Electrocardiography Flashcards
What does an electrocardiogram measure?
Electrical signal produced by action potentials as they move through the heart muscle
ECG Traces
Upward and downward deflections that give information about cardiac function
** What is shown on an electrocardiogram when there is no dipole? **
Flat baseline
** What is shown on an electrocardiogram when there is a depolarization in a cell? **
Deflection above baseline
** What is shown on an electrocardiogram when cells are repolarizing? **
Deflection below baseline
What is represented by the peak of upward or downward deflection on an electrocardiogram?
The largest possible dipole moment where the same number of cells are carrying a negative charge as those carrying a positive charge
What speed do ECG traces typically run at?
25mm/s
How many seconds are represented by one small box on an electrocardiogram?
40 miliseconds / 0.04 seconds
How many seconds are represented by a large box on an electrocardiogram?
200 miliseconds / 0.2 seconds
How many mV are represented by a small box on an electrocardiogram?
0.1mV
How many different setups/leads are used in a clinical electrocardiogram?
12
Why is it important that electrocardiograms are measured in 3D?
Action potentials move through the heart in three dimensions, leads are recorded simultaneously and problems can be identified and their specific location identified
How many electrodes are placed on the body during a 12 lead electrocardiogram?
10 electrodes
Name the 12 leads in an electrocardiogram
Limb Leads:
- I
- II
- III
Augmented limb leads:
- aVF
- aVL
- aVR
Precordial (chest) leads:
- V1
- V2
- V3
- V4
- V5
- V6
Which leads are unipolar and which are bipolar?
Unipolar:
- aVF
- aVL
- aVR
- V1-V6
Bipolar:
lead I, II and III
** Give a general description of the placement of the electrodes in an electrocardiogram **
- 4 electrodes placed on each limb (RA, RL, LA, LL)
- 6 electrodes placed across chest (V1-V6)
** Explain the concept of vector addition to interpret ECGs **
Heart’s Electrical Activity: Represented as vectors (magnitude & direction).
Lead Projections: Each ECG lead records the component of the overall vector along its axis.
Net Vector: Summing individual vectors gives the net vector (overall electrical activity).
Clinical Insight: The net vector’s direction/magnitude helps determine the heart’s electrical axis and detect abnormalities.
** What electrodes do the augmented limb leads use? **
- aVR: positive electrode at RA with reference to the average of LA and LL
- aVL: positive electrode at LA with reference to the average of RA and LL
- aVF: positive electrode at LL with reference to the average of RA and LA
** What electrodes do leads I - III use? **
lead I - potential difference btwn LA and RA
lead II - potential difference btwn LL and RA
lead III - potential difference btwn LL and LA
** How do electrocardiograms collect information from 3 dimensions? **
Frontal Plane (Coronal View):
Limb Leads (I, II, III, aVR, aVL, aVF): record electrical activity from different angles around the heart.
Horizontal Plane (Transverse View):
Precordial Leads (V1–V6) capture front-to-back and right-left electrical activity.
Vector Addition:
Each lead records a projection of the heart’s electrical activity.
By combining these projections, ECG creates a 3D representation of depolarization and repolarization.
** How does perspective influence wave amplitudes? **
Depolarization Toward an Electrode → Positive Wave
If an action potential moves toward a lead’s positive electrode, the ECG records an upward (positive) deflection.
Depolarization Away from an Electrode → Negative Wave
If the impulse moves away from a lead’s positive electrode, the ECG records a downward (negative) deflection.
What does the highlighted part of the ECG show?
P wave
What does the highlighted part of the ECG show?
QRS complex
What does the highlighted part of the ECG show?
T wave
Explain what is happening in the heart during the P wave
- flat line - SA node spontaneously generates an action potential (too small for reading on ECG, thus flat line)
- Beginning of upward deflection - action potential sweeps from right atrium to left atrium
- return to baseline - dipole decreases as cells become fully depolarized
- flat line - all cells fully depolarized, atria contracting, action potential passed to AV node where conduction is slowed and too small for ECG detection thus the line stays at baseline (action potential also passed to purkinje during flat line - too small for detection)
Explain what is happening in the heart during the Q wave
- beginning of ventricular depolarization
- depolarization moves toward negative pole, resulting in a downward deflection
Explain what is happening in the heart during the R wave
- depolarization travels down the interventricular septum toward the apex of the heart
- ventricular depolarization (deflection appears as positive, because despite depolarizations moving to right ventricle technically being negative, the left ventricle is much larger resulting in overall positive vector)
Explain what is happening in the heart during the S wave
- depolarization sweeps up ventricular walls to squeeze blood into blood vessels
- negative deflection because action potentials moving toward negative pole
what is happening during the T wave
ventricular repolarization
** What causes the wave forms in lead II in an ECG? **
Lead II Orientation:
Positive electrode: Left leg
Negative electrode: Right arm
Views the heart’s electrical activity along the natural depolarization path (RA → LL), making it ideal for detecting overall conduction patterns.
See cards on why the waves appear as they do
** Describe the path of action potentials through the heart **
Sinoatrial (SA) Node (Pacemaker) → Atria
Location: Right atrium (near the SVC).
Spread: Depolarization moves right to left and downward, causing the P wave (atrial depolarization).
Atrioventricular (AV) Node → Delay
Location: Between atria and ventricles.
Effect on ECG: PR interval represents this delay.
Right & Left Bundle Branches → Septal Depolarization
Left bundle branch depolarizes the septum first, spreading left to right, creating the Q wave in lateral leads.
Purkinje Fibers → Ventricles Depolarize
Path: Impulses spread fast through Purkinje fibers to ventricular myocardium, contracting ventricles from apex upward.
ECG Effect: R wave (ventricular depolarization).
Repolarization → T Wave Formation
Starts at the epicardium (outside) and moves inward.
Opposite direction of depolarization → T wave is upright in most leads.
hyperkalemia results in
- inactivates Na + channels; decreased atrial contraction
- Na + channels in ventricles less affected (eventually knocked out too)
- K + increases V m just enough to inactivate Na + channels
anisotropy refers to:
properties of the cardiac tissue
What effect does fibrosis have on the heart?
- disrupts cell-cell conduction
- fractionated signals
How can the lungs affect the heart’s ability to conduct signals?
- blood has low resistivity compared to lungs
- lungs can interfere with signals
- ie: if lungs are sitting on top of the heart this can impede electrical conduction to the skin
How did the first ECG work?
- used buckets of saline solution in place of electrodes
- one hand and one foot placed in the saline solutions
- electrical impulses measured
Were holters effective in the diagnosis / detection of arrhythmias or other heart conditions?
- No - monthly holter use over 1 yr missed 33% of cases
- long-term holter has poor compliance
How do defibrillators work?
- shock the heart at a specific time during the ECG trace
- Shock goes to body, then to heart (cannot be directed)
- all cells in the heart get depolarized at once and heart will hopefully regain its rhythm on its own
How do pacemakers work?
deliver low electrical beats to ensure proper beating of the heart
What is the difference between an insertable cardiac monitor and pacemakers or defibrillators?
ICDs actually monitor the heart and deliver a shock if an abnormal rhythm is detected
