Graph analysis of blood pumping of the ventricles

2021-06-02 03:15 PM

The systolic pressure curve was determined by recording the systolic pressure achieved when the ventricles contract at each volume filled.

The figure shows a graph that is used especially in explaining the pumping mechanism of the left ventricle.

The most important components of the graph are the two curves "diastolic pressure" and "systolic pressure". These curves are volume-pressure curves.

The diastolic pressure curve is determined by filling the heart with an increasing volume of blood and then assessing the diastolic pressure directly before ventricular contraction occurs, which is the end-diastolic pressure of the ventricle.

The systolic pressure curve was determined by recording the systolic pressure achieved when the ventricles contract at each volume filled.

Until the volume without ventricular contraction peaks at about 150 ml, the "diastolic" pressure does not rise. Thus, reaching this volume, blood can flow easily through the ventricles from the atria. At 150 ml, the diastolic pressure rises rapidly, partly because the fibrous tissue in the heart is not stretched much and partly because the pericardium begins to stretch close to its limit.

While the ventricles contract, the systolic pressure increases even when the ventricular volume is low and reaches a maximum of about 150 - 170 ml. After that, the volume still increases, and the systolic pressure decreases under some conditions, as evidenced by a decrease in the systolic pressure curve, because at this large volume, the actin and myosin filaments of the myocardium being pulled away, causing the strength of the contraction fibres to become weaker than optimal.

Figure. Relationship between left ventricular volume and left ventricular pressure in diastole and systole. The red line is a “volume-pressure plot, showing the variation of intraventricular volume and pressure during the normal cardiac cycle. EW, external public; PE, potential energy.

Of note in the figure, the maximum systolic pressure with a normal left ventricle is between 250-300 mmHg, but this value is broader for each degree and strength of cardiac excitation by the cardiac nerve. With a normal right ventricle, the systolic pressure is between 60-80 mmHg.

“Volume-pressure graph” in the tom cycle, the work of the heart

The red line in the figure forms a ring called the volume-pressure plot of the normal cardiac cycle in the left ventricle. Some details of this graph are shown in the figure below. This graph is divided into 4 phases.

Phase I: The ventricular filling phase. Phase I in the volume-pressure plot begins at a ventricular volume of about 50 ml and a diastolic pressure of 2-3 mmHg.

The amount of blood remaining in the ventricles after the previous heartbeat is 50 mmHg, which is called the end-systolic volume. As venous blood flows into the ventricles from the atria, the normal ventricular volume increases by 70 ml to about 120 ml, the so-called end-diastolic volume. Thus, the volume-pressure plot in phase I extends in Figure 9-9 denoted "I", and from point A to point B, with volume increasing to 120 ml and diastolic pressure increasing to approx. 5-7 mmHg.

Phase II: The phase of isovolumetric contraction. During isovolumic contraction, ventricular volume remains constant because all valves are closed. However, the intraventricular pressure increases until it equals the pressure in the aorta, about 80 mmHg, described in point C.

Phase III: The ejection phase. During this phase, the systolic pressure rises even higher as the ventricles continue to contract. At this point, ventricular volume decreases because the aortic valve has opened, and blood flow is forced out of the ventricle into the aorta.

Figure. The volume-pressure plot shows the change in volume and pressure during a cycle (red line). The shaded area represents the external work (EW) produced by the left ventricle during the cardiac cycle.

The symbol curve “III” is the “ejection phase”, showing the change in volume and systolic pressure during the ejection phase.

Phase IV: isovolumic expansion phase. At the end of the ejection phase (point D), the aortic valve closes, and the systolic pressure returns to the diastolic pressure. The symbol line “IV” shows the decrease in interior pressure without much change in volume. Thus, the ventricle returns to its starting point, which is about 50 ml of blood in the left ventricle, and the atrial pressure is between 2 and 3 mmHg.

The area surrounded by a volume–pressure function plot (shaded area, denoted “EW”) represents the external work done by the ventricles during the cardiac cycle. In experimental studies of the cardiac cycle, this graph is used to calculate cardiac work.

When the heart pumps a large amount of blood, the area of ​​the graph of work becomes wider. It expands further to the right because the ventricles are filled with blood during diastole, it is increased more by the greater pressure of the ventricles, and it usually expands more to the left because the ventricles contract with a higher volume. smaller volume - especially if the ventricles are stimulated to increase activity by the sympathetic nervous system.

Concept of Preload and Afterload

In assessing contractility, it is important to determine the amount of tension when the muscle begins to contract, which is preload, and to determine the load that the muscle must exert force to resist as afterload.

When the heart contracts, preload is usually thought of as the end-diastolic pressure when the ventricles begin to fill.

Afterload of the ventricles is the pressure in the arteries received from the ventricles. The corresponding systolic pressure is the phase III curve of the volume-pressure plot. (sometimes afterload is less thought of as circulatory resistance versus pressure).

The importance of the concepts of preload and afterload is that in many abnormal states of cardiac or circulatory function, the pressure in the ventricular filling (preload), the arterial pressure against the heart's contraction (afterload), or both vary from normal to more severe.

The chemical energy needed for the heart to contract: the heart's use of oxygen

Cardiac muscle, like skeletal muscle, uses chemical energy to provide work for contraction. About 70-90% of this energy is normally obtained from fatty acid oxidation, with about 10-30% from other nutrients, especially from lactate and glucose. Thus, the rate of oxygen used by the heart is best measured by the chemical energy released while the heart does work.

Experimental studies have shown that the oxygen used by the heart and the chemical energy used in the contraction of the heart are directly related to the total shaded area in the figure. This region is divided into parts including external work (EW) as explained above and another part called potential energy, denoted “PE”. Potential energy represents the extra work that can be done by ventricular contraction if the ventricles should be completely emptied of blood into the heart chambers with each contraction.

The oxygen used is also expressed as roughly proportional to the tension present in the heart muscle during contraction multiplied by the time limit for contraction, known as the stress-time index. Due to the large strain when the systolic pressure is large, corresponding to more oxygen is used. Similarly, much of the chemo is used even when systolic pressure is normal when the ventricles are abnormally dilated because myocardial tension during contraction is proportional to pressure times the ventricular diameter. Of particular importance in heart failure when the ventricles are dilated and paradoxically the amount of chemical energy required for a large amount of contractile work is greater than normal even when the heart is failing.

Contractile performance of the heart

During contraction, most of the chemical energy used is converted to heat, and a small part is successfully converted into contraction. The ratio of contractile work to total chemical energy expended is called the contractile efficiency of the heart, or rather, the efficiency of the heart. The maximum efficiency of the normal heart is between 20-25%. For people with heart failure, this efficiency can be as low as 5-10%.