3 Electrical Axis and Chamber Enlargment Flashcards
The heart’s normal axis is …
Down and to the patient’s left
Vectors of the left ventricle are …
Larger and persist longer than those of the right ventricle
The mean QRS axis can be approximated by looking at…
The QRS complexes in Leads I and aVf
If the predominant direction is positive in both of these leads, the axis is normal
Normal Axis range extends from ______ to ______
0˚ to 90˚
3 o’clock to 6 o’clock
In Left Axis Deviation (LAD), the mean QRS axis is ….
Between 0˚ and -90˚ (pointing toward left shoulder)
In Right Axis Deviation (RAD), the mean QRS axis is …
Between +90˚ and +180˚ (pointing toward right foot)
If the mean QRS deflection is positive in Lead I, the axis is…
Between -90˚ and + 90˚
If mean QRS deflection is positive in aVf, the axis is …
Between 0˚ and +180˚
If mean QRS deflection is positive in both Lead I AND aVF, the axis is…
In the normal range (0˚ to +90˚)
Besides the Four Quadrant method, what’s another way to estimate the axis?
Identify the tallest QRS complex in the frontal plane
—> axis points in the direction of that lead
Find a lead with an EQUIPHASIC QRS complex
—> axis is perpendicular to that lead
How to use an equiphasic lead to determine the axis
Find an equiphasic lead (positive component = negative component on QRS)
The axis is perpendicular to that lead
Locate a positive lead to determine which direction along the perpendicular line corresponds with the axis
What is the Hexaxial Reference System?
For normal axis (upright QRS in both Leads I and aVF), the relative deflection of Lead I and aVF determines a closer approximation of the axis
Positive QRS deflection of Lead I = Lead aVF
Mean QRS axis is ~ +45˚
Positive QRS deflection of Lead I > Lead aVF
Mean QRS lies closer to Lead I (0˚to 45˚)
Positive QRS deflection of Lead I < Lead aVF
Mean QRS axis lies closer to Lead aVF (+45˚ to +90˚)
Normal P wave axis should be…
0˚ to 75˚
Normal T wave axis should be …
Close to the QRS axis
If the R wave in aVL > R wave in Lead 1….
STRONG LAD (associated with LAHB)
If R wave in Lead III > R wave in aVF
STRONG RAD (associated with LPHB)
What are some causes of Left Axis Deviation?
Left anterior hemiblock
Left ventricular hypertrophy
Q waves of inferior myocardial infarction
Chronic coronary artery disease
Diffuse myocardial disease (ie cardiomyopathy, amyloidosis, myocarditis)
Some cases of hyperkalemia
Wolff-Parkinson-White Syndrome with a right-sided accessory pathway
What are some causes of Right Axis Deviation?
Normal finding in children and tall then adults
Right ventricular hypertrophy
Chronic lung disease even without pulmonary hypertension
Anterolateral myocardial infarction
Left posterior hemiblock
Pulmonary embolus
Dextrocardia
RAD can be normal in…
Children and tall thin adults
Tall, thin individuals may have a more ______ heart
Vertical - with a shift in the QRS axis toward +90˚
Obese or pregnant individuals may have a more ______ heart
Horizontal - the mean QRS axis shifted more leftward (close to 0˚)
Thickening of the wall(s) of a cardiac chamber
Hypertrophy
Hypertrophy is usually due to…
Increased work of the heart against higher pressure (ie - HTN, aortic stenosis)
Stretching of the wall(s) of the cardiac chamber
Enlargement (dilation)
Enlargement/dilation of a chamber of the heart often occurs as a result of…
Volume overload (ie LAE due to mitral insufficiency/regurg
_____ tend to dilate rather than hypertrophy
Atria
______ is more common in the ventricles
Hypertrophy
______ provide evidence for Atrial Enlargement
P waves
Tall p waves —> RAE
Wide p waves —> LAE
________ provides evidence for ventricular hypertrophy
QRS complex
Leads ____ and _____ are used to assess Atrial Enlargement
Leads II and V1
P wave amplitude > 2.5mm
RAE (P pulmonale)
P wave duration > 0.10s
LAE (P mitrale)
Increased P wave amplitude and duration
Biatrial Enlargement
Criteria for RAE
P wave is >2.5 mm tall
OR
IF V1 P wave is BIPHASIC, the initial component of the P wave is larger than the terminal component (upward deflection > downward deflection)
Clinical conditions with RAE
Pulmonic stenosis
Tricuspid stenosis
Tricuspid regurgitation
Clues:
• Presence of RVH
• RAE
Criteria for LAE
P wave > 0.10s
OR
The terminal portion of a biphasic P wave in V1 is NEGATIVE with a duration ≥0.04s and depth ≥1mm (one small block)
Clinical conditions with LAE
Mitral stenosis
Mitral regurgitation
Ventricular hypertrophy is assessed by …
Viewing the QRS complex in several leads
Normal ventricular depolarization moves…
Downward, to the left, and posteriorly in the direction of the thicker left ventricular muscle
It moves away from Lead V1, which is the reason the R wave in V1 is small
Why is the R wave usually small in V1?
Because normal ventricular depolarization moves downward, to the left and posteriorly — AWAY from Lead V1
With ventricular hypertrophy, the direction of _____ may change
Depolarization
Large negative R wave in V1 and large positive R wave in V6
LVH
Equiphasic RS complex in V1
RVH
Common causes of LVH
Hypertension
Valvular heart disease
______ leads are more sensitive and helpful in diagnosing LVH
Precordial
LVH criteria
Sum of the deepest S in V1 or V2 plus the tallest R in V5 or V6 >35mm
OR
R in aVL >11mm
OR
R in Lead I plus S in Lead III >25mm
Which is more common: RVH or LVH
LVH
RVH is most often due to …
Pulmonary HTN or pulmonic stenosis
With normal hearts, the R waves get ______ when comparing V1-V6
Progressively more positive
In RVH, what happens to the R waves when comparing V1-V6?
They get progressively smaller rather than getting progressively more positive
RVH Criteria
Right axis deviation (> +90˚)
OR
R wave > S wave in V1 (R wave usually 7mm tall)
OR
S wave > R wave in V6
