Of those children with conduction abnormalities born to mothers with Ro/La antibodies, most will have first-degree block
. However, progression to second-degree and irreversible third-degree block in the postnatal period is well documented (4).
In sinus tachycardia with first-degree block, the P wave may fall within the T wave and not be detected on surface ECG or be mistaken for a retrograde P wave.
The wire study can be used to exclude sinus tachycardia with first-degree block by careful comparison of RP and PR intervals during atrial and/or ventricular pacing (Figure 6).
The significance of first-degree block is not firmly established (see below).
PR intervals greater than 150 msec (mean +3 SD) were considered abnormally prolonged, consistent with first-degree block; 92 fetuses had normal PR intervals throughout the study.