The Bix rule

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The patient was a 64-year-old man with a history of myocardial infarction and subsequent episodes of atrial fibrillation and sinus bradycardia. A permanent ventricular inhibited rate responsive pacemaker was inserted 6 months before the outpatient visit, during which the electrocardiogram (ECG) shown in Figure 1 was recorded. He had been taking sotalol 160 mg twice per day for maintenance of sinus rhythm.

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• Fig 1. 

  ECG obtained at routine outpatient visit. See text for discussion.

The patient was asymptomatic apart from somewhat reduced exercise tolerance. His pacemaker was functioning as originally programmed. No changes were made, and he was sent home with instructions for routine follow-up.

The cardiogram was originally thought to show sinus rhythm 67 beats/min with marked (0.44 sec) first-degree arteriovenous block and an appropriate paced escape beat at the end of the recording; deep symmetric T-wave inversion in multiple leads reflected his previous non–Q-wave infarction.

The elementary step in diagnosing sinus rhythm is examination of all the leads. Some may or may not show sinus P waves, but most should. In this case, only V₁ (and its rhythm strip) shows any discernible atrial activity. This is not sinus rhythm.

The next step requires the awareness of what is seen in V₁ and no other lead. It is the most useful lead in times of uncertainty; its monitoring equivalent, the MCL₁, is rightly called the Marriott lead. Although most of its value is in distinguishing various aberrant, ectopic, or anomalous QRS morphologies, it is also an excellent lead for characterizing atrial activity, including sinus P waves. One arrhythmia tends to show itself only (or best) in V₁:¹ atrial tachycardia with block (“PAT with block”).

The peak of each P wave is exactly halfway between the peaks of the flanking R waves. It is possible that another P wave is hidden in each QRS complex. This is the moment of the Bix rule: If P waves are halfway between the QRS complexes, there should be others “lurking within the QRS complexes.”² It is not just possible, it is probable.

Most people, including myself, know of Harold Bix, a Viennese cardiologist in Baltimore with “encyclopeædic knowledge of arrhythmias,” from Marriott², ³ or the classic article on fusion beats they wrote together.⁴

Fig 1 contains the evidence that the atrial rate is twice the native ventricular rate, right at the very end. The penultimate atrial wave fails to be conducted, and another wave starts just before the pacemaker spike, enabling one to directly measure the atrial rate: 134 beats/min. This illustrates Marriott's epigrammatic dig the break advice: “It is at a break in the rhythm that you are most likely to find the solution.”⁵

Another ECG, taken previously, was found in response to the final diagnosis of the one in Fig 1. It confirmed the diagnosis (Fig 2). Here the atrial rate can be determined in several locations both before and during ventricular pacing. It was decided—again, but on different grounds—to leave well enough alone.

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• Fig 2. 

  Earlier ECG recovered for comparison with Fig 1. See text for discussion.

PAT with block is a great masquerader and is “frequently missed or confused with other arrhythmias, either atrial or ventricular.”¹ It has a tendency to vary its rate, conduction, and even the morphology of its atrial waves. It is an autonomous focus tachycardia strongly associated with digitalis toxicity. In this case, there was no exposure to digoxin. The rate is undoubtedly slower than usual because of sotalol, although rates as slow as 109 beats/min have been reported.¹

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References 

1. Lown B, Wyatt NF, Levine HD. Paroxysmal atrial tachycardia with block. Circulation. 1960;21:129–143
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2. Marriott HJL. Chapter 4. The Bix rule. In: Pearls and pitfalls in electrocardiography. Philadelphia: Lea & Febiger; 1990:8-9.
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3. Wagner GS. Marriott's practical electrocardiography. ed. 9.. Baltimore: Williams & Wilkins; 1994;218.
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4. Marriott HJL, Schwartz NL, Bix HH. Ventricular fusion feats. Circulation. 1962;26:880–884
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5. Marriott HJL. Practical electrocardiography. ed 7.. Baltimore: Williams & Wilkins; 1983;122-3
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