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The most important determinant in the survival of
patients who have sustained hemodynamically
compromising ventricular tachycardia or ventricular fibrillation
is the interval from arrhythmia onset
to delivery of a cardioversion or defibrillation shock. RHYTHMx
ECD™ proprietary software was
designed, created and patented to provide all cardiac patients with
treatment efficacy equivalent to
the proven performance of implantable cardioverter defibrillators
(ICD's).
RHYTHMx devices will automatically deliver external
shocks when needed, according to fully
programmable parameters. These devices significantly decrease the
amount of time a patient will
be in a ventricular tachyarrhythmia, which Cardiac Science believes
will result in decreased morbidity
and mortality, even in highly monitored hospital areas, where current
survival rates for Sudden
Cardiac Arrest are approximately 15%1.
This paper will describe the RHYTHMx algorithm's function
and illustrate and explain the algorithm's
classification of the following shockable rhythms:
· Ventricular Tachycardia (VT)
· Ventricular Fibrillation (VF)
· Ventricular Flutter
· Fine VF
and several non-shockable rhythms, including:
· Asystole
· Torsades de Pointes
· Bundle Branch Block (BBB)
· and Atrial Fibrillation (A-Fib)
Before introducing the arrhythmias, it is useful to
understand the fundamentals of how the
algorithm continuously monitors the patient.
The algorithm demonstrated 100% sensitivity (correct
identification of shockable episodes) and
99.4% specificity (correct identification of non-shockable episodes)
in clinical trials2. Its strength
lies in its ability to properly filter ECG signal for noise rejection,
appropriately detect QRS
complexes, calculate the heart rate, and classify rhythms as shockable
or non-shockable based on
programmed parameters or pre-set default settings.
The patient's ECG signal is filtered to reduce artifact
and wandering baseline. The rate between two adjacent QRS complexes
is calculated, then six of every eight consecutive QRS rates are
averaged, and the two outliers are discarded. If the running average
rate in beats per minute (BPM) is greater than the programmed Detection
Rate (DR), then the rhythm is classified as TACHY. If a TACHY rhythm
persists for the duration of a programmed delay, cardioversion/defibrillation
therapy is automatically delivered (or advised based on clinician
preference) at the end of the delay period. If the running average
is lower than the DR, the rhythm is considered non-shockable.
RHYTHMx device therapy is non-committed. Following
detection of an arrhythmia, the device
delivers or advises a shock only if the last two QRS intervals occur
at a rate which exceeds the
detection rate. Otherwise, therapy is withheld. Subsequently, the
device will internally discharge
the energy if the arrhythmia spontaneously converts to a non-shockable
rhythm.
Synchronization (cardioversion) is always attempted
for a maximum of 2.5 seconds. If the
algorithm fails to identify an appropriate electrical event _ in
other words, the rhythm is likely
ventricular fibrillation _ then the RHYTHMx device delivers an asynchronous
(defibrillation) shock.
This feature eliminates the need for a manual synchronization button.
RHYTHMx
Algorithm
Method of
Analysis
Filtered ECG (3-33 Hz)
QRS Detection
Heart Rate Calculation
SVT Feature Extraction
Rhythm Classification
Algorithm Overview
Occasionally, patients may have a supraventricular
tachycardia that is relatively well tolerated, but
occurs faster than VT. In these cases, an SVT rate higher than the
Detection Rate may be programmed. RHYTHMx analysis is the same,
except that between the programmed DR and the maximum SVT Rate,
RHYTHMx examines the morphology of the waveform to attempt to differentiate
VT/VF from SVT:
· If the averaged rate is between the Detection rate and
the maximum SVT Rate, therapy is
. delivered only if rhythm is classified
as VT (i.e., the QRS complex begins to widen).
· If the averaged rate is greater than the programmed SVT
Rate, therapy is always delivered.
. Thus, if a rhythm occurs at a rate
higher than the maximum SVT Rate setting, it will always
. be shocked.
· The RHYTHMx algorithm is conservative in its rhythm classification.
It will always shock
. VT/VF at the risk of shocking an
SVT. Below is an illustration of the concept.
In the example below, the Detection Rate on the RHYTHMx
device is programmed at 160 BPM, and the SVT Rate is programmed
at 200 BPM. The SVT occurs at 187 BPM, and is correctly classified
as non-shockable. It is important to note that the algorithm will
respond similarly to most narrow-complex SVT's.
It is important to note that RHYTHMx devices require
ECG signal with peak-to-peak amplitude greater than.7
mV in order to acquire and analyze the patient rhythm. If at any
time during monitoring, the signal amplitude falls to below <0.2
mV, the algorithm employs special rhythm classifications:
· If the rhythm is preceded by a non-shockable
rhythm (i.e., running average BPM is lower than
. the programmed Detection Rate), then
RHYTHMx classifies the low-amplitude rhythm as
. ASYSTOLE, a non-shockable rhythm.
· If the rhythm is preceded by a shockable
rhythm (i.e., running average BPM is higher than the
. programmed Detection Rate), then
RHYTHMx classifies the low-amplitude rhythm as FINE VF,
. and will administer a shock as programmed.
ECG Signal that becomes saturated, that is, the signal
exceeds preset frequency and amplitude thresholds, is classified
as UNUSABLE SIGNAL, and is automatically rejected for analysis.
This helps prevent double-counting, which could lead to inappropriate
therapy delivery.
See the strip below for illustration.
VT is generally characterized by rates between 150
and 250 BPM. RHYTHMx devices classify VT as
shockable (TACHY) when it exceeds the programmed Detection Rate
(DR). Therapy is delivered or
advised at the end of the programmed delay.
If historical information regarding the patient's
ventricular arrhythmia is available, it can be especially helpful
in programming RHYTHMx devices. A patient with a history of symptomatic
VT at 175 BPM, for example, might have the DR programmed to 160
BPM. Some patients may tolerate their VT for extended periods or
at markedly higher rates. Further, they may spontaneously convert
to a sinus rhythm. In such cases, the Detection Rate can be increased
or the time delay to shock can be extended in order to avoid premature
therapy.
V-Flutter occurs at rates between 250-350 BPM, and
appears as a series of waves of similar amplitude. Because of the
excessive rate and the accompanying hemodynamic compromise, the
rhythm should be treated immediately. Please see the illustration
below.
Note that the QRS complex is still identifiable in both VT and V-Flutter
(although the P-waves are
hidden), evident by the tick marks beneath each R-wave.
VF is an erratic arrhythmia that causes the heart muscle to quiver,
as expressed in the random absence of identifiable QRS complexes
on the ECG. Although VF generally occurs at rates exceeding 350
BPM, RHYTHMx devices are rate-driven, and count the detected electrical
events as indicated by the tick marks. Therefore, while the BPM
indicates a rate lower than 350 BPM in the strip below, this rate
clearly exceeds the programmed Detection Rate of 180.
Fine VF is a deterioration of amplitude in VF. RHYTHMx
devices classify rhythms as Fine VF when signal amplitude is less
than .2 mV and is preceded by a shockable rhythm. Fine VF is considered
shockable.
Asystole is ventricular standstill. There is no cardiac output,
and no electrical activity in the heart.
RHYTHMx devices classify rhythms as asystole when signal amplitude
is less than .2 mV and the
rhythm is preceded by a non-shockable rhythm. Asystole is non-shockable.
In this rapid ventricular rhythm, the complexes appear
to deflect upward then downward, in a
twisting pattern. Rates of Torsades can exceed 200 BPM, and if left
untreated, the rhythm could
deteriorate to VF. RHYTHMx devices will deliver therapy to rates
exceeding the programmed
Detection Rate (see exceptions for SVT discussed on page 2). In
patients whose history indicates
possible paroxysmal Torsades where cardioversion or defibrillation
is not desired immediately, it is
important to program a sufficient delay to allow the clinician to
evaluate treatment options.
Alternatively, RHYTHMx devices are equipped with an ADVISORY mode,
in which the device will
charge the defibrillator but await the clinician, who delivers the
shock when appropriate.
The heart rate in the illustration above is 228. Since
this rate exceeds the programmed Detection
Rate of 170, the rhythm is classified as shockable.
Normally, the electrical conduction impulse travels
down both the right and left branches at the same speed, so that
both ventricles contract at the same time. When a block occurs in
one of the branches, the impulse on the blocked side may be delayed
with respect to the normally conducting branch. The delay is visible
on the ECG.
RHYTHMx devices may be used to monitor patients with
a bundle branch block or related
intraventricular conduction defect. It is important, however, to
verify that the QRS complexes are
triggering properly, and the T-waves are not being counted, as is
the case in the strip chart below.
A-Fib is characterized by irregular ventricular activity,
an almost indiscernible atrial rate, and P-waves that are absent,
replaced by fine fibrillatory waves. The following strip shows A-Fib
resulting in a ventricular rate of 111 BPM with a programmed Detection
Rate of 160 BPM; it is, therefore, non-shockable. If a rapid ventricular
response to the atrial fibrillation occurs, then the same considerations
apply to programming A-Fib as with other SVT's.
It is important to be familiar with the normal function of the
patient's pacemaker prior to attaching a RHYTHMx device. Stimulation
artifact that is visible on the device screen or on the strip chart
printout is a contraindication for the use of a RHYTHMx device.
Note that the device filtering capability might be sufficient to
suppress this stimulation artifact, even if it is visible on a 12-lead
ECG.
Some pacemakers will revert to unipolar stimulation
after exposure to a high energy shock. In this situation, if the
patient receives an appropriate shock from the RHYTHMx device and
the pacemaker then reverts to unipolar mode, then multiple detection
errors may occur.
This document is intended to illustrate the response
of the RHYTHMx algorithm on selected rhythm
types. Please consult with appropriate physician advisors for additional
guidance in programming
Rhythmx devices. For more information from Cardiac Science, Inc.,
please call (949) 587-0357 or visit the website at www.cardiacscience.com
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