Page 714 - Cardiac Nursing

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690 PA R T I V / Pathophysiology and Management Disease
DISPLAY 28-7 The Makings of an ICD
Leads: Leads are insulated wires made from either silicone rubber or polyurethane, they connect the ICD to the patient’s
heart. There are five major components to a lead: (1) the electrode(s), (2) the conductor(s), (3) insulation, (4) connector
pins, and (5) the fixation mechanism. The ICD system can have up to three leads placed, dependent on the system.
Casing: The casing, or outer shell of the ICD is made from titanium. Titanium is biocompatible, and highly resistant to pene-
tration by body fluids. Titanium is stronger than steel, but up to 45% lighter.
Header: The top portion of the ICD is called the header; it is made of a see-through epoxy. The header has ports, which the
connector pins of the lead(s) are inserted. The lead is secured into the header with the setscrew.
Setscrews: The leads are connected securely to the ICD with a small screw to ensure electrical contact between the lead and
the ICD. At implant, a small sterile screwdriver is packaged with the ICD.
Circuitry: The ICD contains complex microelectronics (very small computer chips) that allows the ICD to process incoming
signals, store information, and produce a response dependent on the signals that are processed. The microprocessors
can respond to programming instructions that allow for changes after implantation. The circuits contain both read only
memory (ROM) and random access memory (RAM). Just as in new computers, the amount of RAM in new ICDs is increas-
ing rapidly, allowing for increased diagnostic information to be stored.
Battery: The internal power source for the ICD is the battery. Most ICD batteries are made from lithium silver vanadium ox-
ide. The battery longevity is close to 5 years for most ICDs. Battery depletion is monitored regularly at follow-up visits.
Once the battery reaches the elective replacement indicator (ERI), the entire ICD is replaced, not just the battery.
Capacitors: The ICD is able to generate enough energy to deliver a shock because the capacitors store an electrical charge.
The capacitor is made of multiple conductors separated by insulators. Capacitors with high-voltage capabilities can
charge up to 830 V in order to deliver a high-energy shock. It can take up to 15 seconds for the device to fully charge to
the highest energy level, usually around 35 J. 73,74

capabilities similar to pacing leads but also has a large electrical ing oversensing of T waves during repolarization and P waves
surface area for delivering high-energy shocks. The ventricular during atrial depolarization. Other incoming signals that can be
lead is usually a dual coil lead. When the lead is placed in the sensed include low-frequency noise, skeletal myopotentials, and
heart, the proximal coil is positioned at the level of the superior EMI. ICD systems have either an automatic gain control or an
vena cava and the distal coil is positioned in the right ventricle. auto-adjusting threshold feature that helps with proper sens-
The defibrillation pathway that is used with most ICD im- ing. 54,72
plants today involves the titanium case of the pulse generator as The ICD primarily detects arrhythmias by looking at the cycle
the current flows from the distal defibrillation coil simultane- length, which is the time between R waves produced by ventricu-
ously to the ICD and the proximal coil. The generator is often lar depolarization. The cycle length represents the heart rate. The
referred to as an “active can” or “hot can.” Using the generator ICD can also be programmed to look at signal morphology, which
to complete the defibrillation circuit has helped lower defibril- helps in arrhythmia detection. 72 For the VF zone; ICD devices
lation thresholds known as defibrillation threshold testings use rate criteria as the sole detection method. The use of rate cri-
(DFTs). 57,72 Early ICDs delivered shocks with a monophasic teria results in maximal sensitivity. The ICD charges the capacitor
waveform, which was a single pulse at a given polarity and du- once the programmed amount of intervals is met (e.g. 8 to 12 in-
ration. Today’s ICDs deliver shocks with a biphasic waveform. tervals of a rate of 180 bpm). The ICD then delivers the shock af-
A biphasic shock has a negative and positive pulse, which low- ter reconfirming the rate. If rate criteria are not met, the shock is
ered DFTs significantly. Lower thresholds result in higher rates aborted. The reconfirmation prevents unnecessary shocks for non-
of successful defibrillation, a higher margin of safety, and pro- sustained events.
longed battery life. 78 A VT zone can also be programmed into the ICD. Once again,
If a dual-chamber ICD is placed, a second lead is placed in the rate is the primary detection method, but other detection en-
atrium. A dual-chamber system may be preferable in patients with hancements can be programmed to increase specificity of VT
a history of atrial fibrillation, for discrimination of arrhythmias. If detection, thus decreasing inappropriate shocks for supraventricu-
the patient requires CRT therapy, a third lead is placed into the lar tachycardia and atrial fibrillation. One of the pitfalls of ICD
coronary sinus, which would then allow biventricular pacing (Fig. detection is inappropriate shocks delivered for supraventricular ar-
28-38). rhythmias. These optional detection features include sudden-onset
criterion, an R-R interval stability criterion, an electrogram width
Sensing and Detection Enhancements criterion, and sustained rate duration. Dual-chambered pace-
maker defibrillators also use atrial rate data (Fig. 28-39) and com-
Recognizing ventricular arrhythmias is essential for the ICD; it pare the atrial versus ventricular rate to help deliver appropriate
is the sensing that measures the intracardiac electrograms signal therapy. 54 When VT episodes were reviewed in a dual-chamber
from the lead electrodes. The sensing electrodes transmit each ICD, the ventricular rate was noted to be faster than the atrial rate
ventricular depolarization (R-wave) signal to the sense amplifier 80% of the time in most studies. ICDs either directly or indirectly
of the ICD. The main challenge for the sensing system is two compare atrial and ventricular rate as a first step in distinguishing
fold. It must detect the very low amplitudes of VF while avoid- SVT from VT. 79