Cardiac Cycle: All You Need to Know

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Cardiac cycle appearing from the beginning of one heart beat to the beginning of next heart beat and repeating themselves successively.

Cardiac cycle time (H.Rate = 75/min) : 0.8 sec
Systole : 0.27 sec
Diastole: 0.53 sec
Systole : Contraction and emptying
Muscle stimulated by action potential and contracting
Diastole : Relaxation and filling
Muscle reestablishing Na+/K+/Ca++ gradient and is relaxing

  •  Atria and ventricles go through separate cycles of systole and diastole.
  • Atria contract prior to the ventricular contraction i.e. when atria contract ventricles relax and vice versa.
  • Each ventricle expels same volume of blood/beat.

Systole:

1) Isovolumic contraction
A-V valves close;
because (ventricular press > atrial press)
2) Aortic valve opens
3) Ejection phase
4) Aortic valve closes

Diastole

1) Isovolumic relaxation
2) A-V valves open
3) Rapid inflow
4) Diastasis – slow flow into ventricle
5) Atrial systole – extra blood in and this just
follows P wave. Accounts for 25% of filling
Atrial Pressure/Jugular Venous Pressure Waves
Atrial pressure waves:
a-wave = atrial contraction
c-wave = ventricular contraction
(A-V valves bulge)
v-wave = flow of blood into atria
x- descent = falling Rt atrial pressure due to atrial diastole
y- descent = falling Rt atrial pressure during ventricular filling

Ejection Fraction

• End diastolic volume = 120 ml
• End systolic volume = 50 ml
• Ejection volume (stroke volume) = 70 ml
• Ejection fraction = 70ml/120ml = 58% (normally 60%)
• If heart rate (HR) is 70 beats/minute, what is cardiac
output?
• Cardiac output = HR x stroke volume
= 70/min. x 70 ml
= 4900ml/min.
• If HR =100, end diastolic volume = 180 ml,
end systolic vol. = 20 ml, what is cardiac output?
C.O. = 100/min. x 160 ml = 16,000 ml/min.

Aortic Pressure Curve

Aortic pressure starts increasing during systole
after the aortic valve opens.
• Aortic pressure decreases toward the end of the
ejection phase.
• After the aortic valve closes, an incisura occurs
because of sudden cessation of back-flow toward
left ventricle.
• Aortic pressure decreases slowly during diastole
because of the elasticity of the aorta.

Valvular Function

• To prevent back-flow.
• Chordae tendineae are attached to A-V valves.
• Papillary muscle, attached to chordae tendineae, contract during systole and help prevent back-flow.
• Because of smaller opening, velocity through aortic and pulmonary valves exceeds that through the A-V valves.

Events in Cardiac cycle (Left ventricle)

Systole : 0.27 Sec or 0.3 sec
LV : 120 mmHg
RV : 25 mmHg
1. Onset of ventricular contraction.
2. A-V valves closure.
3. Isovolumetric ventricular contraction.
(LV = 60 msec, RV = 15 msec)
4. Semilunar valves open.
5. Ventricular ejection. (LV= 200 msec, RV= 270 m sec)
a. Rapid ejection 1/3rd (70%)
b. Slow ejection 2/3rd (30%)
ESV = 65 ml.
6. Semilunar valves close.
Events in Cardiac cycle (Left ventricle)
Diastole: 0.53 sec or 0.5 sec
7. Isovolumetric relaxation (100 msec)
8. A.V valves open
9. Ventricular filling (500 msec)
a. Rapid filling 60-65%
b. Slow filling (Diastasis) 5-10%
c. Rapid filling again (Atrial systole) 20-25%
‘Atrial kick’
• EDV = 135 ml
• Atrial Systole:
a. RA = 4-6 mmHg
b. LA = 7-8 mmHg

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