Pressure waveforms and cardiac output

What is the cardiac cycle?

  • Everything that happens in the heart within a single heart beat
  • The cardiac cycle involves the regular process of relaxation and filling of the heart with blood (diastole), followed by contraction and ejection of blood into the great vessels (systole)
    • Diastole
      • Isovolumetric (ventricular) relaxation
        • The start of isovolumetric relaxation is marked by the second heart sound of the aortic and pulmonary valves closing
          • The closure of the aortic valve gives a brief rise in aortic pressure known as the dicrotic notch
            • The v wave on the CVP is due to the atria filling with closed tricuspid and mitral valves
      • Rapid relaxation and filling
        • There is a pressure gradient between the atria and the now relaxed ventricles, leading the atrioventricular valves to open, and blood to flow rapidly into the ventricles, causing the y descent on the JVP
      • Slow relaxation and filling
        • Aortic valve closed, mitral valve open
          • Low pressure gradient between atria and ventricles results in slow blood flow
            • CVP is rising, filling the atria
      • Atrial contraction
        • Once atria full, p wave fires triggering atrial contraction
          • Usually this only accounts for 10-15% of ventricular filling, but during tachycardia this can be up to 30-40%, which is why patients with atrial fibrillation tend to decompensate when they develop a significant tachycardia
    • Systole
      • Isovolumetric contraction
        • Ventricles contract and generate higher pressure than in the atria, causing closure of atrio-ventricular valves, (heard as the first heart sound – LUB)
          • The pressure continues to increase, causing the AV valves to bulge back into the atria, accounting for the c wave on the central venous pressure wave form
            • The pressure finally reaches and exceeds the aortic pressure, causing the aortic valve to open, and blood to eject into the aorta
      • Rapid ejection
        • There is initially a large pressure gradient between ventricle and aorta, meaning there is fast flow of blood
          • The right ventricle contracts, dragging the right atrium down, and reducing central venous pressure – seen as the x descent
      • Reduced ejection
        • As the ventricle empties, the pressure initially rises more slowly, and then starts to decrease, meanwhile the aortic pressure steadily rises
          • Once aortic pressure rises beyond ventricular pressure and the ventricle starts to relax, the aortic valve slams shut, heard as the second heart sound – DUB