Main Abnormalities:

• Irregularly irregular broad complex tachycardia.
• Extremely rapid ventricular rates — up to 300 bpm in places (RR intervals as short as 200ms or 1 large square).
• Beat-to-beat variability in the QRS morphology, with subtle variation in QRS width.

Explanation of ECG Findings:

• Irregularly irregular rhythm is consistent with atrial fibrillation.
• There is a left bundle branch block morphology to the QRS complexes.
• However, the ventricular rate is far too rapid for this to be simply AF with LBBB.
• The rates of 250-300 bpm and the variability in QRS complex morphology indicate the existence of an accessory pathway between the atria and ventricles.

Diagnosis:

• These findings indicate atrial fibrillation in the context of Wolff-Parkinson-White syndrome.

• Broad complex irregular tachycardia at very rapid rates? -> Suspect AF with WPW!
• These patients can rapidly become haemodynamically unstable.
• The options for chemical cardioversion are very limited, favouring DC cardioversion.

I would recommend immediate DC cardioversion in this patient. My approach would be to fluid load (0.5 – 1L crystalloid bolus), add in a push-dose pressor to elevate the BP (e.g. Metaraminol 0.5 – 1mg IV) sedate with something that has minimal effects on BP (e.g. fentanyl or ketamine in cautious doses), and then shock at 200j biphasic. Consider using an AP pad position for maximal 1st shock success.