What emergency drugs are likely to be effective in the patient with a transplanted heart?
Only direct-acting inotropes, chronotropes, and vaso-constrictors will be immediately effective. Epinephrine is usually the direct-acting inotrope of choice. Direct-acting chronotropes include epinephrine, isoproterenol, and dobutamine. Pacing (via external pacing pads or a trans-venous pacing wire) is also an option to increase heart rate. For reasons previously discussed, atropine is not likely to increase heart rate in a denervated graft. Direct-acting vasoconstrictors include phenylephrine and perhaps vaso-pressin. For the majority of situations, small bolus doses of epinephrine and phenylephrine are effective and are used as first-line agents when necessary.
Ephedrine is a noncatecholamine sympathomimetic agent with both direct and indirect actions on α- and β-adrenergic receptors. The indirect actions of ephedrine come from enhanced release of norepinephrine. While ephedrine can be used advantageously in the patient with a transplanted heart, it is not usually considered a first-line emergency drug because at least part of its desired action must await the subsequent release of norepinephrine.
Dopamine, a drug which also has both direct and indirect effects, is the immediate precursor of, and causes the release of, norepinephrine. In the cardiac transplantation patient, dopamine would initially be expected to exert only its direct dopaminergic effects (e.g., coronary and splanchnic vaso-dilatation) through dopaminergic receptors. The vasocon-strictive α-effects and the desired β-adrenergic effects on heart rate and contractility are only going to come from sub-sequently released norepinephrine. When all this is taken into account, dopamine may not be the optimal first-line choice when inotropy or chronotropy is urgently needed.