Effects of rapid stimulation on the transmembrane action potentials of rabbit sinus node pacemaker cells

Pharmacological tests of the mechanism of the periodic rhythm caused by veratramine in the sinoatrial node of the guinea pig

1. We investigated the effects of several drugs and extracellular ions on the periodic sinoatrial node rhythm caused by high concentrations of veratramine (>2 microM) in isolated guinea pig sinus atria. 2. During the active phase of this rhythm, pacemaker activity appeared to be due to transient afterdepolarizations resembling the delayed afterdepolarizations attributed to Ca++-induced Ca++ release in cardiac tissue. 3. Ryanodine (200-2200 nM) did not decrease the transient afterdepolarizations, and instead increased the heart rate during the active phase, prolonged the active phase, and sometimes caused conversion to regular rhythm. 4. Dichlorobenzamyl (10-110 microM), a blocker of electrogenic Na+-Ca++ exchange, did not slow or stop beating during periodic rhythm, but rather increased average heart rate and, at a higher concentration, caused conversion to regular rhythm. 5. Ouabain (0.1 microM), an inhibitor of the sodium pump and electrogenic Na+-K+ exchange, had little effect on veratramine periodic rhythm, but at higher concentrations it caused increased average heart rate and conversion to regular rhythm. 6. The chronotropic effect of Ca++ was normally weakly positive; however, in the presence of veratramine, and before the appearance of periodic rhythm, the chronotropic effect of Ca++ was weakly negative, and was associated with destabilization of the heart rate, leading to frequency oscillations or periodic rhythm. 7. Veratramine changed the chronotropic effect of K+ from weakly negative to moderately positive. 8. When half the Na+ or Cl- in the bathing medium was replaced by an impermeant ion, in the absence of veratramine the average heart rate was slightly decreased, whereas, in the presence of veratramine and periodic rhythm the average rate was increased, although the increase was not statistically significant in the case of low Na+. 9. These observations indicate that Ca++-induced Ca++ release, Na+-Ca++ exchange, and probably electrogenic Na+-K+ exchange play no important role in generation of periodic rhythm. The increased K+ dependence suggests an altered pacemaker mechanism.

Inhibition by E-4031 of the prolongation of the first returning cycle length after overdrive in anesthetized dog hearts

Prolongation of the functional recovery of the sinoatrial (SA) nodal pacemaker activity after overdrive, "overdrive suppression," is determined by intrinsic pacemaker activity, pacemaker site, and SA conduction. We investigated the effects of E-4031, a blocker of a rapid type of the delayed rectifier K+ current (Ikr) and stimulation of the intracardiac parasympathetic nerves to the SA nodal region (SAP) on the prolongation of the first returning cycle length (1st RCL) after overdrive in autonomically decentralized hearts of open-chest anesthetized dogs. Second and third RCLs also were measured. We determined SA node recovery time (SNRT) and corrected SNRT (CSNRT) after atrial pacing at rates of 120, 150, and 200% of the control rate for 1 min and also determined SA conduction time (SACT) by the constant-atrial-pacing method. E-4031 (0.1-3 micromol/kg i.v.) increased the sinus cycle length (SCL) and SNRT dose dependently. However, E-4031 decreased CSNRT when the pacing rate was low or the number of pacing stimuli was small, although the agent did not induce a significant change in CSNRT when sufficient pacing stimuli were applied. E-4031 decreased SACT dose dependently. After E-4031 treatment. we observed changes in atrial electrical configurations, suggesting the possibility of pacemaker shift. When SAP stimulation increased SCL, SNRT, CSNRT, and SACT, E-4031 selectively inhibited the prolongation of SCL by SAP stimulation but did not affect the prolongation of CSNRT or SACT. These results suggest that functional recovery of the SA nodal pacemaker activity after overdrive is regulated by Ikr at least in part and that Ikr inhibition attenuates prolongation of the SCL but not the 1st RCL induced by parasympathetic nerve activation in the heart in situ.

Cellular electrophysiologic mechanisms of cardiac arrhythmias

Cardiac arrhythmias are caused by alterations in the electrophysiologic properties of the cardiac cells, which affect the characteristics of the transmembrane potentials. The electrophysiologic properties that cause arrhythmias are automaticity, triggered activity, and reentrant excitation. Each of these mechanisms is described in terms of the characteristics of the transmembrane potentials and how these influence the appearance of the arrhythmia on the electrocardiogram.

Effect of stimulus intensity on atrial refractoriness and sinus node recovery

INTRODUCTION

Prior studies of sinus node function in man stated that the stimulus intensity of overdrive pacing has no effect on the response of the sinus node to overdrive suppression; however, data documenting these statements were lacking. Previous studies have also suggested that drive train stimulus intensity can alter ventricular refractoriness, but similar studies have not been performed on the human atrium. The purpose of this study was to evaluate the effects of drive train stimulus intensity on atrial effective refractory period and sinus node recovery time.

METHODS AND RESULTS

The effect of drive train stimulus intensity on atrial effective refractory period and sinus node recovery time was studied in 42 patients undergoing clinical electrophysiologic tests. The atrial effective refractory period was shorter at 10 mA (221 +/- 20 msec) and 5 mA (232 +/- 25 msec) than at a drive train stimulus intensity of 1.5 times late diastolic threshold (248 +/- 24 msec, P < 0.05 for pairwise comparison). The sinus node recovery time did not demonstrate a similar effect in the baseline state, following beta-adrenergic blockade, or following combined parasympathetic and beta-adrenergic blockade. However, following isolated parasympathetic blockade with atropine, the corrected sinus node recovery time shortened from 88 +/- 51 msec at 1.5 times late diastolic threshold to 48 +/- 55 msec at 10 mA (P < 0.05). Significant variability was present in sinus node recovery time measurements at baseline and following beta blockade; this variability decreased following parasympathetic blockade.

CONCLUSION

These data suggest that drive train stimulus intensity can affect the electrophysiologic properties of sinus node and atrial tissue. This effect appears to be mediated by local catecholamine and acetylcholine release and provides further evidence that the interaction between pacing stimuli and the cardiac autonomic system may need to be considered in evaluating electrophysiologic effects.

Complete sinoatrial block in two patients with bradycardia-tachycardia syndrome

Electrophysiologic studies with recordings of sinus node electrograms were performed in two patients with bradycardia-tachycardia syndrome. In both patients, the rest electrocardiogram showed apparent sinus bradycardia. Patient 1 had frequent paroxysms of atrial tachycardia with long pauses of up to 10 seconds; Patient 2 had paroxysmal atrial flutter and atrial pauses of up to 8 seconds. Multiple, repetitive, low frequency deflections, with a cycle length ranging from 730 to 960 ms in Case 1 and 570 to 750 ms in Case 2, suggestive of sinus node electrograms, were recorded at a critical area at the junction between the superior vena cava and the right atrium. These low frequency deflections had no relation to spontaneous junctional beats or the spontaneous atrial beats that showed high frequency deflections on the atrial electrogram. However, they could be suppressed by spontaneous or paced atrial beats. Pharmacologic interventions in Case 2 showed that the cycle length of the low frequency deflections shortened after administration of isoproterenol and did not change after propranolol or atropine. Thus, complete sinoatrial exit block with intact entrance conduction can occur in patients with bradycardia-tachycardia syndrome. Under such circumstances, the surface electrocardiographic manifestation of sinus bradycardia may not be of sinus origin.

Effect of basic pacing cycle length on sinus node refractoriness in the rabbit

The effect of basic pacing cycle length on sinus node refractoriness was investigated. In 18 rabbit right atrial preparations, the sinus node effective refractory period (SNERP) was measured at multiple basic pacing cycle lengths. In 14 experiments SNERP was measured at basic pacing cycle lengths of 400, 350 and 300 ms. The mean SNERP (+/- standard deviation) prolonged from 168 +/- 31 ms at 400 ms to 181 +/- 37 ms at 350 ms to 196 +/- 40 ms at 400 ms (p less than 0.001). To rule out the possibility that rapid stimulation might release acetylcholine and thus prolong refractoriness, 4 more experiments were conducted in the presence of atropine (2 X 10(-6) M), and similar results were obtained. The spatial orientation of refractoriness was examined in 7 experiments. At the same premature interval, shorter basic pacing cycle lengths resulted in block of the premature impulse at a greater distance from the sinus node. Therefore, in sinus node tissue refractoriness increases with shortening of basic pacing cycle length, a response similar to that of the atrioventricular node.

Regional differences in the electrical activity of the rabbit sinus node

In order to study regional differences in membrane properties within the sinus node, we have investigated the electrical activity of small specimens of tissue (approximately 0.3 mm in diameter) that have been prepared from the central, transitional and peripheral areas of the rabbit sinus node. The action potential upstroke velocity and overshoot as well as the maximum diastolic potential were significantly greater in tissue from the peripheral area. Paradoxically, the rate of spontaneous activity was slowest in tissue from the central area (normally the leading pacemaker site in the intact heart). Recovery of excitability was slowest in tissue from the central area and fastest in tissue from the peripheral area. The depression of electrical activity produced by rapid stimulation (overdrive) was most marked in the central region. Raising the bathing K concentration depresses spontaneous activity and this effect was again most marked in the central region. Tissue from the central area was the least sensitive to acetylcholine in five out of six hearts. Pacemaker shift can occur within the sinus node in response to various interventions, and differences in electrical activity as observed in this study may underlie such changes.

Sinus node recovery times following the spontaneous termination of supraventricular tachycardia and following atrial overdrive pacing: a comparison

We compared the pauses that followed the spontaneous termination of supraventricular tachyarrhythmias with the pauses that followed the cessation of atrial overdrive pacing in 21 patients. In 10 patients with abnormal sinus node function and in 11 patients with normal sinus node function we recorded the spontaneous termination of supraventricular tachyarrhythmia in the clinical electrophysiology laboratory; a strong correlation (r = 0.94) was found between the maximal spontaneous sinus node recovery time and the maximal paced sinus node recovery time. A weaker correlation was found between the paced and spontaneous sinus node recovery times (r = 0.57) when the spontaneous termination of supraventricular tachyarrhythmia was recorded during ambulatory electrocardiographic recording in seven patients. Spontaneous sinus node recovery times were significantly shorter than maximal paced sinus node recovery times (p less than 0.001). However, no significant difference was detected between the paced and spontaneous sinus node recovery times when atrial pacing was performed at the rate of the tachycardia. We conclude that spontaneous and postpacing sinus node recovery times are closely correlated. The paced sinus node recovery time is, however, frequently more prolonged than spontaneous sinus node recovery time because of probable atriosinus entrance block during rapid supraventricular tachycardia.

Comparative electrophysiologic effects of digoxin in the nonsedated chronically instrumented puppy

The electrophysiologic effects of acute but not chronic administration of cardiac glycosides have been studied. Nineteen chronically instrumented beagle puppies underwent 2-week courses of parenteral digoxin in three dosage regimens: six received digoxin, 0.04 mg/kg/day; seven received 0.03 mg/kg/day; and 11 received 0.02 mg/kg/day. Mean serum concentrations were 3.2 ng/ml, 1.3 ng/ml, and 1.0 ng/ml, respectively. Significant electrophysiologic effects on sinus node function were produced only by the highest dose. Atrioventricular node conduction was significantly delayed among animals receiving both high and middle dosages. All three regimens significantly effected atrioventricular specialized conduction system functional refractory periods. Atropine decreased digoxin-induced effects on all measured parameters but totally eliminated the digoxin effect on the corrected sinus node recovery time.