Evaluation of isoproterenol as a method of stress testing

Arbutamine stimulation detects viable myocardium 4 weeks after coronary occlusion

At low doses, dobutamine has potent inotropic, but limited chronotropic, effects-properties that may be necessary for detection of hibernating myocardium. The efficacy of other catecholamines, which have more closely coupled inotropic and chronotropic effects, for the detection of viable myocardium is unknown. This study evaluated the efficacy of arbutamine, a catecholamine with potent chrono-tropic effects, for the detection of viable myocardium in a canine model of hibernating myocardium. Contractile reserve was assessed during stepwise arbutamine infusion (dosages of 2.5, 5, 10, 50, and 100 ng/kg/min) at 3 days (early) and 4 weeks (late) after coronary ligation. Segment shortening, wall thickening, and segmental wall motion were assessed by sonomicrometry and echocardiography. After 4 weeks of occlusion, functional recovery was assessed after revascularization. During the early arbutamine study, the sensitivity for predicting functional recovery was highest at a dosage of 50 ng/kg/min, which also produced tachycardia. The sensitivity was 50% for segment shortening, 20% for wall thickening, and 75% for wall motion score. The late arbutamine study had improved sensitivity. The sensitivity was 100% for segment shortening, 80% for wall thickening, and 90% for wall motion score at a dosage of 50 ng/kg/min. At the late arbutamine study, myocardial perfusion reserve in the ischemic zone of dogs with functional recovery was only mildly reduced (2.0 versus 2.6 in nonischemic zones, P =.53). After coronary occlusion, viable myocardium can be detected with high doses of arbutamine that produce tachycardia. However, the sensitivity of arbutamine stimulation for predicting functional recovery is low early after occlusion, but it is improved by 4 weeks after occlusion with adequate perfusion reserve.

A novel catecholamine, arbutamine, for a pharmacological cardiac stress agent

Arbutamine, developed for use as a cardiac stress agent, was compared with isoproterenol and dobutamine in anesthetized dogs for cardiovascular actions prior to and after beta-adrenergic blockade with propranolol. The efficacy and safety of arbutamine were also evaluated in a canine model of myocardial ischemia obtained by partially occluding the left anterior descending coronary artery. Comparison of hemodynamic variables in normal dogs showed that arbutamine was approximately equipotent to isoproterenol in increasing heart rate and cardiac contractility, and in decreasing total peripheral vascular resistance and mean arterial blood pressure. Arbutamine was 210 times more potent than dobutamine in increasing cardiac contractility by 70%; however, at this dose dobutamine exhibited a negative chronotropic response. Beta-adrenergic blockade with propranolol shifted the agonist's dose-response curves for heart rate and contractility to the right; however, low doses of dobutamine exhibited a negative chronotropic effect and increased the total peripheral vascular resistance. In dogs subjected to partial left anterior descending coronary artery occlusion, arbutamine produced significant ST-segment deflections, beginning at a dose of 0.1 nmol/kg/min. Impairment of segment shortening, reflecting cardiac wall motion abnormality, was evident at a dose of 0.3 nmol/kg/min. Isoproterenol did not cause significant changes in these parameters. These results show that arbutamine is capable of producing graded increments in cardiac contractility and rate before and after beta-adrenergic blockage in normal dogs. In dogs subjected to coronary artery occlusion, it is capable of provoking myocardial ischemia at dose levels devoid of toxicity.

Chronically decreased aortic distensibility causes deterioration of coronary perfusion during increased left ventricular contraction

OBJECTIVES

This study investigated the long-term effects of decreased aortic distensibility on the heart in relation to coronary perfusion.

BACKGROUND

Aortic distensibility is decreased in patients with atherosclerosis and hypertension and in the elderly. However, the effect of a long-term decrease in aortic distensibility on coronary perfusion has not been fully investigated.

METHODS

Twelve anesthetized dogs underwent thoracotomy and were allocated to two groups: Group I included six control dogs with a normal aorta; Group II included six dogs with decreased aortic distensibility produced by banding the descending aorta. After 4 to 6 weeks, the dogs had a second operation to measure coronary artery flow and transmural flow distribution. Because the effect of decreased aortic distensibility on coronary perfusion may be affected by ventricular contractility, measurements were performed at baseline and during increased ventricular contraction induced by isoproterenol infusion.

RESULTS

At baseline, arterial compliance was reduced by 35% in Group II, but there was no change in total mean arterial resistance. Hemodynamic variables, regional wall motion and coronary flow were also similar in both groups. However, during isoproterenol infusion, coronary flow increased more in Group II than in Group I (p < 0.01), and the coronary flow reserve ratio (maximal peak hyperemic flow divided by rest flow) decreased more in Group II than in Group I (mean [+/- SD] 1.9 +/- 0.4 vs. 2.4 +/- 0.3, p < 0.05). Moreover, although the transmural flow distribution was similar in the two groups at baseline, during isoproterenol infusion the endocardial flow increased less in Group II than in Group I (p < 0.05), and the endocardial/epicardial flow ratio was significantly decreased in Group II compared with Group I (mean [+/- SD] 0.70 +/- 0.18 vs. 0.99 +/- 0.22, p < 0.05). The subendocardial electrocardiogram showed ST segment elevation during isoproterenol infusion in Group II (p < 0.05) but not in Group I.

CONCLUSIONS

These results demonstrate that during increased ventricular contraction, chronically decreased aortic distensibility contributes to a further decrease in the coronary flow reserve ratio, impairs endocardial blood flow and may induce subendocardial ischemia even in the absence of coronary artery stenosis.

Dobutamine digital echocardiography for detecting coronary artery disease

To assess the value of dobutamine echocardiography for detecting coronary artery disease (CAD), 70 men (mean age 62 +/- 8 years) presenting for coronary angiography were prospectively studied. Dobutamine (2.5 to 40 micrograms/kg/min) was infused in 3-minute stages. Digital echocardiograms were recorded on-line at baseline, during low- and high-dose dobutamine infusion, and at recovery. An echocardiogram positive for CAD was defined as one showing a new wall motion abnormality induced by dobutamine. Compared with coronary angiography, the overall sensitivity of dobutamine echocardiography for detecting CAD was 86%, specificity 95% and accuracy 89%. The sensitivity for detecting 3-vessel CAD was 100%, 89% for 2-vessel and 69% for 1-vessel CAD. The accuracy of predicting multivessel disease by 2 methods was 71% and 84%, respectively. Heart rate at the echocardiographic ischemic threshold was lower in patients with 3- and 2-vessel CAD versus 1-vessel CAD (89 +/- 17, 95 +/- 18 and 118 +/- 18 beats/min, respectively, p less than 0.01); rate-pressure product was also lower in patients with 3- and 2-vessel CAD versus 1-vessel CAD (12.7 +/- 3.6, 13.7 +/- 2.8 and 18.9 +/- 44 x 10(3) beats/min x mm Hg, respectively, p less than 0.01). Heart rate was the most important physiologic determinant of ischemia induced by dobutamine. There were no major complications during the study. Thus, dobutamine digital echocardiography is an excellent test for identifying CAD and should be beneficial in patients unable to exercise.

Echocardiographic study of left ventricular function in type 1 diabetes mellitus: hypersensitivity of beta-adrenergic stimulation

Systolic left ventricular function was investigated by echocardiography in 23 young, type 1 diabetics and 11 control subjects. A stimulation by isoproterenol was performed in order to study beta-adrenergic cardiac responsiveness. M-mode recordings were digitized and analyzed by computer. Systolic parameters such as left ventricular fractional shortening and mean velocity of circumferential shortening were not different, but maximal velocity of shortening was increased both at rest (P less than 0.01) and with isoproterenol (P less than 0.05) in the diabetics. An abnormal systolic anterior motion of the mitral valve was found during administration of isoproterenol in 65% of the diabetics and in only one control. These findings are suggestive of a hyperkinetic state, associated with a poor metabolic control (high value of HbA1), together with adrenergic hypersensitivity in type 1 diabetes mellitus.

Evaluation of coronary artery disease in the patient unable to exercise: alternatives to exercise stress testing

Exercise stress testing is a well-established method for the diagnostic, prognostic, and functional assessment of patients with known or suspected CAD. A variety of alternative tests have been described in patients unable to perform leg exercise. Atrial pacing and dipyridamole imaging have been evaluated most extensively, and results compare favorably with those of exercise testing for diagnosing the presence of CAD. Both tests may be used to assess prognosis after myocardial infarction, and dipyridamole imaging may be useful in patients undergoing preoperative evaluation. The use of the cold pressor test and isometric handgrip exercise have also been described. However, the value of both tests is limited by a relatively low sensitivity for detecting the presence of CAD. Other testing modalities--arm ergometry, intravenous infusion of beta-adrenergic agonists, and transthoracic pacing--show promise but require further assessment to confirm their value.

The effects of nitroglycerin on regional myocardial contractile dysfunction produced by treadmill exercise or isoprenaline stimulation in dogs

1. To compare different methods of cardiac stress testing that are clinically applied in the management of coronary heart disease, 2 groups of dogs each were chronically instrumented and subjected to treadmill exercise or isoprenaline infusion in the presence of coronary stenosis. 2. It was of interest to determine differences in haemodynamic and regional myocardial contractile parameters, the response to antianginal therapy (nitroglycerin 15 micrograms kg-1 15 min-1, i.v.), and, in particular, whether this response differed according to the mode of cardiac stimulation, i.e. treadmill exercise or isoprenaline infusion. 3. After stenosis of the circumflex branch of the left coronary artery which affected resting myocardial function only minimally, treadmill exercise or isoprenaline infusion induced transient regional contractile dysfunction. Heart rate, arterial blood pressure, left ventricular end-diastolic pressure and left ventricular dp/dtmax were registered and myocardial oxygen demand was calculated. Regional contractile performance was assessed by ultrasonic distance measurement in the underperfused and in a normally perfused area. 4. Treadmill exercise led to an increase in systolic arterial and left ventricular end-diastolic pressure. In contrast, isoprenaline-induced stimulation led to a decrease in diastolic arterial and left ventricular end-diastolic pressure. Regional contractile function in the critically underperfused area showed a deterioration during both modes of stress. Nitroglycerin completely abolished stress-induced contractile dysfunction only in the group where treadmill exercise was employed for stimulation. 5. The inability of nitroglycerin to prevent myocardial dysfunction in the isoprenaline group may be due to exhaustion of the arterial and/or venous vasodilator potency of nitroglycerin in the presence of adrenoceptor vasodilatation induced by isoprenaline. 6. These findings indicate that clinical antianginal drug testing and the evaluation of the course of disease in patients with coronary heart disease may be highly dependent on the test method chosen.

Relation between graded, subcritical impairments of coronary flow reserve and regional myocardial dysfunction induced by isoproterenol infusion in dogs

Isoproterenol has been used experimentally and clinically to elicit ischemia. The usefulness of this approach, however, in eliciting regional dysfunction in the presence of mild to moderate single-vessel coronary disease quantitated on the basis of coronary flow reserve measurements has not been previously defined. Open-chest, anesthetized dogs were instrumented with an electromagnetic flow probe, high-fidelity micromanometers, and subendocardial ultrasonic crystals. A rigid, screw occluder was used to produce five subcritical coronary stenoses in each dog associated with varying impairment of postocclusion reactive hyperemia at rest but no impairment of resting coronary blood flow. Regional function at rest and in response to the isoproterenol challenge (0.25 micrograms/kg/min) in nonstenotic and stenotic conditions was assessed. Relative regional function was maintained during the infusion until nearly total loss of coronary flow reserve. With this near-critical stenosis, function was lower than in the nonstenotic state but remained greater than resting control values. Moderate impairments of coronary flow reserve were not associated with isoproterenol-induced deterioration of regional function. In conclusion, detection of impaired coronary flow reserve at rest is a more sensitive index of the severity of a coronary stenosis than is detection of regional dysfunction during isoproterenol challenge. Failure to maintain the expected isoproterenol-induced increase in regional function is manifested only when stenoses are associated with nearly total loss of resting coronary flow reserve. This suggests that the clinical use of isoproterenol challenge is not effective in eliciting regional dysfunction when mild coronary disease is present.

Relation of coronary blood flow and reactive hyperemia to regional dysfunction induced by dopamine infusion in dogs: limitations in detecting subcritical coronary stenoses

Sympathomimetic agents have been used clinically to elicit regional dysfunction or heterogeneity of coronary blood flow to detect coronary artery lesions. However, the usefulness of this procedure in detecting mild to moderate coronary stenoses has not been defined previously. This was investigated in 10 open chest anesthetized dogs using subendocardial ultrasonic crystals to measure segment lengths. An electromagnetic flow probe was placed on the proximal left anterior descending artery, and graded coronary stenoses were created using a cuff occluder. In the first phase of the study, subcritical coronary stenoses were created which impaired maximal postocclusion reactive hyperemia, but not coronary blood flow at rest. A constant infusion of dopamine (average 15 micrograms/kg per min) was then administered. Regional dysfunction during dopamine infusion was not consistently observed despite production of coronary stenoses resulting in total loss of reactive hyperemia at rest. Regional function during dopamine treatment was, however, critically related to the actual changes in coronary blood flow during the infusion. In the second phase of the study, regional function during dopamine challenge was investigated during progressive impairments of coronary blood flow. Regional function was maintained until coronary blood flow during the infusion was reduced to near rest levels below which regional function deteriorated rapidly.(ABSTRACT TRUNCATED AT 250 WORDS)

Speed of onset of pharmacodynamic activity of propranolol, practolol, oxprenolol and metoprolol after intravenous infection in man

1 The speed of onset of the pharmacodynamic activity of intravenous propranolol, practolol, oxprenolol and metoprolol was determined, using attenuation of isoprenaline-induced tachycardia as the end-point, in 16 patients with clinically coronary heart disease. 2 Antagonism was evident within 15 s of injection into the central circulation of all four drugs. The time to maximum attenuation of isoprenaline tachycardia was significantly more rapid with propranolol and oxprenolol than with practolol and metoprolol.

Response of the left ventricle to stress: effects of exercise, atrial pacing, afterload stress and drugs

A variety of tests are being utilized today to diagnose the presence of ischemic heart disease, assess the prognosis of myocardial and valvular heart disease and evaluate the effects of various pharmacologic agents on cardiac performance. This review summarizes the current evidence regarding the response of left ventricular performance and size to atrial pacing, afterload stress and various forms of exercise. The responses in normal persons and in subjects with coronary heart disease is reviewed and, when applicable, the effects of various pharmacologic agents on exercise performance in these patient groups are examined.