What does chronaxie have to do with His Bundle pacing?

 What does chronaxie have to do with His Bundle Pacing (HBP), for that matter what does chronaxie have to do with anything?

The minimal voltage amplitude that can capture a nerve or muscle at an infinitely long pulse duration is called the rheobase.More simply rheobase is the minimal voltage necessary to stimulate the tissue. The chronaxie is defined as the pulse duration required for capture with a voltage of twice the rheobase. 

The chronaxie is important in regard to programing pulse generators. The chronaxie approximates the point on the strength duration curve at which there is minimal energy expended. The strength duration curve intersects the energy curve at the chronaxie, where energy equals V2/r X I .

Quoting Ellenbogen, fifth edition " As a practical point when programming the pulse duration of a  pulse generator,chronaxie is an excellent choice to minimize energy,limit charge drained from the battery... 

That sentence was written at a time before there was clinical application of His Bundle pacing. Is there any reason to believe that that rule of thumb would be no longer applicable? Is the chronaxie an excellent choice to minimize energy and limit battery depletion in patients with His Pacing? If so does that apply to both selective and non-selective His pacing?

Dr. Marek Jastrzebski and his colleagues in Krakow along with Dr. Pugazhendi  Vijayaraman from Geisinger Commonwealth School of Medicine (1)have studied the strength duration curves of the His bundle and the adjacent right ventricular muscle and make suggestions regarding programming .

They determined the HB and adjacent Right ventricular muscle chronaxies in 127 patients finding that in patients with selective HB pacing that the His bundle had a shorter chronaxie than the adjacent RV muscle while in non selective HBP patients the RV and HBP chronaxies did not differ.

The authors say that it is their practice to promote selective HB capture by empirically programming the pulse duration  (pd) to 0.2 -0.3 ms.However,for non selective , particularly in cases of distal AV block a longer pd ( i.e. 1.0 ms) would offer greater safety. They suggest that sometimes (often ?) the default setting from the factory is often 1.0 ms, which is far from optimal. 

 

1) Jastrzebski,M et al His bundle has a shorter chronaxie that does the adjacent ventricular myocardium:Implications for pacemaker programming. Heart Rhythm 2019;16:1808-1810.

2) Clinical Cardiac pacing,defibrillation,and resynchronization therapy, Ellenbogen,Wilkoff,Kay,Lau and Auricchio FifthEdition