Delta AH Interval

From Ask Dr Wiki

• Used to differentiate Long RP Tachycardias such as Atypical AVNRT, Atrial Tachycardia, or PJRT

How to Perform

1. Pace HRA at Cycle Length of Tachycardia during Sinus Rhythm and measure AH Interval
2. Pace HRA 10-20 ms faster than Tachycardia Cycle Length during Tachycardia and measure AH Interval
3. Substract Delta AH during SR - Delta AH during tachycardia. If Delta AH Interval is
   1. >40 ms AVNRT or Atypical AVNRT
   2. <20 ORT
   3. < 10 Atrial Tachycardia. The exception to the rule is Atypical AVNRT if tachycardia is initiated during pacing at tachycardia cycle length the Delta AH Interval will be < 10.

Mechanism

• During Tachycardia you go down the fast and up the slow. If the ERP of the fast is longer than the slow then during pacing during SR you will go down the slow so the Delta AH Interval will be > 40.
• If you are able to induce the tachycardia during atrial pacing at the TCL then the ERP of the slow pathway must be the same as the ERP of the Fast Pathway thus you will go down the fast pathway with atrial pacing which will result in the exception to the rule which will be a short (<10) Delta AH Interval

References

1. Man et al. Comparison of Atrial-His Intervals During Tachycardia and Atrial Pacing in Patients with Long RP Tachycardia. JCE 1995.9; 700-710