physiology

Cardiac mechanics

By Dr. Stefano Casali

The right venticolo is characterized by a triangular section, with a bellows movement divided into three stages:

Shortening of free ventricular wall;

Ventricular wall displacement towards interventricular septum (bellows dragging);

Free ventricular wall entrainment towards interventr septum. for left ventricle contraction

The left ventricle. it has a cylindrical cavity, with very developed oblique muscle bundles. The characteristic contraction of the transverse diameter, with a slight shortening of the longitudinal axis, gives the configuration of a high-pressure pump, capable of pushing small quantities of blood into a high-pressure circle. The structure is such as to speak of a volume pump for the right ventricle, able to push high quantities of blood into the pulmonary circulation, characterized by low resistance. Pressure pump instead for the left ventricle, apt to determine the release of moderate amounts of blood inside the systemic circle, characterized instead by strong resistance.

Cardiac cycle (0.9 sec.)

The cardiac cycle consists of various phases between which systole and diastole indicate contraction and relaxation respectively. The phases are:

1) Ventricular isovolumic contraction (preload): the contraction of the muscular bundles determines an increase in intraventricular pressure that leads to the closure of the mitral valve, and subsequently an increase in intra-atrial pressure. The cavity therefore appears as a closed chamber, the two valves being closed; this involves an increase in pressure, first slow then rapid, without shortening of the fibers. This phase ends at a pressure of 80 mmHg.

2) Outflow phase: compared the aortic pressure, the aortic valve opens, causing the ventricle to empty, divided into two phases. Fast efflux up to a peak of 125 mmHg, followed by a slow efflux which stops at a P of 100 mmHg. These two phases are obviously marked by a shortening of the muscle bundles. The second phase (called protodiastole) is instead determined by the decrease of pressure gradient between ventricle and aortic bulb (highly elastic structure) with intraventricular pressure drop, and by the gradual inactivation of the muscle bundles activated first. The end of this phase coincides with the end of the systole and with the incisura, that is the closure of the aortic valve.

3) Isometric release: there is a further pressure drop to 0, with 40% of the diastolic blood still contained inside.

4) Rapid diastolic ventricular filling: following the falling pressure of the isometric relaxation, the mitral valve opens, allowing a blood column to pass towards the released ventricle, without yet comparing the gradient difference. Only one third of the past blood is due to the presystole, the rest in fact passes through a pressure gradient dictated by the ventricular diastole.

5) Slow diastolic ventricular filling or diastasis: this is the phase in which the pressure gradients equalize, therefore following the systole, closure of the mitral valve and resumption of the cardiac cycle.

The only difference observed in the right ventricle concerns, besides the clearly lower pressure values, the outflow trend, where instead of observing a peak, there is a plateau. This event is attributable to the low resistance of the pulmonary circulation.

Heart tones.

The tones are noises produced by the heart following movement or stopping of blood. They are 4, only the first two are auscultable.

1st TONE: marks the beginning of the ventricular systole and decreases during the outflow phase. He listens well to the apical region of the heart. It is due to the impact of the blood towards the closed atrioventricular valve, to the vibrations of the tendinous cords, to the turbulent flow of the blood that passes to the aortic bulb.

2nd TONE: coincides with the notch, is shorter than the first, and is determined by the closure of the aortic and pulmonary valves. It is auscultated on the left and right of the sternum in the 2nd intercostal space.

3rd TONE: of very short duration, it is given by the impact of blood towards the interventricular wall during the rapid ventricular filling phase.

4th TONE: starts in the middle of the presystole, and is determined by the vibrations of the blood and the cardiac apparatus.