Background. Pathogenesis of cardiac contusion involves primary traumatic and secondary hypoxic mechanisms of myocardial contractile function depression as well as body reactions aimed at adapting to altered environment. A significant part of these reactions is realized in the context of stress. The intensity of the stress component in myocardial dysfunction is largely determined by the individual stress reactivity of the body. Objectives. To assess the contractile function and functional reserves of the myocardium of rats with a high and low level of stress resistance in the post-traumatic period of cardiac contusion. Methods. A preclinical experimental randomized trial involved 134 white mature nonlinear male rats weighing about 270 g. The animals were divided by sequentially performed Open Field and Porsolt Forced Swim tests into 2 groups: group 1 — animals with low stress resistance, group 2 — animals with high stress resistance. Within each group, the animals were randomized into control and experimental subgroups. In the experimental subgroups, 6, 12 and 24 hours after simulating cardiac contusion, the force and rate indicators of myocardial contractility were evaluated using the Fallen isolated heart model. The summary measures of the study included assessing the contractile force and rate of isolated hearts of rats with low and high stress resistance, particularly under conditions of high-frequency rhythm load (ranging from 4.0 to 8.3 Hz) during the post-traumatic period of cardiac contusion. Data analysis was performed using software packages MS Office 2013 (Microsoft Corporation, USA) and Statistica, v. 10 (StatSoft, USA). The differences were considered to be statistically significant at p < 0.05. Results. 6, 12 and 24 hours after simulating a cardiac contusion, contractile force and rate of isolated hearts decreased in group 1 and group 2. In low stress-resistant animals, immediately following the stabilization period and during high-frequency rhythm test, the contractility force and rate in isolated hearts were statistically significantly lower (p = 0.0008) compared to those recorded in highly stress-resistant individuals. During the stimulation of a high-frequency rhythm, isolated hearts in the experimental group revealed diastolic dysfunction at all time points. In highly stress-resistant animals, diastolic dysfunction occurred at a heart rate of 300 min-1 and above, whereas in low stress-resistant animals, it manifested at a heart rate of 240 min-1 and above. At the same heart rate, diastolic dysfunction in low stress-resistant animals was statistically significantly greater (p = 0.0008) compared to that of highly resistant animals. Conclusion. The post-traumatic period following experimental myocardial contusion is characterized by a reduction in the force and rate of myocardial contractility, as well as a decrease in functional reserves of the myocardium, regardless of stress resistance. High stress resistance is associated with better preservation of cardiac contractile function and contractility reserves, whereas low stress resistance correlates with a more pronounced degree of myocardial dysfunction and a significant reduction in functional reserves of the contused heart. Differences in the severity of contractile dysfunction under conditions of high and low body resistance to stress can be attributed to varying degrees of secondary myocardial damage in the contused area, resulting from the misbalance between stress-activating and stress-limiting mechanisms involved in the development of secondary damage.
