Fick’s Principle Of Cardiac Output Measurement

Fick’s Principle Of Cardiac Output Measurement

Define the terms cardiac output and cardiac index. Mention methods for mention the factos measuring cardiac output that influence cardiac output? (or) Explain Fick’s Principle. (or) What is normal cardiac output? How it is determined in man?
Answer:

Definitions:

1. Cardiac Index:

• It is defined as the amount of blood pumped out of ventricle per minute per square meter of the body surface area.

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Methods for measuring cardiac output:

1. Direct methods – used in animals.

• By using cardiometer.
• By using flow meter.

How to measure cardiac output using Fick’s principle

2. Indirect methods – used in animals and humans.

• By using Fick’s principle.
• Indicator or dye dilution method.
• Thermodilution technique.
• Ballistocardiography.

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Direct fick method:

• According to Fick’s principle, the amount of a substance taken up by an organ (or by the whole body) or given out in a given unit of time is the product of amount of blood flowing through the organ and the arteriovenous difference of the substances across the organ.
• Amount of substance taken or given = Amount of blood flow/min arteriovenous differene.
• This principle can be used to determine cardiac output.
• Cardiac output = \(\frac{Amount of substance taken/given by organ/minute}{Arteriovenous difference of substance across the organ}\)

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Dye-dilution method:

• A solution containing 5 mg of dye is injected rapidly into a vein.
• Immediately after the injection, samples of arterial blood are taken serially at intervals of 0.5 – 2 sec.
• Each is analysed for concentration of dye.
• Values are plotted on semi-logarithmic paper with concentration of dye on Y-axis and time on X-axis.
• The resultant curve shows.
  1. Initial increase in concentration of dye till it reaches a peak.
  2. Then steadily declines.
  3. Next, rises again indicating recirculation of the blood containing the dye.

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