Isovolumetric Contraction

Intervals are used to segment the cardiac cycle. The valve movement is as follows:

First, the mitral valve shuts down followed by contractions in the heart in an isovolumetric manner. It leads to ventricular ejection at a rapid rate and then both valves get closed, causing the heart to relax at the same time. After this, the ventricular chambers are filled up, leading to diastasis. In this article, we will discuss Isometric contraction in detail. 

Isometric contraction occurs in early systole when the ventricles contract, but there is no commensurate volume change. While all of the heart’s valves are closed, this brief period of the cardiac cycle occurs.

Meaning of Isovolumetric Contraction

The open atrioventricular valves allow blood to enter the atria and flow to the ventricles in a healthy young adult. An increase in atrial contraction quickly follows, causing the heart to begin actively pumping around 30% of the blood that has returned to it. Atrioventricular valves close as diastole ends and ventricular pressure rises due to contraction, preventing atria from being recirculated. The aortic and pulmonary semilunar valves are also closed at this point, which corresponds to the R peak on an ECG. 

Even while contraction raises ventricular pressure to dangerous levels, the closed valves prevent any overall change in volume. As a result of the semilunar valves being opened during the isovolumetric contraction phase, which only lasts around 0.03 s, the resulting high pressure finally surpasses that in the aorta and [pulmonary trunk]. It maintains a proper blood flow through the circulatory system as a result of this technique.

How Does Isovolumetric Contraction Occur? 

• The QRS complex of the ECG, which shows ventricular depolarisation, is the first sign of this phase of the cardiac cycle. Excitation-contraction coupling occurs, myocytes contract and intraventricular pressure rises rapidly. During this phase, the pace at which the pressure builds is at its peak. Maximal dP/dt is the term for this.

• Atrial pressure is lower than intraventricular, and thus, the AV valves close. The contraction of the papillary muscles with their chordae tendineae linked to the valve leaflets is also triggered by the contraction of the heart. To avoid them bulging back into the atria and becoming ineffective (i.e. “leaking”), this strain is applied to the AV valve leaflets. 

• The first cardiac sound is made when the AV valves close (S1). Because the mitral valve closes before the tricuspid, this sound is generally divided (0.04 seconds).

• Ventricular pressure rises rapidly without a change in ventricular volume when the AV valves close and the aortic and pulmonic valves open (i.e., no ejection occurs). The volume of the ventricles does not alter because all of the valves are shut. As a result, contraction is referred to as “Isovolumetric” or “isovolumetric”. 

• Myocyte contraction is not always isometric because the length of each myocyte is constantly changing. A single fibre can shorten concentrically (i.e., contract isotonically) while others shorten isometrically (i.e., contract eccentrically) or isometrically (i.e., lengthening contraction). This increases in circumference and decreases in atrial base-to-apex length as the heart becomes more spheroid in shape.

• Mechanisms that govern excitation-contraction coupling determine the rate at which pressure increases in the ventricular ventricles. dP/dtmax is the name given to the pressure change rate that is the fastest during this phase.

Isovolumetric Relaxation

• S2 is the second heart sound, and isovolumetric relaxation begins when intraventricular pressures drop enough after phase 4. The aortic and pulmonic valves suddenly collapse (aortic precedes pulmonic). The aortic and pulmonary artery pressure traces show a characteristic notch when a valve closes, along with a slight backflow of blood into the ventricles.

• Diarrhoea occurs after the closure of the aortic and pulmonary valves and is followed by a gradual decrease in pressure.

• Lussitropy, the rate at which muscle fibres relax, determines how quickly the ventricular pressure drops. The sarcoplasmic reticulum, which is in charge of swiftly re-sequestering calcium after contraction, is a major regulator of this relaxation.

• There is no change in volume because all valves are closed during this period. End-systolic volume is the volume of blood remaining in a ventricle, which is around 50 ml in the left ventricle. 

• Vein return from the lungs continues to raise the LAP in the patient. The v-wave is the name given to the final LAP peak in this phase.

Conclusion

Ventricular contraction comprises two phases and takes around 0.35 s to complete. The isovolumetric contraction lasts around 0.05 s, followed by an ejection phase that lasts about 0.30 s. To seal the mitral valve and produce the first cardiac sound, a left ventricular isovolumetric contraction raises its pressure above the atrial pressure. When left ventricular pressure surpasses aortic pressure, the aortic valve is opened after isovolumetric contraction. The heartbeat lasts around 0.05 s as the mitral and tricuspid valves close, as well as the aortic and para-aortic valves shut.