The cardiac cycle

Question 1

Define “Cardiac Cycle”

Answer 1

the orderly depolarisation and repolarisation sequence that triggers a cycle recurring of atrial and ventricular contraction and relaxation

Question 2

What is Passive filling? (phase 1 of cardiac cycle)

Answer 2

• pressure in atria and ventricles close to 0
• AV valves open so venous return flows into ventricles
• Aortic pressure becomes 80mmHg and aortic valve closes
• Ventricles become 80% full

Question 3

What is atrial contraction? (phase 2 of cardiac cycle)

Answer 3

• P wave signals atrial depolarisation
• atria contract between the P wave and QRS complex
• atrial contraction completes the EDV (130ml in normal)

Question 4

What is isovolumetric ventricular contraction? (phase 3 of cardiac cycle)

Answer 4

• starts after the QRS complex
• ventricular pressure rises and when it exceeds atrial pressure the AV valves shut
• FIRST HEART SOUND
• shut valves cause tension around the closed volume (contraction)
• ventricular pressure rises steeply

Question 5

What is Ventricular Ejection? (phase 4 of cardiac cycle)

Answer 5

• ventricular pressure exceeds aorta/pulmonary pressure so aortic/pulmonary valves open
• SV is ejected by each ventricle (leaving behind EDV)
• aortic pressure rises
• T wave signals ventricular repolarisation
• ventricles relax and pressure falls below aortic/pulmonary pressure so valves shut and produce SECOND HEART SOUND
• the valve vibration produces the dicrotic otch in aortic pressure curve

Question 6

What is Isovolumetric Ventricular Relaxation? (phase 5 of cardiac cycle)

Answer 6

• closure of aortic/pulmonary valves signals the start of this
• ventricle is again a closed box and tension falls around this closed volume
• when ventricular pressure falls below atrial pressure, AV valves open and the heart begins a new cycle