Lack of diurnal variation in maximal symptom-limited exercise test response in chronic stable angina

QT restitution properties of middle-aged women with different exercise capacities

BACKGROUND

Framingham Study data indicate an incremental risk of cardiovascular death in relation to low exercise capacity in women. The reason for death is still not clear.

METHODS

QT restitution properties in 80 middle-aged women were investigated to confirm whether the cardiac restitution property was affected by the exercise capacity. Exercise tests were performed according to the Bruce protocol. Seventy cases were divided into the low-exercise capacity group (LEC group) and high-exercise capacity group (HEC group) by median exercise capacities. Sequential QT intervals and their preceding TQ intervals were measured, and the QT restitution curve (QTRC) was constructed. Two exponential equations were used to match the data and calculate the maximum slope (Smax) of QTRC, respectively.

RESULTS

With elevation in the exercise level, the women in the LEC group had a higher change rate of QT intervals (41 ± 10 vs 29 ± 3 ms/Met, P = .001) and TQ intervals (46 ± 12 vs 41 ± 7 ms/Met, P = .046); and the ratio of QT interval alterations to TQ interval alterations increased (0.41 ± 0.09 vs 0.36 ± 0.07, P = .003). The Smax of the QTRC in the LEC group was higher than that in the HEC group (1.43 ± 0.44 vs 1.13 ± 0.34, P = .002). There was an inverse relationship between Smax and exercise capacity (r = -0.43, P = .001).

CONCLUSION

Middle-aged women with low exercise capacity have steeper QTRCs than those with high exercise capacity, denoting a more unstable alternation of QT interval with elevation in exercise level.

Circadian variations in ischemic threshold and their relation to the occurrence of ischemic episodes

BACKGROUND

The occurrence of ischemic episodes during daily activity has been reported to exhibit a bimodal circadian distribution, yet its relation to the ischemic threshold (heart rate at 1-mm ST segment depression) has not been explored.

METHODS AND RESULTS

To determine whether the ischemic threshold during daily activity exhibits a circadian pattern that might relate to the frequency of occurrence of ischemic episodes, we studied the time of occurrence and the heart rate at onset of ischemia in 1,371 ischemic episodes recorded in 41 patients with stable coronary disease, positive exercise testing, and repeated ischemic episodes during ambulatory ECG monitoring (AEM). All patients had 7 days of AEM; 23 were off any anti-ischemic therapy, while 18 were on low dose of beta-blockers. The occurrence of ischemic episodes exhibited the typical bimodal circadian distribution with a prominent peak between 7:00 and 11:00 AM and a second less prominent peak between 6:00 and 9:00 PM. The threshold of myocardial ischemia exhibited a different single-peaked circadian distribution; it was lowest between 1:00 and 3:00 AM and highest between 10 AM and 1 PM. Time series analyses indicated a strong hour-by-hour trend of each of the two circadian distributions, whereas the two series cross-correlated maximally at a lag of zero hours (p < 0.01), indicating a complex interplay between myocardial oxygen demand and supply in determining the occurrence of ischemic episodes during daily activity. The morning increase in the frequency of ischemic episodes could not be attributed to a reduced threshold but rather to an increase in demand. The low threshold at night-time might probably indicate that the mechanism of ischemia during these hours is reduced coronary flow due to increased coronary tone. Secondary analyses for several predefined patients' subsets gave similar results. Patients who received low-dose beta-blockers maintained the bimodal circadian distribution of the occurrence of ischemic episodes, whereas the ischemic threshold exhibited a constant pattern with no circadian changes.

CONCLUSIONS

Our results demonstrate that myocardial oxygen demand is a major determinant of daily ischemia, yet changes in the ischemic threshold that probably reflect dynamic changes in coronary tone play also an important role. The relative contribution of increased demand and decreased threshold to the genesis of ischemic episodes during daily activity can be assessed by AEM and may help to optimize medical therapy.

24-hour anti-ischaemic action with once daily nifedipine. Experience obtained with a fatty-alcohol matrix tablet in patients with coronary artery disease

The ability of a fatty-alcohol matrix, slow-release tablet of nifedipine 60 mg to maintain a 24-hour anti-ischaemic action in the fixed dose of 60 mg once daily has been investigated in a randomised, placebo-controlled, double-blind trial. 12 normotensive patients with angiographically proven coronary artery disease (stenosis of at least one major vessel > or = 70%) were studied. The anti-ischaemic response was assessed over a period of 4 days as changes in the exercise-induced ST-segment depression 6 h and 24 h post-dose, and ST-segment changes in 24-h ambulatory ECGs. A measurable anti-ischaemic response was observed in 8 of the 12 patients. Exercise-induced ST-segment depression 6 h after the administration of nifedipine was reduced by 30% compared to placebo, and there was still a measurable anti-ischaemic response 24-h post-dosing. Both responses were independent of changes in exercise blood pressure. In 7 patients with ischaemic episodes in the 24-h ECGs, nifedipine treatment had only a minor effect on the intensity and duration of ischaemia. It is concluded that a significant anti-ischaemic effect lasting 24 h could be demonstrated using effort-induced ST-segment changes in patients with angiographically proven coronary heart disease, who were treated once daily with nifedipine 60 mg as a fatty-alcohol slow release tablet.

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

BACKGROUND

There is a circadian pattern in the occurrence of cardiac events in patients with coronary artery disease. Whether changes in coronary vascular tone contribute to these phenomena is unknown. We measured the ischemic threshold, defined as either the heart rate or rate-pressure product at 1-mm ST segment depression during treadmill exercise and used it as an index of the lowest coronary vascular resistance; the premise was that when ischemic threshold became lower, coronary vascular resistance was higher, and vice versa.

METHODS AND RESULTS

Fifteen patients (group A) with stable coronary artery disease underwent four identical treadmill exercise tests in 24 hours, and ischemic threshold was measured as the heart rate at the onset of 1-mm ST depression. Before each treadmill test, postischemic forearm vascular resistance was measured after 5 minutes of forearm occlusion, using strain-gauge plethysmography. Sixteen additional patients (group B) underwent two treadmill tests at 8 AM and 1 PM, and ischemic threshold was measured as the heart rate-blood pressure product at 1-mm ST depression. A circadian variation was noted: In group A, the heart rate-derived ischemic threshold was lower at 8 AM and 9 PM compared with noon and 5 PM (p less than 0.03). Also, in group B, the rate-pressure product-derived ischemic threshold was 8 +/- 2% lower at 8 AM compared with 1 PM (p = 0.008). A circadian variation parallel to the observed variation in ischemic threshold was also noted in the postischemic forearm blood flow, which was lower in the morning and at night (p less than 0.004). There was a strong correlation between postischemic forearm blood flow and ischemic threshold (p less than 0.0001), such that ischemic threshold was lower at the time of day when postischemic forearm blood flow was lower, and vice versa.

CONCLUSIONS

A lower ischemic threshold in the morning suggests that the ischemia-induced coronary vascular resistance is increased at this time, a finding supported by a similar variation in postischemic forearm vascular resistance. Parallel changes in forearm and coronary resistance suggest that generalized (neural or humoral factors) rather than local factors are responsible for the observed circadian changes. Increased coronary tone in the mornings may not only contribute to the higher incidence of transient ischemia but may help trigger acute cardiac events at this time.

Chronotherapy in coronary heart disease: comparison of two nitrate treatments

The purpose of this study was to assess the ischemic burden and the hemodynamic changes during daily activities in patients with coronary heart disease. Three exercise tests were performed during the day (10:00 a.m., 2:00 p.m., 6:00 p.m.), recording ST-segment depression, pulmonary artery pressure, pulmonary wedge pressure, and cardiac output as well as heart rate and systemic blood pressure during placebo and nitrate therapy. With placebo as well as nitrate therapy there was a gradual increase of ischemia and preload and a decrease of cardiac output during the day. High nitrate concentrations led to a significant reduction of both preload and ST depression with a marked circadian phase dependency of cardiovascular effects.

Changes in myocardial ischemic threshold during daily activities

This study assesses the variations in myocardial ischemic threshold (heart rate at the onset of ischemia) during daily activities in patients with ischemic episodes on Holter monitoring. Eighty patients with known coronary artery disease, positive treadmill stress test results and greater than or equal to 2 ischemic episodes during a 24-hour period of Holter monitoring were studied. The lowest and the highest ischemic thresholds were determined for each patient. The mean lowest ischemic threshold was 85 beats/min, and the mean highest ischemic threshold was 109 beats/min. The highest ischemic threshold was identical to ischemic threshold values noted during exercise. Of the 895 ischemic episodes, 654 (74%) were preceded by a moderate (greater than 10%) increase in heart rate. The variability of ischemic threshold (difference in percentage between the highest and lowest ischemic thresholds) increased with the number of ischemic episodes (range 2 to 60%). However, in different patients with a similar number of ischemic episodes, different variability was observed. These differences in ischemic thresholds are probably indirect indicators of the vasomotor activity of the coronary arteries in different patients.

Interval treatment of coronary artery disease with sustained-release isosorbide-5-mononitrate

The possibility of maintaining preload reduction and enhancement of exercise tolerance during an interval treatment with 50 mg/day of sustained-release isosorbide-5-nitrate (IS-5-N) was investigated in 13 patients (aged 54.4 +/- 7.9 years [mean +/- standard deviation]) with angiographically confirmed coronary artery disease and chronic stable angina pectoris. The effects of a single dose (acute test) were compared with those following an 8-day (chronic) regimen of mononitrate administration. Two hours after administration of 50-mg sustained-release IS-5-N, mean resting pulmonary arterial pressure (PAP), measured with a Swan-Ganz catheter, was reduced by 27% (p less than 0.01), and at submaximal exercise level (50 W, 3 minutes) by 34% (p less than 0.001). At individually highest comparable work loads mean PAP was reduced by 28% (p less than 0.01), and at maximal work load the PAP reduction was 21% (p less than 0.05). At the end of 1 week of therapy with sustained-release IS-5-N a slight, clinically irrelevant reduction of hemodynamic effects was recorded. Work capacity increased after 1 hour by 60% (408 +/- 104 vs 653 +/- 147 W x min, p less than 0.001)--still significantly above baseline 10 hours after nitrate administration. No difference from baseline was demonstrable 24 hours after medication. During interval therapy the improved work capacity was fully maintained (chronic, 1 hour: 417 +/- 93 vs 679 +/- 160 W x min, p less than 0.001). During interval therapy with sustained-release IS-5-N, hemodynamics and exercise tolerance were durably improved.(ABSTRACT TRUNCATED AT 250 WORDS)

Lack of diurnal variation in the onset of non-Q wave infarction

Data concerning the time of onset of myocardial infarction were obtained for 540 of the 544 patients with creatinine kinase (CK)-MB-confirmed non-Q wave myocardial infarction enrolled in the multicenter Diltiazem Reinfarction Study. Data were also collected for 627 patients who were screened but excluded. Among the 1,167 patients, no diurnal pattern of onset could be found at either 2- or 6-hour intervals. Among the 540 patients enrolled in the trial, no pattern could be found at these intervals either, although at 8-hour intervals, 27% of infarctions occurred between midnight and 8:00 AM, compared with 37% between 8:00 AM and 4:00 PM and 36% between 4:00 PM and 12:00 AM (p = 0.02). In contrast to the patterns previously noted for Q wave myocardial infarction, there was no preponderance of non-Q wave infarction in the late morning. Circadian rhythm was also absent among patients not treated with beta-blockers as well as among patients presenting with ST segment elevation on their enrollment electrocardiograms. Diabetics, women, and patients with first infarction were more likely to present during the afternoon hours. We conclude that the late morning preponderance seen for Q-wave myocardial infarction is not discernable in patients with non-Q wave myocardial infarction. This observation suggests that the pathogenesis of these two infarct subtypes is different or that the process of thrombotic coronary occlusion in Q wave infarction (sustained) differs from that in non-Q wave infarction (nonsustained).

Effects of glutamate on exercise tolerance and circulating substrate levels in stable angina pectoris

The effects of glutamate on exercise tolerance, ischemic threshold and venous substrate concentrations were studied in 20 patients with stable angina pectoris and positive stress tests. Each patient underwent 4 upright bicycle exercise tests on consecutive days. The first and fourth tests were performed without medication while the second and third tests were preceded by a low and high bolus dose of monosodium glutamate, either 0.8 and 1.5 mg/kg body weight intravenously (10 patients) or 40 and 80 mg/kg orally (10 patients). Comparison of the first and fourth tests revealed good reproducibility of electrocardiographic, hemodynamic and metabolic data. Glutamate increased exercise duration (p less than 0.05) in a dose-related way when given intravenously (59 +/- 14 and 153 +/- 14 seconds) and when given orally (53 +/- 21 and 90 +/- 23 seconds; all data are mean +/- standard error of the mean). It also delayed the onset of ST-segment depression (p less than 0.05) by 73 +/- 19, 120 +/- 23, 62 +/- 27 and 80 +/- 30 seconds, respectively. Hemodynamics were not changed by glutamate at rest or at comparable workloads, but at onset of ST-segment depression the heart rate-blood pressure product was increased (p less than 0.05). Glutamate administration induced dose-related 1.5- to 10-fold elevations in plasma glutamate, 15 to 50% decreases in plasma free fatty acids (p less than 0.05) and 5 to 30% increases in plasma alanine contents. Circulating levels of glucose, lactate, citrate and albumin were not modified by glutamate.(ABSTRACT TRUNCATED AT 250 WORDS)