Circadian variation in ischemic threshold. A mechanism underlying the circadian variation in ischemic events

Effect of morning versus evening dosing of diltiazem on myocardial ischemia detected by ambulatory electrocardiographic monitoring in chronic stable angina pectoris. Dilacor XR Ambulatory Ischemia Study Group

Myocardial ischemia occurs frequently during daily life and has a circadian pattern similar to that reported for myocardial infarction and sudden death. Because of the increased risk of myocardial ischemia in the morning hours, it has been suggested that the administration of anti-ischemic medication before bedtime may be more effective than the traditional morning dosing. This randomized, double-blind, placebo-controlled, crossover study evaluated the effects of 480-mg/day diltiazem (given either in the A.M. or the P.M.) on myocardial ischemia using ambulatory electrocardiographic monitoring in 68 patients with chronic stable angina and > or = 2 minutes of ischemia per 48 hours. During treatment with diltiazem, the duration and number of myocardial ischemic episodes were reduced by 45% (94 to 52 minutes, p <0.004) and by 40% (4.5 to 2.7 episodes, p <0.003), respectively. The duration and number of myocardial ischemic episodes during daytime (6 A.M. to 6 P.M.) hours were also reduced by 52% (74 to 36 minutes, p <0.002) and by 48% (3.1 to 1.6 episodes, p <0.001), respectively. There was no significant difference between A.M. and P.M. dosing. Morning ischemia (6 A.M. to noon), considered separately from daytime ischemia, was also significantly reduced by both A.M. and P.M. dosing regimens, with no difference between the regimens. The results of this study showed that both A.M. and P.M. dosing of long-acting diltiazem were equally effective in suppressing episodes of ambulatory myocardial ischemia at all times.

Automatic measurement of corrected QT interval in Holter recordings: comparison of its dynamic behavior in patients after myocardial infarction with and without life-threatening arrhythmias

This study was designed to determine the value of automatic corrected QT-interval measurement in Holter tapes in patients after myocardial infarction as a marker of life-threatening ventricular arrhythmias. We compared the corrected QT interval, automatically measured in 24-hour Holter recordings, in two groups of patients after myocardial infarction: group I was composed of 14 patients admitted consecutively to our hospital for documented sustained ventricular tachycardia or out-of-hospital cardiac arrest. Group II consisted of 28 patients with previous myocardial infarction with characteristics similar to those of group I, but without malignant ventricular arrhythmias in the follow-up. The global mean 24-hour corrected QT interval was longer in group I (425 +/- 20 msec) than in those patients after myocardial infarction without arrhythmias (group II) (405 +/- 17 msec; p < 0.01). Furthermore, a significant proportion of patients of group I (seven of 14) exhibited more peaks of corrected QT longer than 500 msec compared with patients of group II (two of 28; p < 0.005). A circadian rhythm of corrected QT peaks was observed in group I, having a significantly higher incidence from 11 PM to 11 AM (p < 0.05). We conclude that automatic corrected QT-interval measurement on Holter electrocardiogram is now available and feasible. Our results suggest that this is a marker for risk assessment of life-threatening ventricular arrhythmias. Large-scale trials are needed to confirm these results and to determine the predictive value of this technique for risk stratification.

New insights into the pathogenesis and prevention of acute coronary syndromes

Several recent studies have shown that 60-70% of coronary occlusions that cause acute coronary syndromes (such as unstable angina, myocardial infarction, or sudden ischemic death) evolve from atherosclerotic plaques that are only mildly to moderately obstructive. Numerous studies have demonstrated that coronary thrombosis, the immediate cause of acute coronary syndromes, is a consequence of plaque disruption. Most thrombotic events are related to deep plaque fissure, while superficial plaque erosion is the cause in a significant minority of cases. Thus, the mechanisms by which stable coronary artery disease (CAD) evolves into an unstable and potentially lethal acute coronary syndrome are related to plaque disruption and thrombosis. The vulnerability of a plaque to disruption appears to be determined by the presence of a large lipid-rich core, a thin fibrous cap, and an inflammatory cellular infiltrate, rather than by the size of the plaque or the severity of stenosis caused by a plaque before disruption. In addition to plaque disruption and thrombosis, enhanced vasoconstriction--a characteristic feature of CAD and dyslipidemia-may contribute to the clinical manifestations of CAD. Angiographic studies have demonstrated that risk factor modification produces a disproportionately greater reduction in ischemic clinical events than in anatomic regression of plaque, suggesting "plaque stabilization" may be the major mechanism of such clinical benefit. The relatively rapid attenuation of endothelial-mediated vasomotor dysfunction with the treatment of dyslipidemia lends credence to this concept.

Heart rate variability reproducibility and stability using commercially available equipment in coronary artery disease with daily life myocardial ischemia

Heart rate variability (HRV) appears to be a strong predictor of death. The reproducibility of HRV measurements in patients with stable coronary artery disease (CAD) who have daily life myocardial ischemia, however, is unknown. Thirty patients with stable CAD (25 men and 5 women; aged 62 +/- 8 years) with daily life ischemia were studied with 2 consecutive 24-hour Holter monitoring recordings. Intra- and interobserver reproducibility of the HRV measures was high, with correlations ranging from 0.990 to 0.999 (p < 0.0001). Strong correlations between time and frequency domain HRV measures were observed (range 0.912 to 0.963; p < 0.0001). Both the frequency and duration of ischemia, measured by ST change, varied significantly by day for each patient (s = 155.5; p < 0.0001; s = 232.5, p < 0.0001, respectively). Correlations for HRV measurements between days remained high (range 0.871 to 0.983; p < 0.0001), despite stratification by magnitude of daily ischemia. Thus, 24-hour HRV measurements are stable in CAD patients with daily life myocardial ischemia over a short period, despite varying magnitudes of daily ischemia. These results support the use of HRV as a clinical tool and an outcome measure in future CAD intervention studies using commercially available equipment.

Frequency, significance, and mechanism of myocardial ischemia during daily activities detected by Holter dynamic electrocardiogram

Exercise testing (EX) and Holter ECG (DCG) were performed consecutively in 52 patients with ischemic heart disease. A total of 100 episodes of myocardial ischemia (IE) were recorded during DCG in 30 patients, who constituted 94% of the patients with myocardial ischemia under 6 metabolic equivalents (METs) during EX. A significant increase in heart rate (HR) was observed before the development of IE. The duration of this increase in HR was longer in IE than in periods in which the HR increased above the ischemic threshold, but without ischemia. The incidence of IE showed two peaks at 8-10 am and 4-6 pm. The frequency of IE among all of the periods with increased HR was highest at 8-10 am (51%). IE in the morning was associated with a lower HR than that in the afternoon, and LF/HF, which reflects sympathetic activity, evaluated using power spectral analysis, increased before IE in the morning. The early appearance of myocardial ischemia in EX can predict its appearance in daily life. The increase in HR and its duration appear to be associated with the development of IE. The increases in sympathetic activity in the morning and the increase in myocardial oxygen demand accompanied by physical activity in the afternoon most likely contributed to the circadian variation in the incidence of ischemic episodes.

Relationships of the circadian rhythms of thrombotic, ischemic, hemorrhagic, and arrhythmic events to blood pressure rhythms

Convincing evidence has recently accumulated that several unfavorable cardiovascular events show a well defined pattern in their occurrence throughout the day. Myocardial angina and infarction, sudden cardiac death, arrhythmias, fatal pulmonary thromboembolism, and ischemic and hemorrhagic cerebrovascular accidents occur more frequently in the morning, after awaking, until noon. Diurnal variations in multiple biologic functions, such as assumption of an upright posture associated with increased platelet aggregability, changes in blood clotting, fibrinolysis, and vascular tone and resistance, may be potentially active triggering factors. Moreover, variations in sympathetic tone, catecholamine secretion, and blood pressure have to be considered. The role of triggering factors and their relationships with blood pressure patterns is discussed in view of an optimized pharmacologic treatment.

Patterns and behavior of transient myocardial ischemia in stable coronary disease are the same in both men and women: a comparative study

OBJECTIVES

This study sought to compare the circadian variations in transient ischemic activity, mean heart rate and ischemic threshold between women and men with coronary artery disease.

BACKGROUND

There is a circadian variation in ischemic activity, onset of myocardial infarction and sudden cardiac death in patients with coronary artery disease, but studies assessing ischemia have incorporated predominantly male subjects.

METHODS

Thirty-one women and 45 men underwent at least 48 h of ambulatory ST segment monitoring.

RESULTS

There was a similar and significant circadian variation in ischemic activity in both women and men (p < 0.0001 and p < 0.0001, respectively), with a trough at night, a surge in the morning and a peak between 1 and 2 PM, corresponding to a similar circadian variation in mean hourly heart rate (p < 0.0001) that was not different between men and women (p = 0.28, power to detect a shift 99.9%). Mean heart rate at onset of ischemia (ischemic threshold) had similar variability in women and men (p = 0.96), and harmonic regression analysis confirmed a significant circadian variation (p < 0.0001), with a trough at night and a peak during activity hours. Heart rate increased significantly in the 5 min before ischemia throughout the 24 h (p < 0.0001), with no gender differences in the pattern of preonset to onset heart rate changes over time (p = 0.52); the smallest differences were recorded in the middle of the night. The majority of ischemic episodes (80%) had a heart rate increase > 5 beats/min in the 5 min before ischemia, but there were no gender differences.

CONCLUSIONS

Women with coronary artery disease have a pattern of ischemic activity and underlying pathophysiologic mechanisms very similar to men. The importance of increase in myocardial oxygen demand in the genesis of ischemia in both men and women is reflected by similar magnitude of heart rate increases before ischemia. The lower ischemic threshold during the nocturnal hours, when blood pressure is also lower, is consistent with a circadian variation in underlying coronary vascular tone.

Hemodynamic changes as triggers of cardiovascular events

Although a large number of clinical studies during the past decade have documented that myocardial ischemia, acute myocardial infarction, and sudden cardiac death have definite circadian patterns, recently published work suggests that it is not necessarily a specific time of the day but the sleep/wake cycle and postural changes as well as the balance between triggers and protective factors that determine the onset of cardiovascular events. A clear understanding of the pathophysiologic processes responsible for ischemic events and the specific roles played by various triggers would be helpful in better defining the therapeutic strategies designed for the prevention and treatment of acute cardiac events. Because changes in hemodynamic parameters are of paramount importance in initiating an ischemic event, it is critical to review their role and relationship in the pathogenesis of acute coronary events.

Triggers of myocardial ischemia during daily life in patients with coronary artery disease: physical and mental activities, anger and smoking

OBJECTIVES

This study assessed the potency of physical and mental activities and emotions (anger and anxiety) and smoking and other substance use as proximate triggers of ischemia in patients with coronary artery disease during daily life.

BACKGROUND

Myocardial ischemia occurs during a wide variety of activities in patients with coronary artery disease, but frequency and relative potency of physical and mental activities, smoking and use of caffeine and alcohol as triggers of ischemia during daily life have not been established.

METHODS

Patients (n = 63) with coronary artery disease and evidence of out-of-hospital ischemia kept a validated structured diary of physical and mental activities and psychologic states while undergoing ambulatory electrocardiographic monitoring for 24 to 48 h.

RESULTS

Ischemia occurred most frequently during moderately intense physical and mental activities. Patients spent the largest proportion of time engaged in low intensity physical and mental activities (p < 0.05), but the likelihood of ischemia was greatest during intense physical (p < 0.0001) and stressful mental activities (p < 0.03). The percentage of time in ischemia was elevated and approximately equivalent for high intensity physical and high intensity mental activities (5%) compared with 0.2% when patients were engaged in low intensity activities. Strenuous physical activity (e.g., effortful walking, p < 0.05) and the experience of intense anger were potent ischemic triggers, and heart rates at onset of ischemia increased with the intensity of physical and mental activity and with anger. Among smokers, ischemia was more than five times as likely when patients smoked than when they did not (during 24% vs. 5% of diary entries, p < 0.0001). Coffee and alcohol consumption were also related to ischemia (p < 0.05), but this association disappeared after controlling for concurrent cigarette smoking.

CONCLUSIONS

Triggers of ischemia in patients with coronary artery disease during daily life include not only strenuous exercise, but also activities involving low levels of exertion, such as anger and smoking. Mental activities appear to be as potent as physical activities in triggering daily life ischemia. Coffee and alcohol consumption are related to ischemia only by virtue of their associations with smoking.

Circadian variation in sudden cardiac death: effects of age, sex, and initial cardiac rhythm

STUDY OBJECTIVE

Previous studies based on data obtained from vital statistics records have demonstrated circadian variation in the occurrence of sudden cardiac death. The purpose of this study was to examine the effects of age, sex, and initial cardiac rhythm on circadian variability in sudden cardiac death.

METHODS

This study employed a retrospective analysis of the records of adult patients with witnessed cardiac arrest who underwent resuscitation in an urban paramedic system during a 5-year period.

RESULTS

The records of 2,250 consecutive patients with witnessed cardiac arrest were reviewed. Spectral analysis was used to decompose the data into frequency components. A circadian variation in the occurrence of sudden cardiac death was demonstrated, with a low occurrence rate between midnight and 6 AM and a 2.4-fold increase between the rate at 6 AM and the rate at noon. The same circadian pattern was noted among both men and women, among both patients aged 18 to 70 and those older than 70 years, and among patients with various initial cardiac arrest rhythms (ventricular tachycardia or fibrillation, asystole, and electromechanical dissociation). However, the outcome of resuscitation in these patients (ie, the rate of successful resuscitation and the rate of survival) did not demonstrate circadian variation.

CONCLUSION

Witnessed out-of-hospital sudden cardiac death demonstrated circadian variation, and this variability was observed regardless of the patient's age, sex, or initial cardiac arrest rhythm. The outcome of resuscitation did not show circadian variability. These results suggest a common pathophysiologic mechanism leading to sudden cardiac death.

Chronobiological aspects of pulmonary thromboembolism

A growing body of evidence has recently accumulated suggesting that several unfavorable cardiovascular events, e.g., myocardial angina and infarction, sudden cardiac death, and stroke, present a specific temporal pattern in their occurrence. This has also been shown to be true for fatal pulmonary thromboembolism, with a high frequence peak in winter and in the morning hours. The authors report the circannual and circadian aspects of pulmonary thromboembolism, analysing both the temporal patterns of the underlying favouring or precipitating factors, and the possible therapeutic implications.

Melatonin receptors mediate potentiation of contractile responses to adrenergic nerve stimulation in rat caudal artery

The hormone melatonin potentiated contractile responses to adrenergic nerve stimulation in isolated ring segments of rat caudal artery. This effect was inhibited by the melatonin receptor antagonist luzindole but not by the serotonin 5-HT2 receptor antagonist ketanserin. Melatonin had no direct effects on vascular tone. Melatonin agonists potentiated contractile responses with a relative order of potency (2-iodomelatonin, EC50 = 0.6 nM; melatonin, EC50 = 4.7 nM; N-acetylserotonin, EC50 = 1.5 microM) that is consistent with the melatonin ML1 receptor subtype. Melatonin also potentiated contractions elicited by exogenous norepinephrine and produced its effects in the absence of an intact endothelium. These data suggest that melatonin acts on receptors in the smooth muscle. The caudal artery provides a useful functional assay for pharmacological analysis of melatonin receptors. Physiologically, melatonin may activate its receptors at night to influence thermoregulation in the rat by enhancing the effects of sympathetic input to the caudal artery.

Circadian variation of ischemic threshold in syndrome X

To evaluate whether the ischemic threshold has a circadian rhythm in patients with syndrome X, we analyzed 90 episodes of ST depression detected on 24-hour Holter recordings of 12 such patients. Ischemic threshold was considered as heart rate (HR) at 1 mm ST depression. To correct for differences in basal HR among patients, however, the ischemic threshold was also calculated as a normalized index of HR at 1 mm ST depression: [(HR at 1 mm ST-24-hour modal HR)/24-hour modal HR]-100. Mean hourly values of both absolute and normalized HRs at 1 mm ST depression were obtained by grouping and averaging respective values of all episodes detected in every hour of the day in all patients. Chronobiologic analysis was performed by single cosinor method. A significant circadian rhythm was found for HR (mesor 76 beats/min, amplitude 10 beats/min, acrophase at 2:16 P.M., p < 0.001), number of episodes of ST depression (mesor 3.75, amplitude 2.9, acrophase at 2:45 P.M., p < 0.001) and cumulative time of ischemia, with a high correlation of distributions. Episodes of ST depression showed a double peak initially in the morning, and again in the afternoon. Both raw and normalized values of HR at 1 mm ST depression also had a significant circadian variation in ischemic threshold, which was lower in the night and early morning hours, progressively increased until the first afternoon hours, and subsequently decreased in the evening.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of amlodipine, atenolol and their combination on myocardial ischemia during treadmill exercise and ambulatory monitoring. Canadian Amlodipine/Atenolol in Silent Ischemia Study (CASIS) Investigators

OBJECTIVES

This study compared the effects of amlodipine, atenolol and their combination on ischemia during treadmill testing and 48-h ambulatory monitoring.

BACKGROUND

It is not known whether anti-ischemic drugs exert similar effects on ischemia during ambulatory monitoring and exercise treadmill testing.

METHODS

Patients with stable coronary artery disease and ischemia during treadmill testing and ambulatory monitoring were randomized to receive amlodipine (n = 51) or atenolol (n = 49). Each group underwent a counterbalanced, crossover evaluation of single drug and placebo, followed by evaluation of the combination.

RESULTS

Amlodipine and the combination prolonged exercise time to 0.1-mV ST segment depression by 29% and 34%, respectively (p < 0.001) versus 3% for atenolol (p = NS). During ambulatory monitoring, the frequency of ischemic episodes decreased by 28% with amlodipine (p = 0.083 [NS]), by 57% with atenolol (p < 0.001) and by 72% with the combination (p < 0.05 vs. both single drugs; p < 0.001 vs. placebo). Suppression of ischemia during exercise testing and ambulatory monitoring was similar in patients with and without exercise-induced angina. Exercise time to angina improved by 29% with amlodipine (p < 0.01), by 16% with atenolol (p < 0.05) and by 39% with the combination (p < 0.005 vs. placebo, atenolol and amlodipine). In patients with angina, total exercise time improved by 16% with amlodipine (p < 0.001), by 4% with atenolol (p = NS) and by 19% with the combination (p < 0.05 vs. placebo and either single drug). In those patients without angina, no therapy significantly improved total exercise time.

CONCLUSIONS

Ischemia during treadmill testing was more effectively suppressed by amlodipine, whereas ischemia during ambulatory monitoring was more effectively suppressed by atenolol. The combination was more effective than either single drug in both settings.

Circadian variation in the occurrence of paroxysmal supraventricular tachycardia in clinically healthy subjects

The purpose of this study was to assess prospectively the circadian distribution of spontaneous paroxysmal supraventricular tachycardia (PSVT) in drug-free subjects with no previous history or symptoms and signs of concomitant heart or lung disease. Of 112,424 presumably diurnally active patients admitted to the Emergency Department of a city hospital during a 2-year period (1990-1991), a total of 185 patients were screened with these characteristics. Time of symptom onset was exactly recordable in 177 (75 men and 102 women). Analysis of variance documented a higher incidence in the morning-afternoon hours. Cosinor analysis, although not a perfect method for the time series analysis, verified circadian rhythmicity with afternoon peak times. Our findings suggest that a circadian pattern in intrinsic electrical instability of the heart conduction system exists irrespective of the circadian fluctuations in the pathophysiologic mechanisms of the cardiovascular or lung diseases most frequently associated with PSVT itself.

Estrogen modulates the inducible expression of platelet-derived growth factor mRNA by monocyte/macrophages

We examined the effects of estrogen, 12-O-tetradecanoylphorbol 13 acetate (TPA), and lipopolysaccharide (LPS) on the gene expression of platelet-derived growth factor (PDGF) by the monocyte/macrophage cell line, THP-1. THP-1 cells were exposed to TPA for 48 or 96 hours to induce differentiation. Some were treated with LPS in the last 3 hours and/or ethinyl estradiol (estrogen) (10(-9) M) in the last 20 hours. Total cellular RNA was isolated and cDNA was synthesized and then coamplified (with an internal control, beta-actin, product size 1126 bp) using polymerase chain reaction (PCR) and a set of primers for PDGF-A (product size 225 bp), PDGF-B (217 bp), or PDGF beta-receptor (PDGF-R) (228 bp). The products were separated on an agarose gel and the ratios of radioactivity incorporated into PDGF PCR products to beta-actin products were used to assess the relative changes in the levels of PDGF mRNA abundance in response to various inducers. TPA induced the expression of PDGF-A mRNA, whereas LPS had no effect. Treatment of TPA-stimulated cells with estrogen caused a 61% and 190% increase in PDGF-A mRNA (p < 0.05) at 48 and 96 hours, respectively. Addition of estrogen to cells treated with both TPA and LPS did not cause any significant change in the amounts of the transcripts. In contrast to PDGF-A mRNA, attempts to visualize and estimate PDGF-B and PDGF-R mRNA were unsuccessful. This was probably due to low levels of these transcripts in THP-1 cells. The results indicate that estrogen modulates PDGF-A gene expression by monocyte/macrophages and suggest that estrogen may influence atherogenesis at the vascular level.

Effect of autonomic nervous system dysfunction on the circadian pattern of myocardial ischemia in diabetes mellitus

OBJECTIVES

The aim of this study was to determine the prevalence and characteristics of ambulatory myocardial ischemia in patients with diabetes mellitus and to delineate the relation between the presence and severity of autonomic nervous system dysfunction and the incidence and time of onset of myocardial ischemia.

BACKGROUND

Conflicting data exist with regard to the circadian pattern of myocardial infarction and other cardiovascular events, such as ambulatory ischemia, in diabetes.

METHODS

We performed ambulatory electrocardiographic monitoring in 60 patients with diabetes and coronary artery disease. Autonomic nervous system testing was performed in a subgroup of 25 patients with myocardial ischemia after discontinuation of all antianginal medications.

RESULTS

Thirty-eight of 60 patients had evidence of ambulatory ischemia; 91% of all ischemic episodes were asymptomatic. The 25 patients with ambulatory ischemia who underwent autonomic nervous system testing had a peak incidence of ischemia between 6 AM and noon (46 of 133 ischemic episodes, p < 0.007), compared with the other three 6-h periods. Fifteen of the 25 patients had no or mild autonomic nervous system dysfunction and demonstrated a similar peak incidence of ischemia between 6 AM and noon (p = 0.0009). However, the 10 patients with moderate to severe autonomic nervous system dysfunction did not experience a morning peak of ischemia, and the number of ischemic episodes was distributed evenly throughout the day (p = 0.4).

CONCLUSIONS

Silent ischemia is highly prevalent among patients with diabetes and coronary artery disease. Time of onset of ischemia in diabetic patients follows a circadian distribution, with a peak incidence in the morning hours. However, patients with significant autonomic nervous system dysfunction did not demonstrate such a peak, suggesting that alterations in sympathovagal balance may have an effect on the circadian pattern of cardiovascular events.

Heart rate variability before the occurrence of silent myocardial ischemia during ambulatory monitoring

Thirty-three ischemic episodes in 19 patients with stable coronary artery disease were studied to clarify changing autonomic nervous system activity during daily life before the occurrence of myocardial ischemia. Nonischemic points were studied for comparison of control data with ischemic episodes. These were defined as (1) patient showing no ischemic ST-T change while having the same heart rate with onset of ischemic episodes, and (2) presence within 1 to 2 hours before or after onset of ischemic episodes in the same patient. We analyzed heart rate (HR) variability during the 30-minute period before the onset and after the end of ischemic episodes during 24-hour monitoring. The period of 30 to 40 minutes before ischemia was regarded as the baseline, and HR variability was analyzed at 10-minute intervals before each ischemic episode and nonischemic point. HR variability was quantified on the band of 2 components: low frequency (0.04 to 0.15 Hz; LF) and high frequency (0.15 to 0.40 Hz; HF). Of the 33 episodes, 24 (73%) had a greater LF/HF value during the 30-minute period before ischemia than that before the nonischemic points. Distribution of the number of the 24 episodes demonstrated circadian rhythm with a peak from 8 to 10 A.M. HF power began to decrease from the last 10 minutes before ischemia, compared with baseline. A significant decrease in HF power with a background of greater value of LF/HF may explain the reduced ischemic threshold for ischemia during daily life.

Role of increases in heart rate in determining the occurrence and frequency of myocardial ischemia during daily life in patients with stable coronary artery disease

OBJECTIVES

The goal of this study was to investigate the role of increases in heart rate in the development of ischemic episodes recorded during ambulatory electrocardiographic (ECG) monitoring in patients with stable coronary artery disease and to establish the importance of such increases in determining the frequency of ambulatory myocardial ischemia.

BACKGROUND

The factors that determine the occurrence and frequency of episodes of myocardial ischemia that patients with stable coronary artery disease experience during daily life have not been clearly defined. In particular, the role of increases in heart rate in the development of myocardial ischemia is controversial.

METHODS

To address these issues, 54 patients (42 men and 12 women, mean age 60.5 +/- 8 years) with proved coronary artery disease who had > or = 1 mm ST segment depression during exercise testing underwent an exercise treadmill test with use of the National Institutes of Health combined protocol and a 48-h period of ambulatory ECG monitoring. The exercise ischemic threshold was determined as the heart rate at the onset of ST segment depression during exercise testing.

RESULTS

During monitoring, 48 (89%) of the 54 patients had at least one episode of ST segment depression (mean +/- SD 6.6 +/- 5 episodes, range 0 to 22). The majority (320 of 359 or 89%) of ischemic episodes were preceded by an increase in heart rate > or = 10 beats/min; the most significant increase (22.3 +/- 10 beats/min) occurred during the 5-min period before the onset of the episode. An ischemic episode occurred 80% of the times the heart rate reached the exercise ischemic threshold. A strong correlation was observed between the number of times the exercise ischemic threshold was reached during monitoring and both the number and the duration of ischemic episodes (r = 0.90 and 0.71, respectively, p < 0.0001).

CONCLUSIONS

Increases in heart rate that exceed the exercise ischemic threshold are commonly observed before the onset of episodes of ambulatory myocardial ischemia in patients with stable coronary artery disease. Moreover, such increases constitute an important determinant of the frequency of myocardial ischemia during daily life. These findings may explain the variability observed in the number of ischemic episodes and may have important implications for the mechanisms that contribute to myocardial ischemia in daily life and for the clinical evaluation of patients with coronary artery disease.