Cardiac output is termed as the measurement of blood that the heart delivers per minute to the aorta. For a normal human being cardiac output is in the range of 4 – 6 liters per minute. During each heart beat blood pumps around 70 to 100 ml of blood. Blood is pumped from the right ventricle into pulmonary circulation. At the same time, some measurable amount of blood is pumped into systemic circulation from left ventricle. Cardiac output can be given as product of stroke volume and heart rate.
It is given as the speed at which heart beats. It is calculates the contractions per minute (bpm). During each ventricular contraction, blood flows through arteries. The flow is detected as one pulse. Heartbeat is affected due to various external factors like age, diseases, exercise, temperature and emotions.
Stroke volume is defined as amount of blood being pumped regularly from left ventricle per beat. Factors affecting stroke volume are blood volume in the body, heart contractility and resistance level from blood vessels. Every change in stroke volume affects the blood pressure.
Conditions Affecting Cardiac Output
No change: Sleep and Moderate temperature changes.
Increased: Anxiety, Exercise, Pregnancy, Anemia.
Decreased: Standing / Sitting, Rapid Changes in Heart Beat, Heart Disease.
Indirect measurements are employed to measure cardiac output as given below.
- Fick’s Method
- Indicator dilution method
- Thermo dilution
- By impedance change
In this method, analysis of gas keeping of the organism is considered to measure cardiac output. Oxygen is continuously infused into the blood or it is removed from the blood. The amount of oxygen left out in the blood before and after the infusion of oxygen into the blood is measured. Let I be the amount of oxygen infused inside or removed per unit time. It will be equal to the difference between the amount of blood that arrives and departs from the measurement point.
Where, Q – Measured cardiac output in liters/minute
CA – Oxygen concentration in arterial (outgoing) blood
CV – Oxygen concentration in venous (incoming) blood
Indicator Dilution Method
In this method, a known amount of dye (indocyanine – cardio green dye) acts as an indicator in the blood circulation. It uses rapid injection. The concentration of indicator per unit time is measured at the measurement point. Doing this helps us to estimate the volume of blood flow. Assume M mg of indicator is injected into the vein or into the right heart directly. From right heart, the dye passes to the lungs, left heart and finally appears at the arterial circulation. An indicator is used at the peripheral artery. A measuring chamber is used to draw an amount of blood from artery and detector continuously analyses the blood samples. The concentration of the indicator is directly related to the detector output. Finally, the detector output is displayed on the chart recorder with respect to the unit time. The major disadvantage of this method is injection of foreign substance into the human blood.
Thermo Dilution Technique
In dye dilution, technique the main drawback is the presence of foreign body inside the blood. Even without taking the blood out for sample analysis, the photoelectric detector can be used to measure the dye concentration. Even radioisotope can be used if radiation detector is used. Again, the foreign body accumulation in the body creates a problem. To avoid this entire problem, Thermo Dilution Techniques are used. At a room temperature 10 ml of 5%, dextrose in water is injected as dye (indicator) into the right atrium. A thermistor is attached to the tip of the catheter. The indicator is mixed and at the pulmonary artery, thermistor is used to detect the temperature. At the pulmonary artery, the thermistor measures the difference in temperature of the dye during injection and its temperature after circulating in the blood. Temperature will be reduced and it is integrated with respect to time. Calibration and corrections are applied to the obtained value, which finally gives the cardiac output.
Where, Q = cardiac output per second
V = volume of dye injected into the body
ρ = density
S = specific heat
T = temperature in oC
= integral of blood temperature change
Subscript i – injectate and b is blood
From the block diagram, to reduce the non-linearity of the thermistor, a resistor is connected parallel to it. This helps to attain a linear relation between temperature and resistance of thermistor. After linearizing, the value is sent to the linearizing amplifier. Next block is the integrator block. This block calculates the integral change of blood temperature with respect to time. The data about density, specific heat capacity of blood and thermal indicator, volume of dye injected is fed to the system. Finally, computer displays the cardiac output in liter / minute.
Impedance Change Technique
Here cardiac output is measured electronically. Four electrode probes are used around the thorax. Electrode pair 1 and 4 acts as current electrode and other pair 2 and 3 picks voltage across the thorax. Assume ρ to be resistivity of the patient, A cross sectional area of thorax, L is the separation length between the 2 and 3 electrodes. Therefore, the resistance of thorax is given as