Changes in myocardial ultrastructure induced by cooling as well as rewarming

The aim of the present study was to investigate if hypothermia and rewarming, without accompanying cardiac ischaemia or cardioplegia, causes myocardial damage. Anaesthetized rats were subjected to a cooling procedure (4 h at 15–13°C) where spontaneous cardiac electromechanical activity was maintained, followed by rewarming. Control rats, hypothermic rats and posthypothermic rats were perfusion-fixed, the hearts removed and the ventricles examined using an electron microscope. Based on morphometric methodology volume fractions as well as absolute volumes of cellular and subcellular components of the ventricles were assessed. In hypothermic hearts capillary volume fraction was significantly decreased, which was probably due to a decrease in perfusion pressure. The cytosolic volume increased in both absolute values and as a fraction of the myocyte: from 25 ± 11 in controls to 43 ± 8 μl and from 0.067 ± 0.023 to 0.102 ±0.013, respectively. There was a corresponding relative decrease in the volume fraction of myofilaments from 0.598 ± 0.030 to 0.548 ± 0.024. In posthypothermic hearts significant tissue swelling was apparent, dominated by a significant increase in myocyte volume from 372 ± 66 in controls to 522 ±166 μl. Similar changes were measured in mitochondrial and cytosolic volumes. In conclusion, the myocardial ultrastructure was altered during hypothermia as well as after rewarming. Posthypothermic myocardium showed generalized cellular swelling and areas of cellular necrosis.