NURS 208 – ELECTROCARDIOGRAM ECG/EKG
October 30, 2020 – Paul Lee
Chapter 20
Electrocardiogram (ECG/EKG)
Graphic recording/display of the biopotentials generated by the myocardium during the cardiac cycle
- Reflects the rhythmic electrical events of depolarization and repolarization wave (action
potential), followed by mechanical events of contraction and relaxation of the atria and
ventricles
- Displays electrical activity of the heart
- Is a vector (physics) both magnitude and direction
Depolarization
- Upward deflection when depolarization wave (+ve charges) moves towards the +ve electrode
Repolarization
- Downward deflection when repolarization wave (-ve charges) moves towards the +ve electrode
Types of Waves
P wave: depolarization of atria
QRS complex: depolarization of ventricles (L and R together)
T wave: repolarization of ventricles
U wave: unknown, possible repolarization of papillary muscles (small)
Standard position (placement) of the ECG recording electrodes:
Normally, ECG contains 6 limb leads (I, II, III, aVR, aVL, and aVF) and 6 chest leads (V1 to V6)
- Lead aVR, aVL, aVF, and V1 to V6 are also referring to as unipolar ECG
- Leads I. II, & III are also know as bipolar limb leads
- Ground electrode (RL) is always connected to the right leg
Bipolar limb leads:
1. Lead I
- -ve lead (RA) at right arm,
- +ve lead (LA) at left arm
2. Lead II
- -ve lead (LA) at left arm
- +ve lead (LL) at left leg
3. Lead 3 III
, - -ve lead (LA) at left arm
- +ve lead (LL) at left leg
These connections are arbitrarily chosen such that the QRS complexes will be upright in all 3 limb leads
in most normal individuals
Unipolar limb leads: (augmented limb leads)
aVR
- Right arm (RA) as +ve, all other leads (LA & LL) serve as –ve electrode
aVL
- Left arm (LA) as +ve, all other leads (RA & LL) serve as –ve electrode
aVF
- Left leg (LL) as +ve, all other leads (RA & LA) serve as –ve electrode
Unipolar chest leads: (precordia leads)
V1
- In the 4th intercostal space (between ribs 4 and 5) just to the right of the sternum
V2
- In the 4th intercostal space (between ribs 4 and 5) just to the left of the sternum
V3
- Between leads V2 and V4
(SLIDE 36)
Normal ECG values:
Paper speed = 25 mm / sec (1mm=0.04 sec)
Amplitude = 1 mm / 0.1 mV
P wave: Height < 2.5 mm in lead II Width < 0.11 sec in lead II
,** PR: interval = Between 0.12 to 0.20 sec
- Short PR interval indicates abnormal impulse conduction from atrium to ventricles without
normal delay (normally by the AV node)
- Long PR interval, consider heart block (disruption of conduction)
QRS complex: = Should be less than 0.12 sec
- Q = down
- R = up
- S = down
- Wide QRS, consider bundle branch block, ventricular rhythm, etc.
- Tall QRS, consider ventricular hypertrophy
QT interval: = Between 0.3 and 0.44 sec
Need to calculate the corrected QT interval (QTc) because it is closely related to HR
- QTc is calculated by dividing the QT interval by the square root of the preceding R-R interval
QTc = QT / √(RR)
Long QTc interval (long QT syndrome) could indicate:
Heritable abnormality in the cardiac ions channels
- One example is the abnormality in the cardiac Na+ channel leads to persistent I(Na) during the
action potential (AP) plateau
- Results in prolongation of the AP duration, reflected as an increased QT-interval on the ECG
Long QT syndrome could be acquired:
- Usually induced by drug therapy such as antiarrhythmic medication
- Other causes include electrolyte disturbances, myocardial ischemia, and use of drugs such as
cocaine
ST segment: SLIDE 43
As the isoelectric line
- Elevation or depression could indicate myocardial ischemia
T wave:
Usually, upright position
- Inverted T wave could indicate myocardial ischemia, intraventricular conduction delay, or
even an anxiety attack
SAMPLE QUESTIONS – 45-47
, Cardiac cycle
Sequence of electrical and mechanical events in one complete heartbeat
SLIDE 49
Cardiac cycle (sequence of electrical vs mechanical )
1. Start of atrial depolarization (P wave)
- Atria are still in full relaxation
2. Atria depolarization complete
- Atrial contraction in progress
3. Start of ventricular depolarization (QRS complex)
- Ventricles are still in full relaxation
4. Ventricular in depolarization complete
- Ventricular contraction is in progress
5. Start of ventricular repolarization (T wave)
- Ventricular contraction is still in progress
6. No electrical or mechanical activity, completion of cardiac cycle
***ELECTRICAL COMES BEFORE MECHANICAL CONTRACTION
Pressure block Cardiac cycle (only consider left atrium & ventricle) SLIDE 50
Red dotted line (top) = aortic pressure
Black solid line = left ventricular pressure
Blue dotted line (bottom) = left atrial pressure
2 main events for a complete cardiac cycle
A) Ventricular systole (contraction)
1. Isovolumic ventricular contraction
2. Ventricular ejection
i) Rapid ejection phase
ii) Reduced ejection phase
B) Ventricular diastole (relaxation)
1. Isovolumic ventricular relaxation
October 30, 2020 – Paul Lee
Chapter 20
Electrocardiogram (ECG/EKG)
Graphic recording/display of the biopotentials generated by the myocardium during the cardiac cycle
- Reflects the rhythmic electrical events of depolarization and repolarization wave (action
potential), followed by mechanical events of contraction and relaxation of the atria and
ventricles
- Displays electrical activity of the heart
- Is a vector (physics) both magnitude and direction
Depolarization
- Upward deflection when depolarization wave (+ve charges) moves towards the +ve electrode
Repolarization
- Downward deflection when repolarization wave (-ve charges) moves towards the +ve electrode
Types of Waves
P wave: depolarization of atria
QRS complex: depolarization of ventricles (L and R together)
T wave: repolarization of ventricles
U wave: unknown, possible repolarization of papillary muscles (small)
Standard position (placement) of the ECG recording electrodes:
Normally, ECG contains 6 limb leads (I, II, III, aVR, aVL, and aVF) and 6 chest leads (V1 to V6)
- Lead aVR, aVL, aVF, and V1 to V6 are also referring to as unipolar ECG
- Leads I. II, & III are also know as bipolar limb leads
- Ground electrode (RL) is always connected to the right leg
Bipolar limb leads:
1. Lead I
- -ve lead (RA) at right arm,
- +ve lead (LA) at left arm
2. Lead II
- -ve lead (LA) at left arm
- +ve lead (LL) at left leg
3. Lead 3 III
, - -ve lead (LA) at left arm
- +ve lead (LL) at left leg
These connections are arbitrarily chosen such that the QRS complexes will be upright in all 3 limb leads
in most normal individuals
Unipolar limb leads: (augmented limb leads)
aVR
- Right arm (RA) as +ve, all other leads (LA & LL) serve as –ve electrode
aVL
- Left arm (LA) as +ve, all other leads (RA & LL) serve as –ve electrode
aVF
- Left leg (LL) as +ve, all other leads (RA & LA) serve as –ve electrode
Unipolar chest leads: (precordia leads)
V1
- In the 4th intercostal space (between ribs 4 and 5) just to the right of the sternum
V2
- In the 4th intercostal space (between ribs 4 and 5) just to the left of the sternum
V3
- Between leads V2 and V4
(SLIDE 36)
Normal ECG values:
Paper speed = 25 mm / sec (1mm=0.04 sec)
Amplitude = 1 mm / 0.1 mV
P wave: Height < 2.5 mm in lead II Width < 0.11 sec in lead II
,** PR: interval = Between 0.12 to 0.20 sec
- Short PR interval indicates abnormal impulse conduction from atrium to ventricles without
normal delay (normally by the AV node)
- Long PR interval, consider heart block (disruption of conduction)
QRS complex: = Should be less than 0.12 sec
- Q = down
- R = up
- S = down
- Wide QRS, consider bundle branch block, ventricular rhythm, etc.
- Tall QRS, consider ventricular hypertrophy
QT interval: = Between 0.3 and 0.44 sec
Need to calculate the corrected QT interval (QTc) because it is closely related to HR
- QTc is calculated by dividing the QT interval by the square root of the preceding R-R interval
QTc = QT / √(RR)
Long QTc interval (long QT syndrome) could indicate:
Heritable abnormality in the cardiac ions channels
- One example is the abnormality in the cardiac Na+ channel leads to persistent I(Na) during the
action potential (AP) plateau
- Results in prolongation of the AP duration, reflected as an increased QT-interval on the ECG
Long QT syndrome could be acquired:
- Usually induced by drug therapy such as antiarrhythmic medication
- Other causes include electrolyte disturbances, myocardial ischemia, and use of drugs such as
cocaine
ST segment: SLIDE 43
As the isoelectric line
- Elevation or depression could indicate myocardial ischemia
T wave:
Usually, upright position
- Inverted T wave could indicate myocardial ischemia, intraventricular conduction delay, or
even an anxiety attack
SAMPLE QUESTIONS – 45-47
, Cardiac cycle
Sequence of electrical and mechanical events in one complete heartbeat
SLIDE 49
Cardiac cycle (sequence of electrical vs mechanical )
1. Start of atrial depolarization (P wave)
- Atria are still in full relaxation
2. Atria depolarization complete
- Atrial contraction in progress
3. Start of ventricular depolarization (QRS complex)
- Ventricles are still in full relaxation
4. Ventricular in depolarization complete
- Ventricular contraction is in progress
5. Start of ventricular repolarization (T wave)
- Ventricular contraction is still in progress
6. No electrical or mechanical activity, completion of cardiac cycle
***ELECTRICAL COMES BEFORE MECHANICAL CONTRACTION
Pressure block Cardiac cycle (only consider left atrium & ventricle) SLIDE 50
Red dotted line (top) = aortic pressure
Black solid line = left ventricular pressure
Blue dotted line (bottom) = left atrial pressure
2 main events for a complete cardiac cycle
A) Ventricular systole (contraction)
1. Isovolumic ventricular contraction
2. Ventricular ejection
i) Rapid ejection phase
ii) Reduced ejection phase
B) Ventricular diastole (relaxation)
1. Isovolumic ventricular relaxation
