Cardiac Acceleration Research Articles

Unpredictable threat increases early event-related potential amplitudes and cardiac acceleration: A brain-heart coupling study.

In the face of unpredictable threat, rapid processing of external events and behavioral mobilization through early psychophysiological responses are crucial for survival. While unpredictable threat generally enhances early processing, it would seem adaptive to particularly increase sensitivity for unexpected events as they may signal danger. To examine this possibility, n = 77 participants performed an auditory oddball paradigm and received unpredictable shocks in threat but not in safe contexts while a stream of frequent (standard) and infrequent (deviant) tones was presented. We assessed event-related potentials (ERP), heart period (HP), and time-lagged within-subject correlations of single-trial EEG and HP (cardio-EEG covariance tracing, CECT) time-locked to the tones. N1 and P2 ERP amplitudes were generally enhanced under threat. The P3 amplitude was enhanced to deviants versus standards and this effect was reduced in the threat condition. Regarding HP, both threat versus safe and unexpected versus expected tones led to stronger cardiac acceleration, suggesting separate effects of threat and stimulus expectancy on HP. Finally, CECTs revealed two correlation clusters, indicating that single-trial EEG magnitudes in the N1/P2 and P3 time-windows predicted subsequent cardiac acceleration. The current results show that an unpredictable threat context enhances N1 and P2 amplitudes and cardiac acceleration to benign auditory stimuli. They further suggest separable cortical correlates of different effects on cardiac activity: an early N1/P2 correlate associated with threat-effects on HP and a later P3 correlate associated with expectedness-effects. Finally, the results indicate that unpredictable threat attenuates rather than enhances the processing of unexpected benign events during the P3 latency.

A pilot study investigating affective forecasting biases with a novel virtual reality-based paradigm

A body of research indicates that people are prone to overestimate the affective impact of future events. Here, we developed a novel experimental paradigm to study these affective forecasting biases under laboratory conditions using subjective (arousal and valence) and autonomic measures (skin conductance responses, SCRs, and heart rate). Thirty participants predicted their emotional responses to 15 unpleasant, 15 neutral, and 15 pleasant scenarios (affective forecasting phase) to which they were then exposed in virtual reality (emotional experience phase). Results showed that participants anticipated more extreme arousal and valence scores than they actually experienced for unpleasant and pleasant scenarios. The emotional experience phase was characterized by classic autonomic patterns, i.e., higher SCRs for emotionally arousing scenarios and greater peak cardiac acceleration for pleasant scenarios. During the affective forecasting phase, we found only a moderate association between arousal scores and SCRs and no valence-dependent modulation of cardiac activity. This paradigm opens up new perspectives for investigating affective forecasting abilities under lab-controlled conditions, notably in psychiatric disorders with anxious anticipations.

Cardiac sensitivity to rewards in cognitively inflexible nonclinical participants.

In psychopathologies characterized by compulsive decision-making, core impairments include cognitive inflexibility and excessive sensitivity to rewards. It has been posited that traits shared by nonclinical individuals and psychiatric patients could help explain the pathogenesis of compulsive decision-making. To investigate whether cognitive inflexibility predisposes nonclinical individuals to poor choices and hyper-reactivity to reward, we recruited people with high and low scores for cognitive persistence and used the Iowa Gambling Task to assess decision-making and cardiac reactivity to monetary gains/losses. As is frequently observed in psychophysiological research, the data indicated discrepancies among self-reports, behavior, and physiology. Cognitive inflexibility was not related to worse performance; however, monetary gains, in line with the literature, prompted marked cardiac accelerations. Consistent with our research goal, only inflexible participants showed large cardiac accelerations during the largest monetary wins. Taken together, the data confirm an association between cognitive persistence and physiological reward sensitivity in a nonclinical population. The findings are in line with recent theories on the development of compulsive behaviors that consider cognitive inflexibility as a transdiagnostic impairment and predisposing factor for excessive reactivity to rewards, and might act both as a preexisting individual trait and drug-induced deficit.

AUditive Direct in Utero Observation (AUDIO): A Randomized Controlled Trial for a Prenatal Demonstration of Fetal Hearing.

Introduction: The objective of this randomized controlled study was to demonstrate whether acoustic stimulation in utero is associated with fetal reactivity which is documentable by cardiotocography. Materials and methods: A monocentric randomized controlled trial was performed at a single university tertiary hospital between September 2016 and July 2017. This study was registered as a randomized clinical trial on clinicaltrail.gov (registration number NCT04622059). Unselected pregnancies at term of gestation were consecutively recruited for the purpose of this study. After 10 min of normal cardiotocography without accelerations (non-stress-test with a basal frequency between 110 and 150 beats/min, normal variability between 6 and 15 b/min, no accelerations, and no fetal movements), fetuses were randomized at a 1:1 ratio to either of the two groups. Fetuses in group A (n = 105) received acoustic stimulation after 10 min from the beginning of the CTG, whereas fetuses in group B received no stimulation (n = 105). The outcome variables investigated were the lapse of time between the beginning of the CTG and the occurrence of the first acceleration, and the lapse of time between the beginning of the CTG and the first fetal movement noticed. Results: The lapse of time between the beginning of the CTG and the occurrence of the first acceleration was significantly shorter in the group with acoustic stimulation compared to the other group (14.87 ± 5.01 vs. 21.90 ± 6.94 min, p-value < 0.001 log-rank test). Similarly, the lapse of time between the beginning of the CTG and the occurrence of the first fetal movement was significantly shorter in group A compared to group B (17.77 ± 7.62 vs. 23.28 ± 7.61 min, p-value < 0.001, log-rank test). Fetal cardiac acceleration and the occurrence of a fetal movement during the first 20 min of the CTG were more frequently recorded in group A compared to group B (respectively, 15% vs. 5% and 20% vs. 8%). Conclusion: This RCT showed an early fetal reaction following auditive stimulus, documentable by cardiotocography. Further research is needed to investigate a possible role of acoustic stimulation in utero for the prenatal diagnosis of congenital hypoacusis.

Threat induces cardiac and metabolic changes that negatively impact survival in flies

SummaryAdjusting to a dynamic environment involves fast changes in the body’s internal state, characterized by coordinated alterations in brain activity and physiological and motor responses. Threat-induced defensive states are a classic case of coordinated adjustment of bodily responses, cardiac regulation being one of the best characterized examples in vertebrates. A great deal is known regarding the neural basis of invertebrate defensive behaviors, mainly in Drosophila melanogaster. However, whether physiological changes accompany these remains unknown. Here, we set out to describe the internal bodily state of fruit flies upon an inescapable threat and found cardiac acceleration during running and deceleration during freezing. In addition, we found that freezing leads to increased cardiac pumping from the abdomen toward the head-thorax, suggesting mobilization of energy resources. Concordantly, threat-triggered freezing reduces sugar levels in the hemolymph and renders flies less resistant to starvation. The cardiac responses observed during freezing were absent during spontaneous immobility, underscoring the active nature of freezing response. Finally, we show that baseline cardiac activity predicts the amount of freezing upon threat. This work reveals a remarkable similarity with the cardiac responses of vertebrates, suggesting an evolutionarily convergent defensive state in flies. Our findings are at odds with the widespread view that cardiac deceleration while freezing has first evolved in vertebrates and that it is energy sparing. Investigating the physiological changes coupled to defensive behaviors in the fruit fly has revealed that freezing is costly yet accompanied by cardiac deceleration and points to heart activity as a key modulator of defensive behaviors.

Development of a Classical Conditioning Task for Humans Examining Phasic Heart Rate Responses to Signaled Appetitive Stimuli: A Pilot Study.

Cardiac responses to appetitive stimuli have been studied as indices of motivational states and attentional processes, the former being associated with cardiac acceleration and latter deceleration. Very few studies have examined heart rate changes in appetitive classical conditioning in humans. The current study describes the development and pilot testing of a classical conditioning task to assess cardiac responses to appetitive stimuli and cues that reliably precede them. Data from 18 adults were examined. They were shown initially neutral visual stimuli (putative CS) on a computer screen followed by pictures of high-caloric food (US). Phasic cardiac deceleration to food images was observed, consistent with an orienting response to motivationally significant stimuli. Similar responses were observed to non-appetitive stimuli when they were preceded by the cue associated with the food images, suggesting that attentional processes were engaged by conditioned stimuli. These autonomic changes provide significant information about classical conditioning effects in humans.

Threat Induces Changes in Cardiac Activity and Metabolism Negatively Impacting Survival in Flies

Adjusting to a dynamic environment involves fast changes in the body’s internal state, characterized by coordinated alterations in brain activity, physiological and motor responses. Threat-induced defensive states are a classic example of coordinated adjustment of bodily responses, being cardiac regulation one of the best characterized in vertebrates. A great deal is known regarding the neural basis of invertebrate defensive behaviours, mainly in Drosophila melanogaster. However, whether physiological changes accompany these remains unknown. Here, we set out to describe the interna bodily state of fruit flies upon an inescapable threat and found cardiac acceleration during running and deceleration during freezing. In addition, we found that freezing leads to increased cardiac pumping from the abdomen towards the head-thorax, suggesting mobilization of energy resources. Concordantly, threat-triggered freezing reduces sugar levels in the hemolymph and renders flies less resistant to starvation. The cardiac responses observed during freezing were absent during spontaneous immobility, underscoring the active nature of freezing response. Finally, we show that baseline cardiac activity predicts the amount of freezing upon threat. This work reveals a remarkable similarity with the cardiac responses of vertebrates, suggesting an evolutionarily convergent defensive state in flies. Our findings are at odds with the widespread view that cardiac deceleration while freezing has first evolved in vertebrates and that it is energy sparing. Investigating the physiological changes coupled to defensive behaviours in the fruit fly has revealed that freezing is costly, yet accompanied by cardiac deceleration, and points to heart activity as a key modulator of defensive behaviours.

Attentional processing of information related to illness: Biases and associations with emotional response in young adults with different levels of illness anxiety.

This study investigated attentional processing of illness-related information and associations with emotional reactivity. 100 young adults with low to high illness anxiety levels underwent free and cued viewing tasks, while eye-tracking and emotional reactivity were recorded. During free viewing, participants showed early orienting bias and sustained vigilance bias toward illness vs neutral pictures. Increased illness anxiety predicted vigilance bias to illness vs fearful pictures. During cued viewing, participants showed avoidance bias for illness vs neutral pictures, predicted by greater cardiac acceleration. Task nature appears to influence attentional processing patterns of illness stimuli. Preliminary evidence supports that attention allocation may be an emotion regulation mechanism.

Adaptive Non-Immersive VR Environment for Eliciting Fear of Cockroaches: A Physiology-Driven Approach Combined with 3D-TV Exposure.

Non-immersive VR environments are related to the least interactive application of VR techniques, such that interaction with the VR environment can occur commonly by 3D-TV without full immersion into the environment. This study presents how 3D-TV exposure combined with physiology recording can elicit fear of cockroaches among individuals with different levels of fear. Thirty-six participants, set apart into three fear groups (low vs. moderate vs. high), were exposed to VR environment with cockroaches for 4 minutes while recording and using cardiac activity as input to the VR environment. Results revealed significant effects on self-report measures and heart rate between different fear groups. Moreover, participants with higher levels of fear were more likely to trigger cockroaches into the scenario due to their cardiac acceleration. Overall results suggest that our physiology-driven VR environment is valid for fear elicitation while having potential use in therapeutic domain.

Integration of three investigations of Novelty, Intensity, and Significance in dishabituation paradigms: A study of the phasic Orienting Reflex

Data from three published studies examining autonomic and ERP measures in variants of a dishabituation paradigm were re-analysed to clarify ambiguous novelty results. The three studies manipulated 1. Novelty, 2. Novelty and Intensity, and 3. Novelty and Significance, in auditory dishabituation paradigms at very long interstimulus intervals (ISIs). The question of whether any single ERP matches SCR as the benchmark for the phasic Orienting Reflex (OR) was also addressed. Finally, we aimed to align the re-analysed measures of this and the previous analyses with processes of Preliminary Process Theory (PPT). The SCR demonstrated decrement, recovery, and dishabituation. A summary temporal PCA extracted PN (Processing Negativity), P3a, P3b, Novelty P3, and classic SW for detailed analysis. P3b and SW showed decrement but no recovery at the change trial, while Respiratory pause (RP) and Novelty P3 demonstrated decrement and recovery, but no dishabituation. Post hoc exploration of observed power versus sample size for each of these findings confirmed their robustness. No decrement for PN was also confirmed. Five autonomic and ERP groupings emerged and aligned with modules of processing in PPT: ECR1 (cardiac deceleration), P1, N1-3, and PN – stimulus registration; RP and Novelty P3 – Novelty registration; Peripheral Vasoconstriction (PVC) and P3b – Intensity registration; and ECR2 (cardiac acceleration) and classic SW – Response system. The SCR was confirmed as the sole index of the phasic OR. The pattern of results for the Late Positive Complex (LPC) components (P3a, P3b, Novelty P3, and SW) suggests each is differentially sensitivity to selective determinants of the phasic OR, and consequently the summary LPC is presented as the most appropriate central index of the phasic OR.

Influence of physical rehabilitation on heart rate dynamics in patients with idiopathic pulmonary fibrosis.

This study sought to determine whether a 6-week physical rehabilitation programme has a measurable influence on heart rate responsiveness to changing metabolic demand in patients with pulmonary fibrosis. Fifteen individuals (13 with idiopathic pulmonary fibrosis and two with pulmonary fibrosis associated with rheumatoid disease) underwent assessments of physical function (6-min walk test), ventilatory function (spirometry), and cardio-respiratory function (respiratory gas analysis and electrocardiogram [ECG] recording during a protocol consisting of periods of rest, incremental bicycle exercise to maximal effort, and post-exercise recovery). RR (beat-to-beat cardiac intervals) data were derived from the ECG and used to quantify (a) heart rate variability (HRV) and (b) cardiac acceleration (AC) and deceleration capacities (DC) (via phase rectified signal averaging). Following the rehabilitation programme, heart rate was elevated by 11%–18% during exercise and recovery states. HRV was not statistically influenced by rehabilitation during any stage of the assessment protocol; however, qualitative changes were apparent with HRV increasing by 68%–75% during all stages of the protocol. Statistically, AC and DC were similar pre- and postrehabilitation (AC=−2.7 and −3.2 msec; DC=2.3 and 3.2 msec, respectively) but again we observed qualitative improvements in these indices of 19% and 38%, respectively. These results provide initial evidence that physical rehabilitation improves heart rate dynamics (via modulation of autonomic control of heart rate) in idiopathic pulmonary fibrosis patients, even when respiratory and physical functions are unchanged or diminished.

The effect of plot explicit, educational explicit, and implicit inference information and coviewing on children's internal and external cognitive processing

ABSTRACTParental coviewing – the act of being present when a child is watching television – can influence the child’s cognitive processing and emotional reactions. This study investigated the role coviewing has on the child’s cognitive processing – which is evidenced by the phasic psychophysiological orienting response to three types of information: plot explicit, educational explicit, and implicit inference. An experiment was conducted that measured the heart rate of children (N = 88; mean age = 9.12 years) while watching messages either with or without a parent present in the room. It was predicted, and found, that coviewing leads to greater resource allocation to encoding the message – as indicated by phasic cardiac deceleration, and that information that required internal processing, such as plot explicit or implicit inferential content, leads to greater resources allocated to internal processing – as indicated by phasic cardiac acceleration. Implications for parental mediation strategies and educational television programming are given.

Long-Term Metal PM2.5 Exposures Decrease Cardiac Acceleration and Deceleration Capacities in Welders.

The aim of the study was to clarify whether long-term metal particulates affect cardiac acceleration capacity (AC), deceleration capacity (DC), or both. We calculated chronic exposure index (CEI) for PM2.5 over the work life of 50 boilermakers and obtained their resting AC and DC. Linear regression was used to assess the associations between CEI PM2.5 exposure and each of AC and DC, controlling for age, acute effects of welding exposure, and diurnal variation. Mean (standard deviation) CEI for PM2.5 exposure was 1.6 (2.4) mg/m-work years and ranged from 0.001 to 14.6 mg/m-work years. In our fully adjusted models, a 1 mg/m-work year increase in CEI for PM2.5 was associated with a decrease of 1.03 (95% confidence interval: 0.10, 1.96) ms resting AC, and a decrease of 0.67 (95% confidence interval: -0.14, 1.49) ms resting DC. Long-term metal particulate exposures decrease cardiac accelerations and decelerations.

Are the Associations of Cardiac Acceleration and Deceleration Capacities With Fine Metal Particulate in Welders Mediated by Inflammation?

The aim of this study was to investigate whether associations of acceleration capacity (AC) and deceleration capacity (DC) with metal-PM2.5 are mediated by inflammation. We obtained PM2.5, C-reactive protein, interleukin (IL)-6, 8, and 10, and electrocardiograms to compute AC and DC, from 45 male welders. Mediation analyses were performed using linear mixed models to assess associations between PM2.5 exposure, inflammatory mediator, and AC or DC, controlling for covariates. The proportion of total effect of PM2.5 on AC or DC (indirect effect) mediated through IL-6 on AC was 4% at most. Controlling for IL-6 (direct effect), a 1 mg/m increase of PM2.5 was associated with a decrease of 2.16 (95% confidence interval -0.36 to 4.69) msec in AC and a decrease of 2.51 (95% confidence interval -0.90 to 5.93) msec in DC. IL-6 may be mediating the effect of metal particulates on AC.

HEMODYNAMIC MECHANISMS OF CLINICAL EFFECTS OF CYCLIC STAY OF A HUMAN IN NORMOBARIC HYPOXIC ENVIRONMENT

Purpose: in order to assess the adaptive hemodynamic responses formed in humans during cyclic stay (daily, for 2 hours, 15 sessions in total) in normobaric hypoxic gaseous environment with oxygen content in nitrogen of 15% (NHGE-15). Materials and methods: a study has been conducted with participation of 12 male test subjects aged 25-52. In course of cyclic stay in NHGE-15, systemic and cerebral blood flow parameters were evaluated in the test subjects. Results: in response to stay in NHGE, at the initial stage of the testing procedure, the test subjects showed hyperkinetic systematic circulation reactions: cardiac acceleration, increase in systemic blood pressure and cardiac output, with decrease in peripheral vascular resistance. At the same time, an increase in blood flow to cerebral vessels and hypersthenia in major cerebral vessels were revealed. Further stay in NHGE-15 was accompanied by a statistically significant reduction in severity of the above responses reflecting a gradual decrease in «stressfulness» of hypoxic effect due to formation in the body of primary adaptational structural and functional alterations in oxygen transport systems, regulatory and plastic processes. The fact of systemic and regional hemodynamics parameter optimizing was also revealed, recorded under normal conditions of gaseous environment, in course of the cyclic hypoxic impact. Summary: hypoxic exposure on the human body leads to centralization of the circulatory system, aimed at the maintenance of oxygen supply of vital organs. The result of hypoxic training is to reduce the severity of the reaction by increasing the reliability of functioning of the system of hemodynamics and its regulatory mechanisms.

Short-term metal particulate exposures decrease cardiac acceleration and deceleration capacities in welders: a repeated-measures panel study

ObjectiveAcceleration (AC) and deceleration (DC) capacities measure heart rate variability during speeding up and slowing down of the heart, respectively. We investigated associations between AC and DC with occupational short-term...

Flecainide attenuates rate adaptation of ventricular repolarization in guinea-pig heart

Objectives: Flecainide is class Ic antiarrhythmic agent that was found to increase the risk of sudden cardiac death. Arrhythmic responses to flecainide could be precipitated by exercise, suggesting a role played by inappropriate rate adaptation of ventricular repolarization. This study therefore examined flecainide effect on adaptation of the QT interval and ventricular action potential duration (APD) to abrupt reductions of the cardiac cycle length. Design: ECG and ventricular epicardial and endocardial monophasic APD were recorded in isolated, perfused guinea-pig heart preparations upon a sustained cardiac acceleration (rapid pacing for 30 s), and following a single perturbation of the cycle length evoked by extrasystolic stimulation. Results: Sustained increase in heart rate was associated with progressive bi-exponential shortening of the QT interval and APD. Flecainide prolonged ventricular repolarization, delayed its rate adaptation, and decreased the amplitude of QT interval and APD shortening upon rapid cardiac pacing. During extrasystolic stimulation, flecainide attenuated APD shortening in premature ventricular beats, with effect being greater upon using a longer basic drive cycle length (S1-S1=550 ms versus S1-S1=300 ms). Conclusions: Flecainide-induced arrhythmia may be partly accounted for by attenuated adaptation of ventricular repolarization to sudden changes in cardiac cycle length provoked by transient tachycardia or ectopic beats.

Brain sparing effect in growth-restricted fetuses is associated with decreased cardiac acceleration and deceleration capacities: a case-control study.

Phase rectified signal averaging (PRSA) is a new method of fetal heart rate variability (fHRV) analysis that quantifies the average acceleration (AC) and deceleration capacity (DC) of the heart. The aim of this study was to evaluate AC and DC of fHR [recorded by trans-abdominal fetal electrocardiogram (ta-fECG)] in relation to Doppler velocimetry characteristics of intrauterine growth restriction (IUGR). Prospective case-control study. Single third referral centre. IUGR (n=66) between 25 and 40 gestational weeks and uncomplicated pregnancies (n=79). In IUGR the nearest ta-fECG monitoring to delivery was used for PRSA analysis and Doppler velocimetry parameters obtained within 48hours. AC and DC were computed at s=T=9. The relation was evaluated between either AC or DC and Doppler velocimetry parameters adjusting for gestational age at monitoring, as well as the association between either AC or DC and IUGR with or without brain sparing. In IUGRs there was a significant association between either AC and DC and middle cerebral artery pulsatility index (PI; P=0.01; P=0.005), but the same was not true for uterine or umbilical artery PI (P>0.05). Both IUGR fetuses with and without brain sparing had lower AC and DC than controls, but this association was stronger for IUGRs with brain sparing. Our study observed for the first time that AC and DC at PRSA analysis are associated with middle cerebral artery PI, but not with uterine or umbilical artery PI, and that there is a significant decrease of AC and DC in association with brain sparing in IUGR fetuses from 25weeks of gestation to term. Brain sparing in IUGR fetuses is associated with decreased acceleration and deceleration capacities of the heart.

Prevalence of Abnormal Systemic Hemodynamics in Veterans With and Without Spinal Cord Injury

ObjectiveIncreased prevalence of heart rate and blood pressure abnormalities are evident in persons with spinal cord injury (SCI), but age, comorbid medical conditions, and prescription medication use may contribute. To determine differences in the prevalence of cardiac acceleration (heart rate ≥80 beats per minute), hypotension (blood pressure ≤110/70mmHg), orthostatic hypotension (OH) (−20/−10mmHg with upright positioning), and hypertension (HTN) (blood pressure ≥140/90mmHg) in veterans with and without SCI. DesignObservational trial. SettingMedical center. ParticipantsSubjects included veterans with SCI (n=62; cervical: tetraplegia, C3-8; high thoracic, T1-5; low thoracic, T7-L2) and veterans without SCI (n=160). InterventionsNone. Main Outcome MeasuresWe assessed medical history, prescription medication use, and heart rate and blood pressure during a routine clinical visit. Prevalence rates of cardiac acceleration, hypotension, OH, and HTN were calculated using binary logistic regression analysis with 95% confidence intervals. The influence of SCI status, age, smoking status, cardiovascular diagnoses, and use of prescribed antihypertensive medications on the prevalence of abnormal heart rate and blood pressure recordings was determined. ResultsThe diagnosis of HTN was reduced in the high thoracic and tetraplegia groups compared with the non-SCI and low thoracic groups. Use of antihypertensive medications was increased in the low thoracic group compared with the other 3 groups and was increased in the non-SCI group compared with the tetraplegia group. The prevalence of cardiac acceleration was reduced, and the prevalence of systolic hypotension was increased in the tetraplegia group. The prevalence of diastolic hypotension was increased in all SCI groups compared with the non-SCI group. For all analyses, increased prevalence of abnormal heart rate and blood pressure recordings was not further explained by the covariates, with the exception of age, cardiovascular diagnoses, and antihypertensive medications in the cardiac acceleration model; however, SCI status remained significant and was the dominant predictor variable. ConclusionsOur data suggest that SCI status contributes to the prevalence of cardiac acceleration and systolic and diastolic hypotension regardless of cardiovascular medical conditions or prescription antihypertensive medication use.

Reduced intrinsic heart rate is associated with reduced arrhythmic susceptibility in guinea-pig heart

Objectives. In the clinical setting, patients with slower resting heart rate are less prone to cardiovascular death compared with those with elevated heart rate. However, electrophysiological adaptations associated with reduced cardiac rhythm have not been thoroughly explored. In this study, relationships between intrinsic heart rate and arrhythmic susceptibility were examined by assessments of action potential duration (APD) rate adaptation and inducibility of repolarization alternans in sinoatrial node (SAN)-driven and atrioventricular (AV)-blocked guinea-pig hearts perfused with Langendorff apparatus. Design. Electrocardiograms, epicardial monophasic action potentials, and effective refractory periods (ERP) were assessed in normokalemic and hypokalemic conditions. Results. Slower basal heart rate in AV-blocked hearts was associated with prolonged ventricular repolarization during spontaneous beating, and with attenuated APD shortening at increased cardiac activation rates during dynamic pacing, when compared with SAN-driven hearts. During hypokalemic perfusion, the inducibility of repolarization alternans and tachyarrhythmia by rapid pacing was found to be lower in AV-blocked hearts. This difference was ascribed to prolonged ERP in the setting of reduced basal heart rate, which prevented ventricular capture at critically short pacing intervals required to induce arrhythmia. Conclusions. Reduced basal heart rate is associated with electrophysiological changes that prevent electrical instability upon an abrupt cardiac acceleration.