Introduction
24.The Cardiac Cycle, Heart Sounds, Pressure-Volume Diagram, and Cardiac Work
1. The Cardiac Cycle: A Symphony of Coordinated Events
The cardiac cycle encompasses all the mechanical and electrical events occurring within the heart during a single heartbeat. It's a continuous process with distinct phases, best understood in relation to changes in pressure and volume within the left ventricle, as depicted in the pressure-volume diagram.
1. Atrial Systole :
Triggered by the SA node firing, causing atrial depolarization, seen as the P wave on an ECG .
Atria contract, pushing the remaining blood into the ventricles, bringing the ventricular volume to its maximum, known as the End-Diastolic Volume (EDV), typically around 130 ml .
2. Isovolumetric Contraction :
Ventricular depolarization begins, marked by the QRS complex on the ECG.
Ventricles contract, causing a rapid rise in ventricular pressure. This pressure closes the AV valves (mitral and tricuspid), preventing backflow into the atria.
The first heart sound (S1) is heard due to the closure of the AV valves.
Ventricular volume remains constant (isovolumetric) as all four valves are closed.
3. Ventricular Ejection :
Ventricular pressure surpasses aortic pressure, forcing the aortic valve open.
Blood is rapidly ejected into the aorta, causing a sharp decrease in ventricular volume and a rise in aortic pressure.
The amount of blood ejected is the Stroke Volume (SV), usually about 70 ml at rest.
The remaining blood in the ventricle is the End-Systolic Volume (ESV).
4. Isovolumetric Relaxation:
Ventricular repolarization occurs, reflected as the T wave on the ECG.
Ventricles relax, leading to a rapid drop in ventricular pressure.
The aortic valve closes as ventricular pressure falls below aortic pressure, generating the second heart sound (S2).
AV valves remain closed, resulting in constant ventricular volume (isovolumetric relaxation).
5. Ventricular Filling:
Ventricular pressure drops below atrial pressure, causing the AV valves to open.
Rapid ventricular filling occurs as blood flows from the atria into the ventricles.
Ventricular volume increases while ventricular pressure remains low due to ventricular relaxation and compliance.
Note- The heart spends more than half of the cardiac cycle in diastole, the period of relaxation and filling.
2. Heart Sounds-
Heart sounds are produced by the closure of heart valves during the cardiac cycle.
S1: The "lub" sound, associated with the closure of the AV valves (mitral and tricuspid) at the beginning of ventricular systole.