I think this is preexcited atrial fibrillation. Shortest V to V is less than 200 ms. This puts him at high risk for SCD. If clinically stable, Procainamide IV. If not, then DC CV. Next step is ablation as an in-patient.

This ECG is quite scary! Upon a closer look, we can conclude that this is an IRREGULAR WIDE COMPLEX TACHYCARDIA.

Irregular because there is beat to beat variability(RR variability is seen throughout the entire tracing)
Wide because the QRS duration is more than 3 small boxes or 120 ms
Tachycardia because heart rate is more than 100 beats per minute

When there is no discernible atrial electrical activity(P waves) seen, or when you are unsure whether they are present. There are 4 differential diagnosis that you will need to consider when it comes to irregular WCT

1)AFib with wide QRS as a result of aberrant conduction or pre-existing bundle branch block
2) AFib with an accessory pathway(WPW)
3) Polymorphic VT or Torsade De Pointes(if there is documented QT prolongation)
4) Normal VT

The conventional teaching is that when you see a wide complex tachycardia, think about VT,VT and VT first before anything else especially when it is regular

4) Although VT can sometimes be slightly irregular, we exclude this from our differential diagnosis because of the marked persistent irregularity seen in this tracing. If you can appreciate in the long rhythm strip, some of the RR interval is twice the duration of others(some of them is approximately 2 big box some of them is 1 big box)

3) we rule out polymorphic VT or Torsade because the QRS morphology looks quite consistent throughout. And there is no twisting of the axis too!

1 and 2 can be difficult to differentiate. The importance of differentiating lies in the fact that normal AV nodal blocking drugs used recommended for controlling ventricular response of AFib are CONTRAINDICATED because they will accelerate conduction down the accessory pathway precipitating hemodynamic compromise

If this is AFib, the rate is much faster than usual...200 to 300 per minute in various parts of the tracing. The RR interval appears to be in between 1 to 1.5 big box in most part of the tracing. This rate is just too fast for normal conduction to occur down normal AV pathway(the AV node is like a red light in the conduction system and it usually does not allow more than 200 impulses to be conducted each time). Therefor, there must be a bypass tract in the heart that allows impulses to bypass the AV node. Some say if you find AFib with wide QRS at an exceedingly fast rate..consider WPW syndrome

I can't remember the Specific treatment for this as most beta blockers and CCB are contraindicated.Procainamide seems to be quite effective BUT ACLS recommends expert consultation(which I think should be done ASAP) and a synchronized cardioversion should den done at any point the patient becomes unstable.

Changes when normal rhythm is restored. shortened PR,widened QRS complexes, delta waves

Excellent points by Dr. RShama & CheeYongChuan (both of whom make Jason's job easy). CheeYongChuan covers highlights in excellent fashion. I'll add just a few points:

• Re Treatment: My review of the literature suggests that in addition to Procainamide - that either Amiodarone or Ibutilide could also be used. Each drug has its advocates - and none is perfect. This is a difficult subject to study, given the acute situation of patients on presentation (not the type of patient you'd want to enroll in a controlled, prospective trial giving full time for drug loading without option to emergently cardiovert .... ) - and studies I've seen are problematic in methodology ... (My write-up in ACLS Comments #5 ). As per Dr. RShama - cardioversion may be needed - and these patients should be promptly referred to EP for ablation (which may be curative).
• Excellent discussion by CheeYongChuan - which I'd amend by fast-track to the conclusion that once you become familiar with the "snapshot" that Jason shows - your diagnosis should immediately be WPW with Very Rapid AFib. This rhythm can't be anything else! It is way too fast for normal AFib (300/min in various parts of the tracing) - the QRS is very wide - and WPW with AFib characteristically shows the periodic relative slowing seen here that punctuates periods of excessive rapidity. Challenge yourself on that concept with this tracing (ECG Blog #37) - which Jason assisted me with on my interpretation.
• Finally - Regarding Jason's question about use of Verapamil (or for that matter Diltiazem) for this rhythm? - that would be a "No-No", with potentially disastrous consequence since drugs like Verapamil/Diltiazem/Digoxin (and ? Beta-Blockers) may speed up forward conduction down the accessory pathway, thereby precipitating deterioration to VFib ... (ECG Blog #18).

GREAT tracing from the "endless" files of Jason Roediger!

I actually think the first 1/3 of the ECG shows atrial flutter with 2:1 conduction and pre-excitation. It looks to be pretty regular at 150 bpm. From about the 1/3 point it seems to devolve into 1:1 conduction, which continues to devolve either into atrial fibrillation with 1:1 conduction or just atrial flutter with variable conduction down the AVN and AP.

Otherwise I agree with everything else!