Biomedical Engineering Reference
In-Depth Information
12.2.7 Interrogation and Testing of Implanted
Device
Table 12.1
Recommended electrical performance
Implant
Chronic
Sensing amplitude
Atrium
Interrogate the device by means of the programmer before
opening its sterile package. To ensure the correct measure-
ment of parameters, heat the device to room temperature.
Perform tests of the pacing or tachycardiac modes, form the
condenser manually, check the state of the battery, and reset
the counters - all of this depends on the type of device. If the
actual state of the battery indicates something other that
“beginning of life,” the device must not be implanted.
Defibrillators also indicate so-called monitored battery volt-
age that must be at the maximal level determined by the
manufacturer before implantation. Before connecting the
leads, especially in patients who are dependent on cardiac
pacing, the device must be programmed and switched over
from the storage mode to ensure immediate pacing after the
connection.
³
1 mV
³
1 mV
Ventricle
³
5 mV
³
5 mV
Impedance
Pacing circuit
300-2,000 Ω
300-2,000 Ω
Shock circuit
20-80 Ω
20-80 Ω
Pacing threshold
Voltage (at 0.5 ms)
Atrium
£
1.0 V
£
1.5 V
Ventricle
£
1.0 V
£
1.5 V
Current (at 0.5 ms)
Atrium
£
1.5 mA
£
6 mA
Ventricle
£
1.5 mA
£
6 mA
Slew rate
Atrium
³
0.3 V/s (mV/ms)
³
0.3 V/s (mV/ms)
Ventricle
³
1.0 V/s
³
0.5 V/s
Retrograde conduction time
100-400 ms
12.3
Connecting Leads to the Device
or pectoral muscle stimulation in the pocket where the device
is implanted.
Later measurements by means of the implanted devices
needn't strictly correspond to the measurements established
by means of the PSA as a consequence of the signal filtering.
However, the basic measurement should correspond to the
recommended values stated in Table
12.1
.
Electrical performance of the lead should be measured
after allowing sufficient time for the effect of local tissue
trauma to subside (about 10 min). If the measurements do
not correspond to the values stated above, it is necessary to
reposition the lead and secure it again using the procedures
previously discussed.
The initial electrical measurements may differ from the
long-term recommendation because of acute cellular trauma.
The values might be dependent on patient-specific factors,
such as the state of tissue, the balance of electrolytes, and
medicamentous interactions. Lower internal potentials and
higher pacing thresholds might be a marker of the lead posi-
tion in the ischemic or cicatricial tissue. Because the signal
quality might worsen over time, it is necessary to change the
lead position so that the obtained signal has the highest pos-
sible amplitude, the shortest possible period, and the lowest
possible pacing threshold. In the permanent state, R wave
amplitudes lower than the recommended value might cause
incorrect determination of heart action rate, which might
lead to failure of tachycardia detection or a false-positive
diagnosis of normal rhythm. In addition, a low value of the
pacing threshold requires a safety limit because the pacing
threshold might increase after implantation. Signal periods
that exceed the programmed refractory period of the device
might cause incorrect determination of the cardiac action
rate, unsuitable delivery of the high-voltage shock, or both.
Insert the lead connector straight into the lead port without
bending the lead near the lead-header interface. Incorrect
insertion might cause damage to the lead's insulation. If
required, or in accordance with manufacturer's instructions,
lubricate the lead connector sparingly using sterile water or
gel to make insertion easier. If the lead connector pin is not
connected to the device at the time of implantation, the lead
connector must be capped before closing the pocket incision.
Tie down a suture around the lead cap to keep it in place.
With regard to the patient's anatomical structure and the size
and movement of the device, gently coil the excess lead wire
and place it in the pocket mostly under the device. It is
important to place the lead into the pocket in a manner that
minimizes tension and torsion, the cranking to the closed
angle, and reduces pressure.
Implantable systems are equipped with various types of
lead connectors. IS-1 connectors are most common for car-
diac pacing leads. Before inserting a lead's connector to the
device header port, it is necessary to check whether the set-
screws are extended enough to enable insertion of the connec-
tor. If the IS-1 connector is connected to the device correctly,
the pin must visibly project at least 1 mm behind the connec-
tor block. For correct connection of the older 3.2-mm connec-
tor (different than the IS-1 type), the pin must reach less than
1 mm from the socket end after inserting the lead. Then, insert
the torque wrench into the slot in the center of the pothole in
the seal plug positioned near the tip of the completely extended
lead's connector. The device is equipped with setscrews that
are to be tightened using a bidirectional torque wrench that is
a part of the supplied package. While tightening the setscrews,
push the lead so that it stays completely inserted in the socket.
