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Electrocardiogram
(ECG)
PhillipN, (2007) Display device of a medical monitor as used in anesthesia [photograph]. Retrieved from
https://en.wikipedia.org/wiki/Monitoring_(medicine)#/media/File:Monitor_(medical).jpg
Summary
•
•
•
•
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Clinical Use
•
Specifications
•
History
Principles of Operation •
•
Block Diagram
Commercial Examples
Preventive
Maintenance
Common Problems
Test Procedures
Typical Heart Rate
Normal heart rate – 60 to 80 beats/min (5 liters/min)
During Exercise – 120 to 160 beats/min (15 to 25 liters/min)
Tachycardia - heart rate more than 100 beats/min (resting).
Bradycardia - heart rate less than 60 beats/min (resting).
Maximum Heart Rate = 220 – Age
Athletes - Max HR = 160 to 220
Circulation
Two separate pumps:
Right Side
moves deoxygenated blood to the
lungs for oxygenation
Left Side
moves oxygenated blood to the
body
Which leads to two distinct circulatory
systems:
Pulmonary
vessels to and from the lungs
Systemic
vessels to and from the rest of the
body
Vessels that move blood away from the
heart are called arteries and vessels that
return blood to the heart are called veins.
Blausen Medical Communications, Inc. (2013), Cardiovascular
System [image]. Retrieved
fromhttps://en.wikipedia.org/wiki/Talk:Circulatory_system#/medi
a/File:Blausen_0168_CardiovascularSystem.png
The Nernst Equation
EK= -61.5  log (Ki/Ko)
Membrane potential depends of ions concentration: Na+, K+, ClIonic Current
Animation:
http://outreach.mcb.harvard.edu/animations/actionpotential.swf
Cell Depolarization
Villetakanen (Own work), Nerve Cell Depolarization [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons
Cell Depolarization
• The cardiac pacemaker cell action
potential:
Silvia3 (2010), Pacemaker Action
Potential [image]. Retrieved
fromhttps://en.wikipedia.org/wiki/Cardiac
_action_potential#/media/File:Pacemaker
_potential_annotated.gif
Heart’s Anatomy
Wikipedia. “Heart.” Wikipedia, p. 1-12. Retrieved from:
https://en.wikipedia.org/wiki/Heart
Heart’s Anatomy
Blood Flow in Heart:
Four-chambered muscular vessel:
Two Atria (left and right)
Two Ventricles (left and right)
Atrium:
Filling chamber
Pushes blood into ventricle
Ventricle:
Pressurization chamber
Ejects blood into circulation
Wikipedia. “Heart.” Wikipedia, p. 1-12. Retrieved from:
https://en.wikipedia.org/wiki/Heart
Chambers separated by heart valves:
One-way flow valves
Four in total (tricuspid, pulmonary, mitral, aortic)
Circulatory Pressures
Yaddah (2006), View from the front [image]. Retrieved from
https://en.wikipedia.org/wiki/Circulatory_system#/media/File:Diagram_of_the_human_heart_(cropped).svg
Clinical Use
• Measures
– Rate and regularity of heartbeats
– Size and position of the chambers
– Presence of any damage to the heart
– Effects of drugs
Specifications
• Input: Voltage (biopotential)
• Output :
– Electronically (display)
– Paper
Principles of Operation
Einthoven’s triangle
Kychot (2009), Einthoven Triangle [image]. Rwtrieved from https://commons.wikimedia.org/wiki/File:ECG-Einthoven-triangle.svg
ECG Signal
ECGpedia.org, “Basics.” Wikipedia. Retrieved from: http://en.ecgpedia.org/wiki/Basics
ECG Signal
Openstax College. “The Cardiac Cycle, Fig. 40.14.” From the publication: Biology. Rice University: 2013, pgs. 1205
.
ECG Signal
• P – atrial
depolarization
• QRS complex –
ventricular
depolarization
• T – ventricular
repolarization
Anthony Atkelski (2007), Schematic diagram of a normal sinus rhythm [image].
Retrieved from
https://en.wikipedia.org/wiki/Electrocardiography#/media/File:SinusRhythmLabels.svg
Signal of Heart Diseases
Atrial Fibrillation
Michael Rosengarten BEng, MD.McGill [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
Ventricular Fibrillation
Jer5150 (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
Sensor
Electrodes
Disposable Ag/AgCl surface electrode
Malkin, Robert. Medical Instrumentation in the Developing World. Engineering World Health, 2006.
Electrodes
TYPES:
• Disposable: $.05 to $.11 each
• Reusable
• Plate electrode
Gel is important for
a good contact
with skin!
Diagnostic ECG
ECGpedia.org,.“ECG.Reference.Card.”.Wikipedia..Retrieved.from:.
http://www.ecgpedia.org/A4/ECGpedia_on_1_A4En.pdf.
Cooper, Justin and Alex Dahinten for EWH. “Electrocardiogram (ECG)
Preventative Maintenance.” From the publication: Medical Equipment
Troubleshooting Flowchart Handbook. Durham, NC: Engineering World Health,
2013.
Wave Forms
Npatchett (2015), Derivation of the limb leads [image]. Retrieved from https://en.wikipedia.org/wiki/Electrocardiography#/media/File:Limb_leads_of_EKG.png
Wave Forms
Malkin, Robert. Medical Instrumentation in the Developing World. Engineering World Health, 2006.
Block Diagram for an analog
ECG
Amplifier
VOUT = (V1 – V2)R2/R1
Amplifies a difference.
VOUT = AC(V1 + V2) + AD(V1 – V2)
AD:differential (signal) gain, AC:common mode (noise) gain.
The ratio AD/AC (Common Mode Rejection Ratio – CMRR) is
a very important parameter. Ideally CMRR →∞
Amplifier
ECG amplifier (instrumentation amplifier)
Input impedance >100 MΩ
Range: 0.05–150 Hz
Differential amplifiers with high gains (1000)
CMMRs: 80 – 120 dB
Differential Amplifier
Arthur Ogawa (2014), Differential Amplifier with non-ideal on amp [image]. Retrieved from
https://en.wikipedia.org/wiki/Differential_amplifier#/media/File:Op-Amp_Differential_Amplifier_input_impedence_and_common_bias.svg
Sampling
The heartbeat rate itself is
typically in the 60 to 200
beats per minute
(1 Hz to 3.3 Hz)
4 Hz
Range: 0.05–150 Hz
512 Hz sampling (2 ms)
10 samples at the QRS
complex 12 bit resolution in
analog-to-digital conversion
150 Hz
ECGpedia.org,.“ECG.Reference.Card.”.Wikipedia..Retrieved.fro
m:. http://www.ecgpedia.org/A4/ECGpedia_on_1_A4En.pdf.
Sources of Interference
AC interference:
Cooper, Justin and Alex Dahinten for EWH. “Electrocardiogram (ECG) Preventative Maintenance.” From the
publication: Medical Equipment Troubleshooting Flowchart Handbook. Durham, NC: Engineering World Health,
2013.
Muscular interference
Commercial Examples
PhillipN, (2007) Display device of a medical monitor as used
in anesthesia [photograph]. Retrieved from
https://en.wikipedia.org/wiki/Monitoring_(medicine)#/media/File:Monitor_
(medical).jpg
DiverDave (2010), Bispectral index monitor indicating a nearly
isoelectric pattern of electroencephalographic activity
[photograph]. Retrieved from
https://en.wikipedia.org/wiki/Bispectral_index#/media/File:BIS
_Monitor-Burst_Suppression.JPG
Commercial Examples
Pollo (2010), Biphasic defibrillator [photograph]. Retrieved from
https://en.wikipedia.org/wiki/Defibrillation#/media/File:Defibrillator_monitor_Lifepak_12.jpg
Defibrillators also have a wave II
ECG signal
Patient’s Safety
• Leak currents
–Sometimes ECG may be used in
direct contact with the heart
–Even a small current running
through the lead wires can
cause death (ventricular
fibrillation)
Defibrillators display ECG leads I, II & III
Preventive Maintenance
• Calibration
• Clean reusable electrodes with alcohol and cloth
• physical checks-lead wires, Welch cups (if used),
case damage, connectors, keypad/switches
• operation-paper speed, calibration pulse,
trace/printout quality, front panel buttons and lights,
battery life (if applicable)
• cleaning
• electrical patient safety
Common Problems
• Weak or poor signal:
–
–
–
–
–
–
User error (may be no manual)
User error in position of electrodes
Loose wire
Dry disposable electrode
AC inference
Interference from other machines
Common Problems
• No signal:
– No contact at Electrode (fallen off, no gel,
broken wire)
– Saturated Amp (gain too high)
– Problem with cable connection
– Damaged amplifier due to defibrillator
Common Problems
• User error
– Manual lack or disregard
– Complex user interface
– Older models may require to adjust the gain
for proper rate reading
– Stylus position if applicable
Common Problems
• User error
– Other electrode devices (ESU,
bioimpedance) may interfere
with each other
– Wrong brightness settings – no display trace
Common Problems
• User error
– Wrong electrodes positioning
Symptom: Signal is saturated or distorted by
power line noise
Rules, electrodes should not be placed :
1 - On scar tissue
2 - Over a lot of body hair
3 - Closer than 2 inches from each other may
Common Problems
• Lead wires and main cord damage
• Lead interference
Common Problems
• AC interference
– 50 Hz/60 Hz: Check for a filter switch
Common Problems
• AC interference
Line fluctuation
Procedure:
Test the machine in
another room
Connect the equipment
to a voltage regulator ligia diniz [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons
Common Problems
• Movement Artifact
–Check to see if the patient
is cold or nervous (muscle
tremors interfere)
–Patient or nurse touching
any metal or wall
Common Problems
• Lack of disposable electrodes and gel
– One can make cheap electrodes
with sewing snaps and tape
– Gel can be replaced by an ion free
solution (Ex: salt water) or hand
cream, shampoo, etc…
By Grkauls (Own work) [Public
domain], via Wikimedia
Commons
Common Problems
• Paper feed problems
• No trace in the printer
–Newer devices: clean the print
head with alcohol gently with a
fine soft clean cloth or Q-tip
–Older devices (use alcohol to
clean clotting in the ink tube)
Test Procedures
• Test on yourself
• Check the heart rate alarms:
– Lower than your own heart rate
(upper rate alarm)
– Higher than your own heart rate
(lower rate alarm).
• Check battery (if present) charging circuit
Test Procedures
• Electrode lead leakage. (connect 1 KΩ
resistor from electrode wire to ground,
measure voltage (< 50 mV) in resistor and
compute current (should be < 50 µA)
ECG Lead
< 50 mV
< 50 µA
1 KΩ
David R. (Own work) [CC BY-SA 3.0
(http://creativecommons.org/licenses/by-sa/3.0)
or GFDL (http://www.gnu.org/copyleft/fdl.html)],
via Wikimedia Commons
By Thiagoalmeidasa (Own work) [Public domain], via Wikimedia Commons
PhillipN, (2007) Display device of a medical monitor as used
in anesthesia [photograph]. Retrieved from
https://en.wikipedia.org/wiki/Monitoring_(medicine)#/media/Fil
e:Monitor_(medical).jpg
Troubleshooting
Cooper, Justin and Alex Dahinten for EWH. “Electrocardiogram (ECG) Troubleshooting Flowchart.” From the publication: Medical Equipment
Troubleshooting Flowchart Handbook. Durham, NC: Engineering World Health, 2013.