Electrocardiographic changes in induced hyperkalemia in ponies.

: In 11 crossbred ponies, hyperkalemia was induced by acute intracardiac infusion of a 0.4M solution of potassium chloride. The ECG changes noticed can be assigned to 4 stages: (1) a widening and a lowering of amplitude followed by inversion and final disappearance of the P wave; (2) an increase in the amplitude of the T wave; (3) an increase in the QRS interval, with some irregularity in the ventricular rate; and (4) periods of cardiac arrest that became terminal or were followed by ventricular fibrillation. At plasma potassium values of greater than 7.5 mM/L, the T-wave amplitude and duration of QRS increased. In most ponies, a 50% increase in the QRS duration was present when K+ values were 9 to 10 mM/L. The P waves are absent at plasma K+ values of 9.4 to 12.6 mM/L. Sinusoidal-shaped QRS-T complexes and periods of cardiac arrest occurred before the development of the terminal dysrhythmia. At plasma K+ values of 11.3 to 17.3 mM/L, cardiac standstill or ventricular fibrillation occurred. The Q-0 Tc and Q-Tc intervals were not altered by hyperkalemia.