Physiologic Basis for ECG - Steve Flashcards
What does the ECG measure?
Potential differences between recording electrodes generated by electrical currents projecting from the heart to body surface during depolarization and repolarization
What 3 things contribute to the reproducible rate and rythm observed on the ECG?
(1)Automaticity
Cyclic depolarization of autorhythmic cells
(2) Conduction System
Order and timing of electrical events
(3) Functional Syncytium
Current conducted cell-to-cell via gap junctions
Detail the following aspects of this ECG…
P wave = ?
QRS complex = ?
T wave = ?
P wave = atrial depolarization
QRS complex = ventricular depolarization (atria repolarizing simultaneously)
T Wave = Ventricular repolarization
Where does the P wave begin? Where does it go?
Begins at SA node, spreads towards AV node
Describe the QRS sequence of ventricular depolarization!
Depolarization starts at the interventricular septum (Q wave) and endocardial surfaces.
At the end of depolarization phase, current reverses, flows toward the outer walls of the ventricles near the base (S wave).
Average (vector) of current flow: Base à Apex (R wave)
Why does left ventricular electrical activity dominate the R wave?
What is the average vector of current in ventricular depolarization?
Because there are many more myocytes associated with the left ventricle vs. the right ventricle.
•Average vector of current is R à L and inferiorly oriented (Base à Apex)
Why is the normal Mean QRS Vector oriented in the Normal Quadrant in the frontal plane?
–Muscle mass of left ventricle
–Normal sequence and distribution of conduction system
–Anatomical position of the heart in the chest
If the first deflection is downward =
Q wave
The first upward deflection =
R wave
If there is a second upward deflection =
R’ wave
The first downward deflection following an upward deflection =
S Wave
If the first wave of the complex is an R wave, the following downward deflection =
–S wave, not Q wave
A downward deflection only =
–a Q wave if it is the first wave of the complex
If the entire configuration consists solely of one downward deflection =
QS wave
What are the arrows indicating?
S.A. Node
Atrial Muscle
AV node
Common Bundle
Bundle branches
purkinje fibers
ventricular muscle
What is the normal range for the vector of ventricular depolarization?
What range might you see in some individuals?
–Normally, between 0° to +90°
(May range from -30° to +110°, depending on source/person)
Name the waves
QS
R
Name the waves
RS
QR
Name the waves
QRS
RSR’
What is the average vector of the T wave?
•Average vector of current is L -> R and superiorly oriented (Apex -> Base)
What is the PR interval?
Atrial depolarization and AV nodal delay
What is the QT interval? How about the ST segment?
•QT interval = Ventricular
depolarization/repolarization
•ST segment = Ventricular contraction & ejection (isoelectric line)
- RR interval =
- TP interval =
- RR interval = Heart rate
- TP interval = Ventricular relaxation & filling
•Horizontal: Time
5 large squares = ?
–5 large squares = 1 second (5 × 0.2 seconds/large square)
•Vertical: Relative Voltage
2 large squares = ?
•Vertical: Relative Voltage
–2 large squares = 1 mV (2 x 0.5 mV/large square)
What is the normal PR interval?
Must know this!
What is this important for?
•Normally: 0.12 - 0.2 seconds
(3 to 5 mm on the EKG paper)
•Important for identifying
conduction blocks
•AV blocks: 1°, 2° (Mobitz type I and II), 3°
•
•Deviation from isoelectric line indicates…
•Deviation from isoelectric line indicates myocardial ischemia or injury
What are the mechanisms behind the two congenital Long QT syndromes and the acquired Long QT syndrome?
Congenital:
Mutant Na+, Ca2+ channels (improper inactivation)
Mutant K+ channels (defective repolarization)
Acquired
Electrolyte imbalances, pharmacologic, etc.
What are the two methods of determining heart rate?
- How many large boxes between QRS
- Tic marks are 3 seconds
An 86 y/o female presents with complaint of chest pain. She rates her pain as a 4 on a scale of 0 to 10. Her BP is 142/72. What is her HR?
60
An 83 y/o male complains of chest pain. He had a pacemaker implanted 5 days earlier. His BP is 148/60. What is his HR? What is this arythmia?
•75 – 80 bpm
Atrial fibrillation
•A 1-month-old infant has a 3-minute seizure. What is the HR?
130 - 140 bpm,
Wave of depolarization traveling toward a positive electrode records a
positive voltage
Wave of depolarization traveling away from a positive electrode records a
negative voltage
1.Wave of repolarization traveling away from a positive electrode records a…
a) Wave of repolarization traveling toward a positive electrode records a
positive voltage
negative voltage
1.Waves traveling perpendicular to the lead axis record…
no net voltage (isoelectric)
What does the electrode record?
Positive deflection
What does the electrode record?
Negative deflection
2) Wave of repolarization moving away from a positive electrode records a…
2a) Wave of repolarization moving toward a positive electrode records a…
3) Wave of repolarization perpendicular to positive electrode records a…
positive deflection
negative deflection
biphasic wave
Describe the leads of a 12 lead ECG.
Is a lead an electrode?
•Bipolar Standard Limb Leads (1 +, 1 - )
–Leads I, II, III
•Unipolar Augmented Leads (1 +, 2 - )
–Leads aVL, aVR, aVF
•Unipolar Chest Leads
–V1 – V6
Leads ≠ Electrodes
How many limb leads?
How many precordial leads?
6 Limb Leads
•3 standard leads
3 augmented leads
•Arms: 2 electrodes
Legs: 2 electrodes
6 Precordial Leads
•Chest: 6 electrodes
What do limb leads record?
•Record electrical activity moving up/down and left/right in the coronal plane
What is meant by angle of orientation?
How is it expressed?
(Critical concept)
- Lead’s unique view of the heart
- Line from negative electrode(s) → positive electrode(s)
•Expressed in degrees, superimposed on a 360° circle of the frontal plane
What is this known as?
Einthoven’s Triangle
What is the angle of orientation for limb lead one?
•Lead I: LA (+) RA (-); Angle of orientation = 0°
What is the angle of orientation for limb lead 2?
•Lead II: LL (+) RA (-); Angle of orientation = 60°
What is the angle of orientation for limb lead 3?
•Lead III: LL (+) LA (-); Angle of orientation = 120°
For each aVL…
1 (+) limb electrode
2 (-) electrodes
Angle of orientation for aVL?
•Lead aVL: LA (+) RA, LL (-); Angle of orientation = −30°
Angle of orientation for aVR
•Lead aVR: RA (+) LA, LL (-); Angle of orientation = −150°
What is the angle of orientation for aVF?
•Lead aVF: LL (+) RA, LA (-); Angle of orientation = +90°
What leads have an anterior view of the heart?
V2,3,4
What leads have left lateral view of heart?
I, aVL, V5, V6
What leads have an inferior view of the heart?
II, III, aVF
What leads have a right ventricular view of the heart?
aVR, V1
Which leads in the frontal plane likely record a positive deflection for the P wave?
•Left lateral leads
–I and aVL
•
•Inferior leads
–II, III and aVF
•What does Lead aVR record for the P wave?
–Most rightward (−150°) of all the frontal leads
–Atrial depolarization moves away from aVR
–Records a negative deflection
Which leads in the horizontal plane would the wave of atrial depolarization be moving towards?
•Left lateral leads
–V5 and V6
–
•Anterior leads
V2-V4 are variable
P wave amplitude is generally most positive in _______ and most negative in lead _______
P wave amplitude is generally most positive in lead II and most negative in lead aVR
•What does V1 record for the P wave?
–Right ventricular lead; perpendicular to the atrial current
–May record a biphasic P wave
•Which leads in the frontal plane can record large positive deflections (R waves)? (-30° - +90°)
–Left lateral (I, aVL) and inferior leads (II, aVF, III)
•Which lead in the frontal plane will normally record a deep negative deflection (S wave)?
aVR
Which leads in the horizontal plane are expected to record tall positive deflections during ventricular depolarization (R waves)?
–V5 and V6 (over left ventricle)
•Which leads in both the frontal & horizontal plans would be expected to record positive T waves?
–Inferior & Left Lateral Leads (I, II, III, aVF, aVL, V5, V6)
–Depends on axis
Which lead in the horizontal plane is expected to record a negative deflection (S wave) ?
–V1 (over right ventricle)
•Which precordial lead and which limb lead would be expected to record negative T waves?
–V1 and aVR
If the vector representing the mean current flow in the ventricles is:
•Perpendicular to the axis of a limb lead, there will be
an isoelectric voltage recorded for that lead.
If the vector representing the mean current flow in the ventricles is:
•Parallel and in the same direction as the axis of a limb lead, there will be
a large positive voltage recorded for that lead.
If the vector representing the mean current flow in the ventricles is:
•Parallel and in the opposite direction as the axis of a limb lead, there will be
a large negative voltage recorded for that lead.
•Normal:
AVF mostly ?
LL I mostly ?
+, +
•LAD:
AVF mostly ?
LL I mostly ?
-
+
•RAD:
AVF mostly ?
LL I mostly ?
+
-
•Extreme RAD:
AVF mostly ?
LL I mostly ?
-
-
What is the mean QRS axis?
0 degrees
•What kind of general changes might cause a MEA deviation?
–Physical position of the heart
–Hypertrophy
–Infarction
–Conduction
What three primary things might drive a LAD?
What could be the cause for each of these three things?
–Physical Shift
•Pregnancy
•Obesity
•End of deep expiration
•Recumbent position
–Left Ventricular hypertrophy* (maybe)
•Chronic systemic hypertension
•Aortic valve stenosis, regurgitation
•Athlete
–Infarction of Right Ventricle
What might cause a RAD?
–Physical Shift
•Tall and lean
•End of deep inspiration
•Standing position
–Right Ventricular hypertrophy*
•Pulmonary valve stenosis, regurgitation
•Pulmonary hypertension
–Infarction of Left Ventricle
Pregnancy, obesity, end of deep expiration or recumbant position could result in what impact to MEA?
LAD
What impact might these have on MEA?
•Pulmonary valve stenosis, regurgitation
•Pulmonary hypertension
RAD - due to right ventricular hypertrophy
What impact on MEA might these have?
- Tall and lean
- End of deep inspiration
- Standing position
RAD
What impact might the following have on MEA?
- Chronic systemic hypertension
- Aortic valve stenosis, regurgitation
- Athlete
LAD - due to left ventricular hypertrophy
Axis rotation refers to a…
How does the rotation tend to occur?
shift in the horizontal plane.
“Rotate toward hypertrophy and away from infarction”
Right ventricular hypertrophy will show what on the precordial leads?
Large positive deflection in V1
–Followed by progressive decrease in amplitude towards the left chest leads (vs. normal “R-wave progression”)
What two lead recordings do you sum to determine left ventricular hypertrophy?
What is the cut-off?
V1 and V5
More than 35 mm = LVH
Tricuspid stenosis would likely drive?
dilation or hypertrophy, right atrium
Mitral stenosis would drive?
Dilation or hypertrophy, left atrial
What are the 9 steps of ECG interpretation?
- Calibration
- Rythm
- Heart rate
- Intervals
- Mean QRS axis
- P wave abnormalities
- QRS wave abnormalities
- ST segment or T wave abnormalities
- Compare with patient’s previous ECG’s
