Sinus bradycardia

Tachy-brady syndrome: Electrophysiology and evolving principles of management

Sudden alterations in the heart rate may be associated with diverse symptoms. Sinus node dysfunction (SND), also known as sick sinus syndrome, is a sinoatrial (SA) node disorder. SND is primarily caused by the dysfunction of the pacemaker, as well as impaired impulse transmission resulting in a multitude of abnormalities in the heart rhythms, such as bradycardia-tachycardia, atrial bradyarrhythmias, and atrial tachyarrhythmias. The transition from bradycardia to tachycardia is generally referred to as "tachy-brady syndrome" (TBS). Although TBS is etiologically variable, the manifestations remain consistent throughout. Abnormal heart rhythms have the propensity to limit tissue perfusion resulting in palpitations, fatigue, lightheadedness, presyncope, and syncope. In this review, we examine the physiology of tachy-brady syndrome, the practical approach to its diagnosis and management, and the role of adenosine in treating SND.

Explainable artificial intelligence for heart rate variability in ECG signal

Electrocardiogram (ECG) signal is one of the most reliable methods to analyse the cardiovascular system. In the literature, there are different deep learning architectures proposed to detect various types of tachycardia diseases, such as atrial fibrillation, ventricular fibrillation, and sinus tachycardia. Even though all types of tachycardia diseases have fast beat rhythm as the common characteristic feature, existing deep learning architectures are trained with the corresponding disease-specific features. Most of the proposed works lack the interpretation and understanding of the results obtained. Hence, the objective of this letter is to explore the features learned by the deep learning models. For the detection of the different types of tachycardia diseases, the authors used a transfer learning approach. In this method, the model is trained with one of the tachycardia diseases called atrial fibrillation and tested with other tachycardia diseases, such as ventricular fibrillation and sinus tachycardia. The analysis was done using different deep learning models, such as RNN, LSTM, GRU, CNN, and RSCNN. RNN achieved an accuracy of 96.47% for atrial fibrillation data set, 90.88% accuracy for CU-ventricular tachycardia data set, and also achieved an accuracy of 94.71, and 94.18% for MIT-BIH malignant ventricular ectopy database for ECG lead I and lead II, respectively. The RNN model could only achieve an accuracy of 23.73% for the sinus tachycardia data set. A similar trend is shown by other models. From the analysis, it was evident that even though tachycardia diseases have fast beat rhythm as their common feature, the model was not able to detect different types of tachycardia diseases. The deep learning model could only detect atrial fibrillation and ventricular fibrillation and failed in the case of sinus tachycardia. From the analysis, they were able to interpret that, along with the fast beat rhythm, the model has learned the absence of P-wave which is a common feature for ventricular fibrillation and atrial fibrillation but sinus tachycardia disease has an upright positive P-wave. The time-based analysis is conducted to find the time complexity of the models. The analysis conveyed that RNN and RSCNN models could achieve better performance with lesser time complexity.

Solution-Processed Large-Area Nanocrystal Arrays of Metal-Organic Frameworks as Wearable, Ultrasensitive, Electronic Skin for Health Monitoring

Pressure sensors based on solution-processed metal-organic frameworks nanowire arrays are fabricated with very low cost, flexibility, high sensitivity, and ease of integration into sensor arrays. Furthermore, the pressure sensors are suitable for monitoring and diagnosing biomedical signals such as radial artery pressure waveforms in real time.

Sinus node dysfunction in 128 patients. A retrospective study with follow-up

A retrospective study covering all admissions during a 6-year period revealed 128 patients with sinus node dysfunction (SND). The patients were grouped according to the ECG criteria chosen: group I 33 patients with sinus bradycardia, group II 37 with sinoatrial block/sinus arrest, group III 58 with brady-tachy syndrome. Additional heart disease, predominantly ischaemic, was found in 56%. The frequency and severity of symptoms increased from group I to group III. Pacemaker treatment was given to 40% of the cases, while medical treatment alone was successful in 17%. A follow-up including 104 patients was carried out after a mean observation period of approximately three years. Sixteen patients had died. The cause of death may have been SND per se in only one case. Five patients died of apoplectic insults or complications to such. In total, nine possible or proven systemic embolic events were found--all occurring in patients with brady-tachy syndrome. A progression of the ECG abnormality from a lower to a higher group took place in nine patients during the observation period. It is concluded that SND is a condition with a broad clinical spectrum and a stationary or slowly progressive course. In general, it carries a good prognosis. A substantial number of deaths of disabilities in patients with brady-tachy syndrome may be ascribed to systemic embolism. Long-term anticoagulant therapy is proposed in this subgroup of patients with SND.

Embolism in sinoatrial disease

Among a series of consecutive patients treated with permanent pacemaker between 1965 and 1976, 43 had sinoatrial disease with paroxysmal tachycardia (group A), 30 sinoatrial disease without tachycardia (group B) and 165 atrioventricular block (group C). A retrospective study showed systemic vascular events compatible with embolism in 35%, 7% and 10% in groups A, B and C, respectively. The groups were comparable as regards age, other diseases and duration of pacemaker therapy. Because of the high incidence of embolism in group A, anticoagulant therapy should be evaluated in patients with the brady-tachycardia syndrome.

Administration of atropine in the setting of acute myocardial infarction: potentiation of the ischemic process?

Atropine has also been suggested to potentially worsen the ischemic situation in patients who are in the midst of acute coronary ischemia. We report the case of a female patient with ischemic chest pain and third degree atrioventricular block who developed acute myocardial infarction (AMI) immediately after atropine administration. The use of atropine in this instance remains a reasonable option and should be strongly considered-despite this apparent complication. Undoubtedly in some cases, acute ischemia is intensified by hypoperfusion attributable to vagally mediated bradyarrhythmia; atropine is the antidote for such situations. An awareness of this potential adverse reaction coupled with a prudent selection of candidates for atropine therapy will show the risk/benefit ratio in each individual patient and, therefore, guide the clinician.

Acute myocardial infarction complicated by hemodynamically unstable bradyarrhythmia: prehospital and ED treatment with atropine

The purpose of this study was to investigate the therapeutic response to atropine of patients experiencing hemodynamically compromising bradyarrhythmia related to acute myocardial infarction (AMI) in the prehospital (PH) setting and the therapeutic impact of the PH response to atropine on further Emergency Department (ED) care. In addition, the prevalence of AMI in patients presenting with atrioventricular block (AVB) is noted. Retrospective review of PH, emergency department (ED), and hospital records. PH patients, with hemodynamically compromising bradycardia or AVB with evidence of spontaneous circulation, who received atropine as delivered by emergency medical services (EMS) personnel, were used. Urban/suburban fire department-based emergency medical services (EMS) system with on-line medical control serving a population of approximately 1.6 million persons. Hemodynamic instability was defined as the presence of any of the following: ischemic chest pain, dyspnea, syncope, altered mental status, and systolic blood pressure less than 90 mm Hg. Bradycardia was defined as sinus bradycardia, junctional bradycardia, or idioventricular bradycardia (grouped as bradycardia), whereas AVB included first-, second- (types I and II), or third-degree (grouped as AVB). The response that occurred within 1 minute of atropine dosing was recorded as none, partial, complete, or adverse. Comparisons were made between patients with AMI and non-AMI hospital discharge diagnoses. The diagnosis of AMI was confirmed by abnormal elevations in creatinine phosphokinase MB fraction. One hundred seventy-two patients meeting entry criteria were identified. Of these, 131 (76.1%) had complete PH, ED, and hospital records and were used for data analysis. Forty-five patients (34.3%) had a primary hospital discharge diagnosis of AMI; the remaining patients had a non-AMI discharge diagnosis. AMI patients were significantly younger (67 +/- 12 v 73 +/- 13 years, P = .025), were less likely to have a history of heart disease (35.5% v54.7%, P = .038), and were more likely to present with chest pain (68.9% v24.4%, P < .001) or hypotension (60% v37.2%, P = .013) compared with non-AMI patients. Forty-five of 131 patients presented with AVB, of which 25 had a hospital discharge diagnosis of AMI (55.6%). The mean time from first dose of atropine to ED arrival and the total dose of atropine received in the PH setting did not differ between AMI and non-AMI groups (15.2 +/- 7.7 v 16.2 +/- 8.7 minutes, P= .5; and 0.9 +/- 0.49 v 1.0 +/- 0.58 mg, P = .25). The likelihood of achieving normal sinus rhythm in the PH setting did not differ between AMI and non-AMI groups (40% v 18.6%, P = .07). No differences were found between AMI and non-AMI groups in the amount of additional atropine given (1.2 +/- 0.58 v 1.3 +/- 1.1 mg, P = .58) or the use of other resuscitative therapies after ED arrival (isoproterenol, 13.3% v12.8%, P = .93; dopamine, 28.9% v26.7% P = .79; transcutaneous pacing, 26.7% v26.7%, P = .99; transvenous pacing, 8.9% v5.8%, P = .51), with the exception of thrombolytic therapy (24.4% v 0%, P< .001) and cardiac catheterization (22.2% v3.4%, P = .001). Despite a lack of significant difference in achieving a normal sinus rhythm in the prehospital or ED setting, AMI patients were more likely to achieve a normal sinus rhythm over the total course of PH and ED care than non-AMI patients (44.4% v24.4%, P = .019). Hemodynamically unstable (by ACLS criterion) AVB presenting in the PH setting is associated with a hospital diagnosis of AMI in most (55.6%) patients in this study. AMI patients with hemodynamically unstable AVB or bradycardia are no more likely to respond to atropine therapy in the PH setting than patients with non-AMI hospital diagnoses. Finally, although there is no difference in the treatment of compromising AVB or bradycardia received by AMI versus non-AMI patients in the PH or ED setting, AMI patients are more likely to achieve a normal sinus rhythm over the t

The efficacy of atropine in the treatment of hemodynamically unstable bradycardia and atrioventricular block: prehospital and emergency department considerations

OBJECTIVE

To determine the efficacy of atropine therapy in patients with hemodynamically compromising bradycardia or atrioventricular block (AVB) in the prehospital and emergency department settings.

METHODS

DESIGN

Retrospective review of prehospital, emergency department, and hospital records.

PARTICIPANTS

Prehospital patients with hemodynamically compromising bradycardia or AVB with evidence of spontaneous circulation who received atropine as delivered by emergency medical services personnel (advanced life support level).

SETTING

Urban/suburban fire department-based emergency medical service system with on-line medical control serving a population of approximately 1.6 million persons.

DEFINITIONS

Hemodynamic instability was defined as the presence of any of the following: ischemic chest pain, dyspnea, syncope, altered mental status, and systolic blood pressure less than 90 mmHg. Bradycardia was defined as sinus bradycardia, junctional bradycardia, or idioventricular bradycardia (grouped as bradycardia) while AVB included first-, second- (types I and II), or third-degree (grouped as AVB). The response that occurred within one minute following each dose of atropine was defined as none, partial, complete, or adverse.

MAIN RESULTS

Of 172 patients meeting entry criterion complete data was available for 131 (76.1%) and constitutes the study population. The mean age was 71 years. Fifty-one percent were female. Forty-five patients had AVB and 86 bradycardia. Patients with AVB were more likely to have a presenting systolic blood pressure less than 90 mmHg than those with bradycardia. In the 131 patients, responses to atropine were as follows: 26 (19.8%) = partial, 36 (27.5%) = complete, 65 (49.6%) = none, and 4 (2.3%) = adverse. Patients presenting with bradycardia (compared to AVB) more commonly: (1) received a single dose of atropine; (2) a lower total dose of atropine in the prehospital interval; (3) were more likely to arrive in the ED with a normal sinus rhythm; and (4) were less likely to receive additional atropine or isoproterenol in the ED. Those patients who achieved normal sinus rhythm over the total course of care were likely to have achieved that rhythm during the prehospital interval. There was no difference between groups in the likelihood of leaving the ED with a normal sinus rhythm achieved during the ED interval. Acute myocardial infarction was more common in patients presenting with AVB (55.5%) than with bradycardia (23.2%, P = 0.001).

CONCLUSIONS

Approximately one-half of patients who received atropine in the prehospital setting for compromising rhythms had either a partial or complete response to therapy. Adverse responses were uncommon. Those patients who presented with hemodynamically unstable bradycardia to EMS personnel responded more commonly to a single dose and a lower total dose of atropine compared to similar patients with AVB. Those patients who achieve normal sinus rhythm by ED discharge were likely to have achieved it during the prehospital interval.

Postexercise electrocardiographic and clinical changes in patients with sick sinus syndrome

Forty patients with sick sinus syndrome, 15 women and 25 men with a mean age of 53.83 +/- 13.34 years, were studied using a maximal graded bicycle stress test. None of the patients were using a pacemaker or being treated with drugs that would interfere with the sinus node function; one patient had family myocardiopathy and eight suffered from essential hypertension. All patients, including those suffering from very marked bradycardia (less than 40 beats/min) responded to the increased effort with increased heart rate. The exercise test was stopped in 22 patients (55%) after the appearance of clinical signs and in 4 (10%) after ST-segment depression greater than 1 mm. Eight (20%) finished the stress test after reaching the maximal heart rate according to age, due to an increase in sinus rate. The exercise produced or increased extrasystoles in five patients (12.5%), but only one was forced to suspend the test. The Q-T interval, corrected for heart rate according to Bazett's formula (QTc), was measured on the resting ECG before the start of the test and on the ECG recorded immediately following the end of the exercise in all patients, except one with atrial fibrillation. In 24 patients (60%), a QTc mean increase of 0.040 +/- 0.022 sec was observed at the end of the stress test. Fourteen (35%) had the usual shortening due to the increase in heart rate. One patient showed no variation of the QTc. A lengthening of the QTc at the end of the exercise in more than half of the patients was the most intriguing electrocardiographic change.(ABSTRACT TRUNCATED AT 250 WORDS)

Exercise testing in the diagnosis of sick sinus syndrome in the elderly: implications for treatment

Twenty-three elderly patients underwent treadmill exercise testing to fatigue; 11 patients had sick sinus syndrome (7 males, 4 females; age range 71-91 years; mean, 78 years); 12 elderly persons were controls (5 males, 7 females; age range 70-93 years; mean, 76 years). All were euthyroid. The study showed that patients with sick sinus syndrome compared with the control group demonstrated a lower resting heart rate, a smaller increase in heart rate during exercise, poorer exercise tolerance, and differences in heart rate during the recovery phase. The value of the effort test in relation to the diagnosis of sick sinus syndrome is discussed, together with its possible implications in the choice of pacemaker.

Cardiovascular effects of adenosine in man; possible clinical implications

The results summarized above indicate that adenosine is a physiologically relevant modulator of the cardiovascular system in man. The levels of adenosine are low during resting conditions, but may increase during conditions of oxygen and/or substrate deficiency. Already the basal concentration seems to be sufficient to affect regional flow in vital organs such as the heart. Several drugs may act by increasing the levels of adenosine or by influencing its receptors. In addition, adenosine may be used in many clinical situations as a vasodilator, antiaggregatory compound as well as an antiarrythmic agent. Its effect is easy to control due to the extremely short plasma half-life. The dose range for the clinical effects are summarized in Table 6. Both the physiological and pharmacological aspect of adenosine are subject to intense study in several laboratories.

Sick sinus syndrome: an adenosine-mediated disease

It is suggested that adenosine is a mediator of the paroxysmal bradycardias and tachycardias characterising the sick sinus syndrome. Increased sensitivity to adenosine could account for the arrhythmias, which might therefore be suppressed by an adenosine antagonist such as theophylline.

Pacemaker therapy in the seventies: interaction of patient age, time of implant, and indications for pacing

In the 1970s, 707 patients received a primary pacemaker implant in our hospital. An analysis of the indications for pacemaker therapy revealed that one-third had third degree heart block, one-third had sick sinus syndrome and one-third had other indications. The distribution of indications was identical in all age groups. Furthermore, this distribution did not change from the first half of the decade to the second.

Survival in sinoatrial disorder (sick-sinus syndrome)

A total of 381 patients with established (156) or potential (225) sinoatrial dysfunction were included in a 10-year prospective survey to determine the course of the disease and the benefits of pacing. With the exclusion of nine patients who were lost to follow-up, 61 were fitted with pacemakers. The overall survival of patients with established and potential dysfunction was similar and apparently indistinguishable from that of the normal population. Pacemaker implantation had little discernible effect on mortality though it reduced some incapacitating symptoms. These findings suggest that sinoatrial dysfunction is a relatively benign condition. Hence pacing should probably not be adopted as a routing measure but be reserved for patients with troublesome symptoms.

Characterisation of heart rate response to exercise in the sick sinus syndrome

Patients with sick sinus syndrome have abnormalities of the sinoatrial node. We have measured the heart rate response to exercise in 7 patients with sick sinus syndrome without significant associated heart disease (group A) mean age 53.4 years, and compared this with 7 'normal' patients who were age-matched to within 5 years (group B), and 7 younger, well-trained subjects (group C). All underwent maximal treadmill exercise. Although maximum oxygen consumption (VO2max), 1/min per kg, in group A was not significantly different from group B (23.8 +/- 4.7 vs 19.9 +/- 0.8, mean +/- SE) maximum heart rate, beats/min, in group A was significantly lower than in group B (124 +/- 8.9 vs 163 +/- 3.7, P less than 0.001). At the end of 3 minutes of Bruce Stage I exercise, group A patients had a heart rate less than 130/minute (95% confidence level), whereas group B patients had heart rates greater than 134/minute. VO2 was plotted against heart rate (HR). Patients in group A had a significantly lower slope (deltaHR 5.20 +/- 0.33/delta1 ml VO2/kg per min, P less than 0.001). There was no significant difference in the slopes between groups A and C. On exercise patients with sick sinus syndrome have a normal VO2, but a reduced heart rate response as compared with age-matched normal patients. This abnormal heart rate response to the physiological stimulus of exercise may be of help in the evaluation of patients with sick sinus syndrome who do not have significant underlying heart disease.

Electrophysiologic effects of hydralazine on sinoatrial function in patients with sick sinus node syndrome

The electrophysiologic effects of hydralazine were evaluated in nine hypertensive patients with sinoatrial dysfunction. Intravenous hydralazine, 0.15 mg/kg, caused no significant reduction in arterial blood pressure. Yet this dose of hydralazine increased heart rate from 61.9 +/- 4.1 beats/min (mean +/- standard error of the mean) to 68.6 +/- 4.9 (P less than 0.001). Sinus nodal recovery time upon termination of atrial pacing shortened from 3,207 +/- 1,098 to 2,064 +/- 573 msec (P less than 0.05) and second escape cycles shortened as well (P less than 0.025). Acceleration of heart rate and abbreviation of recovery time did not closely correlate with change in blood pressure (r = 0.41 and 0.18, respectively). Junctional escape beats became more frequent and junctional escape time shortened from 2,525 +/- 692 to 1,705 +/- 382 msec (P less than 0.05). Sinoatrial conduction time tended to shorten, but a significant change was not observed. Atrial tachyarrhythmias did not occur and atrial refractoriness was unchanged. Thus, a minimal blood pressure response to hydralazine was associated with enhanced automaticity. Hydralazine merits clinical trial for treatment of sick sinus syndrome with concomitant hypertension.

Pathological studies in sinoatrial disorder (sick sinus syndrome)

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Symptomatic sinus arrest in a young girl

Sinus node dysfunction, previously unreported in girls, occurred in a 13-year-old girl who required permanent pacemaker implantation because of recurrent syncopal attacks. In addition to periodic sinus arrest, the presence of significant atrioventricular conduction disturbance was also documented. Although she had had a persistent ductus arteriosus divided at an earlier age, the disturbance of rhythm and conduction cannot be ascribed to a congenitial or haemodynamic abnormality. An acquired origin can only be postulated.

Graded exercise testing in patients with sinus node dysfunction

Serial measurements of heart rate and oxygen uptake were obtained before and during maximal upright graded bicycle stress testing in 16 patients, 10 to 77 years old (mean 46 years), with sinus node dysfunction; five had permanent and two had temporary demand ventricular pacemakers. In 15 patients, including those with pacemakers, maximal exercise was performed before and after the intravenous administration of 1 mg atropine. Maximal exercise was terminated because of cerebral symptoms in seven (three had effort-induced tachyarrhythmias and one had autonomic insufficiency), fatigue in five (one had effort-induced heart block), heart failure in three and angina pectoris in one. With maximal exercise, patients with sinus node dysfunction were unable to obtain maximal heart rates or oxygen uptakes comparable to age- and sex-matched control subjects. Additionally, maximal oxygen uptake did not differ significantly between patients with or without pacemakers even when ventricular pacing rates were increased (two instances). The administration of atropine increased the resting heart rate, but the maximal heart rate and oxygen uptake achieved during maximal exercise did not differ significantly from those obtained before the administration of atropine in the patient and control groups. Physically active patients with sinus node dysfunction have diminished exercise capacity due in part to cardiac arrhythmia, latent or overt cardiac failure, or autonomic dysfunction.

Electrophysiologic evaluation of sinus node function in patients with sinus node dysfunction

Twenty patients of mean age 66.2 years, with suspected sinus node dysfunction, underwent extensive electrophysiologic study. Sinus bradycardia (18), sinus pauses (3), and sinoatrial block (1) were identified in their ECGs prior to study. Also 11 patients had some abnormality of atrioventricular nodal and/or intraventricular conduction prior to study. At the time of electrophysiological study, 10/20 patients (50%) had a mean cycle length exceeding 1000 msec, and mean P-V interval exceeded 210 msec in 7/20 (35%). The estimated "sinoatrial conduction time" exceeded 215 msec in 6/16 (38%) patients. The maximum first escape cycle following pacing at six different rates exceeded a value equal 1.3 X the mean value of the control cycle length + 101 msec (slope of regression line + Y intercept + 1 SD) in 13/9 (68%) patients. Nineteen patients received 1 mg atropine intravenously and mean cycle length decreased by 19%, from 891 +/- 175.8 msec to 718 +/- 182.9 msec. Graded infusion of isoproterenol was employed in 19 patients; four patients required an infusion rate greater than 28.3 ng/kg/min to produce a 20% decrease in spontaneous sinus cycle length. These data would indicate that a variety of interventions are required to characterize the disturbance of sinus node automaticiy and sinoatrial conduction in patients with sinus node dysfunction.

First-degree sinoatrial heart block: sinoatrial block in the sick-sinus syndrome

Sinoatrial conduction time (SACT) was estimated from the delay in the atrial recovery period after premature depolarization applied in that portion of atrial diastole when increasing prematurity resulted in a constant recovery interval. In 20 normal patients SACT was 169 msec. +/- 91 (2 S.D.). At least nine of 19 patients with "sick-sinus syndrome" (SSS) demonstrated SACT that were longer than seen in these normal subjects. SACT was prolonged in seven of nine SSS patients with abnormal A-V nodal conduction. Among 10 SSS patients with normal A-V conduction, only two had prolonged SACT. This study identifies first-degree sinoatrial block as a frequent manifestation of SSS associated with the presence of A-V node conduction abnormalities.

Sick sinus syndrome in children

The ages of 6 male patients with the sick sinus syndrome ranged from 10-15 years when their symptoms began. At rest all had a heart rate of 60/min or less. Two had syncopal attacks which threatened life; 1 had only attacks of dizziness; the other 3 had no syncopal attacks but had recurrent attacks of supraventricular tachycardia ('brady-tachycardia syndrome') which were more resistant to drug therapy than is usual in childhood. They were not controlled or suppressed by digoxin when it was given. Substernal pain occurred in 2 patients who had syncope. In all patients the heart rate remained inappropriately slow after exercise and atropine. Cardiac pacemakers were used in the 2 patients with life-threatening syncope. Any patient who has dizziness or syncopal attacks and an inappropriately slow heart rate should have electrocardiograms recorded at rest and after exercise to record the heart rate and to look for abnormal P-waves.

Treatment of sinoatrial syndrome with permanent cardiac pacing in 90 patients

Out of a pacemaker population of 392 patients, 90 (23%) have been found to have sinoatrial syndrome. Their ages ranged from 22 to 86 years, and averaged 66 for men and 70 for women. The male-to-female ratio was 1:1.6. Before pacemaker implantation, syncopal attacks had occurred in 54%, dizziness without syncope in 31% and tachyarrhythmias in 57%. Atrial or paroxysmal supraventricular tachycardia had been recorded in 33%, atrial fibrillation or flutter in 28%, and ventricular tachycardia in 11%. First and/or second degree AV block was found in 36%. Coronary heart disease was present in 61% and 20% had had myocardial infarction. Cardiomyopathy and previous carditis were other associated heart diseases. Sinoatrial syndrome was the only manifestation of heart disease in 20%. Follow-up time after pacemaker implantation ranged from 3 months to 7 years, mean 23 months. Syncopal attacks were stopped in 48 of 49 patients, dizziness was relieved in all 28 patients and tachyarrhythmias were controlled by combined drug treatment in 43 of 51 patients. Nineteen patients died during the follow-up, most of them of cerebrovascular events or myocardial infarction. Associated coronary heart disease was especially frequent in this group. The death of one patient was caused by a run-away pacemaker. Other pacing failures were due to electrode movement or premature battery exhaustion. There was no mortality associated with pacemaker implantations or replacements. These results strongly support the view that pacemaker treatment most effectively controls symptoms of sinoatrial syndrome when drug treatment fails.

"Paradoxical" prolongation of sinus nodal recovery time after atropine in the sick sinus syndrome

A case of symptomatic sick sinus syndrome is presented with confirmation of sinus nodal dysfunction established by functional testing. The validity of such provocative testing and the criteria for abnormality are discussed. A newly recognized, seemingly "paradoxical" and potentially detrimental effect of atropine noted in this patient is examined. Despite an increase in sinus rate and an improvement in sinoatrial conduction time after administration of atropine, a markedly prolonged sinus recovery time after rapid atrial pacing occurred, and atrial quiescence for more than 10 seconds was seen. Possible electrophysiologic mechanisms for this phenomenon, such as decreased atriosinus entrance block, concealed sinoatrial reentry or enhanced intranodal depolarization, are discussed and potential clinic correlates are made.

Digitalis in the sick sinus syndrome. The effects of digitalis on sinoatrial automaticity and atrioventricular conduction

The effect of digitalis in patients with sinoatrial node dysfunction was studied. Atrial pacing studies were performed while His bundle electrograms were recorded in fourteen patients with sinus bradyeardia, syncope, or related symptoms. Measurements were repeated after 0.01 mg/kg ouabain, followed in some by 1 mg atropine. Ouabain produced a significant shortening of sinoatrial recovery time, but no significant change in heart rate. Atropine normalized sinoatrial recovery times in most patients. This study demonstrates that digitalis causes an increase in automaticity in the sick sinus syndrome as reflected by a shortened sinoatrial recovery time. The efficacy of contemplated therapeutic agents in sick sinus syndrome can be evaluated by rapid atrial pacing. When clinically indicated, digitalis may be used for congestive heart failure or tachyarrhythmias in the sick sinus syndrome in the absence of significant A-V conduction disease.

Adams-Stokes syndrome caused by sinoatrial block

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Tape monitoring of the electrocardiogram in ambulant patients with sinoatrial disease

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Assessment of sinus node function in patients with the sick sinus syndrome

Thirty-one patients with symptomatic sinus node dysfunction were evaluated with electrocardiograms, Holter monitor recordings, exercise, isoproterenol infusions, atropine administration, Valsalva maneuvers, carotid sinus massage, and overdrive pacing. Four basic clinical subsets were recognized: (1) carotid sinus hypersensitivity (2) bradycardia-tachycardia syndrome, (3) episodic sinus arrest, and (4) persistent symptomatic sinus bradycardia. The study group demonstrated a normal heart rate response to exercise and isoproterenol infusion (%Δ = +95 exercise, +144 isoproterenol) in the face of diminished responsiveness to atropine administration (%Δ = +23). Marked carotid sinus hypersensitivity was demonstrated in eight patients, and four patients demonstrated slight abnormalities during performance of Valsalva maneuvers. Significant suppression of sinus node dysfunction was observed following atrial overdrive in the study group (postpacing pause = 3087 ± 464 msec) as compared to patients without significant sinus node function (postpacing pause = 1073 ± 63 msec) ( P < 0.001). In patients with intact V-A conduction, ventricular overdrive also resulted in sinus node suppression (postpacing pause = 1901 ± 357 msec). There was a marked decrease in the degree of sinus node depression following atropine administration. Ten of 31 patients demonstrated various degrees of A-V block following atrial pacing at rates less than 100 beats/min.

It is concluded that the present methods of evaluation of sinus function, especially sinus node recovery time following overdrive pacing, may prove of value in the investigation of patients with syncope of unknown etiology.