The Cardiac Cycle Flashcards
Why is the heart referred to as a pump?
- Two pumps acting in series
- Output of left and right sides over time must be equal
- Artria act as priming pums for ventricles
What pressures do the systemic and pulmonary circulation work at?
Systemic circulation = HIGH pressure
Pulmonary circulation = LOW pressure
What is systole?
Contraction and ejection of blood from ventricles
What is diastole?
Relaxation and filling of ventricles
What is the normal stroke volume for a 70kg man?
At rest each ventricle pumps ~70ml blood per beat
How does heart muscle exert force?
- Specialised form of muscle
- Discrete cells but interconnected electrically via gap junctions forming a functional syncytium
- Cells contract in response to action potential in the membrance which causes a rise in intracellular calcium
How long are cardiac action potentials and how do they spread through the heart?
- Cardiac action potentials are relatively long, as they last for the duration of a single contraction of the heart (~ 280ms) as the heart needs to beat in synchrony
- Action potentials are triggered by the spread of excitation from cell to cell
What are the four valves that determine the pathway of blood through the heart?
Right side of the heart:
IN: Tricuspid valve
OUT: Pulmonary valve
Left side of the heart
IN: Mitral valve
OUT: Aortic valve
Why decides if a valve is open or closed?
- Valves open or close depending on differential blood pressure on each side
- Valve cusps are pushed open to allow blood flow
- Close together to seal and prevent backflow
How is inversion of the valves prevented?
- Cusps of mitral and tricuspid valves attach to papillary muscles via chordae tendineae
How is conduction passed through the heart?
- Pacemaker cells in sinoatrial node generate an action potential
- Activity spreads over atria (atrial systole)
- Reaches the atrioventricular node and delayed for ~120ms as atria need to contract fully before the ventricles can
- From av node excitation spreads down septum between ventricles
- Next spreads through ventricular myocardium from inner (endocardial) to outer (epicardial) surface
- Ventricle contracts from the apex up forcing blood through the outflow valves
What 7 stages can the cardiac cycle be split into?
1) Atrial contraction
2) Isovolumetric contraction
3) Rapid ejection
4) Reduced ejection
5) Isovolumetric relaxation
6) Rapid filling
7) Reduced filling
How long does diastole and systole last?
Systole lasts for ~0.35 ms (covers steps 2-4 of the cardiac cycle)
Diastole lasts for ~0.55 ms (covers steps 5-7 and 1 of the cardiac cycle)
How is the time for the cardiac cycle affected by changes in heart rate?
When heart rate increases, the time for diastole decreases
What is the Wiggers diagram?
Reflects the pressure (mmHg) and volume (ml) of the left side of the heart
PRESSURE: Aortic pressure, Left atrial pressure, Left ventricular pressure
VOLUME: Left ventricular volume, Electrocardiogram, Phonocardiogram
What happens in the Wiggers diagram during Phase 1 Atrial contraction?
- LEFT ATRIAL PRESSURE: Atrial pressure rises due to atrial systole. Called A wave
- Atrial contraction accounts for final ~10% of ventricular filling. This value varies with age and exercise
- P wave on ECG signifies the onset of atrial depolarisation
- At the end of phase 1 ventricular volumes are maximal (End-Diastolic Volume (EDV))
What happens in the Wiggers diagram during Phase 2 isovolumetric contraction?
- Mitral valve closes as intraventricular pressure exceeds atrial pressure
- Rapid rise in ventricular pressure as ventricle contracts
- Closing of mitral valve causes the C wave in the atrial pressure curve
- Isovolumetric since there is no change in ventricular volume (all valves are closed)
- QRS complex in ECG signifies onset of ventricular depolarisation
- Closure of the mitral and tricuspid valves results in the first heart sound (S1)
What happens in the Wiggers diagram during phase 3: rapid ejection?
- Ejection begins when the intraventricular pressure exceeds the pressure within the aorta. This causes the aortic valve to open
- Atrial pressure initially decreases as the atrial base is pulled downward as ventricle contracts. This is called the X descent
- Rapid decrease in ventricular volume as blood is ejected into aorta
What happens in the Wiggers diagram during phase 4: reduced ejection?
- Repolarisation of ventricle leads to a decline in tension and the rate of ejection begins to fall
- Atrial pressuer gradually rises due to the continued venous return from the lungs. This is called V wave
- Ventricular repolarisation depicted by T wave of ECG
What happens in the Wiggers diagram during phase 5: isovolumetric relaxation?
- When intraventricular pressure falls below the aortic pressure, there is a brief backflow of blood which causes the aortic valve to close
- ‘Dicrotic notch’ in aortic pressure curve caused by valve closure
- Although rapid decline in ventricular pressure, volume remains constant since all values are closed. Hence isovolumetric relaxation
- End systolic volume (ESV) occurs in this stage
- Closure of the aortic and pulmonary valves results in the second heart sound (S2)
What happens in the Wiggers diagram during Phase 6: rapid filling?
- Fall in atrial pressure that occurs after opening of mitral valve is called the Y descent
- When the intraventricular pressure falls below atrial pressure, the mitral valve opens and rapid ventricular filling occurs
- Ventricular filling is normally silent. However third heart sounds can be heard (S3).
- S3 sounds are normal in children but can be a sign of pathology in adults
What happens in the Wiggers diagram during Phase 7: reduced filling?
- Rate of filling slows down (diastasis) as ventricle reaches its inherent relaxed volume.
- Further filling is driven by venous pressure
- At rest the ventricles are ~90% full by the end of phase 7
What are the two types of abnormal valve function?
- Stenosis
- Regurgitation
What happens when valves don’t open enough?
- Obstruction to blood flow when valve normally open –> Stenosis
- Left side of the heart is more problemationas it is working under high pressure
