Summary This study aimed to examine acute effects of photon radiation on the isolated cardiac contractile apparatus. This objective is based on successful clinical trials on the subject of intracoronary brachytherapy and the vision of also using radiation in cardiac surgery, in addition to conventional revascularization techniques to treat highly affected coronary vessels, in which bypass grafting alone would be hopeless. For this, however, it would require larger radiation fields than in conventional intracoronary brachytherapy; thus adjacent myocardium could be more affected and depression of ventricular function with hemodynamic consequences could occur. Elementary knowledge about acute myocardial reaction to radiation does not currently exist; there are only considerations about depressing radiation effects.
The model of myocardial demembranized, “skinned” fibers was used to analyze the isolated contractile apparatus under conditions of equilibration and without regulation systems of the living cell. A total of 48 right atrial appendages were obtained from patients undergoing coronary artery bypass grafting. Atrial trabeculae were irradiated with one single dose of 2, 4, 10, 20, 40, or 80 Gy, respectively, or kept unirradiated as the control. Accordingly, from each appendage, there were both trabeculae being radiated and those not being radiated. Trabeculae were demembranized with Triton-X-100. Force development and cross bridge kinetics were analyzed under isometric conditions by approximating the force courses on an exponential function. Muscle stiffness was registered to evaluate the filaments’ mechanic and elastic properties. The statistics were performed using a Repeated Measures Analysis of Variance model.
No significant differences were found between control and irradiated fibers concerning mean force amplitudes related to cross-sectional area, time constants and stiffness, regardless of the delivered dose. Up to 80 Gy, the cardiac contractile apparatus maintained its structural und functional integrity.
The absence of acute radiation effects on the contractile filaments’ function argues against acute myocardial protein deformation during brachytherapy. Thus, it is possible that potential acute radiation effects on the heart could be caused at higher levels, like vessels, changes of membrane’s properties or effects on the cardiac conduction system. Subsequently, acute depression of myocardial function caused by radiation-induced failure of the contractile apparatus has not to be expected during brachytherapy with large radiation fields. Nevertheless, long-term effects remain to be analyzed.