peas in a pod….

the case.

an 81 year old male is referred by his GP for ongoing tachycardia & progressive hypotension. He has been gradually taken off his antihypertensives (telmisartan & prazosin) over the past 2-3 weeks as his BP has consistently been 80-90mmHg systolic.

This is his ECG…

ECG #1

[DDET Describe & interpret this ECG…]

  • Rate.
    • 126/min
  • Rhythm.
    • Regular.
  • Axis.
    • Normal [-27*]
  • Intervals.
    • PR ~ 200msec
    • QRS ~ 110msec [RBBB pattern]
    • QTc ~ 420msec
  • Segments.
    • Isoelectric ST segments
  • Others.
    • P-wave axis appears normal .
    • Low voltage QRS, no electrical alternans.


Narrow complex regular tachycardia with incomplete right bundle branch block.

The differential diagnoses in this instance is broad. How can we narrow this down further ??


[DDET Let’s have a closer look…]

Firstly a reminder of the electrocardiographic anatomy of supraventricular tachycardias….

Electrophysiology of supraventricular dysrhythmias. Image courtesy of ECGpedia.
Electrophysiology of supraventricular dysrhythmias. Image courtesy of ECGpedia.

The RP interval.

This additional assessment of the P:QRS relationship assists in differentiating “supraventricular tachycardias”.

Short RP = P wave immediately follows the QRS.
Long RP = an interval P wave precedes the QRS.

Alternatively; if the RP > PR it is a long RP interval.

Courtesy of cyberounds.com.
Courtesy of cyberounds.com.


Differential Dx of Short RP-interval SVT.

  • Typical AV-nodal reentry tachycardia (‘Slow-Fast’)
  • Atrio-ventricular reentry tachycardia
  • Atrial tachycardia with long first-degree AV block
Reentry pattern seen in Typical AVNRT. Note the "short RP interval". Image courtesy of ECGpedia.
Reentry pattern seen in Typical AVNRT. Note the “short RP interval”. Image courtesy of ECGpedia.


Differential Dx of Long RP-interval SVT.

  • Atypical AV-nodal reentry tachycardia (‘Fast-Slow’)
  • Atrial tachycardia
  • Sinus tachycardia
  • SA-nodal reentry
  • Permanent junctional reciprocating tachycardia (PJRT) 


So, let’s get back to our original ECG…

Vertical blue lines indicating P-waves. Note that the RP interval is > 50% of the RR interval.
Vertical blue lines indicating P-waves.
Note that the RP interval is > 50% of the RR interval.

We are dealing with a long RP-interval tachycardia. 

Now, let us look at the P waves !!

P waves - negative in aVR; positive in II/aVF.
P waves – negative in aVR; positive in II/aVF.

The P-wave axis appears ‘normal’; ie. heading in a superior to inferior direction, suggesting that the P-wave is originating near the SA-node.

This makes both atypical AVNRT & AVRT very unlikely & hence narrows down our list of differential diagnoses to sinus tachycardia, sinoatrial nodal reentry or atrial tachycardia (with a high right-atrial focus).


[DDET What is your approach to this patient ?]

An elderly patient with tachycardia, hypotension and low-voltage on ECG.

  • Must consider & investigate for pericardial effusion.
  • In this instance, bedside ECHO showed no pericardial fluid.

Over the initial 45 minutes of evaluation in the ED, the heart did not halter from 125-130 bpm. This was despite IV fluid boluses for hypotension & analgesia.

  • This increased the suspicion of an atrial tachycardia or SA-nodal reentry.
  • Electrolytes normal.

We consider the idea of slowing the AV-node to see if we can get some more answer, however with ongoing hypotension we are loathed to try medications. After a quick listen to the neck for a bruit, we gave a trial run of carotid sinus massage.

  • A small pause… (no flutter waves) – not captured on paper, sorry.
  • Two ventricular ectopics
  • Return to regular sinus rhythm (Heart rate of 90-100 bpm)


[DDET This is his repeat ECG…]

ECG #2

  • Rate.
    • 96/min
  • Rhythm.
    • Regular. Sinus.
  • Axis.
    • Normal [90*]
  • Intervals.
    • PR ~ 180 msec [shorter than ECG #1]
    • QRS ~ 110 msec [RBBB pattern]
    • QTc ~ 400 msec.
  • Segments.
    • Isoelectric ST segments
  • Others.
    • Slightly different P wave morphology to ECG #1
    • Low voltage QRS, no electrical alternans.

Interpretation sinus rhythm with incomplete right bundle branch block & low-voltage QRS

I believe the initial ECG is an atrial tachycardia that reverted with carotid sinus massage.


[DDET Putting this together…]

Atrial Tachycardia.

Responsible for only ~10% paroxysmal SVTs.

Typically results from a single ectopic atrial pacemaker (triggered by digoxin toxicity or increased automaticity) or reentry of the atrial tissue.

  • Occurs in people with normal hearts & those who structurally abnormal hearts (eg. congenital heart disease or valvular heart surgery).

Usually episodic or paroxysmal.

  • Enhanced automaticity results in non-sustained but repetitive tachycardias
  • Reentrant forms [micro-reentry] may be continuous & sustained.

Clinical signs & symptoms;

  • Tachycardia – rapid & regular.
    • Can be irregular in the setting of variable AV-conduction
  • Sudden onset of palpitations
  • Warm-up phenomena.
    • Tachycardia gradually speeds-up over time.
    • Less clinically apparent.
  • Dyspnoea, dizziness, fatigue, chest tightness.
  • Syncope.
  • Heart failure symptoms.
    • Especially in those with tachycardia-induced cardiomyopathy [frequent, incessant tachycardia].


  • As with all dysrhythmias; ensure the patient is not anaemic & that there are no electrolyte disturbances.
  • TFTs
  • Check a digoxin-level if appropriate
  • ECG.
    • Should be scrutinised for alternate diagnoses, eg. MAT
  • ECHO.


  • Correct hypoxia.
  • Correct electrolytes.
  • Carotid sinus massage → can revert paroxysmal forms of atrial tachycardia
  • Rate-control.
    • Beta-blockers vs Calcium-channel blockers.
    • Caution with negatively inotropic agents (esp. verapamil) → Can result in profound cardiovascular collapse !
    • Consider ultra-short acting agents such as esmolol or adenosine; Typically adenosine will not,  but can revert atrial tachycardia – instead allowing you to see the ongoing P-waves march through.
  • Cardioversion.
    • May not be successful in incessant forms (or with MAT).
  • Maintenance pharmacotherapy.
    • Verapamil seems adequate at controlling atrial tachycardia from triggered activity.
    • Whilst beta-blockers are recommended for AT from enhanced automaticity, their overall success rates are low.
    • Class III antiarrhythmics can be used to maintain sinus rhythm.
  • Cardiology consultation.
    • However most patients should be able to be discharged if systemically well.


[DDET References]

  1. Chan TC, Brady WJ, Harrigan RA, Ornato JP, Rosen P. ECG in Emergency Medicine and Acute Care. Elsevier Mosby 2005.
  2. Wellens HJ. The value of the ECG in the diagnosis of supraventricular tachycardias. Eur Heart J. 1996 Jul;17 Suppl C:10-20.
  3. ECGpedia
  4. A Rational Approach to the Diagnosis and Treatment of Narrow QRS Complex Tachycardias @ Cyberounds
  5. Atrial tachycardia @ eMedicine/Medscape
  6. Determinants of “P” wave location in narrow qrs tachycardia ? @ Dr.S.Venkatesan MD.


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