Critically analyse two commonly used techniques for the measurement of cardiac output.

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College Answer

Commonly used* techniques to measure cardiac output are few. Many techniques can be used, 
these include: intermittent thermodilution (inject cold fluid•), "continuous" thermodilution 
(heated*), use of doppler ( echocardiograhic probes• : transthoracic, trans-oesophageal. suprasternal, 
oesophageal), transthoracic bioimpedance•, and calculated using the Fick equation• (intermittent 
mixed venous oxygen, continuous SVO2 catheter), and variations on arterial waveform analysis. 
Appropriate critical analysis will include balances between advantages and disadvantages 
(mcluding invasiveness, insertion, limitations, misinterpretation, other data obtained etc.), and detail 
of accuracy (bias and precision) as well as indicator of reproducibility ( eg .. coefficient of variation).

Discussion

Again, a question which would be well suited to a tabulated answer.

Technique of cardiac output monitoring

Advantages

Disadvantages

PA catheter

“Gold standard” of CO monitoring
Easy to insert
Interpreter-independent

Risk of vascular access
Unreliable with septal defects or tricuspid regurgitation
Thrombotic complications
Potential for valve damage

PiCCO

Easy to insert
Interpreter-independent

Risk of vascular access
Unreliable with septal defects , tricuspid regurgitation or arrhytmia
Invalidated by rapid changes in vascular tone

Transthoracic Doppler

Non-invasive

Interpreter-dependent
Poor reproducibility in serial assessments
Depends heavily on image quality

Oesophageal Doppler

Minimally invasive

Positional; risk of gastric or oesophageal perforation

SvO2 measurments

Easy to insert 
Interpreter-independent

No information on  regional oxygen extraction
Assumptions regarding cardiac output

Pulse dye densitometry

Interpreter-independent

Difficult to perform
Specialized equipment required
Exposure  to dye may be undesirable 
Studies of validity give conflicting results

Bioimpedance cardiography

Non-invasive

Thus far not validated for clinical use

References

References

Mathews, Lailu, and Kalyan RK Singh. "Cardiac output monitoring." Annals of cardiac anaesthesia 11.1 (2008).

de Waal, Eric EC, Frank Wappler, and Wolfgang F. Buhre. "Cardiac output monitoring." Current Opinion in Anesthesiology 22.1 (2009): 71-77.

Pinsky, Michael R. "Hemodynamic evaluation and monitoring in the ICU."CHEST Journal 132.6 (2007): 2020-2029.