ECG of the month. Pleural effusion with hyperthyroidism.
Riesen SC, Lombard CW.
J Am Vet Med Assoc. August 2005;227(4):556-8.

In cats, ventricular preexcitation is a very rare conduction disturbance that is characterized by AV conduction via an abnormal accessory pathway in addition to the usual AV-nodal conduction that is dormant. Extranodal pathways lie outside the AV node and are commonly referred to as bundles of Kent. Some accessory pathways connect the atrial with the bundle of His (atriohisian fibers) or, alternatively, connect the proximal portion of the AV node with the bundle of His (intranodal or nodohisian fibers). Other accessory pathways connect the AV node with ventricular muscle (nodoventricular fibers). The combination of hypothermia, hyperthyroidism, and voluminous pleural effusion (15% of body weight), is likely to have led to the conduction abnormalities detected in the cat of this report. We speculate that these 3 factors influenced the intra- and extracellular ionic milieux considerably, facilitating abnormal conduction velocities and propagation of electrical impulses. Furthermore, atrial myocardial changes were likely to have contributed to the observed abnormal P-wave morphology (e.g., slow upstroke of the initial part) and facilitated the generation of atrial premature contractions. Because the P-wave amplitude was reduced by approximately half in the follow-up ECG, the slow upstroke was not visible as well as it had been previously. Generally, most ECG abnormalities resolve with successful management of thyrotoxicosis. In the cat of this report, the conduction disturbance resolved and there was notable regression of the previously increased R-wave amplitude after an increase of the methimazole dose.