The clinical significance of bradycardic rhythms in acute myocardial infarction

Coronary artery lesion distribution in patients with chronic kidney disease undergoing percutaneous coronary intervention

Purpose

To determine the location of coronary atherosclerosis distribution observed in patients with chronic kidney disease (CKD).

Methods

A cross-sectional study was conducted using the database of cardiovascular medicine data from Saitama Sekishinkai Hospital to clarify the association between renal function and angiographic characteristics of coronary atherosclerosis. In total, 3268 patients who underwent percutaneous coronary intervention were included. Propensity score matching revised the total to 1772. The association of renal function with the location and/or distribution of coronary atherosclerosis lesions was then examined.

Results

Overall, coronary lesion was observed in the left anterior descending coronary artery (LAD) in 56% patients, whereas 28% and 22% were in the right coronary artery (RCA) and left circumflex coronary artery (LCX), respectively. LAD was most affected and observed in 57% patients with stage 1 CKD. RCA was second-most affected, at 26% CKD stage 1, but it increased to 31%, 38%, and 59% in CKD 3, 4, and 5, respectively. In CKD 5 patients, the RCA was the most affected artery (59%), with 41% LAD lesions. Logistic regression analysis after propensity score matching showed that the odds ratios for an RCA lesion was 3.658 in CKD 5 (p = .025) compared with CKD 1 after adjusting for traditional risk factors.

Conclusion

The prevalence of RCA lesions, but not LAD or LCX lesions, increased with increasing CKD stage. The pathophysiology of coronary atherosclerosis may differ by lesion location. Deterioration of renal function may affect progression of atherosclerosis more in the RCA than in the LAD or LCX.

A 56-Year-Old Female With Acute ST-Segment Elevation Myocardial Infarction, Complete Heart Block, and Hemodynamic Instability

Chest pain is a common emergency department complaint, but a small percentage of patients with this complaint experience acute coronary syndrome, with a still smaller percentage having ST-elevation myocardial infarction (STEMI) with hemodynamic instability and arrhythmia. A 56-year-old female presented to our emergency department with acute chest pain. She was diagnosed with inferior wall STEMI, had complete heart block and hemodynamic instability, and underwent emergent reperfusion via coronary catheterization. This combination of signs and symptoms required thoughtful assessment and treatment along with diagnostic accuracy and proper disposition. This case offers a review of this uncommon presentation, including pathophysiology and treatment.

Conduction Disorders in the Setting of Acute STEMI

ST-elevation myocardial (STEMI) is frequently associated with conduction disorders. Regional myocardial ischemia or injury may affect the cardiac conduction system at various locations, and neural reflexes or changes in the balance of the autonomous nervous system may be involved. Sinoatrial and atrioventricular blocks are more frequent in inferior than anterior STEMI, while new left anterior fascicular block and right bundle branch block indicate proximal occlusion of the left anterior descending coronary artery. New left bundle branch block is associated with multi-vessel disease. Most conduction disorders associated with STEMI are reversible with reperfusion therapy, but they may still impair prognosis because they indicate a large area at risk, extensive myocardial infarction or severe coronary artery disease. Acute STEMI recognition is possible in patients with a fascicular or right bundle branch block, but future studies need to define the cut-off values for ST depression in the leads V1-V3 in inferolateral MI and for ST elevation in the same leads in anterior STEMI. In the left bundle branch block, concordant ST elevation is a specific sign of acute coronary artery occlusion, but the ECG feature has low sensitivity.

Contemporary practice pattern of permanent pacing for conduction disorders in inferior ST-elevation myocardial infarction

BACKGROUND

Currently, there is no clear consensus regarding the optimal waiting period before permanent pacemaker implantation in patients with conduction disorders following an inferior myocardial infarction.

HYPOTHESIS

We aimed to elucidate the contemporary practice pattern of pacing, especially the timing of pacemaker implantation, for sinoatrial node and atrioventricular (AV) conduction disorders following an inferior ST-elevation myocardial infarction (STEMI).

METHODS

Using the National Inpatient Sample database from 2010 to 2014, we identified patients with a primary diagnosis of inferior STEMI. Primary conduction disorders were classified into: (a) high-degree AV block (HDAVB) consisting of complete AV block or Mobitz-type II second-degree AV block, (b) sinoatrial node dysfunction (SND), and (c) no major conduction disorders.

RESULTS

Among 66 961 patients, 2706 patients (4.0%) had HDAVB, which mostly consisted of complete AV block (2594 patients). SND was observed in 393 patients (0.6%). Among the 2706 patients with HDAVB, 267 patients (9.9%) underwent permanent pacemaker. In patients with HDAVB, more than one-third (34.9%) of permanent pacemakers were placed within 72 hours after admission. The median interval from admission to permanent pacemaker implantation was 3 days (interquartile range; 2-5 days) for HDAVB vs 4 days (3-6 days) for SND (P < .001). HDAVB was associated with increased in-hospital mortality, whereas SND was not.

CONCLUSIONS

In patients who developed HDAVB following an inferior STEMI, only one in 10 patients underwent permanent pacemaker implantation. Despite its highly reversible nature, permanent pacemakers were implanted relatively early.

Correlation of Admission Heart Rate With Angiographic and Clinical Outcomes in Patients With Right Coronary Artery ST-Segment Elevation Myocardial Infarction Undergoing Primary Percutaneous Coronary Intervention: HORIZONS-AMI (The Harmonizing Outcomes With Revascularization and Stents in Acute Myocardial Infarction) Trial

BACKGROUND

Bradycardia on presentation is frequently observed in patients with right coronary artery ST-segment elevation myocardial infarction, but it is largely unknown whether it predicts poor angiographic or clinical outcomes in that patient population. We sought to determine the prognostic implications of admission heart rate (AHR) in patients with ST-segment elevation myocardial infarction and a right coronary artery culprit lesion.

METHODS AND RESULTS

We analyzed 1460 patients with ST-segment elevation myocardial infarction and a right coronary artery culprit lesion enrolled in the randomized HORIZONS-AMI (Harmonizing Outcomes with Revascularization and Stents in Acute Myocardial Infarction) trial who underwent primary percutaneous coronary intervention. Patients presenting with high-grade atrioventricular block were excluded. Outcomes were examined according to AHR range (AHR <60, 61-79, 80-99, and ≥100 beats per minute). Baseline and procedural characteristics did not vary significantly with AHR except for a more frequent history of diabetes mellitus, longer symptom-to-balloon time, more frequent cardiogenic shock, and less frequent restoration of thrombolysis in myocardial infarction 3 flow in patients with admission tachycardia (AHR >100 beats per minute). Angiographic analysis showed no significant association between AHR and lesion location or complexity. On multivariate analysis, admission bradycardia (AHR <60 beats per minute) was not associated with increased 1-year mortality (hazard ratio 1.33; 95% CI 0.41-4.34, P=0.64) or major adverse cardiac events (hazard ratio 1.08; 95% CI 0.62-1.88, P=0.78), whereas admission tachycardia was a strong independent predictor of mortality (hazard ratio 5.02; 95% CI 1.95-12.88, P=0.0008) and major adverse cardiac events (hazard ratio 2.20; 95% CI 1.29-3.75, P=0.0004).

CONCLUSIONS

In patients with ST-segment elevation myocardial infarction and a right coronary artery culprit lesion undergoing primary percutaneous coronary intervention, admission bradycardia was not associated with increased mortality or major adverse cardiac events at 1 year.

CLINICAL TRIAL REGISTRATION

URL: https://www.clinicaltrials.gov. Unique identifier: NCT00433966.

Accelerated sinus rhythm prevents catecholaminergic polymorphic ventricular tachycardia in mice and in patients

RATIONALE

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is caused by mutations in cardiac ryanodine receptor (RyR2) or calsequestrin (Casq2) genes. Sinoatrial node dysfunction associated with CPVT may increase the risk for ventricular arrhythmia (VA).

OBJECTIVE

To test the hypothesis that CPVT is suppressed by supraventricular overdrive stimulation.

METHODS AND RESULTS

Using CPVT mouse models (Casq2(-/-) and RyR2(R4496C/+) mice), the effect of increasing sinus heart rate was tested by pretreatment with atropine and by atrial overdrive pacing. Increasing intrinsic sinus rate with atropine before catecholamine challenge suppressed ventricular tachycardia in 86% of Casq2(-/-) mice (6/7) and significantly reduced the VA score (atropine: 0.6±0.2 versus vehicle: 1.7±0.3; P<0.05). Atrial overdrive pacing completely prevented VA in 16 of 19 (84%) Casq2(-/-) and in 7 of 8 (88%) RyR2(R4496C/+) mice and significantly reduced ventricular premature beats in both CPVT models (P<0.05). Rapid pacing also prevented spontaneous calcium waves and triggered beats in isolated CPVT myocytes. In humans, heart rate dependence of CPVT was evaluated by screening a CPVT patient registry for antiarrhythmic drug-naïve individuals that reached >85% of their maximum-predicted heart rate during exercise testing. All 18 CPVT patients who fulfilled the inclusion criteria exhibited VA before reaching 87% of maximum heart rate. In 6 CPVT patients (33%), VA were paradoxically suppressed as sinus heart rates increased further with continued exercise.

CONCLUSIONS

Accelerated supraventricular rates suppress VAs in 2 CPVT mouse models and in a subset of CPVT patients. Hypothetically, atrial overdrive pacing may be a therapy for preventing exercise-induced ventricular tachycardia in treatment-refractory CPVT patients.

Factors associated with the occurrence of cardiac arrest during hospitalization for acute myocardial infarction in the second national registry of myocardial infarction in the US

Cardiac arrest can occur as a complication of acute myocardial infarction (AMI). To date, few studies have described factors associated with cardiac arrest occurrence and survival during hospitalization for treatment of AMI. We used data from a large national registry of hospitalized AMI patients to identify these factors. Data were collected from 1073 participating institutions, representing 14.4% of US hospitals. Hospital site coordinators conducted periodic chart reviews for AMI patients and data were submitted to an independent center for periodic review. Univariate analysis and multivariate logistic regression were used to identify factors associated with cardiac arrest. We found that cardiac arrest occurred in 4.8% (14,725/305,812) of hospitalized AMI patients. The survival rate to hospital discharge for these individuals was 29.4%. Sustained ventricular tachycardia or fibrillation (VT/VF) was present in 34.7% and was associated with a higher rate of survival to hospital discharge compared to cardiac arrest patients without a ventricular tachyarrhythmia (47.5 vs. 19.8%, P < 0.00001). Hypotension (initial systolic BP < 90 mmHg), q-wave AMI, old age, heart failure and initial heart rate abnormalities (bradycardia or tachycardia) were associated with a higher prevalence of cardiac arrest. A higher percentage of women compared to men experienced cardiac arrest (6.0 vs. 4.41%, P < 0.0001). Cardiac arrest prevalence was lower in patients with inferior wall infarction than in other types of ST-elevation infarction. Use of reperfusion therapy (PTCA or tPA) was associated with improved survival compared to hospitalized AMI patients who did not receive such therapy.

Sinus bradycardia as a predictor of right coronary artery occlusion in patients with inferior myocardial infarction

Differentiation of right coronary artery (RCA) from left circumflex artery (LCxA) occlusion may be difficult since both can present an electrocardiographic pattern of inferior myocardial infarction (IMI). We studied 133 patients with IMI, 92 patients with RCA occlusion and 41 patients with LCxA occlusion. Risk factors such as previous MI, arterial hypertension, diabetes, smoking, and dislipemia, were similar for RCA and LCxA occlusions. Patients with RCA occlusion had a higher incidence of isolated IMI than patients with LCxA occlusion, 50% vs. 17%, respectively (P<0.001). Arterial hypotension was more prevalent (P<0.05) among patients with RCA (18%) rather than those with LCxA occlusion (2%). RCA occlusion presented an association with sinus bradycardia, an association not observed with LCxA occlusion (15% vs. 0%, respectively; P<0.01). Total atrioventricular block was only present among patients with RCA (18%). Proximal occlusions of the RCA presented lower heart rates (sinus bradycardia) than medial and distal occlusions (13% vs. 1% and 1%, respectively; P<0.0001 and P<0.001). Therefore, regarding patients with IMI: (1) sinus bradycardia is more frequent when the infarct-related artery is the RCA; (2) proximal occlusions of the right coronary predispose low heart rates; and (3) occlusion of the LCxA rarely induces sinus bradycardia.

Arrhythmias after acute myocardial infarction. Evaluation and management of rhythm and conduction abnormalities

Patients with myocardial infarction can experience a wide range of arrhythmias and conduction abnormalities, from transient and relatively innocuous sinus bradycardia to life-threatening ventricular fibrillation. This nut-and-bolts article covers all the possibilities, emphasizing the clinical significance of the various arrhythmias and their evaluation and treatment. Also included are indications for temporary and permanent pacemaker placement based on the revised ACC/AHA guidelines.

Factors associated with late onset of advanced atrioventricular block in acute Q wave inferior infarction

To elucidate the clinical characteristics associated with advanced atrioventricular (AV) block that appears relatively late (more than 24 hours) after the onset of myocardial infarction (MI), 101 patients with acute Q wave inferior MI were studied. Fourteen patients had late-onset advanced AV block, and 87 patients were free of AV block. The hospital mortality rate was 11%. Multivariate analysis was performed to determine the important variables associated with the occurrence of late advanced AV block and hospital mortality rates based on 12 clinical variables. Colloid osmotic pressure, right atrial pressure, serum potassium level, and number of segments with advanced asynergy were the significant factors associated with the occurrence of late advanced AV block, whereas advanced asynergic segments and alveolar arterial oxygen difference were important in the consideration of hospital mortality rates. Therefore not only the extent of myocardial ischemia but also the increases in the extracellular potassium level and interstitial fluid are some of the factors that are associated with the occurrence of late advanced AV block in acute inferior MI. Late advanced AV block, in itself, has no significant influence on hospital mortality rates.

Analysis of ambulatory electrocardiograms in 14 patients who experienced sudden cardiac death during monitoring

The Holter monitors of 14 patients (out of 58,000 Holter recordings performed between 1978 and 1984) who experienced cardiac arrest and expired during the recording period were analyzed. Tachyarrhythmic arrest patients frequently had coronary heart disease, congestive heart failure, and prolonged QTc intervals. The highest incidence of intermediately frequent premature ventricular complexes (PVCs) occurred between 15 and 6 hours prior to death. The frequency of ventricular couplets increased toward the time of arrest. The hours with greatest frequency of ventricular tachycardia (VT) were found to be the last 5 hours of life. An increasing incidence of ST-segment changes greater than 2 mm was noted throughout all of the risk periods until the third hour prior to arrest when the incidence diminished. Conversely, the incidence of lower amplitude ST-segment changes (usually elevation) increased over the final 6 hours. The mean time of death was 0228 hours +/- 5:20. In conclusion, we observed two patterns of Holter-monitored changes which usually occurred prior to death and may represent predictors of sudden death: (1) an increasing incidence of intermediately frequent isolated PVCs followed by increased ventricular couplets and runs of VT; (2) return of high amplitude ST-segment changes toward baseline. To our knowledge, the temporal relationship of the degree of ST-segment deviation to sudden death and the time of sudden death have not been reported in large studies of Holter-monitored sudden death patients.

Cardiac receptors: their function in health and disease

Cardiac receptors include both mechanically and chemically sensitive receptors located in atria and in ventricles. Atrial receptors innervated by myelinated vagal afferent fibers reflexly regulate heart rate and intravascular volume. On the other hand, stimulation of ventricular receptors can cause either reflex bradycardia and hypotension or, alternatively, excitation of the cardiovascular system. The former response is mediated by vagal afferents, whereas the latter is mediated by sympathetic (spinal) afferents. Under normal circumstances, cardiac receptors sense changes in wall motion or diastolic pressure and perhaps provide a fine tuning of the cardiovascular system. However, under certain pathological conditions such as coronary ischemia, which cause release of substances such as bradykinin and prostaglandins, there is an exaggerated response of the ventricular receptors. Because these receptors cause a reflex depression of the cardiovascular system and, in particular, induce renal vasodilation, they may protect the heart and kidney by lessening myocardial oxygen requirements and by increasing renal blood flow. In the situation of heart failure both atrial and ventricular receptors are reset and therefore provide for an exaggerated neurohumoral discharge. Finally, patients with aortic stenosis may demonstrate a paradoxical vasodilation and syncope during exercise when there likely is excessive stimulation of left ventricular receptors by the high transmural pressure.

Sudden death during ambulatory monitoring. Analysis of six cases

The ambulatory electrocardiographic recordings of six patients with coronary artery disease who died during monitoring were analyzed. In four patients, sinus rhythm was interrupted by sinoatrial, atrioventricular, nodal, or infra-His conduction abnormalities leading to bradyarrhythmic sudden death. Two patients died of sustained ventricular tachycardia or ventricular fibrillation. These data emphasize that the arrhythmias involved in the sudden death syndrome may be more heterogenous than currently appreciated.

Effects of acute vagally-mediated bradycardia on systemic hemodynamics and coronary blood flow before and after coronary stenosis

The effects of short episodes (1 min) of vagally-mediated bradycardia were studied in 9 anesthetized dogs utilizing vagal stimulation and slow atrial pacing (120 and 80 beats/min) before and after graded coronary constriction of the left anterior descending (LAD) and the left circumflex (CCA). In the presence of 90% LAD stenosis, bradycardia tended to restore both the elevated total LAD coronary vascular resistance (CVR-LAD) and the reduced, total CVR-CCA towards control levels obtained at corresponding slow rates in the absence of coronary stenosis; as a result, LAD coronary flow (F-LAD) was relatively less reduced and the accessory rise of F-CCA disappeared. In the presence of combination of 90% LAD plus 70% CCA stenosis, the effects of bradycardia on total CVR-LAD and F-LAD were similar to those obtained with single 90% LAD stenosis, but the accessory flow through the CCA was abolished resulting in no significant difference of the rate-dependent alterations of total CVR-CCA and F-CCA as compared with those observed in the absence of coronary stenosis. In the presence of single or combined coronary stenosis, bradycardia restored the depressed aortic pressure and cardiac output towards control values obtained at comparable slow rates before coronary stenosis. The results support the concept that in the presence of 90% LAD stenosis vagally-mediated bradycardia 1) decreases the tension-time index (myocardial nutritional demand) shifting cardiac performance to less expensive "flow work" and 2) facilitates antegrade flow through the highly stenotic LAD thereby inhibiting accessory flow through the nonstenotic CCA.

Atropine-induced ventricular fibrillation: case report and review of the literature

A bradycardic and mildly hypotensive acute myocardial infarction patient developed sinus tachycardia, ventricular tachycardia, flutter, and fibrillation following intravenous atropine. Previous case reports are reviewed and the literature regarding the advisability of this mode of therapy is discussed. In the light of conflicting opinion as to the necessity of atropine in the mildly hypotensive and bradycardic acute myocardial infarction patient, and in view of its potentially deliterious effects on ischemic myocardium, a cautious and selective application of this drug is advised.

Clinical evaluation of the enhancement of vagal tone in acute myocardial infarction by edrophonium hydrochloride: effects on ventricular arrhythmias, His bundle electrography, and left ventricular function

Enhanced electrical stability of acutely ischemic myocardium with vagal stimulation and acetylcholinesterase inhibition has been demonstrated experimentally. To extend these findings clinically, within 24 hours of acute myocardial infarction, 11 patients underwent continuous 10 hour Holter monitoring: 2.5 hour control before and after 5 hour constant edrophonium infusion (0.25 to 2.00 mg./minute). Continuous infusion of the agent lowered heart rate 92 to 78 b.p.m. (p less than 0.01). Although mean total ventricular extrasystoles (PVC's) per 5 hours per patient (131) and PVC's per 1,000 beats (4.7) were unchanged (p greater than 0.05), potentially lethal tachyarrhythmias (malignant PVC's: multifocal, R on T, paried, greater than 5 per minute or ventricular tachycardia) were terminated in six of 10 patients by edrophonium. However, serious ventricular arrhythmias continued in three patients and appeared in four despite the agent. Ventricular fibrillation did not occur during the 10 hour period of study. In addition, the patients were evaluated hemodynamically and by His bundle electrograms before and after a 10 mg. bolus of edrophonium prior to the 10 hour constant infusion: heart rate declined (88 to 72 b.p.m., p less than 0.01), while mean arterial pressure (98 mm. Hg), left ventricular filling pressure (14 mm. Hg), cardiac index (2.4 L. per minute per square meter), and stroke work index (36 Gm.m./M.2) were unchanged (p greater than 0.05). The edrophonium bolus prolonged the A-H interval (117 to 135 msec., p less than 0.01) while the H-Q interval was unaltered (48 msec; p greater than 0.05). It is concluded that increased vagal tone with edrophonium did not reduce the over-all presence of premature ventricular contractions in the entire study group; however, the malignant nature of PVCs and ventricular tachycardia appeared to be lessened by the parasympathomimetic agent in certain patients. In addition, no adverse hemodynamic or intraventricular conduction effects were produced by edrophonium administration.

Prognostic features of ventricular tachycardia complicating acute myocardial infarction

Prognostic features of 115 patients with ventricular tachycardia complicating acute myocardial infarction were analyzed. Age, sex, infarct location and peak CPK levels were not significantly different when comparing survivors (S) and non-survivors (NS). Highly significant clinical characteristics of NS compared to S were: heart rate, presence of cardiogenic shock and a poor response to lidocaine therapy (P less than 0.0001, 0.0003 and 0.001 respectively). Electrocardiographic features distinguishing S and NS were: coupling intervals (S = 522.9, NS = 389.9, P less than 0.004), prematurity index (S = 1.36, NS = 1.04, P less than 0.001), ventricular tachycardia rate (S = 132, NS = 174, P less than 0.0013) and number of episodes of ventricular tachycardia (S = 4.04, NS = 6.75, P less than 0.0058). These findings have importance for the evaluation of newer active and prophylactic therapies.

New observations on the effects of atropine on the sinoatrial and atrioventricular nodes in man

Previous observations of slowing of the heart rate after administration of atropine in doses smaller than 0.4 mg and recent reports of development of rhythm disorders in patients with acute myocardial infarction given atropine prompted us to evaluate systematically the effects of various doses of atropine (0.1 to 0.8 mg) on the response of the sinoatrial (S-A) and atrioventricular (A-V) nodes in healthy volunteers. The response of the S-A node to atropine was characteristically bimodal, slowing at smaller doses and accelerating at larger doses. In contrast, the A-V node showed acceleration of conduction in response to all doses of atropine used. A hypothesis based on current understanding of the electrophysiologic parameters governing impulse formation and impulse conduction is advanced to explain the apparent paradox in the S-A and A-V nodal responses to small doses of atropine. The results suggest the need for caution and continuous rhythm monitoring when giving atropine to patients with acute myocardial infarction.

Bradycardia-hypotension syndrome in acute myocardial infarction. Reappraisal of the overdrive effects of atropine

Sixty-eight (17 per cent) of 380 patients with acute myocardial infarction had the bradycardia-hypotension syndrome (ventricular rate below 60/min and systolic blood pressure less than 100 mm Hg) during the first 24 hours of admission to a large general hospital. In 61 of the 68 patients, the administration of atropine significantly increased the heart rate (from 46 plus or minus 14 to 79 plus or minus 12/min) (p less than 0.01) and systolic blood pressure (from 70 plus or minus 15 to 105 plus or minus 13 mm Hg) (p less than 0.001). In 26 of the 68 patients, ventricular premature complexes decreased from 9.4 plus or minus 3/min to 2.4 plus or minus 0.7/min (p less than 0.001) after the administration of atropine. It is concluded that the bradycardia-hypotension syndrome is not an uncommon complication following acute myocardial infarction and that selected doses of atropine may have a beneficial effect without significant complications.

Influence of atropine and of vagally mediated bradycardia on the occurrence of ventricular arrhythmias following acute coronary occlusion in closed-chest dogs

In contrast to previous opinions, recent investigations have suggested that increasing heart rate (HR) with atropine when moderate sinus bradycardia accompanies acute myocardial infarction is not necessarily beneficial. To further characterize the influence of vagally mediated changes in HR during ischemia, we evaluated the effects of atropine and of electric stimulation of the vagus nerves on the incidence of ventricular arrhythmias during acute coronary occlusion in closed-chest dogs. Protection from occlusion-induced arrhythmia was not observed when 28 dogs receiving atropine were compared with 27 control dogs. Rather, the total incidence of ventricular arrhythmias was significantly higher ( P < 0.05) and ventricular fibrillation tended to occur more frequently in the atropine-treated group. Moreover, fewer ventricular arrhythmias (and total absence of ventricular fibrillation or close-coupled premature beats) were noted in 12 control animals with spontaneous bradycardia (HR<60 beats/min) compared with 15 nonbradycardic animals. When vagal stimulation produced bradycardia (HR = 40-60 beats/min) during coronary occlusion, occurrence and character of ventricular arrhythmias were the same as in dogs with normal rates (HR = 80-100 beats/min). Although these results may not necessarily apply to man, further studies are needed before it can be assumed that all individuals with moderate bradycardia during acute myocardial infarction should receive vagolytic agents.

Artificial pacemakers: indications and management

The development of artificial pacemakers for the electrical control of the cardiac rhythm has greatly enhanced the physician's ability to treat cardiac dysrhythmias. Pacemakers have been useful in treating Stokes-Adams syndrome and symptomatic bradyeardias; they have helped control the occurrence of tachyarrhythmias and have played an important role in the management of arrhythmias accompanying myocardial infarctions. With their more frequent use, pacemakers have contributed to our knowledge of underlying conduction and natural pacemaker disorders. As new indications for artificial pacemaking have been elucidated, more complex pulse generators have been developed, and newer technics found for their insertion. In spite of recent development the pulse generators in general use have a limited useful lifetime.

This paper reviews the indications for pacemaker insertion that are commonly employed. In addition, an approach to the problem of pulse generator replacement is presented.

Electrical stability of acutely ischemic myocardium. Influences of heart rate and vagal stimulation

Previous investigations have shown that a slower heart rate (HR) and myocardial ischemia independently diminish the electrical stability of the heart. It therefore was suggested that increasing heart rate during myocardial infarction might diminish the incidence of serious ventricular arrhythmias. However, since increased HR during experimental acute myocardial ischemia augments the degree of ischemia, an evaluation of the presumed "protective" effects of increased HR on the electrical stability of acutely ischemic myocardium was undertaken. The differences in refractory periods (RP) of eight contiguous areas of the left ventricle were determined as a function of HR. In nonischemic myocardium, the disparity of RP was less at an HR of 180 than 60. However, in ischemic myocardium the disparity increased in three of six animals as the HR was increased from 60 to 90, in seven of 10 animals as HR was increased from 60 to 120, and in all animals when the HR was increased from 60 to 180. The increased disparity of RP is believed to favor development of reentrant arrhythmia. The vulnerability of the heart to develop ventricular fibrillation was assessed by determining ventricular fibrillation threshold (VFT). During ischemia, VFT was not only an inverse function of HR but also was found to be independently influenced by electrical stimulation of the cervical vagus nerves. In the absence of vagal stimulation VFT was lowered in only one of four dogs as HR was increased from 50 to 90, but decreased 30% ( P < 0.01) as HR reached 120 and 74% at 180 beats/min. When vagal stimulation was used to control HR VFT was lowered 37% as HR was increased from 50 to 60 to 90 ( P < 0.05). We conclude that increasing HR within a physiologic range by diminishing vagal tone during myocardial ischemia decreases electrical stability of the ventricle by (1) increasing ischemia consequent to the rate-induced increase in myocardial oxygen requirements, and (2) a direct electrophysiologic action of the vagus on the ventricular myocardium.

Hemodynamic sequelae of cardiac arrhythmias

The hemodynamic consequences of cardiac arrhythmias depend on various factors, including the ventricular rate and the duration of the abnormal rate, the temporal relationship between atrial and ventricular activity, the sequence of ventricular activation, the functional state of the heart, the irregularity of the cycle length, associated drug therapy, the peripheral vascular vasomotor system, disease in organ systems other than the heart, and the degree of anxiety caused by the disease processes. Sinus bradycardia, even with rates as low as 40 beats/min, may not be associated with significant hemodynamic consequences unless the stroke volume is limited by myocardial or valvular disease, as in acute myocardial infarction. Cardiac output usually, but not invariably, falls when atrial fibrillation replaces normal sinus rhythm, even at comparable ventricular rates, both at rest and during exercise. Similar observations have been made during the development of atrial flutter despite the persistence of effective mechanical atrial activity in at least some cases. Marked hemodynamic changes are frequent in the course of ventricular tachycardia with systemic arterial hypotension, a decrease in cardiac output, and evidence of cerebral, coronary, and renal vascular insufficiency. Cyclic variations in systemic and pulmonary arterial pressures are common during atrioventricular dissociation. Cardiac output is generally depressed during the severe bradycardia of acquired complete heart block with evidence of atrioventricular valvular insufficiency. Increase of the heart rate by ventricular pacing reverses all or some of these abnormalities. The changes in congenital complete heart block are considerably less severe because myocardial insufficiency is less frequently seen in congenital complete heart block.

Coronary artery occlusion in the conscious dog. Effects of alterations in heart rate and arterial pressure on the degree of myocardial ischemia

Bradycardia, with or without hypotension, frequently occurs in the early phases of acute myocardial infarction. To determine the relative effects of alterations in heart rate and blood pressure on the degree of ischemic injury, the left anterior descending coronary artery was occluded for 15-min periods in closed-chest conscious dogs by inflating a balloon cuff previously implanted around the artery. The degree of myocardial ischemia was estimated by summating the S-T elevation recorded from 12 myocardial electrodes. Heart rate was increased by atropine or pacing and decreased by electrical stimulation of the vagus nerve. Hypotension was produced by venesection (average decrease in mean BP, 56 mm Hg). At normal arterial pressures there was a positive correlation between percent change in heart rate (range 30-215 beats/min) and percent change in S-T elevation (y = 0.75 X + 30.2, r = 0.93, P < 0.01). When myocardial ischemia was induced during hypotension and bradycardia, S-T elevation totaled 68 mv at 15 min of ischemia. When heart rate was increased to control levels in the presence of hypotension S-T elevation during myocardial ischemia was greater (mean difference 29 mv, P < 0.05). In contrast, when blood pressure was increased to control in the presence of bradycardia, S-T elevation in seven of 10 dogs was less than during hypotension and bradycardia. Thus, during experimental acute myocardial ischemia, hypotension induced by hemorrhage increases ischemic injury, and bradycardia reduces it. It is concluded that in acute myocardial ischemia increases in heart rate, even from slow baseline rates; may be deleterious to the myocardium. It remains to be determined whether alterations in the degree of myocardial ischemia induced by hemorrhagic-hypotension are analogous to those caused by the type of hypotension that often accompanies bradycardia occurring during acute myocardial infarction in man.

Prehospital care of acute myocardial infarction

It has been estimated recently that half of deaths among the patients with acute myocardial infarction occur before they ever reach the hospital for definitive medical care. One of the major reasons for the high incidence of death was attributed to delays in reaching medical care, involving patients themselves, physicians, transportation, and receiving areas of the hospital. In order to reduce this high mortality substantially we should focus our attention on the prehospital care of this disease, which may include several major approaches.

A. To shorten the delay in securing medical care:

1. Public education, with emphasis on the early warning symptoms and signs of acute myocardial infarction, and the need and importance of seeking early medical care, with special attention directed toward the high-risk coronary patients.

2. Professional education.

3. Mechanisms to direct and/or bring patients with suspected or proven acute myocardial infarction promptly to the system of medical care with special emphasis on the utilization of a telephone information center and provision of rapid transportation.

B. Establishment of emergency life-support stations for screening, monitoring, and early stabilization of cardiac arrhythmias:

1. Fixed.

a. Emergency department of hospitals, preferably with an attached precoronary care area.

b. Areas where many employees are working on weekdays.

c. Areas where many people move in and out from day to day.

d. Areas where there is periodic concentration of mass population.

2. Mobile.

a. Mobile coronary care unit.

b. Mobile intensive care unit.

C. Prevention of sudden death:

1. Study of mechanism and clinical environment of sudden death.

2. Development of techniques for early diagnosis and management of acute myocardial infarction prior to inception of symptoms.

3. Identification of individuals prone to sudden death.

4. Preventive measures directed toward the individuals prone to sudden death.

a. Effective control of risk factors.

b. Intermittent or periodic electrocardiographic monitoring in these individuals.

c. Long-term antiarrhythmic therapy for these individuals.

d. Possibility of implanting an automatic defibrillator in patients with previous myocardial infarction or in those recovered from ventricular fibrillation.

5. Early administration of antiarrhythmic drugs in patients with suspected or proven acute myocardial infarction.