Hyperkalemia is occasionally encountered in veterinary medicine. The most profound effects of hyperkalemia are on the cardiac system. These include decreased myocardial excitability, increased myocardial refractory period, and slowed conduction; all effects which can cause critical cardiac disturbances. Severe hyperkalemia is a potentially life-threatening complication in a variety of conditions. Because of time it takes to get potassium concentration determined, early diagnosis of severe hyperkalemia can be made using an ECG. What is often underappreciated is that potassium is not the sole determinant of ECG changes seen in hyperkalemia."Classic" electrocardiographic findings in hyperkalemic patients include bradycardia, peaked T waves, decreased R and P wave amplitudes, prolonged QRS intervals, prolonged PR intervals, flat or invisible P waves, and deviation of the ST segment. The expected progression of electrocardiographic changes associated with hyperkalemia is well described in human medical literature. In general, the first change to occur is the development of a peaked T-wave. Next the P-R interval becomes prolonged as does theQRS duration. After this, the P-wave becomes smaller or absent due to lack of atrial depolarization. The sinus node still fires, leading to a sino-ventricular rhythm. This can then progress to a sine wave type configuration followed by ventricular fibrillation. Other findings include complete heart block and ventricular arrhythmias. What is essential to note is that the relationship between potassium levels and electrocardiographic changes may vary between individual human patients. There is also the realization that the ECG is not always a reliable marker of hyperkalemia. Normal ECG's, ECG's with minimal abnormalities, and those with "atypical" results can be obtained in human patients with severe hyperkalemia.
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