Stroke and minute volumes of blood circulation (heart)
The amount of blood ejected by the ventricle of the heart into the arteries per minute is an important indicator of the functional state of the cardiovascular system (CVS) and is called minute volume blood (IOC). It is the same for both ventricles and at rest is 4.5–5 liters.
An important characteristic of the pumping function of the heart gives stroke volume , also called systolic volume or systolic ejection . Stroke volume- the amount of blood ejected by the ventricle of the heart into the arterial system in one systole. (If we divide the IOC by the heart rate per minute, we get systolic volume (CO) of blood flow.) With a contraction of the heart equal to 75 beats per minute, it is 65-70 ml, during work it increases to 125 ml. In athletes at rest, it is 100 ml, during work it increases to 180 ml. The definition of IOC and CO is widely used in the clinic.
Ejection Fraction (EF) - expressed as a percentage of the ratio of the stroke volume of the heart to the end-diastolic volume of the ventricle. EF at rest in a healthy person is 50-75%, and during exercise it can reach 80%.
The volume of blood in the cavity of the ventricle, which it occupies before its systole is end-diastolic volume (120-130 ml).
End-systolic volume (ESO) is the amount of blood remaining in the ventricle immediately after systole. At rest, it is less than 50% of the EDV, or 50-60 ml. Part of this blood volume is reserve volume.
The reserve volume is realized with an increase in CO at loads. Normally, it is 15-20% of the end-diastolic.
The volume of blood in the cavities of the heart, remaining with the full implementation of the reserve volume, at maximum systole is residual volume. CO and IOC values are not constant. With muscular activity, the IOC increases to 30-38 liters due to the increased heart rate and the increase in COQ.
A number of indicators are used to assess the contractility of the heart muscle. These include: ejection fraction, the rate of expulsion of blood in the phase of rapid filling, the rate of pressure increase in the ventricle during the period of stress (measured by probing the ventricle) /
The rate of expulsion of blood changed by Doppler ultrasound of the heart.
Pressure increase rate in the cavities is considered ventricular is considered one of the most reliable indicators of myocardial contractility. For the left ventricle, the value of this indicator is normally 2000-2500 mm Hg / s.
A decrease in ejection fraction below 50%, a decrease in the rate of blood ejection, and a rate of pressure increase indicate a decrease in myocardial contractility and the possibility of developing insufficiency in the pumping function of the heart.
The IOC value divided by the body surface area in m 2 is defined as cardiac index(l / min / m 2).
SI \u003d IOC / S (l / min × m 2)
It is an indicator of the pumping function of the heart. Normally, the cardiac index is 3–4 l / min × m 2.
IOC, UOC and SI are united by a common concept cardiac output.
If the IOC and blood pressure in the aorta (or pulmonary artery) are known, it is possible to determine the external work of the heart
P = IOC × BP
P is the work of the heart in minutes in kilogram meters (kg / m).
IOC - minute volume of blood (l).
BP is the pressure in meters of water column.
During physical rest, the external work of the heart is 70-110 J, during work it increases to 800 J, for each ventricle separately.
Thus, the work of the heart is determined by 2 factors:
1. The amount of blood flowing to it.
2. Vascular resistance during expulsion of blood into the arteries (aorta and pulmonary artery). When the heart cannot pump all the blood into the arteries with a given vascular resistance, heart failure occurs.
There are 3 types of heart failure:
1. Insufficiency from overload, when excessive demands are placed on the heart with normal contractility in case of defects, hypertension.
2. Heart failure in case of myocardial damage: infections, intoxications, beriberi, impaired coronary circulation. This reduces the contractile function of the heart.
3. A mixed form of insufficiency - with rheumatism, dystrophic changes in the myocardium, etc.
The whole complex of manifestations of the activity of the heart is recorded using various physiological methods - cardiography: ECG, electrokymography, ballistocardiography, dynamocardiography, apical cardiography, ultrasound cardiography, etc.
The diagnostic method for the clinic is the electrical registration of the movement of the contour of the heart shadow on the screen of the X-ray machine. A photocell connected to an oscilloscope is applied to the screen at the edges of the heart contour. When the heart moves, the illumination of the photocell changes. This is recorded by the oscilloscope in the form of a curve of contraction and relaxation of the heart. This technique is called electrokymography.
Apical cardiogram is registered by any system that captures small local displacements. The sensor is fixed in the 5th intercostal space above the site of the cardiac impulse. Characterizes all phases of the cardiac cycle. But it is not always possible to register all phases: the cardiac impulse is projected differently, part of the force is applied to the ribs. The record for different individuals and for one person may differ, depending on the degree of development of the fat layer, etc.
Research methods based on the use of ultrasound are also used in the clinic - ultrasound cardiography.
Ultrasonic vibrations at a frequency of 500 kHz and above penetrate deeply through tissues being formed by ultrasound emitters applied to the surface of the chest. Ultrasound is reflected from tissues of various densities - from the outer and inner surfaces of the heart, from vessels, from valves. The time of reaching the reflected ultrasound to the catching device is determined.
If the reflective surface moves, then the return time of the ultrasonic vibrations changes. This method can be used to record changes in the configuration of the structures of the heart during its activity in the form of curves recorded from the screen of a cathode ray tube. These techniques are called non-invasive.
Invasive techniques include:
Cardiac catheterization. An elastic probe-catheter is inserted into the central end of the opened brachial vein and pushed to the heart (into its right half). A probe is inserted into the aorta or left ventricle through the brachial artery.
Ultrasound Scan- the source of ultrasound is introduced into the heart using a catheter.
Angiography is a study of the movements of the heart in the field of x-rays, etc.
Mechanical and sound manifestations of cardiac activity. Heart sounds, their genesis. Polycardiography. Comparison in time of periods and phases of the cardiac cycle of ECG and FCG and mechanical manifestations of cardiac activity.
Heart push. During diastole, the heart takes the shape of an ellipsoid. During systole, it takes the form of a ball, its longitudinal diameter decreases, and its transverse diameter increases. The apex during systole rises and presses against the anterior chest wall. In the 5th intercostal space, a cardiac impulse occurs, which can be registered ( apical cardiography). The expulsion of blood from the ventricles and its movement through the vessels, due to reactive recoil, causes oscillations of the whole body. Registration of these oscillations is called ballistocardiography. The work of the heart is also accompanied by sound phenomena.
Heart sounds. When listening to the heart, two tones are determined: the first is systolic, the second is diastolic.
systolic the tone is low, drawn out (0.12 s). Several layering components are involved in its genesis:
1. Mitral valve closure component.
2. Closing of the tricuspid valve.
3. Pulmonary tone of expulsion of blood.
4. Aortic tone of blood expulsion.
The characteristic of the I tone is determined by the tension of the cusp valves, the tension of the tendon filaments, the papillary muscles, the walls of the myocardium of the ventricles.
Components of blood expulsion occur when the walls of the main vessels are tense. I tone is well heard in the 5th left intercostal space. In pathology, the genesis of the first tone involves:
1. Aortic valve opening component.
2. Opening of the pulmonic valve.
3. Tone of stretching of the pulmonary artery.
4. Tone of aortic distension.
Amplification of the I tone can be with:
1. Hyperdynamia: physical activity, emotions.
In violation of the temporary relationship between the systole of the atria and ventricles.
With poor filling of the left ventricle (especially with mitral stenosis, when the valves do not fully open). The third variant of amplification of the first tone has significant diagnostic value.
Weakening of the I tone is possible with mitral valve insufficiency, when the leaflets do not close tightly, with myocardial damage, etc.
II tone - diastolic(high, short 0.08 s). Occurs when the semilunar valves are closed. On the sphygmogram, its equivalent is - incisura. The tone is higher, the higher the pressure in the aorta and pulmonary artery. Well heard in the 2nd intercostal space to the right and left of the sternum. It increases with sclerosis of the ascending aorta, pulmonary artery. The sound of I and II heart sounds most closely conveys the combination of sounds when pronouncing the phrase "LAB-DAB".
Home / Lectures 2nd year / Physiology / Question 50. Coronary blood flow. Systolic and minute blood volume / 3. Systolic and minute blood volume
Systolic volume and minute volume- the main indicators that characterize the contractile function of the myocardium.
Systolic volume- stroke pulse volume - the volume of blood that comes from the ventricle in 1 systole.
Minute volume- the volume of blood that comes from the heart in 1 minute. MO \u003d CO x HR (heart rate)
In an adult, the minute volume is approximately 5-7 liters, in a trained one - 10-12 liters.
Factors affecting systolic volume and minute volume:
body weight, which is proportional to the mass of the heart. With a body weight of 50-70 kg - the volume of the heart is 70 - 120 ml;
the amount of blood entering the heart (venous blood return) - the greater the venous return, the greater the systolic volume and minute volume;
heart rate affects systolic volume, and rate affects minute volume.
Systolic volume and minute volume are determined by the following 3 methods.
Calculation methods (Starr formula): Systolic volume and minute volume are calculated using: body weight, blood mass, blood pressure. A very approximate method.
concentration method- knowing the concentration of any substance in the blood and its volume - calculate the minute volume (inject a certain amount of an indifferent substance).
Variety- Fick method - the amount of O 2 that enters the body in 1 minute is determined (it is necessary to know the arteriovenous difference in O 2).
Instrumental- cardiography (curve of recording the electrical resistance of the heart). The area of \u200b\u200bthe rheogram is determined, and according to it - the value of the systolic volume.
Stroke and minute volumes of blood circulation (heart)
Stroke or systolic volume of the heart (VV)- the amount of blood ejected by the ventricle of the heart with each contraction, minute volume (MV) - the amount of blood ejected by the ventricle per minute. The value of SV depends on the volume of the cardiac cavities, the functional state of the myocardium, and the body's need for blood.
Minute volume primarily depends on the body's needs for oxygen and nutrients. Since the body's need for oxygen is constantly changing due to changing conditions of the external and internal environment, the value of the cardiac output of the heart is very variable.
The change in the value of the IOC occurs in two ways:
through a change in the value of UO;
through changes in heart rate.
There are various methods for determining the stroke and minute volumes of the heart: gas analytical, dye dilution methods, radioisotope and physico-mathematical.
Physical and mathematical methods in childhood have advantages over the others due to the absence of harm or any concern for the subject, the possibility of arbitrarily frequent determination of these hemodynamic parameters.
The magnitude of the stroke and minute volume increases with age, while the VR changes more noticeably than the minute volume, since the heart rate slows down with age. In newborns, the SV is 2.5 ml, at the age of 1 year - 10.2 ml, 7 years - 23 ml, 10 years - 37 ml, 12 years - 41 ml, from 13 to 16 years - 59 ml (S. E. Sovetov , 1948; N. A. Shalkov, 1957).
In adults, the UV is 60-80 ml. The parameters of the IOC, related to the body weight of the child (per 1 kg of weight), do not increase with age, but, on the contrary, decrease.
3. Systolic and minute blood volume
Thus, the relative value of the IOC of the heart, which characterizes the body's need for blood, is higher in newborns and in infants.
Stroke and minute volumes of the heart are almost the same in boys and girls aged 7 to 10 years. From the age of 11, both indicators increase in both girls and boys, but in the latter they increase more significantly (MOC reaches 3.8 liters by the age of 14-16 in girls, and 4.5 liters in boys).
Thus, gender differences in the considered hemodynamic parameters are revealed after 10 years. In addition to stroke and minute volumes, hemodynamics is characterized by a cardiac index (CI - the ratio of the IOC to the body surface), CI varies in children over a wide range - from 1.7 to 4.4 l / m 2, while its relationship with age is not detected ( the average value of SI for age groups within school age is approaching 3.0 l / m 2).
"Pediatric Thoracic Surgery", V.I.Struchkov
Popular section articles
Calculation of the work of the heart. Static and dynamic components of the heart. Heart power
The mechanical work done by the heart develops due to the contractile activity of the myocardium. Following the spread of excitation, there is a contraction of myocardial fibers.
Systolic blood volume
The work done by the heart is expended, firstly, in pushing blood into the main arterial vessels against pressure forces and, secondly, in imparting kinetic energy to the blood. The first component of work is called static (potential), and the second - kinetic. The static component of the work of the heart is calculated by the formula: Ast = PcpVc, where Pav is the average blood pressure in the corresponding main vessel (aorta - for the left ventricle, pulmonary arterial trunk - for the right ventricle), Vc - systolic volume. . The mechanical work done by the heart develops due to the contractile activity of the myocardium. A=Nt; A-work, N-power. It is spent on: 1) pushing blood into the main vessels 2) giving blood kinetic energy.
Rav is characterized by constancy. IP Pavlov attributed it to the homeostatic constants of the body. The value of pav in the systemic circulation is approximately 100 mm Hg. Art. (13.3 kPa). In a small circle pav = 15 mm Hg. Art. (2 kPa),
2) Static component (Potential). A_st=p_av V_c ; p_av - mean blood pressure Vc - static volume Rav in a small circle: 15 mm Hg (2 kPa); p_cpv large circle: 100 mm Hg (13.3 kPa). Dynamic component (Kinetic). A_k=(mv^2)/2=ρ(V_c v^2)/2; p-blood density(〖10〗^3kg*m^(-3)); V-blood flow velocity (0.7 m * s ^ (-1)); In general, the work of the left ventricle in one contraction at rest is 1 J, and the right one is less than 0.2 J. Moreover, the static component dominates, reaching 98% of the entire work, then the kinetic component accounts for 2%. With physical and mental stress, the contribution of the kinetic component becomes more significant (up to 30%).
3) The power of the heart. N=A/t; Power shows how much work is done per unit of time. The average myocardial power is maintained at 1 W. Under load, the power increases to 8.2 W.
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Some indicators of hemodynamics
1. Calculation of heart rate is usually done by palpation of the pulse on the radial artery or directly from the heart beat.
To exclude the emotional reaction of the subject, the calculation is carried out not immediately, but after 30 seconds. after compression of the radial artery.
2. The determination of blood pressure is carried out by the Korotkov auscultatory method. The values of systolic (SD) and diastolic (DD) pressures are determined.
The calculation of hemodynamics is carried out according to Savitsky.
3. The value of PD - pulse pressure, and SDD - average dynamic pressure is obtained by the formula:
PD=SD-DD (mm Hg)
SDD=PD/3+DD (mmHg)
In healthy people, PP ranges from 35 to 55 mm Hg. Art.. The idea of the contractility of the heart is associated with it.
Mean dynamic pressure (DDP) reflects the conditions of blood flow in the precapillaries; this is a kind of potential of the circulatory system that determines the rate of blood flow into the tissue capillaries.
SDD slightly increases with age from 85 to 110 mm Hg. In the literature, there is an opinion that DDS is below 70 mm Hg. indicates hypotension, and above 110 mm Hg.
HEART WORK
about hypertension. Being the most stable of all indicators of blood pressure, SDD changes slightly under various influences. During exercise, fluctuations in SDD in healthy people do not exceed 5-10 mm Hg, while SD under these conditions increases by 15-30 mm Hg and more. Fluctuations in DDS, exceeding 5-10 mm Hg, as a rule, are an early sign of a disorder in the circulatory system.
4. Systolic volume of blood flow (SVK), or systolic output (stroke volume) is determined by the amount of blood that is ejected by the heart during systole. This value characterizes the contractile function of the heart.
Minute volume of blood flow (minute volume of the heart or cardiac output) is the volume of blood that the heart ejects in 1 minute.
The calculation of SOC and IOC is carried out according to the Starr formula, using indicators of SD, DD, PD, heart rate, taking into account the age (B) of the subject:
SOC \u003d 100 + 0.5 PD-0.6 DD - 0.6 V (ml)
In a healthy person, the SOC averages 60-70 ml.
IOC \u003d JUICE * HR
At rest, in a healthy person, the IOC, on average, is 4.5-5 liters. With physical activity, the IOC increases by 4-6 times. In healthy people, an increase in the IOC occurs due to an increase in the SOC.
In untrained and sick patients, the IOC increases due to increased heart rate.
The value of the IOC depends on gender, age, body weight. Therefore, the concept of minute volume per 1 m 2 of the body surface was introduced.
5. Cardiac index - a value that characterizes the blood supply to a unit of body surface per 1 minute.
SI \u003d IOC / PT (l / min / m 2)
where PT is the surface of the body in m 2, determined according to the Dubois table. SI at rest is 2.0-4.0 l/min/m 2 .
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Systolic or stroke volume (SO, SV) is the volume of blood that the heart ejects into the aorta during systole, at rest about 70 ml of blood.
Minute volume of blood circulation (MOV) - the amount of blood ejected by the ventricle of the heart per minute. The IOC of the left and right ventricles is the same. IOC (l / min) \u003d CO (l) x heart rate (bpm). On average 4.5-5 liters.
Heart rate (HR). Heart rate at rest is about 70 beats / min (in adults).
Regulation of the heart.
Intracardiac (intracardiac) mechanisms of regulation
9. Systolic and minute volume of the heart.
Heterometric self-regulation - an increase in contraction force in response to an increase in the diastolic length of muscle fibers.
Frank-Starling law: the force of myocardial contraction in systole is directly proportional to its filling in diastole.
2. Homeometric self-regulation - an increase in contractility without changing the initial length of the muscle fiber.
a) Anrep effect (dependence force-velocity).
With an increase in pressure in the aorta or pulmonary artery, an increase in the force of myocardial contraction occurs. The rate of shortening of myocardial fibers is inversely proportional to the force of contraction.
b) Bowditch ladder (chronoinotropic dependence).
Increase in the force of contraction of the heart muscle with an increase in heart rate
Extracardiac (extracardiac) mechanisms of regulation of the activity of the heart
I. Nervous mechanisms
A. Influence of the autonomic nervous system
The sympathetic nervous system has the following effects: positive chronotropic ( increase in heart rate ), inotropic(increased force of heart contractions), dromotropic(increased conductivity) and positive bathmotropic(increased excitability) effects. The mediator is norepinephrine. Adrenoreceptors α and b-types.
The parasympathetic nervous system has the following effects: negative chronotropic, inotropic, dromotropic, bathmotropic. The mediator is acetylcholine, M-cholinergic receptors.
B. Reflex influences on the heart.
1. Baroreceptor reflex: with a decrease in pressure in the aorta and carotid sinus, an increase in heart rate occurs.
2. Chemoreceptor reflexes. In conditions of lack of oxygen, an increase in heart rate occurs.
3. Goltz reflex. With irritation of the mechanoreceptors of the peritoneum or abdominal organs, bradycardia is observed.
4. Danini-Ashner reflex. When pressing on the eyeballs, bradycardia is observed.
II. Humoral regulation of the heart.
Hormones of the adrenal medulla (adrenaline, norepinephrine) - the effect on the myocardium is similar to sympathetic stimulation.
Hormones of the adrenal cortex (corticosteroids) - a positive inotropic effect.
Hormones of the thyroid cortex (thyroid hormones) - positive chronotropic.
Ions: calcium increases the excitability of myocardial cells, potassium increases myocardial excitability and conductivity. A decrease in pH leads to inhibition of cardiac activity.
Functional groups of vessels:
1. Cushioning (elastic) vessels(aorta with its departments, pulmonary artery) turn the rhythmic ejection of blood into them from the heart into a uniform blood flow. They have a well-defined layer of elastic fibers.
2. Resistive vessels(resistance vessels) (small arteries and arterioles, precapillary sphincter vessels) create resistance to blood flow, regulate the volume of blood flow in various parts of the system. In the walls of these vessels there is a thick layer of smooth muscle fibers.
Precapillary sphincter vessels - regulate the exchange of blood flow in the capillary bed. Contraction of the smooth muscle cells of the sphincters can lead to occlusion of the lumen of small vessels.
3.exchange vessels(capillaries) in which the exchange between blood and tissues takes place.
4. Shunt vessels(arteriovenous anastomoses), regulate organ blood flow.
5. capacitive vessels(veins), have high extensibility, carry out the deposition of blood: veins of the liver, spleen, skin.
6. return vessels(medium and large veins).
Determination of cardiac output
An accurate determination of the minute volume of the heart is possible only if there is data on the oxygen content in both arterial and venous blood of the heart cavities. Therefore, this method is not applicable as a general clinical research method.
However, it is possible to make a rough estimate of the adaptive capacity of a normal heart during physical work, if we assume that the fluctuations in the product of the pulse rate and the reduced arterial pressure occur in parallel with changes in minute volume.
Reduced arterial pressure = amplitude of arterial pressure * 100 / mean pressure.
Mean pressure = (systolic + diastolic pressure) / 2.
Example. At rest: pulse 72; blood pressure 130/80 mm; reduced blood pressure = (50*100)/105 = 47.6; minute volume \u003d 47.6 * 72 \u003d 3.43 liters.
After exercise: pulse 94; blood pressure 160/80 mm; reduced blood pressure = (80*100)/120 = 66.6; minute volume \u003d 66.6 * 94 \u003d 6.2 liters.
It goes without saying that with this method it is possible to obtain not absolute, but only relative indicators. It should be added to this that the calculation according to Liljestrand and Zander, although it allows to some extent to judge the adaptive capacity of a healthy heart, nevertheless, under pathological conditions of the blood circulation, allows for a wide possibility of errors.
The average minute volume of the heart in people with a healthy heart is 4.4 liters. More reliable data are provided by the Birgauz method, in which the products of the amplitude of blood pressure and the pulse rate before and after exercise are compared with the normal values of these quantities established by Wetzler. At the same time, the nature of the load (climbing stairs, squatting, moving arms and legs, raising and lowering the upper half of the body in bed) does not play any role, however, it is necessary that the subject after the load show obvious signs of fatigue.
Execution technique. After a 15-minute stay at rest in bed, the subject's pulse rate and blood pressure are measured 3 times; the smallest values are taken as initial values.
After that, a test with a load is carried out, as indicated above. Immediately after the load, measurements are taken again, and the blood pressure is determined by the examining doctor, and the pulse rate is simultaneously determined by the nurse.
Calculation. The index of cardiac output (QV m) is determined by the following formula:
QV m = (resting amplitude * resting heart rate)/(normal amplitude * normal heart rate)
(see table).
In the same way, the determination is carried out after the load (in this case, only the numerator of the fraction changes, and the denominator remains constant):
QV m = (amplitude under exercise * heart rate under exercise) / (normal amplitude * normal heart rate)
(see table).
Age-related changes in heart rate and blood pressure (according to Wetzler)
Grade. Normal: QVm at rest is about 1.0.
Indicators of the work of the heart. IOC
After loading, the increase is not less than 0.2.
Pathological changes: the initial value of the index at rest is below 0.7 and above 1.5 (up to 1.8). Decrease in the index after the load (danger of collapse).
The Birghaus test is often used as a preoperative circulatory test.
At the same time, according to Meissner, one should be guided by the following general provisions: there are no circulatory disorders in patients with an index of 1.0 - 1.8, which increases after exercise.
Patients with an index above 1.0, but without increasing it after exercise, need measures aimed at improving blood circulation. The same is necessary for the index below 1, but not below 0.7, if after the load it rises by at least 0.2.
In the absence of an increase, these patients need preliminary intensive treatment until these conditions are met.
Determining the minute volume of the heart, including the time of blood circulation, is also possible by determining the period of tension and the period of expulsion of the left ventricle, since, according to Blumberger, the electrocardiogram, phonocardiogram and carotid pulse are in a certain relationship.
But this requires appropriate equipment, which allows using this method only in large clinics.
It is equal to the product of the volume of blood ejected with each contraction (systole) times the heart rate. A person at rest is ok. 5 l, during physical work up to 30 l.
Big Encyclopedic Dictionary. 2000 .
See what "MINUTE HEART VOLUME" is in other dictionaries:
- (syn.: minute volume of blood, volumetric rate of blood ejection, cardiac output, cardiac output minute) indicator of heart function: the volume of blood ejected by the ventricle in 1 minute; expressed in l/min or ml/min… Big Medical Dictionary
Big Medical Dictionary
- (minute volume of blood flow), the amount of blood ejected by the heart in 1 minute. It is equal to the product of the volume of blood ejected with each contraction (systole) times the heart rate. A person at rest has about 5 liters, during physical work up to ... ... encyclopedic Dictionary
- (minute volume of blood flow), the amount of blood ejected by the heart in 1 minute. It is equal to the product of the volume of blood ejected with each contraction (systole) times the heart rate. A person at rest is ok. 5 l, with physical work up to 30 l ... Natural science. encyclopedic Dictionary
Minute volume of the heart- - the amount of blood ejected by the ventricles of the heart in 1 min at rest is the same for both ventricles; is, l: horse 20 30, cow 35, sheep up to 4, dog up to 1.5 l; minute volume of blood... Glossary of terms for the physiology of farm animals
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CIRCULATION- BLOOD CIRCULATION. Contents: I. Physiology. The plan for constructing the K system ....... 543 Driving forces of K ............... 545 The movement of blood in the vessels ........ 546 The speed of K ....... .......... 549 Minute blood volume .......... 553 Blood circulation rate … Big Medical Encyclopedia
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Key points . Along with arterial pressure, for a sufficient supply of the peripheral parts of the body, the minute volume of the heart (MOV), i.e., the mass of blood involved in the circulation for 1 minute, is of decisive importance. It can be measured in three different ways:
- - according to the Fick method;
- - according to the indicator dilution method;
- - using rheocardiography.
While the Fick and indicator dilution methods belong to bloody methods that require access to the vascular bed, rheocardiography belongs to non-invasive non-bloody methods of measurement.
Fick method . To determine the minute volume of the heart (MOV) according to the Fick method, it is necessary to measure the absorption of oxygen and the arterial oxygen content difference (avD-O 2). MOS is determined by the formula:
If we assume that there is the same absorption of oxygen, then a large difference in avD-O 2 according to this formula is equivalent to a small MOC and, conversely, a small avD-O 2 means a large MOC. Based on these relationships between avD-O 2 and MOS, some authors limit themselves to measuring avD-O 2 and refuse to calculate MOS.
The oxygen content in arterial and mixed venous blood necessary to determine avD-O 2 can be measured directly or calculated from the hemoglobin concentration and oxygen saturation of arterial and mixed venous blood. For this determination, blood must be taken from a. pulmonalis and from the artery of the systemic circulation (Fig. 3.5).
To determine oxygen consumption, it is necessary to measure the oxygen content in the inhaled and exhaled air. For this purpose, it is best to collect air in breathing gas bags (Douglas bags). The Fick method is characterized by high measurement accuracy, which becomes even more accurate with decreasing MOC. Thus, the Fick method for measuring MOS in shock is the most appropriate. It is not suitable only in the presence of defects - shunts, since then part of the blood does not pass through the lungs. The technical costs of measurement, especially given the need to determine the oxygen content of the inhaled air, are so significant that they make the Fick method for practical control in shock rarely applicable.
Indicator dilution method . When determining the MOC by the method of diluting the indicator, a certain amount of the indicator is injected into the patient's vein, and after mixing with the blood, the remaining concentration of this indicator in the outflowing blood is determined. The introduction of the indicator and the measurement of concentration should be carried out in one of the main vascular highways (right ventricle, a. pulmonalis, aorta). With a large MOS, a strong dilution occurs, and with a small one, on the contrary, a small dilution of the indicator. If the indicator concentration curve is recorded simultaneously, then in the first case there is a slight, and in the second - a sharp rise in the curve. A prerequisite for using the method is thorough mixing of blood and indicator and avoiding any loss of indicator.
The calculation of MOS is carried out according to the formula:
MOC = Amount of indicator injected/Area of concentration curve over time
The MOC can be calculated using a small computer into which the required data is entered. Coloring substances, isotopes or cold solutions can be used as indicator substances.
In the practice of intensive care, the method of cold dilution (thermodilution) is most widely used. In this method, a cold solution is injected into vena cava superior or into the right atrium and register the change in blood temperature caused by it in a. pulmonalis(Fig. 3.6). With a catheter floating in a. pulmonalis, equipped with a temperature probe at the end, using a small computer, you can quickly calculate the MOC. The thermodilution technique has become a routine method used in the clinic at the patient's bedside. The details of the method are described below. When using the method of diluting paints, the coloring matter is injected into a. pulmonalis. The concentration of the dye is measured in the aorta or in one of the large arterial trunks (Fig. 3.7). A significant disadvantage of the dye dilution method is that the dye remains in the circulation for a long time and therefore this remaining amount of the substance must be taken into account in subsequent measurements. For the dye dilution method, a computer can also be used to calculate the MOC.
Rheocardiography . Refers to indirect non-invasive measurement methods and also makes it possible to determine the stroke volume of the heart. The method is based on registration of changes in bioelectrical resistance in the chest resulting from ischemic changes in the volume of blood in the heart. Removal of rheographic curves is carried out using circular tape electrodes, which are fixed on the neck and chest (Fig. 3.8). Stroke volume is calculated simply by the level of amplitude of the rheographic curve, by the time of expulsion of blood from the heart, by the distance between the electrodes and by the main resistance. When recording rheographic curves, certain external measurement conditions must be observed (location of the electrodes, position of the patient, breathing cycle), otherwise the comparison of the measured values will become impossible. According to the experience gained in the clinic, rheocardiography is especially suitable for current monitoring in the same patient, but for the absolute determination of stroke and cardiac output in shock, it is very conditionally applicable.
Normal values . The normal values of MOS at rest, depending on the height and weight of the patient's body, are 3-6 l/min. With significant physical exertion, MOS increases to 12 l / min.
Since there are close relationships between growth and MOS value, it is recommended to take into account the corresponding surface of the patient's body when obtaining data on MOS. With this kind of recalculation, the measured value of MOS is divided by the value of the body surface, obtaining the so-called index of cardiac output, or more simply, the cardiac index, which indicates the value of MOS per 1 m 2 of body surface. The normal values of the MOS index are at rest 3-4.4 l/min m 2 . The surface of the body is determined by the nomogram of the values of height and body weight. According to the MOS index, there is also a stroke volume index. In the same way, the stroke volume is recalculated to the value of the body surface in 1 m 2. Normal values are 30-65 ml per 1 m 2 of body surface.
During the initial phase of shock, MOS should be measured at intervals of 30-60 minutes. If, as a result of anti-shock therapy, hemodynamics is stabilized, then measurements at intervals of 2-4 hours are sufficient (Fig. 3.9).
The systolic (stroke) volume of the heart is the amount of blood ejected by each ventricle in one contraction. Along with heart rate, CO has a significant effect on the value of the IOC. In adult men, CO can vary from 60-70 to 120-190 ml, and in women - from 40-50 to 90-150 ml (see Table 7.1).
CO is the difference between end-diastolic and end-systolic volumes. Therefore, an increase in CO can occur both through greater filling of the ventricular cavities in diastole (increase in end-diastolic volume), and through an increase in the force of contraction and a decrease in the amount of blood remaining in the ventricles at the end of systole (decrease in end-systolic volume). CO changes during muscular work. At the very beginning of work, due to the relative inertia of the mechanisms leading to an increase in the blood supply to the skeletal muscles, venous return increases relatively slowly. At this time, the increase in CO is mainly due to an increase in the force of myocardial contraction and a decrease in end-systolic volume. As the cyclic work performed in the vertical position of the body continues, due to a significant increase in blood flow through the working muscles and activation of the muscle pump, venous return to the heart increases. As a result, the end-diastolic volume of the ventricles in untrained individuals rises from 120-130 ml at rest to 160-170 ml, and in well-trained athletes even up to 200-220 ml. At the same time, there is an increase in the force of contraction of the heart muscle. This, in turn, leads to a more complete emptying of the ventricles during systole. End-systolic volume during very heavy muscular work can decrease to 40 ml in untrained people, and up to 10-30 ml in trained people. That is, an increase in end-diastolic volume and a decrease in end-systolic volume lead to a significant increase in CO (Fig. 7.9).
Depending on the power of work (O2 consumption), rather characteristic changes in CO occur. In untrained people, CO increases as much as possible compared to its level m at rest by 50-60%. For most people, when working on a bicycle ergometer, CO reaches its maximum at loads with oxygen consumption at the level of 40-50% of the MIC (see Fig. 7.7). In other words, with an increase in the intensity (power) of cyclic work, the mechanism for increasing the IOC primarily uses a more economical way to increase the ejection of blood by the heart for each systole. This mechanism exhausts its reserves at a heart rate of 130-140 beats/min.
In untrained people, the maximum CO values decrease with age (see Fig. 7.8). In people over 50 years of age, performing work with the same level of oxygen consumption as 20-year-olds, CO is 15-25% less. It can be assumed that the age-related decrease in CO is the result of a decrease in the contractile function of the heart and, apparently, a decrease in the rate of relaxation of the heart muscle.
