Commentary
The possible options are:
- Electromagnetic interference (EMI)
- Lead noise (lead fracture / insulation failure)
- Loose set screw
- Diaphragmatic myopotential.
- Pectoral myopotential.
Few key observations in the tracings are:
- There is more noise in the near field (NF, tip to ring) than far field
(FF, Can to coil). In fact, there is hardly any QRS / clear
electrogram in NF in spite of 10 times more magnification of NF
electrogram [0.1 vs 1 mV scale].
- The oversensing in the marker channel is intermittent although there
is background noise throughout. As the intensity of background noise
varies, the oversensing varies simultaneously, making it intermittent
in the marker channels.
- The characteristic of the noise (which is clearer in panel 1B) is like
myopotential (high frequency, low amplitude) as compared to make and
break signals of lead fracture (high frequency and high amplitude).
Based on the first point, EMI becomes unlikely in which both channels
have nearly equal noise in a majority of cases [1]. Noises related
to lead integrity issues have highly varying amplitude unlike a uniform
noise like the index case. The lead impedance was within normal range.
Loose set screw can have similar EGM quality and commonly present in
perioperative period. Moreover, abnormal impedance is often detected in
set screw related oversensing [2]. In the index case, prominent
noise in the NF channel makes an issue related to lead tip much more
likely [1].
Myopotential appears to be the most likely possibility as per the high
frequency, low amplitude uniform nature of the noise. Both pectoral and
diaphragmatic myopotentials (DMP) are non-cyclical but can have
variation of the noise related to arm movement / respiration
respectively. However, pectoral myopotential tends to have more noise in
FF / leadless EGM as the IPG (Can) is nearby [1,3]. As ICDs do not
use FF signals as default sensing channel, pectoral myopotentials do not
lead to oversensing ( unless the sensing polarity is manually changed)
[2]. In the index case, NF predominant noise indicates oversensing
related DMP. Real time EGM during deep inspiration also confirmed the
same.
When oversensing is suspected due to DMP, lead perforation shall be the
prime suspect. This can be confirmed by signs
of pericarditis or pericardial effusion [1]. With intact lead, DMP
oversensing is highly unlikely with true bipolar sensing in dedicated
bipolar leads [4]. It can however take place when sensitivity is
maximum e.g. after long diastolic intervals or ventricular paced events,
and often ends with a sensed R wave, which reduces sensitivity abruptly
[5]. Thus, it commonly occurs in pacing dependent ICD patients, in
whom inhibition of pacing maintains high ventricular sensitivity,
resulting in persistent oversensing as well as inappropriate detection
of VF [1].
Our case had dedicated bipolar lead without pacing requirement, hence,
perforation was suspected upfront. However, echocardiography did not
show any pericardial effusion. The lead could be traced only upto the
apex. But the device parameters were grossly abnormal. R wave amplitude
(sensing) was < 1mV and loss of capture (LOC) was evident even
at highest output. In fact, the lowermost panel in Fig 1 shows
undersensing along with LOC (true / functional) in the Vp beat.
Interestingly, pacing and shocking impedance trends were all within
normal range. Finally, chest X ray (CXR) and fluoroscopy confirmed a
lead perforation as the lead was noted outside the cardiac silhouette
almost reaching the inner chest wall (Fig. 2).
Skeletal myopotentials including DMP have predominant frequencies in the
range of 75 Hz, but they have a proportion of frequency content up to
100 to 200 Hz and some as low as 20 Hz. In ICDs, low-pass filters in the
range of 40 to 80 Hz can attenuate high-frequency components, but at
times sufficient high-frequency signals can still bypass these filters
to give myopotential electrograms a distinctive appearance like our case
[1]. Newer ICDs have in-built algorithms to reliably suspect and
withhold therapies related to noise. In fact, the index patient had a
several more episodes of oversensing where therapy was aptly withheld
due to noise detection algorithm.
After explaining the scenario to the patient’s relatives, she was
scheduled for lead extraction in a hybrid operation theater (OT). In the
OT under general anesthesia, the lead was unscrewed and pulled out.
Luckily no evidence of pericardial effusion/ tamponade was noted on
intraoperative trans-esophageal echocardiography (TEE). After waiting
for 10 min, a new single coil ICD lead was positioned into lower-mid
septum on the same sitting. Perioperative period was uneventful. To
summarise, this unique case describes one of the rarer causes of
inappropriate ICD discharges. This case also highlights the need for
careful implantation ICD leads which being heavier and stiffer, have
more preponderance for perforation then softer pacing leads.