Lecture 23: ECG 1 and 2 Flashcards
What do recording electrodes detect?
A small portion of the electrical activity that has reached the body surface
ECG
The graphic record of the electrical activity that reaches the surface of the body as a result of cardiac depolarization and repolarization
Where can electrodes be placed to record the electrical activity of the heart
Anywhere on the surface of the body, however waves will look differently on left vs right
Why do waves look different on left versus right?
- There is a positive deflection on the ECG when a wave of depolarization moves towards a positive electrode
- A negative deflection when a wave of depolarization moves away from a positive electrode
What waves do electrodes record?
Depolarization and repolarization
What are used to monitor electrical forces in the heart?
ECG leads (sets of reference electrodes)
What depend on how the electrical forces align to the leads of the ECG recording
direction, magnitude, and deflections
Lead
When an ECG machine is connected between recording electrodes at two points on the body, the specific arrangement of each pair of connections
What is the right leg electrode used for?
An electrical ground, NOT for measurement
The standard ECG consists of how many leads?
12
Each lead views the heart at a
Unique Angle (Angle of Orientation)
What are the locations of the 12 leads?
6 are limb, 6 are precordial (chest)
In vet med, what leads are used most frequently
The three limb standard leads
How is an electrode designated as positive or negative?
Automatically by the ECG machine
The angle of each lead can be determined by
Drawing a line from the negative to the positive electrode
Bipolar limb leads
One limb electrode is the (+) pole and another is the (-) pole
Augmented (unipolar) limb leads
- Single electrode is chosen as the (+) pole
- There is no single (-). Rather, all the other electrodes are averaged together to create a composite (-) reference
- When the active electrode is the RA, LA, or LF, the lead is designated aVR, aVL, or aVF respectively.
What does aV stand for?
Augmented voltage
Axial Reference System
Angles of orientation of limb leads
Each ECG electrode records only the ____ _____ ____ at any given moment
average current flow
Although tiny swirls of current may be going off in every direction, each lead only records
the instantaneous average of these forces
The average movement of forces is represented by
a single arrow or vector
Describe ECG paper
A long, continuous roll of graph paper, with light and dark lines running vertically and horizontally
What do light lines of ecg circumscribe
1mm x 1mm
What do dark lines of ecg circumscribe
5mm x 5mm
Horizontal axis of ecg paper measures
time (duration)
The width of each square represents
0.04 seconds
The vertical lines measure
voltage and amplitude
Voltage calibration
A 1mV electrical signal will produce a deflection measuring 10 mm
Waves that appear on an ECG reflect
the electrical activity of myocardial cells which comprise the vast bulk of the heart
What generally aren’t seen by the ECG
pacemaker activity and transmission by the conducting system
Three main characteristics of waves
- Duration
- Amplitude
- Configuration
Duration
measured in fractions of a second
Amplitude
measured in mV
Configuration
The shape and appearance of a wave
Three things happen to a wave on the ECG when a chamber hypertrophies (enlarges):
- The chamber may take longer to depolarize, therefore the ecg wave may therefore increase in duration
- The chamber may generate more current and thus a larger voltage, therefore the ecg wave may increase in amplitude
- A larger percentage of the total electrical current may move through the expanded chamber, therefore the main electrical vector of the ECG wave may shift
The first step in determining the rhythm of the heart is to determine the
heart rate
First vector represents
septal depolarization
Each successive vector represents
progressive depolarization of the ventricles
The vectors swing progressively leftward because
the electrical activity of the much larger ventricle increasingly dominates the ECG
Mean vector
The average vector of all instantaneous vectors
Mean electrical axis
- The direction of the mean vector
- The average of all the instantaneous vectors that are generated as the ventricles depolarize
Each species has a normal range for its mean electrical axis. Therefore values outside this range suggest
a myocardial abnormality
