The electrocardiogram (ECG) is a record of the average electrical potential generated in the heart muscle at the body's surface.
The electrocardiogram (ECG) is a record of the average electrical potential generated in the heart muscle at the body's surface. The electrocardiograph amplifies and filters these small electrical signals and graphs this signal in voltage and time. These electrical signals are created by intracellular and extracellular ionic gradients that move across semipermeable membranes and result in cellular transmembrane action potentials. The action potentials occur in myocardial and automatic tissues and vary based on inherent characteristics of the tissue.
A) Purpose of running an electrocardiogram (ECG)
a. To detect arrhythmias and conduction disturbances
b. To detect heart enlargement (used as an adjunct)
c. To evaluate cardiac therapy
d. To evaluate prognosis
e. To evaluate the progression of the disease
f. To monitor patients during anesthesia
g. To indicate certain metabolic disturbances
i. Electrolyte imbalances
ii. Acid-base disorders
B) Limitations of an ECG
a. Does not tell functional capacity of the heart
b. Records only pathology of the heart muscle and conduction system
c. Not as sensitive for heart enlargement as for arrhythmias
PR interval – From the start of the P wave to the start of the QRS complex.
Time of conduction from the SA node to the ventricles. The AV node represents 75% of this interval.
QRS complex – Ventricular depolarization:
Q wave- 1st negative deflection ~ interventricular septal activity
R wave – 1st positive deflection following the P wave. Predominant waveform in left facing leads (I, II, aVF. aVL, CV6LL, CV6LU).
S wave –1st negative deflection following a positive deflection. With right heart enlargement a larger than normal S wave may be seen.
T wave -- Ventricular repolarization. Can be altered with electrolyte abnormalities such as hyperkalemia.
QT interval—Time of total ventricular activity Altered with electrolyte abnormalities.
ST segment – Area from the end of the QRS to the beginning of the T wave.
Assess for elevation or depression that might indicate ischemia
A) Note the Basics:
a. Paper speed- usually 50mm/sec or 25 mm/sec (See Figure 11)
i. Standard sensitivity = 10mm/mV
ii. Half sensitivity = 5mm/mV
iii. Double sensitivity = 20mm/mV
c. Which leads are provided?
B) Calculate the Heart Rate: (Figure 13)
a. Instantaneous Heart Rate Calculation:
i. Count the number of millimeters between two consecutive R waves and divide this number into 3000 (@ 50mm/s) or 1500 (@25mm/s)
ii. Convenient method because it is fast and can be used to calculate the rate of arrhythmias of short duration.
iii. Inaccurate with irregular rhythms
b. Standard Heart Rate Calculation:
i. Count number of R waves in a given period of time and multiply by and appropriate integer to equal 60 seconds. (i.e. 3 or 6 seconds times 20 or 10 respectively)
ii. Helpful with gradual rate changes over time
iii. Inaccurate for very short lasting arrhythmias
C) Determine the Overall Rhythm
a. Regular – R-R interval does not vary much (less than 10 %)
b. Irregular- Variation in the R-R interval
i. Regular- Pattern to the variation of the R-R interval
1. Sinus arrhythmia-
a. Variation in heart rate with respiration (increases with inspiration and decreases with expiration). Normal in dogs
2. Irregular - Absolutely no pattern to the R-R variation
a. i.e. atrial fibrillation
We will come back to rhythms we will discuss some of the more common rhythms that can be seen on the ECG following the interpretation section.
D) Identify Specific Waveforms:
a. Where is the P wave? Where is the QRS? Where is the T wave?
i.There must be a T wave after every QRS complex thus this helps identify the QRS complex.
ii. P waves generally look the same except with atrial premature contractions and respiratory sinus arrhythmia
iii.P waves and T waves may overlap in very atrial tachycardias
b. Is there a P for every QRS and a QRS for every P wave? Are they related? Are they related at the same PR interval and is this interval reasonable.
c. Are there any complexes that come early or late? Are they supraventricular (normal QRS morphology) or ventricular (typically wide and bizarre) in origin?
Normal hierarchy of cardiac pacemakers:
Ectopic beats originate from somewhere other than the sinus node. They can occur late (escape beats) or early (premature)
Originates from cells in the conduction system because these cells have automaticity
An escape rhythm is never a diagnosis by itself- it is always secondary to something else such as sinus arrest or third degree AV block
Junctional – originates above the ventricle. QRS complexes typically narrow (normal appearing)
Ventricular – Originates in the Purkinje fibers. QRS complexes are typically wide and bizarre
Abnormal depolarization that happens BEFORE the next sinus beat was due
Origin can be distinguished by QRS morphology as with escape rhythms.
Supraventricular origin and ventricular origin
The remainder of this talk will go through some specific rhythms that may be commonly seen when monitoring patients with ECG and how you come to the final diagnosis.
References available upon request.