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A R T I C L E R E P R I N T
Pacemaker: Narrow Pulses Generation for Design and Sensitivity TestA pacemaker is a small device that helps the heart to beat more regularly and properly. It does this with a small electric stimulation that controls the heartbeat. This paper outlines pulse generation and sensitivity test of a pacemaker. Designers require a generated signal to simulate the real pacemaker pulse signal to use for sensitivity testing of the pacemaker. Next generation of function generators of fer the best signal generators that can simulated a pulse signal as narrow as 1 µsec pulse width and arbitrary cardiac signal. The new digital or mixed signal oscilloscope, with its high resolutions acquisition, allows designers to analyze both low amplitude (0.5 mV and 1 mV) ECG signal and pulse signal stimulus simultaneously, to make real-time adjustments to their pacemaker designs.
Pacemaker: Narrow Pulses Generation for Design
and Sensitivity Test
Kah-Meng Chew
General Electronics Measurement Solutions
Keysight Technologies Sdn. Bhd.
Bayan Lepas, Malaysia
Doris Lau
General Electronics Measurement Solutions
Keysight Technologies Sdn. Bhd.
Bayan Lepas, Malaysia
Abstract— A pacemaker is a small device that helps the heart
to beat more regularly and properly. It does this with a small
electric stimulation that controls the heartbeat. This paper
outlines pulse generation and sensitivity test of a pacemaker.
Designers require a generated signal to simulate the real
pacemaker pulse signal to use for sensitivity testing of the
pacemaker. Next generation of function generators offer the best
signal generators that can simulated a pulse signal as narrow as
1usec pulse width and arbitrary cardiac signal. The new digital
or mixed signal oscilloscope, with its high resolutions acquisition,
allows designers to analyze both low amplitude (0.5 mV and 1
mV) ECG signal and pulse signal stimulus simultaneously
allowing designers to make real-time adjustments to their
pacemaker designs.
Keywords— pacemaker, pulse signal, pacing, ECG, narrow pulse,
pacemaker sensitivity, cardiac
Introduction
What is a pacemaker? According to the American Heart
Association, a pacemaker is a small device that helps the heart
beats more regularly and properly. It does this with a small
electric stimulation that helps control the heartbeat. An
engineer may refer to a pacemaker device as a “pulse
generator”. Pacemakers perform four major critical functions:
• Stimulate cardiac depolarization;
• Sense intrinsic cardiac function;
• Respond to increase metabolic demand by providing
rate responsive pacing; and
• Provide diagnostic information stored by the
pacemaker
In clinical use, three common types of pacemakers are [1]
1. Implantable,
• Long-term permanent use
2. External, and
• Miniaturized, transistorized, battery-
powered
3. Console
• Battery- or AC-powered defibrillators
In addition, other sub-options includes unipolar pacing,
bipolar pacing, asynchronous (fix rate), synchronous (on
demand), single or dual chamber, programmable and non-
programmable.
Pacing system
In the typical pacing system, the pacemaker provides the
voltage and the current (electrons) flows down the conductor
through the electrode, and towards the tip of the lead (positive
terminal, cathode). The tip of the lead touches the myocardium
(the muscular tissue of the heart) where the electrical
resistance is produced and stimulates the heart. Then the
current flows through the body tissue to the tip of the lead
(negative terminal, anode) and back to the pacemaker.
Electrical Testing of Pacemaker
Electrical testing parameters involve in testing a pacemaker,
include battery testing, pacing impedance and pulse
generation. Within pulse generation, there are pulse
generation, sensitivity test, timing test, mode switching and
rate adaptive sensor. The focus in this paper is on pulse
generation and sensitivity test of the pacemaker.
I. PULSE GENERATION
The pacemaker designer needs to allow for many adjustments,
including the signal’s amplitude and pulse width. These are
the most important settings used in the pacemaker pacing.
Pulse width is the time or duration of the pacing pulse and is
expressed in milliseconds. Pulse width must be wide enough
to allow for the delivery of each pacing pulse and must be
long enough for the depolarization to disperse into the
surrounding muscle tissues. Widening the pulse width
improves the ability of the pacemaker to capture the cardiac
signal and maintains synchronization. Narrowing the pulse
width improves battery life span and reduces side effects of
electrical signals; these side effects include twitching of the
pectoral muscles and interrupting the normal breathing pattern
by stimulating the diaphragm. The pacemaker produces a
narrow pulse signal approximately 71 beats per minute and a
pulse width around 0.5 msec. [2]
Amplitude is the amount of voltage delivered to the heart by
the pacemaker. Amplitude reflects the strength or height of the
pulse signal. The amplitude of the pulse must be large enough
to cause depolarization (i.e. to “capture” the heart), “capture”
is the nomenclature for effective stimulation of cardiac
depolarization by the pacemaker. On top of that, the amplitude
of the pulse must be sufficient to provide an appropriate
pacing safety margin. The pacemaker produces a pulse burst
amplitude approximately 5.0V and this corresponds to a
narrow “spike” in the ECG.
Figure 1 [3]: This is the output pulse of the pacemaker. The
voltage of the pacemaker is referring to the amplitude of the
leading edge. The droop is influenced by many factors
including the electrode’s lead impedance. The output pulse
corresponds to a narrow pulse or “spike” on the ECG signal.
A. Pulse Generations Solutions
A function generator is the best solution for a pacemaker
designer to simulate the narrow pulses. Most of the new
function generators have built-in pulse capability to generate
basic pulses. For older function generators, the designer can
use the built-in square wave capability to generate pulse signal
by varying the duty cycle between 20 and 80 percent. Some
designers also use the arbitrary waveform generator (AWG) to
generate a pulse because it allows them to top up with
additional customizations such as adding overshoot or
undershoot into the pulse signal.
Questions might arise on why the designer does not use a PC
sound card to generate a pulse signal. The disadvantages of
using a PC sound card include poor waveform quality,
distortion, noise, signal ringing and imprecise amplitude.
These disadvantages far outweigh the main advantages of
sound card’s low cost and availability.
B. Narrower Pulses
Pacemaker designers require narrow pulses for testing. This is
achieved by using the built-in BURST mode capability in the
function generator or arbitrary function generator. BURST
mode capability allows the user to configure the instrument to
output a waveform for a specific number of cycles. The user
can control the amount of time that elapses between bursts
with the internal timer or external triggering.
For example, the designer can output a very narrow pulse
signal of 1us pulse width by adjusting the frequency range
value and duty cycle of the signal, then switch on the BURST
mode feature to adjust the burst rate to achieve the narrow
pulse.
Figure 2: 1µsec pulse signal generate from function generator
II. SENSITIVY TEST FOR PACEMAKER
What is the sensitivity of a pacemaker? [4] This is defined as
the minimum myocardial voltage required to be detected as a
P wave or R wave, measured in mV. Why is this important? If
the pacemaker is overly sensitive, any random fluctuations of
electrical signal from cardiac activity could be mistaken by the
pacemaker and leads the pacemaker to keep pacing
continuously. This can lead to ”madness”. On the other hand,
the pacemaker would not fire at all because the cardiac
activity is considered within normal range. Hence proper
timing and sensitivity need to be calibrated appropriately to
avoid unnecessary complications later.
Figure 3: Typical setup for testing the sensitivity of the
pacemaker
Figure 3 shows the typical test setup for testing the sensitivity
of the pacemaker. In this test, the designer sets the pacemaker
to output a signal of 70 beats per minute. The function
generator produces the cardiac signal to simulate the heart
signal to the pacemaker. The designer tunes the amplitude of
the cardiac signal from the function generator (in mV level)
and the sensitivity of the pacemaker is measured using the
oscilloscope. This minimum sensitivity value is the sensitivity
threshold. Next-generation function generators does not need
an attenuator because they are able to generate an accurate
signal at low amplitude.
Figure 4: This is an oscilloscope screen capture showing both
the cardiac ECG signal and the pacemaker pulse signal.
Showing two signal at the same time allow the user to analyze
their design and make adjustments.
To automate a sensitivity test, the designer needs to write a
program to control the function generator to transmit the
signal burst to the pacemaker and measure the pacemaker
sensitivity level, output pulse width, and beats per minute;
these settings are customized for each patient.
III. SUMMARY
A. Solution for Narrow Pulse Signal Generation
Keysight’s 33622A function generator device is an example of
a state-of-the-art function generator to create pulses for many
applications and is a good solution for pacemaker pulse
simulation. The burst mode allows designers to generate low
duty-cycle pulses from a general-purpose function generator
rather than using a dedicated pulse generator. This device also
allows pacemaker designer to generate cardiac signal and
provides sequencing capability for seamlessly transitioning
through various cardiac signal conditions.
Keysight’s 33622A Waveform Generator (Figure 5) was used
to generate the narrow pulse signal in Figure 2 and the cardiac
signal plus narrow pulse in Figures 4.
Figure 5: Keysight’s 33622A Functional/Arbitrary Waveform
Generator
B. Solution for Sensitivity Test Analysis
Keysight’s InfiniiVision 4000 X-series Oscilloscope is an
example of a next generation oscilloscope in analyzing very
low amplitude signal (such as ECG signals) with the accuracy
needed for medical application. The high update rate allows
the designer to analyze the full spectrum of the signal in high
resolutions acquisition. The signal analysis is also economical
in that it does not require additional accessories to achieve
high vertical resolution for the ECG signal amplitude. In
additional to this, the oscilloscope allows the designer to view
the ECG cardiac signal and the pulse signal stimulus from the
pacemaker.
Keysight’s InfiniiVision MSO-X 4104A (Figure 6) was used
to make measurements in Figures 2 and 4.
Figure 6: Keysight’s InfiniiVision 4000 X-series Oscilloscope
REFERENCES
[1] Doris J. W. Escher, M.D., Types of Pacemakers and their Complications, 1973; 47: 1119-1131
[2] Generating Narrow Pulses with Function Generator, Agilent Technologies, 5898-8033EN, 2008; 1
[3] S. Serge Barold, Roland X. Stroobandt, Alfons F. Sinnaeve, Cardiac Pacemakers Step-by-Step: An Illustrated Guide, 2008; 23
[4] Sensitivity and output settings of the temporary pacemaker, http://www.derangedphysiology.com/main/core-topics-intensive-care/mechanical-haemodynamic-support/Chapter%202.5.3/sensitivity-and-output-settings-temporary-pacemaker
Sponsor by Keysight Technologies Inc.
This information is subject to change without notice. © Keysight Technologies, 2018, Published in USA, July 24, 2018, 5992-3044EN
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This article was originally published in © 2017 IEEE. Reprinted, with permission, from IEEE International Symposium on MeMeA (Medical Measurements and Applications).