Pulse index Continuous Cardiac Output (PiCCO)

This chapter is relevant to Section G6(iii) of the 2023 CICM Primary Syllabus, which expects the exam candidate to "describe the methods of measurement of cardiac output, including limitations, potential sources of error, the need for calibration and the values obtained". This chapter refers specifically to the method of measuring cardiac output by transpulmonary dilution, which can be thermodilution (PiCCO) or lithium dilution (LiDCO). 

From the exam preparation perspective, the First Part candidate can safely ignore this entire section, as it has never really appeared in enough past papers to be of any real relevance. In the Second Part, Question 4 from the second paper of 2024 asked the candidates to choose one from PiCCO, LiDCO and FloTrac  to explain their various properties, and in Question 10 from the second paper of 2017 asked the candidates to choose two methods of measuring cardiac output to compare and contrast, opening the possibility of using PiCCO as one and perhaps the PA catheter as the other. Question 19 from the first paper of 2014 also vaguely mentioned cardiac output measurement for 30% of the mark. Ed Litton's old 2012 paper on this subject remains an excellent starting point, and is probably enough for the time-poor candidate.

Principles of PiCCO

• PiCCO is a combination of transpulmonary thermodilution and pulse contour analysis. 
• This technique uses a cold injectate delivered proximally to the central venous circulation, and the measurement of the temperature change is performed peripherally in a distal artery
• The resulting thermodilution curve is used to derive cardiac output by the Stewart Hamilton method, as well as several calculated variables such as SVR
• The exponential decay time of the diastolic arterial pressure downslope and the SVR can be used to calculate the aortic compliance C_w (where C_w = exponential decay time divided by the SVR)
• SVR, Cw and aortic impedance Z₀ (the opposition to pulsatile flow) form the three elements required to calculate flow from pressure using the  model by Wesseling et al (1993), which computes the SV from the 

Calibration of the PiCCO device

• PiCCO is calibrated using transpulmonary thermodilution 
• This allows the pulse contour to be used as a cardiac output measure
• This is based on the relationship between stroke volume and the area under the systolic portion of the aortic pressure waveform
• It depends on the compliance of the arterial circulation
• The algorithm used by each manufacturer is proprietary (The PiCCO monitor market is largely cornered by Pulsion, but other devices also exist, eg. a Philips and a Drager options are available). 

Advantages of the PiCCO method

• The method, for all its flaws, is old and well-validated
• It is close to PA catheter measurement, and is not far from the other available devices in terms of accuracy (Hadian et al, 2010)
• The cardiac output monitoring is continuous with the pulse contour analysis
• The arterial and venous access required are both likely to already be indicated
• There is no large sheath to insert into a vein, and there is no risk of pulmonary artery rupture
• The transpulmonary dilution can generate interesting parameters such as extravascular lung water, GEDV, etc

Limitations of the PiCCO method

• A large arterial catheter is required, which has implications for vessel damage, ease of insertion, and for the care that needs to be taken when removing the catheter afterwards.
• A CVC is required
• This catheter cannot be inserted at the same site as the CVC
• The pulmonary pressure or wedge pressure cannot be measured
• Pulse contour analysis of cardiac output cannot be relied upon where the patient has a balloon pump or is in atrial fibrillation
• Pulse contour analysis tends to drif from calibration and needs to be recalibrated regularly as the patient's condition changes
• The thermodilution measurement may be confused by the presence of large bodies of fluid in the chest that act as thermal sinks, eg. pleural or pericardial effusions (Oren-Grinberg, 2010)