Chp. 11: Electrocardiography Flashcards
Equivalent Dipole Theory
The heart can be approximated to a dipole; the force that is generated has a direction and magnitude and can be represented by a vector
When is the ECG wave of maximum amplitude?
When the cardiac depolarization vector is parallel to the lead
Einthoven’s Triangle
Electrodes are placed on the right forelimb, left forelimb, and left hindlimb. The three potential differences created by this arrangement are leads I, II, and III.
Lead I
Measures potential difference between right and left forelimb with left considered positive
Lead II
Measures potential difference between right forelimb and left hindlimb with hindlimb considered positive
Lead III
Measures potential difference between left forelimb and left hindlimb with hindlimb considered positive
How are unipolar leads created?
By comparing the three electrodes to a reference voltage created by the monitor (“Wilson’s central terminal”).
aVL (left forelimb), aVR (right forelimb), aVF (left hindlimb)
What plane do the precordial leads provide information on?
The transverse, rather than frontal, plane
Why is a multilead ECG of less value in horses?
Due to extensive branching of the Purkinje system in the ventricles
Base-apex lead
Lead I setting
RA over jugular furrow or manubrium
LA over left apex of heart at olecranon
LL on either side of neck
Wave vs. Segment vs. Interval
“Wave” represents an electrical event in the myocardium and is a deflection above or below baseline
“Segment” is a line joining two waves
“Interval” is the time between two events
P wave
Atrial depolarization
PQ (or PR) interval
Time for wave of depolarization to propagate from atria through the ventricular conduction system
QRS complex
Three waves representing ventricular depolarization
T wave
Ventricular repolarization
ST segment
Segment between end of QRS (J point) and T wave, corresponding to ventricular systole
J wave and U wave
Early and late ventricular repolarization, respectively
J wave in normal heart or with hypothermia, hypercalcemia, or myocardial ischemia
U wave with bradycardia or hypokalemia
50 vs. 25 mm/s paper speed
50mm/s is a faster paper speed where each small box (1mm) equals 0.02s
25mm/s is a slower paper speed where each small box (1mm) represents 0.04s
ECG heart rate calculation
Simplest: Count number of R waves in 3s and multiply by 20
More accurate: Count R-R interval, then multiply by 0.02 or 0.04 depending on paper speed, then divide by 60 for bpm
Systematic approach to ECG evaluation
1) Determine HR
2) Determine if ventricular and atrial rates match (P-P interval and R-R interval match)
3) P for every QRS
4) QRS for every P
5) Are QRS complexes supra ventricular or ventricular
Mean Electrical Axis
Direction and orientation of ventricular depolarization; refers solely to the QRS complex.
Normally to the LEFT
Lead I MEA assessment
If the net deflection of QRS in lead I is positive, MEA lies to the left. If net deflection is negative, MEA lies to the right.
In what species is the QRS complex deflection largely negative in Lead II?
Horses, pigs, ruminants
Respiratory Sinus Arrhythmia
Normal in dogs.
Rate accelerates during inspiration and decelerates during expiration.
Wandering Pacemaker
Changes in site of impulse origin within sinus node and/or changes in exit pathway from sinus node
Arrhythmias in normal, resting horses
Sinus arrhythmia, sinus block, sinus arrest, sinus bradycardia, wandering pacemaker, first and second-degree AV block
Bandpass filter
Filter on ECG that allows signal frequencies within a specific range to be displayed while removing electrical activity above and below this range
What inhalant sensitizes the heart to certain arrhythmias?
Halothane
