Use the ‘Bix Rule’ while interpreting SVT

CME INDIA Case Presentation by Dr D. P. Khaitan, MD (Medicine), FCGP (Ind), FIAMS (Medicine), FICP, Gaya (Bihar).

CME INDIA Case Study

Let us learn following 3 ECGs:

ECG No 1

How you interpret this ECG?

ECG findings:

Heart rate = 150/min, midway P’ (best seen in inferior leads II, III and aVF with prominent negativity in somewhat sinuous pattern): Bix rule applicable.
Simultaneous appearance of intermittent short spell of atrial flutter with variable degree of AV conduction in the same ECG (V1 to V6).

Other findings (QRS axis = about –120⁰ with high left anterior AV block, associated with atypical RBBB: underlying conduction defect with Cardiomyopathy)

Narrow complex tachycardia (150/min) with mid P’ between two QRS complexes and with intermittent short run of atrial flutter with variable degree of AV conduction over precordial leads in the same ECG.

ECG No 2

Ventricular rate = 150/min with midway P’ in between two QRS complexes. On the subsequent ECG atrial flutter with variable degree of AV conduction.

History:

A known case of HCM presented 1.5 years back with AF; reverted back, now taking tablet Prolomet XL 25mg and tablet Cordarone 200mg daily.

Further presented with Palpitation with the above ECG, Inj. Amiodarone Bolus IV 150 given. The subsequent ECG is given below:

ECG -A – Ventricular rate 150/min with midway P’ in between two QRS complex, clearly seen over V1 (situated as a sharp P over T) and hidden P’ seen peeping as a tiny wavelet through the base of QRS complex (see V5) (other finding deep Q over lead I and aVL suggestive of HCM).

ECG -B – Atrial Flutter with variable degree of AV conduction.

ECG No 3

ECG findings:

Heart rate = 150/min, midway P’ obvious over V1 with long RP interval (other findings – low voltage in limb leads, interpolated ventricular premature beat occasional and single over precordial leads)
Well – argued in a lucid way by Dr. Prof. Narendra Kumar, eminent cardiologist – Atrial tachycardia (long RP interval)
Concluded by the opinion of Dr. Yash Lokhanwala, eminent cardiologist: Atrial tachycardia with IVCD of LBB type.
(V1 is the most useful lead in view of uncertainty, also excellent lead for characterizing atrial activity).

CME INDIA Learning Points

(By Dr D.P. Khaitan)

If one thinks over and over almost every time while encountering a rhythm of SVT and adopts ‘Bix rule’ as a habit to pickup the P’ wave situated midway between two ventricular complexes – one may become a Rockstar picking up an atrial flutter / atrial tachycardia with 2:1 conduction, otherwise this is being missed.
Practicing makes a man perfect. And one starts to feel that regular practicing is the starting point of the practical implication of whatever knowledge one possesses. The appreciation of ‘Bix rule’ opens a door while interpreting SVT.

QRS –P’– QRS

(P’)

Bix rule – If one is dealing with supraventricular tachycardia in which visible P’ wave is situated midway between two ventricular complexes, there will be a probability that there is a P’ wave lurking within the next QRS complex. This rule is mainly helpful in identifying atrial flutter with 2:1 AV conduction but also helpful in atrial tachycardia with 2:1 conduction

A basic concept of atrial flutter and atrial tachycardia

Atrial flutter: Macro Reentry

Saw-tooth appearance without intervening isoelectric line
Rate 250-350/min, usually around 300

Atrial tachycardia: Enhanced Automaticity (others- triggered or micro entry)

‘Electrophysiological Foundation’

SVT.2:1 AV conduction.

QRS –P’– QRS

(P’)

The decremental conduction across AV Node does not usually tolerate supraventricular increment beyond a limit if directly imposed upon it – the comfortable ventricular zone is 150 bpm, so is due to 2:1 AV conduction.

Thus, with 2:1 AV conduction one atrial beat is allowed to pass through AV node in a sequential way P-QRS-T, but the next activated non-passed atrial beat is just coincidentally coming at a time of QRS so it is hidden within.

Bix rule – know the breaking point of rhythm pattern on ECG

Atrial flutter waves occur at about 250-350/min usually around 300. These flutter waves, especially of newer onset are conducted with 2:1 through AV node resulting in ventricular rate 125-175/min, on average around 150.
Waves of atrial tachycardia with higher rate might also be conducted with 2:1.
SVT with 2:1 AV conduction, the first P’ may be visible midway between two ventricular complexes while the next one is probably lurking within the next QRS complex.

A methodical approach to Bix rule: My mnemonic – Six ‘S’

Suspect any supraventricular tachycardia around 150/min to be atrial flutter / atrial tachycardia with 2:1 AV conduction, unless and until disapproved
Scrutinize every lead of 12 surface ECG to find out the points in favor of ‘Bix rule’, is already mentioned. Is there any transient short spell of atrial flutter visible anywhere on ECG?
Slow the ventricular rate by carotid massage / adenosine whenever needed.
Set Re-ECG tracings: on slowing the ventricular rate, one should make an effort to visualize atrial flutter with more clarity. (Distinction with atrial fibrillation: which may appear sometimes alike atrial flutter but these are irregularly irregular).
Spot Atrial tachycardia: One should see the presence of non-sinus P with long RP interval to make the diagnosis of atrial tachycardia with 2:1 AV conduction.
See the alternate diagnosis.

Short RP may also be expressed as less than 50% of RR interval
Long RP may be also be expressed as more than 50% of RR interval

Two important alternate diagnosis must be excluded (suspicion – the mirage effect as if P’ is situated midway):

AVNRT: Short RP tachycardia (instead of P wave peeping through the distal base of the QRS, there is a short isoelectric line before inscribing the P wave with RP<50% of RR interval – known as short RP interval), pseudo s in inferior leads with r’ in V1/aVR or even P wave may not be visible (hidden within QRS).

AVRT (orthodromic): Long RP tachycardia (sometimes P’ wave might be seen just after T wave with RP>50% of RR interval – known as long RP interval),

± associated with more prominent ST/T changes over inferior/precordial leads, ± electrical alternans: one may see the evidence of WPW syndrome on basal ECG or after its conversion with DC shock.

‘Bix rule’ seems to be useful while interpreting SVT in certain specific situation. A correct diagnosis is very much essential in managing the case. This popular ‘Bix rule’ was termed as such in honor of a Viennese cardiologist Dr. Harold Bix who had an encyclopedic knowledge of arrhythmia. The recognition points of ‘Breaking out of Rhythm Pattern‘ seems fundamental in identifying the cardiac arrhythmia.

Let us review what Bix rule narrates

Bix rule is a phenomenon description to know the point of ‘Breaking out of Rhythm Pattern’ in SVT – P’ wave halfway between the QRS complexes and the next P’ lurking within the next QRS complex. Dr. Bix with ”encyclopedic knowledge of arrhythmia” put this rule before the world to recognise this pattern. The decremental conduction across AV Node does not usually tolerate supraventricular increment beyond a limit if directly imposed on it – the comfortable ventricular zone is 150 bpm, so is due to 2:1 AV conduction.
The science and art of the cardiac arrhythmias lies in identifying the ‘breaking out of rhythm pattern’ wherein one is most likely to find the solution. Every cardiac arrhythmia is having some particular characteristic of identification which keeps it separate and distinct from other arrhythmias. Marriott’s epigrammatic – ‘dig the break’ narrates the same principle.
Bix rule is the very example of ‘dig the break’. A clear vision, backed up by a definite plan paves a path to reach to the diagnosis in a case of arrhythmia with confidence. Thus, the most efficient way by which to make a diagnostic approach to SVT depends upon one’s ability in the identification of this concerned rule, as mentioned above. This rule imparts a learning how to think the relationship of SVT with the next accompanying ventricular complex in a case with 2:1 AV conduction.
If one thinks over and over almost every time while encountering a rhythm of SVT and adopts Bix rule as a habit to pick up the P’ wave situated midway between ventricular complexes – one may become a Rockstar in picking up an atrial flutter / atrial tachycardia with 2:1 conduction, otherwise this is being missed.

A basic concept of Atrial flutter and Atrial tachycardia

This becomes essential to review the basic concept with the differences in between atrial flutter and atrial tachycardia to understand the electrocardiographic implication of Bix rule.

Bix rule – know the breaking point of rhythm pattern on ECG

To see in details, the following points under rectangular box should be kept in mind:

Atrial flutter waves occur at about 250-350/min usually around 300. These flutter waves, especially of newer onset are conducted with 2:1 through AV node resulting in ventricular rate 125-175/min, on average around 150.
Waves of atrial tachycardia with somewhat higher rate might also be conducted with 2:1
SVT with 2:1 AV conduction, the first P’ may be visible midway between two ventricular complexes while the next one is probably lurking within the next QRS complex – ‘Foundation of Bix rule’.

A methodical approach to Bix rule

The following steps should be undertaken in succession (My mnemonic six ‘S’):

Suspicion:

Suspect any supraventricular tachycardia around about 150/min to be atrial flutter/atrial tachycardia with 2:1 AV conduction, unless and until disapproved

Scrutinise every lead

12 lead surface ECG should be thoroughly scrutinized to observe the following changes:

Find out P’ in midway between two QRS
Hide and seek play: Search out the hidden P’ as peeping through the accompanying QRS:
- A slight notch or slurring through the base of QRS
- Is there any transient short spell of atrial flutter in between anywhere?

Slow the ventricular rate

By carotid massage/adenosine whenever applicable.

Set Re-ECG Tracings

On slowing the ventricular rate, one should make an effort to visualize atrial flutter with more clarity.

(One should rule out atrial fibrillation which may appear at times alike atrial Flutter but these are irregular and not all alike (disorganized) – the pattern of atrial fibrillation is irregularly irregular).

Spot Atrial tachycardia

In atrial tachycardia one can see a distinct P’ wave of abnormal morphology (non-sinus) but not alike atrial flutter appearance.

See the alternate diagnosis

With the rate of round about 150/min with a P’ wave either in front of QRS or just after the T wave may at times compel the clinician to think over the alternate diagnosis due to the mirage effect as if P’ is situated midway.

Two important alternate diagnosis must be excluded:

AVNRT: Short RP tachycardia (instead of P wave lurking through the distal limb of the QRS, there is a short isoelectric line before inscribing the P wave – known as short RP interval with less than 50% of RR interval); pseudo s in inferior leads with r’ in V1/aVR or even P wave may not be visible (buried within QRS).
AVRT (orthodromic): Long RP tachycardia (sometimes P’ wave might be seen just after T wave – known as long RP interval with more than 50% of RR interval), ± associated with more prominent ST/T changes over inferior/precordial leads, ± electrical alternans: one may see the evidence of WPW syndrome on basal ECG or after its conversion with DC shock.

Further reading suggested

10 tips to never miss atrial flutter with 2:1 conduction, Dawn B. Altman, RN, EMT- P. website – www.ems1.com>articles.
The Bix rule – Heart & Lung – The Journal of Cardiopulmonary and Acute care- by George Nikolic, MBBS, FRACP, FACC, 2008 – website – www.heartandlung.org>article>pdf.
The Bix Rule – ECG Rhythms – Website: www.ekgrhythm.com>2016/08
Diagnosing Supraventricular Tachycardia when Physical Examination Trumps the Electrocardiogram, Author – James E. Ip, MD; Bruce B. Lerman, MD – AHA Journals – www.ahajournals;.org>doi>pdf.
The 12 Rhythms of Christmas: Atrial Flutter, Source – ems12lead.com.

Use the ‘Bix Rule’ while interpreting SVT

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