ECG/EKG (electrocardiogram)
A recording of the electrical activity of the heart
Dipole
a positive and negative charge separated by a small distance (think
of terminals of a battery). Note that a dipole can generate local current flow and a small electrical field.
individual dipoles close together
in space and time can
summate!
The magnitude of the resulting field may be large enough to be measured.
Volume Conductor
Normally can't record from the site of the dipole.
To record electrical activity at a distance from the source (dipole), the source must be in a conducting medium. The body is a volume conductor, since it is largely made up of a highly conductive material
Vectors
quantities that have both a magnitude and a direction
Electrodes and Leads
An electrode is a sensor; a lead is a combination of two electrodes.
Active (exploring) electrode
an electrode which senses the electrical field.
Passive (indifferent) electrode
a reference electrode (not sensing the field), considered to be at 0 mV.
Unipolar Lead
combination of an active plus a passive electrode.
-Measure the voltage only at the active electrode
Bipolar Lead
combination of two active electrodes.
Measure the voltage difference between the two electrodes (i.e.,
subtract the voltage at the negative electrode from the voltage at the
positive electrode).
When measuring biopolar leads voltage
ALWAYS SUBTRACT NEGATIVE FROM POSITIVE
What is Needed to Record an EKG?
1. A recording system equipped with amplifiers and electrode junction box.
2. Surface electrodes with low and similar impedance (resistance).
3. Recording paper (grid).
4. Subject (patient).
Paper of EKG
Grid consisting of 5 x 5 mm darker squares, subdivided into 1 x 1 mm
lighter squares.
SPEED= 25mm/Sec
Dark lines on grid
0.2 sec (X AXIS)
light lines on grid
0.04 sec (X AXIS)
Voltage gain
(Y AXIS)
voltage gain- 1mV/cm.
Voltage polarity
up= +
down= -
baseline at 0mV is the ISOELECTRIC LINE
standardization of paper
speed, amplitude, placement of electrodes, etc. is essential!
Standard Limb Leads
Known as Einthovens triangle
Consists of one positive(+) electrode, one negative(-) electrode, and one "ground' lead that prevents electrical interference.
Includes leads I-III
Why place electrodes on wrists and ankle?
dont have much fat as well as not having as much hair.
it is also an area of soft skin (vs dead skin that dont have conduction abilities)
Standard Limb Lead Electrode Arrangements
a. Lead I: RA (-) and LA (+)
b. Lead II: RA (-) and LL (+)
c. Lead III: LA (-) and LL (+)
Augmented Limb Leads
another term for unipolar limb leads, reflecting the fact that the ground lead is disconnected, which increases the amplitude of deflection on the ECG tracing
Note that augmented limb leads do
not have a true negative electrode, since they are unipolar.
A
Precordial Leads
Six EKG leads placed on the anterior chest to record electrical activity of the heart, mainly the electrical impulses originating in the ventricles or the heart's anterior wall
perpendicular to the plane of the limb leads. Therefore, these leads view the
Summary of 12-lead System
various leads monitor different axes/planes. The limb leads (standard and augmented) monitor activity the frontal (vertical) plane; the precordial leads monitor activity in the transverse (horizontal) plane.
-allows for a 3D picture for analysis
You can measure either the ________ rate or ________ rate through an EKG trace
atrial rate or ventricular rate but in healthy individuals they should be the same
calculating BMP from EKG trace
1) To determine the ventricular rate, determine (approximately) how many QRS complexes are in
a six second interval. Remember, the first QRS complex is time zero!
(COUNT PEAKS)
2) count the remaining boxes that are before the first QRS and last QRS comple
What considerable information does an EKG give us?
Gives us:
1. Pattern and Frequency of Events:
a. Rate (atrial and ventricular)
b. Rhythm (regularity or irregularity of patterns).
2. Conduction Time:
a. Intervals/segments between waves indicate conduction time between
areas.
b. Width of waveforms indica
Why is an EKG a good choice?
Non-invasive!
What an EKG trace cant tell you
1. It does not tell you much about mechanical activity (contraction, relaxation),
except during ventricular fibrillation.
2. It does not make a diagnosis! It is dangerous to make a diagnosis solely on
the basis of an EKG trace (e.g., "fibrillation" in an