Factors influencing long-term heart failure mortality in patients with obstructive hypertrophic cardiomyopathy in Western Sweden: probable dose-related protection from beta-blocker therapy

Obstructive hypertrophic cardiomyopathy: from genetic insights to a multimodal therapeutic approach with mavacamten, aficamten, and beyond

BACKGROUND

A cardiac condition marked by excessive growth of heart muscle cells, hypertrophic cardiomyopathy (HCM) is a complex genetic disorder characterized by left ventricular hypertrophy, microvascular ischemia, myocardial fibrosis, and diastolic dysfunction. Obstructive hypertrophic cardiomyopathy (oHCM), a subset of HCM, involves significant obstruction in the left ventricular outflow tract (LVOT), leading to symptoms like dyspnea, fatigue, and potentially life-threatening cardiac events. With advancements in genetic understanding and the introduction of novel pharmacologic agents, including cardiac myosin inhibitors like mavacamten and aficamten, there is a paradigm shift in the therapeutic approach to oHCM.

MAIN BODY

The underlying mechanisms of HCM are closely tied to genetic mutations affecting sarcomere proteins, particularly those encoded by the MYH7 and MYBPC3 genes. These mutations lead to disrupted sarcomere function, resulting in hypertrophic changes and LVOT obstruction. While genetic heterogeneity is a hallmark of HCM, clinical diagnosis relies heavily on imaging techniques such as Echocardiography and cardiac magnetic resonance imaging to assess the extent of hypertrophy and obstruction. Current pharmacological management of obstructive HCM (oHCM) focuses on alleviating symptoms rather than modifying disease progression. Beta-blockers and calcium channel blockers are primary treatment options, although their effectiveness varies among patients. Recent clinical trials have highlighted the potential of novel cardiac myosin inhibitors, including mavacamten and aficamten, in enhancing exercise capacity, reducing LVOT obstruction, and improving overall cardiac function. These innovative agents represent a significant breakthrough in targeting the fundamental pathophysiological mechanisms driving oHCM. A comprehensive literature review was conducted, utilizing top-tier databases such as PubMed, Scopus, and Google Scholar, to compile an authoritative and up-to-date overview of the current advancements in the field. This review sheds light on the updated 2024 American Heart Association (AHA) guidelines for HCM management, emphasizing the treatment cascade and tailored management for each stage of oHCM. By introducing a new paradigm for personalized medicine in oHCM, this research leverages advanced genomics, biomarkers, and imaging techniques to optimize treatment strategies.

CONCLUSIONS

The introduction of cardiac myosin inhibitors heralds a new era in the management of oHCM. By directly targeting the molecular mechanisms underpinning the disease, these novel therapies offer improved symptom relief and functional outcomes. Ongoing research into the genetic basis of HCM and the development of targeted treatments holds promise for further enhancing patient care. Future studies should continue to refine these therapeutic strategies and explore their long-term benefits and potential in diverse patient populations. This review makes a significant contribution to the field by synthesizing the most recent AHA guidelines, emphasizing the crucial role of tailored management strategies in optimizing outcomes for patients with oHCM, and promoting the incorporation of cutting-edge genomics and imaging modalities to enhance personalized care.

Advancements in the Diagnosis and Treatment of Hypertrophic Cardiomyopathy: A Comprehensive Review

Hypertrophic cardiomyopathy (HCM) is characterized by excessive growth of myocardial tissue, most commonly due to genetic mutations in sarcomere proteins. This can lead to complications such as heart failure, mitral regurgitation, syncope, arrhythmias, sudden cardiac death, and myocardial ischemia. While we have come a long way in our understanding of the pathophysiology, genetics, and epidemiology of HCM, the past 10 years have seen significant advancements in diagnosis and treatment. As the body of evidence on hypertrophic cardiomyopathy continues to grow, a comprehensive review of the current literature is an invaluable resource in organizing this knowledge. By doing so, the vast progress that has been made thus far will be widely available to all experts in the field. This review provides a comprehensive analysis of the scientific literature, exploring both well-established and cutting-edge diagnostic and therapeutic options. It also presents a unique perspective by incorporating topics such as exercise testing, genetic testing, radiofrequency ablation, risk stratification, and symptomatic management in non-obstructive HCM. Lastly, this review highlights areas where current and future research is at the forefront of innovation in hypertrophic cardiomyopathy.

Promising therapies for adults with symptomatic obstructive hypertrophic cardiomyopathy: 2023 and beyond

INTRODUCTION

Hypertrophic cardiomyopathy (HCM) is a heterogeneous genetic heart disease with an estimated prevalence in the general population of 0.2% to 0.6%. Clinically, HCM can range from no symptoms to severe symptoms such as heart failure or sudden cardiac death. Currently, the management of HCM involves lifestyle modifications, familial screening, genetic counseling, pharmacotherapy to manage symptoms, sudden cardiac death risk assessment, septal reduction therapy, and heart transplantation for specific patients. Multicenter randomized controlled trials have only recently explored the potential of cardiac myosin inhibitors (CMIs) such as mavacamten as a directed pharmacological approach for managing HCM.

AREAS COVERED

We will assess the existing medical treatments for HCM: beta-blockers, calcium channel blockers, disopyramide, and different CMIs. We will also discuss future HCM pharmacotherapy guidelines and underline this patient population's unfulfilled needs.

EXPERT OPINION

Mavacamten is the first-in-class CMI approved by the FDA to target HCM pathophysiology specifically. Mavacamten should be incorporated into the standard therapy for oHCM in case of symptom persistence despite using maximally tolerated beta blockers and/or calcium channel blockers. Potential drug-drug interactions should be assessed before initiating this drug. More studies are needed on the use of CMIs in patients with kidney and/or liver failure and pregnant/breastfeeding patients.

Long-Term Prevalence of Systolic Dysfunction in MYBPC3 Versus MYH7-Related Hypertrophic Cardiomyopathy

BACKGROUND

The 2 sarcomere genes most commonly associated with hypertrophic cardiomyopathy (HCM), MYBPC3 (myosin-binding protein C3) and MYH7 (β-myosin heavy chain), are indistinguishable at presentation, and genotype-phenotype correlations have been elusive. Based on molecular and pathophysiological differences, however, it is plausible to hypothesize a different behavior in myocardial performance, impacting lifetime changes in left ventricular (LV) function.

METHODS

We reviewed the initial and final echocardiograms of 402 consecutive HCM patients with pathogenic or likely pathogenic MYBPC3 (n=251) or MYH7 (n=151) mutations, followed over 9±8 years.

RESULTS

At presentation, MYBPC3 patients were less frequently obstructive (15% versus 26%; P=0.005) and had lower LV ejection fraction compared with MYH7 (66±8% versus 68±8%, respectively; P=0.03). Both HCM patients harboring MYBPC3 and MYH7 mutations exhibited a small but significant decline in LV systolic function during follow-up; however, new onset of severe LV systolic dysfunction (LV ejection fraction, <50%) was greater among MYBPC3 patients (15% versus 5% among MYH7; P=0.013). Prevalence of grade II/III diastolic dysfunction at final evaluation was comparable between MYBPC3 and MYH7 patients (P=0.509). In a Cox multivariable analysis, MYBPC3-positive status (hazard ratio, 2.53 [95% CI, 1.09-5.82]; P=0.029), age (hazard ratio, 1.03 [95% CI, 1.00-1.06]; P=0.027), and atrial fibrillation (hazard ratio, 2.39 [95% CI, 1.14-5.05]; P=0.020) were independent predictors of severe systolic dysfunction. No statistically significant differences occurred with regard to incidence of atrial fibrillation, heart failure, appropriate implanted cardioverter defibrillator shock, or cardiovascular death.

CONCLUSIONS

MYBPC3-related HCM showed increased long-term prevalence of systolic dysfunction compared with MYH7, in spite of similar outcome. Such observations suggest different pathophysiology of clinical progression in the 2 subsets and may prove relevant for understanding of genotype-phenotype correlations in HCM.

Indications and management of implantable cardioverter-defibrillator therapy in childhood hypertrophic cardiomyopathy

Sudden cardiac death is the most common mode of death during childhood and adolescence in hypertrophic cardiomyopathy, and identifying those individuals at highest risk is a major aspect of clinical care. The mainstay of preventative therapy is the implantable cardioverter-defibrillator, which has been shown to be effective at terminating malignant ventricular arrhythmias in children with hypertrophic cardiomyopathy but can be associated with substantial morbidity. Accurate identification of those children at highest risk who would benefit most from implantable cardioverter-defibrillator implantation while minimising the risk of complications is, therefore, essential. This position statement, on behalf of the Association for European Paediatric and Congenital Cardiology (AEPC), reviews the currently available data on established and proposed risk factors for sudden cardiac death in childhood-onset hypertrophic cardiomyopathy and current approaches for risk stratification in this population. It also provides guidance on identification of individuals at risk of sudden cardiac death and optimal management of implantable cardioverter-defibrillators in children and adolescents with hypertrophic cardiomyopathy.

Clinical Features of Heart Failure in Patients With Hypertrophic Cardiomyopathy in a Regional Japanese Cohort - Results From the Kochi RYOMA Study

BACKGROUND

The clinical features of heart failure (HF) in patients with hypertrophic cardiomyopathy (HCM) in Japan have not been fully elucidated.

METHODS AND RESULTS

In 293 patients with HCM (median age at registration, 65 (57-72) years) in a prospective cardiomyopathy registration network in Kochi Prefecture (Kochi RYOMA study), HF events (HF death or hospitalization for HF) occurred in 35 patients (11.9%) (median age, 76 (69-80) years), including 11 HF deaths during a median follow-up of 6.1 years. The 5-year HF events rate was 9.6%. Atrial fibrillation, low percentage of fractional shortening, and high B-type natriuretic peptide level at registration were predictors of HF events. The combination of these 3 factors had a relatively high positive predictive value (55%) for HF events and none of them had a high negative predictive value (99%). There were 4 types of HF profile: left ventricular (LV) systolic dysfunction (40%), severe LV diastolic dysfunction (34%), LV outflow tract obstruction (LVOTO) (20%), and primary mitral regurgitation (MR) (6%). HF deaths occurred in patients with LV systolic dysfunction or LV diastolic dysfunction, but none of patients with LVOTO or primary MR due to additional invasive therapies.

CONCLUSIONS

In a Japanese HCM cohort, HF was an important complication, requiring careful follow-up and appropriate treatment.

Factors associated with excess female mortality in obstructive hypertrophic cardiomyopathy

BACKGROUND

Several studies have reported excess female mortality in patients with hypertrophic cardiomyopathy, but the cause is unknown.

AIMS

To compare risk-factors for disease-related death in both sexes in a geographical cohort of patients with obstructive hypertrophic cardiomyopathy (oHCM).

METHODS AND RESULTS

Data-bases in all ten hospitals within West Götaland Region yielded 250 oHCM-patients (123 females, 127 males). Mean follow-up was 18.1 y. Risk-factors for disease-related death were evaluated by Cox-hazard regression and Kaplan-Meier survival-curves, with sex-comparisons of distribution of risk-factors and therapy in total and age-matched (n = 166) groups. At diagnosis females were older, median 62 y vs. 51 y, (P < 0.001), but not different in outflow-gradients and median NYHA-class. However, septal hypertrophy was more advanced: 10.6 [IQR = 3.2] vs. 9.6 [2.5] mm/m2 BSA; P = 0.002. Females had higher disease-related mortality than males (P = <0.001), with annual mortality 2.9% vs. 1.5% in age-matched groups (P = 0.010 log-rank). For each risk-category identified (NYHA-class ≥ III, outflow-gradient ≥50 mmHg), a higher proportion of females died (P = 0.0004; P = 0.001). Calcium-blocker therapy was a risk-factor (P = 0.005) and was used more frequently in females (P = 0.034). A beta-blocker dose above cohort-median reduced risk for disease-related death in both males (HR = 0.32; P = 0.0040) and in females (HR = 0.49; P = 0.020). Excess female deaths occurred in chronic heart-failure (P = 0.001) and acute myocardial infarctions (P = 0.015). Fewer females received beta-blocker therapy after diagnosis (64% vs. 78%, P = 0.018), in a smaller dose (P = 0.007), and less frequently combined with disopyramide (7% vs. 16%, P = 0.048).

CONCLUSION

Addressing sex-disparities in the timing of diagnosis and pharmacological therapy has the potential to improve the care of females with oHCM.

What Aspects of Phenotype Determine Risk for Sudden Cardiac Death in Pediatric Hypertrophic Cardiomyopathy?

Sudden cardiac death due to hypertrophic cardiomyopathy (HCM), is the most common autopsy-proven cause of unexpected medical death in children after infancy. This mode of death is preventable by implantation of an internal cardiac defibrillator (ICD), a procedure that has considerable morbidity in childhood patients, and even mortality. Since HCM is an inheritable disease (usually autosomal dominant, occasionally recessive), family screening may identify subjects at risk. This review summarizes published studies carried out to identify which phenotypic markers are important risk factors in childhood patients with HCM and reviews the performance of existing risk-stratification algorithms (HCM Risk-Kids, PRIMaCY) against those of single phenotypic markers. A significant proportion of HCM-patients diagnosed in childhood are associated with RASopathies such as Noonan syndrome, but a knowledge gap exists over risk stratification in this patient group. In conclusion, pediatric risk-stratification algorithms for sudden cardiac death perform better in children than adult HCM risk-stratification strategies. However, current multivariable algorithms overestimate risk substantially without having high sensitivity, and remain ‘a work in progress’. To include additional phenotypic parameters that can be reproducibly measured such as ECG-markers, e.g., ECG risk score (which has high sensitivity and negative predictive value), tissue Doppler diastolic function measurements, and quantification of myocardial scarring on cardiac magnetic resonance imaging, has the potential to improve risk-stratification algorithms. Until that work has been achieved, these are three factors that the clinician can combine with the current algorithm-calculated per cent risk, in order better to assess risk.

Sudden cardiac death in childhood hypertrophic cardiomyopathy is best predicted by a combination of electrocardiogram risk-score and HCMRisk-Kids score

AIM

To compare risk algorithms (HCMRisk-Kids, ECG Risk-score) in hypertrophic cardiomyopathy (HCM) without syndrome association (ns-HCM) and with Noonan-like syndromes (RAS-HCM).

METHODS

A national paediatric HCM cohort (n = 151), presenting <19 years of age, mean follow-up 13.3 years, from all Swedish centres of Paediatric Cardiology (presenting 1972-2015), with 41 RAS-HCM patients (61% males), and 110 ns-HCM patients (68% familial; 65% males). The end-point was a composite of sudden cardiac death and resuscitated cardiac arrest (SCD/CA). Risk-factors were studied with Cox-hazard regression, and receiver operating characteristic curve analysis (C-statistic).

RESULTS

There were 33 SCD/CA, 27/110 in ns-HCM and 6/41 in RAS-HCM (p = 0.27). In ns-HCM HCMRisk-Kids ≥6% at diagnosis had C-statistic of 0.69 for predicting SCD/CA during first 5 years of follow-up and positive predictive value (PPV) of 22%. After 7 years of age (HCMRisk-Kids7plus), C-statistic was 0.76. ECG Risk-score ≥6 at diagnosis had C-statistic 0.87 and PPV of 31%. Independent risk factors for SCD/CA were HCMRisk-Kids7plus score (p = 0.005) and ECG risk-score (p < 0.001), whereas early beta-blocker dose (p = 0.001) and myectomy (p = 0.004) reduced risk. The sum of HCMRisk-Kids7yplus and ECG Risk-score7yplus ≥14 best predicted SCD/CA within 5 years in ns-HCM with C-statistic of 0.90 [0.83-0.96], sensitivity 100% and PPV 38%.

CONCLUSION

Combining the ECG Risk-score with HCMRisk-Kids improves risk stratification in ns-HCM and shows promise in RAS-HCM.

Management of cardiac aspects in children with Noonan syndrome - results from a European clinical practice survey among paediatric cardiologists

BACKGROUND

The majority of children with Noonan syndrome (NS) or other diseases from the RASopathy spectrum suffer from congenital heart disease. This study aims to survey cardiac care of this patient cohort within Europe.

METHODS

A cross-sectional exploratory survey assessing the treatment and management of patients with NS by paediatric endocrinologists, cardiologists and clinical geneticists was developed. This report details responses of 110 participating paediatric cardiologists from multiple countries.

RESULTS

Most paediatric cardiologists responding to the questionnaire were associated with university hospitals, and most treated <10 patients/year with congenital heart disease associated with the NS spectrum. Molecular genetic testing for diagnosis confirmation was initiated by 81%. Half of the respondents reported that patients with NS and congenital heart disease typically present <1y of age, and that a large percentage of affected patients require interventions and pharmacotherapy early in life. A higher proportion of infant presentation and need for pharmacotherapy was reported by respondents from Germany and Sweden than from France and Spain (p = 0.031; p = 0.014; Fisher's exact test). Older age at first presentation was reported more from general hospitals and independent practices than from university hospitals (p = 0.031). The majority of NS patients were followed at specialist centres, but only 37% reported that their institution offered dedicated transition clinic to adult services. Very few NS patients with hypertrophic cardiomyopathy (HCM) were reported to carry implantable cardioverter defibrillators for sudden cardiac death prevention. Uncertainty was evident in regard to growth hormone treatment in patients with NS and co-existing HCM, where 13% considered it not a contra-indication, 24% stated they did not know, but 63% considered HCM either a possible (20%) or definite (15%) contraindication, or a cause for frequent monitoring (28%). Regarding adverse reactions for patients with NS on growth hormone therapy, 5/19 paediatric cardiology respondents reported a total of 12 adverse cardiac events.

CONCLUSIONS

Congenital heart disease in patients with NS or other RASopathies is associated with significant morbidity during early life, and specialty centre care is appropriate. More research is needed regarding the use of growth hormone in patients with NS with congenital heart disease, and unmet medical needs have been identified.

The Impact of Glucocorticoid Therapy on Guideline-Directed Medical Treatment Titration in Patients Hospitalized for Heart Failure with Low Blood Pressure: A Retrospective Study

Background

Positive inotropic and renal protective actions of glucocorticoids have been observed clinically. Therefore, glucocorticoids may be used in patients with heart failure and low blood pressure (HF-LBP).

Methods

The medical records of 144 consecutive patients with HF-LBP who received glucocorticoids as an adjunctive treatment to facilitate the up-titration of β-blocker and angiotensin-converting enzyme inhibitor were reviewed.

Results

After four weeks of treatment, the metoprolol and captopril (or equivalent) dosages were progressively and consistently increased from 25 (interquartile range [IQR] = 12.5-75 mg/day) to 100 mg/day (IQR = 50-178.8 mg/day) and from 0 (IQR = 0-25 mg/day) to 12.5 mg/day (IQR = 0-50 mg/day), respectively. There was a remarkable beneficial hemodynamic response to the glucocorticoid treatment signified by an increase in blood pressure and decrease in heart rate. The average heart rate decreased by 6 beat per minute (bpm) (0.5-16 bpm), and the mean arterial blood pressure increased from 74.06 ± 7.81 to 78.85 ± 7.91 mmHg. We also observed an improvement in renal function and an increased diuretic response following glucocorticoid treatment. As a result, the left ventricular ejection fraction increased from 28.92 ± 8.06% to 33.86 ± 8.76%, and the diuretic response increased from 776.7 mL/40 mg furosemide (IQR = 133.8-2000 mL) to 4000 mL/40 mg furosemide on day 28 (IQR = 2200-5925 mL).

Conclusion

The use of glucocorticoid treatment to maintain hemodynamic and renal functional targets when titrating guideline-directed medical treatment in patients with HF-LBP may be safe, effective, and feasible.

Morbidity and resource usage after myectomy- or pacing-treatment in hypertrophic obstructive cardiomyopathy: A case-control study

BACKGROUND

Treatment of left ventricular outflow-obstruction (LVOTO) in hypertrophic obstructive cardiomyopathy (HOCM) by short atrio-ventricular delay pacing has long-term hemodynamic results that are not inferior to myectomy, but publications comparing long-term morbidity following those treatments are lacking.

METHODS

A search for HOCM-patients attending all ten hospitals in the West Götaland Region, Sweden, from 2002 through 2013, identified 251 patients (42 treated with myectomy, 88 with pacing and 121 conservatively). As the age at procedure was significantly lower in the myectomy-group compared to the pacing-group, morbidity was compared by case-control methodology, matching patients for age, maximal wall thickness and LVOT-gradient. We found 31 pairs who constituted the comparison-groups. Post-intervention median follow-up was 15.4 and 10.4 years in pacing- and myectomy-group, respectively. Post-procedural and long-term complications and re-interventions, length of stay, and cost of hospitalization were documented.

RESULTS

Both treatments improved New York Heart Association class and LVOT-gradients significantly. There were fewer peri-procedural complications in the pacing-group compared to myectomy-group (3.2% and 35.5% p < 0.001). During follow-up pacemaker was implanted in 35.5% of myectomy-group for atrio-ventricular block, 9.7% peri-operatively, and 25.8% during late-follow-up. Furthermore, the pacing group had a superior freedom from all re-interventions, 90.3% versus 61.3% in myectomy-group (p = 0.003). Pacing patients had a shorter in-hospital stay (median 4 [IQR = 2] days) compared to myectomy 11 [7] days; P < 0.001). The mean cost of hospitalization was 74,000 ± 16,000 SEK for pacing and 310,000 ± 180,000 SEK for myectomy, p < 0.001.

CONCLUSION

Pacing is a simple and reliable treatment for drug-refractory HOCM-patients with low rate of complications and costs.

A Validated Model for Sudden Cardiac Death Risk Prediction in Pediatric Hypertrophic Cardiomyopathy

BACKGROUND

Hypertrophic cardiomyopathy is the leading cause of sudden cardiac death (SCD) in children and young adults. Our objective was to develop and validate a SCD risk prediction model in pediatric hypertrophic cardiomyopathy to guide SCD prevention strategies.

METHODS

In an international multicenter observational cohort study, phenotype-positive patients with isolated hypertrophic cardiomyopathy <18 years of age at diagnosis were eligible. The primary outcome variable was the time from diagnosis to a composite of SCD events at 5-year follow-up: SCD, resuscitated sudden cardiac arrest, and aborted SCD, that is, appropriate shock following primary prevention implantable cardioverter defibrillators. Competing risk models with cause-specific hazard regression were used to identify and quantify clinical and genetic factors associated with SCD. The cause-specific regression model was implemented using boosting, and tuned with 10 repeated 4-fold cross-validations. The final model was fitted using all data with the tuned hyperparameter value that maximizes the c-statistic, and its performance was characterized by using the c-statistic for competing risk models. The final model was validated in an independent external cohort (SHaRe [Sarcomeric Human Cardiomyopathy Registry], n=285).

RESULTS

Overall, 572 patients met eligibility criteria with 2855 patient-years of follow-up. The 5-year cumulative proportion of SCD events was 9.1% (14 SCD, 25 resuscitated sudden cardiac arrests, and 14 aborted SCD). Risk predictors included age at diagnosis, documented nonsustained ventricular tachycardia, unexplained syncope, septal diameter z-score, left ventricular posterior wall diameter z score, left atrial diameter z score, peak left ventricular outflow tract gradient, and presence of a pathogenic variant. Unlike in adults, left ventricular outflow tract gradient had an inverse association, and family history of SCD had no association with SCD. Clinical and clinical/genetic models were developed to predict 5-year freedom from SCD. Both models adequately discriminated between patients with and without SCD events with a c-statistic of 0.75 and 0.76, respectively, and demonstrated good agreement between predicted and observed events in the primary and validation cohorts (validation c-statistic 0.71 and 0.72, respectively).

CONCLUSION

Our study provides a validated SCD risk prediction model with >70% prediction accuracy and incorporates risk factors that are unique to pediatric hypertrophic cardiomyopathy. An individualized risk prediction model has the potential to improve the application of clinical practice guidelines and shared decision making for implantable cardioverter defibrillator insertion. Registration: URL: https://www.clinicaltrials.gov; Unique identifier: NCT0403679.

Mathematical model of the ventricular action potential and effects of isoproterenol-induced cardiac hypertrophy in rats

Mathematical action potential (AP) modeling is a well-established but still-developing area of research to better understand physiological and pathological processes. In particular, changes in AP mechanisms in the isoproterenol (ISO) -induced hypertrophic heart model are incompletely understood. Here we present a mathematical model of the rat AP based on recordings from rat ventricular myocytes. In our model, for the first time, all channel kinetics are defined with a single type of function that is simple and easy to apply. The model AP and channels dynamics are consistent with the APs recorded from rats for both Control (absence of ISO) and ISO-treated cases. Our mathematical model helps us to understand the reason for the prolongation in AP duration after ISO application while ISO treatment helps us to validate our mathematical model. We reveal that the smaller density and the slower gating kinetics of the transient K⁺ current help explain the prolonged AP duration after ISO treatment and the increasing amplitude of the rapid and the slow inward rectifier currents also contribute to this prolongation alongside the flux in Ca²⁺ currents. ISO induced an increase in the density of the Na⁺ current that can explain the faster upstroke. We believe that AP dynamics from rat ventricular myocytes can be reproduced very well with this mathematical model and that it provides a powerful tool for improved insights into the underlying dynamics of clinically important AP properties such as ISO application.