Effect of potassium excess on pacemaker activity of canine sinoatrial node in vivo

Hyperkalemia secondary to renal hypoperfusion in a dog with third-degree atrioventricular block

OBJECTIVE

To demonstrate a critical role of artificial cardiac pacing to normalize severe bradyarrhythmia-induced hyperkalemia in a dog with third-degree atrioventricular (AV) block.

CASE SUMMARY

A dog was presented for evaluation of severe hyperkalemia and bradyarrhythmia. ECG revealed third-degree AV block with heart rate of 10-20 min, which was lower than usual ventricular escape rate. The etiology of severe hyperkalemia was considered secondary to significantly decreased renal perfusion and potassium excretion as a result of poor cardiac output caused by the AV block. Since potassium disorders may lead to life-threatening conditions such as cardiopulmonary arrest, artificial cardiac pacing was immediately pursued. Within 1 hour, serum potassium concentration was normalized, which confirmed that severe hyperkalemia occurred primarily due to third-degree AV block. The dog was then treated with permanent pacemaker implantation. A recheck evaluation 3-month post pacemaker implantation showed normal serum potassium concentration and the dog was clinically healthy.

NEW OR UNIQUE INFORMATION PROVIDED

This report describes hyperkalemia secondary to significantly decreased cardiac output and renal hypoperfusion caused by third-degree AV block. The authors propose that artificial cardiac pacing must be considered as an emergency treatment for bradyarrhythmia-induced hyperkalemia, which ultimately restores cardiac output and renal perfusion and normalizes serum potassium concentration.

The heart cell--electrophysiological aspects

A short review is given of the ionic fluxes associated with the action potential in Purkinje fibres, and in sinus and AV-node, and of the influence of variations in the extracellular concentrations of K+, Na+, Ca2+ and Mg2+ on basic electrophysiological parameters. Variations in serum potassium concentration often plays an important role in the genesis of arrhythmias by changing several electrophysiological parameters, whereas only extreme variations in the serum calcium level produce electrophysiological effects of clinical importance. Hypo- and hypernatremia within the ranges observed clinically most probably do not produce significant electrophysiological changes. Variations in extracellular magnesium concentration seem to have effects on electrophysiological parameters particularly when they occur simultaneously with changes in the concentrations of calcium and potassium.

Distribution of coronary artery flow to the canine right atrium and sinoatrial node

We assessed segmental distribution of blood flow to the right atrium and the region of the sinoatrial node using micropheres (7-10 mum) in 20 anesthetized dogs. Mean right atrial flow averaged 83+/-7 (SE) ml/min X 100 g, which was 56% of the left ventricular blood flow. The distribution of right atrial flow was not homogeneous. For example, flow to anterior right atriaater (105+/-8 ml/min X 100 g) than mean right atrial flow. Following ligation of the sinus node artery, perfusion of the segment containing the sinus node decreased by only 36%. Relative preservation of perfusion to the sinus node following sinus node artery ligation may explain why ligation of the sinus node artery does not alter heart rate. Furthermore, we also found that cannulation and pump perfusion of the sinus node artery at pressures 10 and 50 mm Hg greater than systolic pressure did not alter the distribution of right atrial flow. Thus, because cannulation and perfusion of the sinus node artery do not artifactually distort regional right atrial blood flow, we conclude that this should be a useful method for evaluating responsiveness of the sinus node to various interventions.

Evaluation of sino-atrial node function in man by overdrive suppression

Sino-atrial node (SAN) function was evaluated in 46 patients, three of whom had the sick sinus syndrome. Patients were paced from the right atrium for 15 to 180 sec at rates of 90, 110, 130, and 150 beats/min. The rapid cessation of pacing was associated with suppression of the SAN at all paced rates and at all durations of pacing. The observed pause was terminated by a sinus beat in all instances. The duration of pacing had little influence on the duration of the observed pause. The pause increased as the pacing rate was increased until, at a rate of 150 beats/min, a marked decrease in the pause was noted. Atropine (1.5-3.0 mg iv) diminished but did not eliminate the SAN suppression. Subthreshold pacing did not suppress SAN function. Three patients with sick sinus syndrome had a greater degree of SAN suppression than normal patients (4732 ± 415 msec [SSS] M ± sem ; 1041 ± 56 msec for normal patients).

The determination of the duration of the pause following cessation of atrial pacing provides a technique for recognition of abnormalities of SAN function.