The Bix Rule
Report:
Sinus tachycardia 104/min
Second degree 2:1 AV block
Comment:
The rule states that if a P wave (or a P’ wave) is half-way between the QRS complexes, it probably has a companion P wave hidden within the QRS complex47. This is demonstrated below (Fig 70a), when the AV block advances beyond 2:1 conduction and junctional escape beats emerge as well as the hitherto “hidden” P waves. (They were not completely hidden: the J-junction of several QRS complexes is obviously distorted by what can only be atrial activity.)
Note that the R-P is exactly the same as the PR interval. The rule does not require such precision: if the atrial wave is approximately half-way through, the flanking QRSs should always be suspected of hiding others.
A similar strip is shown in Fig 70b, from a 61 year old lady with inferior MI. Some of it resembles Wenckebach AV conduction in the top strip, but there is no effect on the QRS rate despite the marked progressive prolongation of the PR intervals. The bottom strip contains nice examples of ventriculophasic sinus arrhythmia in complete AV block.
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