Basic Concepts Of Electrocardiography Flashcards
Magnitude of the potential depends on
Strength of the source
Distance from the electrode to the dipole
Angle of a line form the electrode to the midpoint of the dipole therefore largest deflection when parallel
Lead avR
Right arm is positive
Lead avL
Left arm is positive
Lead avF
Left foot is positive
Electrocardiogram sensitivity and specificity for chamber enlargement
60% sensitivity
90% specificity
Limitations of the ECG
Low sensitivity for chamber enlargement
Intermittent nature of some arrhythmias
- the routine ECG only records about 0.1% of the heart beats in a 24 hour period
Position when recording ECG
Right lateral recumbency
Electrode attachment
Just below elbows or stifles
Standard sensitivity
1 mV = 10 mm
Double sensitivity
1 mV = 20 mm
Half sensitivity
1 mV = 5 mm
Which electrocardiograph paper speed is best for measurements
50 mm/sec
Which electrocardiogram paper speed is best for rhythm analysis (saves paper)
25 mm/sec
Artifacts
ECG abnormalities not due to cardiac disorders
Electrical interference
“60 cycle” due to A/C in electrical wires in ECG machine and room
Most common patent movement and how to correct it
Respirtory
Correct by holding mouth closed for 3 to 4 seconds or by placing leads down on leg farther from chest wall
Things to do first in ECG analysis
Evaluate for artifacts
Try to identify normal vs abnormal complexes
Identify the P QRS and T waves in all leads
Caliper
Four basic determinations from ECG
Heart rate
Rhythm analysis
Measurements for analysis of cardiac size
Mean electrical axis determination (MEA)
A 25 mm/sec 3 sec region is
75 mm
At 50 mm/sec 3 seconds is
150 mm
How to determine instantaneous heart rate
Method 1 - determine the time (in seconds)for a RR interval; divide this number 60 sec /min to get bpm
Method 2 -count the number of small boxes for a RR interval ; divide this number by 1500 if paper speed is 25 mm/sec and 3000 if paper speed is 50 mm/sec
Supraventricular rhythm
Narrow, upright in lead II +/- related P wave >90% like “normal”; rapid conduction system
Ventricular rhythm
Wide, bizarre, no related P wave
Premature beats
Supra ventricular premature complex
Ventricular premature complex
ST segment
Look for depression or elevation relative to baseline
QT interval
Varies inversely with heart rate
Important in people, less so in animals
T wave
Normal variability
Hyperkalemia
Mean electrical axis (MEA)
The average direction that a wavelength of dipoles is moving in the heart
Applied to the QRS complex
Goal is to determine MEA to within 30 degrees ( is the axis normal or shifted right or left)
Several methods
Methods to determine mean electrical axis
The major deflection method
The isoelectric lead method
Isoelectric lead method
Find the most isoelectric lead (net voltage closest to zero)
Find the lead that is perpendicular to this lead
Determine the net valve for this lead - this is the MEA
