Br Heart J: first published as 10.1136/hrt.34.9.901 on 1 September 1972. Downloaded from British Heart Journal, I972, 34, 901-904. Sinus rate in acute myocardial infarction R. M. Norris, C. J. Mercer, and S. E. Yeates From Green Lane Hospital, Auckland, New Zealand The clinical course of sinus bradycardia and sinus tachycardia has been studied in 735 patients owith acute myocardial infarction admitted to a coronary care unit. Sinus bradycardia was con- sidered to have occurred when two or more records showed sinus rhythm with a rate below 6o a minute, and sinus tachycardia when a sinus rate over 0OO a minute occurred in two or more records. Hospital mortality of patients having sinus bradycardia was significantly lower (6%) and of those with sinus tachycardia was significantly higher (26%) than of those who had neither bradycardia nor tachycardia (i5%). Major ventricular arrhythmias were no more common in patients having sinus bradycardia and when they did occur did not usually do so at the time ,'that the bradycardia was present. Sinus bradycardia was more common in cases ofposterior and subendocardial infarction without radiological evidence of cardiac failure, while sinus tachy- cardia was frequent in patients with anterior transmural infarction complicated by heartfailure. The benign course of sinus bradycardia is surprising because experimental evidence suggests that slow heart rates after infarction favour the occurrence of ventricular arrhythmias. Further work is necessary to correlate heart rate with prognosis, particularly in patients seen at the onset ,.f infarction, and before admission to a coronary care unit. Observations of patients in coronary care clinical and experimental evidence, and to units has shown that wide variations in sinus suggest that further studies on the association rate occur in patients who have suffered acute of bradyarrhythmias with ventricular fibrilla- http://heart.bmj.com/ myocardial infarction. Sinus tachycardia has tion are necessary under a wider variety of been found to be associated with a high mor- clinical conditions. lrality, while sinus bradycardia has in general een found to be benign and not associated with serious complications (Lawrie et al., Patients and methods I967; Jewitt et al., I967; Chapman, I97I). Patients were all those admitted to the four- Experiments on dogs (Han et al., i966a, b; bedded coronary-care unit at Green Lane Hos- Han, i969), however, have shown that slow pital over three years. Myocardial infarction was on October 1, 2021 by guest. Protected copyright. heart rates after infarction favour the occur- considered to have occurred if two or more of the rence of ventricular arrhythmias, and on this following criteria were satisfied: (i) Characteristic account aggressive treatment of bradycardia clinical presentation; (2) pathological Q waves, ST elevation, or T wave inversion in the electro- - favoured by some (Lown et al., I967; cardiogram with evolutionary changes; and (3) Gregory and Grace, I968). Moreover, as rise in serum aspartate aminotransferase (SGOT) sinus bradycardia occurs most commonly at to over 40 units/ml. Patients over 70 years of age the onset of infarction before most patients were in general excluded because of shortage of come under medical attention, it has been beds, 53 per cent of patients were admitted within 0 suggested that sinus bradycardia occurring six hours of onset of the most severe chest pain, early after the onset may have a more serious and stay in the unit was three days on average. *ignificance than when the arrhythmia occurs Ten-second electrocardiograms were made later (Adgey et al., i968, I971; Stock, I97I). hourly or whenever an arrhythmia occurred, and The purpose of this paper is to pre- clinical data with details of arrhythmias were re- amplify corded on punch cards (International Business vious observations (Norris, I969a; Norris and Machines). Sinus bradycardia was considered to Mercer, I970) on the benign course of sinus have occurred if two or more records showed sinus bradycardia as seen in a coronary care unit, rhythm with a rate below 6o a minute. In order to draw attention to the conflict between the to study possible effects or associations of sinus bradycardia with prognosis and with other Received 2I December I971. arrhythmias, the findings were compared with Br Heart J: first published as 10.1136/hrt.34.9.901 on 1 September 1972. Downloaded from go2 Norris, Mercer, and Yeates those in patients who had sinus tachycardia (two TABLE I Sinus tachycardia and sinus or more records with a sinus rate over Ioo a bradycardia complicating acute myocardial minute), those who had both sinus bradycardia infarction - incidence, mortality, and and sinus tachycardia and those who had neither of these arrhythmias. Cases of sinus bradycardia association with major ventricular arrhythmias were also examined in relation to the lowest re- Sinus Sinus Sinus Neither sinus corded heart rate, the time of occurrence after bradycardia tachycardia bradycardia bradycardia ' admission to the coronary care unit, and the site and sinus nor sinus of infarction and degree of cardiac failure assessed tachycardia tachycardia from the electrocardiogram and chest x-ray. No. of cases 152 267 41 275 Per cent of total 2I 36 6 37 Results Mortality 9 (6%) 70 (26%) 9 (22%) 42 (15%/o) No. having major ven- During the three years, I223 patients were tricular arrhythmias 39 (26%) 87 (33%) I5 (37%) 64 (23%) admitted to the coronary care unit and, of No. having ventricular I these, 735 had acute myocardial infarction fibrillation II (7%) 29 (II%) 4 (I0%) 20 (7%) according to the above criteria, and were un- selected cases, not referred from other hos- pitals. Of these 735 patients, i52 (2i%) had was between 50 and 6o a minute, in 30 per sinus bradycardia, 267 (36%) had sinus tachy- cent the rate was 40 to 50 a minute, while in cardia, 4I (6%) had both sinus bradycardia I0 per cent of cases it was less than 40 a and tachycardia, and 275 (37%) had neither minute, often with alternation between sinus arrhythmia (Table I). Hospital mortality was nodal significantly lower (6%; P<o-oi) in and pacemakers. Patients in the latter X2=8'i, group usually had an abrupt onset of brady- patients who had sinus bradycardia, and sig- cardia accompanied by faintness, pallor, and nificantly greater (26%; X2=999, P<o-oi) in These attacks those who had sinus tachycardia, when these hypotension. responded promptly to intravenous atropine 0-3 to o-6 groups of patients were compared with those mg, but in several cases when there was an who had neither (mortality i 5%). The incidence of ventricular fibrillation and inadvertent delay in giving atropine there was a spontaneous improvement in heart rate. Of major ventricular arrhythmias in the same I3 patients in whom sinus bradycardia at a groups of patients is also shown in Table i. rate of less than 40 a minute was observed, Major ventricular arrhythmias were classified only one had major ventricular arrhythmias as as salvoes of two or more ventricular ectopic http://heart.bmj.com/ beats, ventricular ectopic beats showing the described above, and none had ventricular fibrillation. The possibility that some cases of R on T phenomenon, ventricular tachycardia, and ventricular fibrillation. We have found sinus bradycardia were due to therapy was also considered, and in 8 cases it was possible the first three of these arrhythmias to be asso- that alprenolol, which was the subject of a ciated with a significantly increased incidence of ventricular fibrillation (R. M. Norris, I968, clinical trial (Briant and Norris, I970), was the cause of the arrhythmia. In some other unpublished observations). There was no patients morphine may have caused the brady- difference in the incidence of ven- significant cardia, particularly when vomiting occurred on October 1, 2021 by guest. Protected copyright. tricular arrhythmias among any of the groups, after giving the drug. though there was a tendency for patients with The relation of sinus bradycardia and sinus sinus tachycardia to have a higher incidence of major ventricular arrhythmias and ventricular tachycardia to the site and extent of infarction fibrillation. Of the ii patients who had both sinus bradycardia and ventricular fibrillation, 7 had ventricular fibrillation while they were TABLE 2 Relation between site and extent of being observed in the coronary care unit. In infarction and occurrence of sinus bradycardia all of these patients ventricular fibrillation and tachycardia l occurred at a time when sinus bradycardia was not present. Moreover, ventricular fibril- Anterior Anterior Posterior Posteriort lation and major ventricular arrhythmias were trans- subendo- trans- subendo- no more common when sinus bradycardia mural cardial mural cardial occurred within one hour of admission Sinus bradycardia I0% 24% 31% 36% (approximately 5o% of cases) than when it Sinus tachycardia 52% 24% 29% 30% occurred later. Sinus bradycardia and sinus c Further analysis of the cases of sinus brady- tachycardia 6% 5% 6% 2% Neither sinus bradycardia nor cardia showed that in approximately 6o per sinus tachycardia 32% 47% 34% 32% M cent of cases the lowest recorded heart rate Br Heart J: first published as 10.1136/hrt.34.9.901 on 1 September 1972. Downloaded from Sinus rate in acute myocardial infarction 903 TABLE 3 Relation between sinus bradycardia slow heart rates favour the occurrence of and tachycardia and occurrence of radiological arrhythmias due to re-entry or spontaneous pulmonary oedema discharge of excitable myocardial cells (Han et al., I966a, b; Han, i969), and the clinical IL No. of No. having evidence that 'over driving' the heart at a cases oedema faster rate can suppress ventricular arrhyth- Sinus bradycardia 58 7 (I2%) mias (Sowton, Leatham, and Carson, i964). Sinus tachycardia I15 53 (46%) The mechanisms of asynchronous repolariza- Sinus bradycardia and sinus tion and spontaneous depolarization of myo- tachycardia I9 7 (37%) Neither sinus bradycardia cardial cells which have been described would nor sinus tachycardia io8 34 (32%) presumably also operate in bradyarrhythmia due to atrioventricular block.