Author + information
- Received December 5, 1996
- Revision received July 7, 1997
- Accepted July 11, 1997
- Published online November 1, 1997.
- Asif Rehman, MDA,
- Gloria Zalos, RNA,
- Neil P Andrews, BMBS, MRCPA,
- David Mulcahy, MD, MRCPA and
- Arshed A Quyyumi, MD, FACCA,* ()
- ↵*Dr. Arshed A. Quyyumi, Cardiology Branch, National Heart, Lung, and Blood Institutes, National Institutes of Health, 10 Center Drive, MSC 1650, Building 10, Room 7B-15, Bethesda, Maryland 20892-1650.
Objectives. We investigated the contribution of changes in systemic blood pressure to the genesis of spontaneous myocardial ischemia.
Background. Although increases in heart rate often precede the development of spontaneous myocardial ischemia, it remains a subject of controversy whether these are accompanied by simultaneous changes in blood pressure.
Methods. Using an ambulatory monitoring device that triggered blood pressure recordings from the level of the ST segment, we documented systolic and diastolic blood pressure and heart rate changes related to episodes of ST segment depression in 17 patients with stable coronary artery disease.
Results. Systolic blood pressure and heart rate, but not diastolic pressure, increased significantly before the onset of ST segment depression and persisted throughout the ischemic episode. There was a significant correlation between the changes in heart rate and systolic blood pressure during episodes of myocardial ischemia (r = 0.5, p = 0.0005) and between heart rate and systolic blood pressure changes at 1-mm ST segment depression during treadmill exercise testing and ambulatory monitoring (r = 0.73, p = 0.0005 for heart rate; r = 0.77, p = 0.0008 for systolic blood pressure), indicating that patients with a low heart rate threshold during ischemic episodes also had a lower systolic blood pressure threshold before ischemia during both tests. Circadian changes in systolic blood pressure paralleled the variations in heart rate and ischemic episodes, with the lowest values at night.
Conclusions. Significant increases in myocardial oxygen demand, including systolic blood pressure, occur during episodes of spontaneous myocardial ischemia. Patients with a lower heart rate threshold during ischemic episodes had a lower systolic blood pressure threshold during both ambulatory monitoring and treadmill exercise. The effects of antianginal therapy on blood pressure changes during ischemia need to be explored further.
Episodes of transient myocardial ischemia are often silent and therefore elucidation of events preceding their onset has been difficult. Examination of time periods prior to the development of ischemia demonstrates the importance of increases in heart rate preceding the onset of a majority, but not all episodes. Based on heart rate analysis it may be concluded that ischemia is precipitated primarily by increases in myocardial oxygen demand [1–6], but in a small number of episodes, reduction in blood flow due to coronary vasoconstriction may be the precipitating event [7–9]. However, myocardial oxygen demand is determined not only by alterations in heart rate, but also by changes in blood pressure and contractility. Thus, it is possible that episodes not associated with increases in heart rate may be accompanied by significant increases in blood pressure, or vice versa.
Studies examining changes in blood pressure in relation to onset of spontaneous ischemic episodes during normal daily activities are sparse [4–10]. Deedwania et al. performed frequent ambulatory blood pressure recordings in patients with multiple episodes of transient ST segment depression and reported increases in blood pressure before the onset of ischemia in the majority of patients . However, blood pressure recordings in that study were random and not triggered by the ischemic episodes. Using a device that is capable of triggering blood pressure recordings from changes in the ST segment, we investigated the relation between alterations in arterial blood pressure to fluctuations in the heart rate and to the onset of myocardial ischemia during normal daily activities.
Thirty-eight patients (34 men, 4 women, mean age 62 ± 10 years) with angiographically proven coronary artery disease (9 with single-vessel disease, 18 with two-vessel disease and 11 with three-vessel disease), a positive treadmill exercise test (≥1 mm ST segment depression) and stable symptoms underwent ambulatory ST segment monitoring. Of these, 17 patients (mean age 65 ± 9 years) had at least one episode of ST segment depression during monitoring. Two patients had single-vessel disease, 9 had two-vessel disease and 6 had three-vessel disease. Rest left ventricular ejection fraction was 56.3 ± 12%, and all patients including two women had abnormal thallium stress tests. None had a recent (<3 months) myocardial infarction or unstable angina. Patients did not have left bundle branch block and were not taking digoxin or diuretic agents. All investigations were performed after withdrawal of antianginal medications for at least 48 h, including beta-blockers in 18 patients, and nitroglycerin was available for pain. The study was approved by the Ethical Committee of the National Heart, Lung and Blood Institute.
1.2 Ambulatory ST Segment and Blood Pressure Monitoring
Patients underwent between 18 and 48 h of ambulatory ST segment monitoring (total 463 h) using a solid-state recorder (Stuart Medical Inc., Smartlink ABP 310) that was programmed to trigger blood pressure cuff measurements when the ST segment changed from 0.5 to 0.8 mm from its rest level. The device was also programmed to obtain blood pressure readings after 3-min intervals during the episode of ST segment depression for a total of 15 min and up to a maximum of between 4 and 6 readings per hour. In addition, the monitor was programmed to trigger blood pressure readings at preset time points, which included 30-min intervals during the day and 1-h intervals during the night. The device was approved by the Food and Drug Administration for measurements of blood pressure, arrhythmias and ST segments. We also ensured reproducibility of blood pressure recordings obtained by the device with manual blood pressure measurements.
Two-channel electrocardiograms (ECGs) were obtained using leads CM5and either modified lead II or CM3. The choice of the second lead was based on the location of maximal ST segment depression on the treadmill exercise test. Patients were instructed to be mobile, carry out normal daily activities and keep a detailed diary of symptoms, nitroglycerin consumption and their activities. An ischemic episode was defined as ≥1 mm planar or downsloping ST segment depression 0.06 to 0.08 s after the J point lasting for ≥1 min. Return of the ST segment level to baseline for 2 to 3 min was required between two episodes. At the time of hook-up, patients were tested for any significant ST segment changes related to position, and they were excluded if there was a positional change of >0.5 mm in the ST segment.
1.3 Data Analysis
All Holter recordings were analyzed at 60 times the normal speed, and segments with ST segment depression where blood pressure recordings were obtained were printed out in real time. When available, blood pressure and heart rate measurements were recorded before the episode at 1 to 4 h, 20 to 59 min, 10 to 19 min, 5 to 9 min and <5 min, and during the episode at <5 min, 5 to 10 min, 11 to 20 min, 21 to 40 min and >40 min after its onset, as well as after the end of the episode of ST segment depression.
1.4 Treadmill Exercise Testing
Treadmill exercise testing was performed using the combined National Institutes of Health (NIH) protocol in 16 patients and the standard Bruce protocol in one patient. The combined NIH protocol employs slow increases in work load, allowing for accurate estimation of heart rate changes at the onset of ECG changes . Patients exercised until at least one of the following end points was achieved: severe chest pain, ST segment depression ≥4 mm, ≥20 mm Hg decline in systolic blood pressure, ventricular arrhythmias, extreme fatigue or shortness of breath. Twelve-lead ECGs were obtained at rest and at every minute during exercise. Lead aVR was replaced by lead CM5. Test results were considered positive for ischemia when planar or downsloping ST segment depression of ≥1 mm at 0.06 s after the J point was observed. Blood pressure was measured using a cuff sphygmomanometer every 3 min. For the purposes of correlating the findings of ambulatory ECG monitoring and exercise testing, the exercise ischemic threshold was measured as the heart rate and blood pressure at 1) the onset of ST segment depression (at a time when the ST segments began to depress from baseline by 0.25 mm in lead CM5, as during Holter monitoring); and 2) at the time of development of 1-mm ST segment depression in lead CM5.
1.5 Statistical Analysis
Mean values during exercise testing and ambulatory monitoring were compared using the Student ttest. Repeated measures analysis of variance was performed to investigate whether there was a significant difference between changes in heart rate, systolic and diastolic blood pressures and rate–pressure product before, during and after termination of episodes of ischemia. The relation between variables during exercise testing and ambulatory monitoring was compared using the Pearson correlation coefficient. A p value of <0.05 was considered significant. All data are expressed as mean ± SD in the text and SEM in the figures.
Of the 38 patients with positive treadmill exercise tests who underwent ambulatory monitoring, 17 had at least one episode of ≥1 mm ST segment depression after 463 h of monitoring (mean 27.2 ± 15 hours/patient). A total of 64 episodes—55 (86%) silent and 9 (14%) symptomatic—were recorded (mean 3.76 ± 3.3 per patient, mean duration 42.9 ± 20 min).
2.1 Blood Pressure Changes During Myocardial Ischemic Episodes
Blood pressure changes surrounding the episodes of ischemia were averaged whenever available, and the mean changes during the 64 episodes are shown in Fig. 1. There was no significant change in the systolic or diastolic blood pressure between a period 4 h before the onset of ischemia and a period 10 min before the onset of ST segment depression. The mean of all recordings during this period was taken as the baseline measurement for comparison with events during ischemia. In the period 10 min before the onset of myocardial ischemia, when patients with 20 episodes had measurements of blood pressure, there was a significant increase in both the systolic blood pressure and rate–pressure product, but not in the diastolic blood pressure (Fig. 1); systolic blood pressure increased by 8.4% (p = 0.03) and the rate–pressure product by 13.7% (p = 0.044). Systolic blood pressure was higher in 70% of the episodes. Within 5 min after the onset of ischemia (mean 2.3 ± 1.4 min), when the ST segments had depressed by a mean of 0.8 ± 0.4 mm below baseline, the increases in systolic blood pressure (13%, p < 0.001) and the rate–pressure product (36.4%, p < 0.001) were more prominent. Diastolic blood pressure remained unchanged (p = 0.8 by analysis of variance), and 85% of episodes were accompanied by an increase in systolic blood pressure compared with baseline. This increase in systolic blood pressure persisted throughout the period of ST segment depression, achieving a 17.3% increase (p < 0.001) when the ST segment depressed by 1 mm a mean of 13.3 ± 16.5 min after the onset of ST segment depression. Systolic blood pressure and the rate–pressure product returned to preischemic levels after the end of ST segment depression (Fig. 1).
2.2 Relation Between Heart Rate and Systolic Blood Pressure Changes During Spontaneous Ischemic Episodes
Measurements of heart rate and blood pressure were made in 15 patients with 44 episodes at or around the time of development of 1-mm ST segment depression. There was a correlation between the percent increase in heart rate and percent increase in systolic blood pressure at 1-mm ST segment depression compared with baseline (r = 0.5, p = 0.0005), suggesting that increases in blood pressure paralleled increases in heart rate during episodes of myocardial ischemia (Fig. 2).
We divided episodes of ischemia into those in which 1-mm ST segment depression occurred at low heart rates (≤90 beats/min, n = 19, 43%) and others in which it occurred at high heart rates (>90 beats/min, n = 25, 57%), and we examined the changes in blood pressure in these two subsets (Fig. 3). Eight patients had only low heart rate episodes, three had only high heart rate episodes, and the remaining six had both types. Rest preischemic heart rates were significantly lower in patients in whom 1-mm ST segment depression occurred at low heart rates (73 ± 8 vs. 85 ± 9 beats/min, p < 0.001). There was a significant increase in heart rate before the onset of ST segment depression and at the development of 1-mm ST segment depression in both groups; however, the increase was significantly lower in the low heart rate episodes (Fig. 3). Preischemic systolic and diastolic blood pressures were similar in both groups, and systolic blood pressure increased at the onset of and at the development of 1-mm ST segment depression in both subsets; however, like heart rate increases, there was a strong trend toward a greater increase in systolic blood pressure during episodes with ischemia at high heart rates (p = 0.07). Thus, the rate–pressure product at 1-mm ST segment depression was 12.7 ± 2.5 mm Hg·beats/min·103in those with low heart rate episodes and 18.4 ± 3.3 mm Hg·beats/min·103(p < 0.001) in those with ischemia at high heart rates. One-mm ST segment depression occurred after 7.4 ± 5.3 min in those with low heart rate ischemia and at 17.8 ± 20.5 min (p < 0.04) in those with high heart rate ischemia. These findings indicate that episodes of ischemia that occur at low heart rates also have a low systolic blood pressure threshold, but there is an increase in myocardial oxygen demand before the onset of both low and high heart rate episodes.
Eight episodes were not accompanied by any diary entries. Of the remaining episodes, only two high heart rate episodes were accompanied by nitroglycerin consumption. Twenty-one percent of low heart rate and 18% of high heart rate episodes occurred at times when there apparently was no significant physical activity according to patient diaries. Only one high heart rate episode occurred with mental stress, and the remaining episodes were related to periods of mild to moderate physical exertion, which included eating, talking, driving and walking.
2.3 Comparison of Hemodynamic Changes Accompanying Ambulatory Ischemic Episodes and Exercise-Induced Ischemia
Fig. 4demonstrates blood pressure changes at the development of 1-mm ST segment depression during spontaneous ischemic episodes and during exercise testing. The mean heart rate and systolic and diastolic blood pressures before (>10 min) and at 1-mm ST segment depression were calculated for each patient during ambulatory ischemic episodes and compared with hemodynamic measurements at rest and after 1-mm ST segment depression during treadmill exercise. There was a significant correlation between the heart rate (r = 0.73, p = 0.007), systolic blood pressure (r = 0.77, p = 0.0008) and rate–pressure product (r = 0.81, p = 0.002) at 1-mm ST segment depression during exercise testing and ambulatory monitoring, indicating that patients who developed ST segment depression during exercise testing at a lower work load also developed spontaneous ischemia at a lower work load, and vice versa (Fig. 4). The magnitude of increase in systolic blood pressure was similar during exercise testing and ambulatory monitoring, when the changes were compared at the onset of 1-mm ST segment depression (14 ± 18% vs. 14.9 ± 15%, respectively). However, the increase in heart rate at 1-mm ST segment depression during exercise testing was greater than the increase during ambulatory monitoring (44 ± 15% vs. 18 ± 11%, p < 0.001).
2.4 Comparison of Hemodynamic Changes in Silent and Symptomatic Ischemia Episodes
The magnitude of increase in the rate–pressure product before the development of 1-mm ST segment depression was not significantly different between painful (n = 9) and silent (n = 55) episodes (40.6 ± 30% vs. 50 ± 51%, p = NS).
2.5 Circadian Variation in Ischemia and Hemodynamic Changes
As described previously, spontaneous episodes of myocardial ischemia were more frequent during the day compared with the night (Fig. 5). Similarly, hourly heart rates were higher during the day and were significantly lower at night. Systolic blood pressure but not diastolic blood pressure changes paralleled changes in heart rate and ischemia (Fig. 5). We compared the circadian changes in heart rate and blood pressure in the 21 patients who had ambulatory blood pressure monitoring but did not have ischemic episodes with those of 17 patients who had episodes (Fig. 5). There were no significant differences in the pattern of change in heart rate and blood pressure in the two groups.
The major findings of this studyare that 1) significant increases in heart rate and systolic but not diastolic blood pressure accompany the onset of a majority of episodes of transient myocardial ischemia in patients with stable coronary artery disease; 2) there is a relation between heart rate and systolic blood pressure changes that accompany ischemic episodes, such that episodes with smaller increases in heart rate have smaller increases in systolic blood pressure; 3) patients with smaller increases in heart rate and blood pressure during spontaneous episodes also have a lower heart rate and blood pressure threshold to ischemia during exercise testing; 4) not only does the circadian variation in heart rate parallel the prevalence of transient ischemia as reported previously, but there is also a similar and parallel diurnal variation in systolic blood pressure with a peak in the daytime hours and a trough at night.
3.1 Blood Pressure Changes with Myocardial Ischemia
There has been an active debate regarding the pathophysiologic findings during episodes of transient myocardial ischemia. Although some studies have shown no changes in heart rate before the onset of myocardial ischemia during ambulatory monitoring [7–9], in recent years, results from our center and other investigators have convincingly demonstrated increases in heart rate before the onset of ST segment depression in the majority of patients [1–6]. The increase in myocardial oxygen demand preceding these episodes is believed to contribute to the precipitation of ischemia, whereas episodes without significant increases in heart rate are believed to be due to vasoconstriction-induced reductions in coronary blood flow. In an investigation where frequent recordings of ambulatory blood pressure were obtained in patients with episodes of ST segment depression, Deedwania et al. demonstrated an increase in systolic blood pressure in the majority of episodes where blood pressure recordings were obtained by chance around the onset of ischemia. In the current investigation, we used an ambulatory monitoring device that triggered blood pressure recordings immediately after the onset of ST segment depression. Using this technique, we were able to record 69 blood pressure recordings in 64 episodes within a 10-min period around the onset of ST segment depression and demonstrated that systolic but not diastolic blood pressure increases occurred before the onset of ST segment depression in the majority of episodes. Within 2.5 min of the onset of ST segment depression, when the ST segment level was a mean 0.8 mm below baseline, the rate–pressure product was higher in 91% of the episodes.
A lack of increase in systolic blood pressure and rate–pressure product observed in a minority of episodes may be due to several reasons. One possibility is that these episodes were precipitated primarily by coronary vasoconstriction [12–14]. Another likelihood is that the heart rate and blood pressure readings used as baseline and obtained during the 4-hour period before ischemia were not at a rest level and were elevated due to daily activities. The latter possibility may be an important reason for an apparent lack of increase in rate–pressure product in some episodes, as demonstrated from the fact that the pre-episoderate–pressure product in patients with no apparent increase before the onset of episodes was higher than that in the same patients before the other episodes where the rate–pressure product increased before ST segment depression. Analysis of the patients’ diaries revealed that at the time when the pre-episode data were obtained, patients were either walking or eating. Finally, the strong correlations between exercise-induced changes in heart rate and systolic blood pressure and the changes observed during spontaneous ischemic episodes suggest that patients with little or no change in rate–pressure product may in fact have a very low threshold for developing ischemia, even during exercise. None of the patients had a fall in rate–pressure product that persisted throughout the episode, a finding that has been reported in a minority of episodes previously .
3.2 Heart Rate Changes Accompanying Spontaneous Ischemic Episodes
Our results are consistent with previous studies demonstrating that the majority of episodes have an increase in heart rate before the onset of myocardial ischemia [1–6, 10, 15]. We have previously reported on the close correlation between the frequency of myocardial ischemia and the number of times the heart rate increased above the exercise ischemic threshold , and also that the variability in myocardial ischemia during a 24-h period and over longer periods correlated with a corresponding variation in exercise ischemic threshold [16–19]. Examination of episodes where myocardial ischemia appeared to occur at low heart rates in this study revealed that there were significant increases in both heart rate and systolic blood pressure before ischemia in these episodes, even though the magnitude of increase was often lower.
3.3 Hemodynamic Changes During Spontaneous Ischemia Compared with Exercise Testing
When heart rate and systolic blood pressure changes during exercise and ambulatory monitoring were compared, it was apparent that there was a close correlation between these variables, such that patients with a low ischemic threshold during treadmill exercise had a lower heart rate and systolic blood pressure threshold during ambulatory monitoring. The strength of the correlations indicates that increases in myocardial oxygen demand, a primary stimulus during exercise-induced ischemia, is also important as a precipitating factor in the majority of spontaneous ischemic episodes.
3.4 Pathophysiologic Findings of Spontaneous Myocardial Ischemia
Although our observations demonstrating heart rate and blood pressure increases at the onset of spontaneous ischemia and the close correlation between ischemic threshold during exercise testing and ambulatory ischemia strongly indicate a crucial role for increases in myocardial oxygen demand in precipitating spontaneous ischemia, it should be emphasized that these findings do not completely rule out the contribution of coronary vasoconstriction in transient myocardial ischemia. That coronary vasoconstrictor tone contributes to myocardial ischemia is suggested by several findings: 1) 9% of episodes did not have an increase in rate–pressure product at the onset of ischemia; and 2) there was a variation in the rate–pressure product at the onset of 1-mm ST segment depression during different episodes in the same patient, suggesting a role for dynamic variation in coronary vascular tone as a contributing factor to the variation in ischemic threshold, a finding also noted in previous studies [17, 20, 21]. Similarly, the circadian variation in ischemic frequency can be explained not merely by the circadian variation in determinants of myocardial oxygen demand, such as heart rate and blood pressure, but also by a circadian variation in coronary and peripheral vascular tone [17, 18], which may, at least partly, be dependent on the diurnal variation in sympathetic nervous system tone .
3.5 Study Limitations
Relatively small number of patients and episodes were analyzed in this study; therefore, the conclusions cannot be extrapolated to all patients with myocardial ischemia, and larger studies need to be conducted to discover variations in the pattern of blood pressure changes with ischemia.
This investigation establishes the role of systolic blood pressure changes during episodes of spontaneous myocardial ischemia. The effect of antianginal therapy on blood pressure changes related to ischemic episodes needs to be investigated further.
We are grateful to William B. Schenke, BS for help with statistical analysis and graphics.
- Received December 5, 1996.
- Revision received July 7, 1997.
- Accepted July 11, 1997.
- The American College of Cardiology
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