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CARDIOVASCULAR INSTRUMENTATION
Techniques and Significance of Threshold Measurement for
Cardiac pacing*
Relationship t o Output Circuit of Cardiac Pacemakers
S . Serge Barold, M .B., F.C.C.P.,
'* and lames A. Winner, B.S.E.E. 1.
Because it determines the Yeserven of the pacemaker's
output circuit, threshold measurement remains the single most important factor that will ultimately ensure
successful pacing. In this review, we describe the various
techniques and significance of threshold measnrement 'in
relation to the property of pacemaker output circuits.
There are two basic types of pacing circuits: (1) those
with coastant current, as in most external (temporary)
pulse generators, and (2) those with constant voltage,
as in many implantable pulse generators. Current-limited
pulse generators have features of both constant-current
and constant-voltage circuitry. The current threshold is
helpful in determining the integrity of the electrode-tissue interface and reflectsthe density of the current, which
is the prime factor responsible for successful stirnulation. Voltage thresholds are useful for information on
lead position and integrity, especially when voltage and
current are measured simultaneously. Impedance (calculated from voltage and current during stimulation)
can be helpful in the diagnosis of lead fractures, insulation breaks, and position problems. Threshold and impedance are entirely unrelated factors, each providing
specific and different information about a pacing 'system. Threshold may also be measured in t e m s of charge
and energy and in relation to the width of the pulse (at
a constant impulse amplitude). The concept of safety
margins is important when measuring long-term threshold at the time of replacement of a pulse generator.
We have analyzed this problem and have attempted to
make acceptable recommendations in the absence of
clear information in the literature about this subject.
roper evaluation of a pacemaker's function rep
quires some knowledge of the basic design
characteristics of the electronic circuits that deliver
The threshold for cardiac pacing may be defined
as the smallest amount of electrical energy that produces consistent capture outside the refractory
period of the heart3 This threshold may be measured by gradually decreasing the output of the
generator to the point where capture no longer
occurs consistently or by increasing the output from
a low value to the point where consistent capture
occurs. The small difference in threshold obtained
by these two techniques is almost always clinically
unimportant. The threshold may vary according to
the site of stimulation, the type of electrode and its
age, the duration of the stimulus, the polarity, and
the wave shape. It may also be influenced by administration of drugs, changes in electrolyte levels,
the state of the myocardium, and the type of measuring equipment u t i l i ~ e d . ~
the pacemaker's impulse. Unfortunately, confusing
terminology has made an already dBcult technical
Because threshold measubject even more -cult.
surement actually determines "reserve" of the output circuit, it remains the single most important
measurement that will ultimately ensure successful pacing, and it is essential for the determination
of optimal electrode position, lead integrity, and the
design and use of low-output pulse generators to
increase longevity.'"
In this review, we dezcribe the various techniques and significance of threshold measurement in
relation to the properties of pacemaker output circuits. This information is essential for trouble-shooting problems &th pacemakers.
'From the Division of Cardiology, Department of Medicine,
the Genesee Hospital and the University of Rochester
School of Medicine and Dentistry, Rochester, NY.
"Chief of Cardiology, the Genesee Hospital, and Associate
Professor of Medicine.
+Engineer, Medtronic, Inc.
Rkptint requests: Dr. Barold, 224 A l e d r Street, Rochester,
New York 14607
Lead impedance in this discussion refers to impedance when the electrode stimulates the heart,
in contrast to source impedance, which is related to
the'sensing property of the electrode. The two are
not the' same, and the complex subject of source
760 BAROLD, WINNER
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CHEST, 70: 6, DECEMBER, 1976
impedance is beyond the scope of this discussion.
The in uiuo impedance (resistance) imposed upon a pulse generator consists of the sum of ( 1) the
resistance of the pacing lead (equivalent to the in
uitro value), and ( 2 ) the impedance of the body
tissue between the electrodes, which is nonlinear
and rises during the pulse. The impedance of the
body tissue can be divided into the following two
parts: ( 1 ) the pure tissue resistance and ( 2 ) the
nonlinear polarization effect, caused by oppositely
charged particles that accumulate at the electrodeheart interface during the pacemaker pulse. This
alignment of charges acts like a capacitor. The effect is essentially zero during the extremely rapid
initial part of the pacemaker's stimulus and increases to the trailing edge, after which most of the
accumulated charges dissipate by diffusion, usually
within 200 msec. Polarization increases as the surface area of the electrode decreases. Polarization
also depends on the mode of stimulation (unipolar
or bipolar), the maturity of the lead, the tissue
chemistry, the material of the electrode, and the
amplitude and duration of the output-pulse
~idth.~-~
VOLTAGE @ 10mA
MEDTRONIC
5880A
LEADING EDGE ONLY
RESISTANCE
The output of a temporary constant-current pulse
generator is independent of the resistance (impedance) to the limits of the power supply, which
is generally about 15 volts ( Fig 1) . Consequently, if
the resistance of the lead increases, the voltage automatically increases (within the range of power
supply) to keep the current constant according to
Ohm's law (V = IR, where V is voltage, I is current, and R is resistance or impedance).
The smaller the required current, the greater the
range of resistances over which current remains
truly constant. Figure 1 shows that when the dial
for current output is set at 1 ma, one commercially
available external pulse generator ( Medtronic
588014) is almost a perfect source of constant current over a wide range of resistances. Because of
design, difficulties in maintaining constant current
may arise when the load exceeds 800 to 1,000 ohms
(Fig 1 ) . Thus, the curves for current (Fig 1) at
various dial settings remain flat in the anticipated
clinical range (300 to 1,000 ohms) but begin to
bend down (ie, amplitude decreases) as the limits
of the power supply are approached. The absolute
limits of the power supply are approximately 15 v
for this particular unit (Medtronic 5880A).
Constant-current circuitry would thus appear to
be advantageous during external ( temporary ) pacing because of its unique compensating feature allowing constant current in the face of a possible
CHEST, 70: 6, DECEMBER, 1976
/
VOLTAGE
/
-
FIGURE1. Current and voltage output curves (leading edge
only) of constant-current pulse generator ( Medtronic 5880A )
at various resistive loads. Corresponding current and voltage
curves are shown for current settings of 1, 2, 5 and 10 ma
( K = 1,000 ohms). Resistance is shown in log scale. In this
and subsequent figures, measurements were made on bench,
and standard technique of voltage drop across a 10-ohm
resistor was used to measure current on oscilloscope. Pulse
retains its square wave form under almost all circumstances.
increase in the resistance of the lead; however, this
is done at the cost of an increased drain on current
and a consequent reduced longevity of the power
sources. Nevertheless, longevity is of little practical
importance during temporary pacing, as power
sources may be readily replaced.
With constant-voltage circuitry, the output of
voltage will remain constant (within limits), regardless of the load imposed on the pulse generator.
Only special pacemakers, used mostly for research,
have true constant-voltage output pulses; however,
most implantable pulse generators are termed constant voltagea because the leading edge of the voltage pulse remains constant, regardless of the load
(Fig 2 ) . Historically, the outputs of pulse generators have been given in terms of current (milliamps), and it is commonly stated that the pulse
generator has an output of 10 ma. This 10 ma is
specified with an arbitrary load of 500 ohms only.
The cardiac load and the resistance of the lead
THRESHOLD MEASUREMENT FOR CARDIAC PACING 761
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MEDTRONlC
XYTRON SOH)
EDGE
VOLTAGE
TRAILING EDGE
won
CURRENT
(I(
IU
101
RESISTANCE --)
FIGURE2. Voltage and current output curves of a constantvoltage implantable pulse generator ( Medtronic Xytron 5950)
at various resistive loads. Both leading and trailing edges are
shown. Note that only leading edge of voltage remains constant with all resistances over 200 ohms, but current falls as
resistance increases. Resistance is shown in log scale.
usually differ from this arbitrary value, and the current output will, therefore, adjust itself according
to Ohm's law and decrease in order to maintain
constant voltage as the resistance increases.
When applied to the leading edge, the concept of
constant-voltage output should not be confused
with changes in voltage that occur during the pacemaker pulse after the constant leading edge. The
voltage of the pulse wave form droops from leading
to trailing edge as a function of the resistance and
the size of the pulse generator's output capacitor
(Fig 2). Any constant-voltage course may cease to
remain constant when the resistance is very low be
cause the battery then becomes unable to supply all
of the current required to maintain a steady voltage
(Fig 2), but such low resistances are not seen
clinically.
The threshold has historically been evaluated
with temporary pulse generators with constantcurrent characteristics, yet many implanted pulse
generators are of the constant-voltage type. At the
time of replacement of a pulse generator, the
threshold measured with an external constant-current device could be 1 ma, and a decision could,
therefore, be made to simply implant a new pulse
generator based only on this information that seemingly indicates a good threshold. If the impedance
is 7,000 ohms because of a partial fracture, the fact
that the compensation feature has increased the
voltage output of the constant-current external
pulse generator to 7.0 v would remain unknown
because the only visible control is the current setting ( V/I = R; 7 v/ 1 ma = 7,000 ohms). This 7-v
stimulating voltage is higher than that available
from conventional implantable (constant-voltage)
pulse generators. Therefore, the implantable pulse
generator would not capture the heart when connected to the lead. This point becomes clearer by
considering yet another example. Assume a current
threshold of 3 ma and a lead impedance of 5,000
ohms. A current setting of 5 ma on one constantcurrent pulse generator (Medtronic 5880A) will,
therefore, deliver the necessary3 ma to capture the
heart, so that pacing will occur at a seemingly acceptable long-term threshold of 5 ma according to
the settings on the pulse generator (Fig 1 ) ; however, pacing will not occur when a constant-voltage
(5-v) generator is connected to the lead, as this
generator can only deliver 1 ma, which is below the
required current threshold at this particular lead
impedance ( I = V/R = 515,000 = 1 ma). These
two examples illustrate that with respect to the
delivery of current, the constant-voltage pulse generator simply cannot compensate for a higher Iead
impedance, whereas adequate compensation may
occur with a constant-current device.O
The true incidence of this type of problem is unknown, but these situations may be easily avoided
by testing thresholds with a constant-voltage device
that matches or closely matches the output characteristics of the implantable unit.O The constantvoltage threshold should be measured routinely
whenever a pulse generator is replaced or a problem with pacing is suspected. Interestingly, studies
by Fontaine and associates4*10have found that voltage thresholds are more consistent and reproducible
than constant-current values for the routine measurement of threshold.
This design aims at increasing the life of an implanted pacemaker by controlling the maximal current flow in the electrode circuit. "Current-limited"
circuitry has features of both constant-current and
constant-voltage circuitry. The "current-limited"
source appears like constant current with low re-
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CHEST, 70: 6, DECEMBER, 1976
Energy Threshold
CORDIS
OMNl STANICOR
HIGH CURRENT SETTING
The energy threshold is usually expressed in
microjoules and gives the total picture of energy
requirement for depolarization; it is an indirectmeasurement calculated from the product of voltage, current, and pulse duration at thre~hold.~l-'~
Due to the capacitive'effects of the electrode-heart
interface, the voltage will lag behind the current in
time. Theoretically,-this'phasedifference enters into
the energy calculation, but it is neglected in practice because the difference is less than 2 percent.12
Because energy = VIT = 12RT (where T is pulse
width), a fourfold increase in curkent measured in
milliamperes would be equivalent to a 1Bfold increase expressed in microjoules.
Strength-DurationCurves (Including Threshold Related to Pulse Width)
lmn
11.
~ O U
RESISTANCE -+
FIGURE
3. Voltage and current output curves for current-lirnited pulse generator (Cordis OmniStanicor at high current
setting). Both leading and trailing edges are shown. Resistance is shown in log scale.
sistances, in that the voltage increases as the resistance increases until the voltage supply limit is
reached (Fig 3). When the limit is reached, the
pulses are more char~cteristicof constant-voltage
circuitry since the voltage of the leading edge remains constant (Fig 3). The shape of the impulse
resembles that of a constant-voltage pacemaker only
when the resistance is high (Fig 3).
OTHER WAYS
OF MEASURING
THRESHOLDS
Charge Thresholds
Charge thresholds are seldom calculated, but this
information could be helpful in the design of lowoutput pulse generators with greater longevity.
Charge threshold is usually expressed in microcoulombs and is the product of the current threshold in milliamperes and the duration of the pulse
in milliseconds. To calculate longevity, the pulse
generator's output charge (and not the charge
threshold) must be compared to the pulse generator's battery charge (cell capacity is rated in amphours; the standard mercury-zinc cell wallory
RMl] has a capacity of 1 amp-hours).
CHEST, 70: 6, DECEMBER, 1976
When the circuitry of the external device measuring threshold matches that of the implanted
pulse generator, a useful strength-duration curve
may be easily plotted. The voltage or current is
plotted on the vertical axis and pulse duration on
the horizontal axis12 (Fig 4). The output of the
pulse generator may then be superimposed on these
curves, both at fuU voltage
and after depletion
of
one cell (eg, mercury-zinc) to assess safety margins.
Noninvasive programmability of the duration of
the pulse is a relatively new approach to conserve
energy and has so far been used with constantvoltage pulse generators.14J5 The shortest duration
of an impulse producing consistent capture (at a
constant impulse amplitude) represents the threshold of stimulation as a function of pulse duration,
with the voltage held constant at the output of the
pulse generat~r.~'.'~
Proper adjustment of pulse
duration allows long-term pacing with conservation
of energy and charge.l5JeThe ability to increase the
output energy of an implanted unit in the face of a
large, but temporary, change in threshold (eg,
anoxia) is an important benefit of noninvasive programming of the width of the pulse. Programmability of the width of the pulse may also be useful in regaining capture and temporarily extending
the useful life of an implanted pulse generator with
a failing battery if pulse duration is increased. Figure 4 illustrates the value of increasing pulse duration with cell depletion. Some implantable pulse
generators automatically adjust the width of their
pulse when battery depletion occurs. This feature
is called "energy compensation." The increase in
the width of the pulse compensates for the drop in
voltage that occurs with the depletion of one cell,
thereby maintaining the energy delivered and the
original safety margins.
THRESHOLD MEASUREMENT FOR CARDIAC PACING 763
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STRENGTH DURATION CURVE
With a constant voltage threshold device incorporating adjustable
pulse width, this strength duration curve can be easily created.
1I
I
ln~ttal
output
One Cell
Safety Margln
pulse width
....
:\
wtth decreasIng output
voltage
I
Typtcal chrontc
threshold curve
0
I
I
I
I
I
05
1.0
1.5
2.0
2.5
Pulse Duration
-
(milliseconds)
FIGURE
4. Typical strengthduration curve for long-term lead.
This illustrates importance of pulse width and principle of
automatic energy compensation that is feature of some implantable pulse generators ( Medtronic ) . Pulse width increases with battery depletion. Initial output refers to output
voltage of implantable pulse generator (5.4 v ) , and "one cell
depleted refers to output voltage with depletion of one mercury-zinc cell ( 3.6 v ) . Dashed cume shows long-term threshold. Assume current at 500 ohms. Initial energy delivered by
generator = VIT = 5.4 x 10.8 x 0.5 =29 microjoules at 500
ohms. Energy delivered with one cell depleted = VIT = 3.6
X 7.2 X 1.5 = 39 microjoules at 500 ohms. Voltage threshold at initial pulse width = 2.75 v. Voltage threshold with
one cell depletion and increased pulse width = 1.25 v. Initial
safety margin = Vo/Vt2 = 3.86/1. Safety margin with one
cell depleted with increased pulse width = (3.6/1.25)* =
8.29/1. Safety margin with one cell depleted, without energy
compensation, by means of increased pulse width = (3.W
2.75)* = 1.71/1.
Safety Margins
The concept of safety margins is important when
measuring long-term threshold at the time of replacement of a pulse generat~r.'~.""~
The threshold
in any given patient can vary physiologically during
the day13J7J8and can be significantly altered by a
number of pharmacologic agents or metabolic abnormalitie~.'~.~~
Much of the original work in this
area was done by measuring changes in threshold
expressed in microjoules. This stems largely from
the work of Preston et al,13who measured threshold
as a percentage of available pacemaker energy.
They also measured physiologic and pharmacologic
changes ( + or -) in threshold as the maximal
percentage of change from the baseline energy
threshold. Their approach and the important clinical
implications become clearer with a specific example.
Let us assume a patient with an average threshold
of 5 microjoules. A 50-percent decrease in energy
threshold gives a value of 2.5 microjoules (2.515 X
100 = 50 percent). A 50-percent increase in energy threshold gives a value of 7.5 microjoules (2.515
X 100 = 50 percent ) . Ordinary physiologic circumstances may increase or decrease the threshold expressed in microjoules, by as much as 50 per~ e n t . ' ~ . ' ~If. ' the
~ patient's threshold is measured at
the lowest point (2.5 microjoules ) in its physiologic
swing, an adequate safety margin must provide at
least 7.5 microjoules to allow for a 50 percent increase
in energy requirement above the "baseline" value of
5 microjoules. Safety margins have been traditionally defined as the ratio of output energy of the
pulse generator (Eo) to the energy at threshold
( E , ) . In a constant-voltage pulse generator, the
output voltage remains constant according to the
number and type of cell (5.4 v with four mercuryzinc cells and 6.7 v with five such cells). This allows simple calculation of safety margins by comparing voltage output ( Vo ) to threshold voltage
( V, ), provided measurements are made at the same
pulse width ( T ) :
Safety margin = Eo = VOIOT
= VOIO
= VOVO
= VOVO
= Vf =
E. V.1.T V.1.
V.V. v:
A
vtvt
-
[?I=
R
According to the previously mentioned considerations in our hypothetical patient, we would recommend (T. A. Preston, personal communication,
1976) an energy safety margin ( & / E t ) of 311,
(7.5/2.5), rather than a 211 ratio.'' A 311 energy
safety margin may be translated into a voltage ratio
of about 1.75 X Vo/V,.
The output voltage of most constant-voltage pulse
generators is about 5 v or slightly higher. A constant-voltage threshold of 3 v is probably the highest acceptable chronic threshold. The range of 3 to
3.5 v is a gray zone and may be acceptable if the
patient is not "pacemaker dependent." Values over
3.5 v are generally unacceptable (G. Fontaine, personal communication, 1976). Firm recommendation about the lower acceptable value for long-term
threshold cannot be made in the absence of clearer
information in the literature about this problem. In
a borderline situation a decision must be made to
replace the system, connect a high-output generator,
or, at worst, consider the safety margin acceptable.
These decisions must be individualized according
to the need of the patient and the pacing svstem.
No chances should be taken in patients who might
be subject to large fluctuations in long-term thresholds because of potentially serious metabolic dis-
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CHEST, 70: 6, DECEMBER, 1976
orders or other conditions liable to cause s i g d c a n t
changes in threshold, especially if the patient is
"pacemaker dependent."
C m ~ c fIMPLICATIONS
i
The current threshold is helpful in determining
the integrity of the electrode-tissue interface and
reflects the density of current, which is a prime
factor responsible for successful stimulation. While
current thresholds yield good information on lead
position, they are often not helpful in determining
lead integrity. Voltage thresholds are useful for information on lead position and lead integrity, especially when voltage and current are measured simultaneously. Energy thresholds may be calculated
when mathematically accurate safety margins are
required.
Impedance is calculated when voltage and current are measured during stimulation of the heart.
Voltage and current wave forms may be displayed
on an oscilloscope, averaged, and then used in
Ohm's law to determine impedance. Average impedance (measured from the ratio of average voltage to average current during the pulse) tends to
fall slightly in the first ten days after initial imof the lead and then stabilizes.~.~o the
absence of lead fracture, infection, or excessive
myocardial scarring,2' long-term values for impedance tend to remain constant and do not differ
significantly from the original ~ a l u e s . ~ .Calcula'~,~~
tion of impedance is obviously more accurate when
both voltage and current are measured simultaneously. Because polarization is a function of pulse
amplitude, the aoerage impedance may vary slightly
(but not significantly for clinical purposes) if
calculated from measurements obtained at threshold, compared with those obtained at the full output of the implantable pulse generator. We recommend that voltage and current be obtained at the
same voltage level as the output of the implantable
pulse generator and be measured near the leading
edge of the pulse, since more information can be
learned about the lead and heart tissue resistance
before the polarization effect begins to influence the
reading significantly. Currently, it is simpler, more
practical, and rob ably more accurate to use devices that measure voltage and current instantaneously and simultaneously very close to the leading
edge and to display one or both values digitally
(some commercially available threshold analyzers
measure voltage and current at an arbitrary time of
90 microseconds into the pulse). Instantaneous display of current when taking the constant voltage
threshold may be the only way to make the diagnosis of an intermittent (and rapidly) changing
lead impedance due to incomplete fracture.
It cannot be overemphasized that threshold and
are
unrelated
each providing
and different
about a pacing system. Determination of impedance
can be helpful in the diagnosis of lead fractures, inTable 1
sulation breaks, and position
Presents some guidelines.
CONCLUSIONS
Proper measurement of stimulation thresholds is
important to ensure good function and safety of a
pacemaker. The techniques for threshold measurement should be standardized to allow meaningful
comparison of published data.4 The external device
Table 1-4uideliner for implantation of Permanent Pacemukerr
Constant
Current
Threshold
Constant
Voltage
Threshold
Impedance*
Increased
Increased
Normal
Displacement ;
high threshold
Reposition or use
high output generator
Increased
Decreased
Decressed
Fracture in lead
insulation
Repair or replace
lead
Normal
(low) or
increased
Increased
Increased
Lead fracture
Repair or replace
lead (consider
unipolarization of
bipolar system)
Decreased
Decreased
Normal
Norma1 situation;
if there h malfunction
of implanted system,
consider faulty pacemaker
lead connection or faulty
implanted pulse generator
Correct connection
or replace pulse
generator
Cause
Treatment
*In general, impedance ranges from 300 to 1,000 ohms. Higher values may occasionally be seen in normally functioning systems,
particularly with ball-tip electrodes. Acceptable ranges of impedance should be obtained from individual manufacturers.
CHEST, 70: 6, DECEMBER, 1976
THRESHOLD MEASUREMENT FOR CARDIAC PACING 765
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for measuring thresholds should preferably use the
same type of output circuitry as the implantable
pulse generator. When voltage and current measurements are known, lead impedance can be calculated. This is essential for the differential diagnosis of obvious or impending lead fracture,
insulation breaks, and lead dislodgment. The ordinary constant-current external pulse generator has
important limitations, and the additional use of a
constant-voltage device for determining threshold is
essential to insure integrity and optimal performance of the pacing system.
ACKNOWLEDGMENT: We are grateful to Larry Shearon,
B.S.E.E. (Medtronic, Inc.) for his contribution and enthusiastic support.
R
E
F
E
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m
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2 Smyth NPD, Keshishian JM, Baker NR, et al: Physiological rationale for the clinical use of low output pacemakers. Med Ann DC 43:257,1974
3 Furman S, Escher DJW: Principles and Techniques of
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4 Fontaine G, Kevorkian M, Bonuet M, et al: Defbition de
la mesure du seuil d'entrainement electrique: A propos
de I'etude par ardinateur de 2400 mesures ( seuil pl.Bcoce
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CHEST, 70: 6, DECEMBER, 1976