Cardiac Cycle Notes

Cardiac Cycle

• One complete heartbeat consists of the contraction/relaxation of both atria, followed by the contraction/relaxation of both ventricles.
• The average duration of a cardiac cycle is 0.8 seconds.

Definitions

• Systole: Phase of myocardial contraction (atrial systole, ventricular systole).
  • During systole, the pressure in a chamber is elevated, and blood is ejected.
• Diastole: Phase of myocardial relaxation.
  • During diastole, the pressure in a chamber falls, and the chamber fills with blood.

Key Points to Remember

1. The myocardium contracts to provide pressure for the ejection of blood (systole) and relaxes to reduce pressure and allow filling (diastole).
2. Blood moves from regions of high pressure to regions of low pressure.
3. The cardiac conduction system coordinates the contraction of the myocardium.
4. The heart valves (atrioventricular and semilunar) ensure blood moves in a forward, not backward, direction.

Duration of Cardiac Cycle Events

• Cardiac cycles per minute: 75
• Duration of one cardiac cycle: 0.8 seconds
• Duration of atrial systole: 0.1 seconds
• Duration of atrial diastole: 0.7 seconds
• Duration of ventricular systole: 0.3 seconds
• Duration of ventricular diastole: 0.5 seconds
• Duration of cardiac quiescence (atria and ventricles in diastole): 0.4 seconds
• Pressure changes in the right heart are approximately 1/5th of those in the left heart.

Cardiac Cycle Phases

• Ventricular Filling (Ventricular Diastole - Late)
  • Passive ventricular filling
  • Active ventricular filling
• Atrial Contraction (Atrial Systole Begins)
• Atrial Systole
• Atrial Diastole
• Isovolumic/Isovolumetric Contraction
• Ventricular Contraction (Ventricular Systole - First Phase)
• Ventricular Ejection (Ventricular Systole - Second Phase)
• Isovolumic/Isovolumetric Relaxation (Ventricular Diastole - Early)

Electrocardiogram and Heart Sounds

• ECG waves: P wave, QRS complex, T wave
• Heart sounds: 1st sound (S1), 2nd sound (S2)

Pressure and Volume Changes

• Atrial Systole:
  • Left atrium pressure increases slightly.
• Left Ventricle:
  • Pressure increases during ventricular systole.
  • Volume increases during ventricular filling and decreases during ventricular ejection.
• Aorta:
  • Pressure fluctuates with ventricular systole and diastole.
• EDV (End Diastolic Volume): ~130 ml
• ESV (End Systolic Volume): ~60 ml
• SV (Stroke Volume): ~70 ml (EDV - ESV)

Valve States

Ventricular Diastole

• Early Ventricular Diastole
  • Passive ventricular filling
  • Blood flow: From atria into ventricles
  • Pressures: Atrial > Ventricular
  • Valve State: A-V Open, Semilunar (S-L) Closed
  • Heart Sound: Yes
  • ECG: None
• Late Ventricular Diastole
  • Active ventricular filling
  • Contraction of atrium pumps additional 25% of blood
  • Atrial pressure > Ventricular pressure
  • A-V open
    • Semilunar (S-L) closed

Ventricular Systole

• Isovolumetric Contraction
  • Ventricles begin to contract – no volume change.
  • Ventricular pressure > Atrial pressure
  • A-V closes
  • S-L closed
  • End Diastolic volume
  • All valves closed. Ventricular volume does not change
  • ECG: QRS
  • Heart Sound: Yes
• Period of Ejection
  • Ventricle continue to contract – blood ejected into aorta
  • Ventricular pressure > aorta
  • A-V closed (pressure in ventricle > atrium)
  • S-L opens
  • ECG: T-wave

Early Ventricular Diastole

• Ventricles begin to relax - no volume change
• Atrial starting to open
• Atrial pressure > Ventricular pressure
• Pressure in aorta > ventricle
• End-systolic Volume
• Period of isovolumetric relaxation – all valves closed.

Mid Ventricular Diastole

• The atria and ventricles are in diastole.
• Atrial pressure is greater than ventricular pressure; thus, blood moves from atria into ventricles through open AV valves.
• The ventricles receive about 75% of its final blood volume during this time.

Late Ventricular Diastole

• The atria contract and eject the remaining volume (25%) of blood into the ventricles.
• The arteries during this entire time period are losing pressure as blood circulates to smaller vessels.

Ventricular Systole

• Ventricular pressure increases as the ventricles contract.
• When ventricular pressure is greater than atrial pressure, the AV valves close.
• When ventricular pressure is greater than arterial pressure, the semilunar valves open, and blood is ejected into the arteries.

Isovolumetric Period of Contraction

• Period of time, during ventricular systole, after the AV valves close but before the semilunar valves open, when ventricular volume remains unchanged.

Early Ventricular Diastole

• Ventricular pressure falls during ventricular diastole.
• When ventricular pressure is less than arterial pressure, the semilunar valves close.
• When ventricular pressure is less than atrial pressure, the atrioventricular valves open; the ventricles begin to fill.

Isovolumetric Period of Relaxation

• Period of time, during ventricular diastole, after the semilunar valve closes but before the AV valve opens, when ventricular volume remains unchanged.

Key Terms

• Dicrotic Notch: A brief rise in arterial pressure results from arterial blood rebounding off newly closed semilunar valves.
• End Diastolic Volume (EDV): The total volume of blood that a ventricle receives during its filling period.
• End Systolic Volume (ESV): The total volume of blood left in a ventricle after ventricular contraction.
• Stroke Volume (SV): The total volume of blood ejected into arteries due to ventricular contraction ($SV = EDV - ESV$).

Heart Sounds

• First Sound (S1):
  • Generated by the closure of the AV valves.
  • Sounds like