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Anti-ischemic therapy and stress testing: pathophysiologic, diagnostic and prognostic implications
© Sicari; licensee BioMed Central Ltd. 2004
Received: 01 June 2004
Accepted: 20 August 2004
Published: 20 August 2004
Anti-ischemic therapy, in particular beta-blockers, is the most commonly employed drug for the control of myocardial ischemia in patients with stable coronary artery disease. Its widespread use also in patients with suspected coronary artery disease has important practical, clinical diagnostic and prognostic implications because diagnostic tests are heavily influenced by its effects. In the present review, the pathophysiological mechanisms of ischemia protection by antianginal therapy are described. Not all stressors are created equal in front of the different classes of antianginal drugs and on their turn the different classes of drugs exert different levels of protection on inducible ischemia. Several clinical implications can be drawn: From the diagnostic viewpoint antianginal therapy decreases test sensitivity, offsetting the real ischemic burden for a too high percentage of false negative tests. From the prognostic viewpoint test positivity in medical therapy identifies a group of subjects at higher risk of experiencing cardiac death and positivity on medical therapy can be considered a parameter of ischemia severity. Nonetheless in patients with known coronary artery disease the ability of antianginal therapy to modify the ischemic threshold at stress testing represent a powerful means to assess therapy efficacy. From a practical viewpoint, the use of antianginal therapy at time of testing has advantages and disadvantages which are largely dependent on the purpose a test is performed: if the purpose of testing is to diagnose ischemia, it should be performed in the absence of antianginal medications. If the purpose of testing is to assess the protective effects of antianginal therapy, the test should be performed on medications.
Anti-ischemic therapy, in particular beta-blockers, is the most commonly employed drug for the control of myocardial ischemia in patients with stable coronary artery disease. Its widespread use also in patients with suspected coronary artery disease has important practical, clinical diagnostic and prognostic implications because diagnostic tests are heavily influenced by its effects. The diagnostic and prognostic impact of anti-ischemic therapy on stress testing is largely ignored but not negligible. The issue raises several questions: How to evaluate patients at time of testing for myocardial ischemia? How to interpret a stress test performed on anti-ischemic therapy? Are the stressors employed for the detection of myocardial ischemia created equal in relation to the different classes of drugs used in clinical practice? Is stress testing able to assess the efficacy of medical therapy in patients with known coronary artery disease? Has the protection of anti-ischemic therapy on inducible myocardial ischemia any impact on long-term survival?
Pathophysiologic implications of anti-ischemic therapy during stress testing
In the presence of coronary atherosclerosis, appropriate arteriolar dilation can paradoxically exert detrimental effects on regional myocardial perfusion, causing overperfusion of myocardial layers or regions already well perfused in resting conditions at the expense of regions or layers with a precarious flow balance in resting conditions . Anti-ischemic therapy can interfere with all the above mentioned mechanisms of ischemia induction, although in a very different fashion. The mechanism of action of anti-ischemic drugs is easily fitted within the familiar framework of supply-demand mismatch. In particular, beta-blockers are credited with reducing exercise-induced ischemia by decreasing myocardial oxygen demand and possibly by increasing supply through a reduction in extravascular forces . Experimental studies demonstrated that beta-blockers reduce dipyridamole-induced ischemia by an alteration of regional myocardial blood flow . However, this straightforward explanation seems inadequate in justifying the protective effects of beta-blockers on dipyridamole-induced ischemia. Experimental data show that beta-blockers do not affect the dipyiridamole-induced increase in flow . On the other hand, the increase in myocardial oxygen consumption does not play any significant role in the induction of dipyridamole-induced ischemia, which is due to an absolute reduction in subendocardial flow (tightly linked to regional wall thickening) mostly for "vertical" and "horizontal" steal phenomena.
However, experimental studies on the model of the exercising dog have shown that beta-blockers protect myocardium from stress-induced myocardial blood flow-function relation: for a given transmural flow, there is a rise of subendocardial and a fall of subepicardial flow, with an improved regional performance . This same mechanism has also been documented with some calcium antagonists, such as diltiazem and may explain, in part, the beneficial effects of this class of drugs on dipyridamole-induced ischemia. Calcium antagonists can effectively prevent ischemia provoked by dipyridamole also through other mechanisms, which they share with nitrates, and they tend to increase the coronary flow supply during stress. In this case, the prevention of steal phenomena may be due to the increase in collateral flow (which has been shown with nitrates and, to a much lesser extent, with some calcium antagonists) . and to the dilation of epicardial coronary lumen size. The pronounced increase in collateral flow can prevent horizontal steal phenomena due to dipyridamole, wheras even a small increase of the coronary diameter can dramatically reduce the blood pressure drop across the stenosis, therefore preventing vertical steal phenomena. Beta-blockers exert a direct and competitive action on beta-1 receptors, as they are employed as specific antagonists of dobutamine-induced ischemia. Dobutamine, through its beta-1 agonist action determines the increase in oxygen consumption, but it induces flow maldistribution through beta-2 arteriolar receptors.
Diagnostic implications of the use of anti-ischemic therapy during stress testing
Prognostic implications of anti-ischemic therapy during stress testing
The protective effect of anti-ischemic therapy on inducible myocardial ischemia might exert a powerful impact on prognosis. From the EPIC-EDIC Data bank, it has been analyzed the prognostic impact of antianginal therapy at time of testing in 7333 patients with suspected or known coronary artery disease undergoing pharmacologic stress echocardiography with either dipyridamole or dobutamine. The results show that a positive test on medical therapy is an additional marker of ischemia severity at stress testing whereas a negative test on medical therapy is less prognostically benign, being a false negative result . (Fig. 8). No prognostic difference was found among the various forms of anti-ischemic drugs at time of testing, but the presence per se of antinaginal therapy at time of testing is an independent predictor of death. It is worth noting that in the study only a very low percentage of patients was taking beta-blockers: if on one side this aspect represent a clear lack of adherence to recommendations ., on the other it is an observed pattern of prescription in our data base, which simply reflected the clinical practice and the lack of a universally accepted policy of testing regarding concomitant therapy [32, 33]. Marwick et al. . have demonstrated a protective effect on mortality of beta-blocker therapy in patients with a negative exercise echocardiography whereas specificity and negative predictive value is increased for the prediction of cardiac events (cardiac death, myocardial infarction and unstable angina) during exercise scintigraphy in patients evaluated off medical therapy at time of testing .
The clinical implications of these results are far-reaching. Inducible myocardial ischemia during pharmacological stress testing on medical therapy identifies the subset of patients at highest risk of death. On these patients an aggressive approach has to be undertaken in order to change the natural history of coronary artery disease. On the far opposite end the incidence of death in patients with a negative pharmacologic test off therapy is so low that no intervention could lower the spontaneous rate of death any further. At intermediate risk are those patients with a negative test on medical therapy or with a positive test off medical therapy. Different clinical scenarios can be foreseen on the basis of the present results: 1) A negative test on medical therapy might represent a false negative result, therefore it is advisable to repeat the test off therapy in order to assess the real ischemic burden through the conventional stress echocardiographic parameters [36, 37]. – i.e. number of ischemic segments, severity of induced dysfunction, (both expressed by peak wall motion score index), pharmacologic load and time of onset of ischemia. This is in line with the recommendations of the American Heart Association in patients with stable angina .; 2) In the case of a positive test off medical therapy, the effect of therapy can be assessed with the advantage of using an objective, primary ischemic end point such as changes in wall motion during stress.
Effects of oral therapy on stress testing sensitivity
Calcium channel blockers
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