Final answer:
In case of asystole where the patient has no pulse and is unresponsive, the nurse would prepare to administer epinephrine and/or vasopressin, which are critical in advanced cardiac life support. Medications like beta-adrenergic blockers, are not indicated in this scenario because they may further decrease heart function.
Step-by-step explanation:
If a nurse observes a flat line on a patient's monitor, indicating asystole (the absence of any electrical activity in the heart), and the patient is unresponsive and without a pulse, the nurse would prepare to administer epinephrine and/or vasopressin. These medications are used in advanced cardiac life support (ACLS) to increase cardiac output and blood pressure during a cardiac arrest. In contrast, beta-adrenergic blockers and some other agents mentioned, like digoxin and calcium channel blockers, can have negative inotropic effects which would not be beneficial in cardiac arrest situations, and can, in high doses or certain conditions, lead to bradycardia or even heart stoppage.
Potassium channel blockers such as amiodarone and procainamide are used to treat cardiac dysrhythmias. However, in the case of a flat line, which represents total cardiac arrest, immediate medications needed are those that can support cardiac function and systemic circulation, mainly epinephrine, which is a first-line medication, and vasopressin, which may also be used as part of ACLS protocol.