Download Electrocardiography

Electrocardiography
(ECG)
An Overview of the Cardiovascular
System
The Heart
Valves of the Heart (1)
Valves of the Heart (2)
Cardiac Conduction System
Cardiac Conduction System
• The sinoatrial (SA) node, located in the wall of the right atrium.
• The atrioventricular (AV) node, located at the junction between the
atria and ventricles.
• Conducting cells interconnect the two nodes and distributethe
contractile stimulus throughout the myocardium. In the atria,
conducting cells are found in internodal pathways, which distribute
the contractile stimulus to atrial muscle cells as this electrical
impulse travels from the SA node to the AV node. (The importance
of these pathways in relaying the signal to the AV node remains in
dispute, because an impulse can also spread from contractile cell to
contractile cell, reaching the AV node at about the same time as an
impulse that travels an internodal pathway.) In the ventricles,
conducting cells include those in the AV bundle and the bundle
branches, as well as the Purkinje( ) fibers, which distribute the
stimulus to the ventricular myocardium.
Impulse Conduction
through the
Heart
Sinoatrial Node and Ventricular
Muscle Fibre Action Potentials
Sinoatrial (SA) node fibres possess
the ability to depolarize
spontaneously until a threshold
potential of about -40 mV is
reached, which generates a new
action potential . These pacemaker
action potentials spread to working
myocardial fibers, resulting in
working myocardium action
potentials. In the ventricle, a
working myocardium action
potential begins with a rapid
reversal of the myocardial cell
membrane potential, from a resting
potential of about -90 mV to the
initial peak of about +20 mV.
Ten Electrode Locations for a
Standard 12-lead Electrocardiogram
•
•
•
•
•
•
•
•
•
•
LA: left arm
RA: right arm
LL: left leg
RL: right leg
V1: Fourth intercostal (between the ribs)
space, right of sternum (breastbone)
V2: Fourth intercostal space, left of
sternum.
V4: Fifth intercostal space, in the same
vertical line as the clavicle (collarbone)
V3: Midway between V2 and V4
V6: Fifth intercostal space, in same
vertical line as armpit fold
V5: Between V4 and V6
Limb Leads
• Lead I = LA - RA
• Lead II = LL - RA
• Lead III = LL - LA
Lead III=Lead II - Lead I
RL is connected to ground.
Vw = (RA+LA+LL)/3
Vw: Wilson's central terminal is
the average of the three limb
leads. This approximates
common, or average, potential
over the body
Augmented Limb Leads
• Augmented voltage right (AVR)
AVR = RA - (LA+LL)/2 = 3(RA-Vw)/2
= -(I+II)/2
• Augmented voltage left (AVL)
AVL = LA - (RA+LL)/2 = 3(LA-Vw)/2
= I-II/2
•
Augmented voltage foot (AVF)
AVF = LL - (LA+RA)/2 = 3(LL-Vw)/2
= II-I/2
Chest Leads (Precordial Leads)
V1, V2, V3, V4, V5 and V6 are placed directly on the
chest. Wilson's central terminal is used for the
negative electrode, and these leads are
considered to be unipolar
Electrocardiograph
(ECG)
An Electrocardiogram
An Electrocardiogram
• The small P wave, which accompanies the depolarization of
the atria. The atria begin contracting about 25 msec after
the start of the P wave.
• The QRS complex, which appears as the ventricles
depolarize. This electrical signal is relatively strong, because
the ventricular muscle is much more massive than that of
the atria. It is also a complex signal, largely because of the
complex pathway that the spread of depolarization takes
through the ventricles. The ventricles begin contracting
shortly after the peak of the R wave.
• The smaller T wave, which indicates ventricular
repolarization. Atrial repolarization is not apparent,
because it takes place while the ventricles are depolarizing,
and the QRS complex masks the electrical events.
Cardiac Muscle Polarization and ECG
Standard ECG Grid
RR interval ~ 5 division
= 1000ms -> 60 bpm
Normal ECG Recordings from the 12
Surface Leads
Electrocardiograph System Diagram
Key Features
•
Input Dynamic Range
– display ± 5 mV to ± 10 mV at a signal varying rate within 320 mV/sec.
– direct current offset voltage ±300 mV.
•
Frequency Response
A general rule of thumb is to preserve 30 harmonics. For example, a heartbeat of 60 bpm
sampling rate is 60 Hz , a heartbeat of 100 bpm, sampling rate is 120 Hz
• System Noise
Not exceed 30 μV p-p over a 10-s period (The special circuit consists of a 51-kΩ resistor in
parallel with a 47-nF capacitor, in series with each patient-electrode connection (AAMI,
2007)).
•
Arrhythmia Detection
– The FDA recommends that ANSI/AAMI EC57:2003 Testing and Reporting Performance
Results of Cardiac Rhythm and ST Segment Measurement Algorithms (AAMI, 2003) be
followed.
•
Leads-Off Detection
– Less than 1 μA of current is transmitted to various pairs of patient electrodes in order to
determine whether each electrode is properly connected to the patient. If an open
circuit is detected, an alarm monitor sounds (AAMI, 2002).
Cardiac Arrhythmias
• A heart rate lower than 60 bpm is referred to as
sinus bradycardia.
• A heart rate higher than 100 bpm is referred to as
sinus tachycardia.
• Arrhythmias can be categorized into five classes:
1. Escape.
2. Irregular rhythms.
3. Premature beats.
4. Tachyarrhythmias.
5. Heart blocks.
Escape (1)
A potential pacemaker may replace a failing SA node. These
potential pacemakers in cardiac tissue are known as ectopic
foci. Escape arrhythmia refers to the ability of an ectopic focus
to “escape” overdrive suppression and pace at its inherent
rate, according to a physiologic hierarchy. The fastest available
pacemaker paces and suppresses slower pacemaking activities,
in what is known as overdrive suppression.
- If the SA node fail, an atrial focus begins to pace at an
inherent rate of 60-80 bpm (Atrial escape: normal P
morphology is lost.
- If atrial foci fail to pace, an AV junctional focus begins to pace
at an inherent rate of 40-60 bpm (Junctional escape a normal P
wave is not seen, may occasionally see a retrograde P wave).
The AV junction is the portion of the AV node possessing
ectopic foci.
- If AV junctional foci fail to pace, a ventricular focus paces at
an inherent rate of 20-40 bpm (Ventricular escape: no P wave,
wide, abnormal QRS). Ventricular foci are composed of
Purkinje cells within the Purkinje fibers.
Escape (2)
A: Sinus rhythm, followed by
atrial escape beats (printed
with Rivertek RSIM).
B: Sinus beat, followed by a
junctional escape beat and
then a junctional escape beat
with retrograde (invertyed) P
wave superimposed on the T
wave
C: Ventricular escape rhythm,
enormous ventricular
complexes, without P waves
Irregular Rhythms
A:Wandering pacemaker. occurs when
pacemaker activity “wanders” from the SA node
to nearby atrial ectopic foci. This results in rate
variation within the normal range and a shape
variation in the P wave.
B:Multifocal atrial tachycardia. multifocal atrial
tachycardia (MAT). Now the rate is greater than
100 bpm, and the P-wave shape varies greatly,
as three or more atrial foci pace.
C: Atrial fibrillation. When multiple atrial
ectopic foci continuously fire, complete
depolarization of the atria may be prevented,
with only occasional depolarization through the
AV node to stimulate the ventricles. Atrial
fibrillation (Afib) is present—chaotic atrial
spikes and irregular ventricular rhythm.
References
1.
2.
3.
4.
Fundamental of Anatomy and Physiology, Frederic H. Martini
Biomedical Instrumentation: Application and Design, John G.
Webster
Medical Device Technologies: A Systems Based Overview Using
Engineering Standards, Gail D. Baura
The Biomedical Engineering Handbook, Joseph D. Bronzino