Cardiac axis Flashcards
What is cardiac axis?
Cardiac axis represents the overall direction of electrical activity. Cardiac axis represents the sum of depolarisation vectors generated by individual cardiac myocytes.
What does “vector” represent?
Both magnitude and direction of the action potential
What can cause the Axis to change?
- Size of myocardium (amount of myocytes)
- Death of myocardium (e.g. due to infarction)
- “Mechanical shift” (physical movement of the heart)
- Conduction defects (e.g. LBBB)
Name causes of left axis deviation
- Normal Variance
- Left Ventricular Hypertrophy
- Conduction Defects (LBBB)
- Inferior Wall MI
- Preexcitation Syndromes (e.g. Wolff-Parkinson White syndrome)
- Congenital Cardiac Defect
- Hyperkalaemia
- Mechanical Shift (pregnancy, ascites, abdominal tumour, organomegaly)
- Paced rhythm
Name causes of right axis deviation
- Normal variation (children and young adults)
- Limb lead reversal
- Right ventricular hypertrophy
- Conduction defects (RBBB)
- Lateral wall MI
- Preexcitation syndromes (WPW)
- Congenital Cardiac Defect
- Dextrocardia
- Pneumothorax
- Mechanical shift (e.g. emphysema)
- Conditions leading to right ventricular strain (pulmonary embolism, chronic lung disease)
What does positive QRS represent?
The vector is moving in the direction of that lead (towards positive electrode)
What does negative QRS represent?
The vector is moving in the opposite direction of that lead (towards negative electrode)
What does isoelectric QRS represent?
The vector is moving perpendicular (90 degrees) to that lead
What is a normal QRS axis?
-30 to +90 degrees
What value is left axis deviation?
-90 to -30 degrees
What value is right axis deviation?
+90 to +180 degrees
What value is extreme axis deviation
-90 to +180 degrees
