1. Cardiac output and venous return

2. Cardiac output
Cardiac output is the quantity of blood pumped into the aorta each minute by the heart. This is also the quantity of blood that flows through the circulation.
For young, healthy men, resting cardiac output averages about 5.6 L/min.
For women, this value is about 4.9 L/min.
The average cardiac output is 5 L/min.

3. Cardiac output varies widely with the level of activity of the body.
The following factors, directly affect cardiac output:
(1) the basic level of body metabolism,
(2) whether the person is exercising,
(3) the person’s age,
(4) size of the body

4. Cardiac output = Stroke volume x Heart rate

5. Cardiac index
Cardiac output per square meter of body surface area is cardiac index.
The normal human being weighing 70 kilograms has a body surface area of about 1.7 square meters, which means that the normal average cardiac index for adults is about 3 L/min/m2 of body surface area

6. Cardiac output increases in proportion to the surface area of the body
At 10 years Cardiac index = 4 liter/meter sq
At 80 years Cardiac index = 2.4 liter/meter sq

8. Cardiac output = Stroke volume x heart rate

10. Factors affecting cardiac output
Age and Sex
Cardiac factors
Nervous control
Condition of myocardium
Non cardiac factors
Metabolic demands of the tissues
Venous return (Pre load)
Total peripheral resistance (After load)

11. Autonomic Nervous System
sympathetic and
Parasympathetic
Condition of myocardium
Athlete’s heart

12. Venous return The Frank-Starling law of the heart,
This law states that when increased quantities of blood flow into the heart, the increased blood stretches the walls of the heart chambers. As a result of the stretch, the cardiac muscle contracts with increased force, and this empties the extra blood that has entered from the systemic circulation.

13. Increase venous return also increases heart rate by
Stretch of SA node
Bainbridge reflex

14. Metabolic demands of the tissues
Greater is the metabolic demand greater is the cardiac output and blood flow to that organ
Exercise

15. Peripheral resistance
Cardiac output is inversely related to total peripheral resistance

16. Conditions resulting in hyper effective heart
Sympathetic stimulation
Hypertrophy of heart in marathon runner

17. Conditions resulting hypoeffective heart
HTN
IHD
Valvular heart disease
Myocarditis

19. Specific conditions

20. Pathological causes of high cardiac output due to decreased peripheral resistance
Beriberi
AV fistula
Anemia
Hyperthyroidism

22. Causes of low cardiac output
Cardiac causes
IHD
Valvular heart disease
cardiomyopathies
Non cardiac causes
Loss of blood volume
Vasovagal attack
Obstruction of veins
Decreased muscle mass
Decreased rate of tissue metabolism
Acute venous dilation

23. Effect of extra cardiac pressure on cardiac output
Increase in intra-pleural pressure
Cardiac tamponade
Opening of thoracic cage
Positive pressure breathing
Negative pressure breathing

25. Venous Return
Venous return is the quantity of blood flowing from the veins into the right atrium each minute.

26. Venous return
Three principal factors that affect venous return to the heart from the systemic circulation.
1. Right atrial pressure, which exerts a backward force on the veins to decrease flow of blood from the veins into the right atrium.
2. Degree of filling of the systemic circulation (measured by the mean systemic filling pressure), which forces the systemic blood toward the heart
3. Resistance to blood flow between the peripheral vessels and the right atrium.

27. Venous return can be calculated by the following formula:
Venous return = Psf– PRA/RVR
VR is venous return,
Psf (mean systemic filling pressure)
PRA (right atrial pressure)
RVR (resistance to venous return).
In the healthy human adult, the values for these are as follows:
venous return equals 5 L/min,
mean systemic filling pressure equals 7 mm Hg, right atrial pressure equals 0 mm Hg, and
resistance to venous return equals 1.4 mm Hg per liter of blood flow.

32. Factors regulating blood flow within the veins
Right atrial pressure
Compression by surrounding tissues (Intra-abdominal pressure as in pregnancy)
Venous pump or Muscle pump and venous valves
Venous tone
Respiratory pump
Effect of gravity

33. How to measure cardiac output
Echocardiography
Fick principle
Dye dilution method

34. Fick principle Amount of oxygen in arterial blood = 200ml/l
Amount of oxygen in venous blood = 160 ml/l
Arteriovenous difference = 40ml
When blood enters lungs
40 ml is added to = 1 l
1 ml is added to = 1/40
200 ml is added to = 1 x200/40 = 5 l

35. Cardiac output = oxygen absorbed/min by lungs/arteriovenous oxygen difference

36. Dye dilution method A known amount of dye is injected into an arm vein
Serial concentration of dye are calculated from an artery and cardiac output is calculated as