To the Editor,
We have read with great interest the article entitled ‘Electrogram-only
guided approach to His bundle pacing with minimal fluoroscopy: A
single-center experience’ by Zanon et al1 in the
latest issue of the journal. We would like to thank the authors for
their study regarding the use of electrograms during implantation of the
His lead with minimal or no fluoroscopy. Related radiation dose
parameters such as fluoroscopy time and dose area product (DAP) were
also presented. Radiation dose related to the fluoroscopy system is
generally dependent on various autoregulated variables including the
tube voltage, the tube current and the pulse duration. In addition,
various programmable settings including the frame rate, collimation, and
magnification are used to regulate the amount of radiation although the
number of variables which can be programmed in the manufacturer specific
system varies. Fluoroscopy time should not be an estimation factor for
radiation dose because its correlation with other standard radiation
measures is weak. Manufacturer specific calculations, either total time
while pressing on the fluoroscopy pedal or only a sum of fluoroscopic
pulses, make the radiation dose difficult to estimate. The air kerma
area product, or DAP (Gy.cm2) is accepted as the most
accurate measurement of radiation energy given for fluoroscopic
purposes. Roughly, the effective dose (ED) (mSv) of fluoroscopic source
of radiation can be calculated using a conversion factor of 0.20 from
the DAP measurement. For instance, routine single/dual-chamber
pacemaker/defibrillator implantation has an average ED of 4 mSv,
compared to an average ED of nearly 20 mSv for a cardiac
resynchronization therapy device implantation.2 The
frame rate, collimation and magnification should be properly set to
reduce the DAP/ED values.
The frame rate seems as the most important programmable factor related
to the radiation dose. Continuous fluoroscopy has generally the frame
rate of ≥30 frames per second (fps), which is associated with increased
radiation exposure. Therefore, the frame rate should be set as low as
achievable. Decreasing the frame rate from 30 fps to 7.5 fps reduces the
ED by 75%. Compensatory tube current increase generally occurs during
pulsed fluoroscopy, which is associated with the same number of x-rays
produced during continuous fluoroscopy. However, significantly decreased
number of pulses cause significant reduction of radiation exposure in a
linear way. The frame rates as low as ≤3 fps should be used during
diagnostic and ablation procedures and device implantations in the
electrophysiology laboratory although reduced temporal
resolution.2,3 Furthermore, configuring the frame rate
≤1 fps using the ECG triggering or pacing artifact can be the best
approach. Configuration of the pulsed fluoroscopy at the lowest frame
rate needed has the greatest potential for ensuring radiation dose at
the lowest level.
Properly configured and applied collimation, reduction of radiation to
an anatomical area that requires imaging, has an important role in
reducing both the direct and the scattered radiation. Collimation,
especially the asymmetric one, reduces visualization area and decreases
the DAP/ED in a linear way. During the procedure, the use of collimation
restricts the view on the cardiac contour. After viewing the whole
anatomy, re-collimation should be performed.
Another option for reducing the radiation dose is to set the largest
field of view, or the smallest magnification, possible. In the smallest
field of view mode, or the largest magnification mode, a smaller
radiation area is projected in the image intensifier and, a larger image
is produced on the screen. This process is related to a lower radiation
output and a dimmer image. The fluoroscopy unit has an automatic system
for controlling the brightness sensed. Therefore, the automatic system
compensates for the lower brightness by increasing radiation exposure
from the tube. With the use of magnification, the radiation dose
increase in flat-panel detectors is lower compared to the
image-intensifiers, however it is still remarkable.4
In the light of all above-mentioned settings, the fluoroscopy time
should not be used as an important variable measuring the radiation
dose. Instead, the DAP/ED values should be reported.
Keywords: dose area product/effective dose; fluoroscopy;
programmable; radiation dose