Physiology Practical 1 Flashcards
What is the mean electrical axis
The mean electrical axis is the average of all the instantaneous mean electrical vectors occurring sequentially during depolarization of the ventricles
what does the R wave represent in an ECG
The R wave represents the maximum potential developed during ventricular systole
how do you calculate the mean electrical axis
1) Measure the height of the R wave on the ECG records in two leads using any arbitrary unit (number of boxes or mm).
2) Note: the direction of the deflection (positive is above baseline and negative below baseline).
3) Mark the height of the measured deflection on the corresponding lead lines on a circle of axis.
4) Note: any unit will do as long as you use the same unit for both markings.
5) Starting at the centre of the circle mark a positive deflection toward the arrow head and a negative deflection towards the arrow tail.
6) Draw lines perpendicular to the lead axes through each of your two marks.
7) Draw a straight line that connects the centre of the circle of axis to the point at which the lines intersect
8) Estimate the axis of the vector which corresponds to the mean electrical axis (as calculated from the R wave).
does the direction of the electrical axis change as you breathe in and out and why
On maximal inspiration we would expect the axis to shift to the right and on expiration to the left.
why is the mean electrical axis clinically important
Less than -30° (or less than 0°) is termed a left axis deviation.
Greater than +90° is termed a right axis deviation.
what are the two approaches of analysis of ECG
Geometric method (quantitative): The axis can be measured within the frontal plane (using limb leads). Inspection method (qualitative):
where does the mean electrical axis usually lie
The mean electrical axis for the heart normally lies between -30 and +90
what are conditions that effects the mean electrical axis
Increased cardiac muscle mass (e.g., left ventricular hypertrophy)
Changes in the sequence of ventricular activation (e.g., conduction defects)
Ventricular regions are incapable of being activated (e.g., dead (infarcted) tissue
what does left axis deviation look like
Left = Leaving
(leads I & III are pointing away from each other or “leaving” though remember that II needs to be negative as well)
lead I is positive
lead III is negative
what does right axis deviation look like
Right = Reaching
(leads I & III are pointing towards each other or “reaching” regardless of what lead II is doing)
lead I is negative
lead III is positive
what is left axis deviation
Left axis deviation (LAD) is a condition whereby the mean electrical axis of ventricular contraction of the heart lies in a frontal plane direction between -30° and -90°.
This is reflected by a QRS complex positive in lead I and negative in leads II and aVF.
