Computer detection of premature ventricular complexes: A modified approach

The accuracy of a data reduction system for arrhythmia detection in identifying premature ventricular complexes was evaluated in continuous tape records of 30 patients in a coronary care unit. Computer analysis was performed with a Honeywell 316 digital computer. Threshold values for dominant complexes were automatically determined and recognition of premature ventricular complexes was based on differences in QRS configuration, timing and T wave configuration from the dominant complexes. Verification of the computer accuracy in detecting premature ventricular complexes was made with visual beat by beat inspection using a two channel strip chart recorder with simultaneous recording of the electrocardiogram and computer signal. This procedure allowed for exact beat to beat correlation and, thus, absolute determination of false positive and false negative identifications.From 0.5 to 6 continuous hours of monitoring per patient (average 3.5 hours) were analyzed for a total of 105 monitoring hours. The basic cardiac rhythms noted were normal sinus rhythm, sinus arrhythmia, sinus tachycardia, demand pacemaker rhythm, atrial fibrillation and atrioventricular (A-V) dissociation with junctional rhythm. Premature ventricular complexes were evident in 28 tapes (93 percent) including 12 (43 percent) with multifocal premature ventricular complexes and 3 (11 percent) with ventricular tachycardia. The visual count of premature ventricular complexes totaled 7,921. Of these, 7,542 (95 percent) were properly classified by the computer. The total computer count was 8,717, representing a 13 percent false positive and 5 percent false negative identification rate. The false positive identifications of premature ventricular complexes occurred during periods of 10 seconds or more of continuous noise artifact and in the presence of atrial premature complexes conducted aberrantly. When these sections of tape were excluded, the computer had a less than 2 percent false negative and 3 percent false positive rate of identification of premature ventricular complexes.     

Unreliability of conventional electrocardiographic monitoring for arrhythmia detection in coronary care units

To evaluate the reliability of conventional coronary care unit electrocardiographic monitoring, a study was made of 31 consecutive patients with uncomplicated verified acute myocardial infarction. All patients were monitored routinely with conventional equipment, and at the same time the electrocardiogram for each patient was recorded continuously on electromagnetic tape and stored for later analysis by an automated arrhythmia detection system. All patients studied were within 24 hours of the onset of chest pain and on entry into study all were free of shock, heart block, bundle branch block, severe heart failure or an existing arrhythmia. By conventional monitoring, premature ventricular contractions were recognized in 64.5 percent of patients compared with 100 percent using the automated detection system (P <0.01). The corresponding percentages for recognition of premature atrial contractions were 45.2 vs. 96.8 percent (P < 0.001); serious ventricular arrhythmias, 16.1 vs. 93.5 percent (P <0.001); multifocal premature ventricular contractions, 6.5 vs. 87.1 percent (P < 0.001); and consecutive premature ventricular contractions, 13.0 vs. 77.5 percent (P < 0.001), respectively. The delay from the time of first occurrence as detected by the automated system to recognition by the conventional monitoring system averaged 18 hours for premature ventricular contractions, 10 hours for serious ventricular arrhythmias and 23 hours for premature atrial contractions. The on-line use of an automated arrhythmia detection system in the coronary care unit is suggested if further improvement in the elimination of arrhythmias as a primary cause of death after myocardial infarction is to be achieved. The presence of serious ventricular arrhythmias in virtually all patients after myocardial infarction suggests that prophylactic antiarrhythmic agents be used in this setting; however, none of the presently available antiarrhythmic agents have been shown to reduce mortality when given prophylactically following myocardial infarction.     

Idioventricular rhythm complicating acute myocardial infarction

The incidence, natural history, prognosis, and electrocardiographic characteristics of idioventricular rhythm complicating acute myocardial infarction are described. It occurred as a transient arrhythmia nearly always within 24 hours of infarction in 61 (8%) of 737 patients, and was characterized by paroxysms of between 6 and 20 beats with widened bizarre QRS complexes at a rate of between 60 and 90 a minute. Most cases showed fusion beats and P waves dissociated from the QRS complexes, and in many cases idioventricular rhythm started during the slow phase of sinus arrhythmia. Though it usually occurred in patients with moderately severe transmural infarcts, the incidence of ventricular fibrillation and subsequent mortality was no greater than in patients with infarcts of equivalent severity who did not have idioventricular rhythm. It is concluded that this rhythm is a common and relatively benign arrhythmia complicating myocardial infarction, and that it should be distinguished from ventricular tachycardia.     

The relation of sinus rate to the frequency of paroxysmal ventricular tachycardia during acute myocardial infarction in man.

Continuous tape recordings of cardiac rhythm were made in 51 male patients with acute myocardial infarction within 24 hours of their infarction. These tracings were analyzed for the incidence of paroxysmal ventricular tachycardia (PVT) and the sinus rate immediately preceding each episode of PVT. In 26 patients, 112 episodes of PVT at a rate greater than 100 beats/min were documented. Although 67 per cent of the episodes of PVT were preceded by sinus rates between 60 and 100 beats/min, 15 per cent occurred at sinus rates below 60 beats/min and 18 per cent occurred at sinus rates above 100 beats/min. The data remained essentially unchanged regardless of whether ventricular tachycardia was defined at rates in excess of 100, 120 or 140 beats/min. The results of this study show that during the early phases of acute myocardial infarction in man, PVT was most common during sinus rates generally thought to be within the normal range (60 to 100 beats/min). A lower, but close to equal incidence of PVT was observed during sinus bradycardia and sinus tachycardia.     

Prophylactic quinidine for the prevention of arrhythmias after acute myocardial infarction

A selected group of 103 patients with uncomplicated myocardial infarction of less than 24 hours′ duration were randomly assigned to two different treatments: 45 patients received quinidine sulfate orally in a dose of 0.4 g every 8 hours and 58 patients received sodium lactate placebo. The period of observation was 72 hours. Ventricular tachyarrhythmias, including ventricular tachycardia, and ventricular premature beats that were multifocal or occurred at a frequency greater than 5/ min occurred in 26 of 58 patients (45 percent) receiving placebo and in 7 of 45 patients (16 percent) treated with quinidine. Quinidine effected an even more significant reduction of ventricular tachycardia; this arrhythmia was observed in 12 of 58 patients receiving placebo and in only 1 of 45 quinidine-treated patients (P < 0.01). Continuous 72 hour electromagnetic tape recordings showed no reduction In the frequency of isolated ventricular premature beats in the quinidine-treated group compared with the control group. Bradyarrhythmias including heart block warranted discontinuance of the trial in 6 of 45 quinidinetreated patients and 2 of 58 patients receiving placebo. There was no difference in mortality in the two groups during the 72 hour study period or during the entire period of hospitalization.     

50th anniversary historical article. Myocardial infarction and coronary care units

The results of treatment of 250 patients with established acute myocardial infarction in a coronary care unit in a university hospital are described. The criteria for diagnosis have been carefully defined. In 62 percent of patients admitted with a tentative diagnosis of acute infarction, the initial impression was confirmed. Fifteen percent of patients admitted to the unit were classified as having possible infarction; in this group, the mortality rate was 3 percent. A classification of functional severity based on clinical evidence of heart failure or shock is presented. Morbidity and mortality in acute myocardial infarction are related to the functional severity of the illness. Although arrhythmia is common, the overriding importance of five life-threatening arrhythmias is emphasized. Mortality of patients in the coronary care unit was not improved in comparison to those treated under regular care until strong central direction of therapeutic programs, immediate treatment of arrhythmia in cardiac arrest, and delegation of some medical authority to trained nurses was accomplished. The change in concept of the purposes and practices of special coronary care from resuscitation to prevention of arrhythmia is emphasized. The mortality in myocardial infarction complicated by shock remains high. In the absence of shock, aggressive medical treatment in the coronary care unit reduced mortality from 26 to 7 percent. The implications of these data in the management of patients admitted to a hospital with a diagnosis of acute myocardial infarction are discussed.     

Prevalence,characteristics and significance of ventricular tachycardia (three or more complexes) detected with ambulatory electrocardiographic recording in the late hospital phase of acute myocardial infarction

A 24 hour electrocardiographic recording was performed before hospital discharge in 430 patients who survived the cardiac care unit phase of acute myocardial infarction. Fifty patients (11.6 percent) had ventricular tachycardia, that is, three or more consecutive ventricular complexes. In 25 (50 percent) of these 50 patients, there was only one episode of ventricular tachycardia and, in 15 patients (30 percent), the longest run of ventricular tachycardia was only three consecutive ventricular premature depolarizations. The average rate of tachycardia was 119/min. Tachycardia rarely started with R on T ventricular premature complexes (4 of 1,370 episodes in 50 patients).There was no difference between the groups with and without ventricular tachycardia with respect to age and sex, but the patients with tachycardia had a significantly greater prevalence of previous myocardial infarction, left ventricular failure in the cardiac care unit, atrial fibrillation, ventricular tachycardia or ventricular fibrillation in the cardiac care unit and significantly more frequent use of digitalis and diuretic and antiarrhythmic drugs at the time of hospital discharge.The group with tachycardia had a 38.0 percent 1 year mortality rate compared with the rate of 11.6 percent in the group without tachycardia. Ventricular tachycardia had a strong association with 1 year mortality (odds ratio = 4.7). Although ventricular tachycardia had a significant association with many other postinfarction risk factors, it was still significantly associated with the 1 year mortality (p < 0.05) when other important risk variables were controlled statistically using a multiple logistic regression model. The 36 month cumulative mortality rate was 54.0 percent in the group with ventricular tachycardia compared with 19.4 percent in the group without tachycardia.     

Association of accelerated idioventricular rhythm and paroxysmal ventricular tachycardia in acute myocardial infarction

Although previous studies have suggested that accelerated idioventricular rhythm rarely coexists with paroxysmal ventricular tachycardia, this relation has not been systematically evaluated in acute myocardial infarction. To examine this relation, the frequency and characteristics of the two arrhythmias were analyzed by performing 24 hour Holter monitoring during the initial 24 hours of acute myocardial infarction in 52 successive patients. Twenty-four of these patients had documented accelerated idioventricular rhythm; 28 patients did not. Paroxysmal ventricular tachycardia occurred in 83 percent of patients with accelerated idioventricular rhythm but in only 18 percent of patients without this arrhythmia (P < 0.001). The results remained at the same level of significance whether paroxysmal ventricular tachycardia was defined by rates greater than 100, 120 or 140 beats/min. These findings suggest that accelerated idioventricular rhythm complicating acute myocardial infarction is not always benign and is frequently associated with more serious forms of ventricular arrhythmia.     

Nontransmural myocardial infarction: A comparison of hospital and late clinical course of patients with that of matched patients with transmural anterior and transmural inferior myocardial infarction

The hospital and long-term course of 67 patients with nontransmural myocardial infarction was compared with that of 66 patients with transmural anterior and 63 patients with transmural inferior infarction matched for age, sex, previous infarction and prior congestive heart failure. During their hospital stay, patients with nontransmural infarction had significantly less congestive heart failure and fewer intraventricular conduction defects than did patients with transmural anterior infarction; fewer atrial tachyarrhythmias and less sinus bradycardia and atrioventricular block than did patients with transmural inferior infarction; and an incidence of hypotension, pericarditis and ventricular irritability similar to that of patients in the other two groups. Patients with nontransmural infarction had a significantly lower coronary care unit mortality rate (9 percent) than that of patients with transmural anterior or transmural inferior infarction (20 and 19 percent, respectively). By 3 months, the mortality rate had risen to 14 percent in patients with nontransmural infarction, but was significantly higher (29 and 27 percent, respectively) in patients with transmural anterior or transmural inferior infarction. Angina was common in all three groups, occurring in more than 50 percent of patients during a mean follow-up period of 28.6 months after hospital discharge.In contrast, the incidence of subsequent myocardial infarction was significantly greater in patients with nontransmural myocardial infarction, occurring in 21 percent at 9 months compared with only 3 percent of patients with transmural anterior (p <0.01) and 2 percent of patients with transmural inferior (p <0.05) infarction. By 54 months, 57 percent of patients with nontransmural infarction had sustained a new infarction contrasted with only 12 percent of patients with transmural anterior (/p <0.001) and 22 percent of patients with transmural inferior (p <0.01) infarction. Late mortality increased in patients with nontransmural myocardial infarction and, although this group had a significantly better survival rate at 3 months, the overall late mortality of the three groups was comparable. The study suggests that nontransmural myocardial infarction is an unstable ischemic event associated with a great risk of later myocardial infarction and high late mortality rate. A more aggressive diagnostic and therapeutic approach may be warranted in patients with nontransmural myocardial infarction.     

Evidence of sinoatrial block as a curative mechanism in radiofrequency current ablation of inappropriate sinus tachycardia

Inappropriate sinus tachycardia is a nonparoxysmal tachycardia characterized by high resting heart rates and a disproportionate response to activity. Sinus node modification with radiofrequency current has been used successfully as treatment for this arrhythmia. However, the electrophysiologic mechanisms leading to successful modification are not yet fully elucidated. We report a case of a patient with drug-resistant inappropriate sinus tachycardia in whom successful treatment of the arrhythmia was achieved by documented sinoatrial exit block induced by radiofrequency current applications.     

Results and significance of Holter monitoring after direct percutaneous transluminal coronary angioplasty for acute myocardial infarction

Whether acute and direct percutaneous transluminal coronary angioplasty improves the incidence of nonsustained ventricular tachycardia in patients surviving acute myocardial infarction is not known. In 400 consecutively studied patients, Lown classification IVb on Holter monitoring was only associated with arrhythmia morbidity, whereas reduced ejection fraction was related to total and cardiac mortality and arrhythmia morbidity.     

EARLY MOBILISATION AND DISCHARGE OF PATIENTS WITH ACUTE MYOCARDIAL INFARCTTON

The operation of a mobile coronarycare unit in addition to a hospital coronary-care unit (C.C.U.) increases the demand for monitoring facilities and convalescent beds. Early mobilisation and early discharge of patients not at risk is therefore important. The results of such a policy in patients with acute myocardial infarction admitted to a C.C.U. are reported. 18% of patients were discharged by the seventh day, and 62% spent ten days or less in hospital. The subsequent mortality and readmission-rate suggest that early discharge was not harmful. Factors which predicted the risk of dying between the end of the first week and three months were assessed. A coronary prognostic index of 6 or more, significant ventricular arrhythmias in the first forty-eight hours, sinus tachycardia, persistent ST-segment elevation, and recurrent ischæmic pain or recurrent arrhythmia were all associated with increased mortality. The last three factors were used to define a group of patients (57%) of whom none had died at three-month follow-up.     

R on T or R on P phenomenon? Relation to the genesis of ventricular tachycardia

Continuous electrocardiographic monitoring of 225 patients with acute myocardial infarction was performed during the initial 48 hours after admission. Two hundred twelve episodes of ventricular tachycardia occurred in 49 subjects, and 8 patients had primary ventricular fibrillation. Most cases of ventricular tachycardia were associated with late coupling of premature ventricular complexes. Of the 212 instances of ventricular tachycardia, 42 (20 percent) were initiated by a premature complex on the T wave (R on T) (R-R'/Q-T less than 1), and 93 (44 percent) had initiating premature complexes that occurred directly after onset of the sinus P wave (R on P). Of eight episodes of ventricular fibrillation, seven were initiated by a premature ventricular complex and in four of these there was associated R on T phenomenon. The influence of atrial contraction and myocardial stretch on reentry or ectopy is proposed as a possible explanation for the relatively high incidence rate of ventricular tachycardia observed after the onset of the sinus P wave.     

Ventricular arrhythmias during treatment with alteplase (recombinant tissue plasminogen activator) in suspected acute myocardial infarction.

Continuous electrocardiography during the first 24 hours of a stay in a coronary care unit was used to record ventricular arrhythmias during treatment with alteplase (recombinant tissue plasminogen activator) or placebo. Recordings were made on 378 of the 436 patients admitted to a double blind trial of alteplase or placebo in one participating centre of the Anglo-Scandinavian study of early thrombosis (ASSET), patients being selected according to the availability of recorders. Of these, 309 (158 given alteplase and 151 placebo) had greater than 5 hours of analysable data. Most of the arrhythmias were recorded in patients with an in hospital diagnosis of myocardial infarction. Ventricular couplets and ventricular tachycardia were significantly more common in the patients treated with alteplase. Further, in patients with myocardial infarction who had ventricular extrasystoles, couplets, or ventricular tachycardia type a, the number of hours in which each arrhythmia was recorded was significantly higher in the alteplase group. The various ventricular arrhythmias in the alteplase group tended to cluster in the first 4-12 hours of the recordings. During the first 24 hours admission there were four episodes of ventricular fibrillation in the alteplase group and five in the placebo group of taped patients. By one month there had been 18 deaths in these 309 patients (alteplase four, placebo 14). These bore no relation to any recorded arrhythmia. Clinical records for the patients with no or minimal tape data yielded six further episodes of ventricular fibrillation during the first 24 hours (three in the alteplase group and three in the placebo group). Of the total 436 patients, 10 of the 218 patients in the alteplase group had died by one month compared with 22 of the 218 patients treated with placebo. The use of alteplase increases the incident of non-life threatening ventricular arrhythmias. These results, however suggest that arrhythmia after thrombolysis in the pre-hospital phase may be less of a problem than it is perceived to be.     

Ventricular arrhythmias during treatment with alteplase (recombinant tissue plasminogen activator) in suspected acute myocardial infarction

Continuous electrocardiography during the first 24 hours of a stay in a coronary care unit was used to record ventricular arrhythmias during treatment with alteplase (recombinant tissue plasminogen activator) or placebo. Recordings were made on 378 of the 436 patients admitted to a double blind trial of alteplase or placebo in one participating centre of the Anglo-Scandinavian study of early thrombosis (ASSET), patients being selected according to the availability of recorders. Of these, 309 (158 given alteplase and 151 placebo) had greater than 5 hours of analysable data. Most of the arrhythmias were recorded in patients with an in hospital diagnosis of myocardial infarction. Ventricular couplets and ventricular tachycardia were significantly more common in the patients treated with alteplase. Further, in patients with myocardial infarction who had ventricular extrasystoles, couplets, or ventricular tachycardia type a, the number of hours in which each arrhythmia was recorded was significantly higher in the alteplase group. The various ventricular arrhythmias in the alteplase group tended to cluster in the first 4-12 hours of the recordings. During the first 24 hours admission there were four episodes of ventricular fibrillation in the alteplase group and five in the placebo group of taped patients. By one month there had been 18 deaths in these 309 patients (alteplase four, placebo 14). These bore no relation to any recorded arrhythmia. Clinical records for the patients with no or minimal tape data yielded six further episodes of ventricular fibrillation during the first 24 hours (three in the alteplase group and three in the placebo group). Of the total 436 patients, 10 of the 218 patients in the alteplase group had died by one month compared with 22 of the 218 patients treated with placebo. The use of alteplase increases the incident of non-life threatening ventricular arrhythmias. These results, however suggest that arrhythmia after thrombolysis in the pre-hospital phase may be less of a problem than it is perceived to be.     

Ablation of cardiac tissues by an electrode catheter technique for treatment of ectopic supraventricular tachycardia in adults

Five patients with chronic or recurrent ectopic supraventricular tachycardias unresponsive to drugs underwent programmed stimulation, endocardial mapping, and attempted catheter ablation of the arrhythmia focus. For attempted ablation, an intracardiac electrode catheter was positioned near the exit point of the tachycardia and served as the cathode while a chest wall patch served as the anode. In two patients with tachycardia originating near the coronary sinus, discharges of 200 or 400 J each were delivered to two electrodes at the earliest area of endocardial activation. These two patients with incessant tachycardia remain free of tachycardia for 17 and 11 months, respectively. In one patient with tachycardia originating from the right atrial appendage, both catheter and surgical ablation proved unsuccessful in that a new focus of atrial tachycardia supervened. This patient subsequently underwent successful catheter ablation of the atrioventricular junction. Two patients with junctional tachycardia underwent catheter ablation of the atrioventricular junction. Complete atrioventricular block followed atrioventricular junctional ablation and these patients required permanent cardiac pacing. The junctional tachycardia was replaced by sinus rhythm with episodes of unsustained atrial tachycardia. However, after 13 +/- 5 months follow-up, neither of the patients require antiarrhythmic drugs. Catheter ablation can be effective for atrial foci near the coronary sinus os, and can be performed with preservation of atrioventricular conduction. Arrhythmia ablation is possible in those with atrioventricular junctional tachycardia but requires the sacrifice of atrioventricular conduction. After ablation, other automatic atrial foci may become operative and complicate use of dual-chamber pacemakers.     

Prognostic Factors in Acute Myocardial Infarction treated in a Coronary Care Unit*

Summary: Prognostic factors in 269 cases of acute myocardial infarction treated in a coronary unit were analysed using a computer. The mortality rate was significantly higher in the elderly (≤60 years) and those with extensive infarction, shown by prolonged cardiac pain (>4 hours) or high serum enzyme levels (SGOT> 200 Sigma-Frankel units/ml; LDH> 2,000 Berger-Broida units/ ml). It was high also with tachycardia (sinus, supraventricular or ventricular), complete heart block, and complete bundle branch block. It increased progressively with severity of myocardial failure. Secondary cardiac arrect had a high mortality. Clinical signs of catecholamine hypersecretion (sinus tachycardia, pallor, sweating), hypoxaemia (central cyanosis), or low cardiac output (peripheral cyanosis, cold extremities, oliguria) greatly increased the mortality rate. Radiological cardiomegaly and pulmonary congestion each doubled it. An insignificant mortality increase accompanied ventricular and supraventricular ectopics, atrial fibrillation, incomplete heart block, and previous myocardial infarction, angina, and hypertension. There was no significant relationship between mortality and admission delay, sex, tobacco consumption, diabetes, or family history of ischaemic disease or diabetes. Only one patient died of primary cardiac arrest. Sinus bradycardia was a good prognostic sign. Analysis of the literature showed its lower mortality rate to be significant. Since intravenous atropine may cause arrhythmias, it should be reserved for sinus bradycardia with hypotension. It was suggested that patients with adverse prognostic signs short of extreme myocardial decompensatiqn should be monitored longer. Younger patients without severe infarction had a low mortality rate. Investigation of these may reveal a group which can be discharged safely from hospital soon after the completion of monitoring.     

Heart insufficiency in acute myocardial infarct. Long-term prognostic implications. Study of 213 cases of myocardial infarct

OBJECTIVE: To evaluate intra-hospital and first year prognosis of the acute myocardial infarction. DESIGN: Univariate analysis of hospital and late mortalities in 21 characteristics on the evolution of the acute phase of myocardial infarction of patients sequentially admitted in a coronary care unit. SETTING: Coronary care unit and out patient clinic of a school hospital. PATIENTS: A group of 213 patients with acute myocardial infarction admitted sequentially in a coronary care unit was studied. MATERIAL AND METHODS: Making use of a computerized information system the following characteristics f the patients were prospectively studied: age, sex, previous history of myocardial infarction, angor, diabetes, hypertension and tabacism, presence of left ventricular dysfunction electrocardiographic localization of the infarction, presence of angor in the acute phase of the infarction, epistenocardic pericarditis, hypertensive reaction, hypotension, sinus tachycardia, sinus bradycardia, supraventricular disrhythmias, ventricular disrhythmias, A-V block, complete right bundle branch block, complete left bundle block and peak of CPK values. Univariate analysis was made between each one of these characteristics and hospitalar and one year mortalities. RESULTS: Statistically significant differences were obtained in the hospitalar mortality in the following characteristics: age (p less than 0.001), sex (p less than 0.03), previous history of diabetes (p less than 0.05) and tabagism (p less than 0.005), left ventricular disfunction (p less than 0.0005), hypotension (p less than 0.005), sinus tachycardia (p less than 0.0005), sinus bradycardia (p less than 0.024), A. V. block (p less than 0.004), and peak of CPK (p less than 0.05). Statistically significant differences were found in one year mortality in the following characteristics: age (p less than 0.001), left ventricular disfunction (p less than 0.02), sinus tachycardia (p = 0.0116) and peak of CPK (p less than 0.05). Conclusion: Influence in the hospitalar mortality was demonstrated by the following characteristics of the patients with myocardial infarction: age, sex, previous history of diabetes and tabagism, left ventricular disfunction, infarct size expressed by the peak values of CPK, sinus tachycardia, sinus bradycardia, hypotensive reaction in the acute phase of the myocardial infarction and A-V block. Mortality in the late phase infarction was influenced by age, left ventricular dysfunction, sinus tachycardia and peak values of CPK.     

Idioventricular rhythm complicating acute myocardial infarction

The incidence, natural history, prognosis, and electrocardiographic characteristics of idioventricular rhythm complicating acute myocardial infarction are described. It occurred as a transient arrhythmia nearly always within 24 hours of infarction in 61 (8%) of 737 patients, and was characterized by paroxysms of between 6 and 20 beats with widened bizarre QRS complexes at a rate of between 60 and 90 a minute. Most cases showed fusion beats and P waves dissociated from the QRS complexes, and in many cases idioventricular rhythm started during the slow phase of sinus arrhythmia. Though it usually occurred in patients with moderately severe transmural infarcts, the incidence of ventricular fibrillation and subsequent mortality was no greater than in patients with infarcts of equivalent severity who did not have idioventricular rhythm. It is concluded that this rhythm is a common and relatively benign arrhythmia complicating myocardial infarction, and that it should be distinguished from ventricular tachycardia.