ISCHEMICALLY INDUCED ALTERATIONS IN ELECTRICAL-ACTIVITY AND MECHANICAL PERFORMANCE OF ISOLATED BLOOD PERFUSED CANINE MYOCARDIAL PREPARATIONS

Study objective - The aim was to investigate the direct effect of ischaemic insult on the electrical activity or mechanical performance of the sinoatrial node, atrioventricular node or Purkinje fibres (ventricular papillary muscle) of the dog heart.Design - The three isolated tissues independently mounted in a water jacketed glass container were cross circulated through their respective arteries at a constant pressure (120 mm Hg) with arterial blood from a heparinised supporting dog under sodium pentobarbitone anaesthesia. Rate of automatic rhythmical contractions (automaticity) or tension development was measured before, during, and after ischaemia (30 min).Experimental preparations - Seven sinoatrial nodes, five atrioventricular nodes, and 11 papillary muscles were prepared from the heart of mongrel dogs, weighing 11.5-15 kg.Measurements and main results - The automaticity of the sinoatrial node and atrioventricular node exhibited a high degree of resistance to ischaemia, whereas both the automaticity (Purkinje fibres) and the tension development of papillary muscle were more susceptible to ischaemia and ultimately disappeared during ischaemia. Upon resumption of the blood supply, contractility rapidly returned to preischaemic levels. However, force-frequency analyses revealed that even after reperfusion, papillary muscle had lost the ability to respond normally to stimulus frequencies greater than 2 Hz.Conclusions - The findings suggest (1) that in the special excitatory and conductive system, the more distal the segment is to the sinoatrial node, the more vulnerable it is to ischaemia; (2) that the myocardium impaired by ischaemic insult lacks functional reserve, as evidenced by enlargement of the differences between the alternate tensions of pulsus alternans; and (3) that cessation of the blood supply does not predominantly damage those tissues whose functions are mainly regulated by a slow inward calcium current.