CARDIAC UNIT: ELECTROCARDIOGRAM Flashcards
Explain the timing and morphology of: P-Wave
Timing: The P-wave represents atrial depolarization, which occurs when the electrical impulse generated in the sinoatrial node (SA node) spreads across the atria, causing them to contract.
Morphology: The P-wave is typically small and rounded.
Explain the timing and morphology of: PR interval
Timing: represents the time it takes for the electrical impulse to travel from the SA node through the atria, pause at the atrioventricular node (AV node), and then travel down the bundle of His and its branches to the ventricles.
Morphology: The PR interval is usually a flat line (isoelectric) with a duration of approximately 120-200 ms
Explain the timing and morphology of: QRS complex
Timing: represents ventricular depolarization, which occurs as the electrical impulse rapidly spreads through the ventricles, causing them to contract.
Morphology: The QRS complex is typically a larger, more complex waveform consisting of a Q-wave, an R-wave, and an S-wave. The exact appearance and duration of the QRS complex can vary. <120 ms
Explain the timing and morphology of: ST segment
Timing: represents the early phase ofventricular repolarization
Morphology: The ST segment is typically a flat, isoelectric line.
Explain the timing and morphology of: T-wave
Timing: represents ventricular repolarization, which is the recovery of the ventricles as they prepare for the next contraction.
Morphology: The T-wave is usually a positive waveform, small and rounded, slightly larger than P-wave waveform
Explain the timing and morphology of: QT interval
Timing: represents the total time it takes for ventricular depolarization and repolarization.
Morphology: The QT interval is measured from the beginning of the QRS complex to the end of the T-wave. Its duration varies with heart rate, and it can be prolonged in certain cardiac conditions. <360-440 ms
What would cause a long PR interval?
Block at the atrioventricular node
What would cause a short PQ interval?
Pre-excitation as seen in Wolff-Parkinson-White (WPW) syndrome
Define: electrocardiogram
Electrocardiogram: measures the movement of positive charge, i.e. propagating depolarization
Describe 3 ECG conventions:
- upwards deflection
- downward deflection
- recording electrode
Upwards deflection: movement of positive charge towards the electrode
Downward deflection: movement of positive charge away from the electrode
Recording electrode: represented by +
Repolarization is represented by a positive upwards deflection (T wave) on an ECG. Why?
The endocardium gets excited before the epicardium but the epicardium action potential is shorter than endocardium.
Epicardium repolarizes first means that as we start to repolarize, the epicardium drops down to -90 mV although endocardium is still at 20mV
This creates a voltage gradient in the direction of the recording electrode which records it as a positive deflection.
What are the advantages of ECG?
- simple and cheap
provides information about:
- anatomical orientation of the heart (its axis)
- chamber sizes (hypertrophy)
- arrhythmias and conduction blocks
- myocardial ischemia (degree and location)
- myocardial infarction (heart attack)
- congenital defects
- changes in cardiac function with time and therapy
List all of the electrodes on a 12 lead ECG
10 electrodes total .
- 3 bipolar limb leads
- Unipolar leads: 3 augmented voltage limb leads; 6 precordial (“chest”) leads
- Reference electrode
Discuss the position of the 3 bipolar limb leads
Lead 1: negative electrode goes on right arm, positive electrode on left arm
Lead 2: negative electrode on right arm, positive electrode on left leg
Lead 3: negative electrode on left arm, positive electrode on left leg
Define: Eithoven’s triangle and Einthoven’s Law
Einthoven’s triangle is an imaginary formation of three limb leads in a triangle used in electrocardiography, formed by the two shoulders and the pubis
Einthoven’s Law: 1+3 = 2
Einthoven’s Law explains that Lead II’s complex is equal to the sum of the corresponding complexes in Leads I and III and is given as II = I + III;
Discuss the 3 augmented voltage unipolar limb leads
Unlike the bipolar limb leads (Lead I, Lead II, and Lead III), which record the voltage difference between two limb electrodes, unipolar limb leads measure the voltage at one limb electrode while referencing an “augmented” or “virtual” central point.
Not actually leads!
aVR: Right arm
aVL: Left arm
aVF: Left foot
Discuss the precordial (“chest”) leads
Gives a transverse view of the heart
V1-V2: Septal
- V1: Placed in the fourth intercostal space just to the right of the sternum.
- V2: Placed in the fourth intercostal space just to the left of the sternum.
- V3-V4: anterior
- V3: Placed midway between V2 and V4.
- V4: Placed in the fifth intercostal space in the midclavicular line (around the level of the nipple).
- V5-V6: lateral
- V5: Placed horizontally in line with V4, at the anterior axillary line.
- V6: Placed horizontally in line with V5, at the midaxillary line.
What does each ECG lead show?
Each lead shows a scalar (1-D) projection in that plane of the 3-D vector
Define: mean electrical axis
average direction of electrical depolarization in the heart during the QRS complex of the cardiac cycle
Describe the quadrant method to determine the mean electrical axis. List the pros and cons
Look at lead 1 and aVF - if upright, individual probably has a normal axis.
Normal axis: 0-90 degrees
Pro: easy and quick to determine if someone has a normal axis
Cons: not very accurate
Describe the geometric method to determine the mean electrical axis. List the pros and cons
- Measure magnitude of QRS
- Mark on circle of axis, +2 units at 60 degrees (lead 2) and 1 unit at 30 degrees (negative direction on aVR)
- Draw two perpendiculars
- Connect center of the circle with intersection of two perpendiculars
- Estimate axis of yellow arrow (about 95 degrees)
Pros: more accurate
Cons: need a protractor or ruler or computer
Describe the isoelectric method to determine the mean electrical axis. List the pros and cons
- Find the limb leads
- Find the lead with the most biphasic QRS complex
- Identify the lead at 90 degrees to this
- Determine if the QRS is an upward or downward deflection
(see example from notes)
Describe a right axis deviation and what can cause it
Right axis deviation: more electrical activity on the right side of the heart; can be determined if aVF is upright deflection and lead 1 is downward deflection
- If you are tall and thin your heart gets less squished by organs. Heart tends to hang more right.
- Common in infants too
- If you have hypertrophy of your right ventricle this can also cause a right axis deviation
Describe a left axis deviation and what can cause it
Left axis deviation: more electrical activity on the left hand upward quadrant; lead 1 is upwards deflecting and aVF is downward deflecting
- Obesity or pregnancy: heart is being squashed by abdominal organs and pushed upward to the left
- Left ventricular hypertrophy
Describe an extreme axis and what can cause it
Mean electrical axis is in the upper right quadrant; lead 1 and aVF have downward deflection
- Dextrocardia: heart is positioned on the right side of the chest instead of its normal position on the left side
- Cardiac pacemaker is placed on the bottom ventricle making the signal go int he other direction
