Electrocardiography Flashcards
What is the purpose of an electrocardiogram?
Recording potential changes of volttage that are generated by the heart during the cardiac cycle and transmitted to the surface of the body
Normal waveform in electrocardiogram
How many leads are used in recording an ECG?
12
What are the ECG leads?
Limb leads:
Bipolar limb leads (use two active electrodes): Lead I, Lead II, Lead III
Unipolar augmented limb leads (Use one electrode): aVR, aVL, aVF
Precordial or Chest Leads:
V1, V2, V3, V4, V5, V6
What does each lead measure the potential difference of?
Lead I: Record potential difference between Right arm (Negative) and Left arm (Positive)
Lead II: Record potential difference between Right arm (negative) Left leg (positive)
Lead III: Record potential difference between Left arm (Negative) and Left leg (positive)
What does Einthoven’s law state?
Voltage in lead II = Voltage in lead I + Voltage in lead III
Einthoven’s triangle (Two arms and the left leg form apices of a triangle around the heart) leads to the law’s occurence
Voltage from each limb lead
Location of the first chest lead (V1)
Location of the second chest lead (V2)
Location of the fourth chest lead (V4)
Location of the third chest lead (V3)
Location of the fifth chest lead (V5)
Location of the sixth chest lead (V6)
ECG from each chest lead
Which lead is used in analysing ECG results?
Lead II
Produces the ECG waveform in the diagram
Significance of the P wave
Caused by Atrial Depolarization
Duration and voltage of the P wave
Duration: 0.12 sec
Voltage: 0.1 and 0.3 millivolts
Which wave shows atrial repolarization?
Atrial Repolarization is not recorded as it is masked by QRS complex
Significance of the QRS complex
Caused by Ventricular Depolarization
Q wave is due to depolarization of interventricular septum
R wave is due to depolarization of the apex and ventricular wall
S wave is due to depolarization of posteriobasal part of the left ventricle and the pulmonary conus
Duration and voltage of the QRS complex
Duration: 0.08 – 0.1 sec.
Voltage: 1.0 to 1.5 mv (Lead II)
Significance of the T wave
Caused by Ventricular Repolarization
Significance of the U wave
Small +ve wave
Caused by Repolarization of the papillary muscles
Generally is not visible on the ECG
Intervals and segments on the ECG
P-R interval
Q-T interval
S-T segment
Extent of the P-R interval
Beginning of P wave and beginning of QRS complex
Duration of the P-R interval
It is around 0.16 sec (<0.2sec/1 large square)
What does the P-R interval represent?
Interval between the beginning of electrical excitation of the atria and the beginning of excitation of the ventricle
In what conditions may the P-R interval be prolonged?
1st degree heart block
Extent of the Q-T interval
Beginning of Q wave to the end of the T wave
Duration of the Q-T interval
0.35-0.4 sec (<2 large squares)
What does the Q-T interval represent?
Ventricular depolarization and repolarization
In what conditions may the Q-T segment be prolonged?
Increases the incidence of ventricular arrhythmias and sudden death
Extent of the S-T segment
End of the S wave and the beginning of T wave
Characteristics of the S-T segment
Normally isoelectric
Axes of an ECG
What does one large square represent?
What does one small square represent?
One large square contains 25 small squares
Formulae for calculating rate on an ECG
ECG for tachycardia
ECG for bradycardia
How can regular and irregular rhythms be distinguished?
Regular: R-R interval is equal
Irregular: R-R interval is not equal
Define the axis of the heart/cardiac axis
Cardiac axis is the general direction in which the heart depolarizes
What is the normal axis of the heart?
-30 to +90˚
Normal heart axis depolarization
Heart axis deviations on ECG
Causes of right axis deviation
Right ventricular hypertrophy
Right bundle branch block
Infant and thin people
Right ventricular load (pulmonary embolism/COPD)
Causes of left axis deviation
Left ventricular hypertrophy
Left bundle branch block
Obesity
Mechanical shift (ascites, pregnancy and abdominal tumors)
