Cardiac output and venous return

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.

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

Cardiac output = Stroke volume x Heart rate

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

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

Cardiac output = Stroke volume x heart rate

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)

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

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.

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

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

Peripheral resistance Cardiac output is inversely related to total peripheral resistance

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

Conditions resulting hypoeffective heart HTN IHD Valvular heart disease Myocarditis

Specific conditions

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

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

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

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

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.

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.

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

How to measure cardiac output Echocardiography Fick principle Dye dilution method

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

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

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