Functions and Electrocardiographic Representation
Diastole is not simply a passive waiting period for the next contraction. It is a critically important stage that performs several key functions simultaneously:
- Provides the myocardium with necessary time for rest.
- Ensures restoration of energy reserves within cardiac muscle cells.
- Guarantees adequate preparation for the next systole through proper chamber filling.
On the electrocardiogram, the relaxation process is closely linked to the isoelectric line. The TP segment, lasting approximately 370 ms, reflects the resting membrane potential of cardiomyocytes and coincides with the phase of total diastole (when both atria and ventricles are relaxed). Adding the P wave of the subsequent cycle to this yields the TP interval, which lasts 470 ms and strictly corresponds to the total duration of ventricular diastole.
Relaxation Period (Phases 6 and 7)
The dynamics of diastole begin with a sharp drop in myocardial tension. The relaxation period is divided into two sequential phases:
- Phase 6: Protodiastolic period. This marks the very beginning of relaxation. In the right ventricle, this process takes 50 ms, and in the left, 40 ms. At this moment, pressure in the aorta and pulmonary trunk continues to decline smoothly as blood flows peripherally toward organs and tissues. Protodiastole ends with the closure of the semilunar valves.
- Phase 7: Isovolumetric (isometric) relaxation.
Mechanism and Hemodynamics: As the ventricles relax, intraventricular pressure drops rapidly and falls below the pressure in the large outflow vessels (aorta and pulmonary trunk). Due to this pressure gradient, blood surges back toward the heart. This backflow instantly snaps the cusps of the semilunar valves shut. The impact of the blood column against the closed cusps causes them to vibrate, which is heard as the second heart sound (S2).
Throughout the entire relaxation period, the atrioventricular valves remain closed.
Filling Period (Phases 8 and 9)
As soon as ventricular pressure drops below atrial pressure, the filling period begins. It is also divided into two phases:
- Phase 8: Rapid filling.
- Phase 9: Slow filling (diastasis).
Mechanism: Filling occurs passively. Blood accumulated in the atria flows down the pressure gradient into the empty, relaxed ventricles.
During this period, the state of the valve apparatus changes: the atrioventricular valves open to allow blood flow, while the semilunar valves remain securely closed, preventing backflow from the arterial bed.