Biomedical Engineering Reference
In-Depth Information
1 1
Cardiac Resynchronization Therapy
Cardiac resynchronization therapy (CRT) is a nonpharmaco-
logical method used to treat chronic heart failure. One symp-
tom of heart failure is ventricular dyssynchrony decreasing
the stroke volume. CRT enables treatment of ventricular dys-
synchrony by means of delivering pacing pulses to the right
and left ventricle. As a consequence, a better heart contrac-
tion mechanism and an increase in cardiac output is achieved.
The clinical efficiency of CRT was confirmed in large ran-
domized studies, which are described in the Chap.
6
,
dealing
with indications for treatment. CRT improves hemodynamic
parameters and quality of life of patients with severe heart
failure. The first CRT systems made use of a left ventricular
(LV) lead inserted via an epicardial approach requiring tho-
racotomy. Today, the lead is inserted via the subclavian vein
through the coronary sinus and into the target coronary vein.
The pacing electrodes of the LV lead should be suitably
located at the place of the most recent ventricular activation
during intrinsic cardiac impulse conduction. In most patients,
this site is assumed to be on the left ventricle lateral wall if
the lateral or posterolateral vein is available.
Biventricular pacing may be delivered using both pace-
makers (CRT pacemakers [CRT-Ps]) and defibrillators (CRT
defibrillators [CRT-Ds]). Technically, pacing components of
CRT-Ds are identical to those of CRT-Ps [80, 81].
pacing pulse width and amplitude, pacing configuration,
and, to a limited extent, timing [ 47 ] may be programmed.
As in conventional dual-chamber pacemakers, various
pacing modes can be chosen in CRT systems. The DDD
mode is suitable for patients with heart failure and sinus bra-
dycardia because it can deliver biventricular pacing synchro-
nous with the atrium at rates above the lower rate limit (LRL)
and AV sequential biventricular pacing at the LRL or a sen-
sor-indicated rate. The VDD mode is suitable for patients
with heart failure with normal sinus rhythm; in this mode,
the biventricular pacing is delivered synchronously with the
atrium, but atrial pacing is excluded. The VDD(R) pacing
can be inappropriate, though, because AV desynchronization
occurs during sensor-controlled ventricular pacing if the
sensor-indicated rate exceeds the intrinsic sinus rhythm. The
VVI(R) modes may be harmful to patients with heart failure
with normal sinus activity but appropriate for patients with
chronic atrial tachycardia. They deliver biventricular pacing
at the LRL or at the sensor-indicated rate. If tracking to ven-
tricles occurs during atrial tachycardia, a higher LRL can be
programmed or a sensor can be turned on.
To secure biventricular pacing, LV and RV pacing must
be applied immediately after a possible sensed RV event.
This basic function of CRT systems may be used in the
modes of tracked or merely ventricular pacing. The pacing
rate is between the LRL and the maximum pacing rate (MPR).
In modes with an adaptive rate, or in tracking modes, maxi-
mum biventricular pacing is limited by the maximum sensor
rate or the maximum tracking rate (MTR).
Biventricular pacing can be interrupted by sensed ventric-
ular events, ventricular extrasystoles, spontaneously tracked
atrial tachycardia, or even sensed atrial pacing in ventricles
(crosstalk). The interruption of biventricular pacing may lead
to symptoms of heart failure, which is why the device, while
sensing the intrinsic ventricular activity when still in the AV
delay interval or in nontracking pacing modes, immediately
triggers ventricular pacing if the MPR is not exceeded.
11.1
Securing Left Ventricular Pacing
Biventricular pacing was first applied in a human as early as
the 1970s in the framework of attempts to suppress ventricu-
lar arrhythmia by overdriving certain indications. For the
sake of saving money, a dual-chamber pacemaker with
minimum atrioventricular (AV) delay was sometimes used
as a biventricular VVI system. At the turn of the millennium,
CRT systems still had electrically connected right- and left-
ventricular channels. Today, right ventricular (RV) and LV
leads may be programmed separately. LV parameters of
