Cardiac output or minute volume in animals
Cardiac output or minute volume in animals is the volume of blood ejected by either the left or the right ventricles into the greater vessels, the aorta or pulmonary artery in a minute.
Cardiac Output (L/min) → Stroke Volume × Pulse Rate
In most animals, the cardiac output is 10% of body weight in kgs. Cardiac out put of right ventricle will be normally equal to that of left ventricle. If this minute volume is expressed on the basis of body surface it is referred to as cardiac index (L / m2 / min).
Cardiac Index → Minute Volume / Surface area of the Body
Venous return is the quantity of blood flowing from the great veins, vena cava in to the right atrium each minute.
Determination of cardiac output
Cardiac output can be measured by- Direct Fick method, Indicator dilution method and Flowmeter method.
1. Direct Fick method
Fick’s principle- The amount of substance taken up by the circulation per unit time equals the arterial level of the substance minus the venous level times the blood flow (Cardiac output). According to Fick’s principle, cardiac output can be determined by measuring oxygen consumption/ minute and arterio-venous oxygen difference.
Samples of blood are collected from artery and mixed venous blood sample drawn from right ventricle or pulmonary artery using catheter. Oxygen consumption/ minute is determined by spirometry.
Amount of O2 present in both arterial and venous samples are determined and the difference in the O2 content, the atrio-venous O2 difference is expressed as per cent volume.
2. Indicator dilution method
Indicator dilution method method is based on the principle that if a known quantity of a substance is mixed with an unknown volume of fluid the volume can be determined from the concentration of the substance in the fluid. The volume equals the amount of substance (in milligrams) divided by concentration of substance (in milligrams/litre).
A known amount of indicator (Cardio green dye) is injected into a vein; arterial blood is sampled continually or at short intervals, frequently. The average concentration of dye in arterial blood is determined by the dye dilution curve, which is drawn using the sampling time interval and the log of dye concentration for each time sample. The duration of the curve for one complete circulation of the dye is determined from the curve.
3. Flowmeter method
Magnetic flowmeter method- The electromagnetic flowmeter used measures the electromotive force generated when the blood flowing within a vessel of known diameter passes through a magnetic field at right angles to the magnetic lines of force. Cardiac output is determined by placing the flowmeter probe around the root of aorta.