The C wave is part of the atrial pressure curve seen in cardiac physiology. It specifically marks a phase of the cardiac cycle:
- The C wave is observed during the phase of iso-volumetric contraction of the heart.
- This occurs shortly after the closure of the atrioventricular (AV) valves, primarily the mitral valve, at the beginning of systole.
- The iso-volumetric contraction phase begins when the ventricles contract with the AV valves closed, causing a small upward bulge in the closed valves. This bulging effect towards the atria is reflected as a slight increase in atrial pressure, known as the C wave.
Therefore, the correct answer is: Iso-volumetric contraction.