Implantable Devices (Pacemaker, ICD, CRT, Loop Recorder)

What is the advantage of using biphasic over monophasic shock waveforms for ICDs and external defibrillators?

The reversing polarity of the devices makes biphasic waveforms more effective such that VF is terminated at lower energies with less injury to the myocardium.

The principle of “(R-wave) synchronization” is needed for what shocking modality?

Cardioversion

During a pacemaker gen change, the leads are disconnected and reconnected. What might have gone wrong during the procedure if, post-procedure, the ventricular impedance reading suddenly skyrockets and we have loss of capture?

The lead pin was not fully (re)-inserted

What is pace-sense conductor fracture?

An episode on an ICD where the near-field channel shows non-physiologic spikes, and the far-field channel shows normal rhythm. There is also a spike in impedance intermittently. This may be provoked by reaching one’s arms upward.

What is SVT discrimination on an ICD?

SVT discriminators work for presenting rhythms that fall within the programmed zone (i.e. think about delivering a shock for VT at or above 170 bpm). Algorithms can reference many things, including: rhythm onset, R-R (ir)regularity, QRS morphology, PR interval, AV association, A and V rates, rhythm onset and in what chamber, etc.

Which ICD ventricular rate sensing configuration is more prone to oversensing diaphragmatic myopotentials?

Integrated bipolar sensing uses the tip as one electrode and the RV shock coil as the second electrode, which widens the sensing field. Thus, diaphragmatic and far-field signals are picked up more easily.

What is chronotropic incompetence, and how can it be managed with a pacemaker?

Chronotropic incompetence means that the heart rate does not increase adequately enough to support a given level of activity or metabolic demand. Usually 80%+ of the age-predicted max is an acceptable heart rate; otherwise, rate-responsive pacing will raise the paced rate with activity.

A stimulator delivers a 5 mA pulse lasting 0.5 ms. How much charge is delivered per pulse?

Q = Current x Time. So Q = (5E-3 A) x (0.5 E-3 s) = 2.5E-6 C = 2.5 microcoulombs

For an ICD, what is non-committed shock reconfirmation?

An ICD will reconfirm that the patient is still in VT/VF before delivering a shock after having charged. If sinus rhythm has returned, the shock is aborted and the charge is dumped internally.

For an ICD: If a high-voltage shock impedance reading is significantly lower than the one before it, what might be wrong?

There is an insulation breach in the shock circuit and some of the current is taking a path of least resistance through a short, so the shock impedance is lowered