ECG
Consider rare causes:
Lyme's disease
Lupus
Myocarditis
Myocardial infarction
Lyme's disease
Lupus
Myocarditis
Myocardial infarction
First degree AV block
Third degree AV block
Second degree AV block type I 
Left bundle branch block
Second degree AV block type II
Wide-complex tachycardia
Narrow-complex tachycardia
AV dissociation without complete heart block