HEART BLOCKS AND CARDIAC PACEMAKERS Arun Abbi Jason Mitchell Jan 21, 2010.
Pacemakers, Cardiac Mappingand EEGin the ......In 1958. cardiac pacing represented a surgical...
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CLINICAL ELECTROENCEPHALOGRAPHY•1979VOL I0.NO3
Pacemakers, Cardiac Mapping and EEG in the
Community Hospital:
a Concise Review of Background and Basic Technical Considerations
Evelyn M. Anderson and Andrew L. Carney
Introduction
In 1958. cardiac pacing represented a
surgical triumph for a catastrophic and
uncommon condition (1). Today, pacemaker
implantation is no longer rare. Ten thousand
pacemakers are implanted monthly and there
are over 225.000 pacemakers outstanding
with the lithium power source alone (2).
Moreover, application has broadened as the
capacity for cardiac rhythm analysis has
increased. Long term rhythm monitoring, in
the coronary care unit and with the
ambulatory Holter monitor, electrophysio-
logical studies and sinus node recovery time
(SNRT) (3-7) are commonly employed in
patient selection.
Diagnosis has become more specific.
Hazardous rhythm or conduction patterns
may be an indication for pacing even in the
absence of symptoms. Temporary
pacemakers are employed to protect the
patient during a period of stress and are then
removed or replaced with permanent units.
The Heart Is A Pump
The fascination for pacemakers has not
subsided. The search for a better power
source has dominated the literature of the past
decade. Miniturization, improved circuitry,
non-invasive programming and trans-
telephonic pacemaker analysis has produced
such a clutter of minutiae, it seems forgotten
that the heart is a pump: if pacing does not
improve the pumping of the heart, the patient
becomes worse and not better The heart as a
pump is the concern of this paper.
Pacemaker Aggravation of Neurological
Symptoms
The patient who has a pacemaker implanted
to relieve syncope or bradycardia may find
himself disabled with dizziness and ataxia that
is pacemaker related. Most pacemakers are
NOT .PHYSIOLOGIC and stimulate the right
ventricle (RV). RV pacing results in reduced
stroke volume by the loss of the atrial
contribution and by mitral and tricuspid
insufficiency (8-10). When the cardiac reserve
is adequate. RV pacing may be well tolerated,
but it can be catastrophic (11) or bizarre (12)
but more commonly worsens the neurological
status of the patient, especially vertebral
basilar symptoms (13). When this occurs,
physiologic pacing may resolve the problem.
An introduction to basic background
information and technical considerations
follows.
Conduction System of the Heart
Normally, the sinus node high in the right
atrium (RA) (Fig. 1) initiates the impulse which
spreads to the atria and the A-V node, down
the Bundle of His to the right and left bundle
branches which result in synchronized
chamber contraction. The atria contract and
fill the ventricles; the papillary muscles
contract, close and stabilize the A-V valves
(mitral and tricuspid) before the ventricles
contraci.
Physiologic Pacing — Not Popular
Transvenous RV pacing is the most popular
because it is the easiest to accomplish, has the
lowest morbidity and mortality and employs
the most reliable components with the fewest
technical problems. But RV pacing results in
Evelyn M Anderson MD is Assistant Clinical Professor
Department ol Eleclroencephalography and Andrew L
Carney M D is Assistant Clinical Professor. Department
ol Thoracic and Cardiovascular Surgery Metro Six Affiliated
Hospitals Abraham Lincoln School of Medicine University
of Illinois Chicago
Please send requests for reprints to Evelyn M Anderson
MD 720 North Michigan Avenue Chicago III 60611
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CLINICAL ELECTROENCEPHALOGRAPHY 1979 VOL 10.NO3
Sinus NodeA-V Node
Cardiac Conduction System
Figure 1. CONDUCTION SYSTEM OF THE HEART
Normally, the stimulating impulse
originates in the sinus node of the right
atrium, proceeds to the AV node and down
the Bundle of His to the right and left
bundle branches.
the LEAST improvement of cardiac output
and often significantly reduces it (8-10).
Right Ventricular Pacing (RV)
The right ventricle is stimulated and the
impulse radiates from that point. If the
conduction system is intact, the impulse
travels retrograde to the atria causing late
contraction. If complete heart block exists,
this does not occur. Not only does RV pacing
lose the atrial contribution, but the ventricular
contraction may occur before the A-V valves
are fully closed permitting regurgitation. The
net result is a reduction of stroke output (8-
10).
Evolving Pacemaker Technology
Demand pacemakers which are inhibited by
spontaneous cardiac activity have largely
replaced fixed rate pacemakers because of
the high mortality associated with competing
rhythms. Programmable units which permit
variation of rate and other parameters have as
their objectives optimal hemodynamic
response and minimal battery depletion (14).
Atrial Pacing — Physiologic Pacing
Arterial pacing simulates normal sinus
rhythm (Fig. 2,5). If the conduction system is
not intact, then the ventricle must also be
stimulated. Three basic types of pacemakers
are available which attempt to utilize the atrial
contribution:
1. ATRIAL — Stimulates only the atrium,
usually the right atrium and is followed by a
normally conducted ventricular contraction
(Fig's. 2.5). Usually the atrium is sensed (15).
2. ATRIAL SYNCHRONOUS — Senses the
atrial contraction and after a time interval
always stimulates the ventricle. Useful in the
presence of heart block but atrial arrhythmia
has necessitated emergency removal.
3. A-V SEQUENTIAL — Senses the ventricle
and stimulates the atrium and ventricle in
succession. Only the ventricular contraction
inactivates the pacemaker (Fig. 2.5). Despite
the wide acceptance of the desirability of atrial
pacing less than one percent of pacemakers
implanted are of the physiologic type (16).
Experience With Atrial and AV Sequential
Pacing
The hemodynamic improvement with AV
sequential pacing for heart block
complicating acute myocardial infarction has
been well documented (17). but it is rarely
used The greatest use of atrial and AV
sequential pacing has been in cardiac surgery
where easy access to the atria simplifies
electrode placement and where external
programmable pacemakers can accomodate
to a large variety of conditions Here diagnosis
and treatment of arrhythmia (18-20) and
improved hemodynamic status has been
documented (21,22). But even here, the
difficulties encountered have been such that
only recently has TEMPORARY atrial pacing
developed a strong following The difficulties,
common to both temporary and permanent
pacing are related to 1 the pacemaker. 2 the
electrode and 3. the pacing site.
1. The Pacemaker
Atrial or A-V pacing may be temporary or
115
CLINICAL ELECTROENCEPHALOGRAPHY : 1979VOL 10.NO3
Vv
H.PARWUL
. ■ vV.'
L OCOPITAL
vV vvV Vv.WVj'v
\. • / \
^ OCCIPITAL
AV
Figure 2. RIGHT ATRIAL AND A-V SEQUENTIAL PACING
Right Atrial Pacing - The atrial pacemaker artifact preceeds the P-wave and is followed by a
normally conducted ORS complex
A-V Sequential Pacing - The atrial pacemaker artifact preceeds the P-wave and the higher
voltage ventricular pacemaker artifact triggers the distorted ORS characteristic of RV pacing
permanent. Temporary pacing is commonly
employed following cardiac surgery (18-22).
myocardial infarction (17), in the
catheterization laboratory (23) and for the
control of tachyarrhythmias (19). The atrial
electrogram (AEG) facilitates diagnosis and
treatment of arrhythmia (18.19.20). The
objectives are to achieve the optimum cardiac
output (4.10.17). to stress the heart (23). to
suppress tachyarrhythrrva (5.18.19) and to
determine the sinus node recovery time (6).
Permanent pacemakers may be for
intermittant use as with rapid atrial stimulation
(to 300 bpm) for control ol tachyarrhythmia
(24) or for constant demand operation to
achieve optimal cardiac output (22).
Flexibility — A Necessity
External programmable units have been
long employed for temporary pacing. Initial
permanent atrial and AV pacemakers were not
programmable and implantation of these units
required the selection of optimal rate and
mode PRIOR to surgery and they could not be
changed without further surgery (4)
Unfortunately, the needs of the patient may
change from day to day and from year to year
A programmable pacemaker permits the
selection of optimal pacing rate and mode
AFTER implantation.
New Pacemaker in Clinical Evaluation
The A-V sequential pacemaker. Medtronic
5992. holds promise. It may function in the
standby mode, or it can pace the atrium or the
atrium and ventricle in sequence or the right
ventricle alone (Fig.5). The interval between
the impulse to the atrium and to the ventricle
as well as the rate can be varied by an external
radiofrequency coupled electromagnetic
device AFTER being implanted This flexibility
permits optimal physiologic pacing under a
wide variety of conditions
116
CLINICAL ELECTROENCEPHALOGRAPHY «1979 VOL 10.NO.3
III
R. OCCIPITAL
Figure 3. RIGHT VENTRICULAR PACING
The ventricular pacemaker artifact is closely associated with a distorted ORS complex which
pervades and distorts the entire EEG.
2. The Atrial Electrode
Transvenous Route
A prime difficulty has been the high degree
of skill required to position and maintain
electrode position. The J-shaped electrode
was developed to hook into the right atrium
when passed transvenously; because
dislodgment was problem, tines were added
(Fig. 4A) and favorable reports have followed
(25). Both atrial and ventricular electrodes are
required for atrial synchronous and AV
sequential pacing. Two veins, then, must be
sacrificed, for if one vein is used for two
electrodes, dislodgment of the first electrode
often results from the manipulation of the
second (22).
Surgical Approach
Steel wires have routinely been affixed to
the ventricle following open heart surgery for
years. Temporary post-operative atrial pacing
(18.19,21) employs these same wires and the
first attempt at permanent atrial pacing
employed electrodes of this type (4).
The Atrial Pinch-On Electrode (Figure 4B),
still in clinical evaluation may prove to be a
significant development, for prior to it, there
has been NO SATISFACTORY PERMANENT
ELECTRODE available for surgical
placement. This electrode permits the atrium
to be paced at any accessible site with
precision, security and minimal trauma.
3. The Pacing Site
Pacemaker Function and Pacing Site
Pacemaker sensing is critical for
performance reliability and is dependent upon
the electrical potential and the area of the
electrode in contact with the tissue (14,26.27).
Low pacing thresholds permit long battery life
by reducing current drain. Pacing thresholds
will vary with the surface area of the electrode,
duration of the stimulus, wave configuration,
type of pacing and the pacing site. Low
threshold sites may not have optimal sensing
capacity. Endocardial and epicardial sites
may be well suited for either pacing or sensing
or both, but not necessarily both.
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CLINICAL ELECTROENCEPHALOGRAPHY •1979.VOL. 10.NO.3
A B C
Figure 4. ATRIAL STIMULATING AND MAPPING ELECTRODES
A. Atrial Tined Electrode (Medtronic 6990) The J-shape has been used to enhance
transvenous placement by hooking it high in the right atrium. The tines have been added to
minimize dislodgement of the electrode and loss of pacing.
B. Atrial Pinch-On Electrode (Medtronic 6995) The pincer jaw is applied surgically to the atrial
wall. Two electrodes are required for a bipolar configuration.
C. Atrial Mapping Electrode (Medtronic 6902) This bipolar endocardial pacing electrode also
serves as a mapping electrode. When the tip is applied to the right atrium, the potentials
recorded and thresholds determined approximate those of the permanent atrial pinch-on
electrode.
Furthermore, thresholds tend to rise and
sensing potentials may fall after acute implant
as the tissue responds to the foreign body.
Electrode design and pacemaker circuitry
must meet the requirement of chronic
thresholds.
Specialized Nature of the Right Atrium
The complex nature of the right atrium must
be appreciated. The highly specialized sinus
and AV nodes, the thin and highly mobile atrial
wall, dysfunction of the sinus node, atrial
dysrrhythmia and the disturbed function
created by metabolic disturbance, e.g.
cardiopulmonary bypass, are very real.
Moreover, slight variation in pacing site may
alter conduction and refractory times (28).
The Right Atrium — All Parts Are NOT Equal
The intrinsic activity of the sinus node
surpasses that of any other part of the heart.
The marked variation of electrical potential
recorded at elective atrial pacemaker
implantation (Fig.6 A.B). correlates well with
endocardial electrophysiological studies (5-
7). In our limited experience the high right
atrium has high electrical potential and high
pacing threshold. It would appear the high RA
118
CLINICAL ELECTFIOENCEPHALOGRAPHY ©1979.VOL.10.NO 3
Sponttneou*
Rhythm
Inhibited Mod* (no output)
B
Induced P-wave
I
IAtrial Pacing Mode (ventricular output Inhibited)
Induced P-wave
AV Sequential Pacing Mode
No Alrlal Outpul
Atrial Disable Mode (ventricular
demand pacing)
Alrtal Escape Interval
Ventricular Escape Interval
AV Sequential Inlarval
Conducted R-wave
Induced R-wave
Induced R-wave
rPulse Generator Operation
FIGURE 5. - MODES OF PROGRAMABLE A-V SEQUENTIAL PACEMAKER
(Medtronic 5992)
- Spontaneous cardiac activity. Pacemaker inactive.
- The atrium is pacemaker stimulated. The QRS complex that follows the
induced P-wave is normally conducted.
- Both the atrium and the ventricle are stimulated in succession by the pacemaker. Only the ventricular electrode can sense and the ventricular R-wave
inhibits the pacemaker.
D. Ventricular Demand - Only the ventricle is stimulated and sensed. II the conduction system is intact.an atrial contraction may occur late in the cardiac cycle.
A. Standby Mode
B. Atrial Pacing
C. A-V Sequential
119
CLINICAL ELECTROENCEPHALOGRAPHY ei979VOL.10.NO3
N48Oj$ALr'a'-/u--^^
SE.K7
Figure 6. EEG RECORDING OF THE ELECTROCARDIOGRAM AND THE RIGHT ATRIAL ELECTROGRAM
A. HIGH ATRIAL ELECTROGRAM
Note the high electrical potential of the high atrium. This finding is noted in endocardial
recordings of the atrium. The pacing threshold at this site was 4.0 V with the exploring
electrode.
B. LOW ATRIAL ELECTROGRAM
The low electrical potential noted correlates with endocardial recordings The pacing
threshold at this site was 0.9 V with the exploring electrode and 0 7V with the permanent atrial
electrode.
120
CLINICAL ELECTROENCEPHALOGRAPHYei979.VOL.10.NO3
might be best for sensing and the low RA with
low potential best for pacing (Fig. 6 A,B).
Cardiac Mapping
Cardiac mapping has been a complex
research tool requiring sophisticated
personnel and equipment which has been
applied to the physiological evaluation of
arrhythmias (29.30), identification of aberrant
conduction pathways (31). and delineating
the extent of myocardial infarction (32). More
recently, endocardia) electrograms (Bundle of
His) have aided in identifying the malfunction
of the sinus node (Sick Sinus Syndrome) (3-7)
and evaluating the endocardial pacing site
(26,33). Mapping of the atrium prior to
permanent (4) or temporary implantation of
electrodes (18,21,22) has not been reported.
Rarely has mapping of the ventricle with
surgical electrode placement been reported
(34), but the sites of highest thresholds were
noted to have the highest electrical potential.
Cardiac Mapping With the EEG
Sophisticated cardiac mapping devices are
expensive and unnecessary for the
pacemaker application. An isolated electrode
system and a precisely calibrated writer are
adequate. We have utilized a technique
employing equipment generally available in
the community hospital. The EEG (Grass
Model 89) with a biopotential isolator (Fig.7) is
satisfactory but a larger ten channel unit may
be preferred. The advantage is that the
equipment and trained personnel are already
at hand, no additional cash outlay is required
and a written record is retained for
subsequent analysis. Threshold determina
tions can be done with the Pacing System
Analyser (Medtronic 5300).
Surgical Approach — Preferred
When the optimal site of pacing is localized,
it should be possible to fix the pacing
electrode to that site. This cannot be done
with a transvenous electrode, therefore a
surgical approach is preferred.
The Exploring Electrode
There is a direct relationship between the
electrode surface area and the electrical
potential measured (26,27). Strictly speaking,
an exploring surface electrode and a
penetrating electrode are not comparable.
But, in our hands, the electrical potentials and
the pacing thresholds derived from the tip of
the bipolar endocardial electrode (Fig.4C)
approximates that of the permanent atrial
"pinch on" electrode (Figure 4B, 6A and B)
secured in the atrial wall, but more data is
required for statistical significance.
Technique — Atrial Mapping
By securing the biopolar electrode to a
surgical forceps (Fig. 8A). positioning of the
tip against the atrial wall, mapping is
facilitated. An alligator clip is applied to the
connection for the tip electrode (Fig. 8B)
which in turn is connected to the isolated jack
box (Fig. 7) of the EEG. Potentials are
recorded in the high, middle and low positions
on the lateral and medial aspects of the right
atrium.
Technique — Threshold Determination
Pacing thresholds are determined by
connecting the negative pole of the external
pacemaker to the electrode tip, while the
positive pole is grounded to the patient.
Pacing thresholds are determined in all
locations. The lowest threshold area is
selected for the pacing site. The permanent
electrodes are fixed to the atrium and
thresholds again determined. The low voltage
atrial pacing spikes can be easily recorded on
the highly sensitive EEG. When the study is
completed, the electrode can be used for
endocardial pacing.
Pacing Thresholds
In a patient having elective atrial pacing,
thresholds may vary by a factor of five in a
given atrium (Fig. 6 A,B). The site of lowest
electrical potential is often the site of lowest
threshold. During atrial fibrillation and
following cardiopulmonary bypass, the atria
may be refractory to stimulation. Excessive
motion or scar from previous open heart
surgery may jeopardize thresholds but
obliteration of the pericardial sac appears to
be no detriment.
Room for Improvement
Modification of EEG gain and calibration
controls would facilitate ease of operation.
Difference in paper speed (ECG 25 mm/sec vs
EEG 30 mm/sec) may initially pose a problem
121
CLINICAL ELECTROENCEPHALOGRAPHYe 1979.VOL. 10.NO.3
r ' ' -v.
Figure 7. BIOPOTENTIAL ISOLATOR
When combined with the EEC cardiac mapping of the right atrium and ventricle becomes
feasible in the community hospital without additional financial outlay.
122
CLINICAL ELECTROENCEPHALOGRAPHY C1979.VOL.10.NO.3
Figure 8. MAPPING THE RIGHT ATRIUM
A. MANIPULATION OF THE ELECTRODE
The endocardial pacing electrode (Medtronic 6902) is affixed to a surgical forceps with a
plastic sleeve. This permits ready application of the tip to the atrial and ventricular apicardial
surface.
B. PROPER CONNECTION CRITICAL
Since the electrode is bipolar, only the pole from the tip with the opaque marker can be used.
The opposite pole reflects the ring. All mapping and threshold determinations use this
connection.
to the cardiologist or cardiovascular surgeon
unless he is familiar with this. More important,
though reference to direct atrial recordings is
rare (18-20, 35), endocardial atrial
electrograms are helpful (5-7). Response of
the P-wave to direct atrial stimulation is also of
interest (19,20). The increased use of atrial
pacing should result in a proliferation of the
literature in this area.
Technical Aids
A scale which facilitates ECG time
measurements at EEG paper speed would be
welcome. The determination of heart rate
from the R-R interval (Figure 9) is facilitated
with a conversion table (Table 1).
Conclusion
The heart is a pump. The objective of
cardiac pacing is to improve the cardiac
output. The most popular method of cardiac
pacing (RV) is not physiologic and results in
reduced stroke output when compared to that
of normal sinus or atrial paced rhythm.
Recognition of pacemaker induced low
output states and pacemaker aggravated
neurological symptoms has quickened
interest in physiologic atrial pacing.
The increased complexity of atrial pacing
has been discouraging in the past, but
technical advances in electrode and
pacemaker design are promising. Selection of
the optimal pacing site appears critical and
123
CLINICAL ELECTROENCEPHALOGHAPHY e 1979 VOL10.NO 3
rwwte&ff1*^^
J^£$p\f~VW-t~J^Tfrr****>W
Figure 9. THE R-R INTERVAL
The time between the R-waves must be expressed in milliseconds in order to calculate heart rate
in beats per minute from Table 1. N.B. The paper speed of the EEG (30 mm/sec) differs from that
of the ECG (25 mm/sec) and should be taken into consideration.
HR
1
2
3
4
5
6
7
8
9
HEART
TABLE 1
RATE and R-R INTERVALS (sec)
The R-R interval in milliseconds is converted directly to heart rate in beats per minute.
10
6.0000
5.4545
5.0000
4.6154
4.2857
4.0000
3.7500
3.5294
3.3333
3.1579
20
3.0000
2.8571
2.7273
2.6087
2.5000
2.4000
2.3077
2.2222
2.1428
2.0689
30
2.0000
.9355
.8750
.8182
.7647
.7140
.6666
1.6216
1.5789
1.5385
40
1.5000
1.4634
1.4286
1.3953
1.3626
1.3333
1.3043
1.2766
1.2500
1.2245
50
1.2000
1.1764
1.1538
1.1320
1.1111
1.0900
1.0714
1.0526
1.0345
1.0169
60
1.0000
0.9836
0.9677
0.9524
0.9375
0.9230
0.9091
0.8955
0.8824
0.8696
70
0.8571
0.8451
0.8333
0.8219
0.8108
0.8000
0.7894
0.7792
0.7692
0.7595
80
0.7500
0.7407
0.7317
0.7229
0.7143
0.7058
0.6977
0.6896
0.6818
0.6742
90
0.6666
0.6593
0.6522
0.6452
0.6383
0.6315
0.6250
0.6186
0.6122
0.6061
100
0.6000
0.5941
0.5882
0.5825
0.5769
0.5714
0.5660
0.5607
0.5555
0.5505
124
CLINICAL ELECTROENCEPHALOGRAPHY 1979.VOL 10.NO.J
TABLE 1 (continued)
HEART RATE and R-R INTERVALS (sec)
HR
1
2
3
4
5
6
7
8
9
110
0.5454
0.5405
0.5357
0.5310
0.5263
0.5217
0.5172
0.5128
0.5085
0.5042
120
0.5000
0.4959
0.4918
0.4878
0.4839
0.4800
0.4762
0.4724
0.4687
0.4651
130
0.4615
0.4580
0.4545
0.4511
0.4478
0.4444
0.4412
0.4379
0.4348
0.4317
140
0.4285
0.4255
0.4225
0.4196
0.4167
0.4137
0.4109
0.4082
0.4054
0.4027
150
0.4000
0.3974
0.3947
0.3916
0.3896
0.3871
0.3846
0.3822
0.3797
0.3773
160
0.3750
0.3727
0.3704
0.3681
0.3658
0.3636
0.3614
0.3593
0.3571
0.3550
170
0.3529
0.3509
0.3488
0.3468
0.3448
0.3428
0.3409
0.3390
0.3371
0.3352
180
0.3333
0.3315
0.3297
0.3279
0.3261
0.3243
0.3226
0.3209
0.3191
0.3175
requires the determination of the focal
electrical potential and stimulating threshold
before electrodes are positioned. With
minimal expense, the EEG can be adapted for
this type of cardiac mapping in the community
hospital.
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