Clinical and experimental aspects of myocardial stunning

Temporary biventricular pacing postcardiopulmonary bypass in patients with reduced ejection fraction

BACKGROUND AND AIM

Patients with low ejection fraction (EF) undergoing myocardial revascularization frequently require ventricular pacing following cardiopulmonary bypass (CPB). While the benefits of chronic biventricular (BiV) pacing in patients with low EF are well established, there are little data on acute effects during heart surgery. This study analyzed the response of BiV versus single ventricle lead pacing on hemodynamics and left ventricular (LV) function immediately following CPB.

METHODS

Ten patients with decreased LV EF (mean = 35 +/- 6%) underwent open-heart surgery with CPB. Temporary pacing electrodes were placed on the right atrium, apex of the right ventricle, and lateral wall of the LV after separation from CPB. The hemodynamic effects of three atrio-ventricular (right, left, and BiV) pacing modes were studied for four minutes each. The pacing sequence was randomly allocated with a resting period of three minutes between each mode. Hemodynamic and echocardiographic data of LV function were collected. Statistical analysis was performed with analysis of variance.

RESULTS

BiV pacing increased cardiac output by 4%, 13%, and 44% over right ventricular pacing, LV pacing, and pre-bypass values, respectively. The fractional area of change increased significantly with BiV pacing compared to right ventricular and LV pacing (36%, 35% to 44%, p < 0.01). An increased propagation velocity of 49 cm/s compared to 38 cm/s and 40 cm/s for right ventricular and LV pacing, respectively, suggested an improvement in diastolic function.

CONCLUSION

In patients with low EF, BiV pacing immediately after CPB significantly improves LV systolic function and cardiac output, and suggests significantly improved diastolic function.

Surgery for myocardial salvage in acute myocardial infarction and acute coronary syndromes

This article addresses the pathophysiology, the treatment options, and their rationale in the setting of life-threatening acute myocardial infarction and acute on chronic ischemia. Although biases may exist between cardiologists and surgeons, with this review, we hope to provide the reader with information that will shed light on the options that best suit the individual patient in a given set of circumstances.

Pulmonary embolism and myocardial hypoxia during extracorporeal membrane oxygenation

The treatment of a newborn with severe meconium aspiration by venoarterial extracorporeal membrane oxygenation (ECMO) was complicated by myocardial hypoxia with a marked decrease of myocardial contractility. The onset of the cardiac hypoxia was related to a pulmonary artery embolus. The origin of the embolus was a deep femoral vein thrombosis, caused by a central vein catheter, which was inserted 1 day before ECMO by venous cutdown. The possible pathophysiology of myocardial hypoxia in this patient is discussed, especially with regard to myocardial perfusion, supporting the hypothesis of coronary perfusion occuring with blood from the left ventricle and not from the arterial cannula in the aorta.

Effect of triiodothyronine administration in experimental myocardial injury

Twelve healthy pigs were subjected to a 20-min, period of regional myocardial ischemia by snaring the left anterior descending coronary artery (LAD) between its first and second diagonal branches. The resulting myocardial injury caused significant acute hemodynamic impairments. Cardiac index declined significantly during reperfusion interval and returned to preischemic level by postoperative day 7. Plasma total triiodothyronine (TT3), free triiodothyronine (FT3) and free fatty acid (FFA) decreased gradually and reached the nadir at 6 h after LAD occlusion. In contrast, plasma reverse triiodothyronine (rT3) increased progressively after LAD occlusion and reperfusion. To investigate the effect of T3 on ischemic myocardium, T3 (0.2 microgram/kg/dose; n = 5) or saline (placebo; n = 6) was administered immediately, 30 min, 60 min, 90 min, and 120 min after reperfusion. Plasma TT3 and FT3 increased dramatically after triiodothyronine supplement but declined to presichemic level at six h after LAD occlusion. The pigs treated with T3 demonstrated a rapid improvement in cardiac index over the reperfusion interval, whereas cardiac index in the placebo group remained depressed. Myocardial oxygen consumption estimated by rate pressure product showed no difference between placebo and T3-treated groups. Oxygen extraction as O2 saturation difference between aorta and coronary sinus was less in T3-treated group. Nine pigs (four in the T3-treated group and five in the placebo group) were subjected to euthanasia with hypertonic KCl solution on postoperative day 7. Myocardial infarct size determined by triphenyltetrazolium chloride (TTC) tissue enzyme staining technique was not significantly different between T3-treated and placebo groups. We concluded that this animal model is a useful model of myocardial injury simulating "euthyroid sick syndrome" as seen in patients with cardiopulmonary bypass, and T3 supplementation after reperfusion significantly enhanced postischemic left ventricular functional recovery but did not affect myocardial oxygen consumption and myocardial infarct size.

Protective effects of adenosine in the reversibly injured heart

BACKGROUND

There is substantial evidence that the nucleoside adenosine reduces postischemic ventricular dysfunction (ie, myocardial stunning). Studies performed in our laboratory have attempted to address the mechanism of adenosine-mediated protection of the reversibly injured heart.

METHODS

Experiments were performed in isolated perfused rat and rabbit hearts and in in situ canine and porcine preparations. The role of adenosine A1 receptors was assessed by using adenosine A1 receptor agonists and antagonists, and by measuring interstitial fluid purine levels with the cardiac microdialysis technique.

RESULTS

In isolated perfused hearts, treatment immediately before ischemia with adenosine and adenosine A1 receptor analogues significantly improved postischemic ventricular function, effects that were blocked by a selective adenosine A1 receptor antagonist. In in situ canine and porcine preparations, pretreatment with adenosine and an adenosine deaminase inhibitor increased preischemic interstitial fluid adenosine levels and attenuated regional myocardial stunning. Adenosine treatment was also associated with improved myocardial phosphorylation potential in isolated guinea pig hearts and in the in situ porcine preparation.

CONCLUSIONS

These results suggest that adenosine-induced attenuation of myocardial stunning is mediated via adenosine A1 receptor activation and enhancement of postischemic myocardial phosphorylation potential.

Ischemic heart disease and antioxidants: mechanistic aspects of oxidative injury and its prevention

The disease state of myocardial ischemia results from a hypoperfusion-induced insufficiency of heart-muscle oxidative metabolism due to inadequate coronary circulation. Myocardial ischemia is an important, lifespan-limiting medical problem and a major economic health-care concern. Reperfusion, although avidly pursued in the clinic as essential to the ultimate survival of acutely ischemic heart muscle, may itself carry an injury component. Cardiac reperfusion injury appears to reflect, at least in part, an oxidant burden established upon reoxygenation of ischemic myocardium. Laboratory evidence demonstrates that oxidative stress to the heart-muscle cell (cardiomyocyte) can elicit the three known types of ischemia-reperfusion injury that directly affect the myocardium: arrhythmia, stunning, and infarction. The limited clinical occurrence of serious reperfusion arrhythmias has restricted the importance of antioxidants as antiarrhythmic agents against this form of myocardial ischemia-reperfusion damage. Despite the utmost clinical significance of lethal cardiomyocyte injury as a negative prognostic indicator for the ischemic heart-disease patient, inconsistent results of antioxidant interventions in reducing infarct size have somewhat tempered interest in antioxidant infarct trials. By contrast, the negative clinical consequences of stunning may indeed be preventable by utilizing antioxidants to help restore postischemic cardiac pump function. Several as yet unanswered questions remain regarding oxidative stress in the reperfused heart, its significance to cardiomyocyte damage, and its ability to elicit specific postischemic myocardial derangements. Targeted mechanistic studies are required to address these questions and to define the pathogenic role of oxidative stress (and, hence, the therapeutic potential of antioxidant intervention) in myocardial ischemia-reperfusion injury. The overall aim of current research in this area is to enable the cardiac surgeon/cardiologist to advance beyond the largely palliative drugs now available for management of the coronary heart-disease patient and attack directly the pathogenic determinants of heart-muscle ischemia-reperfusion injury. Optimal use of antioxidants may help address this important medical need.

Metabolically based treatment of stunned myocardium

Reversible myocardial ischemia is associated with a rapid decrease in contractility and prolonged postischemic ventricular dysfunction, due in part to altered intracellular calcium handling and/or contractile protein dysfunction. The maintenance of intracellular calcium homeostasis and force development by the contractile apparatus are dependent upon the free energy derived from ATP hydrolysis. This energy of hydrolysis is determined by the myocardial phosphorylation potential, an estimate of which can be made from the ratio (CrP)/(Cr) x (P(i)). Results from in vitro and in vivo studies suggest that pyruvate enhances contractility in both normal and stunned myocardium by enhancing myocardial phosphorylation potential. In regionally stunned porcine myocardium, pyruvate infusion increased recovery of regional ventricular function from 33% +/- 4% of preischemic systolic wall thickening to 81% +/- 4% and increased the (CrP)/(Cr) x (P(i)) ratio fivefold from 0.21 +/- 0.04 to 1.05 +/- 0.08. Thus, metabolic substrates that enhance myocardial energetics and ventricular function may be effective agents for attenuating postischemic ventricular function.

Intracoronary injection of basic fibroblast growth factor enhances angiogenesis in infarcted swine myocardium

OBJECTIVES

This study was performed to examine the effect of intracoronary exogenous basic fibroblast growth factor (bFGF) on angiogenesis in infarcted myocardial regions.

BACKGROUND

Exogenous bFGF is a potent promoter of angiogenesis. Little information is available on its effect on myocardial angiogenesis.

METHODS

Myocardial infarction was induced in 10 pigs by intracoronary injection of microscopic beads. Four pigs served as a control group; in six pigs slow-release bFGF was delivered by the beads. Cardiac performance was evaluated by repeated echocardiographic measurement and angiogenesis was evaluated by immunohistochemical studies 14 days later.

RESULTS

As compared with control pigs, pigs treated with bFGF had higher microvessel counts (mean +/- SEM) in both viable tissue (141 +/- 27 per field vs. 39 +/- 4, p = 0.01) and nonviable tissue (329 +/- 26 per field vs. 95 +/- 7, p < 0.001) within the infarct area. No significant differences in total regional left ventricular wall motion were noted between the two groups throughout the 14-day study period.

CONCLUSIONS

In the swine, direct intracoronary application of bFGF to infarcted myocardium enhances myocardial neovascularization within 2 weeks.