Total cardiac output

Methods

A. Flowmeters

• Electromagnetic
  * Based on Faraday's law of electromagnetic induction
• USS
  * Based on Doppler Shift

B. Fick principle

• Direct Fick method
  * use lung's O2 uptake
• Indirect Fick method
  * estimate CO
• Dye-dilution
• Thermodilution

C. Imaging techniques

• Radiographic ventriculography
• Radionuclide ventriculography
• Echocardiography

D. From arterial pressure

• Wesseling method
  * based on Ohm's law (CO=MAP/SVR)

Direct Fick method

Based on Fick principle

i.e. Blood flow
= uptake / a-V concentration difference

Will require:

• Oxygen consumption (i.e. uptake by body tissues)
  --> measured from oxygen uptake from lung
• CaO2
  --> measured from arterial blood
• CvO2
  --> measured from mixed venous blood sampled using pulmonary artery catheter

Therefore,

Cardiac output
= O2 consumption / CaO2-CvO2
= 250(mL/min) / [200 (mLO2/L) -150 (mLO2/L)]
= 250/50
= 5 Lmin

Thermodilution method

Application of Stewart-Hamilton indicator dilution technique
* (modification of Fick principle)

Method

A double-lumen pulmonary artery catheter put in through peripheral vein

--> Advance the tip into the pulmonary artery

At this position,

The cathether opening would be a few inches proximal to the tip, in the right atrium.

Then,

Cold saline of known volume injected

--> Cold saline mixes with blood and temperature change is detected downstream by the thermistor at the catheter tip.

Calculation when dye is used

Quantity of dye used (n)
= mean concentration (c) x Q (t1-t2)

i.e. n = cQ(t1-t2)

Also, mean concentration
= area under curve/t2-t1

Therefore,

Q = n / area under curve

Need to correct for

• Specific gravity of fluid compared to blood
• Specific heat of saline and blood
• Initial blood temperature and initial injectable temperature

Advantage

• Arterial puncture not necessary
• Negligible recirculation
  * Cooled blood warmed up by the time it passes thermister for the second time
• Non-toxic
• Can be repeated rapidly

Disadvantage

• Some heat exchange also take place between blood and the surrounding structures
• Speed of infusion of cold saline might not be uniform