Can you tell us more about the model? That would help to  understand why this might occur. Has it been used previously? 

De fato, depende do seu modelo experimental. Tenho trabalhos publicados com o modelo de isquemia e reperfusão e mesmo antes desse procedimento ocorriam alterações no traçado do eletrocardiográfico. Meus animais eram ventilados mecanicamente, e qualquer alteração na ventilação assim como o uso de determinados sedativos podem causar alterações no ECG. Porém isso não compromete o desenvolvimento do trabalho, uma que fará vários experimentos e obterá uma média e o desvio dessa média.

Recomendo: 

https://www.researchgate.net/publication/318323840_A_Simple_and_Efficient_Methodology_for_the_Study_of_Cardioprotective_Drugs_in_Animal_Model_of_Cardiac_Ischemia-Reperfusion.

https://www.researchgate.net/publication/270964283_Epilepsy-induced_electrocardiographic_alterations_following_cardiac_ischemia_and_reperfusion_in_rats?_sg=6P76v4D11X3rKFoxCNuXjhWc6lz3sZqOAvp3FqxZ2sdKogDTGcTZBTPoUMo9fnQNHQdzhQhUZZhNHA.YG55x4ykAF2_SN_FpiDdqEToEdCA5J8GWr0BMTGxAxKnj64AJA_B7CQPZhsiAyVlHrecIblIuogMzYudFVsY7w&_iepl%5BviewId%5D=bGN6HkkmlkZrs6Z1ImBOZQ8f&_iepl%5BprofilePublicationItemVariant%5D=default&_iepl%5Bcontexts%5D%5B0%5D=prfpi&_iepl%5BtargetEntityId%5D=PB%3A270964283&_iepl%5BinteractionType%5D=publicationTitle 

Article A Simple and Efficient Methodology for the Study of Cardiopr...

Article Epilepsy-induced electrocardiographic alterations following ...

I thank you a lot for your message. We are studying morphological and functional parameters of the myocardium of a cichlid fish after endoscopy-guided heart apical resection. There are no previous studies involving this animal (we are developing the model). Assessments are being conducted in the short, medium and long term, and ECG signals are showing wide variation. In the short-term, for example, ECG intervals are long (versus control), however, in the medium-term the ECG intervals are absurdly smaller than those in the control. In the long-term follow-up, the ECG intervals appear prolonged. We're looking for an explanation! We have some assumptions, but we would like opinions.  

Olá! Obrigada pela literatura recomendada. Olharemos!!!!!

You are welcome. 

I do not know if it applies to fish, but changes in pulmonary ventilation with possible changes in blood pH can influences cardiac electrical activity.