The patient:  This ECG is taken from a 55-year-old man whose wife called 911 because he had a syncopal episode.  When the paramedics arrived, he was conscious and alert, and denied any symptoms.  He gave a history of "cardiac", diabetes, and opiate abuse.  We do not know the nature of his cardiac history or his medications.  

It is difficult to pinpoint a definite diagnosis with this lack of information and a clearly abnormal ECG.  We will limit our discussion to listing the abnormalities seen:

The ECG rhythm:  There is a fast, regular rhythm that is supraventricular in origin (there are P waves).  When a supraventricular rhythm has a rate of about 150 per minute, we should ALWAYS consider ATRIAL FLUTTER WITH 2:1 CONDUCTION.  Atrial flutter produces P waves (flutter waves) at approximately 250-350 per minute.  The normal AV node is able to conduct half of these, at a rate of about 150 per minute. Atrial flutter with 2:1 conduction is the most common presentation of new-onset atrial flutter.  It is often missed by people who expect to see several flutter waves in a row, producing the "sawtooth pattern".  That being said, atrial flutter is usually discernable in at least a few leads if it is present.  We do not see any signs of flutter waves in this ECG.

That leaves us with a differential diagnosis of sinus tachycardia vs. one of the regular supraventricular tachycardias like reentrant tachycardias or atrial tachycardia.  Sinus tachycardia can be recognized by several features. If we are fortunate enough to witness the onset or offset of the fast rhythm, will will recognize sinus tachycardia by a "warm up" or gradual speeding up of the rate, and a "cool down", or gradual slowing.  On the other hand, SVTs often have abrupt onset and offset.  Sinus tachycardia often has a very obvious cause, such as hypovolemia, fever, pain, anxiety, vigorous exercise, or hypoxia.  Sinus tachycardia usually has a distinct, upright P wave in Lead II, and a clearly-seen, often negative, P wave in Lead V1.  This ECG does not show the onset of the tachycardia, and is not long enough to evaluate for rate changes. Lead II appears to have upright P waves on the downslope of the previous T waves. V1 has deeply negative P waves, and V4 has the most clearly-seen P waves.  Without being positive, this looks more like sinus tachycardia than a reentrant tachycardia.  It would help to know more about the patient's condition.

This ECG was taken from an unknown patient.  It shows sinus tachycardia with left bundle branch block. The ECG criteria for left bundle branch block are: 

* Wide QRS (.12 seconds or greater)

* Negative QRS deflection in V1

* Positive QRS in Leads I and V6 

* Supraventricular rhythm

In addition to these criteria, left bundle branch block will cause repolarization abnormalities.  This is because depolarization is altered through the left ventricle, which causes repolarization to also be altered.  Instead of the electrical impulse traveling down the left bundle branch to depolarize the left ventricle, it depolarizes the right ventricle first, then spreads cell-to-cell across the larger left ventricle. The ST and T wave changes caused by left bundle branch block are normally “discordant”.  That is, the ST segment will be elevated in leads with negative QRS complexes, and depressed in leads with positive QRS complexes.  This elevation and depression of the ST segment may “imitate” the changes caused by acute myocardial infarction.  They may also work to conceal M.I. changes, as we may not recognize  STEMI as we attribute the ST changes to the left bundle branch block itself. 

For more on determining the presence of acute M.I. when the patient has left bundle branch block, check out these links:  ECG Guru, LBBB with AMI; Life in the Fast Lane, Sgarbossa Criteria; EMS 12-Lead, Sgarbossa Criteria;  Dr. Smith's Modified Sgarbossa Criteria.