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284 P R I N C I P L E S A N D P R A C T I C E O F C R I T I C A L C A R E

Configuration
Configuration Defib with Single Tach Arrhythmia Sensing Dual Chamber
Detection Criteria SVT Criteria
Fib Detection 330 ms/182 bpm V < A Rate Branch
Tach Detection 375 ms/160 bpm for 12 intervals Morphology Off
SVT Upper Limit Same as Fib Interval Stability On (80 ms), (60 ms), 12 intervals
V = A Rate Branch
1 Morphology Off
Sudden Onset On (100 ms)
2

MTD 2 min (Fib Therapy)
MTF Same as Tach for 40 sec
Tachyarrhythmia Therapy Tach ATP
Fib/MTF: [1] Defib 36.0 J (801 V) Output 7.5 V, 1.0 ms
85%
[2] Defib 36.0 J (801 V) BCL 200 ms
Min BCL
[3] Defib × 4 36.0 J (801 V) 4 No. Bursts 3
Tach: [1] ATP Stimuli 10 stimuli
[2] CVRT 10.0 J (429 V) Scanning 12 ms
[3] CVRT 20.0 J (605 V) Ramp Off
[4] CVRT × 2 36.0 J (801 V) Shock Waveform Stored EGM
A Sense/Pace, ± 3.0 mV
3 Biphasic, Fixed Tilt EGM #1 V Sense/Pace, ± 8.9 mV
EGM #2
RV (+) to SVC/Can (–)
Defib: 65 % / 65 % Events Fib, MTD, Tach
CVRT: Same as Defib Settings Detection, 16 sec Pre, 1 min Max
TM
FIGURE 11.51 Implantable cardioverter defibrillator (ICD) programmed parameter summary report from St Jude Medical ICD Atlas DR Model V-240
(Courtesy St Jude Medical, St Paul, MN): box 1, detection criteria. Arrhythmias are classified first on the basis of ventricular rate as detected by
sensing circuitry. Defining rates for each rhythm classification are programmable. In the example shown above a rate of >182/min or greater is the
cut-off for ‘Fib Detection’, which then initiates treatment following the steps for fibrillation (Fib/MTF) (box 3). ‘Tach Detection’ is classified when the
rate is between 160 and 182, then further information can be sought to differentiate between supraventricular and ventricular arrhythmias and
initiate appropriate treatment (box 2); box 2, SVT criteria. When rhythms fall into the ‘Tach Detection’ rate (here 160–182/min), treatment is momen-
tarily suspended to allow classification as SVT. If ventricular rate >atrial rate, then the rhythm is classified as VT and therapy applied according to
‘Tach’ (box 3). When ventricular rate <atrial rate (V<A in box 2), the rhythm could be either SVT or VT and an assessment of interval stability is
made (with marked irregularity supporting AF and the withholding of treatment). ECG morphology distinction can also be enabled (although here
is OFF) that compares morphology prior to and during tachycardia. If the ventricular and atrial rates are the same (V=A), again the rhythm could
be either VT or SVT and further discrimination is made on morphology and on whether onset was sudden or gradual. This process of rhythm
detection is extremely rapid and does not unnecessarily delay therapy. Additionally, even when SVT criteria are met, they are usually subordinate
to sustained rate in the ‘Tach Detection’ algorithm; thus, if the tachycardia persists for a set qualifying period, therapy is initiated according to the
‘Tach’ algorithm in box 3; box 3, tachyarrhythmia therapy. Usually two treatment arms, each with six steps, are prescribed and are independently
programmable. Tachyarrhythmias in the ‘Fib’ range are managed more aggressively than those in the ‘Tach’ range. Escalating energy settings for
defibrillation may be described, but here all six attempts at defibrillation are at maximum (36 J). Treatment in the ‘Tach’ range usually commences
with one or more attempts at antitachycardia pacing (ATP), progressing to cardioversion; box 4, tach ATP. This describes the parameter setting
during antitachycardia pacing. In this example 7.5 V is delivered to the ventricles at a basic cycle length (BCL), which is 85% of the cycle length
of the tachycardia (pacing is delivered slightly faster than the tachycardia). It will provide up to three ‘bursts’ of pacing, each of 10 beats or ‘stimuli’.
Ramping is turned off in this case, but if ON it permits the pacing rate to increase progressively after each unsuccessful attempt at overdrive.

taking care to avoid positioning paddles over the ICD. This will disable tachycardia therapies so that if the
105
External chest compressions can safely be undertaken by terminal rhythm is VT or VF, therapies will not be
rescuers, including during device therapy. 70 delivered.
Terminal Care and Mechanisms of Death Other than by disabling therapy, cardiac death may occur
by normal mechanisms. Cardiac arrest in the acute
in the Patient with an ICD context, as well as when it occurs as the endpoint of ter-
ICDs often create uncertainty amongst health care minal illness, ultimately occurs when cardiac metabolism
workers as to how death may occur. In the palliative fails or systemic factors cause cardiac depression or
patient, where active resuscitation for cardiac death is arrhythmic irritability. The same remains true of the
not to be pursued, the decision to disable antitachy- patient with an ICD. However, cardiac depressive factors
cardia therapies is often taken. This can be achieved will not cause bradycardia or asystole because of the
by reprogramming the ICD, and there is often sufficient pacemaker function. What would otherwise be a
time to incorporate this step into palliative planning. bradyarrhythmic death will instead become eventual
Alternatively, when active treatment is being withdrawn failure to capture by the pacemaker. Similarly, if the
as a patient progresses more rapidly towards an unex- cardiac impact of acute or terminal illness produce
pected (acute) death, there may be a need to disable tachyarrhythmias, then these same influences will increase
therapy before the availability of personnel to reprogram the defibrillation threshold and antitachycardia therapies
the device. In this context it may be appropriate to will become unsuccessful. Devices offer no protection
secure a ring magnet over the ICD (tape it in place). against pulseless electrical activity.