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

114

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

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vV vvV Vv.WVj'v

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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.

117

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

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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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