Evidence for concealed fasciculo-ventricular connections as revealed by His bundle pacing
Rehan Mahmud1, Eduardo Back Sternick2, Damian Sanchez-Quintana3
1Cardiac Electrophysiology Department, McLaren Bay Region, Bay City, MI, USA.
Insights
Fasciculo-ventricular pathways, once thought rare, are common and activate the septum during His bundle pacing. These pathways, usually hidden, become apparent with His bundle pacing, revealing their role in cardiac electrical conduction.
Area of Science:
- Cardiology
- Electrophysiology
- Cardiac Anatomy
Background:
- Mahaim described paraspecific connections between the ventricular conduction axis and septum ~100 years ago.
- These pathways were believed to be ubiquitous but not fully explored for septal activation during His bundle pacing.
Purpose of the Study:
- To investigate if specialized septal pathways facilitate septal activation during His bundle pacing.
- To correlate histological findings with His bundle pacing electrocardiographic results.
Main Methods:
- Comparison of 22 serial histological datasets with 34 His bundle pacing cases.
- Histological examination for specialized pathways connecting the atrioventricular conduction axis to the ventricular septum crest.
- Analysis of 12-lead electrocardiograms during His bundle pacing to identify fasciculo-ventricular pathways.
Main Results:
- Histologically specialized pathways were found in nearly 80% of datasets.
- Most patients (32/34) showed QRS complexes resembling fasciculo-ventricular pathways during His bundle pacing.
- Pathway capture varied with pacing voltage; ventricular activation was normal at higher voltages but prolonged at lower voltages.
Conclusions:
- Histological findings confirm the ubiquitous nature of fasciculo-ventricular connections.
- Fasciculo-ventricular pathways, concealed during sinus rhythm, manifest during His bundle pacing.
- These pathways are likely responsible for septal activation during His bundle pacing.
Background:
It is almost 100 years ago since Mahaim described the so-called paraspecific connections between the ventricular conduction axis and the crest of the muscular ventricular septum, believing such pathways to be ubiquitous. These pathways, however, have yet to be considered as potential pathways for septal activation during His bundle pacing.
Materials:
So as to explore the hypothesis that specialised septal pathways might provide the substrate for septal activation during His bundle pacing, we compared the findings from 22 serially sectioned histological datasets and 34 different individuals undergoing His bundle pacing.
Results:
We found histologically specialised pathways connecting the branching component of the atrioventricular conduction axis with the crest of the muscular ventricular septum in almost four-fifths of the histological datasets. In 32 of 34 patients undergoing His bundle pacing, the QRS complex closely resembled published images of known conduction through fasciculo-ventricular pathways. In only two patients was a delta wave not seen at any pacing voltages. Capture of these connections varied according to pacing voltage, a finding which correlated with the distance of the pathways from the site of penetration of the ventricular conduction axis. Ventricular activation times remained normal in the presence of the delta wave at higher pacing voltage but were prolonged at lower voltages.
Conclusions:
Our histologic findings confirm fasciculo-ventricular connections, initially described by Mahaim as being paraspecific, are likely ubiquitous. Analysis of 12-lead electrocardiograms leads us to conclude that fasciculo-ventricular pathways, concealed during sinus rhythm, become manifest with His bundle pacing.
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