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