Attention deficit hyperactivity disorder (ADHD) is a prevalent neurodevelopmental disorder with onset typically in childhood with most individuals experiencing symptoms throughout their lifetime [1]. First‐line therapy for ADHD consists of stimulant medication combined with psychotherapy and skill‐building interventions [2]. Stimulant medication therapy is highly effective and considered beneficial in up to 70% of ADHD cases by managing inattention, hyperactivity, and impulsivity [3, 4]. Prescription rates for ADHD medications have increased significantly across all age groups in recent years [5]. Methylphenidate (MPH) and amphetamine are commonly used stimulant medications, often alongside atomoxetine (ATX), an established selective norepinephrine reuptake inhibitor [3, 6]. All three drugs are sympathomimetic amines that increase norepinephrine and dopamine concentration in the prefrontal cortex, stimulating the central nervous system (CNS) [3].

In 2006, Health Canada issued a safety warning regarding rare heart‐related risks associated with ADHD medications after rare reports of sudden deaths in children and adults on stimulants. In 2011, an FDA safety review similarly advised against the use of stimulant medication and ATX in patients with heart disease or intolerance to heart rate and blood pressure changes. Although the American Heart Association issued a statement in 2008 recommending children undergo a thorough cardiac history, physical exam, and electrocardiogram (Class IIa recommendation) prior to starting stimulant therapy [7], subsequent clarification with the American Academy of Pediatrics limited electrocardiogram screening for cases with concerning medical family history or cardiac findings [8]. Similarly, the Canadian ADHD Resource Alliance (CADDRA) guidelines recommend monitoring blood pressure and heart rate in all patients while limiting electrocardiogram monitoring only in patients with known or high‐risk cardiac conditions [9]. In addition, ADHD itself is independently associated with an increased risk of cardiovascular disease, a risk that is significant for all patients regardless of age or medication use [10, 11]. Despite acknowledgement of statistically significant cardiovascular changes and events associated with stimulant medication use, the clinical relevance of stimulant medication‐related changes remains largely unknown.

Patients with pre‐existing arrhythmia syndromes, such as long QT syndrome (LQTS) and Wolff–Parkinson–White (WPW), warrant special consideration when prescribing stimulant medications [12]. These conditions represent two distinct electrophysiological substrates that may be influenced by sympathomimetic medications: abnormal repolarization and abnormal impulse propagation, respectively. In LQTS, medications that prolong repolarization may further extend the QT interval, heightening the risk of torsades de pointes and sudden cardiac death [13, 14]. In contrast, stimulant use in patients with WPW carries a theoretical risk of increasing conduction velocity (CV) through accessory pathways, which could increase susceptibility to tachyarrhythmias [15, 16, 17]. Previous reports suggest that stimulant‐induced catecholamine release may facilitate rapid conduction of atrial fibrillation through an accessory pathway to the ventricles, potentially degenerating into ventricular fibrillation and sudden cardiac arrest [18, 19]. Although enhanced accessory pathway conduction has been proposed as a potential mechanism, the direct effects of stimulant medications on myocardial impulse propagation remain unclear [12]. While WPW and LQTS are frequently discussed in the context of stimulant‐associated cardiac safety, the potential cardiac effects of stimulant medications may have broader relevance beyond these conditions.

Human‐induced pluripotent stem cell‐derived cardiomyocytes (hiPSC‐CMs) have emerged as a valuable in vitro platform for evaluating drug‐induced electrophysiological effects and cardiac safety beyond traditional preclinical models [20, 21]. Previous studies have demonstrated that hiPSC‐CMs exhibit functional adrenergic signaling, whereby sympathomimetic stimulation increases spontaneous beat rate (BR), alters intracellular calcium (Ca²⁺) handling, and modulates repolarization properties in a dose‐dependent manner. For example, β‐adrenergic agonists such as isoprenaline have been shown to enhance automaticity and Ca²⁺ transient amplitude, supporting their utility for studying pharmacological modulation of cardiac electrophysiology [22, 23, 24]. Such findings highlight the ability of hiPSC‐CM models to capture physiologically relevant responses to compounds that influence sympathetic signaling.

Despite the widespread clinical use of CNS stimulants such as MPH, ATX, and dextroamphetamine (DEX), there remains limited experimental data describing their direct effects on cardiomyocyte electrophysiology. To address this gap, we used atrial and ventricular hiPSC‐CM monolayers to characterize the time‐dependent effects of clinically relevant stimulant compounds on CV, BR, and repolarization. These parameters were selected because they reflect key electrophysiological parameters relevant to stimulant‐associated cardiac safety concerns. In particular, CV is relevant to disorders of abnormal impulse propagation such as WPW, while repolarization is relevant to conditions characterized by delayed repolarization such as LQTS. By evaluating multiple electrophysiological endpoints using a hiPSC‐CM model, this study sought to provide insights into cardiac electrophysiology responses that may be relevant to individuals with underlying arrhythmia susceptibility.

A detailed description of the methods is provided in the Supporting Information.

In this study, we investigated the effects of clinically relevant concentrations of stimulant medications, including MPH, DEX, and ATX on atrial and ventricular hiPSC‐CMs using a MEA platform. Given clinical concerns regarding stimulant use in individuals with underlying cardiac conditions such as WPW and LQTS [12], our primary aim was to characterize the effects of these compounds on key electrophysiological parameters relevant to arrhythmia susceptibility, including CV, automaticity, and repolarization.

Our findings demonstrate that hiPSC‐CMs preserve distinct chamber‐specific electrophysiological properties, with aCMs exhibiting higher intrinsic BR, faster CV, and shorter repolarization duration compared to vCMs (Figure 2). These chamber‐specific differences in CV may reflect variations in intrinsic cellular electrophysiology, intracellular coupling, and experimental configuration, as conduction properties across hiPSC‐CM models can be influenced by differences in tissue architecture, maturation state, and connexin expression [43, 44]. These observations are consistent with known electrophysiological differences between atrial and ventricular myocardium and support the utility of chamber‐specific hiPSC‐CMs as a platform for studying drug effects on human CMs. Importantly, accessory pathways in WPW consist of ordinary working myocardium or large fascicles of cardiac muscle that bridge atrium to ventricle in areas where the fibrous layer is fenestrated or incomplete, and in some cases, may take the form of atrio‐fascicular tracts [45, 46, 47]. Our approach enabled assessment of stimulant‐induced changes in CV across atrioventricular myocardial tissue relevant to pre‐excitation syndromes.

In our hiPSC‐CM model, stimulant exposure produced minimal effects on CV across both cell types (Figure 3A & 3B). In cardiac tissue, CV is primarily determined by the kinetics of Na_v1.5‐mediated inward sodium current (I _Na), which underlies the rapid upstroke (V _max) of the action potential depolarization, the passive electrical coupling between CMs via gap junctions, primarily Cx43 in ventricular and Cx40 in atrial tissue, and electrotonic properties of the cardiac tissue network [48, 49, 50, 51, 52]. Modulation of these properties can alter impulse propagations; however, none of the stimulant compounds tested in this study produced sustained changes in CV under the examined conditions. ISO modestly increased CV at early time points, which is consistent with the known effect of β‐adrenergic receptor (AR) stimulation on cardiac electrophysiology. Previous studies have demonstrated that activation of β‐ARs using isoprenaline increases I _Na availability and enhances gap junctional conductance through protein kinase A (PKA)‐dependent signaling pathways [53, 54]. In contrast, a transient early decrease in CV was observed with MPH, which could reflect indirect ionic effects as MPH has no established direct ion channel targets at therapeutic concentrations [55]. ATX, on the other hand, has been shown to block Na_v1.5 channels in a state‐ and use‐dependent manner at concentrations within its therapeutic plasma range; yet we observed no significant CV effects with ATX exposure under our experimental conditions [56]. MEA‐derived conduction measurements may be influenced by electrode geometry and lack the spatial resolution necessary to characterize conduction heterogeneity. While not employed in this study, optical mapping would provide substantially higher spatial and temporal resolution for assessment of activation patterns and monolayer‐level conduction dynamics. While MEA is well suited for comparative drug screening and longitudinal studies, complementary optical mapping may provide greater sensitivity for detecting subtle conduction effects. Overall, none of the stimulants tested (MPH, DEX, ATX, and CAFF) produced significant or sustained effects on CV in aCM and vCM monolayers. These findings are consistent with previous consensus statements indicating that stimulant use in WPW does not appear to increase arrhythmic risk through conduction‐dependent mechanisms [57], and reinforce the importance of pathway‐specific electrophysiologic properties, rather than stimulant exposure, in risk stratification and clinical decision‐making.

While CV was largely preserved, all stimulant compounds produced a rapid, transient increase in BR, most prominently within the first hour of exposure (Figure 3C,D). This response was consistent across both aCMs and vCMs. Following the initial chronotropic response, BR declined progressively over time, potentially reflecting a time‐dependent attenuation of the adrenergic signal. Positive controls, ISO and CAFF, induced the most robust and sustained increases in BR, confirming the chronotropic responsiveness of the hiPSC‐CM model [58, 59]. The positive chronotropic effects of catecholaminergic stimulation in hiPSC‐CMs are mediated through β‐AR activation, which increases intracellular cAMP via adenylyl cyclase and activates PKA. This signaling enhances HCN₄‐mediated pacemaker currents (I _f) and L‐type Ca²⁺ current (I _Ca,L), and accelerates sarcoplasmic reticulum Ca²⁺ cycling, resulting in increased automaticity and faster BR [60, 61]. In our model, the magnitude of the chronotropic response to stimulants was substantially smaller than that observed with ISO, a direct non‐selective β‐AR agonist. Stimulants like MPH, DEX, and ATX enhance noradrenergic signaling at sympathetic nerve terminals through distinct presynaptic mechanisms. MPH inhibits both the norepinephrine and dopamine transporters (NET and DAT, respectively), increasing synaptic catecholamine availability. ATX acts selectively at NET, elevating synaptic norepinephrine. DEX acts primarily as a transporter substrate rather than a reuptake inhibitor, which promotes presynaptic catecholamine release through vesicular displacement (VMAT2‐mediated) and reverse transport through NET. The net effect of all three compounds is an increase in extracellular synaptic norepinephrine availability, representing their principal cardiac effect. Dopamine elevation, while pharmacologically relevant in the CNS, contributes minimally to cardiac adrenergic signaling [3, 62, 63, 64, 65]. The resulting β‐AR stimulation in CMs mimics physiological sympathetic activation, which may underlie the increased incidence of palpitations reported by patients taking these medications [66].

In addition to their effects on conduction and automaticity, we evaluated the impact of stimulant compounds on repolarization, as reflected by changes in FPDc. FPDc measured on the MEA platform is considered as an in vitro surrogate for CM repolarization time, analogous to the rate‐corrected QT interval on the surface ECG, and forms the basis of several pro‐arrhythmia screening platforms [23]. Stimulant exposure produced a consistent directional effect on FPDc across all stimulants, with ATX eliciting the most pronounced FPDc prolongation in both aCMs and vCMs (Figure 3E,F). While direct ionic currents were not measured, previous in vitro studies have shown that ATX directly inhibits hERG‐mediated rapid delayed rectifier potassium current (I _Kr) in a concentration‐dependent manner [67]. Because I _Kr is the primary mediator of Phase 3 cardiac repolarization, its inhibition delays repolarization which prolongs action potential duration and FPD measured on the MEA platform. Inhibition of hERG channels is a well‐established mechanism underlying drug‐induced QT prolongation [68], and likely contributes to the measurable FPDc prolongation observed in our study. MPH and DEX are not reported to exert direct effects on hERG or other cardiac ion channels at therapeutic plasma concentrations. The modest FPDc prolongation observed with these stimulants may, therefore, reflect indirect effects of elevated catecholamines rather than direct ion channel blockade. Increased catecholamine levels activate β‐AR signaling pathways that enhance intracellular Ca²⁺ cycling and modify repolarizing K⁺ currents (I _K ). While acute β‐AR stimulation typically shortens repolarization via PKA‐mediated augmentations of I _Ks (encoded by KCNQ1/KCNE1) and I _Ca,L, sustained adrenergic drive has been shown to induce electrophysiological remodeling, including altered repolarizing I _K or I _Ca balance, which can prolong FPD [69, 70]. At the model level, hiPSC‐CMs have a relatively immature electrophysiological phenotype with attenuated I_K expression, which may limit repolarization response to adrenergic stimulation. This may also account for failure of indirect β‐AR stimulants (MPH, DEX, and CAFF) to accelerate FPDc as seen with ISO, which shortened FPDc in vCMs, consistent with rate‐dependent repolarization effects [71]. Smaller FPDc changes with MPH, DEX, and CAFF may reflect subtle repolarization changes that are often clinically silent but could become relevant in susceptible individuals. The divergence in FPDc responses between stimulants and ISO may reflect distinction between indirect catecholamine elevation and direct receptor agonism in modulation of cardiac repolarization through distinct pathways, warranting further mechanistic investigation.

Given ongoing concerns regarding the use of stimulant medications in children with underlying cardiac conditions such as LQTS and WPW [12], our findings provide insight into the electrophysiological effects of these medications in a human‐relevant cardiomyocyte model. Although the present study focused on conduction, automaticity, and repolarization, the observed responses may have a broader relevance to other inherited or acquired arrhythmogenic conditions in which sympathetic stimulation contributes to arrhythmia susceptibility. Together, these findings support the use of hiPSC‐CM platforms for evaluating cardiac responses to stimulant medications and other sympathomimetic agents.

This study demonstrates the utility of chamber‐specific hiPSC‐CMs as a physiologically relevant platform for evaluating the electrophysiological effects of stimulant medications, particularly in the context of conditions associated with increased arrhythmia susceptibility, such as WPW and LQTS. At clinically relevant plasma concentrations, MPH, DEX, ATX, and CAFF did not produce significant or sustained changes in CV, suggesting that these stimulants are unlikely to substantially alter conduction properties at therapeutic doses. However, the observed effects on automaticity and repolarization indicate that the electrophysiological actions of stimulant medications may extend beyond conduction and highlight the importance of further mechanistic studies to identify the cellular and ionic mechanisms underlying these responses. While caution is warranted when extrapolating in vitro findings to clinical practice, particularly given the developmental phenotype of hiPSC‐CMs, this study provides a foundation for future investigations into stimulant‐associated cardiac safety.

This research was funded by the British Columbia Children's Hospital Foundation.

The authors declare no conflicts of interest.