Chapter 39

### Introduction

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.

### Definitions and Terms

• ▪  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

### Techniques

• ▪  Manual thermodilution bolus CO:
• —A predetermined bolus (5-10 cc) of saline of known temperature (as based on the measurement of the injectate temperature) is injected into the central venous pressure (CVP) port of PAC (Figures 39-1 and 39-2).
• —A CO-analysis computer characterizes the heat change of the blood at the PAC tip as a curve (Figure 39-3), and the area under the curve is used to calculate CO by dividing that area into the amount of indicator, consequently:
• • Lesser area under the curve is consistent with faster blood flow and higher CO (Figure 39-4).
• • Greater area under the curve is consistent with slower blood flow and lower CO (Figure 39-5).
• —A series of determinations are typically averaged together to determine CO after discarding unreliable measurements (Figure 39-6).
Figure 39-1.

Showing injection of indicator in the right atrium or vena cava and flow through the heart.

Figure 39-2.

Showing injection and sensing of the indicator.

Figure 39-3.

Shows a normal CO curve (note that although dye temperature may be lower than that of blood, the output curve is typically represented as a temperature curve above the baseline).

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