Electrocardiographic characterization of myocardial function in normoxic and hypoxic teleosts

Electrocardiography was applied to analyze cardiac function of four teleost species (Piaractus mesopotamicus, Hoplias malabaricus, Hoplias lacerdae and Cyprinus carpio) during normoxia and graded hypoxia. In these species, hypoxic bradycardia consistently occurred during severe hypoxia (below the critical oxygen tension--PCO2) and was accompanied by alterations in the ECG recordings. Three basic ECG alterations were demonstrable: 1) increase in the T wave area and amplitude, being more positive and with symmetrical morphology during severe hypoxia (P. mesopotamicus); 2) negative T wave in normoxia, changing to isodiphasic (just above the PCO2) and positive (below the PCO2; H. malabaricus and H. lacerdae); 3) positive T wave in normoxia, changing to negative in severe hypoxia (5 mmHg; Cyprinus carpio). These findings indicate changes in the direction of ventricular repolarization during exposure to severe hypoxia, and the analysis of the ECGs in relation to the derivation line permitted the estimation of these drifts to be 17 degrees in P. mesopotamicus, 46 degrees in H. malabaricus, 43 degrees in H. lacerdae, and 32 degrees in C. carpio. The changes in the direction of ventricular repolarization were attributed to myocardial impairment due to insufficient oxygen supply, and support the idea of a relationship between cardiac dysfunction and the bradycardia developed during severe hypoxia.