CARDIAC OUTPUT :
Introduction :-
The amount of blood pushed out of each ventricle is known as cardiac yield. More often than not, it’s utilized to depict cleared out ventricular surge by means of the aorta. Since cardiac yield decides the rate at which blood streams through different real parts, it is the most critical component of the cardiovascular framework.
Heart output is often stated in one of three ways:
1. Stroke volume
2. Minute volume
3. Cardiac index.
1. Stroke volume :-
The volume of blood that each ventricle pumps out during a beat is known as the stroke volume.
When the heart rate is normal (72/minute), the usual value is 70 mL (60 to 80 mL).
2. Minute volume :-
The volume of blood pumped out by each ventricle in a minute is known as the minute volume. It is the result of multiplying heart rate with stroke volume:
Stroke volume × Heart rate equals minute volume.
5 L/ventricle/minute is the typical value.
3. Cardiac index. :-
The minute volume expressed in square meters of body surface area is called the cardiac index. It can be expressed as the amount of blood pumped per ventricle, minutes or square meters of body.
For an adult with an average body surface area of 1.734 square meters and a normal minute volume of 5 liters/min, the normal value is 2.8 ± 0.3 liters/m2 body surface area/min.
Distribution of Cardiac output :-
Blood flow through the tissue per minute must likewise be equal to the venous return per minute, which should match the minute output. Put otherwise, five liters of blood leave each ventricle every minute, five liters go through the tissues every minute, and five liters return to the heart every minute. The accuracy with which these consistent relationships between quantity and time are preserved is truly astounding.
The minute volume of the heart is primarily distributed as follows, notwithstanding the fact that complete data are unknown:
1. Kidney flow rate: 1,300 ml/min
2. Brain: between 700 and 800 ml/min
3. 200 ml per minute for the coronary
4. Muscular: 600–900 ml/min
5. Liver: 1,500 ml per minute on average
The total volume of blood flowing through these organs in a minute is not more than 4,500 milliliters. Thus, the skin, bones, and gastrointestinal system receive the remainder.
Cardiac Reserve Values :-
The maximum amount of blood the heart can pump beyond normal is called cardiac reserve.
An important factor in increasing cardiac output under circumstances such as physical activity is cardiac reserve.
The ability to tolerate physical activity is very important.
Cardiac reserve is usually expressed as a percentage.
Young, healthy adults have a cardiac reserve of 300 to 400 percent. As we age, it is about 200-250%.
In athletes, it increases to 500-600%. In heart disease, cardiac reserve is minimal or nonexistent.
Factors Influencing Cardiac Output :-
Understanding the variability in cardiac output is important for understanding how the heart adapts to different physiological and pathological conditions. Here we investigate factors that influence cardiac output and the impact of its variability.
Physiological Variations :-
1. Age: Because their blood volume is less, youngsters have a lower cardiac output. Adults’ cardiac indices are higher due to their smaller bodies.
2. Sex: Because females have smaller blood volumes than males, their cardiac output is lower. Due to a smaller body surface area than in men, the cardiac index is higher in women.
3. Body build: Higher cardiac output is correlated with a larger body build.
4. Diurnal variation: Early in the morning, cardiac output is low, and over the day, it increases. It is dependent upon each person’s baseline circumstances.
5. Ambient temperature: A slight variation in temperature has no effect on cardiac output. Heart rate increases with temperature exceeding 30°C.
6. Emotional states: The release of catecholamines, which raise heart rate and contraction force, causes anxiety, apprehension, and excitement to enhance cardiac output by 50% to 100%.
7. After meals: Heart rate rises throughout the first hour following a meal.
8. Exercise: Due to a rise in heartbeat and contraction force, exercise causes an increase in cardiac output.
9. High altitude: An increase in adrenaline secretion causes an increase in cardiac output at high altitudes. Hypoxia (low oxygen levels) stimulates the release of adrenaline.
10. Posture: The cardiac output falls when shifting from a recumbent to an upright posture.
11. Pregnancy: There is a 40% increase in cardiac output in the final months of pregnancy.
12. Sleep: During sleep, cardiac output either slightly decreases or remains constant.
Pathological Variations :-
1. Heart Failure: The heart’s capacity to pump blood productively is hampered by heart disappointment, which brings down cardiac yield. Expanded heart rate and liquid maintenance are compensatory measures that at first keep CO levels steady but eventually make the illness worse.
2. Shock: Shock conditions counting hypovolemia, cardiogenic stun, septic stun, and others. They are recognized by seriously moo cardiac yield, inadequately tissue perfusion, and disappointment of the organs. The basic causes can shift from distributive anomalies and broad vasodilation to serious liquid misfortune and heart pump disappointment.
3. Arrhythmias: Heart rhythm abnormalities can have a major effect on cardiac output. Bradyarrhythmias cause a direct reduction in HR and Cardiac Output, but tachyarrhythmias may cause a reduction in stroke volume because of inadequate ventricular filling time.
4. Cardiomyopathies: Conditions that affect the heart muscle, such as hypertrophic and dilated cardiomyopathies, change the heart’s compliance and contractility, which affects cardiac output.
Maintaining Factors of Cardiac Output :-
Four elements maintain (determine) cardiac output:
1. Venous return
2. Heartbeat force
3. Heart rate
4. Resistance at the periphery
1. Venous return :-
Anything that modifies the venous return will cause the cardiac output to change correspondingly. Venous return will depend on the following variables:
1. Exercise with muscles: Squeezing the capillaries and venules causes an increase in venous return. Vein valves, that keep blood from flowing back into the capillary bed, help with this.
2. Respiration: Intraabdominal pressure increases and intrathoracic pressure decreases during inspiration. As a result, the thorax draws in venous blood, which the abdomen then pumps out with each inspiration.
3. Disparity in pressure across venules and capillaries:
The capillary area (capillary tone) typically has a modest positive pressure (32–12 mm Hg), but in the large veins, it may even be negative. Muscular exertion causes vascular dilatation, which raises capillary pressure and the venous return without lowering blood pressure overall. Venous return will decrease, however, if it results in a general drop in blood pressure, such as in shock.
4. The venous return is changed by the vasomotor system by adjusting the lumen of the arterioles and venules.
2. Heartbeat force :-
The contraction’s intensity is mostly determined by three factors:
1. The heart muscle’s starting length: More initial length will result in a larger contraction force within physiological bounds an innate, self-regulating system that enables the heart to adapt to varying end-diastolic volumes. Heterometric control of heart rate:
It is clear that the initial length is correlated with the filling level, which is also dependent on the venous return.
2. The diastolic pause’s duration: Diastole is the time for filling, resting, and recovering. Therefore, unless the rate of venous return is increased, the power of contraction will decrease due to a shorter diastolic time, which is insufficient for these.
3. Contractility: Myocardial fibers contract more forcefully at any given length in response to sympathetic stimuli (homeometric modulation of cardiac output).
The change in peak biometric force at a specific beginning fiber length (end diastolic volume) is determined by contractility. Tachycardia or certain medications like digitalis or epinephrine can increase contractility. The produced force and contraction velocity grow incrementally as a result of the positive inotropic effect.
4. Nutrition and oxygen supply: For optimal heart activity, there must be a sufficient supply of both nutrients and oxygen. A robust heartbeat also requires the right balance of inorganic ions, the right temperature, and the right pressure, in addition to the ideal H-ion concentration.
3. Heart rate :-
1. Heart rate influences the diastolic duration, which in turn influences the contraction force and filling volume, affecting overall the stroke volume and minute volume.
2. It should be mentioned that the minutely volume, not the stroke volume, determines blood pressure.
This gets clearer when you consider the following. As long as the venous return is constant, an increase in heart rate will cause the diastolic pause and stroke volume to decrease. It is possible for the product of heart rate and stroke volume to increase and possibly surpass the resting value.
Therefore, even if the stroke volume decreases, the minute volume and, consequently, blood pressure (BP), may increase. This is accompanied by a mild increase in heart rate.
3. However, if the heart rate is excessively high, the minute output will be significantly below normal since the stroke volume would drop. There may be a dip in blood pressure and unconsciousness. This occurs when the frequency of paroxysmal tachycardia abruptly rises to 150–200 beats per minute. Exercise for the muscles is an exception. Here, the quantity of venous return and cardiac cadence both rise.
Increased cardiac filling occurs even during the brief diastolic phase. As a result, the minute output and stroke volume both rise.)
4. On the other hand, even when the stroke volume is significantly more than normal whenever the heart rate becomes extremely slow (as in heart block), the minute volume may decrease because the outcome may be lower than normal. However, the minute volume might not decrease at all with a mild slowdown. It might emerge in some cases (heart failure recovery). As a result, a change in heart rate on one or the other will typically cause the minute volume to increase somewhat. The period of output will decrease after that.
However, the relationship is flipped and the stroke volume falls when normal functioning (the shaded region) extends outside the range.
4. Resistance at the periphery :-
Even in the face of increased peripheral resistance, the heart maintains a consistent cardiac output and blood flow (afterload). Sufficient heart activity requires an ideal blood pressure. Blood pressure will rise if the arterioles are generally constricted. The heart initially cannot pump out all of the blood, but as the regular venous return is added to the remaining blood, the filling increases in the subsequent heartbeat. As a result, the initial length increases, the heart contracts harder, and the output returns to normal.
Factors Affecting Heart Circulationl-
1. Exercise for muscles: An intense workout may produce 30–40 liters, or 6–10 times the typical minute volume (stroke volume: 170-200 ml; heart rate: 150–180 beats per minute).
2. Posture: Because gravity slows the return of blood in the upright position, each minute’s volume is larger in the recumbent position.
3. Neural input also plays a role in determining stroke volume. Sympathetic inputs cause the cardiac muscle fibers to contract more forcefully at any given length, whereas parasympathetic stimuli have the reverse effect. The term “inotropic action” refers to how catecholamines released by sympathetic stimulation affect contraction strength. An increase in contraction strength without corresponding lengthening of the fibers causes a greater amount of blood to be evacuated from the ventricle, a phenomenon known as an increase in the ejection fraction.
Others Causes-
1. Elevated body temperature, hyperthyroidism, excitement (10–25%): These conditions enhance cardiac output, as do adrenaline, meal consumption and digestion (10–20%), anoxia, CO2 excessive amounts, IV (intravenous) saline, pregnancy (45–85% of full term), etc.
2. Heart failure, shock, hemorrhage, hypothyroidism, and other conditions lower cardiac output.
3. Sleep: Usually not much less, although may decrease significantly.