Cardiac Cycle

Describe the normal pressure and flow patterns (including velocity profiles) of the cardiac cycle

The cardiac cycle:

• Describes sequence of events that occur in the heart over one beat
• Consists of two phases divided into six stages
• Typically is descried as beginning in late diastole when the myocardium is relaxed and the ventricles are passively filling

Phases of the cardiac cycle:

• Diastole
  • Isovolumetric Ventricular Relaxation
  • Rapid Ventricular Filling
  • Slow Ventricular Filling
    (The cycle begins here).
  • Atrial Contraction
• Systole
  • Isovolumetric Ventricular Contraction
  • Ejection

Phases of the Cardiac Cycle

Events during each phase of the cardiac cycle are represented on Wigger's Diagram:

Slow Ventricular Filling (Diastasis)

In slow ventricular filling:

• The AV valves are open and the semi-lunar valves are closed
• The ventricle is relaxed completely and fills slowly
  The ventricles have been mostly filled during rapid ventricular filling and so the pressure gradient is reducing.
  • The pressure in each ventricle is almost zero
• Arterial pressure is falling, as it is end-diastole
• CVP is slowly rising as the ventricle and atria fill
  This period occurs after the y descent.
• The ECG will show the beginnings of a P-wave at the end of this phase

Atrial Contraction

The atria contract, and remaining blood in the atria is ejected into the ventricle. This supplies 10% of the ventricular filling at rest, but up to 40% in tachycardia.

In atrial contraction:

• Arterial pressure is still falling
• The CVP waveform demonstrates the a wave as atrial contraction also causes blood to reflux into the SVC
• The ECG will show the PR interval

Isovolumetric Ventricular Contraction

Once the action potential passes through the AV node and bundle of His, ventricular contraction begins.

In isovolumetric contraction:

• Ventricular pressure rises, and the AV valves close
  This gives rise to the first heart sound, S₁.
  • As ventricular pressure is still less than systemic vascular pressure, the semilunar valves remain closed
• Arterial pressure is still falling
• The CVP waveform shows the C (closure) wave, as the tricuspid valve herniates back into the RA during ventricular contraction
  There is a similar spike in LA pressure as the mitral valve also bulges back into the LA.
• The ECG will show the remainder of the QRS or the start of the QT interval
  • Atrial repolarisation occurs at this stage, but is typically masked by ventricular depolarisation

Ejection

When ventricular pressure exceeds arterial pressure, the semilunar valves open and ejection occurs. Initial ejection is rapid, but as ventricular pressure falls and systemic pressure rises the gradient falls ejection becomes slower.

During ejection:

• Arterial pressure rises rapidly, and is slightly less than ventricular pressure during this stage
• The CVP waveform shows the x descent, as the shortening RV pulls the RA down, rapidly lowering CVP
• The ST segment shows on the ECG as the ventricles are fully depolarised, though the T wave may appear in late ejection

Isovolumetric Relaxation

When contraction is complete, the ventricles begin to relax. Inertia means that ejection continues for a short time.

During isovolumetric relaxation:

• The semilunar valves close
  This gives rise to the second heart sound, S₂, and marks the beginning of isovolumetric relaxation.
  • This occurs when ventricular pressure falls below vascular pressure
• Arterial pressure begins to fall, interrupted by the dicrotic notch which is a brief increase in arterial pressure as the semilunar valves close
• The v wave is visible on the CVP waveform
  Due to atria filling against closed AV valves.
• The end of the T wave is visible on the ECG as ventricular repolarisation occurs

Rapid Ventricular Filling

Most of ventricular filling occurs in this phase. This is because in early ventricular diastole the ventricle is still relaxing and so a pressure gradient is maintained between the atria and ventricle.

During rapid ventricular filling:

• The AV valves open and ventricular filling occurs
  This occurs when atrial pressure exceeds ventricular pressure.
• Arterial pressure is falling
• The y descent occurs when the AV valves open, causing a rapid drop in CVP as the ventricles fill
• No electrical activity is produced - the ECG shows the TP interval

References

1. Hall, JE, and Guyton AC. Guyton and Hall Textbook of Medical Physiology. 11th Edition. Philadelphia, PA: Saunders Elsevier. 2011.
2. Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.
3. Wigger's Diagram (with some modifictions) from Wigger's Diagram. 21/3/2012. (Image). By DanielChangMD (revised original work of DestinyQx); Redrawn as SVG by xavax. CC BY 3.0 , via Wikimedia Commons.