100-µg glass microelectrode, a "microdevice," is gently released to remain within the moving organ. The microdevices provide long-term recordings of action potentials, even during millimeter-scale movement of tissue in which the device is embedded. We demonstrate two different glass micropipette electrode holding and detachment designs appropriate for the heart (sharp glass microdevices for cardiac myocytes in rats, guinea pigs, and humans) and the brain (patch glass microdevices for neurons in rats). We explain how microdevices enable measurements of multiple cells within a moving organ that are typically difficult with other technologies. Using sharp microdevices, action potential duration was monitored continuously for 15 min in unconstrained perfused hearts during global ischemia-reperfusion, providing beat-to-beat measurements of changes in action potential duration. Action potentials from neurons in the hippocampus of anesthetized rats were measured with patch microdevices, which provided stable base potentials during long-term recordings. Our results demonstrate that detachable microdevices are an elegant and robust tool to record electrical activity with high temporal resolution and cellular level localization without disturbing the physiological working conditions of the organ.NEW & NOTEWORTHY Cellular action potential measurements within tissue using glass micropipette electrodes usually require tissue immobilization, potentially influencing the physiological relevance of the measurement. Here, we addressed this limitation with novel 100-µg detachable glass microelectrodes that can be precisely positioned to provide long-term measurements of action potential duration during unconstrained tissue movement.

The Role of Transmural Repolarization Gradient in the Inversion of Cardiac Electric Field: Model Study of ECG in Hypothermia

BACKGROUND

The changes in ventricular repolarization gradients lead to significant alterations of the electrocardiographic body surface T waves up to the T wave inversion. However, the contribution of a specific gradient remains to be elucidated. The objective of the present investigation was to study the role of the transmural repolarization gradient in the inversion of the body surface T wave with a mathematical model of the hypothermia-induced changes of ventricular repolarization.

METHODS

By means of mathematical simulation, we set the hypothermic action potential duration (APD) distribution on the rabbit ventricular epicardium as it was previously experimentally documented. Then the parameters of the body surface potential distribution were tested with the introduction of different scenarios of the endocardial and epicardial APD behavior in hypothermia resulting in the unchanged, reversed or enlarged transmural repolarization gradient.

RESULTS

The reversal of epicardial repolarization gradients (apicobasal, anterior-posterior and interventricular) caused the inversion of the T waves regardless of the direction of the transmural repolarization gradient. However, the most realistic body surface potentials were obtained when the endocardial APDs were not changed under hypothermia while the epicardial APDs prolonged. This produced the reversed and increased transmural repolarization gradient in absolute magnitude. The body surface potentials simulated under the unchanged transmural gradient were reduced in comparison to those simulated under the reversed transmural gradient.

CONCLUSIONS

The simulations demonstrated that the transmural repolarization gradient did not play a crucial role in the cardiac electric field inversion under hypothermia, but its magnitude and direction contribute to the T wave amplitude.

Mechanistic insights into hypothermic ventricular fibrillation: the role of temperature and tissue size

AIMS

Hypothermia is well known to be pro-arrhythmic, yet it has beneficial effects as a resuscitation therapy and valuable during intracardiac surgeries. Therefore, we aim to study the mechanisms that induce fibrillation during hypothermia. A better understanding of the complex spatiotemporal dynamics of heart tissue as a function of temperature will be useful in managing the benefits and risks of hypothermia.

METHODS AND RESULTS

We perform two-dimensional numerical simulations by using a minimal model of cardiac action potential propagation fine-tuned on experimental measurements. The model includes thermal factors acting on the ionic currents and the gating variables to correctly reproduce experimentally recorded restitution curves at different temperatures. Simulations are implemented using WebGL, which allows long simulations to be performed as they run close to real time. We describe (i) why fibrillation is easier to induce at low temperatures, (ii) that there is a minimum size required for fibrillation that depends on temperature, (iii) why the frequency of fibrillation decreases with decreasing temperature, and (iv) that regional cooling may be an anti-arrhythmic therapy for small tissue sizes however it may be pro-arrhythmic for large tissue sizes.

CONCLUSION

Using a mathematical cardiac cell model, we are able to reproduce experimental observations, quantitative experimental results, and discuss possible mechanisms and implications of electrophysiological changes during hypothermia.

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.

Hypothermia-induced spatially discordant action potential duration alternans and arrhythmogenesis in nonhibernating versus hibernating mammals

The heart of hibernating species is resistant to lethal ventricular fibrillation (VF) induced by hypothermia. Spatially discordant (SDA) cardiac alternans is a promising predictor of VF, yet its role in the mechanism of hypothermic arrhythmogenesis in both nonhibernating and hibernating mammals remains unclear. We optically mapped the posterior epicardial surface of Langendorff-perfused hearts of winter hibernating (WH, n = 13), interbout arousal (IBA; n = 4), and summer active (SA, n = 6) ground squirrels (GSs; Spermophilus undulatus) and rabbits (n = 10). Action potential duration (APD) and conduction velocity (CV) dynamic restitution and alternans were determined at 37 to 17°C. In all animals, hypothermia induced heterogeneous APD prolongation, enhanced APD dispersion, and slowed CV. In all groups, hypothermia promoted the formation of APD alternans, which was predominantly spatially concordant in GSs and SDA in rabbits (SD of APD dispersion: 4.2 ± 0.4% vs. 2.0 ± 0.3% at 37°C and 7.5 ± 1.1% vs. 3.4 ± 0.5% at 17°C, P < 0.001 for rabbits vs. the WH group, respectively). In rabbits, hypothermia significantly increased the magnitude of SDA, which enhanced the ventricular repolarization gradient, caused conduction delays (CV: 3.2 vs. 8.2 cm/s at 17°C in rabbits vs. the WH group), conduction block, and the onset of VF (0% at 37°C vs. 60% at 17°C, P < 0.01). In contrast, no arrhythmia was observed in GS hearts at any temperature. The amplitude of CV alternans was greater in rabbits (5.2 ± 0.4% versus 4.5 ± 0.3% at 37°C and 35.3 ± 4.2% vs. 14.9 ± 1.5% at 17°C in rabbits vs. the WH group, P < 0.001 at 17°C) and correlated with the amplitude of SDA. In conclusion, the mechanism underlying SDA formation during hypothermia is likely associated with CV alternans conditioned by an enhanced dispersion of repolarization. The factors of hibernating species resistance to SDA and VF seem to be the safe and dynamically stable conduction and the low dispersion of repolarization.

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.

Electrophysiological mechanisms of antiarrhythmic protection during hypothermia in winter hibernating versus nonhibernating mammals

BACKGROUND

Robust cell-to-cell coupling is critically important in the safety of cardiac conduction and protection against ventricular fibrillation (VF). Hibernating mammals have evolved naturally protective mechanisms against VF induced by hypothermia and reperfusion injury.

OBJECTIVE

We hypothesized that this protection strategy involves a dynamic maintenance of conduction and repolarization patterns through the improvement of gap junction functions.

METHODS

We optically mapped the hearts of summer-active (SA) and winter-hibernating (WH) ground squirrels Spermophilus undulatus from Siberia and nonhibernating rabbits during different temperatures (+3 degrees C to +37 degrees C).

RESULTS

Midhypothermia (+17 degrees C) resulted in nonuniform conduction slowing, increased dispersion of repolarization, shortened wavelength, and consequently enhanced VF induction in SA ground squirrels and rabbits. In contrast, wavelength was increased during hypothermia in WH hearts in which VF was not inducible at any temperature. In SA and rabbit hearts, but not in WH, conduction anisotropy was significantly increased by pacing acceleration, thus promoting VF induction during hypothermia. WH hearts maintained the same rate-independent anisotropic propagation pattern even at 3 degrees C. connexin 43 (Cx43) had more homogenous transmural distribution in WH ventricles as compared to SA. Moreover, Cx43 and N-cadherins (N-cad) densities as well as the percentage of their colocalization were significantly higher in WH compared to SA epicardium.

CONCLUSION

Rate-independent conduction anisotropy ratio, low dispersion of repolarization, and long wavelength-these are the main electrophysiological mechanisms of antiarrhythmic protection in hibernating mammalian species during hypothermia. This strategy includes the improved gap junction function, which is due to overexpression and enhanced colocalization of Cx43 and N-cad.

Analyzing the electrophysiological effects of local epicardial temperature in experimental studies with isolated hearts

As a result of their modulating effects upon myocardial electrophysiology, both hypo- and hyperthermia can be used to study the mechanisms that generate or sustain cardiac arrhythmias. The present study describes an original electrode developed with thick-film technology and capable of controlling regional temperature variations in the epicardium while simultaneously registering its electrical activity. In this way, it is possible to measure electrophysiological parameters of the heart at different temperatures. The results obtained with this device in a study with isolated and perfused rabbit hearts are reported. An exploration has been made of the effects of local temperature changes upon the electrophysiological parameters implicated in myocardial conduction. Likewise, an analysis has been made of the influence of local temperature upon ventricular fibrillation activation frequency. It is concluded that both regional hypo- and hyperthermia exert reversible and opposite effects upon myocardial refractoriness and conduction velocity in the altered zone. The ventricular activation wavelength determined during constant pacing at 250 ms cycles is not significantly modified, however. During ventricular fibrillation, the changes in the fibrillatory frequency do not seem to be transmitted to normal temperature zones.

Ventricular repolarization pattern under heart cooling in the rabbit

AIM

Prolongation of ventricular repolarization is characteristic of myocardial cooling. In the present study, we investigated whether this prolongation is uniform or not throughout ventricular epicardium and how these hypothermia-induced changes express in the body surface potential distribution.

METHODS

Epicardial and body surface potential mapping from 64 unipolar leads was carried out in 18 anaesthetized adult rabbits. Mild hypothermia documented by lowering the mediastinal and rectal temperature from 38 to 32 degrees C was elicited by perfusion of the mediastinum with cooled saline. Activation times, repolarization times and activation-recovery intervals were determined in each epicardial lead.

RESULTS

Baseline activation-recovery intervals distributed non-uniformly on the ventricular epicardium, increasing progressively from the apex to the base and from the left ventricular (LV) sites to the right ventricular (RV) sites (P < 0.05), governing the repolarization sequence of ventricular epicardium. Heart cooling from 38 to 32 degrees C produced the heterogeneous prolongation of activation-recovery intervals which was more pronounced at the apex than at the base, and in the LV areas compared to the RV areas (P < 0.05). These nonuniform changes in local repolarization durations resulted in the development of base-to-apex repolarization sequence and inversion of the body surface potential distribution during the T wave.

CONCLUSION

Thus, under cooling the rabbit heart from 38 to 32 degrees C, the nonuniform prolongation of local repolarization durations resulted in the reversal of ventricular epicardial repolarization sequence which, in turn, was responsible for the inversion in the body surface potential distribution during the T wave.

Moderate hypothermia increases the chance of spiral wave collision in favor of self-termination of ventricular tachycardia/fibrillation

In cardiac arrest due to ventricular fibrillation (VF), moderate hypothermia (MH, 33°C) has been shown to improve defibrillation success compared with normothermia (NR, 37°C) and severe hypothermia (SH, 30°C). The underlying mechanisms remain unclear. We hypothesized that MH might prevent reentrant excitations rotating around functional obstacles (rotors) that are responsible for the genesis of VF. In two-dimensional Langendorff-perfused rabbit hearts prepared by cryoablation ( n = 13), action potential signals were recorded by a high-resolution optical mapping system. During basic stimulation (2.5–5.0 Hz), MH and SH caused significant prolongation of action potential duration and significant reduction of conduction velocity. Wavelength was unchanged at MH, whereas it was shortened significantly at SH at higher stimulation frequencies (4.0–5.0 Hz). The duration of direct current stimulation-induced ventricular tachycardia (VT)/VF was reduced dramatically at MH compared with NR and SH. The spiral wave (SW) excitations documented during VT at NR were by and large organized, whereas those during VT/VF at MH and SH were characterized by disorganization with frequent breakup. Phase maps during VT/VF at MH showed a higher incidence of SW collision (mutual annihilation or exit from the anatomical boundaries), which caused a temporal disappearance of phase singularity points (PS-0), compared with that at NR and SH. There was an inverse relation between PS-0 period in the observation area and VT/VF duration. MH data points were located in a longer PS-0 period and a shorter VT/VF duration zone compared with SH. MH causes a modification of SW dynamics, leading to an increase in the chance of SW collision in favor of self-termination of VT/VF.

Hibernator Citellus undulatus maintains safe cardiac conduction and is protected against tachyarrhythmias during extreme hypothermia: possible role of Cx43 and Cx45 up-regulation

BACKGROUND

Most mammals experience cardiac arrest during hypothermia. In contrast, hibernators remain in sinus rhythm even at body temperatures of 0 degrees C.

OBJECTIVES

The purpose of this study was to quantify electrical activity and connexin expression in the heart of hibernating Siberian ground squirrel Citellus undulatus.

METHODS

Optical imaging and microelectrode recordings were conducted in Langendorff-perfused hearts and isolated papillary muscles of summer active (SA, n = 19), winter hibernating (WH, n = 21), interbout arousal (IBA, n = 12), and winter active (WA, n = 3) ground squirrels and rabbits (n = 14) at temperatures from +37 degrees C to +3 degrees C.

RESULTS

All studied SA and WH hearts maintained spontaneous sinus rhythm, safe propagation through the entire conduction system, and normal pattern of ventricular excitation at all temperatures. However, three of the seven IBA and all rabbit hearts lost excitability at 10 degrees C +/- 1 degrees C and 12 degrees C +/- 1 degrees C, respectively. In WH, SA, and IBA ground squirrels, temperature reduction from 37 degrees C to 3 degrees C resulted in a 10-fold slowing of ventricular conduction velocity and increased excitation threshold. At any temperature, WH ventricles had faster conduction velocity and lower excitation threshold compared with SA and IBA. Immunolabeling demonstrated that connexin43 (Cx43) was significantly up-regulated in WH and WA compared with SA myocardium: Cx43 area density was 12.4 +/- 1.3, 15.0 +/- 3.0 and 8.6 +/- 1.1 microm(2)/1,000 microm(2), respectively. Moreover, Cx45 was expressed in the WH but not in the SA or WA ventricles.

CONCLUSION

Hibernator Citellus undulatus has evolved to maintain safe conduction at extreme hypothermia via up-regulation of Cx43 and Cx45 in order to protect the heart against arrhythmia associated with hypothermia.

Measurements of Electrophysiological Effects of Components of Acute Ischemia in Langendorff-Perfused Rat Hearts Using Voltage-Sensitive Dye Mapping

INTRODUCTION

This study was carried out to evaluate optical mapping in the presence of cytochalasin-D as a method for measuring electrophysiological responses in general, and in particular the responses to acute ischemia in the Langendorff-perfused rat heart. Cytochalasin-D is commonly used to reduce contraction for the purpose of suppressing motion artifacts in voltage-sensitive dye recordings of cardiac membrane potential.

METHODS AND RESULTS

Observations using optical mapping were complemented by recordings of the surface electrogram to provide information independent of the optical measurements. Perfusion of Langendorff-perfused rat hearts with 3 microM cytochalasin-D resulted in a 24% prolongation of the QT interval of surface electrograms indicating that cytochalasin-D prolongs the rat ventricular action potential. Individual components of the electrophysiological response to acute ischemia were globally induced as follows: (1) opening of K(ATP) channels was induced by perfusion of 2 micro M P-1,075, (2) accumulation of extracellular K(+) was simulated by increasing perfusate [K(+)] to 12 mM, and (3) acidosis was simulated by reducing perfusate pH to 6.5. The responses to these interventions could be reliably documented using optical recordings, as well as from surface electrograms. Whole-cell patch clamp measurements on isolated rat ventricular myocytes indicate that cytochalasin-D produces an approximately 2.5-fold increase in P-1,075-induced I(K,ATP).

CONCLUSION

These results provide the necessary background information for interpreting electrophysiological measurements during acute ischemia in the presence of cytochalasin-D.

Contrasting effects of ischemia on the kinetics of membrane voltage and intracellular calcium transient underlie electrical alternans

Repolarization alternans has been considered a strong marker of electrical instability. The objective of this study was to investigate the hypothesis that ischemia-induced contrasting effects on the kinetics of membrane voltage and intracellular calcium transient (CaiT) can explain the vulnerability of the ischemic heart to repolarization alternans. Ischemia-induced changes in action potential (AP) and CaiT resulting in alternans were investigated in perfused Langendorff guinea pig hearts subjected to 10–15 min of global no-flow ischemia followed by 10–15 min of reperfusion. The heart was stained with 100 μl of rhod-2 AM and 25 μl of RH-237, and AP and CaiT were simultaneously recorded with an optical mapping system of two 16 × 16 photodiode arrays. Ischemia was associated with shortening of AP duration (D) but delayed upstroke, broadening of peak, and slowed decay of CaiT resulting in a significant increase of CaiT-D. The changes in APD were spatially heterogeneous in contrast to a more spatially homogeneous lengthening of CaiT-D. CaiT alternans could be consistently induced with the introduction of a shorter cycle when the upstroke of the AP occurred before complete relaxation of the previous CaiT and generated a reduced CaiT. However, alternans of CaiT was not necessarily associated with alternans of APD, and this was correlated with the degree of spatially heterogeneous shortening of APD. Sites with less shortening of APD developed alternans of both CaiT and APD, whereas sites with greater shortening of APD could develop a similar degree of CaiT alternans but slight or no APD alternans. This resulted in significant spatial dispersion of APD. The study shows that the contrasting effects of ischemia on the duration of AP and CaiT and, in particular, on their spatial distribution explain the vulnerability of ischemic heart to alternans and the increased dispersion of repolarization during alternans.

Spatial heterogeneity of action potential alternans during global ischemia in the rabbit heart

Cardiac ischemia causes beat-to-beat fluctuation in action potential duration (APD) alternans, which leads to T wave alternans and arrhythmias. Occurrence of APD alternans that is out of phase at two sites is especially important, but most APD alternans studies have involved rapid pacing of normal myocardium rather than ischemia. To determine the spatial features of APD alternans during ischemia, blood-perfused rabbit hearts were stained with 4-[beta-[2(di-n-butylamino)-6-napthyl]vinyl]pyridinium (di-4-ANEPPS) and imaged with a high-resolution camera. Hearts were perfused with oxygenated Tyrode solution at 37 degrees C for staining and then switched to a 50:50% blood/Tyrode mixture. Hearts were paced from the right ventricle at 3/s, and made ischemic by stopping flow for 6 min. Images of 10,000 pixels were obtained at 300 frames/s. Motion artifact was controlled by immobilization and by manual selection of undistorted single-pixel records. Upstroke propagation and conduction isochrones were displayed by computerized image processing. APD alternans was demonstrated in six of seven hearts, and was out of phase in different regions of the image in three hearts. The largest spatial variation in the onset of depolarization to 50% repolarization (APD50) was 155%. This caused beat-to-beat reversal of repolarization. An alternans map could be constructed for well-immobilized portions of the image. There were discrete regions of APD alternans separated by a boundary, as occurs with intracellular Ca2+ concentration alternans. Pixels as close together as 1.1 mm showed an APD alternans that was out of phase. The out-of-phase APD alternans was not due to conduction alternans, as shown by upstroke intervals and conduction isochrones. This contrasts with rapid pacing, where a causal relationship appears to exist. These new observations suggest distinct mechanisms for the genesis of arrhythmias during ischemia.

Sympathetic nerve sprouting, electrical remodeling, and increased vulnerability to ventricular fibrillation in hypercholesterolemic rabbits

Whether hypercholesterolemia (HC) can induce proarrhythmic neural and electrophysiological remodeling is unclear. We fed rabbits with either high cholesterol (HC, n=10) or standard (S, n=10) chows for 12 weeks (protocol 1), and with HC (n=12) or S (n=10) chows for 8 weeks (protocol 2). In protocol 3, 10 rabbits were fed with various protocols to observe the effects of different serum cholesterol levels. Results showed that the serum cholesterol levels were 2097+/-288 mg/dL in HC group and 59+/-9 mg/dL in S group for protocol 1 and were 1889+/-577 mg/dL in HC group and 50+/-21 mg/dL in S group for protocol 2. Density of growth-associated protein 43- (GAP43) and tyrosine hydroxylase- (TH) positive nerves in the heart was significantly higher in HC than S in protocol 1. Compared with S, HC rabbits had longer QTc intervals, more QTc dispersion, longer action potential duration, increased heterogeneity of repolarization and higher peak calcium current (ICa) density (14.0+/-3.1 versus 9.1+/-3.4 pA/pF; P<0.01) in protocol 1 and 2. Ventricular fibrillation was either induced or occurred spontaneously in 9/12 of hearts of HC group and 2/10 of hearts in S group in protocol 2. Protocol 3 showed a strong correlation between serum cholesterol level and nerve density for GAP43 (R2=0.94; P<0.001) and TH (R2=0.91; P<0.001). We conclude that HC resulted in nerve sprouting, sympathetic hyperinnervation, and increased ICa. The neural and electrophysiological remodeling was associated with prolonged action potential duration, longer QTc intervals, increased repolarization dispersion, and increased ventricular vulnerability to fibrillation.

Effects of acute reduction of temperature on ventricular fibrillation activation patterns

Because of its electrophysiological effects, hypothermia can influence the mechanisms that intervene in the sustaining of ventricular fibrillation. We hypothesized that a rapid and profound reduction of myocardial temperature impedes the maintenance of ventricular fibrillation, leading to termination of the arrhythmia. High-resolution epicardial mapping (series 1; n = 11) and transmural recordings of ventricular activation (series 2; n = 10) were used to analyze ventricular fibrillation modification during rapid myocardial cooling in Langendorff-perfused rabbit hearts. Myocardial cooling was produced by the injection of cold Tyrode into the left ventricle after induction of ventricular fibrillation. Temperature and ventricular fibrillation dominant frequency decay fit an exponential model to arrhythmia termination in all experiments, and both parameters were significantly correlated (r = 0.70, P < 0.0001). Termination of the arrhythmia occurred preferentially in the left ventricle and was associated with a reduction in conduction velocity (-60% in left ventricle and -54% in right ventricle; P < 0.0001) and with activation maps predominantly exhibiting a single wave front, with evidence of wave front extinction. We conclude that a rapid reduction of temperature to <20 degrees C terminates ventricular fibrillation after producing an important depression in myocardial conduction.

Mechanisms of shock-induced arrhythmogenesis during acute global ischemia

Little is known about the mechanisms of vulnerability and defibrillation under ischemic conditions. We investigated these mechanisms in 18 Langendorff-perfused rabbit hearts during 75% reduced-flow ischemia. Electrical activity was optically mapped from the anterior epicardium during right ventricular shocks applied at various phases of the cardiac cycle while the excitation-contraction decoupler 2,3-butanedione monoxime (BDM; 15 mM) was used to suppress motion artifacts caused by contraction of the heart. During ischemia, vulnerable window width increased [from 30-90% of the action potential duration (APD) in the control to -10 to 100% of the APD in ischemia]. Moreover, arrhythmia severity increased along with the reduction of APD (176 +/- 9 ms in control and 129 +/- 26 ms in ischemia, P < 0.01) and increased dispersion of repolarization (45 +/- 17 ms in control and 73 +/- 28 ms in ischemia, P < 0.01). Shock-induced virtual electrode polarization was preserved. Depolarizing (contrary to hyperpolarizing) response time constants increased. Virtual electrode-induced wavefronts of excitation had much more tortuous pathways leading to wavefront fractionation. Defibrillation failure at all shock strengths was observed in four hearts. Optical mapping revealed that the shock extinguished the arrhythmia; however, the arrhythmia self-originated after an isoelectric window of 339 +/- 189 ms. In conclusion, in most cases, virtual electrode-induced phase singularity (VEIPS) was responsible for shock-induced arrhythmogenesis during acute global ischemia. Enhancement of arrhythmogenesis was associated with an increased dispersion of repolarization and altered deexcitation. In four hearts, arrhythmogenesis could not be explained by VEIPS.

Null mutation of connexin43 causes slow propagation of ventricular activation in the late stages of mouse embryonic development

Connexin43 (Cx43) is the principal connexin isoform in the mouse ventricle, where it is thought to provide electrical coupling between cells. Knocking out this gene results in anatomic malformations that nevertheless allow for survival through early neonatal life. We examined electrical wave propagation in the left (LV) and right (RV) ventricles of isolated Cx43 null mutated (Cx43(-/-)), heterozygous (Cx43(+/)(-)), and wild-type (WT) embryos using high-resolution mapping of voltage-sensitive dye fluorescence. Consistent with the compensating presence of the other connexins, no reduction in propagation velocity was seen in Cx43(-/-) ventricles at postcoital day (dpc) 12.5 compared with WT or Cx43(+/)(-) ventricles. A gross reduction in conduction velocity was seen in the RV at 15.5 dpc (in cm/second, mean [1 SE confidence interval], WT 9.9 [8.7 to 11.2], Cx43(+/)(-) 9.9 [9.0 to 10.9], and Cx43(-/-) 2.2 [1.8 to 2.7; P<0.005]) and in both ventricles at 17.5 dpc (in RV, WT 8.4 [7.6 to 9.3], Cx43(+/)(-) 8.7 [8.1 to 9.3], and Cx43(-/-) 1.1 [0.1 to 1.3; P<0.005]; in LV, WT 10.1 [9.4 to 10.7], Cx43(+/)(-) 8.3 [7.8 to 8.9], and Cx43(-/-) 1.7 [1.3 to 2.1; P<0.005]) corresponding with the downregulation of Cx40. Cx40 and Cx45 mRNAs were detectable in ventricular homogenates even at 17.5 dpc, probably accounting for the residual conduction function. Neonatal knockout hearts were arrhythmic in vivo as well as ex vivo. This study demonstrates the contribution of Cx43 to the electrical function of the developing mouse heart and the essential role of this gene in maintaining heart rhythm in postnatal life.

Defibrillation energy requirements and electrical heterogeneity during total body hypothermia

OBJECTIVE

Determine the effects of hypothermia on defibrillation energy requirements and cardiac electrophysiology.

DESIGN

Prospective randomized acute intervention trial.

SETTING

Medical center animal laboratory.

SUBJECTS

Fifteen domestic farm swine.

INTERVENTIONS

Swine were randomized to a hypothermia group (n = 8) or a control group (n = 7). All animals were instrumented with a transvenous defibrillation system connected to a defibrillator that delivers a biphasic-truncated waveform. Values for defibrillation energy requirements were measured at baseline (normothermia, 38-40 degrees C) and during treatment with total body hypothermia (30 degrees C) or no temperature change (sham). Hypothermia was induced by circulating ice-water through anterior and posterior surgical thermal blankets.

MEASUREMENTS AND MAIN RESULTS

Defibrillation energy requirement values at 20%, 50%, and 80% were determined by using an up/down method. In the hypothermia group, defibrillation energy requirement values at baseline did not significantly change during hypothermia (defibrillation energy requirements 50% = 14 +/- 2 J vs. 15 +/- 2 J, respectively). Similarly, the defibrillation energy requirement values in the control group did not change from baseline to sham phase (defibrillation energy requirements 50% = 12 +/- 1 J vs. 13 +/- 1 J, respectively). Hypothermia profoundly affected cardiac electrophysiology, decreasing ventricular fibrillation threshold by 72%, conduction velocity by 25% (p < .01), and tissue excitability, while it prolonged ventricular repolarization and refractoriness by 7.5% to 15%, respectively (p < .05).

CONCLUSIONS

Total body cooling to 30 degrees C was highly arrhythmogenic, although this unstable electrophysiological state did not alter ventricular defibrillation energy requirements. These data suggest that hypothermia may be used to slow metabolic processes without concern over the ability to successfully defibrillate and treat hypothermia-induced arrhythmias.

Understanding conduction of electrical impulses in the mouse heart using high-resolution video imaging technology

The conduction of electrical impulses in the heart depends on the ability to efficiently transfer excitatory current between individual myocytes. Several recent studies have focused on the use of optical mapping techniques to determine the electrophysiological consequences and the proarrhythmic effects of reducing intercellular coupling in newly developed connexin knockout mice. This work has begun to unravel important questions regarding the role of connexins in intercellular coupling and propagation of electrical impulses in the heart. The purpose of this review is to discuss the techniques and unique issues involved in imaging electrical wave propagation in the heart. In addition, we will review recent experimental studies that address the role of intercellular communication in the development of cardiac arrhythmias.

Reentry and fibrillation in the mouse heart. A challenge to the critical mass hypothesis

The idea that fibrillation is only possible in hearts exceeding a critical size was introduced by W. Garrey >80 years ago and has since been generally accepted. In ventricular tissue, this critical size was originally estimated to be 400 mm(2). Recent estimates suggest that the critical size required for sustained reentry is approximately 100 to 200 mm(2), whereas 6 times this area is required for ventricular fibrillation. According to these estimates, fibrillation is not possible in the mouse heart, where the ventricular surface area is approximately 100 mm(2). To test whether sustained ventricular fibrillation could be induced in such an area, we used a high-speed video imaging system and a voltage-sensitive dye to quantify electrical activity on the epicardial surface of the Langendorff-perfused adult mouse heart. In 6 hearts, measurements during ventricular pacing at a basic cycle length (BCL) of 120 ms yielded maximum and minimum conduction velocities (CV(max) and CV(min)) of 0.63+/-0.04 and 0.38+/-0.02 mm/ms, respectively. At a BCL of 80 ms, CV(max) and CV(min) changed to 0.55+/-0.03 and 0. 34+/-0.02 mm/ms. Action potential durations (APDs), measured at 70% repolarization at those pacing frequencies were found to be 44.5+/-2. 9 and 40.4+/-2.6 ms, respectively. The wavelengths (CVxAPD) were calculated to be 28.6+/-3.4 mm in the CV(max) direction and 16.8+/-1. 5 mm in the CV(min) direction at BCL 120 ms. Wavelengths were significantly reduced (P<0.05) at BCL 80 ms (CV(max), 22.2+/-1.8 mm; CV(min), 13.7+/-0.9 mm). In 5 hearts, stationary vortex-like reentry organized by single rotors (4 of 5 hearts) or by pairs of rotors (1 of 5 hearts) was induced by burst pacing. In the ECG, the activity manifested as sustained monomorphic tachycardia. Detailed analysis showed that the local CVs were reduced in the vicinity of the rotor center, which allowed the reentry to take place within a smaller area than was calculated from wavelength measurements during pacing. In 4 of 7 hearts, burst pacing resulted in a polymorphic ECG pattern indistinguishable from ventricular fibrillation. These data challenge the critical mass hypothesis by demonstrating that ventricular tissue with an area as small as 100 mm(2) is capable of undergoing sustained fibrillatory activity.

Effects of IKr and IKs heterogeneity on action potential duration and its rate dependence: a simulation study

BACKGROUND

A growing body of evidence suggests that heterogeneity of ion channel expression and electrophysiological characteristics is an important property of the ventricular myocardium. The 2 components of the delayed rectifier potassium current, IKr (rapid) and IKs (slow), play a dominant role in the repolarization of the action potential and are important determinants of its duration.

METHODS AND RESULTS

In this report, the effects of heterogeneities of IKr and IKs on action potential duration (APD) and its rate dependence (adaptation) are studied with the use of the LRd model of a mammalian ventricular cell. Results demonstrate the importance of IKs density variations in heterogeneity of repolarization. Cells with reduced IKs (eg, mid-myocardial M cells) display long APD and steep dependence of APD on rate. Mechanistically, accumulation of IKs activation and increased sodium calcium exchange current, INaCa, secondary to Na+ accumulation at a fast rate underlie the steep APD-rate relation of these cells. When cells are electrotonically coupled in a multicellular fiber through resistive gap junction, APD differences are reduced. The results demonstrate strong dependence of APD heterogeneity on the degree of intercellular coupling even in the normal physiological range. Highly reduced coupling maximizes APD heterogeneity.

CONCLUSIONS

Heterogeneity of IKs:IKr density strongly influences APD and its rate dependence. However, in the intact myocardium, the degree of gap-junction coupling may be an important factor that determines the manifestation of APD heterogeneity and dispersion of repolarization. The clinical significance of this study is in the context of repolarization abnormalities and associated arrhythmias (eg, long QT syndrome and torsade de pointes).