Cardiac output and right atrial pressure: assessment of cardiac output and venous circulation

2021-05-24 03:49 PM

The sympathetic system affects both the heart and the peripheral circulation, making the heart beat faster and stronger, increasing the mean vascular pressure because of vasoconstriction, especially the veins, and increased resistance to the venous circulation. 

The above factors of cardiac output regulation are quite adequate in most common cases. However, to further elucidate cardiac regulation in stressful situations, such as heavy physical activity, heart failure, and circulatory failure, discuss the following.

In quantitative methods, we need to clarify the following two basic factors: in the regulation of cardiac output: (1) the pumping capacity of the heart, represented by the cardiac output curve (2) the factors Peripheral factors affect the return of blood to the heart, represented by the venous circulation curve. Then, combine the two curves by the same quantification to see the relationship between cardiac output, venous circulation, and right atrial pressure at the same time.

Figure. Assess cardiac output and venous circulation in the normal state.

When an additional 20% of the blood volume is infused into the systemic circulation, the venous circulation is loose and the equilibrium point A shifts to point B.

In a complete circulatory system, the heart and vascular system work together to transport blood.

This requires two factors: (1) the right venous return equals cardiac output (2) the right inflow pressure equals the pressure in the right atrium.

Therefore, one can evaluate cardiac output and right atrial pressure in two ways: (1) measure cardiac output at a given time interval and then plot it on a cardiac output curve, (2) measure cardiac output venous return to the heart at the same time and plot on the venous circulation curve, (3) then plot these two curves on the same graph.

The graph shows the cardiac output curve (red) and the venous circulation curve (blue). The intersection of these two lines is at point A when venous circulation equals cardiac output, just as right atrial pressure equals venous return. Thus, point A simultaneously represents normal cardiac output, venous circulation, and right atrial pressure, known as the 'equilibrium', when cardiac output is 5L/min, pressure Right atrial force is 0 mmHg.

Effect of volume expansion on cardiac output

A sudden 20% increase in circulating volume can increase cardiac output 2.5 to 3 times. This relationship, when a large amount of blood is transfused into the body, the increased circulating volume causes the mean vascular system pressure to increase by 16 mmHg, the venous circulation graph shifts to the right. Simultaneously, increasing the circulating volume causes vasodilation, reducing vascular resistance, or reducing venous resistance, causing the venous circulation curve to shift upward. From the above two mechanisms, the new curve will shift to the right, and the equilibrium point will become point B, when cardiac output and venous circulation both increase 2.5 to 3 times, and right atrial pressure equal to 8 mmHg.

The compensatory mechanism regulates the increase in circulating volume

The increase in cardiac output with increased intravascular blood volume occurs within minutes because it is controlled by the following compensatory mechanisms:

1. Increasing cardiac output increases capillary circulation, percolating fluid into body tissues, so blood flow decreases to normal.

2. Increased intravascular pressure leads to gradual varicose veins, called pressure dilation, causing blood stasis in the periphery, especially in blood-containing organs such as the liver and spleen, leading to a decrease in systemic pressure. medium vascular system.

3. The increase in peripheral circulation leads to an increase in vascular resistance by a self-regulating mechanism, which increases the resistance to the venous circulation.

The above factors will reduce the mean systemic vascular pressure gradually return to normal and increase systemic circulatory resistance. Therefore, after about 10 to 40 minutes, the cardiac output should return to normal.

Sympathetic nerve stimulation affects cardiac output

The sympathetic system affects both the heart and the peripheral circulation, (1) making the heartbeat faster and stronger (2) increasing the mean vascular pressure by constricting the vessels, especially the veins, and increasing the blood pressure. resistance to the venous circulation.

The figure shows normal cardiac output and venous circulation. At equilibrium point, A, cardiac output and venous circulation are 7 mmHg and right atrial pressure is 0 mmHg. With maximal sympathetic stimulation (blue line), mean systemic vascular pressure can be increased up to 17 mmHg (the time point described as venous circulation approaching zero), and at the same time increases the likelihood of pumping the heart's blood by almost 100%. As a result, cardiac output increases from equilibrium point A to point D by a double value, but right atrial pressure remains almost unchanged. Thus, varying degrees of sympathetic stimulation can increase cardiac output to double values ​​for a short time, until compensatory mechanisms are involved after a few seconds, up to a few minutes. control cardiac output back to normal.

Figure. Assess cardiac output regulation when (1) sympathetic nerve stimulation (from point A to point C); (2) maximal sympathetic stimulation (point D); (3) Sympathetic blockade with general spinal anaesthesia (score B).

Sympathetic inhibition affects cardiac output

Sympathetic nerve activity can be inactivated by general spinal anaesthesia or using drugs, such as hexamethonium, which inhibit the conduction of nerve impulses to the sympathetic ganglia. The lowest curve with sympathomimetic blockade with total spinal anaesthesia, showing (1) mean systemic vascular pressure reduced to 4 mmHg, (2) cardiac pumping capacity reduced by 80% compared to physiological value. The cardiac output then decreases from point A to point B and decreases by about 60% of its value.

Arteriovenous catheterization affects cardiac output

The figure depicts the change in circulatory activity after the great artery communicates with the great vein.

1. Two red lines with the intersection at A are physiological curves.

2. Two curves with the intersection at B are curves after arteriovenous catheterization. We find (1) a rapid and sudden decrease in the venous return curve because of the significant reduction in peripheral resistance as blood flows directly from the great artery to the venous system, which largely ignores the resistance of the venous system. In the peripheral circulation, (2) the cardiac output curve is slightly increased because when resistance is combined with a decrease in arterial blood pressure, the heart contracts more easily. Experimentally, at point B, cardiac output increased from 5 L/min to 13 L/min, right atrial pressure increased by about 3 mmHg.

Figure. Changes in right atrial pressure and cardiac output after arteriovenous catheterization. Point A is the physiological balance point, Point B is immediately after the arteriovenous catheterization; Score C is 1 minute after an arteriovenous catheterization or when the sympathetic nervous system begins to activate; Score D is after several weeks when circulating volume has increased and myocardial hypertrophy is present.

3. Point C is after 1 minute when sympathetic nervous system regulation helps restore arterial blood pressure and cause 2 other effects: (1) increase in mean vascular system pressure (because of constriction of blood vessels). whole arteries and veins) from 7 to 9 mmHg, the cardiac output curve shifts to the right by 2 mmHg (2) increases cardiac output (due to cardiac stimulation). At this point, cardiac output increases to 16 L/min, and right atrial pressure increases to 4 mmHg.

4. Score D is that after several weeks, circulating volume increases because of a slight decrease in arterial blood pressure, while the sympathetic nervous system temporarily reduces urea secretion, increasing salt and water retention. The mean systemic vascular pressure is now 12 mmHg, and the venous circulation curve shifts by 3 mmHg to the right. On the other hand, long-term stress on the heart leads to myocardial hypertrophy, and increased output. find another small value range. Thus, at point D, cardiac output increases by about 20 L/min, and right atrial pressure increases to 6 mmHg.

Other evaluation factors

Cardiac output regulation during physical activity, and cardiac output regulation in different stages of congestive heart failure.

Figure. Blood flows through the root of the aorta through an electromagnetic flowmeter.