A 19 year old woman presents to your ED with two hours of rapid palpitations and lightheadedness. She has not had chest pain and has no past medical history of note. On arrival, she is alert, afebrile with a supine BP of 85/60 mmHg.
Describe and interpret her initial ECG
ECG 1
This is a difficult ECG to describe. Initially the rhythm strip shows a narrow, regular, extremely fast rhythm at 300/minute most consistent with atrial flutter with 1:1 conduction. In the middle of the rhythm strip there is a small positive deflection after each QRS as is often seen in AVNRT or AVRT with retrograde P waves, but with unchanged QRS morphology. At the end of the rhythm strip there is some irregularity briefly with the suggestion of flutter waves at 2:1 conduction, but atrial fibrillation is a differential. The ST segments are difficult to assess at this rate but appear depressed in many leads. Overall I think the main rhythm disorder here is atrial flutter, but the rate is very concerning.
Ventricular rates of 300/minute are rarely encountered in emergency medicine and in this context are highly suggestive of the presence of an accessory AV pathway. This is because the normal AV node usually acts as a gatekeeper to prevent passage of rates much greater than around 200/minute, with the exception perhaps of the very young, and highly trained. Rates of 300/minute are generally unstable, poorly tolerated, and may progress to VF, so a plan for rhythm stabilisation is required urgently.
If significant clinical or haemodynamic compromise is present, an immediate synchronised cardioversion should be performed in the safest manner possible. However in this patient, her clinical status appeared surprisingly stable with the concerns being a borderline normal BP and the possible ST depression. In this setting, confirmation of the provisional rhythm diagnosis of atrial flutter might be a reasonable next action by either performing a vagotonic manoeuvre or administration of Adenosine intravenously. (That being said, an eminently reasonable and defensible “safe mode” in any uncertain tachyarrhythmia setting is to proceed straight to electrical cardioversion)
As happens in ED frequently, the situation suddenly just sorted itself out. During the siting of the IV (large bore cannula, proximal ACF vein), the vagotonic stimulus provoked a rhythm change.
ECG 2
Now there is definitely atrial flutter with 2:1 AV conduction, the more commonly encountered rate for this arrhythmia. This is so called “typical” flutter, which has negative flutter waves in the inferior leads and is caused by a counterclockwise re-entry mechanism in the right atrium. The ST segments look much more normal here too.
The short duration of symptoms, clinical stability and lack of obvious precipitants was discussed with the cardiologist and patient, and a plan was agreed to press on and attempt cardioversion chemically with intravenous amiodarone. Other anti-arrhythmics such as procainamide, or electrical cardioversion and procedural sedation would have also been reasonable options. Reversion to sinus rhythm occurred an hour later, with the following post-reversion ECG.
ECG 3
Here there is sinus rhythm, P mitrale, and a suggestion of slurred upstroke of the R wave in the chest leads especially V2-4. These findings support the possible presence of an accessory AV tract or structural heart disease. Given the potentially life threatening nature of her presenting arrhythmia, admission for further investigation was arranged. Goals of investigation include detection of potential precipitants (eg infection, thyroid disease, ischaemia), and more detailed assessment of myocardial function (eg echocardiography and EP studies.)
Key points
1. Heart rates of 300/minute are rare in ED, and usually clinically unstable
2. Rapid rate control +/- reversion is the immediate goal. A definitive rhythm diagnosis can be made later by the electrophysiologist
3. Atrial flutter can manifest varying AV conduction, but 2:1 is most common