Inclusion Criteria:

• Patients presenting to Charleston Area Medical Center (CAMC) General or Memorial Hospital ED with a primary diagnosis of AF/FL
• Patients with a mean ventricular rate of 100 beats per minute or more within one hour of presentation

Exclusion Criteria:

• Under age 18 years
• A diagnosis of acute coronary syndrome (ACS) made by the admitting ED physician (ST elevation myocardial infarction, non-ST elevation myocardial infarction, unstable angina) (beta blockers are a Class I medications for ACS)
• Known history of heart failure with an ejection fraction <50%
• Known ejection fraction <45%, regardless of a history of heart failure. Heart failure and a history of heart failure with an ejection fraction of 40-50% may occur with a normal ejection fraction now referred to as Heart Failure With Preserved Ejection Fraction (HFpEF) or "diastolic dysfunction". A low ejection fraction is not always associated with heart failure. Our technology of measuring ejection fraction is by no means perfect. It is acceptable to use MET in larger than usual starting doses of MET for rate control or patients with "diastolic dysfunction", but not systolic dysfunction. Thus, a patient who has an ejection fraction of 42% may possibly have an ejection fraction of 37%, possible lower. Thus the investigators want to avoid the possibility of a patient with a history of heart failure does not receive MET unless the investigators feel systolic heart failure is not present.
• Systolic blood pressure <90 mm Hg or between 90-99 AND patient is experiencing symptoms of dizziness
• Known allergy or adverse reactions to diltiazem or metoprolol. This is very rare.

Exclusions from ECG readings:

• Current Atrioventricular (AV) block (2nd or 3rd degree)
• Pre-excitation syndromes - Wolfe Parkinson White (WPW) (Accelerated AV conduction- a rare condition where MET and DT are not advised)
• Pulse rate less 100/minute on ED admission (already at rate control)
• Cardiogenic shock or heart failure requiring inotropic agents or intubation
• Respiratory failure requiring intubation
• Pregnancy or lactation (neither pregnancy or lactation are listed as definitely safe for either medication)
• Asthma, defined as (asthma is a relative contraindication for MET:
• current use of inhaler
• use of steroids for dyspnea
• history of being treated for asthma
• Inability or unwillingness to provide informed consent
• Physician decision
• If either medication is a relative contraindication, the patient cannot be randomized.