Limitations
No histological assessment of the ablation lesions was possible to
confirm that impedance drop is a reliable marker of lesion size.
Tissue impedance at the catheter-tissue interface was classically
measured as transthoracic impedance of the energy delivery pathway from
the ablation catheter tip electrode to an indifferent electrode on the
skin. This approach is known to be influenced by variation of thoracic
impedance and in fact local tissue impedance measurements, now provided
by innovative catheters with miniature electrodes, seem to correlate
better with lesion size 20. However, impedance changes
during RF ablation are almost entirely due to variations in local tissue
impedance, and thus the variations of impedance and especially the
percentage variations of impedance should remain a useful measure.
The use of different catheter irrigation rates for different RF powers,
as per Tacticath manufacture instructions, could have had an impact on
the percentage impedance drop values recorded with the different powers.
Irrigation may impact on impedance drop by preventing heating of the
endocardial surface and therefore making impedance drop less predictive
of lesion depth progression. However, the greater impedance fall in the
higher power lesions despite higher irrigation rate suggest that this is
a genuine effect of high power compared to low power.