Effects of graded pleural effusion on QRS in the dog

Six anesthetized healthy dogs were placed in right lateral recumbency. Electrocardiographic leads I, aVF, V10, rV3, V3, and V5 were obtained during a control period and after introduction of pleural effusions ranging between 0.66 and 55 ml of isotonic saline solution/kg of body weight. Peak-to-peak amplitudes of QRS were measured, and the effusion required to decrease this amplitude by more than 1 SD from the mean of healthy dogs was determined for each lead tested. Sensitivity of detecting pleural effusion was greatest in lead I in which an effusion of 2.75 ml/kg could be detected. The QRS amplitudes in leads V10 and aVF were decreased to a significant level only at large effusions. Leads V3 and V5 were "blind," even to the greatest pleural effusions. Radiographs taken with the graded pleural effusion documented that the heart "floated" away from rV3 and toward V3 and V5, thus explaining the sensitivity of rV3 and insensitivities of V3 and V5. It seems that pleural effusion can be detected by using these leads in dogs in right lateral recumbency, that detection is best if serial recordings are taken, and that relatively high sensitivities are achieved using lead I.