What factors predispose patients to digoxin toxicity?
If a patient is on a loop diuretic, such as frusemide (lasix) which decreases the level of K+ ions throughout the body rapidly, this can lead to digoxin toxicity. Digoxin acts by inhibiting the Na+ K+ pump causing an increase in NA+ in the intercellular fluid, which results in a reduced Na+ Ca++ exchange by anti-port mechanism, causing an increase in Ca++ ion accumulation in the myocardial sarcoplasmic reticulum (Totora and Grabawski 1996, p. 63). This increase in Ca++ ions will increase the force of myocardial contraction. This is because ‘Ca+ is necessary for normal muscle contration and thus elevated Ca+ levels increase force of contraction’ (Galbraith 1997, p. 378). Therefore a decrease in myocardial K would cause a further decrease in Na+K+ pump activity therefore decrease AV node activity causing an increase in ectopic pacemaker activity and would also cause an increase in vagal stimulation, causing bradycardia.
Furthermore, if a patient is on Slow K this can cause a patient to develop a K+ electrolyte imbalance. If he has not been taking enough K+ he could become hypokalaemic which would ‘increase the effects of diogoxin, causing toxicity and braydcardia’or alternatively, he could have been incorrectly dosed and given too much K+ causing hyperkalaemia which in turn could make the ‘effects of digoxin to be decreased and he could become tachycardic’ (Galbraith 1997, p. 379) although his assessment findings are not indicative of hyperkalaemia.
Because digoxin has a very low therapeutic range of 1-2mcg/l plasma concentration (Oh 1997, p. 968) any incorrect dosing or unconsidered metabolic or physical variances in this gentleman could cause him to become overdosed with digoxin.
Advanced age predisposes a person to Digoxin toxicity. This is because any ‘increase in age, especially greater than 45 years of age, declines the functional capacity of most major organ including kidneys, liver and cardiac function’ (Sander, 2001, p262). A decrease in Glomerulus’s Filtration Rate (GFR) in elderly patients makes it hard to determine the exact dose, as their rate of excretion may be hard to establish.