Question 4

College answer

Syllabus topic/section:

2.1.4 Cardiovascular Intensive Care: Haemodynamic monitoring: L1

Discussion: 

Candidates scored poorly in this question due to a lack of understanding of the topic and the principles of continuous cardiac output (CCO) monitoring. Many candidates didn’t answer the question that was asked or interpreted the question incorrectly and provided information about all 3 devices rather than just 1 device which was explicitly stated in the question with underlining and bolded. Candidates are advised to read the instructions carefully for maximum time efficiency. The question required candidates to understand that these devices use analysis of the pulse pressure waveform and a proprietary algorithm to generate a CCO and some (PiCCO/LiDCO) combine this with thermodilution. Candidates that scored well in this question understood the basic principles of pulse pressure waveform analysis as outlined above, internal vs external calibration, and that limitations relate to the various elements of a device. These limitations include the actual line (art or CVC), transducer system, thermodilution techniques and patient specific factors (pulse waveform analysis is not validated in certain patient populations like ECMO or post pneumonectomy). Candidates that considered the individual components of a device were able to use this to structure their answer for all 3 parts of the question. The marking rubric is included to aid the candidate’s future study.

Discussion

This exhaustive rubric leaves little room for frivolous misinterpretation. A model answer is probably not necessary for something like this, but here goes anyway (the main challenge being to get this into under 200 words for each device). 

PiCCO:

• Principles:
  • A combination of transpulmonary thermodilution and pulse contour analysis
  • A known volume of cold injectate is delivered via CVC
  • temperature change is measured by a thermistor tipped arterial catheter
  • The resulting thermodilution curve is used to derive cardiac output by the Stewart-Hamilton formula, as well as several calculated variables
• Calibration:
  • Calibrated using transpulmonary thermodilution 
  • The algorithm 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
  • Needs to be recalibrated every 6-8 hrs
• Limitations:
  • A CVC is required
  • A large arterial catheter 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)

LiDCO:

• Principles:
  • Transpulmonary i​​​​ndicator dilution method of cardiac output monitoring using the Stewart Hamilton method
  • A small amount of lithium chloride (0.002 to 0.004 mmol/kg) is injected into a vein
  • A lithium-sensitive electrode then senses the lithium in the arterial circulation
  • The resulting dilution curve is used to derive cardiac output by the Stewart-Hamilton formula, as well as several calculated variables.
• Calibration:
  • The algorithm 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 the manufacturer is proprietary
  • "a beat-to-beat cardiac output monitor that calculates stroke volume from the arterial pressure waveform using an autocorrelation algorithm"
  • Needs to be recalibrated every 6-8 hrs
• Limitations:
  • Inaccurate results if there are intracardiac shunts
  • Patients on lithium therapy will have inaccurate results
  • “electrode drift” can occur if there are high doses of muscle relaxants present (the quaternary ammonium residues interfere with the lithium sensor)
  • You do end up disposing of some blood each time you sample.
  • The inputs into the equations that run the cardiac output monitoring require you to input serum sodium and haematocrit measurements
  • The system does not report intrathoracic blood volume index
  • The support in the literature is smaller in terms of the number of studies as compared to PiCCO or the PA catheter

FloTrac:

• Principles:
  • an uncalibrated pulse contour analysis monitor
  • works on the premise that, if the compliance of the arterial circulation is known, then the stroke volume can be calculated from the pulse pressure
  • estimates the compliance on the basis of the waveform contour and some loookup tables from empirical measurements
• Calibration:
  • ​​​​​​​No need to calibrate - therefore, instantly useable from insertion
  • No additional data beyond patient demographics is needed
  • Patient haemodynamic variables are used to estimate arterial compliance
• Limitations:
  • ​​​​​​​Dependent on good trace quality
  • Affected by damping and resonance
  • Obviusly impossible to use in atrial fibrillation, IABP, VA ECMo with nonpulsatile flow
  • Severe derangedments in arterial compliance (extremes of vasodilation or vasoconstriction) are likely to produce inaccurate readings
  • The patient population in the data set used to generate the data may not be representative of the patient being monitored, eg. it does not cover severe sepsiss liver failure, etc

As one might imagine, the act of putting together something succinct has left this author with the frustrated need to deep dive into the plie fascinating information that was left on the cutting room floor. Large swaths of the PiCCO, LiDCO and FloTrac chapters were renovated in the course of this process. Of the random directions into which this travelled the most interesting was probably the long painful explanation of how the pulse contour is magically transformed into the cardiac output.