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Introduction

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Cardiac output (CO) can be determined by indicator dilution (typically thermodilution), where CO is a function of the quantity of the indicator divided by the area under the dilution curve as measured at a downstream location. In practice, a bolus of cold fluid is injected into the circulation in the vena cava through the pulmonary artery catheter (PAC) and the area is measured under the temperature-change curve in the pulmonary circulation at the PAC balloon tip. This chapter will describe the bolus method to characterize the general technique of thermodilution, although continuous CO measurement is increasingly prevalent.

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Definitions and Terms

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  • ▪  Thermodilution: Measurement of blood flow in the circulation based on an induced change in the heat content of blood flowing downstream from the heat change.
  • ▪  Thermistor: A temperature sensing resistor integrated into a PAC.
  • ▪  Bolus CO: In which a bolus of indicator (typically cold saline) is injected into the circulation as the indicator.
  • ▪  Continuous CO: In which the blood is heat in pulses upstream of the thermistor and CO is determined by a mathematical transformation using the heat current changes (continuous cardiac output calculation is performed automatically by a computer attached to the pulmonary arterial catheter).
  • ▪  Fick method: An approach to calculating CO relying on assumptions about patient systemic oxygen consumption, using the following formula:
    • —Cardiac out put = ((125 mL/min oxygen × body surface area (m2)/(arteriovenous oxygen difference)) × 100

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Techniques

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