Continuous ECG measurements of intracardiac activity from the surface of Langendorff-perfused guinea pig hearts

Breathable, Self-Adhesive Dry Electrodes for Stable Electrophysiological Signal Monitoring During Exercise

On-skin electrodes with high air permeability, low thickness, low elastic modulus, and high adhesion are essential for biomedical signal recordings, which provide data for sports management and biomedical applications. However, nanothickness electrodes interacting with the skin by van der Waals force can be interfered with by sweating, and elastomers with high adhesion prepared by modification are not satisfactory in terms of air permeability. Here, a dry electrode with high stretchability (598%), low elastic modulus (5 MPa), high air permeability (726 g m^-2 d^-1), and high adhesion (6.33 kPa) was fabricated by semi-embedding Ag nanowires into nonyl and glycerol-modified polyvinyl alcohol. Furthermore, a small amount of 40 wt % ethanol was sprayed on the skin to facilitate microdissolution of the substrate and form immediate conformability with skin texture. The dry electrodes can record high-quality electrocardiogram and electromyogram signals through a robust contact with the skin under skin deformation, with a water stream, or after running for 1 h. The film can also be served as the substrate for self-adhesive strain sensors to monitor motion with higher quality than nonadhesive polydimethylsilane-based sensors.

Cardiodynamics and infarct size in regional and global ischemic isolated heart model: comparison of 1 hour and 2 hours reperfusion

Background and Objectives

We investigated whether 1 hour reperfusion is enough to assess cardiodynamics and infarct size in both regional ischemia (RI) and global ischemia (GI) in isolated rat heart models.

Materials and Methods

Hearts were randomly assigned to one of the following groups (each n=14): 1) Sham hearts for 1 hour; 2) Sham hearts for 2 hours; 3) 30 minutes RI followed by 1 hour reperfusion; 4) 30 minutes of RI followed by 2 hours reperfusion; 5) 30 minutes GI followed by 1 hour reperfusion; and 6) 30 minutes GI followed by 2 hours reperfusion.

Results

There were no significant differences in infarct size between 1 hour and 2 hours reperfusion in both RI and GI. Left ventricular developed pressure was significantly decreased at both 1 hour and 2 hours reperfusion in groups of RI and GI compared to baseline (p<0.01). Rate-pressure product and +dP/dt_max also significantly decreased compared to baseline level at both 1 hour and 2 hours reperfusion in groups of RI and GI (p<0.05).

Conclusion

There was no significant difference in infarct size between 1 hour and 2 hours reperfusion in groups of RI and GI. Cardiodynamic variables measured at 1 hour and 2 hours reperfusion significantly decreased compared to baseline level. Our data suggests that reperfusion of 1 hour is sufficient to assess cardiodynamics in both regional and global ischemic isolated hearts model.

Sensitivity and specificity of isolated perfused guinea pig heart to test for drug-induced lengthening of QTc

INTRODUCTION

The purpose of this study was to determine the sensitivity and specificity for predicting the liability of a compound to lengthen QTc using isolated, perfused guinea pig hearts (Langendorff preparation).

METHODS

QTc (Fridericia correction) was calculated from bipolar transventricular electrograms. Hearts were exposed to escalating concentrations of 26 compounds thought to lengthen, and 13 compounds thought not to lengthen, QTc in humans.

RESULTS

In this preparation, QTc was found to lengthen in 26 of 26 compounds thought to be positive (sensitivity 1.00) and not to lengthen or to lengthen insignificantly in 13 of 13 compounds thought to be negative (specificity 1.0) in man. Probucol and ontazolast could not be studied because of limited solubility. Successful experiments were conducted on over 98% of guinea pigs anesthetized.

DISCUSSION

We believe that the isolated perfused guinea pig heart is an in vitro preparation that could be utilized early in preclinical testing for identifying a liability to lengthen QTc in humans, but we do not believe--as is true also for other in vitro methods--that the concentration at which the liability is demonstrated in vitro necessarily predicts the concentration at which a liability exists in man.

Comparison of time- and voltage-dependent K+ currents in myocytes from left and right atria of adult mice

Consistent differences in K+ currents in left and right atria of adult mouse hearts have been identified by the application of current- and voltage-clamp protocols to isolated single myocytes. Left atrial myocytes had a significantly (P < 0.05) larger peak outward K+ current density than myocytes from the right atrium. Detailed analysis revealed that this difference was due to the rapidly activating sustained K+ current, which is inhibited by 100 muM 4-aminopyridine (4-AP); this current was almost three times larger in the left atrium than in the right atrium. Accordingly, 100 muM 4-AP caused a significantly (P < 0.05) larger increase in action potential duration in left than in right atrial myocytes. Inward rectifier K+ current density was also significantly (P < 0.05) larger in left atrial myocytes. There was no difference in the voltage-dependent L-type Ca2+ current between left and right atria. As expected from this voltage-clamp data, the duration of action potentials recorded from single myocytes was significantly (P < 0.05) shorter in myocytes from left atria, and left atrial tissue was found to have a significantly (P < 0.05) shorter effective refractory period than right atrial tissue. These results reveal similarities between mice and other mammalian species where the left atrium repolarizes more quickly than the right, and provide new insight into cellular electrophysiological mechanisms responsible for this difference. These findings, and previous results, suggest that the atria of adult mice may be a suitable model for detailed studies of atrial electrophysiology and pharmacology under control conditions and in the context of induced atrial rhythm disturbances.

Magnesium abolishes inadequate kinetics of frequency adaptation of the Q-aT interval in the presence of sotalol

OBJECTIVE

It has been well established that class III antiarrhythmic drugs can also induce ventricular arrhythmias. Marked changes in the QT interval are correlated with an increased dispersion of repolarization which is an important factor for the induction of ventricular arrhythmias. The aim of the present study was to investigate the effects of sotalol alone and in combination with MgSO4 and the Q-aT interval during abrupt changes in heart rate.

METHODS

The experiments were performed on isolated guinea-pig hearts perfused by the method of Langendorff. The rate adaptation of the Q-aT interval was estimated after abruptly changing the ventricular pacing rate from 220 to 180 ms and back to 220 ms.

RESULTS

In the presence of 10 microM sotalol, at a constant pacing cycle length of 220 ms, the QT interval was prolonged significantly (P < 0.01) from 152 +/- 4 to 166 +/- 3 ms (mean +/- s.e.m., n = 8 in each group). The addition of 3.4 mM MgSO4 caused a slight further prolongation of the QT interval. After abruptly shortening the pacing cycle length from 220 to 180 ms, the Q-aT interval shortened within 2 min by 11.3 +/- 0.5 ms with a time constant (tau) of 77 +/9 beats under control conditions, by 15.4 +/- 0.9 ms (P < 0.05 vs. control with tau = 52 +/- 7 beats (P < 0.05 vs. control) in the presence of sotalol, and by 13.1 +/- 1.2 ms with tau = 158 +/- 13 beats under the combination of sotalol (10 microM) and MgSO4 (3.4 mM). After abrupt shortening of the pacing cycle length the Q-aT interval of the first beat was shortened by 3.3 +/- 0.3 ms under control conditions, by 7.1 +/- 0.2 ms (P < 0.01 vs. control) under sotalol, and by 4.2 +/- 0.2 ms with the combination of sotalol and MgSO4. If the pacing cycle length was abruptly increased from 180 to 220 ms, the effects were comparable to those described above.

CONCLUSIONS

Sotalol led to inadequate kinetics of fate adaptation of the Q-aT interval indicated by a high amplitude of Q-aT interval change, especially within the first beat after abrupt change in the pacing rate. MgSO4 abolished this effect of sotalol. These findings suggest that MgSO4 could reduce sotalol-induced inadequate kinetics of rate adaptation and therefore also dispersion of repolarization, which may result in a reduction of sotalol-induced ventricular arrhythmias.

Effects of AWD 23-111, a new antiarrhythmic substance, on cardiac conduction and refractoriness

In isolated spontaneously beating guinea pig hearts, the effects of AWD 23-111 (N-(dicyclohexylcarbamoylmethyl)-N-(3-diethylamino-propyl)-4-nit robenzamid -hydrochloride), a new synthetic class III antiarrhythmic agent with sodium antagonistic properties, were investigated on cardiac electrophysiological parameters, that is, conduction and refractoriness. Concentration-dependent prolongation of the atrioventricular, intraventricular, and His bundle conduction times and of sinus node cycle length were present. At 0.3 microM the repolarization period was prolonged significantly. No reverse use-dependent effect on the repolarization period was observed. During rapid pacing (pacing cycle length = 120 ms for the ventricle and 180 ms for the atrium) the rate-dependent intraventricular (QRS) or atrioventricular conduction time (AVCT) prolongation follows an exponential function of the beat number and is characterized by a drug-specific time constant. The time constant for the intraventricular conduction time prolongation in the presence of 0.1 microM AWD 23-111 was very long at 150 +/- 29 beats (mean +/- SEM; n = 6), indicating a slow binding kinetic to the sodium channel. At 0.1 microM AWD 23-111, a significant increase in the ventricular effective refractory period was reached when the interstimulus interval (S1-S1) was 120 ms and the number of conditioning stimuli (S1) was higher than the time constant. The time constant for the rate-dependent AVCT prolongation in the presence of 0.3 microM AWD 23-111 was 34 +/- 6 beats (n = 6). The effective refractory period of the atrioventricular conduction significantly increased with the number of conditioning stimuli (S1), until the number was comparable with the time constant. In conclusion, AWD 23-111 exerts a wide variety of actions on the cardiac conduction system. Its combined effects on the potassium and sodium channels seem to be responsible for the marked rate-dependent effect on ventricular refractoriness and for the lack of a reverse use-dependency on JT prolongation.

Effects of semotiadil, a novel Ca2+ channel antagonist, on the electrical activity of Langendorff-perfused guinea pig hearts in comparison with diltiazem, amlodipine and nifedipine

Semotiadil, a new Ca2+ antagonist with a high vasoselectivity, in high concentrations depresses AV nodal conduction in a frequency-dependent manner. The aim of the present study was to investigate the effects of semotiadil on intact cardiac conduction and the pacemaker system in comparison with diltiazem, amlodipine and nifedipine. The effects were studied in isolated guinea pig hearts perfused by the method of Langendorff. Both semotiadil and diltiazem decreased markedly the sinus rate in a concentration-dependent manner whereas this was not the case in the presence of amlodipine and nifedipine. Semotiadil (10 microM) markedly prolonged sinus node recovery time and in the presence of diltiazem (10 microM) in 5 out of 7 experiments an intermittent sinus node arrest occurred. Atrioventricular conduction and the effective refractory period of the AV node were most affected by diltiazem and semotiadil. The Ca2+ channel blocking compound semotiadil showed the most pronounced rate-dependent effects on the AV node. In the presence of diltiazem the QT interval became even shorter than in untreated hearts. In contrast, semotiadil did not act on the QT interval. In conclusion, as semotiadil exerts a clear rate-dependent effect on AV nodal conduction with a long time constant, it mimics the electrophysiological behavior of a substance of the verapamil type.

Stereoselective electrophysiological effects of propafenone in Langendorff perfused guinea pig hearts

The present study was focused on the stereoselective electrophysiological effects of (R)- and (S)-propafenone.HCl evaluated in isolated Langendorff perfused guinea pig hearts. Conduction intervals were measured using an ECG-recording method of high resolution. Refractory periods of the different parts of the myocardium were determined by stimulation with premature stimuli, as well as by stimulation with increasing pacing rate (rate-dependent/refractory periods). Drug concentrations of 0.1, 1 and 3 microM were tested. Both compounds induced a dose-dependent increase in AV-nodal, His-bundle, and intraventricular conduction time which reached significance (p less than 0.01) following 3 microM of either compound. Sinus rate was also dose-dependently and significantly reduced. (R)- and (S)-propafenone.HCl induced a marked prolongation of the rate-dependent refractory period of sino-atrial (by 140 +/- 22%, p less than 0.01 and by 141 +/- 14%, p less than 0.01, respectively) and AV-nodal (by 34 +/- 22%, p less than 0.01 and by 42 +/- 15%, p less than 0.01, respectively) conduction and of the atrial (by 182 +/- 21%, p less than 0.01 and by 195 +/- 15%, p less than 0.01, respectively) and ventricular (by 93 +/- 16%, p less than 0.01 and by 88 +/- 16%, p less than 0.01, respectively) myocardium. The effective refractory periods evaluated by stimulation with premature stimuli were also significantly prolonged under the influence of (R)- and (S)-propafenone.HCl, except the ventricular myocardial refractoriness by (R)-propafenone.HCl (increase to 114 +/- 23%, n.s.). Both compounds showed a strong rate-dependence of their effects and, thus, the refractory periods evaluated by stimulation with increasing pacing rate were significantly more prolonged than the refractory periods evaluated by stimulation with premature stimuli. The main difference between the effects of (R)- and (S)-propafenone.HCl on the cardiac electrical activity is the lack of effect of (R)-propafenone.HCl on the ventricular myocardial refractoriness evaluated by stimulation with premature stimuli.

Protective effects of manganese, cobalt, nickel, and barium against a calcium paradox in the isolated frog heart

The effect of inorganic slow channel blockers on the calcium paradox in the frog heart was examined. Addition of the divalent cations of manganese, cobalt, nickel, or barium during calcium depletion protected the frog heart against a calcium paradox. This protective effect was indicated by reduced protein release, maintenance of electrical activity, and recovery of mechanical activity during reperfusion. Tissue calcium determination results showed that in the control paradox in the absence of divalent cations, there is an efflux of calcium from myocardial cells during calcium depletion and a massive influx of calcium during the following reperfusion, leading to a calcium overload. Divalent cations protected frog myocardial cells, when present in the calcium-free perfusion medium, by reducing both calcium efflux during calcium depletion and the massive calcium influx during reperfusion. The effectiveness of the added divalent cations showed a strong dependence upon their ionic radius. The most potent inhibitors of the calcium paradox in the frog heart were the divalent cations having an ionic radius closer to the ionic radius of calcium. These results are discussed in terms of the possible mechanism involved in the protective effect of manganese, cobalt, nickel, and barium.

Acute effects of the ACE inhibitor lisinopril on cardiac electrophysiological parameters of isolated guinea pig hearts

Angiotensin-converting enzyme (ACE) inhibitors are of benefit in life-threatening ventricular arrhythmias in patients with congestive heart failure and ventricular tachycardia caused by the onset of myocardial ischemia as well as by reperfusion of an ischemic area. The aim of the present study was to investigate whether direct electrophysiological effects are responsible for these observations. Therefore, we investigated the electrophysiological effects of lisinopril on the whole cardiac conduction and pacemaker system in isolated guinea pig hearts perfused by the method of Langendorff at concentrations of 0.01, 0.1, 1, and 10 microM. Lisinopril did not affect heart rate, atrioventricular, His bundle, or intraventricular conduction at any of the concentrations tested. Likewise the frequency-dependent QT duration was unchanged. At a concentration of 10 microM, lisinopril prolonged effective refractory period evaluated by premature beats (46 +/- 15%, n = 8, p less than 0.01) as well as the rate-dependent effective refractory period (31 +/- 12, n = 8, p less than 0.01) of the atrioventricular conduction. These effects can be explained by lisinopril's action as a minor calcium antagonist at a toxic concentration of 10 microM. The present results show that electrophysiological parameters are not substantially altered by lisinopril. Therefore, several other mechanisms such as the unloading of the left ventricle and/or the suppression of noradrenalin release and the electrolyte (potassium and magnesium) repletion and/or regression of left ventricular hypertrophy as long-term effects may play a major role in the antiarrhythmic efficacy of ACE inhibitors.

Comparison of acute electrophysiological effects of amiodarone and its metabolite desethylamiodarone in Langendorff perfused guinea pig hearts

During long-term treatment with amiodarone, slowing of conduction through the atrioventricular node, a prolongation of the QT-interval, and a prolongation of the atrial and ventricular myocardial refractoriness always developed. During short-term treatment, these effects were not found, except for depression of the AV-nodal conduction. This led to the suggestion that the electrophysiological effects of amiodarone during long-term treatment might be partly the result of the accumulation of its metabolite desethylamiodarone. Therefore, we examined the electrophysiological effects of amiodarone and desethylamiodarone on conduction and refractoriness in isolated spontaneously beating guinea pig hearts perfused by the method of Langendorff. Within 1 h of perfusion, desethylamiodarone caused a more pronounced prolongation of the AV-nodal, His-bundle, and intraventricular conduction intervals than did amiodarone. Desethylamiodarone, but not amiodarone led to a prolongation of the QT-interval. The refractoriness of sinoatrial-, AV-nodal conduction, and of the atrial myocardium were significantly more prolonged by amiodarone than by desethylamiodarone. Both compounds showed a comparable strong rate-dependent effect on AV-nodal refractoriness. The ventricular refractoriness was similarily prolonged by either compound. These results show that for the class-III effects (i.e., prolongation of repolarization period) observed under chronic treatment of amiodarone the metabolite desethylamiodarone may be responsible. Desethylamiodarone also exerts more pronounced effects on the fast-channel-dependent parts of the conduction system than does amiodarone, a fact indicated by a higher prolongation of His-bundle and intraventricular conduction.

Comparison of acute effects of anthracyclines on cardiac electrophysiological parameters of isolated guinea-pig hearts

This study was performed to evaluate the acute effects of two anthracycline derivatives, doxorubicin and 4'O-tetrahydropyranyl-doxorubicin [(THP)-doxorubicin], on the conduction intervals, heart rate and refractoriness of isolated spontaneously beating guinea-pig hearts using a high-resolution ECG recording technique (SST-ECG). Doxorubicin as well as (THP)-doxorubicin were added to the perfusate in increasing concentrations of 0.1, 1 and 10 microM. Doxorubicin did not significantly alter the heart rate or conduction intervals. Only the rate-dependent QT interval was significantly shortened under the influence of 10 microM doxorubicin. In contrast, 10 microM (THP)-doxorubicin led to a significant reduction in the heart rate (-13% +/- 3%; P less than 0.01, n = 7) and to a prolongation of atrioventricular conduction time (24% +/- 10%; P less than 0.05, n = 7). The rate-dependent repolarization period (QT interval) was only insignificantly shortened in the presence of 10 microM (THP)-doxorubicin. The maximal following frequencies of each part of the conduction system were not changed by 10 microM doxorubicin. In the presence of (THP)-doxorubicin, the maximal following frequency of the ventricular myocardium was increased by as much as 36% +/- 8% (P less than 0.01, n = 7), indicating a shortening of the effective refractory period of the ventricular myocardium (V-ERP). These results show that the activation of (THP)-doxorubicin resembles the effects of Ca-antagonistic compounds on the heart (i.e. decrease in the spontaneous sinus rate and prolongation of the AV-nodal conduction interval). Changes in the QT interval exerted by doxorubicin and the shortening of the ventricular effective refractory period by (THP)-doxorubicin may indicate an alteration of the K(+)-conductance of the membrane. As the acute electrophysiological effects of doxorubicin and (THP)-doxorubicin are modest and occur only at excessive concentrations (10 microM), a direct influence on the generation of arrhythmias in healthy hearts is unlikely.

Detection and continuous monitoring of intracardiac low-level potentials from the surface of the Langendorff-perfused heart

By appropriate placement of two electrodes on the epicardiac surface of Langendorff-perfused hearts, His-bundle and preatrial signals can be recorded by the surface electrogram (S-ECG). These signals are difficult to detect because of their low amplitude of a few microvolts. To improve the monitoring of these low-level potentials we studied the His-bundle spike as detected by intracardiac electrodes and by epicardial records (S-ECG) and compared these signals in the time and frequency domain. The frequency spectra of these cardiograms were used to develop appropriate filters and high-gain amplifiers for a continuous monitoring of the His-bundle signal from the epicardiac surface. By means of such a monitoring system, high-frequency components of preatrial activities could be resolved also. The time coincidence of these spikes to the simultaneously recorded intraatrial electrogram from electrodes positioned near the sinus node and near the His-bundle is demonstrated. Hence, the early atrial signals likely yield information about sinoatrial conduction. Application examples of monitoring His-bundle signals and preatrial signals in a beat-to-beat manner are described also for various conduction blocks or arrhythmias. With this monitor the evaluation of characteristic parameters of the conduction system of the heart like HV-, AH- and A'H-time, and likely, SACT can easily be performed for every heartbeat on a digital oscilloscope with low resolution or a two-channel chart recorder. Small and intact hearts can be used with this system to detect intracardiac low-level potentials during the heart beat.

The effects of the propranolol enantiomers on the intracardiac electrophysiological activities of Langendorff perfused hearts

The optical isomers of the beta blocking agent propranolol exert beta receptor blocking as well as membrane stabilizing effects. The latter is thought to be responsible for the antiarrhythmic effect of the drug. In this study we quantified the electrophysiological effects of both isomers of propranolol on the conduction and pacemaker system of the heart. The experiments were performed on isolated hearts using a special ECG recording and stimulation technique. To abolish isoproterenol's beta adrenergic stimulatory effect on heart rate, 30-times higher concentrations of (+)propranolol were necessary than of (-)propranolol in order to be consistent. Both isomers caused a similar and marked slowing of conduction velocity through the bundle of His and ventricular myocardium. Also, heart rate, as well as atrio-ventricular conduction velocity were significantly slowed by a concentration of 10 microM of either drug, (-)propranolol being slightly more effective. Only in the presence of (-)propranolol did significant changes of atrio-ventricular and His-bundle conduction occur at a concentration of 1 microM. During programmed stimulation sinus node recovery time was more prolonged by (-)propranolol than during perfusion with (+)propranolol. The highest rate of pacing with 1:1 conduction of the sino-atrial conduction, the atrial and ventricular myocardium was significantly depressed to a comparable degree by either isomers of propranolol. These effects appear to be primarily responsible for the antiarrhythmic effects of both isomers. Because of the minor effects of (+)propranolol on sinus- and AV-node activity, as well as on beta adrenergic receptors, this isomer may have potential clinical importance in the treatment of arrhythmias.

Assessment of the conduction of the cardiac impulse by a new epicardiac surface and stimulation technique (SST-ECG) in Langendorff perfused mammalian hearts

The assessment of the effects on the conduction of the cardiac impulse and of refractoriness simultaneously from multiple cardiac structures is important to evaluate the mode of action of new compounds, as well as to investigate undesired cardiac side effects. The measurements of intracardiac electrical activities of special structures have required catheter methods or averaging techniques to produce clear deflections of low-level potential wave forms. We have developed an epicardial surface electrocardiogram (S-ECG) recording technique to detect continuously sinus node and His-bundle activities in spontaneously beating Langendorff perfused heart preparations. A bipolar surface stimulation technique electrocardiogram (ST-ECG) could be combined with this S-ECG method to assess the effect of pacing at maximal rate on the refractoriness of the sino-atrial, AV-nodal, and His-bundle conduction, as well as that of the atrial and ventricular myocardium. Epicardial surface electrodes were also used for vector analysis of the signals, which include those of the sinus node and His-bundle potentials, in addition to the usual atrial and ventricular loops. The present results demonstrate that this new epicardial surface ECG recording technique, combined with the described SST-ECG, provided a way to improve or replace more elaborate intracardiac techniques used in isolated hearts or in animal experiments.

The influence of elevated Mg2+ concentrations on cardiac electrophysiologic parameters

This study was performed to evaluate the direct effects of magnesium ions on cardiac conduction velocities and on refractoriness using a modified Langendorff perfusion system and a special ECG recording and stimulation technique. An increase of the MG2+ concentration to 2.3 mM had no detectable effects. Higher concentrations of up to 4.6 mM decreased the spontaneous sinus rate and the conduction velocity through the AV node and the His bundle in a concentration-dependent manner. During stimulation with premature beats, in the presence of 4.6 mM of Mg2+, the effective and relative refractory periods (ERP, RRP) of sinoatrial, AV nodal, and His bundle conduction and the ERP of the ventricular myocardium were significantly prolonged, whereas the ERP of the atrial myocardium remained unaffected. The refractoriness of sinoatrial, AV nodal, and His bundle conduction, and of the ventricular myocardium, were also prolonged during pacing with a stepwise increased pacing rate. These observations suggest a marked rate-dependent inhibitory effect of magnesium on the refractoriness of the His-bundle conduction and of the ventricular myocardium, which might be an important factor in the antiarrhythmic effectiveness of magnesium in ventricular tachyarrhythmias.

Modulation of cardiac impulse generation and conduction by nifedipine and verapamil analyzed by a refined surface ECG technique in Langendorff perfused guinea pig hearts

Using a modified Langendorff system, a special ECG recording technique and appropriate placement of two silver wire electrodes, early atrial and His bundle activity can be detected continuously from the surface of intact and spontaneously beating guinea pig hearts. This new method was applied to measure the direct and inhibitory effects of nifedipine and verapamil on impulse generation and conduction in isolated and perfused guinea pig hearts. Depression of sinoatrial conduction was the most prominent effect of nifedipine. In all concentrations applied (10(-7) M, 10(-6) M, 10(-5) M) nifedipine predominantly led to sinoatrial blocks of different degrees. Heart rate decreased slightly in a dose-dependent manner. PQ and HV duration remained essentially constant. In the highest concentration of nifedipine (10-5) M), sinus node activity was so depressed that AV dissociation or ventricular rhythm developed. Only in one out of eight experiments with cumulative increase of nifedipine concentrations to 10(-5) M was the AV node affected by nifedipine and a second-degree AV block developed (10(-6) M). Verapamil's inhibitory effects on the rate of impulse initiation in the sinus node were more pronounced than those of nifedipine, but the inhibition of sinoatrial conduction by verapamil was less marked. At 10(-6) M verapamil, the incidence of sinoatrial blocks and of ventricular rhythm was similar to the incidence of first degree AV blocks. PQ time (+14%) but also HV time (+12%) were prolonged under the influence of this concentration of verapamil. At the highest concentration of verapamil (10(-5) M) applied for 10 min, ventricular rhythm developed in five out of eight experiments, as well as one second and two third-degree AV blocks. The results confirm that the simultaneous measurements of sinus node activity of sinoatrial and atrioventricular conduction and of HV duration is feasible with this ECG technique, to evaluate the inhibitory effects of Ca-antagonists on sinus and AV node activity in the intact heart.