Electrophysiologic mechanisms for ventricular arrhythmias in left ventricular dysfunction: electrolytes, catecholamines and drugs

Primary hypothyroidism with an episode of ventricular tachycardia in a patient with COVID-19: A case report

RATIONALE

Coronavirus disease 2019 (COVID-19) is a systemic disease with major clinical manifestations in the respiratory system. However, thyroid involvement has also been reported. We present a case of hypothyroidism with ventricular tachycardia following diagnosis with COVID-19.

PATIENT CONCERNS

A 77-year-old man was admitted to the isolation ward due to COVID-19. After respiratory support and medical treatment, the patient was successfully weaned off the ventilator. However, an episode of short-run ventricular tachycardia was detected, and primary hypothyroidism was also diagnosed.

DIAGNOSIS

Ventricular tachycardia was detected by electrocardiography.

INTERVENTIONS

Intravenous amiodarone administration and oral levothyroxine replacement.

OUTCOMES

No arrhythmia detected following treatment.

LESSONS

Awareness of the association between hypothyroidism and COVID-19 is important in preventing possible complications.

Relationship between Atrial Tachyarrhythmias and Intrathoracic Impedance in Patients with a Pacemaker and Preserved Ejection Fraction

Atrial fibrillation (AF) is responsible for significant morbidity and mortality in patients with heart failure (HF). Modern pacemakers provide an index of intrathoracic fluid status (OptiVol fluid index-OVFI) by measuring daily intrathoracic impedance. This study aimed to determine whether OVFI is associated with increased atrial tachycardia/fibrillation (AT/AF) events in patients with a preserved ejection fraction (EF). We retrospectively reviewed data from patients with Medtronic Advisa pacemakers between 2012 and 2014 in our hospital. The association and temporal relationship between OVFI and AT/AF events were determined. A total of 150 patients with 211 follow-up visits (mean 1.4 visits per patient) were evaluated. The device-detected AT/AF prevalence was 47%. Device-measured OVFI ≥ 20 Ω-days was significantly associated with the onset of AT/AF ≥ 4 h. OVFI threshold crossing preceded AT/AF events in 55.1% of cases, followed by AT/AF events in only 18.7%. Fluid overload represented by OVFI may trigger AT/AF episodes in patients with a preserved EF more often than that previously reported in patients with a reduced EF. Our findings support the view that worsening pulmonary congestion is associated with increased AT/AF frequency and suggests that fluid overload could trigger and perpetuate AT/AF events in patients with a preserved EF.

Association between changes in the intrathoracic impedance and ventricular arrhythmias in patients with heart failure

BACKGROUND

Ventricular arrhythmias (VAs) are independently related to mortality risk in patients with heart failure (HF). The wide availability of implantable cardioverter defibrillators and cardiac resynchronization therapy devices now offers an opportunity to clinically correlate the two disease processes. We hypothesized that there is an association between changes in the intrathoracic impedance and episodes of VA.

METHODS

Nonconcurrent prospective study of adults (age >20 years) with known HF with reduced ejection fraction (<35%). The OptiVol threshold was categorized as follows: 0-30 Ω-days, 31-60 Ω-days, 61-90 Ω-days, 91-120 Ω-days, and >120 Ω-days. Patients with OptiVol values at 0-30 Ω-days were used as the reference group. Receiver operating characteristic analysis was used to estimate the sensitivity and specificity at each threshold.

RESULTS

Of the 87 eligible patients, 65.5% were males. The mean age of the sample was 73.3 years (±12.7). Compared to patients in the 0-30 Ω-days category, those in the 31-60, 61-90, 91-120, and >120 Ω-days groups had, on average, 1.48, 1.64, 2.24, and 1.6 more VAs, respectively (P = 0.002, 0.009, 0.010 and 0.009, respectively). The sensitivity and specificity of each threshold were as follows: 82.6% and 61.7% at 31-60 Ω-days, 43.5% and 78.3% at 61-90 Ω-days, 30.4% and 85.0% at 91-120 Ω-days, and 21.7% and 88.3% at >121 Ω-days.

CONCLUSION

Our study found a significant positive relationship between changes in intrathoracic impedance and episodes of VAs in patients with HF.

Arrhythmic effects of Epac-mediated ryanodine receptor activation in Langendorff-perfused murine hearts are associated with reduced conduction velocity

Recent papers have attributed arrhythmic substrate in murine RyR2‐P2328S hearts to reduced action potential (AP) conduction velocities (CV), reflecting acute functional inhibition and/or reduced expression of sodium channels. We explored for acute effects of direct exchange protein directly activated by cAMP (Epac)‐mediated ryanodine receptor‐2 (RyR2) activation on arrhythmic substrate and CV. Monophasic action potential (MAP) recordings demonstrated that initial steady (8 Hz) extrinsic pacing elicited ventricular tachycardia (VT) in 0 of 18 Langendorff‐perfused wild‐type mouse ventricles before pharmacological intervention. The Epac activator 8‐CPT (8‐(4‐chlorophenylthio)‐2′‐O‐methyladenosine‐3′,5′‐cyclic monophosphate) (VT in 1 of 7 hearts), and the RyR2 blocker dantrolene, either alone (0 of 11) or with 8‐CPT (0 of 9) did not then increase VT incidence (P>.05). Both progressively increased pacing rates and programmed extrasystolic (S2) stimuli similarly produced no VT in untreated hearts (n=20 and n=9 respectively). 8‐CPT challenge then increased VT incidences (5 of 7 and 4 of 8 hearts respectively; P<.05). However, dantrolene, whether alone (0 of 10 and 1 of 13) or combined with 8‐CPT (0 of 10 and 0 of 13) did not increase VT incidence relative to those observed in untreated hearts (P>.05). 8‐CPT but not dantrolene, whether alone or combined with 8‐CPT, correspondingly increased AP latencies (1.14±0.04 (n=7), 1.04±0.03 (n=10), 1.09±0.05 (n=8) relative to respective control values). In contrast, AP durations, conditions for 2:1 conduction block and ventricular effective refractory periods remained unchanged throughout. We thus demonstrate for the first time that acute RyR2 activation reversibly induces VT in specific association with reduced CV.

Intracellular angiotensin-(1-12) changes the electrical properties of intact cardiac muscle

In the present work, the influence of intracellular injection of angiotensin-(1-12) [Ang-(1-12)] on the electrical properties of the intact left ventricle of Wistar Kyoto rats was investigated with electrophysiological methods. Particular attention was given to the role of chymostatin on the effect of the peptide. The results indicated that intracellular administration of the peptide elicited a depolarization of the surface cell membrane and an increase of duration of the action potential followed by the generation of early afterdepolarizations. The increment of action potential duration caused by Ang-(1-12) (100 nM) was due to a decrease of total potassium current recorded from single cardiomyocytes using the whole cell configuration of pCAMP. The decrease of potassium current was related to the activation of protein kinase C (PKC) because the specific inhibitor of kinase C, Bis-1 (10^-9 M), abolished Ang-(1-12) effects on the potassium current. The question of whether the effect of Ang-(1-12) was related to the formation of Ang II by chymase was investigated.The results revealed that the intracellular administration of chymostatin, a chymase inhibitor (10^-9 M) abolished the effect of intracellular Ang-(1-12) on the potassium current. Moreover, intracellular Ang II (100 nM), by itself, reduced the potassium current, an effect decreased by intracellular valsartan (100 nM). Valsartan (10-9 M) dialyzed into the cell abolished the effect of Ang-(1-12) (100 nM). These observations demonstrate that the effect of Ang-(1-12) on potassium current was related to the formation of Ang II and that the peptide has arrhythmogenic properties.

Early afterdepolarizations promote transmural reentry in ischemic human ventricles with reduced repolarization reserve

AIMS

Acute ischemia is a major cause of sudden arrhythmic death, further promoted by potassium current blockers. Macro-reentry around the ischemic region and early afterdepolarizations (EADs) caused by electrotonic current have been suggested as potential mechanisms in animal and isolated cell studies. However, ventricular and human-specific arrhythmia mechanisms and their modulation by repolarization reserve remain unclear. The goal of this paper is to unravel multiscale mechanisms underlying the modulation of arrhythmic risk by potassium current (IKr) block in human ventricles with acute regional ischemia.

METHODS AND RESULTS

A human ventricular biophysically-detailed model, with acute regional ischemia is constructed by integrating experimental knowledge on the electrophysiological ionic alterations caused by coronary occlusion. Arrhythmic risk is evaluated by determining the vulnerable window (VW) for reentry following ectopy at the ischemic border zone. Macro-reentry around the ischemic region is the main reentrant mechanism in the ischemic human ventricle with increased repolarization reserve due to the ATP-sensitive potassium current (IK(ATP)) activation. Prolongation of refractoriness by 4% caused by 30% IKr reduction counteracts the establishment of macro-reentry and reduces the VW for reentry (by 23.5%). However, a further decrease in repolarization reserve (50% IKr reduction) is less anti-arrhythmic despite further prolongation of refractoriness. This is due to the establishment of transmural reentry enabled by electrotonically-triggered EADs in the ischemic border zone. EADs are produced by L-type calcium current (ICaL) reactivation due to prolonged low amplitude electrotonic current injected during the repolarization phase.

CONCLUSIONS

Electrotonically-triggered EADs are identified as a potential mechanism facilitating intramural reentry in a regionally-ischemic human ventricles model with reduced repolarization reserve.

Ca²⁺-induced delayed afterdepolarizations are triggered by dyadic subspace Ca2²⁺ affirming that increasing SERCA reduces aftercontractions

Ca(2+)-induced delayed afterdepolarizations (DADs) are depolarizations that occur after full repolarization. They have been observed across multiple species and cell types. Experimental results have indicated that the main cause of DADs is Ca(2+) overload. The main hypothesis as to their initiation has been Ca(2+) overflow from the overloaded sarcoplasmic reticulum (SR). Our results using 37 previously published mathematical models provide evidence that Ca(2+)-induced DADs are initiated by the same mechanism as Ca(2+)-induced Ca(2+) release, i.e., the modulation of the opening of ryanodine receptors (RyR) by Ca(2+) in the dyadic subspace; an SR overflow mechanism was not necessary for the induction of DADs in any of the models. The SR Ca(2+) level is better viewed as a modulator of the appearance of DADs and the magnitude of Ca(2+) release. The threshold for the total Ca(2+) level within the cell (not only the SR) at which Ca(2+) oscillations arise in the models is close to their baseline level (∼1- to 3-fold). It is most sensitive to changes in the maximum sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) pump rate (directly proportional), the opening probability of RyRs, and the Ca(2+) diffusion rate from the dyadic subspace into the cytosol (both indirectly proportional), indicating that the appearance of DADs is multifactorial. This shift in emphasis away from SR overload as the trigger for DADs toward a multifactorial analysis could explain why SERCA overexpression has been shown to suppress DADs (while increasing contractility) and why DADs appear during heart failure (at low SR Ca(2+) levels).

Mechanosensitive-mediated interaction, integration, and cardiac control

This review covers aspects of the cardiac mechanotransduction field at different levels, and advocates the possibility that mechanoelectro-chemical transduction forms part of a network of mechanically linked integration in heart-mechanically mediated integration (MMI). It assembles evidence and observations in the literature to promote this hypothesis. Mechanical components can provide the bond between interactions at molecular, cellular, and macro levels to enable the integration. Stretch-activated channels (SACs) exist in the heart, but stresses and strains can affect other membrane channels or receptors. A cellular mechanical change can thus promote several ionic or downstream changes. Cell signal cascades have been implicated and can affect membrane electrophysiology. MMI could shape intracellular and downstream signals using the cytoskeleton and intracellular Ca(2+). MMI also spans other regulatory systems and processes such as the autonomic nervous system (ANS) and operates throughout the whole heart as an integrative system. Finally, supporting the hypothesis, if elements of the normal integration become deranged it contributes to cardiovascular disease and, potentially, lethal arrhythmia.

Mechanosensitivity as an integrative system in heart: an audit

This review examines a manifold of apparently loosely linked observations and mechanisms, from membrane to man, and assembles them to support the notion that mechanoelectric transduction is an integrative regulatory system in the heart. For this, the assemblage has to satisfy, at least to some extent, criteria that apply to other integrative regulatory systems such as the endocrine and nervous systems. The integrative effectors in the endocrine system are chemical linkages, circulating hormones: in the nervous system the linkage is a network of cables, nerve conduction and neurotransmitters. Mechanical integration is would be effected through mechanical machinery, cardiac contractile and hydraulic function with attendant stress and strain transmitted via "tensegrity". This can, through the cytoskeleton, begin with membrane integrins and transmit intracellularly for example via F actins to reach the rest of the membranous integrins. Further transmission to the organ is via cell-to-cell adhesion complexes and the extracellular matrix. This tensegrity facilitates integration of force and strain changes from area to area. In consequence, and analogous to the neurendocrine system, mechanoelectric transduction should, and does (1) operate at the molecular or membrane level--this would be via mechanotransducers affecting transmembrane ionic flow; (2) operate in the cell--to influence electrophysiology; (3) have a multicellular expression--e.g. mechanical distortion of one cell can raise intracellular calcium of an adjacent cell; (4) express in the intact organ--e.g. an increase in venous return hydraulically distends the sinoatrial node, steepening its pacemaker potential, thus increasing heart rate. It should also (5) demonstrate elements of a feedback system--"mechanoelectric feedback", and (6) interact with other systems--the cytoskeleton incorporates cell signalling complexes intersecting with other signal cascades. Finally, (7) it can malfunction to produce clinical abnormality--it contributes electrophysiologically to lethal cardiac arrhythmia. This anatomical and functional behaviour of mechanoelectric transduction could sanction the prospect of viewing it as analogous to the other integrative physiological systems.

Role of intracellular calcium in the antiarrhythmic effect of procainamide during ventricular fibrillation in rat hearts

Increased intracellular calcium (calcium overload) is considered one of the factors that can initiate ventricular fibrillation. In addition, ventricular fibrillation itself can cause and possibly maintain calcium overload. The goal of this study was to determine whether the class IA antiarrhythmic agent procainamide can reduce calcium overload during ventricular fibrillation and, if so, whether this reduction could be responsible for the recovery of the left ventricular function after defibrillation. For this purpose, the effects of 0.1 mmol/L of procainamide perfusion on left ventricular developed pressure, cardiac rate, and intracellular calcium during pacing-induced ventricular fibrillation were measured in isolated perfused rat hearts. Intracellular calcium was assessed by surface fluorometry after indo 1 loading. The concentration of procainamide was selected such that approximately half of the hearts would functionally recover from fibrillation. Cardiac rate and intracellular calcium were compared among four groups, depending on both the perfusate used and the recovery of left ventricular developed pressure at the end of the experiment. We found that procainamide reduced intracellular calcium to steady-state levels in hearts in which left ventricular function completely recovered (developed pressure > 67% of the steady-state value). However, intracellular calcium remained elevated in partially recovered hearts (33% < or = pressure < or = 67%) and in nonrecovered hearts (pressure < 33%). Thus procainamide can reduce calcium overload during ventricular fibrillation, and this reduction could be responsible for the recovery of left ventricular function after defibrillation. This reduction was use dependent, that is, dependent on high cardiac rates during fibrillation rather than on the decrease of cardiac rates before or during defibrillation.

Cardiac arrhythmia therapy

Cardiac dysfunction is often manifested as arrhythmia, with disruption of the normal periodicity and regularity of electromechanical activity. The therapy for arrhythmia begins with proper diagnosis, since many pharmacological interventions are themselves arrhythmogenic. Intervention for acute arrhythmia involves correction of underlying systemic conditions by ensuring adequate oxygenation, ventilation, acid-base homeostasis, electrolyte balance, and fluid status. Classification of antiarrhythmic agents assists in a structured treatment approach that utilizes different agents based on the etiology of the arrhythmia and the drug's mechanism of action. A deliberate treatment strategy guided by the morphological criteria of the arrhythmia modified by the rate and duration of complexes, noting symptoms and hemodynamic stability, is desirable.

An experimental model of the production of early after depolarizations by injury current from an ischemic region

An ischemic myocardial region contains cells with a depolarized resting membrane potential. This depolarization leads to an intercellular current flow between the ischemic region and the surrounding normal myocardial cells which has been termed an "injury current". We have devised an experimental model system in which an isolated guinea pig ventricular cell is electrically coupled to a model depolarized cell in order to evaluate the effects of this injury current on the electrical properties of a normal ventricular cell exposed to drugs which increase calcium current or decrease potassium current. Using low doses of isoproterenol, forskolin, or Bay K 8644 (or 8-bromo-cyclic adenosine monophosphate in the pipette) we found that the action potential duration of the isolated cell was lengthened, but that early afterdepolarizations (EADs) were not produced unless the cell was also coupled to a depolarized cell model representing an adjacent ischemic region. A similar prolongation of the action potential was produced by low doses of quinidine, but EADs were not produced unless coupling to a depolarized cell model was added. EADs could not be produced in any cells in the absence of the drugs even though the coupling to the depolarized cell model was increased up to the level at which the action potential was indefinitely prolonged. At higher isoproterenol concentrations, EADs or spontaneous activity were produced without coupling to the depolarized cell model. Under these conditions, coupling of the cell to a cell model with normal resting membrane potential stopped the spontaneous activity and prevented the occurrence of EADs even with high levels of resistive coupling.(ABSTRACT TRUNCATED AT 250 WORDS)

The dose-related hyper-and-hypokalaemic effects of salbutamol and its arrhythmogenic potential

1. The hypokalaemic effect of salbutamol after more than 30 min of administration has been well described. A hyper-and-hypokalaemic effect for adrenaline has been reported, but no such hyperkalaemic effect for salbutamol. 2. The possible hyper-and-hypokalaemic effects of salbutamol with the concomitant potential for pro-arrhythmia were assessed in the baboon (Papio ursinus). 3. Male and female baboons were anaesthetized with ketamine (15 mg kg-1) and maintained with 6% pentobarbitone as spontaneously breathing animals. Six baboons in each group received either 10, 100 or 500 micrograms kg-1 salbutamol i.v. Lead II of the ECG and femoral i.a. blood pressure were recorded continuously for 10 min. Arterial blood samples were collected at 0 min and then after 3 and 10 min of salbutamol administration. 4. All the animals developed sinus tachycardia (above 200 beats min-1) within 30 s of each dose of salbutamol administration and the high heart rate persisted throughout the experiment. All the animals were hyperkalaemic after 3 min and hypokalaemic after 10 min for each dose of salbutamol. Left ventricular conduction defects were seen in 3 animals during the hyperkalaemic phase. No arrhythmia was seen during the hypokalaemic phase. 5. Salbutamol has a transient hyperkalaemic and a more prolonged hypokalaemic effect in the baboon. The hypokalaemia could not be associated with arrhythmia although conduction defects were associated with the hyperkalaemia. 6. Since salbutamol is used as a bronchodilator in asthmatic patients and to treat acute hyperkalaemia, it is suggested that caution should be exercised when using salbutamol in high doses to treat acute asthma especially during the first few minutes of administration. The finding of hyperkalaemia with salbutamol questions its use in the treatment of hyperkalaemia.