Search PubMed⌕ Search

Biomedical subjects

R S Bexton

Publications and source records attributed to R S Bexton.

70 records · Page 4Linked to original sources
PubMed1983

Retrograde gap in fast pathway conduction accentuated by the class I antiarrhythmic agent, flecainide.

A case is reported of a patient with functional duality of AV nodal conduction in whom, during ventricular extrastimulus testing, there was a gap in retrograde fast pathway conduction which allowed the temporary expression of retrograde slow pathway conduction. The administration of the antiarrhythmic agent flecainide, which has disparate effects on retrograde fast and slow pathway conduction characteristics, accentuated this phenomenon. The electrophysiological basis of gap phenomena is discussed.

Adult↗
PubMed1983

Effect of beta blockade on exercise response after cardiac transplantation.

Six cardiac transplant recipients underwent maximal exercise testing before and after the administration of intravenous propranolol to assess the effect of beta blockade on their exercise heart rate response and exercise capacity. Before propranolol the patients were capable of a mean of 6.8 minutes of exercise and heart rate increased from a resting value of 102 +/- 25 a minute to 138 +/- 34 at peak exercise--a mean increase of 35%. All tests were terminated because of tiredness or muscle weakness. After one hour's rest, intravenous propranolol (0.2 mg/kg over 10 minutes) was administered with a reduction in resting heart rate from 109 +/- 28 a minute to 83 +/- 16. During the repeat exercise test the patients were capable of a mean of 4.5 minutes of exercise and all tests were terminated by extreme exhaustion and/or unsteadiness requiring immediate cessation of the treadmill. Heart rate increased from a resting value of 83 +/- 16 a minute to 96 +/- 18 at peak exercise. The exercise capability of the denervated heart is conspicuously reduced by beta blockade, presumably because of its reliance on circulating catecholamines.

Adult↗
PubMed1983

Electrophysiological abnormalities in the transplanted human heart.

Fourteen relatively long term survivors of cardiac transplantation underwent systematic electrophysiological evaluation and ambulatory electrocardiographic monitoring. Six patients had prolonged conduction intervals during sinus rhythm. Sinus node function could be assessed in all donor atria and in 10 recipient atria. Sinus node recovery times were prolonged in four of the donor atria and in six recipient atria. In the donor atria abnormalities of sinus node automaticity were invariably associated with abnormalities of sinoatrial conduction. Four patients showed functional duality of atrioventricular nodal conduction during programmed extrastimulation, but no patient developed re-entrant arrhythmia. During ambulatory electrocardiographic monitoring no pronounced tachyarrhythmias were recorded. Three patients showed abnormalities of sinus node impulse formation. All three patients had abnormal sinus node recovery times during their electrophysiological study. Long term survivors of cardiac transplantation have a high incidence of electrophysiological abnormalities. Abnormalities of donor sinus node function are probably of clinical significance. The clinical significance of abnormalities detected within the atrioventricular conduction system of the denervated heart remains to be elucidated.

Adult↗
PubMed1983

Interference with the pacemakers of two workers at electricity substations.

Pacemaker function was tested in two electricity substation workers exposed to high tension electric fields. High intensity electric fields induced reversion to the interference mode, producing in one case competitive rhythm and in the other inappropriately slow pacing which resulted in asymptomatic pauses of up to 2.5 s. A suit designed to shield the body from the effects of high intensity electric fields was tried and proved to be effective in protecting the pacemaker, allowing it to function normally in the substations.

Adult↗
PubMed1983

The direct electrophysiologic effects of disopyramide phosphate in the transplanted human heart.

To evaluate the direct electrophysiologic effects of i.v. disopyramide phosphate and to differentiate these effects from its autonomically mediated actions, we administered the drug (2 mg/kg over 5 minutes) during electrophysiologic study to eight cardiac transplant recipients who had documented functional cardiac denervation. After disopyramide, the cycle length of the denervated donor right atrium increased from 626 +/- 129 to 716 +/- 148 msec (mean +/- SD, p less than 0.001), whereas that of the innervated recipient atrium decreased from 846 +/- 195 to 659 +/- 99 msec (p less than 0.02). There were small increases in both the sinus node recovery time (1128 +/- 616 to 1198 +/- 592 msec, p less than 0.05) and corrected sinus node recovery time (440 +/- 418 to 489 +/- 409 msec, p less than 0.02) of the donor atrium, whereas the recovery times of the recipient atrium shortened (sinus node recovery time, 1298 +/- 218 to 1218 +/- 196 msec; corrected sinus node recovery time, 464 +/- 108 to 410 +/- 115 msec). Disopyramide markedly prolonged all conduction intervals. The PA interval increased from 47 +/- 16 to 54 +/- 17 msec (p less than 0.01), the AH interval from 55 +/- 12 to 78 +/- 12 msec (p less than 0.001), the HV interval from 38 +/- 9 to 58 +/- 13 msec (p less than 0.001), the QRS duration from 93 +/- 18 to 129 +/- 34 msec (p less than 0.001) and the QT interval from 339 +/- 23 to 403 +/- 39 msec (p less than 0.001). There was no significant change in the effective refractory period of the atrium, ventricular or atrioventricular node. The functional refractory period of the atrioventricular node increased from 369 +/- 34 to 395 +/- 31 msec (p less than 0.001). The electrophysiologic effects of disopyramide in the denervated heart are markedly depressant. In the innervated normal heart, the majority of these effects are counteracted by the drug's autonomically mediated anticholinergic actions.

Adult↗
PubMed1982

Initial experience with a fully implantable, programmable, scanning, extrastimulus pacemaker for tachycardia termination.

A fully implantable automatic scanning pacemaker designed for tachycardia termination has been used in three patients with regular paroxysmal supraventricular tachycardia. The pacemaker recognizes tachycardia and delivers one or two extrastimuli which automatically scan inwards if tachycardia continues. A memory is incorporated to retain and immediately reuse a successful pacing sequence if tachycardia recurs. Ventricular pacing has been used in two patients and atrial stimulation in one. Although all had suffered frequent attacks of tachycardia after implantation no sustained episodes of tachycardia have been appreciated. No unwanted arrhythmias have been induced and drug treatment has been stopped in all three patients. Fully implantable scanning pacemakers which automatically recognize and revert tachycardia offer an effective and versatile form of treatment of recurrent paroxysmal tachycardias.

Aged↗
PubMed1982

Electrophysiological characteristics of junctional pacemakers in congenital A-V block and following His bundle cryoablation.

The characteristics of the escape mechanism following surgically induced permanent A-V block were investigated and compared with those seen in congenital complete heart block (CHB). Six patients had undergone elective cryothermal ablation of the His bundle for supraventricular arrhythmias unresponsive to pharmacological and pacemaker techniques (group A) and 12 patients had congenital CHB (group B). In the 12 patients in group B the site of block was localized by His bundle electrocardiography to be proximal to the point of recording of the His potential. In 3 patients in group A in whom intracardiac studies were performed it was impossible to record an His potential. There were no significant differences between the control escape rate, junctional recovery time (JRT), and corrected junctional recovery time (cJRT) of the two groups. In group A there were no significant changes in these parameters following the administration of atropine, whereas isoproterenol significantly increased the rate of the subsidiary pacemaker (p less than 0.001) and shortened the JRT (p less than 0.02) and cJRT (p less than 0.02). In group B both atropine (A) and isoproterenol (I) significantly increased escape rate (A, p less than 0.001; I, p less than 0.001) and shortened JRT (A, p less than 0.01; I, p less than 0.001) and cJRT (A, p less than 0.01; I, p less than 0.001). It is concluded that the escape focus in patients with congenital CHB is situated in the A-V node. The escape rhythm following His bundle section is unpredictable and elective permanent pacemaker implantation is indicated.

Adolescent↗
PubMed1982

Implantable automatic scanning pacemaker for termination of supraventricular tachycardia.

Thirteen patients suffering from reentrant supraventricular tachycardia have undergone implantation of a scanning extrastimulus pacemaker. This pacemaker is fully implanted and automatic, and it requires no external control device to activate or control it. The pacemaker is activated when tachycardia occurs. After four cycles an extrastimulus is induced with a preset coupling time from a sensed intracardiac potential, and every four cycles thereafter a further extrastimulus occurs, but on each occasion there is a decrement in coupling cycle by 6 ms until 90 ms of the cardiac cycle has been scanned by extrastimuli. When necessary, two extrastimuli can be introduced with a fixed but preset coupling time between them. Every four beats two extrastimuli are induced but the coupling time between the spontaneous cardiac potential and the first stimulus is decreased by 6 ms until 90 ms of the cardiac cycle has been scanned. The coupling time between the two stimuli is fixed throughout the scan. When termination of tachycardia occurs the successful timing variables are retained in the pacemaker memory so that at the onset of the next episode of tachycardia these settings are used first. Pacemaker pulse width, sensitivity, tachycardia trigger rate, coupling intervals for both stimuli and the use of single or double extrastimuli are all programmable transcutaneously. Three patients required single, and seven patients double ventricular premature stimuli; three patients required double atrial premature stimuli for termination of tachycardia. Despite frequent attacks of tachycardia before implantation, only two patients had a sustained attack of tachycardia after pacemaker implantation.

Adolescent↗
PubMed1982

The use of active fixation electrodes for permanent endocardial pacing via a persistent left superior vena cava.

Two patients underwent permanent endocardial pacing for complete atrioventricular block. In each case a persistent left superior vena cava was either suspected or known to be present. An active fixation electrode was passed down the left superior vena cava and the tip positioned in the apex of the right ventricle. Stable ventricular pacing was achieved for the follow-up period of approximately six months. With the availability of such active fixation electrodes the presence of a persistent left superior vena cava no longer mandates insertion of an endocardial electrode via the right superior vena cava, when present, or implantation of an epicardial pacing system.

Adolescent↗
PubMed1982

Electrophysiological effects of sotalol--just another beta blocker?

The electrophysiological effects of intravenous sotalol hydrochloride (0.4 mg/kg) were assessed in 24 patients, including 13 with the Wolff-Parkinson-White syndrome, undergoing routine electrophysiological study. Fifteen to 30 minutes after sotalol administration there was a significant increase in sinus cycle length and in sinus node recovery time. There was a small increase in the AH interval, but the HV interval was unchanged. The QT and JT intervals, measured during sinus rhythm, were both increased. The atrial, ventricular, and atrioventricular nodal effective refractory periods were all prolonged, as was the atrioventricular nodal functional refractory period. In 13 patients with ventricular pre-excitation there was an increase of the accessory pathway anterograde and retrograde effective refractory periods. In 12 of these 13 sotalol was given during atrioventricular re-entrant tachycardia, resulting in termination in five. Tachycardia cycle length increased in all patients, with the major effect being in the atrioventricular direction. Though some of the effects seen in these patients are consistent with the beta adrenergic antagonist properties of sotalol, the effect on atrial, ventricular, and accessory pathway effective refractory periods and on ventricular repolarisation is not typical of that observed with other beta blockers but may be the result of lengthening of the action potential duration. These findings suggest that sotalol may be a more versatile antiarrhythmic agent than other beta receptor antagonists.

Adolescent↗
PubMed1982

Acute electrophysiological effects of flecainide acetate on cardiac conduction and refractoriness in man.

The electrophysiological effects of flecainide acetate (2 mg/kg as an intravenous infusion over five minutes) were assessed in 47 patients undergoing electrophysiological study. Seven patients had normal electrophysiology, 16 had a direct accessory atrioventricular pathway, 12 had dual atrioventricular nodal (AH) pathways, five had paroxysmal ventricular tachycardia, six had conduction system disease, and one patient had a left atrial tachycardia. No significant change occurred in sinus cycle length. The PA interval, AH interval, and HV interval were all significantly prolonged. The QRS complex duration increased significantly. The QT interval showed slight prolongation due entirely to the increase in QRS duration. Refractoriness of the atrial and ventricular myocardium was slightly prolonged, but was significant only at ventricular level. No significant change occurred in refractoriness of the normal atrioventricular node. Pronounced prolongation of retrograde "fast" AH pathway refractoriness was observed in those patients with dual AH pathways. Anterograde and retrograde accessory pathway refractoriness were both greatly increased. These electrophysiological properties strongly suggest that flecainide will be useful in the management of a wide variety of cardiac arrhythmias. It should be administered, however, with caution to patients with pre-existing conduction system disease. Because repolarization is not delayed flecainide is unlikely to induce ventricular arrhythmias related to prolongation of the QT interval.

Adolescent↗
PubMedDate not supplied

Noninvasive recording of the His bundle electrogram in neonates.

Twenty-seven normal, nonsedated neonates had high-resolution electrocardiography performed during the first four days of life. Using high amplification, analogue and digital filters, and signal averaging, easily defined His bundle deflections of 0.75-7.75 (mean 2.5) microV were detected in 25 (92.6%) of the 27 babies. The PH interval was 60-105 (mean 83) ms and the HV interval 10-25 (mean 17) ms. Surface His bundle electrocardiography is easily performed in the neonate and, as might be predicted, conduction intervals are considerably shorter than those seen in older age groups.

Bundle of His↗
PubMedDate not supplied

Differences in left ventricular filling patterns in heart and heart-lung transplant recipients as assessed by Doppler echocardiography of transmitral flow.

The conventional surgical technique of orthotopic heart transplantation involves reconstruction of atria from donor and recipient components. The mechanical function of the atria is thus likely to be altered either by this anatomic disruption or by the autonomic denervation, which also occurs as a result of transplantation. We investigated 18 subjects with Doppler echocardiography of transmitral flow, to assess the contributions of passive and active filling of the left ventricle in six normal subjects, six heart transplant recipients, and six heart-lung transplant recipients. The ratio of passive transmitral flow to active flow was significantly higher in isolated heart transplant recipients than in normal subjects (p < 0.001), suggesting a reduced active, late diastolic component to left ventricular filling. This was not due to denervation, because the ratio in heart-lung recipients was lower than that in normal subjects (p < 0.01). These data suggest that anatomic rather than neural dysfunction is responsible for a major change in transmitral flow characteristics after isolated heart transplantation; whether the resulting change in pattern of left ventricular filling is clinically important deserves further investigation.

Blood Flow Velocity↗
PubMedDate not supplied

Esophageal electrode for recording recipient atrial activity.

Recording of recipient atrial electrical activity may be difficult, and on occasion impossible, on the surface electrocardiogram. A case is described in which distinct recipient atrial activity was recorded in the presence of a donor atrial dysrhythmia using an esophageal electrode. This simple, noninvasive technique is easy to perform and may prove to be extremely useful in the clinical and research electrophysiologic investigation of heart transplant recipients.

Adult↗