Variability of transient myocardial ischemia in ambulatory patients with coronary artery disease

Ambulatory electrocardiographic (ECG) monitoring of patients with chronic stable angina has demonstrated frequent and prolonged episodes of ischemic ST segment depression, but its clinical use requires an understanding of the components and extent of variability. Therefore, variations in the frequency and duration of episodes of ST segment depression were evaluated with ambulatory ECG recording at daily, weekly, and monthly intervals in 42 patients with chronic stable angina and known coronary artery disease. Data were analyzed with a nested analysis of variance design that yields estimates of variance components. From the estimates of variance components, power calculations and minimum significant percent reductions in frequency and duration of ischemia were derived. During 4,656 hours of ambulatory ECG monitoring, 1,262 episodes of ischemic ST segment depression were detected. The frequency of episodes was 6.3 +/- 0.45/24 hr (mean +/- SEM), and the duration of episodes was 18.3 +/- 2.8/24 hr. Because of variability over time, the ability to detect significant changes was dependent upon the number of subjects, length of monitoring period, and intervals between monitoring periods. In a clinical trial, for example, a sample size of 25 patients monitored for 48 hours with 1 week between control and test conditions would require a 65% reduction in frequency, whereas a sample size of 50 patients monitored under similar conditions would require a 46% reduction in frequency, to attribute the change with 90% power to a therapeutic intervention rather than to a spontaneous variation. When monitoring a single patient for 48 hours with 1 week or 1 month between control and repeat monitoring sessions, episodes of ischemic ST depression must be eliminated to detect significant therapeutic changes in ischemic activity at the 95% confidence level.(ABSTRACT TRUNCATED AT 250 WORDS)

Responsiveness of atrial natriuretic factor to reduction in right atrial pressure in patients with chronic congestive heart failure

In patients with congestive heart failure, atrial natriuretic factor may serve as a counter-regulatory hormone, offsetting the vasoconstrictive and volume-retentive effects of the sympathetic nervous system, the renin-angiotensin-aldosterone system and vasopressin. Indeed, the plasma levels of atrial natriuretic factor and the vasoconstrictor hormones are often simultaneously elevated in these patients. It is not known, however, whether atrial natriuretic factor remains responsive to sudden reductions in atrial pressure in patients with chronic heart failure, or is unresponsive like the vasoconstrictor systems. To examine this issue, the plasma concentrations of atrial natriuretic factor and the vasoconstrictor hormones were measured in 20 normal subjects and 12 patients with chronic congestive heart failure during incremental lower body negative pressure, an intervention that lowers atrial pressure. In the normal subjects, incremental lower body negative pressure at -10, -20 and -40 mm Hg decreased central venous pressure and pulse pressure. At maximal lower body negative pressure, plasma atrial natriuretic factor levels decreased from 51 +/- 5 to 27 +/- 3 pg/ml (p less than 0.01), whereas increases occurred in plasma levels of norepinephrine (194 +/- 11 to 385 +/- 70 pg/ml, p less than 0.01), renin activity (1.4 +/- 0.2 to 3.9 +/- 0.1 ng/ml per h, p less than 0.01) and vasopressin (1.3 +/- 0.1 to 6.4 +/- 2.4 pg/ml, p less than 0.05). In the patients with congestive heart failure, lower body negative pressure also reduced central venous pressure. Baseline plasma atrial natriuretic factor levels were markedly elevated, averaging 438 +/- 138 pg/ml, and decreased to 317 +/- 87 pg/ml at maximal lower body negative pressure (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Detection of pacing-induced myocardial ischemia by endocardial electrograms recorded during cardiac catheterization

Rapid atrial pacing confirms myocardial ischemia in patients with coronary artery disease when angina is provoked, and is accompanied by an increase in left ventricular end-diastolic pressure. In such cases, abnormalities in the surface electrocardiogram (ECG) are often not apparent. To enhance detection of subendocardial ischemia during rapid atrial pacing, local unipolar electrograms were recorded from the tip of a 0.025 in. (0.064 cm) diameter guidewire positioned against the endocardial surface of potentially ischemic regions. Endocardial electrograms, left ventricular end-diastolic pressure and multiple surface ECG leads were recorded during rapid atrial pacing in 21 patients with coronary artery disease. Before pacing, endocardial electrograms in all 21 patients were free of ST elevation. Marked ST elevation was apparent in 17 of the 21 patients after rapid atrial pacing and could be abolished by nitroglycerin. Moreover, in several patients, endocardial ST elevation after rapid atrial pacing was abolished after successful percutaneous transluminal coronary angioplasty of the critically stenosed artery supplying the ischemic region of myocardium. It is concluded that ST elevation in the endocardial electrogram after rapid atrial pacing is a reflection of myocardial ischemia and may be a sensitive marker of pacing-induced ischemia appearing earlier than angina, postpacing increase in left ventricular end-diastolic pressure or ST depression in the surface ECG.

Frequency of ST-segment depression produced by mental stress in stable angina pectoris from coronary artery disease

Physical exertion is a well-documented trigger of transient myocardial ischemia in patients with coronary disease. More recently, studies have shown that mental stress may also be a cause of myocardial ischemia. The purpose of this study was to examine the relationship of physical activities and perceived mental states to myocardial ischemia while patients were going about their normal daily activities. Twenty-eight patients with documented coronary artery disease underwent ambulatory monitoring of the electrocardiogram. Physical activity and perceived mental status were recorded by patients in a diary which was then graded according to intensity of the activity. Analyses of the continuous electrocardiographic recordings were done separately from the analysis of the diaries. The time of each episode of ischemia, the duration of each episode in minutes and the number of episodes in each 24-hour period were calculated. A total of 372 episodes of ST-segment depression occurred in 912 hours of monitoring. Ischemic events occurring during usual physical and usual mental activities were most frequent (36%). Twenty-six percent of ischemic episodes occurred during increased physical activity, but usual mental activities. Interestingly, 22% of the ischemic events occurred at high levels of mental stress, but low physical activity. Ten percent of episodes occurred during sleep. Although the majority of events occurred during usual daily activities, when duration of ischemia was normalized for time spent in each category, increasing physical or mental activity was associated with an increasing duration of ischemia per unit (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Na+-Ca2+ exchange in cultured vascular smooth muscle cells

Vascular smooth muscle cells (VSMC) contract as intracellular free calcium ([Ca2+]i) rises. While Na+-Ca2+ exchange has been proposed to contribute to transmembrane Ca2+ flux, its role in cultured VSMC is unknown. Accordingly, we have investigated the role of Na+-Ca2+ exchange in unidirectional and net transmembrane Ca2+ fluxes in cultured rat aortic VSMC under basal conditions and following agonist-mediated stimulation. Transmembrane Ca2+ uptake was significantly increased in response to a low external Na+ concentration ([Na+]o) compared with 140 mM [Na+]o. Na+-dependent Ca2+ uptake in response to low [Na+]o was further increased by intracellular Na+ loading by preincubation of the VSMC with 1 mM ouabain. Under steady-state conditions, Ca2+ content varied inversely with [Na+]o, increasing from 1.0 nmol Ca2+/mg protein at 140 mM [Na+]o to 4.0 nmol Ca2+/mg protein at 20 mM [Na+]o. Increasing [K+]o to 55 mM also enhanced Na+-dependent Ca2+ influx. Augmentation of Ca2+ uptake with K+ depolarization was not significantly inhibited by the calcium channel antagonist verapamil. Transmembrane Ca2+ efflux was increased in response to 130 mM [Na+]o compared with zero [Na+]o (iso-osmotic substitution with choline+), and was further stimulated by the vasoconstrictor angiotensin II, which is known to elevate [Ca2+]i. These changes in [Ca2+]i were studied directly using fura-2 fluorescence measurements. Elevated [Ca2+]i levels returned to baseline more rapidly in the presence of normal (130 mM) [Na+]o compared with zero [Na+]o (iso-osmotic substitution with choline+). These findings suggest that a bidirectional Na+-Ca2+ exchange mechanism is present in cultured rat aortic VSMC.(ABSTRACT TRUNCATED AT 250 WORDS)

Responses of coronary arteries of cardiac transplant patients to acetylcholine

Accelerated coronary atherosclerosis is a major cause of graft failure after heart transplantation. Graft atherosclerosis is typically diffuse and difficult to detect even with coronary arteriography. Recently, acetylcholine was shown to dilate blood vessels by releasing a vasorelaxant substance from the endothelium (endothelium-derived relaxing factor). We have demonstrated paradoxical vasoconstriction induced by acetylcholine both early and late in the course of coronary atherosclerosis in patients, suggesting an association of endothelial dysfunction and atherosclerosis. In this report, we tested the hypothesis that coronary arteries of heart transplant patients can show endothelial dysfunction before or in the early stages of angiographically evident coronary atherosclerosis. Acetylcholine was infused into the left anterior descending artery of 13 heart transplant patients at 12 (n = 9) and 24 (n = 4) mo after transplantation. Vascular responses were evaluated by quantitative angiography. Among patients with angiographically smooth coronary arteries, relatively few (6/25) arterial segments had preserved vasodilator responses, while the majority failed to dilate (10/25) or paradoxically constricted (9/25). Angiographically irregular coronary arteries were present in three patients, in whom 8/10 segments showed marked paradoxical constriction and the remaining 2/10 failed to dilate. Only 1 of 13 patients retained appropriate dilation to acetylcholine in all segments. Nitroglycerin, which acts directly on vascular smooth muscle, dilated nearly all segments. No clinical features of the patients, including myocardial rejection appeared to correlate with the impaired functional response of vessels. Thus impaired response to acetylcholine is a common early finding in heart transplant patients and emphasizes the potential importance of endothelial dysfunction in the development of atherosclerosis.

Dilation of normal and constriction of atherosclerotic coronary arteries caused by the cold pressor test

Increased vascular constriction has been observed at the site of atherosclerotic lesions, suggesting an association between atherosclerosis and altered vascular tone. While atherosclerosis may increase sensitivity to exogenous vasoconstrictors, little is known about the response of normal and atherosclerotic coronary arteries to an exogenous stimulus that excites the sympathetic nervous system. Therefore, we studied the response to cold pressor test (CPT) using quantitative angiography and Doppler flow velocity measurements in eight patients with angiographically normal coronary arteries (group I), nine patients with mild coronary atherosclerosis (less than 50% diameter narrowing) (group II), and 13 patients with advanced coronary stenoses (greater than 50% diameter narrowing) (group III). In 31 segments of angiographically smooth arteries in group I, the CPT produced vasodilation from a control mean diameter of 2.68 +/- 0.09 (mean +/- SE) to 2.99 +/- 0.09 mm at peak CPT (p less than 0.001), a 12 +/- 1% increase in diameter. In group II, 27 irregular segments constricted to peak CPT from a mean control diameter of 1.82 +/- 0.12 to 1.66 +/- 0.12 mm (p less than .001), a 9 +/- 1% decrease, while 10 smooth segments dilated from a mean control diameter of 1.98 +/- 0.11 mm to 2.34 +/- 0.15 mm (p less than .01), a 19 +/- 2% increase in diameter. Likewise, in group III, the 17 stenotic segments constricted from 1.16 +/- 0.09 to 0.89 +/- 0.09 mm (p less than .001), a 24 +/- 6% decrease; the irregular segments also constricted from 2.44 +/- 0.11 to 2.22 +/- 0.12 mm (p = .002), a 10 +/- 2% decrease. In contrast, two smooth segments dilated from 2.98 to 3.23 mm (mean), an 8% increase in diameter. Coronary blood flow increased 65 +/- 4% (mean) during CPT in group I, it increased 15 +/- 6% in group II, and it decreased 39 +/- 8% in group III. The vasodilator response in four normal patients was partly inhibited by the administration of intracoronary propranolol (17 +/- 3% increase during control, 10 +/- 2% increase after propranolol, 41% less dilation; p = .002). We conclude that the response of normal coronary arteries to the CPT test is dilation, in part related to beta-adrenoreceptor stimulation and possibly flow-mediated endothelial dilation or alpha 2-adrenergic activity. The paradoxical vasoconstrictor response induced by atherosclerosis may represent altered catecholamine sensitivity and/or a defect in endothelial vasodilator function. The presence of atherosclerosis impairs vasodilator responses and thus may contribute to the pathogenesis of myocardial ischemia.

Ambulatory monitoring of the digitized electrocardiogram for detection and early warning of transient myocardial ischemia in angina pectoris

Patients with coronary artery disease have frequent asymptomatic episodes of ischemic ST-segment depression. A new solid-state recorder that analyzes the digitized electrocardiogram (ECG) in real time has been tested for accuracy in detection and quantitation of these events by comparison with a direct ECG during exercise testing and a frequency-modulated recorder during ambulatory monitoring. The device can also be programmed to produce a tone at the onset of ischemic ST-segment depression, and this tone is used to initiate self-administration of nitroglycerin in an attempt to control myocardial ischemic events. The solid-state recorder demonstrated adequate sensitivity and specificity during exercise (100% and 92%, respectively) for detection and quantitation of episodes of ischemic ST-segment depression. In addition, Monitor One accurately measured the duration of ischemic events during exercise testing (r = 0.95, p = 0.001) and the number (r = 0.92, p = 0.001) and duration (r = 0.97, p = 0.001) of ischemic events during ambulatory monitoring. When the audible tone was used to initiate use of nitroglycerin, the duration of episodes of ischemic ST-segment depression was reduced from 105 +/- 74 min/24 hours to 58 +/- 49 min/24 hours (p = 0.02). Thus, a solid-state ECG recorder capable of long-term monitoring appears to accurately detect and quantify episodes of ischemic ST-segment depression in patients with coronary artery disease. Also, patients can interact with an audible tone to key self-administration of therapy in an attempt to control asymptomatic episodes of transient myocardial ischemia.

Relationship of cardiac chamber volume to baroreflex activity in normal humans

The objectives of this study were to examine the effect of incremental lower body negative pressure (LBNP) on cardiac chamber volume and assess the relationship between cardiac chamber volume and baroreflex activation of the neurohormonal axis. Accordingly, echocardiographic determination of cardiac chamber volume and neurohormonal responses were studied in 14 normal subjects during incremental LBNP. LBNP -10 mm Hg decreased left atrial diameter and left ventricular systolic volume index, but did not alter heart rate, systolic or pulse pressure, or stroke volume. During LBNP -10 mm Hg, plasma norepinephrine levels increased, suggesting activation of the sympathetic nervous system. LBNP -40 mm Hg caused a significant decrease in left atrial diameter and left ventricular systolic, diastolic, and stroke volume indices. During LBNP -40 mm Hg, heart rate increased, and systolic and pulse pressure fell. With this more negative level of LBNP, norepinephrine, angiotensin II, aldosterone, and arginine vasopressin concentrations and PRA all increased. The findings that left atrial diameter decreased and plasma norepinephrine concentration increased during LBNP -10 mm Hg suggest that the sympathetic nervous system is sensitive to changes in atrial receptor activity. At higher levels of LBNP (-40 mm Hg), activation of the renin-angiotensin system and release of vasopressin were associated with a fall in left ventricular diastolic volume as well as a decrease in the pressure input to the arterial baroreceptors. Under this condition, the differential contribution of the cardiopulmonary and arterial baroreceptors to the regulation of the renin-angiotensin system and vasopressin release cannot be distinguished.

Characteristics and significance of ischemia detected by ambulatory electrocardiographic monitoring

Ambulatory electrocardiographic (ECG) monitoring of ischemia in patients with coronary artery disease (CAD) provides a new technique for the assessment of ischemic activity and the evaluation of therapies outside of the hospital. Numerous studies have demonstrated that the majority of patients with CAD have episodes of symptomatic and asymptomatic ST segment depression during routine daily activities. Rubidium-82 positron-emission tomographic studies have provided evidence for decreased myocardial perfusion during these episodes of ST segment depression. The prognostic importance of asymptomatic ischemia has been shown in patients with unstable angina to be a marker for early unfavorable cardiac events. Preliminary results suggest a poorer outcome for those patients with chronic stable angina who show episodes of ischemia as well. Ambulatory monitoring studies suggest that total ischemic activity may be underestimated by conventional testing. Whether all ischemic activity detected by ambulatory monitoring requires treatment awaits further study.

Circadian rhythms and coronary artery disease

Circadian variations have long been observed in several metabolic functions, many of which are directly or indirectly related to the cardiovascular system and its pathophysiology. Recent reports linking circadian patterns to the development of symptomatic and asymptomatic myocardial ischemia, sudden cardiac death and myocardial infarction raise important questions concerning the mechanisms of myocardial ischemic activity in patients with coronary artery disease, and they have implications for prognosis, therapy and further research.

Features of the exercise test that reflect the activity of ischemic heart disease out of hospital

To better understand the relationship between the transient myocardial ischemia seen during an exercise test and ischemic activity out of hospital, 39 patients with well-documented coronary artery disease underwent standard treadmill exercise testing (Bruce protocol) and 24 to 48 hr of continuous ambulatory electrocardiographic monitoring during normal daily activities. A total of 245 episodes of transient ischemia were recorded in 21 of 32 patients with positive exercise electrocardiograms (group I), whereas seven patients with negative test results (group II) had no episodes of transient ischemia, during monitoring out of hospital (p less than .01). Certain measures in the exercise test were related to the severity of ischemia out of hospital: there were more episodes and a greater total duration of transient ischemia per 24 hr of ambulatory monitoring in patients who developed ischemic electrocardiographic changes before 6 min of exercise (p less than or equal to .021) or at a heart rate of less than 150 beats/min (p = .005) and in those in whom these ST segment changes persisted for more than 5 min after exercise (p less than or equal to .016). In contrast, there was no relationship between transient ischemia out of hospital and the commonly quoted exercise variables: chest pain, total exercise duration, and the maximum levels of heart rate, systolic blood pressure, and double product. Thus, patients with coronary artery disease and negative exercise electrocardiograms are most unlikely to experience active ischemia during normal daily life.(ABSTRACT TRUNCATED AT 250 WORDS)

Active transient myocardial ischemia during daily life in asymptomatic patients with positive exercise tests and coronary artery disease

The management of asymptomatic patients with coronary artery disease (CAD) is controversial, and the level of "activity" of their CAD during normal daily life remains largely unknown. To examine this question, ambulatory Holter monitoring of ST segments was performed out of the hospital in 7 asymptomatic subjects with CAD during normal daily activities. Their condition was detected because they all had a silent positive exercise test and angiographically proved CAD. During a total of 384 hours of monitoring, 37 asymptomatic episodes of ST depression (1 mm or greater and lasting at least 30 seconds) were recorded in 5 of the patients. Most episodes (68%) were 10 minutes or less in duration but ranged from 1 to 253 minutes, and most (70%) had a maximal ST depression of 1 to 2 mm. A small increase in heart rate, ranging from 1 to 34 beats/min, preceded 65% of the episodes, but 35% were associated with no change or even a decline before the onset of ischemia. Fifty-four percent of the episodes occurred during rest or usual light physical activity, 8% during sleep and only 38% during exercise, including 1 prolonged bout while jogging. During 78% of the episodes, the subjects rated their mental activity as usual and only 14% occurred during mental stress. In addition, a distinct diurnal variation was noted with 57% of the ischemia occurring between 0600 and 1200 hours (p = 0.008). Therefore, most asymptomatic subjects had active transient ischemia during daily life, with many of the characteristics already described in symptomatic subjects with CAD.

Factors determining the activity of ischemic heart disease

Transient regional myocardial ischemia appears to underlie symptoms such as angina pectoris and represents a key pathophysiologic step, since it is an objective marker of disease activity and is capable of causing disabling symptoms and damage to left ventricular myocardium. A study of the characteristics of transient ischemia in and out of the hospital has shown that symptoms are an inconsistent underestimation of these events. Ischemia is generally prolonged, mostly asymptomatic, and usually accompanied by a regional decrease in myocardial perfusion. Studies out of the hospital have also shown that these episodes are frequently triggered by a wide range of ordinary everyday activities. These new features of transient ischemia are worth noting when searching for relevant causes that are present during everyday life and when trying to choose more rational therapy. More detailed studies of patient activity have shown that different levels of mental arousal are the most common triggering mechanism causing ischemia out of the hospital. In addition, the occurrence of transient ischemia during everyday life displays a circadian rhythm, with an increase and peak occurrence between 6:00 A.M. and 12 noon each day. The day-to-day variability of ischemia is marked, indicating functional disturbances of coronary stenoses against a background of a severe reduction in cross-sectional area. The examination of proximal stenoses has shown that the reduction in cross-sectional area is usually underestimated by conventional angiography; pressure gradients across coronary stenoses are common and, with reduced poststenotic blood pressure, can jeopardize perfusion; disturbances of vessel caliber and antegrade flow can accompany many of the ordinary everyday activities known to trigger ischemia detected in Holter tapes studied out of the hospital; and there is clear-cut evidence of endothelial dysfunction in these patients, with reversal of the normal dilator response to acetylcholine and paradoxical constriction of stenoses. This evidence of endothelial dysfunction in humans could be central to the problems of atheromatous narrowing, thrombus, and disturbed vasomotion.

Pathophysiology of experimental renovascular hypertension

The genesis of renovascular hypertension follows a continuum from an acute to a chronic phase. Reduction in renal perfusion initiates renin release and angiotensin-mediated systemic vasoconstriction. Aldosterone secretion, sodium and water retention, and expansion of the extracellular volume ensue. Sustained hypertension is further maintained by interacting physiologic mechanisms including increased angiotensin II sensitivity, vasopressin, ouabain-like substance, the sympathetic nervous system, CNS mechanisms, autoregulation, and structural changes.