CVS Physiology III

Cardiac Cycle

Faiz Baherin MBBSMmed (Emergency) Training Programme USM

Supervisor : A.Prof Nasir

Outline

• Introduction• Relationship between pressure,

volume, heart sounds in the left atrium and ventricle, aorta and jugular vein in one cardiac cycle

• Applied physiology – changes during arrhythmia

Cardiac Cycle

• One complete sequence of ventricular systole and diastole

• Cycle of events that occurs as the heart contracts and relaxes

• Both sides of heart• Takes approximately 0.8 secs at a

heart rate of 72 beats per minute

Cardiac Cycle

• Divided into seven phases (and duration of phases)

1)Atrial Systole (0.11s – 0.53s)2)Isovolumetric Ventricular Contraction

(0.05s)3)Rapid Ventricular Ejection (0.22-0.27s)4)Reduced Ventricular Ejection (0.22-0.27s)5)Isovolumetric Ventricular Relaxation (0.08s )6) Rapid Ventricular Filling (0.11s)7) Reduced Ventricular Filling (0.19s)

Cardiac Cycle

1) Atrial Systole

- Mitral valve is already open and blood passively flows into the ventricle (from previous cycle) approx 80 %

- During this phase, contraction of the atrium tops up the ventricular volume – remaining 20% EDV - 120ml

- Ventricle relaxes

1) Atrial Systole

-The "a" wave occurs when the atrium contracts, increasing atrial pressure.-Blood arriving at the heart flows back up the jugular vein, causing the first wave in the jugular venous pulse.-Ventricle pressure is not raised

1) Atrial Systole – ECG & Heart sounds

-Preceded by p wave on ECG-marks the depolarization of atria

-S4, heard in ventricular hypertrophy-Atria contracts against a stiffened ventricle

2) Isovolumetric Ventricular Contraction

-Left ventricles begins to contract-Increase in LVP-Mitral valve closes-Ventricular volume remains the same (approximately 120ml)

2) Isovolumetric Contraction

- LV pressure builds up

- LV volume remains constant (both valves are close)

- C wave – bulging of mitral valve back to atrium – slight increase in pressure

2) Isovolumetric Contraction – ECG and heart sounds

-QRS Complex-Represents the ventricular depolarization

-1st heart sound – S1-Closure of the mitral valve

3) Rapid Ventricular Ejection

-Ventricular continues contracting-Pressure increased, greater than the aortic pressure (80mmhg)-Opening of aortic valve-Most of stroke volume (almost 70ml) ejected during this phase, -reduce LV volume

3) Rapid Ventricular Ejection- Increase in LVP- Increase in

Aortic pressure due to volume of blood ejected

- Reduce in LV Volume

4) Reduced ventricular Ejection

-Ventricular pressure falls-Aortic pressure falls-Remaining of LV volume still being ejected, due to kinetic energy, with reduced rate

4) Reduced ventricular Ejection

-Reduced both aortic and ventricular pressure-Reduced LV volume-Slight increase in LA pressure

-T wave-ventricles starts repolarizing

5) Isovolumetric Ventricular Relaxation

-Left ventricle relaxes-Reduce pressure in LV-Aortic valve closes-LV volume constant – End systolic Volume (approximately 50ml)

SV = EDV – ESV = 70ml

5) Isovolumetric Ventricular Relaxation

- Reduced LV Pressure

- Incisura in aorta- LV Volume

constant- LA pressure

increases

5) Isovolumetric Ventricular Relaxation – Heart Sound

- S2 due to aortic valve closure- Splitting of heart sounds – during

inspiration – decrease in intrathoracic pressure – increase in venous return to right side of hard, increase in stroke volume, prolongs ejection time, delays closure of pulmonary valve

6) Rapid Ventricular Filling

-LV relaxes-LV pressure falls to its lowest level and constant-Mitral valve opens-LV volume increases rapidly

6) Rapid Ventricular Filling

- LV pressure reduces and remains the same(high compliance)

- Aortic pressure decreases

- LA pressure decreases

- LV volume increases

6) Rapid Ventricular Filling – heart sound

- S3 – normal in children but not adult

- Indicates volume overload as in CCF, mitral regurge

- Occurs due to passive, rapid ventricular filling

- No ECG deflection

7) Reduced Ventricular Filling

- Diastasis- Longest phase in

cardiac cyle- Final portion of

ventricular filling, slower rate

- LV volume increases- Increase in heart

rate alters this phase

Arrhythmia

• Tachycardia of atrial or ventricular origin reduces stroke volume and cardiac output particularly when the ventricular rate is greater than 160 beats/min.

• The stroke volume becomes reduced because of decreased ventricular filling time and decreased ventricular filling at high rates of contraction

Arrhythmia

• If the tachyarrhythmia is associated with abnormal ventricular conduction, the synchrony and effectiveness of ventricular contraction will be impaired leading to reduced ejection

Hence 1) reduced filling time 2) reduced filling 3) reduced ejectionAnd all will contribute to less cardiac output

Arrhythmia – Atrial Fibrillation

• Concept of atrial kick – contribution of atrial systole in ventricular filling – added 10-20% of ventricular volume, during exercise, up to 40%

• Therefore, atrial fibrillation generally has relatively minor hemodynamic consequence at rest, but can significantly limit normal increases in ventricular stroke volume and cardiac output during exercise.

Arrhythmia – Atrial Fibrillation

• In hypertrophy – reduce compliance - increased ventricular stiffness impairs passive filling, atrial contraction contributes significantly to ventricular filling even at rest.

• In AF, loss of atrial kick, reduced filling time, reduced filling, ineffective ejection - CO is significantly affected – hemodynamic instability

Thank you

Reference

• Physiology, Linda S. Costanzo 4th Edition