Which of the following is a contraindication for digoxin administration?
A. Blood pressure of 140/90.
B. Heart rate above 80.
C. Heart rate below 60.
D. Respiratory rate above 20.
Correct Answer: C. Heart rate below 60.
The apical heart rate must be monitored during therapy with digoxin, and the drug held for a pulse below 60 and above 120. Remember that digoxin lowers the heart rate; therefore, the choice that reflects a low heart rate is the best selection. Digoxin toxicity is clinically relevant as it can lead to fatal cardiac arrhythmias. The estimated frequency is at about 0.8 to 4% of patients on steady digoxin therapy. The rate of toxicity increases as serum digoxin concentration reaches over 2.0 ng/ml.
Option A: However, toxicity can also occur at lower levels, especially in the setting of other risk factors such as low body weight, advanced age, decreased renal function, and hypokalemia.
Option B: It increases the force of contraction of the heart by reversibly inhibiting the activity of the myocardial Na-K ATPase pump, an enzyme that controls the movement of ions into the heart. Digoxin induces an increase in intracellular sodium that will drive an influx of calcium in the heart and cause an increase in contractility. Cardiac output increases with a subsequent decrease in ventricular filling pressures.
Option D: Digoxin has vagomimetic effects on the AV node. By stimulating the parasympathetic nervous system, it slows electrical conduction in the atrioventricular node, therefore, decreases the heart rate. The rise in calcium levels leads to prolongation of phase 4, and phase 0 of the cardiac action potential thus increases the refractory period of the AV node. Slower conduction through the AV node carries a decreased ventricular response.