Magnetophysiologic and echocardiographic comparison of blocked atrial bigeminy and 2:1 atrioventricular block in the fetus

Abstract

Background: Blocked atrial bigeminy (BAB) and second-degree atrioventricular block with 2:1 conduction block (2:1 AVB) both present as ventricular bradycardia and can be difficult to distinguish by echocardiography. Since the prognosis and clinical management of these rhythms are different, an accurate diagnosis is essential.

Objective: To identify magnetic and mechanical heart rate and rhythm parameters that could reliably distinguish BAB from 2:1 AVB.

Methods: A retrospective study of ten BAB and seven 2:1 AVB subjects was performed, using fMCG and pulsed Doppler ultrasound.

Results: Distinguishing BAB from 2:1 AVB by using fMCG was relatively straightforward because in BAB the ectopic P wave (P') occurred early, resulting in a bigeminal (short-long) atrial rhythm. The normalized coupling interval of the ectopic beat (PP' of the blocked beat to PP of the conducted beat) was 0.29 ± 0.03. In contrast, the echocardiographic assessment of inflow-outflow gave a normalized mechanical coupling interval (AA'/AA) near 0.5, which made it difficult to distinguish BAB from 2:1 AVB. Heart rate distinguished most subjects with BAB from those with 2:1 AVB (82 ± 5.7 beats/min vs 69 ± 4.2 beats/min), but was not a completely reliable indicator. In most subjects, BAB alternated with sinus rhythm or other rhythms, resulting in complex heart rate and rhythm patterns.

Conclusions: Fetal BAB and 2:1 AV block can be difficult to distinguish using echocardiography because in many fetuses with BAB the mechanical rhythm does not accurately reflect the magnetic rhythm. fMCG provides a more reliable means of making a differential diagnosis.

Keywords: 2-to-1 atrioventricular block; 2:1 AVB; AA; AA′; AV; AVB; Atrioventricular block; BAB; BAC; Bigeminy; Blocked atrial bigeminy; FHR; Fetal arrhythmia; Fetal magnetocardiography; PAC; PP; PP′; QTc; SVT; VSA; atrioventricular; atrioventricular block; blocked atrial bigeminy; blocked atrial couplet; corrected QT interval; fMCG; fetal heart rate; fetal magnetocardiography; interval between A waves of consecutive sinus beats; interval between P waves of consecutive sinus beats; interval from the A wave of the sinus beat to the A wave of the premature beat; interval from the P wave of the sinus beat to the P wave of the premature beat; premature atrial contraction; supraventricular tachycardia; ventriculophasic sinus arrhythmia.

Figure 1

PP′ intervals were measured from the onset of the first significant positive or negative deflection. A: Subject with blocked atrial bigeminy with a PP′ interval of 196 ms. B: Subject with 2:1 atrioventricular block with PP′ interval of 440 ms. PP′ = interval from the P wave of the sinus beat to the P wave of the premature beat.

Figure 2

A negative linear correlation exists between PP′ and percent time in blocked atrial bigeminy. R= −.48; P = .027. PP′ = interval from the P wave of the sinus beat to the P wave of the premature beat.

Figure 3

Representative fetal heart rate tracings. A: Typical subject with blocked atrial bigeminy (BAB). B: Subject with BAB. C: Subject with BAB with occasional atrial trigeminy and conducted premature atrial contractions (PACs). D: Subject showing alternating periods of BAB, sinus rhythm, and oscillating heart rates due to conducted PACs. E: Subject showing BAB with occasional conducted beats during region 1, supraventricular tachycardia during region 2, normal sinus rhythm during region 3, and BAB during region 4. F: Typical subject with 2:1 atrioventricular block (AVB). G: Subject with Wenckebach second-degree AVB accompanied by premature ventricular contractions.

Figure 4

Fetal magnetocardiography tracings of rhythms other than blocked atrial bigeminy: A: Typically aberrantly conducted premature atrial contractions (PACs), indicated by asterisks, in subject 4. B: Aberrantly conducted PACs in subject 10 with a relatively long PP′ interval (300 ms). This unusually long PP′ interval and aberrant QRS complex may imply underlying conduction disease. C: Blocked atrial trigeminy in subject 9. D: Blocked atrial couplets (double arrows) in subject 2. The couplets reduced the fetal heart rate from 85 to 59 beats/min. E: Wolff-Parkinson-White syndrome in subject 10. F: Supraventricular tachycardia with aberrancy at initiation and near termination in subject 5. PP′ = interval from the P wave of the sinus beat to the P wave of the premature beat.

Figure 5

A: Inflow-outflow measurement of the AA′ interval. B: Fetal magnetocardiography measurement of the PP′ interval for subject 9. AA′ = interval from the A wave of the sinus beat to the A wave of the premature beat; PP′ = interval from the P wave of the sinus beat to the P wave of the premature beat.