Lecture 5: The Heart Axis (revisited) And intro to blood flow through the heart

Question 1

Differentiate between instantaneous and mean axis

Answer 1

 Instantaneous- reading from any one point of the ECG
 Mean – used peaks of QRS complex to calculate mean amplitude for different lead-> L1 +aVF used for calculated cardiac axis

Question 2

Explain how the mean cardiac axis is calculated

Answer 2

• Calculate mean heights of R waves of QRS complex for L1 + aVF-> used as arrows + can be moved around at equal proportion w/o changing their magnitude
• Using L1 (A) and avF (O) as components for trig-> calculate angle using tan -1

Question 3

Explain the cardiac cycle

Answer 3

Diastole
- Ventricles + atria relaxed
- Higher pressure in aorta + pulmonary artery= closes semilunar valves-> prevent backflow to ventricles
- Blood returns to heart-> atria filled again due to higher pressure in vena cava + pulmonary vein
- Pressure in atria increases
- Ventricles continue to relax= pressure of ventricles decrease volume + increase pressure of chambers= pushes rest of blood into ventricles
- Slight increase in ventricular pressure + volume->receive ejected blood from contracting atria
Ventricular systole
- Atria relaxed
- Ventricle contract-> decrease volume + increases pressure
- Ventricular pressure>atrial pressure= bicuspid/tricuspid (AV) valves close->prevent backflow
- Ventricular pressure> aorta + pulmonary artery pressure= semilunar valves open
- Blood forced out into arteries

Question 4

Draw wigger’s diagram

Answer 4

Look at notes

Question 5

Explain and annotate wigger’s diagram

Answer 5

atrial contraction:
- atria contraction + pumping blood to ventricle -> ventricle already filled with 80% of blood
- mitral valve remains open + SL valve closed
(ventricles)Isovolumetric contraction:
- mitral valve closed when-> p. atria=p. ventricle
- SL valve remains closed
- Ventricle starts contracting-> pressure starts to increase
Rapid ventricular ejection phase
- Ventricles continue to contraction-> as pressure of ventricle builds up
- SL valve opens-> blood rushes out of ventricle into aorta
Slow ventricular ejection phase
- Blood continues to move from ventricles to aorta-> not as rapid= pressure in ventricle slowly starts to drop
= smaller pressure gradient
Isovolumetric relaxation
- Ventricles start relaxing
- Pressure in ventricle drops to 0
- SL valve closes+ mitral valve closed
- Pressure in aorta> pressure in ventricle
Rapid passive ventricular filling phase
- Mitral valves open again
- Blood moves passively into ventricle
- Portion of blood left in ventricle= end systolic volume
- ESV remains + doesn’t change
Slow passive ventricular filling phase
- Mitral valve still open
- Blood moves slower passively into ventricle