Published on Wednesday, February 27th, 2019 by Tamara Gonzalez Benitez.
As a Clinical Specialist I often discuss the different monitoring systems we have available and how to best use them to achieve their full potential. One of the most common questions I face is about the differences between the calibrated (LiDCOplus and LiDCOunity calibrated) and uncalibrated systems (LiDCOrapid and LiDCOcnap). To be able to fully comprehend their differences, it is important to understand the concepts of trends, accuracy and precision. During our daily life conversations most people use the terms accuracy and precision as equals. I, myself, found it difficult to differentiate at the beginning and that lead me to write this blog.
Accuracy, Precision and Trend
Accuracy refers to the closeness of a measured value to a standard or known value. For example, if a patient’s CO measure with a PAC catheter (goal standard method) is 6 L/min and then a different method is used and the number obtained is 4 L/min, the alternative method is not accurate as the measure it is not close to the known value.
Precision refers to the closeness of two or more measurements to each other. Using the same example, if using the alternative method to measure CO three times, all three times your measure is the same, 4 L/min, then the alternative method is highly precise.
Trend is the general direction in which something is developing or changing. This is an important concept to understand how and when to use a non calibrated system.
You can be precise but inaccurate. You can also be accurate but imprecise. The closeness to values obtained by gold standard methods and the closeness of several measures between themselves is used to validate a new technology available in the market (Jeleazcov et al, 2010). An easy way to understand accuracy and precision is with a shooting target.
How does this relate to the different LiDCO monitoring options available?
LiDCOplus or LiDCOunity calibrated Vs LiDCOrapid and LiDCOcnap
Calibrating the LiDCOplus or LiDCOunity with Lithium Indicator dilution, enables a correction for differences in aortic compliance. When combined with continuous arterial waveform analysis, both highly accurate and highly precise data is obtained (Linton et al, 1997). The LiDCOrapid incorporates an age, height and weight nomogram. When the height and weight is accurately inputted, the aortic compliance scaling factor is estimated from previous population studies. This means that highly precise trend data is obtained (Costa et al, 2007).
LiDCO offers two core technology types; trend based monitoring with the LiDCOrapid, and calibrated with the LiDCOplus and LiDCOunity. We recognise that different patient groups in different clinical settings have different monitoring needs. Trend monitoring allows you to understand how you patient is changing and act accordingly to it. If the monitor is showing a decreasing CO, you should act on the CO drop independently of the number displayed. In the same way, when using the event response option to track changes in stroke volume (SV), what is important while using the monitor is the percentage of change observed, rather than the SV value displayed. If the event response shows an increase of 13% in SV, your patient had a positive response to the fluids independently of the SV value displayed on the monitor which may or may not be within normal limits.
Absolute value monitoring using calibrated technology is very useful when it is very important to identify the patients specific cardiac output. For example, I often hear that on patients receiving high dose Noradrenaline, knowing the accurate cardiac output is important to ensure the patient is not over squeezed and the cardiac output reduced.
Calibrated data is highly recommended when you have a patient with a condition that may involve big haemodynamic changes and/or instability like sepsis; or if your patient needs increasing/high doses of inotropes, vasopressors or combined infusions. In this case you would like to know the exact numbers from your patient and not a generic value from a trend (Linton et al, 1997). Uncalibrated data is useful to assess and treat your patient when they are not requiring higher interventions and medication support like during surgical procedures or post-operative GDT when only fluids and/or low amount of inotropes or vasopressors are required to maintain haemodynamic stability. In this case, trend values for early assessment of changes could be all you need (Costa et al, 2007).
1. Jeleazcov et al. (2010) Precision and accuracy of a new device (CNAPTM) for continuous non-invasive arterial pressure monitoring: assessment during general anaesthesia. British Journal of Anaesthesia 105 (3): 264–72 (2010)
2. Linton et al (1997) Lithium dilution cardiac output measurement: A comparison with thermodilution. Critical Care Med 1997 Vol. 25, No. 11
3. Costa et al (2007) Continuous and intermittent cardiac output measurement in hyperdynamic conditions: pulmonary artery catheter vs. lithium dilution technique. Intensive Care Med. DOI 10.1007/s00134-007-0878-6