Background: In transient episodes of bradycardia (TEBs) the P axis (Pax) gives information about the pacemaker site. Atrial escape (AER) and AV nodal escape rhythm (AVER) have a different Pax than sinus rhythm (SR).Methods: 19 spontaneously breathing healthy preterm infants were studied: GA 29.2 ± 1.9 w; BW 1154 ± 264 g; postnatal age range 3-28 d. 5-Lead ECGs were registered (mean registration time 2.5 h/infant) and sampled by a Physics Data Acquisition System (PhyDAS). To evaluate our methodology the PhyDAS Pax was compared with a standard ECG (stdECG) Pax during SR. TEBs were defined as periods of 3 or more RR-intervals being at least 25% longer than the preceding baseline intervals. During TEBs the PhyDAS Pax was used to classify bradyarrhythmias. A TEB with a Pax range from 0 to+90 ° was considered to be a sinusbradycardia (SB). A Pax range from +90 to +360 ° indicated AER. The absence of P wave or P wave following QRS complex indicated AVER. The RR-interval was expressed as the instantaneous heart rate (IHR). The RR-interval preceding AER or AVER was normalized for its mean baseline interval (nrmRR). Statistics: results as mean ± SD, Wilcoxon test. Mann-Whitney test. Results: During SR the PhyDAS Pax was correlated with the stdECG Pax (50 ± 11 degrees: range 29-68; r=0.82; p<0.0001). In 11 pts 60 TEBs were observed. In 28 TEBs (47%) the Pax remained between 0 to +90 °. During 20 TEBs (33%) in 9 pts the Pax deviated outside the 0 to +90 ° range. During 5 TEBs (8%) in 4 pts AVER developed. 7 TEBs (12%) could not be classified. IHR was significantly lower in AVER (46 + 5 bpm) than in SB (80 ± 18 bpm; p=0.001) or AER (69± 15 bpm; p=0.004) The nrmRR was significantly longer preceding AVER(3.5 ± 0.4) than in SB (2.0 + 0.6: p=0.002) or before AER (2.3 ± 0.6; p=0.003).

Conclusion: In one third of TEBs a wandering pacemaker was observed. Sinus pauses of more than three times the baseline intervals predispose to AVER.