Final answer:
Patients with a prolonged QT interval should not use certain potassium channel blockers like amiodarone or procainamide and beta blockers like propranolol, as they can further extend the QT interval and risk arrhythmias.
Step-by-step explanation:
The two cardiac drugs that should NOT be used in patients with a prolonged QT interval are certain potassium channel blockers and beta blockers. Drugs such as amiodarone and procainamide, which are potassium channel blockers, can further prolong the QT interval and increase the risk of life-threatening arrhythmias. Similarly, early beta blockers such as propranolol can exacerbate an already prolonged QT interval.
The potassium channel blockers affect the repolarization phase of the action potential, which can contribute to the prolongation of the QT interval. Calcium channel blockers, while not directly implicated as contraindicated in prolonged QT, also have a negative inotropic effect, which can be detrimental in certain cardiac conditions. When treating patients with cardiac dysrhythmia, it is crucial to monitor the QT interval and adjust the medication regimen accordingly.