In this article we will discuss about:- 1. Introduction to Cardiac Output 2. Normal Values of Cardiac Output 3. Distribution 4. Control 5. Methods of Measuring 6. Factors Influencing.
Introduction to Cardiac Output:
At each beat certain amount of blood is pumped out by each ventricle into the circulation. This is called cardiac output.
Two terms are used:
1. Stroke volume or systolic discharge (output).
2. Minute volume (minute output).
Stroke volume means the output per ventricle per beat. Minute volume means the output per ventricle per minute. Hence, minute volume = stroke volume × heart rate. As the volume of blood put out by both sides of the heart is same, cardiac output is to be multiplied by 2 so as to calculate the quantity of blood pumped by the heart as a whole.
Normal Values of Cardiac Output:
In adults the average stroke volume is 70 ml and the minute volume about 5-6 litres. In other words, the amount of blood expelled per ventricle per minute, is approximately the same as the total blood volume of the body.
The output is directly proportional to the metabolic rate, and as such, to the surface area and body weight. The cardiac output per minute per square metre of body surface is known as cardiac index. The average value is 3.3 litres. The surface area of an average-sized adult is about 1.7 m2.
Accordingly, the average cardiac index is about 3.3 litres/min/m2 (5.6/1.7).The stroke volume per square metre of body surface is known as stroke volume index. The average value is 47 ml. Anything that increases or diminishes basal metabolic rate, body weight or surface area, also alters the minute volume proportionally.
Distribution of Cardiac Output:
Since venous return per minute should be the same as the minute output, it follows that blood flow through the tissue per minute must also be the same. In other words, 5 litres of blood passes out per ventricle per minute, 5 litres of blood flows through the tissues per minute and the same 5 litres come back to heart per minute. It is really astonishing, how accurately these constant relations of time and quantity are maintained.
Although full data are not known, yet the minute volume of heart is mainly distributed as follows:
i. Kidneys —1,300 ml per minute.
ii. Brain — 700-800 ml per minute.
iii. Coronary—200 ml per minute.
iv. Muscle—600-900 ml per minute.
v. Liver— about 1,500 ml per minute.
Total quantity of blood distributed in these organs does not exceed 4,500 ml per minute. So the remaining amount is distributed to the skin, bones and gastro-intestinal tract.
Translocation of Blood:
For a short period of time, left and right ventricles may pump different amount of blood. When the cardiac output of left ventricle is greater than that of the right, there is translocation of blood from pulmonary circulation to systemic circulation. If the cardiac output of right ventricle is greater than that of the left, there may be translocation of blood to the pulmonary vessels.
It is the capacity of heart to generate sufficient energy for expelling a large quantity of blood and for raising blood pressure above the basal level during emergency. Generally, the normal heart expels about 5 to 6 litres of blood per minute per ventricle and during exercise this amount may be about 30 to 40 litres per minute. According to Starling’s law, it is nothing but physiological capacity of the heart.
Control of Cardiac Output:
Cardiac output depends upon the following four factors:
1. Venous return.
2. Force of heartbeat.
3. Frequency of heart beat.
4. Peripheral resistance.
1. Venous Return:
Anything that increases or diminishes the venous return will alter the cardiac output accordingly.
Venous return depends upon the following:
i. Muscular Exercise:
When muscles contract, they squeeze the capillaries and venules and help in venous return. This is aided by the valves of veins, which prevent the passage of blood back towards the capillary bed.
During inspiration intrathoracic pressure falls and intra-abdominal pressure rises. Hence, with each inspiration venous blood is sucked up by the thorax and is pumped out by the abdomen.
iii. Pressure Difference between Capillaries and Venules:
Normally, there is a slight positive pressure (32-12 mm of Hg) in the capillary area (capillary tone), while in the great veins it may be even negative. Vascular dilatation without fall of general blood pressure will increase the capillary pressure and thereby raise the venous return (viz., muscular exercise). But if it causes a general fall of blood pressure-as in shock, venous return will fall.
iv. The vasomotor system adjusts the lumen of the arterioles and venules and thereby alters the venous return.
2. Force of Heart Beat:
The strength of contraction depends mainly on three factors:
i. The Initial Length of the Cardiac Muscle:
Within physiological limits, greater the initial length, stronger will be the force of contraction [Starling’s law (Fig. 7.81)—which is an inherent, self-regulating mechanism that permits heart to adjust to changing end-diastolic volumes]. It is obvious that the initial length is proportional to the degree of filling, which again, depends on the venous return.
ii. The Length of Diastolic Pause:
Filling, rest and recovery take place during diastole. Hence, with a shorter diastolic period which is inadequate for these, the force of contraction will diminish unless the rate of venous return is raised.
iii. Nutrition and Oxygen Supply:
An adequate supply of nutrition and oxygen is essential for efficient cardiac activity. In addition to this an optimum H-ion concentration, a proper balance of inorganic ions and an appropriate temperature and pressure, are also required for strong heartbeat.
3. Frequency of Heart Beat:
Heart rate affects both stroke volume and minute volume by altering the length of diastole and thereby the degree of filling and force of contraction. It should be noted that blood pressure depends upon the minute volume and not on the stroke volume. The following consideration will clarify. Venous return remaining constant, the rise of heart rate will reduce the diastolic pause and therefore the stroke volume.
But the product-stroke volume x heart rate-may not fall; even it may rise above the resting value. Thus minute volume and therefore blood pressure (B.P) may rise even if the stroke volume falls. This happens with a moderate rise of the heart rate. But if the heart rate be too high, the stroke volume becomes so low that the minute output falls far below the normal (Fig. 7.82).
Blood pressure drops and the subject may be unconscious. This happens in paroxysmal tachycardia when the frequency suddenly becomes 150-200 per minute. [Muscular exercise is an exception. Here, both the frequency of heart beat and the rate of venous return increase. Cardiac filling becomes more than normal even during the short diastolic period. Hence both stroke volume and minute output increase.]
On the other hand, when the heart rate becomes very slow (as in Heart Block)—although the stroke volume is much bigger than normal, yet the minute volume may fall, because the product may be less than normal. But with a moderate slowing the minute volume may not fall at all. In some instances it may rise (recovery from heart failure). Thus alteration of heart rate on either side will generally raise the minute volume up to a certain extent. Beyond that, the minute output will fall.
4. Relation with Peripheral Resistance:
Heart maintains a constant cardiac output and blood flow even against increased peripheral resistance. An optimum blood pressure is essential for adequate cardiac activity. General vasoconstriction of the arterioles will cause an increase in blood pressure. Heart at first fails to expel all its blood but in the next heart beat the filling becomes more, because the normal venous return is added upon the residual blood. Consequently, the initial length becomes bigger, the heart contracts with greater force and the normal output is restored.
Methods of Measuring Cardiac Output:
In animals the output can be measured with the help of:
ii. Heart-Lung preparation (Fig. 7.83)
iii. Dye method
iv. Fick principle using O2 or CO2
v. Physical method (Ballistocardiography).
Knowlton-Starling Heart-Lung Preparation:
This is a very useful method for studying cardiac output under different experimental conditions, viz., variation of pressure, temperature, H-ion concentration, inorganic ions, etc.
The procedure is as follows:
Thorax of an animal is opened, heart is exposed under anaesthesia. Artificial respiration is maintained. Both vagi are severed to prevent the variation of heart rate. The branches arising from the arch of the aorta, viz., the branchiocephalic and left subclavian arteries (in the dog) are ligated. A three-way cannula is inserted in the brachiocephalic artery. The cannula is connected with a mercury manometer to record the mean arterial pressure and through another three-way tube with a bottle containing air.
The air of the bottle due to its compressing capacity serves the purpose of elasticity of the arterial wall. The other limb of the three way tube is connected with a glass tube lined with a thin-walled rubber tube. The pressure inside this tube can be varied at will. This is an artificial resistance. The glass tube has got two side tubes—one of which is connected with a manometer for recording the pressure inside it and the other is connected with the pressure bottle from where air may be pumped inside it.
This rubber-lined glass tube is connected with a spiral heater which opens in the venous reservoir filled with heparinised blood. The reservoir is connected through a rubber tube with a three-way cannula one limb of which inserted into the superior vena cava. A thermometer is kept in the cannula to record the temperature. The inferior vena cava is connected with a water manometer to record the pressure of the right atrium.
There is a screw clamp on the rubber tube connecting the venous reservoir with the three-way cannula which adjusts the rate of flow from the venous reservoir into the right atrium. From the venous reservoir blood enters the right atrium, right ventricle and then through the lungs, left atrium, into the left ventricle. From the left ventricle through the brachiocephalic artery, resistance tube, spiral heater, blood comes again into the venous reservoir and the cycle is repeated.
The output of the left ventricle can be determined by opening the clamp on the tube in between the spiral heater and venous reservoir and collecting blood for specified period in a measuring cylinder. The blood flow through the coronary arteries must be added to give a correct figure of the output of the left ventricle. This experiment does not give any indication regarding the output in the human beings or in the intact animal.
Stewart and Hamilton’s Dye Dilution Method:
There are certain difficulties invariably in introducing cardiac catheterisation. So the dye method is often the method of choice. A known quantity of Evans blue (non-diffusible dye, known as T-1824) is injected into the basilic vein. The dye will circulate through the heart, lungs and appear in the carotid artery.
The concentration of the dye when it first appears in the artery is determined with the help of colorimeter from the samples of arterial blood taken every few second interval. The concentration of each sample of blood is determined by photo-colorimeter and plotted on a semi-logarithmic paper.
The dye concentration rapidly rises to a peak, then falls and rises again due to recirculation of dye. The mean concentration of the dye is mined with the help of the following formula; F = D/ct, where F = volume flow in litres per second, D is the quantity of dye (Evans blue) injected, c is the mean concentration of the dye and t is the duration in seconds of the first passage of dye through the artery.
It was shown by Fick that cardiac output can be calculated by noting certain data about O2 or CO2 exchange.
When applied to oxygen it will be as follows:
O2% of mixed venous blood is determined, say 15 ml.
O2% of arterial blood is determined, say 19 ml.
Arteriovenous O2 difference per 100 ml of blood is therefore, 4 ml.
Total O2 consumption per minute (Douglas bag) say 200 ml.
Hence each 100 ml of venous blood while passing through lungs, takes away 4 ml of O2. Therefore, 200 ml of O2 will be carried away by 100/4 × 200 ml = 5 litres of blood. Obviously, this is the minute output of right ventricle, which is same as that of left ventricle. Similar figures will be obtained by applying the principle to CO2.
So the cardiac output can be determined with the help of the following formula:
In man there is difficulty in applying direct Fick method during heavy exercise. So the other method, viz., Dye method is preferred.
i. Fick Principle using O2:
(a) Total O2 consumption per minute is determined with Douglas bag.
(b) O2 content of arterial blood is calculated from Hb% assuming 95% saturation, and
(c) A sample of mixed venous blood is collected from the right atrium by introducing a fine rubber catheter into the cubital vein and pushing it gradually into the right atrium (Cournand’s method) and its O2% is determined. From these data minute volume can be calculated as shown above.
ii. Fick Principle using CO2:
This method can be readily adopted at the bedside.
The steps are as follows:
(a) CO2 output per minute is determined with Douglas bag.
(b) Alveolar air is collected and its CO2 tension— which is identical with that of arterial blood, is determined, and
(c) Alveolar air is again collected after holding the breath for 5 seconds. The CO2 tension of this air is same as that of venous blood.
CO2% of arterial and venous blood is then determined from the CO2 dissociation curves. From these data minute volume can be calculated.
Another method of determining cardiac output in human beings is the Ballistocardiography which was originally devised by Henderson and later on modified by Starr and his associates. The method is based on the principle of Newton’s third law of motion —”every reaction has an equal and opposite reaction.”
Ballistocardiogram is a record of the recoil of the body caused by the movement of heart and blood within it in opposite direction. It can be recorded by allowing subject to lie on a suitably suspended table. The more convenient method is the recording of the movement of a steel rod kept on the stretched legs while lying supine on a fixed table. The movement of the rod can be recorded by suitable sensitive electronic instrument.
The normal Ballistocardiogram shows following distinct waves; H I J K L M N, of which H I J is the systolic waves. H wave is the small positive wave and occurs due to head-ward movement of the body in response to foot-ward movement of the heart and blood within it during isometric phase of systole. I wave is the negative wave and is inscribed due to foot-ward movement of the body in response to ejection of blood into the aorta during ejection phase. J wave is a large positive wave occurring during head-ward movement of the body due to rapid flow of blood through descending arota (Fig. 7.84).
The stroke volume can be calculated by applying the formula:
Where I and J are areas of the waves, A is the diameter of the aorta and C is the duration of cardiac cycle. Stroke volume multiplied by the heart rate gives the minute output. This method is not accurate.
Factors Influencing Cardiac Output:
i. Muscular Exercise:
In heavy exercise the output may be 30-40 litres, i.e., 6-10 times the normal minute volume (stroke volume 170-200 ml; heart rate 150-180 per minute).
The minute volume is greater in the recumbent posture than in standing, because gravity retards venous return in the latter.
iii. Fever, Hyperthyroidism, Excitement:
(10-25%) adrenaline, ingestion and digestion of food (10-20%), anoxia, CO2 excess, intravenous (I.V.) saline, pregnancy (45-85% of full term), etc., increase cardiac output.
iv. Hypothyroidism, Hemorrhage, Shock, Heart-Failure:
Hypothyroidism, hemorrhage, shock, heart-failure, etc., reduce cardiac output.