Straightforward Guide to the Science Behind Arrhythmogenesis
Straightforward Guide to the Science Behind Arrhythmogenesis
Abnormal impulse generation includes triggered activities and automaticity disorders, providing extra-systoles that further trigger arrhythmias (atrial or ventricular, tachycardia, fibrillation) which manifest on the surface ECG but can vary depending on the maintenance mechanisms.
The slope of repolarisation of cardiac cells is a crucial determinant of arrhythmogenicity as delayed repolarisation exposes the myocardium to electrical instability. Premature cardiac depolarisation can provoke an ectopic or echo beat if the amplitude and conduction velocity (CV) reach threshold values. When developing during phase 2 or 3, these membrane oscillations are called EADs (figure 5). They are the result of decreased outward potassium currents, most commonly the primary potassium channel in the ventricle, IKr, the rapid component of the delayed inward rectifier potassium channel. Its α subunit (pore forming) is encoded by the human ether-a-gogo related gene (hERG). Molecules or diseases can directly disturb the IKr current, increase APD, prolong the refractory period and QTc interval on ECG. A trafficking reduction of the hERG-forming proteins from the endoplasmic reticulum to the cell membrane has also been described in the LQT2 syndrome, but also with non-cardiovascular drugs, such as Hsp90 inhibitors.
(Enlarge Image)
Figure 5.
Key ionic disturbances involved in mechanisms for abnormal impulse generation including factors (triggers, substrates, modulators) involved in the development or impacting different types of cardiac arrhythmias.
Mechanistically, EADs result from the reactivation of Cav1.2 (eg, catecholamine-driven), increased net current through NCX, increased late sodium current as observed in the congenital Long QT3 syndrome, or under ischaemic conditions. EADs develop in tissues where the repolarisation reserve is decreased and proarrhythmic events prone to develop: when another potassium current, IKs, is weak or more generally, under pathophysiological conditions, such as heart failure (HF) or ventricular hypertrophy. EADs are considered as the events triggering the life-threatening polymorphic ventricular tachycardia, Torsade de Pointes (TdP). EADs are favoured by important electrolytes imbalance, increased cathecolamines or bradycardia. Suppressor factors are elevation of serum potassium concentration, infusion of magnesium and/or molecules able to accelerate the rate of repolarisation (eg, mexiletine or verapamil).
TdP is frequently observed in the context of a delayed repolarisation phase, that is, QTc interval prolongation on ECG (temporally, ST segment corresponds to the plateau phase of APs. A more precise analysis of T wave duration from its peak to its end ('Tpeak-to-Tend') has been demonstrated to be a reliable marker for ventricular tachycardia.
QT Interval Assessment Guidelines. If the proarrhythmogenicity of antiarrhythmic compounds prolonging the refractoriness by IKr block (eg, d-sotalol, dofetilide) was not completely unexpected in patients with cardiovascular diseases (SWORD, Survival with oral D-sotalol, trial), real concerns were identified when treatment with non-cardiovascular drugs (eg, prokinetic, psychiatric drugs) were reported to promote EADs and TdP. QTc interval prolongation has become the topic of intensive discussions between health authorities and pharmaceutical industry as considered to be the best marker of the TdP liability by regulators, while others highlight, alternatively, scrutinisation of the causes of cardiac arrhythmogenicity. The first step was the release of the non-clinical ICHS7B guideline focusing on QT interval prolongation, and the clinical E14 guideline outlining standards for thorough QT assessment (TQT study) during clinical development. S7B and E14 became very effective at reducing the number of compounds with QT interval prolongation liability submitted for approval. However, it also suppressed early compounds, long before the potentially beneficial therapeutic effects, or clinical effect on repolarisation, were known. This resulted in the IQ-Cardiac Safety Research Consortium (CSRC) collaboration, assessing the possibility of collecting adequate ECG data based on the earliest clinical studies (ie, exposure-effect analysis and QT study with small patient numbers).
While evaluation of ECG waves is mentioned in another non-clinical guidance (ICHS7A), it is necessary to highlight the absence of a guideline dedicated to drug proarrhythmogenicity. Calculation methods have even been proposed to better refine a drug therapeutic benefit compared with its potential proarrhythmogenicity. However, the most difficult aspect in arrhythmia prediction is to characterise the static elements (eg, hypokalaemia), and also the dynamicity (dispersion of repolarisation, beat-to-beat variability, of the combined elements interacting under conditions, such as altered autonomic tone or increased cathecolamines that ultimately provoke arrhythmias. Various experimental animal models were developed to better characterise conditions contributing to arrhythmias.
The DADs represent triggered electrical activities arising from phase 4 of the AP (figure 5). They develop in the context of intracellular calcium overload following calcium release from the sarcoplasmic reticulum (SR) during diastole. Calcium is thereafter exchanged for sodium ions by the NCX (figure 4), generating a net inward transient current (referred to as ITi), which may depolarise the cells and, when large enough, trigger an extra-systole.
Typically, DADs are observed after toxic exposure to cardiac glycosides (reduction of the sodium/potassium ATPase), during ischaemia, hypokalaemia, increased catecholamine levels (exercise increases the amplitude of DADs). During electrophysiological tests, DADs-dependent arrhythmias can be initiated by either overdrive pacing (increased likelihood) or programmed premature stimulation (in common with re-entry arrhythmias).
Whether a DAD can cause an ectopic AP or not, may depend on densities and functionalities of potassium currents (eg, IK1) to produce an outward current and counteract ITi. In HF, ventricular myocytes display smaller IK1 and a higher density of NCX generating a larger ITi current (impaired contraction), larger DADs which are therefore more likely to produce an AP, and trigger ventricular arrhythmias. During HF, cytosolic calcium is increased while intracellular calcium sequestration during diastole is decreased due to low levels of SERCA2a. Hyperactivation of type 2 ryanodine receptors (RyR2) may be caused by sympathetic neurons and increased levels of catecholamines, leading to hyperphosphorylation of RyR2 and dissociation of its associated protein calstabin, which can be counteracted by beta-adrenoceptor-blockers. DADs are also believed to trigger automatic AV nodal tachycardia and atrial fibrillation (AF). A recent study demonstrated the implication of DADs-related triggered activities in proarrhythmic events observed in the atrial myocardium of chronic AF patients. DADs can be suppressed by β-adrenergic receptor blockers (eg, propranolol) and calcium channel antagonists (eg, verapamil).
Proarrhythmogenicity? Considering the diversity of combined mechanisms and conditions, the prediction of drug arrhythmogenicity is not an easy task. Multi-ion channel blockers may either be less or even more proarrhythmic, depending on the kinetics of interaction with different ion channels. Compounds suppressing potassium and late sodium current/calcium current reduce their arrhythmogenicity, while reducing potassium and fast sodium currents are more likely to provoke arrhythmia by promoting re-entries (eg, quinidine). The CSRC in conjunction with the Food and Drug Administration (FDA) and the Health and Environmental Sciences Institute (HESI) has recently launched the Comprehensive In vitro Proarrhythmia Assay (CiPA) initiative. This aims to improve the detection of drug arrhythmogenicity by proposing approaches which incorporate new methodologies such as multi-ion channel studies, in silico cardiac cellular simulations or human cardiac myocytes derived from iPS cells.
Spontaneous cardiac pacemaker activity results from a diastolic slope (phase 4) which causes the membrane to reach a threshold voltage, allowing for re-excitation. Latent automaticity also characterises AV nodal cells and Purkinje fibres at a lower rate. This activity only manifests when the SA fails to generate impulses, or when APs fail to propagate. An increase in automaticity can arise from other excitable cells (eg, His Bundle) that suddenly depolarise much faster than the SA node to take control of the heart rhythm.
In the SA node, automaticity is under the influence of ion channels and pumps ('voltage clock'). The main ion channels involved in the pacemaker AP are shown in figure 3. The hyperpolarisation-activated pacemaker current, If, a net inward mixed sodium/potassium current, is the main contributor to the diastolic slope. It is controlled by the intracellular cyclic AMP levels, and activated from −110 to −40 mV. In parallel, spontaneous rhythmic submembrane local calcium release from SR activates NCX and depolarises the membrane further (referred to as 'calcium clock'. When the cell membrane reaches −40 mV, Cav1.2 channels activate, calcium entry opens RyR2, triggering the mass release of calcium (CICR) from the SR, which generates an AP, and initiates the contraction of the myocardium (figure 4). To initiate relaxation, calcium ions are pumped back to the SR by SERCA2a pump and extruded from the cytosol by NCX. Notably, while the Nav1.5 current is absent from the SA node, it transmits the electrical influx to the rest of the atria. Nav1.5 reduction (eg, loss of function through mutations in SCN5A) can lead to a widening of the P-wave or prolongation of PR interval.
Sinus node dysfunctions (SND) include sinus bradycardia, sinus pause/arrest, chronotropic incompetence, and sinoatrial exit block. SND can be associated with conduction system disease and supraventricular arrhythmias, such as AF and atrial flutter. When associated with dizziness or syncope, it is referred to as a sick sinus syndrome which is partially due to dysregulation of If. If is modulated by the central nervous system and selectively blocked by ivabradine, which is used in the treatment of angina, coronary artery disease, and/or HF. Other arrhythmias can result from direct or indirect sinus node modulation (ie, atrial tachycardia, atrial bradycardia) including suppression of impulse generation or conduction disturbances leading to ectopic pacemaker activity (ie, AV junctional rhythms). Abnormal automaticity can also arise from spontaneous activity in atrial or ventricular myocardium where the resting membrane potential is reduced (eg, after myocardial ischaemia or infarction).
Disorders in Impulse Generation/Abnormal Impulse Formation
Abnormal impulse generation includes triggered activities and automaticity disorders, providing extra-systoles that further trigger arrhythmias (atrial or ventricular, tachycardia, fibrillation) which manifest on the surface ECG but can vary depending on the maintenance mechanisms.
Early afterdepolarisations
The slope of repolarisation of cardiac cells is a crucial determinant of arrhythmogenicity as delayed repolarisation exposes the myocardium to electrical instability. Premature cardiac depolarisation can provoke an ectopic or echo beat if the amplitude and conduction velocity (CV) reach threshold values. When developing during phase 2 or 3, these membrane oscillations are called EADs (figure 5). They are the result of decreased outward potassium currents, most commonly the primary potassium channel in the ventricle, IKr, the rapid component of the delayed inward rectifier potassium channel. Its α subunit (pore forming) is encoded by the human ether-a-gogo related gene (hERG). Molecules or diseases can directly disturb the IKr current, increase APD, prolong the refractory period and QTc interval on ECG. A trafficking reduction of the hERG-forming proteins from the endoplasmic reticulum to the cell membrane has also been described in the LQT2 syndrome, but also with non-cardiovascular drugs, such as Hsp90 inhibitors.
(Enlarge Image)
Figure 5.
Key ionic disturbances involved in mechanisms for abnormal impulse generation including factors (triggers, substrates, modulators) involved in the development or impacting different types of cardiac arrhythmias.
Mechanistically, EADs result from the reactivation of Cav1.2 (eg, catecholamine-driven), increased net current through NCX, increased late sodium current as observed in the congenital Long QT3 syndrome, or under ischaemic conditions. EADs develop in tissues where the repolarisation reserve is decreased and proarrhythmic events prone to develop: when another potassium current, IKs, is weak or more generally, under pathophysiological conditions, such as heart failure (HF) or ventricular hypertrophy. EADs are considered as the events triggering the life-threatening polymorphic ventricular tachycardia, Torsade de Pointes (TdP). EADs are favoured by important electrolytes imbalance, increased cathecolamines or bradycardia. Suppressor factors are elevation of serum potassium concentration, infusion of magnesium and/or molecules able to accelerate the rate of repolarisation (eg, mexiletine or verapamil).
TdP is frequently observed in the context of a delayed repolarisation phase, that is, QTc interval prolongation on ECG (temporally, ST segment corresponds to the plateau phase of APs. A more precise analysis of T wave duration from its peak to its end ('Tpeak-to-Tend') has been demonstrated to be a reliable marker for ventricular tachycardia.
QT Interval Assessment Guidelines. If the proarrhythmogenicity of antiarrhythmic compounds prolonging the refractoriness by IKr block (eg, d-sotalol, dofetilide) was not completely unexpected in patients with cardiovascular diseases (SWORD, Survival with oral D-sotalol, trial), real concerns were identified when treatment with non-cardiovascular drugs (eg, prokinetic, psychiatric drugs) were reported to promote EADs and TdP. QTc interval prolongation has become the topic of intensive discussions between health authorities and pharmaceutical industry as considered to be the best marker of the TdP liability by regulators, while others highlight, alternatively, scrutinisation of the causes of cardiac arrhythmogenicity. The first step was the release of the non-clinical ICHS7B guideline focusing on QT interval prolongation, and the clinical E14 guideline outlining standards for thorough QT assessment (TQT study) during clinical development. S7B and E14 became very effective at reducing the number of compounds with QT interval prolongation liability submitted for approval. However, it also suppressed early compounds, long before the potentially beneficial therapeutic effects, or clinical effect on repolarisation, were known. This resulted in the IQ-Cardiac Safety Research Consortium (CSRC) collaboration, assessing the possibility of collecting adequate ECG data based on the earliest clinical studies (ie, exposure-effect analysis and QT study with small patient numbers).
While evaluation of ECG waves is mentioned in another non-clinical guidance (ICHS7A), it is necessary to highlight the absence of a guideline dedicated to drug proarrhythmogenicity. Calculation methods have even been proposed to better refine a drug therapeutic benefit compared with its potential proarrhythmogenicity. However, the most difficult aspect in arrhythmia prediction is to characterise the static elements (eg, hypokalaemia), and also the dynamicity (dispersion of repolarisation, beat-to-beat variability, of the combined elements interacting under conditions, such as altered autonomic tone or increased cathecolamines that ultimately provoke arrhythmias. Various experimental animal models were developed to better characterise conditions contributing to arrhythmias.
Delayed Afterdepolarisation
The DADs represent triggered electrical activities arising from phase 4 of the AP (figure 5). They develop in the context of intracellular calcium overload following calcium release from the sarcoplasmic reticulum (SR) during diastole. Calcium is thereafter exchanged for sodium ions by the NCX (figure 4), generating a net inward transient current (referred to as ITi), which may depolarise the cells and, when large enough, trigger an extra-systole.
Typically, DADs are observed after toxic exposure to cardiac glycosides (reduction of the sodium/potassium ATPase), during ischaemia, hypokalaemia, increased catecholamine levels (exercise increases the amplitude of DADs). During electrophysiological tests, DADs-dependent arrhythmias can be initiated by either overdrive pacing (increased likelihood) or programmed premature stimulation (in common with re-entry arrhythmias).
Whether a DAD can cause an ectopic AP or not, may depend on densities and functionalities of potassium currents (eg, IK1) to produce an outward current and counteract ITi. In HF, ventricular myocytes display smaller IK1 and a higher density of NCX generating a larger ITi current (impaired contraction), larger DADs which are therefore more likely to produce an AP, and trigger ventricular arrhythmias. During HF, cytosolic calcium is increased while intracellular calcium sequestration during diastole is decreased due to low levels of SERCA2a. Hyperactivation of type 2 ryanodine receptors (RyR2) may be caused by sympathetic neurons and increased levels of catecholamines, leading to hyperphosphorylation of RyR2 and dissociation of its associated protein calstabin, which can be counteracted by beta-adrenoceptor-blockers. DADs are also believed to trigger automatic AV nodal tachycardia and atrial fibrillation (AF). A recent study demonstrated the implication of DADs-related triggered activities in proarrhythmic events observed in the atrial myocardium of chronic AF patients. DADs can be suppressed by β-adrenergic receptor blockers (eg, propranolol) and calcium channel antagonists (eg, verapamil).
Proarrhythmogenicity? Considering the diversity of combined mechanisms and conditions, the prediction of drug arrhythmogenicity is not an easy task. Multi-ion channel blockers may either be less or even more proarrhythmic, depending on the kinetics of interaction with different ion channels. Compounds suppressing potassium and late sodium current/calcium current reduce their arrhythmogenicity, while reducing potassium and fast sodium currents are more likely to provoke arrhythmia by promoting re-entries (eg, quinidine). The CSRC in conjunction with the Food and Drug Administration (FDA) and the Health and Environmental Sciences Institute (HESI) has recently launched the Comprehensive In vitro Proarrhythmia Assay (CiPA) initiative. This aims to improve the detection of drug arrhythmogenicity by proposing approaches which incorporate new methodologies such as multi-ion channel studies, in silico cardiac cellular simulations or human cardiac myocytes derived from iPS cells.
Abnormal Automaticity
Spontaneous cardiac pacemaker activity results from a diastolic slope (phase 4) which causes the membrane to reach a threshold voltage, allowing for re-excitation. Latent automaticity also characterises AV nodal cells and Purkinje fibres at a lower rate. This activity only manifests when the SA fails to generate impulses, or when APs fail to propagate. An increase in automaticity can arise from other excitable cells (eg, His Bundle) that suddenly depolarise much faster than the SA node to take control of the heart rhythm.
In the SA node, automaticity is under the influence of ion channels and pumps ('voltage clock'). The main ion channels involved in the pacemaker AP are shown in figure 3. The hyperpolarisation-activated pacemaker current, If, a net inward mixed sodium/potassium current, is the main contributor to the diastolic slope. It is controlled by the intracellular cyclic AMP levels, and activated from −110 to −40 mV. In parallel, spontaneous rhythmic submembrane local calcium release from SR activates NCX and depolarises the membrane further (referred to as 'calcium clock'. When the cell membrane reaches −40 mV, Cav1.2 channels activate, calcium entry opens RyR2, triggering the mass release of calcium (CICR) from the SR, which generates an AP, and initiates the contraction of the myocardium (figure 4). To initiate relaxation, calcium ions are pumped back to the SR by SERCA2a pump and extruded from the cytosol by NCX. Notably, while the Nav1.5 current is absent from the SA node, it transmits the electrical influx to the rest of the atria. Nav1.5 reduction (eg, loss of function through mutations in SCN5A) can lead to a widening of the P-wave or prolongation of PR interval.
Sinus node dysfunctions (SND) include sinus bradycardia, sinus pause/arrest, chronotropic incompetence, and sinoatrial exit block. SND can be associated with conduction system disease and supraventricular arrhythmias, such as AF and atrial flutter. When associated with dizziness or syncope, it is referred to as a sick sinus syndrome which is partially due to dysregulation of If. If is modulated by the central nervous system and selectively blocked by ivabradine, which is used in the treatment of angina, coronary artery disease, and/or HF. Other arrhythmias can result from direct or indirect sinus node modulation (ie, atrial tachycardia, atrial bradycardia) including suppression of impulse generation or conduction disturbances leading to ectopic pacemaker activity (ie, AV junctional rhythms). Abnormal automaticity can also arise from spontaneous activity in atrial or ventricular myocardium where the resting membrane potential is reduced (eg, after myocardial ischaemia or infarction).
