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
The recorded ECG waveforms and intervals include:
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
Approach to the ECG:
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: