Ischemic preconditioning: clinical relevance and investigative studies

Experimental animal studies have shown that repetitive brief coronary occlusions render the heart resistant to myocardial infarction from subsequent, more prolonged, coronary occlusions. This phenomenon in animal models has been called ischemic preconditioning. In a number of clinical scenarios, the second in a series of ischemic episodes appears to be less severe than the first, suggesting that ischemic preconditioning also occurs in humans. If the mediator of preconditioning could be identified, it is conceivable that this agent could be administered to patients with coronary artery disease as a myocardial protectant. However, the definite clinical relevance of this interesting experimental finding remains unknown. Unlike the case in animal models subjected to an abrupt occlusion, preconditioning is difficult to study in the clinical setting. This article reviews the findings and limitations of the relevant clinical studies looking for ischemic preconditioning in humans.

Detailed angiographic analysis of women with suspected ischemic chest pain (pilot phase data from the NHLBI-sponsored Women's Ischemia Syndrome Evaluation [WISE] Study Angiographic Core Laboratory)

The purpose of this study is to provide a contemporary qualitative and quantitative analysis of coronary angiograms from a large series of women enrolled in the Women's Ischemia Syndrome Evaluation (WISE) study who had suspected ischemic chest pain. Previous studies have suggested that women with chest pain have a lower prevalence of significant coronary artery disease (CAD) compared with men. Detailed analyses of angiographic findings relative to risk factors and outcomes are not available. All coronary angiograms were reviewed in a central core laboratory. Quantitative measurement of percent stenosis was used to assess the presence and severity of disease. Of the 323 women enrolled in the pilot phase, 34% had no detectable, 23% had measurable but minimal, and 43% had significant ( > 50% diameter stenosis) CAD. Of those with significant CAD, most had multivessel disease. Features suggesting complex plaque were identified in < 10%. Age, hypertension, diabetes mellitus, prior myocardial infarction (MI), current hormone replacement therapy, and unstable angina were all significant, independent predictors of presence of significant disease (p < 0.05). Subsequent hospitalization for a cardiac cause occurred more frequently in those women with minimal and significant disease compared with no disease (p = 0.001). The common findings of no and extensive CAD among symptomatic women at coronary angiography highlight the need for better clinical noninvasive evaluations for ischemia. Women with minimal CAD have intermediate rates of rehospitalization and cardiovascular events, and thus should not be considered low risk.

American College of Cardiology/ European Society of Cardiology international study of angiographic data compression phase I. The effects of lossy data compression on recognition of diagnostic features in digital coronary angiography

OBJECTIVES

This study intended to determine the effect of varying degrees of lossy Joint Photographic Experts Group (JPEG) compression on detection of coronary angiographic features. Background Compression of digital coronary angiograms facilitates playback of images and decreases cost. There are little data on the effect of compression on the accuracy of coronary angiography.

METHODS

At six centers, 71 angiographers each reviewed a set of 100 angiographic sequences. The 100 sequences were divided into four, 25-sequence subsets. Each subset of 25 was displayed either as original images or at one of three compression ratios (CRs) (6:1, 10:1 or 16:1). The effect of lossy compression on the sensitivity and specificity for detection of diagnostic features was determined. The effect of compression on subjective measures of image quality graded by the angiographers was also examined.

RESULTS

Lossy compression at a ratio of 16:1 decreased the sensitivity for the detection of diagnostic features (76% vs. 80%P=0.004). The largest effect was in the detection of calcification (52% vs. 63% at 16:1 compression vs. original images, P<0.001). Subjective indicators of image quality indicated a reduction in confidence in interpretation at CRs of 10:1 and 16:1.

CONCLUSIONS

With increased ratios of lossy compression, a degradation of digital coronary angiograms occurs that results in decreased diagnostic accuracy. The sensitivity for detection of common diagnostic features was decreased, and subjective assessment of image quality was impaired. Caution is warranted in the interpretation of coronary angiograms that have been subjected to lossy JPEG compression beyond a ratio of 6:1.

American College of Cardiology/European Society of Cardiology International Study of Angiographic Data Compression Phase I: The effect of lossy data compression on recognition of diagnostic features in digital coronary angiography

OBJECTIVES

This study intended to determine the effect of varying degrees of lossy Joint Photographic Experts Group (JPEG) compression on detection of coronary angiographic features.

BACKGROUND

Compression of digital coronary angiograms facilitates playback of images and decreases cost. There are little data on the effect of compression on the accuracy of coronary angiography.

METHODS

At six centers, 71 angiographers each reviewed a set of 100 angiographic sequences. The 100 sequences were divided into four, 25-sequence subsets. Each subset of 25 was displayed either as original images or at one of three compression ratios (CRs) (6:1, 10:1 or 16:1). The effect of lossy compression on the sensitivity and specificity for detection of diagnostic features was determined. The effect of compression on subjective measures of image quality graded by the angiographers was also examined.

RESULTS

Lossy compression at a ratio of 16:1 decreased the sensitivity for the detection of diagnostic features (76% vs. 80% p = 0.004). The largest effect was in the detection of calcification (52% vs. 63% at 16:1 compression vs. original images, p < 0.001). Subjective indicators of image quality indicated a reduction in confidence in interpretation at CRs of 10:1 and 16:1.

CONCLUSIONS

With increased ratios of lossy compression, a degradation of digital coronary angiograms occurs that results in decreased diagnostic accuracy. The sensitivity for detection of common diagnostic features was decreased, and subjective assessment of image quality was impaired. Caution is warranted in the interpretation of coronary angiograms that have been subjected to lossy JPEG compression beyond a ratio of 6:1.

Effect of theophylline on the warm-up phenomenon

This study examined whether the adenosine receptor antagonist theophylline prevents the warm-up phenomenon in patients with stable angina undergoing serial exercise tests. Our findings offer evidence that adenosine does not play a role in the warm-up phenomenon, and indirectly suggest that the warm-up phenomenon does not represent ischemic preconditioning in humans.

Worse six-month outcome for patients with multiple stents in a single coronary artery

BACKGROUND

Before the "era" of optimal stent deployment, very few data concerning multiple stents in a single coronary artery showed restenosis rates up to 60%.

OBJECTIVE

To evaluate the 6-month outcome of patients receiving multiple Palmaz-Schatz stents (> or =2 stents) in a single coronary artery compared to those receiving single stents.

METHODS

Three hundred and forty-eight patients having multiple stents were compared to 174 patients receiving single stents during a 6-month follow-up.

RESULTS

Repeat target lesion revascularization (RTLR), either repeat PTCA or CABG, was 10.4% in the single-stent group, 22.6% in the two-stent group, and 23.1% in the > or =2 stent group (p = 0.001, single versus 2 or > or =2 stents). There was not a significant difference between single stent and multiple stent groups in myocardial infarction and death during 6-month follow-up. Multivariate analysis showed multiple stents, diabetes mellitus, and type C lesion to be predictors of RTLR.

CONCLUSIONS

Placement of two or more stents was associated with a significantly higher RTLR compared with single stent placement. The optimal approach to diffuse coronary artery disease remains to be defined.

Factors influencing clinical outcomes after revascularization in the asymptomatic cardiac ischemia pilot (ACIP). ACIP Study Group

BACKGROUND AND AIM

The Asymptomatic Cardiac Ischemia Pilot is the first randomized trial where revascularization involved choice of either coronary bypass or angioplasty used in an early or a delayed symptom-driven approach. One-year outcomes were favorable (reduced recurrent ischemia and adverse outcomes) for an early revascularization strategy (within 4 weeks), compared with an early medical strategy when revascularization was delayed until symptom-driven. This ancillary study examined variables influencing outcomes after these 2 revascularization approaches (early vs. delayed until symptom-driven).

METHODS

Participants were clinically stable coronary disease patients with stress-induced and daily life ischemia who underwent revascularization. Characteristics associated with clinical outcomes occurring within the year following revascularization were examined using Cox regression analysis.

RESULTS

A total of 262 patients received revascularization; 170 in the early approach and 92 in the delayed symptom-driven approach. Thirty-three patients had adverse outcomes (death, nonfatal myocardial infarction, or repeat revascularization) during 1-year follow-up. The most important independent predictor of improved outcome during the follow-up year was attempted revascularization of > or = 66% of vessels with significant stenosis for the early (risk ratio [RR] 0.25, 95% confidence interval [CI] 0.09-0.67) and the delayed (RR 0.21, CI 0.08-0.58) approaches. Factors such as age, stress test results, and coronary angiographic findings did not predict clinical outcome.

CONCLUSIONS

Our findings are important in the planning of a large trial with longer follow-up.

Myocardial infarction related atrial fibrillation: role of endogenous adenosine

Exogenous administration of adenosine induces atrial fibrillation in up to 7.0% of patients. Animal studies affirm endogenous adenosine released in response to tissue hypoxia may play a mechanistic role in arrhythmias associated with myocardial ischaemia or hypoxia. Therefore, atrial fibrillation occurring early after the acute phase of myocardial infarction involving atrial tissue may be secondary to an excessive accumulation of adenosine that leads to a shortening of atrial refractory period. Early in the course of acute inferior myocardial infarction, two patients (males aged 45 and 68) suffered new onset sustained atrial fibrillation that was abrupt in onset and complicated their clinical management. They were administered 250 mg theophylline as a slow intravenous injection at a rate of 100 mg/min or until conversion to normal sinus rhythm occurred. Both patients converted to normal sinus rhythm within five minutes of the administration of theophylline. In up to 52 hours of continuous ECG monitoring after the theophylline administration the atrial fibrillation did not recur. Neither patient experienced any adverse outcome from theophylline administration. These observations are the first reported in humans or laboratory animals to suggest that atrial fibrillation, presumably due to elevated interstitial atrial concentration of adenosine caused by myocardial ischaemia, can be terminated with an adenosine receptor antagonist. However, the hypothesis that excessive accumulation of endogenous adenosine in atrial tissue may induce atrial fibrillation is well substantiated by other investigators. Thus, A1 adenosine receptor antagonists may prove to be valuable in the management of ischaemia related atrial fibrillation.

Differential antagonism of cardiac actions of adenosine by theophylline

OBJECTIVE

To determine the relative sensitivity of cardiac A1- and A2-adenosine receptor-mediated effects to antagonism by theophylline in man.

METHODS

Baseline measurements of the A-H interval (A1-adenosine receptor-mediated effect) and coronary blood flow (A2-adenosine receptor-mediated effect) were made in 10 patients with angiographically normal coronary arteries. Adenosine was then administered as a continuous intravenous infusion followed by a rapid intravenous bolus, and measurements repeated. Theophylline (5 mg/kg i.v.) was then administered, and the adenosine infusion repeated. To corroborate the results found in man, the cardiac A1- and A2-adenosine receptor-mediated effects were measured in guinea pig isolated hearts exposed to increasing concentrations of adenosine, in the absence and presence of theophylline (60 microM).

RESULTS

Compared to baseline, adenosine infusion and bolus caused significant prolongation of the A-H interval (109 +/- 41 vs. 116 +/- 44 vs. 168 +/- 57 ms, respectively), and increase in coronary blood flow (46 +/- 37 vs. 86 +/- 71 vs. 172 +/- 98 ml/min, respectively). Theophylline abolished the prolongation of the A-H interval during adenosine infusion and bolus (99 +/- 36 and 107 +/- 44 ms, respectively), yet had minimal effect on the increase in coronary blood flow (63 +/- 51 and 136 +/- 121 ml/min, respectively). In guinea pig isolated hearts, theophylline was shown to significantly antagonize the A2-adenosine receptor-mediated effects only when the concentrations of adenosine were < or = 1.0 microM.

CONCLUSIONS

In man, theophylline completely antagonizes the A1-adenosine receptor-mediated prolongation of the A-H interval, but has minimal effect on the A2-receptor-mediated coronary vasodilation, particularly when adenosine concentrations exceed 1.0 microM.

Echocardiographic assessment of wall motion in ST-segment alternans during coronary angioplasty

ST alternans occurs in the setting of severe and extensive myocardial ischemia. In this particular case, ST alternans occurred well after the onset of ischemia-induced left ventricular dysfunction. There is no echocardiographic evidence of beat-to-beat variation in contractility to correspond to this ST-segment phenomenon. Further studies of left ventricular function during ST alternans in patients are needed to determine whether this finding applies to all patients with ischemic heart disease.

Lysis of intravascular thrombus prior to coronary stenting using the dispatch infusion catheter

The presence of angiographic evidence of thrombus is generally thought to be a contraindication to coronary stent placement. This report describes four patients in whom angiographic thrombus was lysed using the Dispatch infusion catheter prior to coronary stenting. Urokinase was infused via the Dispatch catheter with resolution of angiographic evidence of thrombus in all cases. No complications were encountered using this technique, and all patients had excellent angiographic results after stenting. We conclude that lysis of intracoronary thrombus using the Dispatch infusion catheter is feasible and appears safe in this small study. Further trials are needed to determine if this technique reduces the acute stent thrombosis rate compared to other techniques for stent deployment in the presence of angiographic evidence of thrombus.

Selective attenuation by N-0861 (N6-endonorboran-2-yl-9-methyladenine) of cardiac A1 adenosine receptor-mediated effects in humans

BACKGROUND

To determine the adenosine receptor subtype selectivity of the novel antagonist N-0861, the A1 and A2 receptor-mediated cardiac effects of adenosine were investigated in 13 patients during continuous intravenous infusion and boluses of adenosine before and after intravenous infusion of N-0861.

METHODS AND RESULTS

Measurements of the the atria-to-His (A-H) interval, chest pain severity, and coronary blood flow velocity were made before and after low-dose (69 microg x kg(-1) x min(-1)) intravenous infusion and bolus (2.5 mg) adenosine. Two doses of N-0861 were infused intravenously, and the adenosine protocol was repeated. N-0861 0.25 mg/kg abolished the negative dromotropic effect (A-H interval prolongation) and chest discomfort experienced during infusion of adenosine and attenuated discomfort observed during the boluses of adenosine; however, the increase in coronary blood flow velocity was not significantly affected.

CONCLUSIONS

These actions of N-0861 support the concept that the negative dromotropic effect and anginalike pain caused by adenosine are A1 adenosine receptor-mediated, whereas the increase in coronary blood flow velocity is due to activation of A2 adenosine receptors. N-0861 appears to be an effective and selective A1 adenosine receptor antagonist in humans.

Venous activated clotting time after intra-arterial heparin: effect of site of administration and timing of sampling

It is not known how the site of arterial administration of heparin and the timing of the activated clotting time (ACT) measurement affect the ACT during coronary interventions. We measured serial femoral venous ACTs after heparin was administered either via the angioplasty guiding catheter into the central aorta or peripherally via a sheath into the femoral artery. When heparin was administered into the central aorta, the ACT rose gradually and by 60 sec plateaued without further increase. When heparin was given into the femoral artery, the ACT displayed a marked "overshoot" early (20-270 sec after heparin) and did not plateau until sometime between 270 and 800 sec after heparin administration. We conclude that the site of administration and timing of venous sampling markedly affect the measured ACT during coronary interventions. Operators should be aware of these effects when assessing the accuracy of the ACT during coronary interventions.

Antegrade filling of an occluded right coronary artery via collaterals from a separate conus artery, a previously undescribed collateral pathway

The conus artery is known to be a frequent supplier of collaterals to the LAD and distal marginal branches of the RCA. In this report we describe a patient with an ostial RCA occlusion who was found to have, during selective conus artery injection, excellent collaterals directly to the proximal RCA. This anatomy was initially mistaken for diffuse disease of the ostium and proximal portion of the RCA.

Decrease in hematocrit after coronary stent placement and dextran therapy

In summary, dextran 40, when given after coronary stent placement, results in a marked decrease in hematocrit within 24 hours. Hematocrit often returns to near baseline levels within 48 hours of stopping dextran. This phenomenon most likely reflects dextran-related hemodilution. This hemodilutional decrease in hematocrit is often misinterpreted as acute blood loss and may result in blood transfusion in patients with low baseline hematocrit. However, far less aggressive anticoagulation regimens, which do not include dextran, are under investigation in patients undergoing coronary stent placement.

The effects of intracoronary adenosine on preconditioning during coronary angioplasty

There is evidence that the first balloon inflation during coronary angioplasty provides a preconditioning stimulus leading to decreased ischemia during subsequent balloon inflations. Endogenous adenosine release may play a role in ischemic preconditioning. Therefore, intracoronary adenosine administration prior to the first balloon inflation during percutaneous transluminal coronary angioplasty (PTCA) might modify the preconditioning response to the first balloon inflation. Forty-one patients underwent double-blind randomization to treatment with 100 mcg of intracoronary adenosine or placebo prior to coronary angioplasty. Twenty patients (11 adenosine, 9 placebo) had complete resolution of ischemia between inflations allowing comparison between the first and second inflation. An angioplasty guidewire was used to obtain an intracoronary electrocardiogram. The mean reduction in ST elevation during the second inflation compared with the first was 4.8 mm in the placebo group and -0.8 in the adenosine group (p < 0.05 placebo vs. adenosine). Seven of 9 placebo patients had a decrease in ischemia during the second inflation compared with the first, while only 2 of 11 adenosine patients showed a reduction. It was concluded that (1) the first inflation during PTCA is a preconditioning stimulus leading to a decrease in ischemia during later inflations, and (2) intracoronary adenosine administration prior to PTCA modifies the preconditioning effect of the first inflation. These data suggest that adenosine plays a role in ischemic preconditioning in humans.

Constrictor and dilator responses to intracoronary acetylcholine in adjacent segments of the same coronary artery in patients with coronary artery disease. Endothelial function revisited

BACKGROUND

In patients with angiographically detectable atherosclerosis or in those with risk factors for coronary artery disease, intracoronary acetylcholine causes coronary constriction instead of endothelium-derived relaxing factor-mediated dilation. Therefore, it has been hypothesized that diffuse endothelial dysfunction precedes development of coronary atherosclerosis. We tested this hypothesis in a systematic investigation of the effects of ascending doses of acetylcholine on the diameters of nonstenotic segments of the left coronary artery in patients with advanced atherosclerosis and coronary risk factors.

METHODS AND RESULTS

Effects of intracoronary infusion of acetylcholine (10(-6) to 10(-4) mol/L) on diameters of proximal, middle, and distal nonstenotic segments of the left coronary artery were studied in 28 consecutive patients with chronic stable angina, positive exercise tests, and angiographic evidence of obstructive atherosclerosis (> or = 50% reduction in lumen diameter in at least one vessel). Two patterns of response to the maximal acetylcholine dose (10(-4) mol/L) were observed. In 21 patients (group 1), only constriction was observed in all left anterior descending and circumflex artery segments studied (16 +/- 3%, 19 +/- 4%, and 23 +/- 4%, respectively; P < .01 compared with control). In 7 other patients (group 2), both constriction and dilation were observed in adjacent segments of the same vessel; maximal acetylcholine dose caused constriction in 14 left anterior descending artery segments from a control diameter of 1.94 +/- 0.19 to 1.33 +/- 0.26 mm (37% reduction, P < .01) and dilation in 16 other segments from 1.63 +/- 0.22 to 1.93 +/- 0.21 mm (25% increase, P < .01). In the circumflex artery, this dose caused constriction in 16 segments from a control diameter of 1.88 +/- 0.14 to 1.33 +/- 0.17 mm (31% reduction, P < .01) and dilation in 12 segments from 1.37 +/- 0.12 to 1.71 +/- 0.09 mm (34% increase, P < .01).

CONCLUSIONS

In 25% of patients studied with advanced angiographic coronary atherosclerosis and coronary risk factors, coronary segments with acetylcholine-inducible dilatation are present. In these patients, the endothelium is not diffusely dysfunctional as currently believed but rather shows marked segmental heterogeneity in the response to acetylcholine reflecting degrees of endothelial dysfunction.