Can sudden cardiac death be predicted from the T wave of the ECG? A critical examination of T wave alternans and QT interval dispersion

Bacterial toxins and heart function: heat-labile Escherichia coli enterotoxin B promotes changes in cardiac function with possible relevance for sudden cardiac death

Bacterial toxins can cause cardiomyopathy, though it is not its most common cause. Some bacterial toxins can form pores in the membrane of cardiomyocytes, while others can bind to membrane receptors. Enterotoxigenic E. coli can secrete enterotoxins, including heat-resistant (ST) or labile (LT) enterotoxins. LT is an AB5-type toxin that can bind to specific cell receptors and disrupt essential host functions, causing several common conditions, such as certain diarrhea. The pentameric B subunit of LT, without A subunit (LTB), binds specifically to certain plasma membrane ganglioside receptors, found in lipid rafts of cardiomyocytes. Isolated guinea pig hearts and cardiomyocytes were exposed to different concentrations of purified LTB. In isolated hearts, mechanical and electrical alternans and an increment of heart rate variability, with an IC50 of ~0.2 μg/ml LTB, were observed. In isolated cardiomyocytes, LTB promoted significant decreases in the amplitude and the duration of action potentials. Na+ currents were inhibited whereas L-type Ca2+ currents were augmented at their peak and their fast inactivation was promoted. Delayed rectifier K+ currents decreased. Measurements of basal Ca2+ or Ca2+ release events in cells exposed to LTB suggest that LTB impairs Ca2+ homeostasis. Impaired calcium homeostasis is linked to sudden cardiac death. The results are consistent with the recent view that the B subunit is not merely a carrier of the A subunit, having a role explaining sudden cardiac death in children (SIDS) infected with enterotoxigenic E. coli, explaining several epidemiological findings that establish a strong relationship between SIDS and ETEC E. coli.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12551-023-01100-6.

Complexities in cardiovascular rhythmicity: perspectives on circadian normality, ageing and disease

Biological rhythms exist in organisms at all levels of complexity, in most organs and at myriad time scales. Our own biological rhythms are driven by energy emitted by the sun, interacting via our retinas with brain stem centres, which then send out complex messages designed to synchronize the behaviour of peripheral non-light sensing organs, to ensure optimal physiological responsiveness and performance of the organism based on the time of day. Peripheral organs themselves have autonomous rhythmic behaviours that can act independently from central nervous system control but is entrainable. Dysregulation of biological rhythms either through environment or disease has far-reaching consequences on health that we are only now beginning to appreciate. In this review, we focus on cardiovascular rhythms in health, with ageing and under disease conditions.

Deciphering Arrhythmia Mechanisms - Tools of the Trade

Pathophysiological remodeling of cardiac function occurs at multiple levels, spanning the spectrum from molecular and sub-cellular changes to those occurring at the organ-system levels. Of key importance to arrhythmias are changes in electrophysiological and calcium handling properties at the tissue level. In this review, we discuss how high-resolution optical action potential and calcium transient imaging has advanced our understanding of basic arrhythmia mechanisms associated with multiple cardiovascular disorders, including the long QT syndrome, heart failure, and ischemia-reperfusion injury. We focus on the role of repolarization gradients (section 1) and calcium mediated triggers (section 2) in the initiation and maintenance of complex arrhythmias in these settings.

Preventing tomorrow's sudden cardiac death today: part I: Current data on risk stratification for sudden cardiac death

Accurate and timely prediction of sudden cardiac death (SCD) is a necessary prerequisite for effective prevention and therapy. Although the largest number of SCD events occurs in patients without overt heart disease, there are currently no tests that are of proven predictive value in this population. Efforts in risk stratification for SCD have focused primarily on predicting SCD in patients with known structural heart disease. Despite the ubiquity of tests that have been purported to predict SCD vulnerability in such patients, there is little consensus on which test, in addition to the left ventricular ejection fraction, should be used to determine which patients will benefit from an implantable cardioverter defibrillator. On July 20 and 21, 2006, a group of experts representing clinical cardiology, cardiac electrophysiology, biostatistics, economics, and health policy were joined by representatives of the US Food and Drug administration, Centers for Medicare Services, Agency for Health Research and Quality, the Heart Rhythm Society, and the device and pharmaceutical industry for a round table meeting to review current data on strategies of risk stratification for SCD, to explore methods to translate these strategies into practice and policy, and to identify areas that need to be addressed by future research studies. The meeting was organized by the Duke Center for the Prevention of SCD at the Duke Clinical Research Institute and was funded by industry participants. This article summarizes the presentations and discussions that occurred at that meeting.

Molecular/genetic determinants of repolarization and their modification by environmental stress

Although a variety of factors, inherited or environmental, can influence expression of ion channel proteins to impact on repolarization, that environment can affect genetic determinants of repolarization for intervals of varying duration is a concept that is not as generally appreciated as it should be. In the following pages we review the molecular/genetic determinants of cardiac repolarization and summarize how pathologic events and environmental intrusions can affect these determinants. Understanding the chains of events involved should yield insights into both the causes and potential avenues of treatment for abnormalities of repolarization.

ECG repolarization waves: their genesis and clinical implications

The electrocardiographic (ECG) manifestation of ventricular repolarization includes J (Osborn), T, and U waves. On the basis of biophysical principles of ECG recording, any wave on the body surface ECG represents a coincident voltage gradient generated by cellular electrical activity within the heart. The J wave is a deflection with a dome that appears on the ECG after the QRS complex. A transmural voltage gradient during initial ventricular repolarization, which results from the presence of a prominent action potential notch mediated by the transient outward potassium current (I(to)) in epicardium but not endocardium, is responsible for the registration of the J wave on the ECG. Clinical entities that are associated with J waves (the J-wave syndrome) include the early repolarization syndrome, the Brugada syndrome and idiopathic ventricular fibrillation related to a prominent J wave in the inferior leads. The T wave marks the final phase of ventricular repolarization and is a symbol of transmural dispersion of repolarization (TDR) in the ventricles. An excessively prolonged QT interval with enhanced TDR predisposes people to develop torsade de pointes. The malignant "R-on-T" phenomenon, i.e., an extrasystole that originates on the preceding T wave, is due to transmural propagation of phase 2 reentry or phase 2 early afterdepolarization. A pathological "U" wave as seen with hypokalemia is the consequence of electrical interaction among ventricular myocardial layers at action potential phase 3 of which repolarization slows. A physiological U wave is thought to be due to delayed repolarization of the Purkinje system.

Neurohormonal risk stratification for sudden death and death owing to progressive heart failure in chronic heart failure

BACKGROUND

This study tested various neurohormones for prediction of heart failure death (death owing to progressive deterioration of ventricular function; HFD). Moreover, B-type natriuretic peptide (BNP) as a predictor of sudden death (SD; as reported previously) and the best predictor of HFD were combined for a simple risk stratification model.

DESIGN

BNP, the N-terminal fragment of BNP (N-BNP), and of the atrial natriuretic peptide (N-ANP) and big endothelin levels were obtained from 452 patients with a left ventricular ejection fraction </= 35%. Outcome was documented during an observation period up to 3 years.

RESULTS

Two hundred and ninety-eight patients survived without heart transplantation, 65 patients underwent heart transplantation and 89 patients died (SD 44 patients, HFD 31 patients, other causes 14 patients). The only independent predictor of SD was BNP (as reported), and the best independent predictor of HFD was N-ANP (P = 0.0001). Analyzing 293 survivors and 44 patients with SD, fewer patients with BNP < 130 pg mL(-1) (Group A, n = 110) died [1%] as compared with patients with BNP > 130 pg mL(-1) and N-ANP < 6300 fmol mL(-1) (Group B, n = 177; 18%; P = 0.0001) and patients with BNP > 130 pg mL(-1) and N-ANP > 6300 fmol mL(-1) (Group C, n = 50; 19%; P = 0.0001). Analyzing 293 survivors and 31 patients with HFD, fewer patients died in Group A (n = 109; 0%; P = 0.0001) and Group B (n = 153; 6%; P = 0.0001) as compared with patients of Group C (n = 62; 34%).

CONCLUSION

Prognostic power of neurohormones depends on the mode of death. The combined determination of BNP and N-ANP identifies patients with minimal risk of death, elevated SD but low HFD risk as well as elevated SD and HFD risk.

Hysteresis effect implicates calcium cycling as a mechanism of repolarization alternans

BACKGROUND

T-wave alternans is due to alternation of membrane repolarization at the cellular level and is a risk factor for sudden cardiac death. Recently, a hysteresis effect has been reported in patients whereby T-wave alternans, once induced by rapid heart rate, persists even when heart rate is subsequently slowed. We hypothesized that alternans hysteresis is an intrinsic property of cardiac myocytes, directly related to an underlying mechanism for repolarization alternans that involves intracellular calcium cycling.

METHODS AND RESULTS

Stepwise pacing was used to induce alternans in Langendorff-perfused guinea pig hearts from which optical action potentials were recorded simultaneously at 256 ventricular sites with voltage-sensitive dyes and in whole-cell patch-clamped cardiac myocytes treated with or without BAPTA-AM (1,2-bis[2-aminophenoxy]ethane-N,N,N',N'-tetraacetic acid tetrakis [acetoxymethyl ester]). Alternans hysteresis was observed in every isolated heart: threshold heart rate for alternans was 280+/-12 bpm, but during subsequent deceleration of pacing, alternans persisted to significantly slower heart rates (238+/-5 bpm, P<0.05). Optical mapping showed that this effect also applied to the threshold for spatially discordant alternans (313+/-2.2 bpm during acceleration versus 250+/-6.6 bpm during deceleration, P<0.05). Alternans hysteresis was also observed in isolated cardiac myocytes. Moreover, calcium chelation by BAPTA-AM raised the threshold for alternans and inhibited hysteresis in a dose-dependent manner with no effect on baseline action potential duration.

CONCLUSIONS

Alternans hysteresis is an intrinsic property of cardiac myocytes that can lead to persistence of arrhythmogenic discordant alternans even after heart rate is slowed. These results also support an important underlying role of calcium cycling in the mechanism of alternans.

Ventricular repolarization components on the electrocardiogram: cellular basis and clinical significance

Ventricular repolarization components on the surface electrocardiogram (ECG) include J (Osborn) waves, ST-segments, and T- and U-waves, which dynamically change in morphology under various pathophysiologic conditions and play an important role in the development of ventricular arrhythmias. Our primary objective in this review is to identify the ionic and cellular basis for ventricular repolarization components on the body surface ECG under normal and pathologic conditions, including a discussion of their clinical significance. A specific attempt to combine typical clinical ECG tracings with transmembrane electrical recordings is made to illustrate their logical linkage. A transmural voltage gradient during initial ventricular repolarization, which results from the presence of a prominent transient outward K(+) current (I(to))-mediated action potential (AP) notch in the epicardium, but not endocardium, manifests as a J-wave on the ECG. The J-wave is associated with the early repolarization syndrome and Brugada syndrome. ST-segment elevation, as seen in Brugada syndrome and acute myocardial ischemia, cannot be fully explained by using the classic concept of an "injury current" that flows from injured to uninjured myocardium. Rather, ST-segment elevation may be largely secondary to a loss of the AP dome in the epicardium, but not endocardium. The T-wave is a symbol of transmural dispersion of repolarization. The R-on-T phenomenon (an extrasystole originating on the T-wave of a preceding ventricular beat) is probably due to transmural propagation of phase 2 re-entry or phase 2 early after depolarization that could potentially initiate polymorphic ventricular tachycardia or fibrillation.

Circadian rhythm in the cardiovascular system: chronocardiology

BACKGROUND

We reviewed recent progress in the study of the chronobiological aspects of the cardiovascular system.

METHODS

Medline was used as the main search tool, and the full texts of selected papers were obtained.

RESULTS

More than 300 references were found, and 52 of them, representing the major findings in this field, were included in the reference list. Results of these studies confirm that most cardiovascular physiological parameters (such as heart rate, blood pressure, electrocardiogram indices) and pathophysiological events (myocardial ischemia/infarction, sudden cardiac death) show circadian rhythms. Results also suggest that consideration of these rhythms is important for the diagnosis and treatment of cardiovascular disorders and that restoration of normal circadian rhythms may be associated with clinical improvement.

CONCLUSION

The study of circadian rhythms in the cardiovascular system is emerging as an important area of investigation because of its potential implications for patient management.

B-type natriuretic peptide predicts sudden death in patients with chronic heart failure

BACKGROUND

Given the high incidence of sudden death in patients with chronic heart failure (CHF) and the efficacy of implantable cardioverter-defibrillators, an appropriate tool for the prediction of sudden death is desirable. B-type natriuretic peptide (BNP) has prognostic significance in CHF, and the stimuli for its production cause electrophysiological abnormalities. This study tests BNP levels as a predictor of sudden death.

METHODS AND RESULTS

BNP levels, in addition to other neurohormonal, clinical, and hemodynamic variables, were obtained from 452 patients with a left ventricular ejection fraction (LVEF) < or =35%. For prediction of sudden death, only survivors without heart transplantation (HTx) or a mechanical assist device and patients who died suddenly were analyzed. Up to 3 years, 293 patients survived without HTx or a mechanical assist device, 89 patients died, and 65 patients underwent HTx. Mode of death was sudden in 44 patients (49%), whereas 31 patients (35%) had pump failure and 14 patients (16%) died from other causes. Univariate risk factors of sudden death were log BNP (P=0.0006), log N-terminal atrial natriuretic peptide (P=0.003), LVEF (P=0.005), log N-terminal BNP (P=0.006), systolic blood pressure (P=0.01), big endothelin (P=0.03), and NYHA class (P=0.04). In the multivariate model, log BNP level was the only independent predictor of sudden death (P=0.0006). Using a cutoff point of log BNP <2.11 (130 pg/mL), Kaplan-Meier sudden death-free survival rates were significantly higher in patients below (99%) compared with patients above (81%) this cutoff point (P=0.0001).

CONCLUSION

BNP levels are a strong, independent predictor of sudden death in patients with CHF.

Effect of metoprolol and d,l-sotalol on microvolt-level T-wave alternans. Results of a prospective, double-blind, randomized study

OBJECTIVES

The study evaluated the effects of metoprolol, a pure beta-blocker, and d,l-sotalol, a beta-blocker with additional class III antiarrhythmic effects, on microvolt-level T-wave alternans (TWA).

BACKGROUND

Assessment of TWA is increasingly used for purposes of risk stratification in patients prone to sudden death. There are only sparse data regarding the effects of beta-blockers and antiarrhythmic drugs on TWA.

METHODS

Patients with a history of documented or suspected malignant ventricular tachyarrhythmias were eligible. All patients underwent invasive electrophysiologic (EP) testing including programmed ventricular stimulation and determination of TWA at increasing heart rates using atrial pacing. Reproducibility of TWA at two consecutive drug-free baseline measurements was tested in a random patient subset. Following baseline measurements, all patients were randomized either to double-blind intravenous infusion of sotalol (1.0 mg/kg) or metoprolol (0.1 mg/kg). Results of TWA assessment at baseline and after drug exposure were compared.

RESULTS

Fifty-four consecutive patients were studied. In 12 patients, repetitive baseline measurement of TWA revealed stable alternans voltage (V(alt)) values (9.1 +/- 5.8 microV vs. 8.5 +/- 5.7 microV, p = NS). After drug administration, V(alt) decreased by 35% with metoprolol (7.9 +/- 6.0 microV to 4.9 +/- 4.2 microV; p < 0.001) and by 38% with sotalol (8.6 +/- 6.8 microV to 4.4 +/- 2.3 microV; p = 0.001). In eight patients with positive TWA at baseline, repeated measurement revealed negative test results.

CONCLUSIONS

In patients prone to sudden cardiac death, there is a reduction in TWA amplitude following the administration of antiadrenergic drugs. This result indicates that TWA is responsive to the pharmacologic milieu and suggests that, to assess a patient's risk of spontaneous ventricular arrhythmia, the patient should be tested while maintaining the pharmacologic regimen under which the risk of arrhythmia is being assessed. This applies particularly for beta-blocker therapy.

Ventricular hypertrophy amplifies transmural repolarization dispersion and induces early afterdepolarization

The effects of left ventricular hypertrophy (LVH) on the generation of phase 2 early afterdepolarization (EAD) and transmural dispersion of repolarization (TDR) were assessed using arterially perfused rabbit ventricular wedge preparations. Transmembrane action potentials from epicardium, subendocardium, and endocardium were simultaneously recorded together with a transmural ECG. Transmural action potential duration (APD) was also mapped. LVH (renovascular hypertension model) produced significant prolongation in ventricular APD and QT interval. Preferential APD prolongation in subendocardium and endocardium was associated with a marked increase in TDR. Phase 2 EADs were generated from subendocardium or endocardium in all LVH rabbits (15 of 15) in the absence of APD prolonging agents at basic cycle lengths of 2,000-4,000 ms. Phase 2 EAD could produce "R on T" extrasystoles, initiating polymorphic ventricular tachycardia (VT). This study provides the first direct evidence from intracellular recordings that phase 2 EAD could be generated from rabbit intact hypertrophied LV wall in the absence of APD prolonging agents, resulting in R on T extrasystoles capable of initiating polymorphic VT under enhanced TDR.