Transpulmonary thermodilution assessments: precise measurements require a precise procedure
© BioMed Central Ltd 2011
Published: 12 October 2011
When incorporating the values of a hemodynamic parameter into the care of patients, the precision of the measurement method should always be considered. A prospective analysis in the previous issue of Critical Care showed that the precision of transpulmonary thermodilution (TPTD) allows for reliable mean values if a standardised procedure is used. The present finding has a physiological basis, as TPTD requires a more prolonged transit time, which in turn reduces the effects that airway pressure and arrhythmia have on venous return-cardiac output steady states. Moreover, this result suggests that the current accepted threshold value of a 15% increase in cardiac output to identify a positive response to a fluid challenge could be reduced in the future. Indeed, this value is mainly related to the precision of the pulmonary artery catheter.
The first principle is that you must not fool yourself - and you are the easiest person to fool.
Richard Feynman (1918 to 1988)
When using a monitoring device for critically ill patients, inadequate precision of the technique can result in serious misinformation. Indeed, the definition of precision is the variability of values due to random errors of measurement. Semantically, it refers to how closely individual measurements agree with each other - that is, the minimum number of measurements necessary to reach a reliable mean value. In this regard, some questions have remained unresolved concerning the measurement of cardiac output (CO) by transpulmonary thermodilution (TPTD) and derived parameters such as global end-diastolic volume (GEDV) and extra-vascular lung water (EVLW). Indeed, TPTD precision has never been prospectively investigated in situations characterized by (1) constant conditions of measurements, (2) the same instrument and operator, and (3) repeated measurements during a short time period.
In the present issue of this journal, Monnet and colleagues  illustrated the precision of the TPTD technique and the number of cold boluses needed to achieve an acceptable level of precision when measuring CO, GEDV and EVLW in 91 critically ill patients. In this elegant study, the authors performed a series of five successive thermodilution measurements and then analyzed the precision of each sequence to demonstrate that a precision of less than 10% for CO, GEDV and EVLW could be reached with only three cold bolus injections of 15 ml each . In addition, the least significance change (LSC) associated with three boluses was less than 12% for these three parameters, even if the patients underwent arrhythmia, were spontaneously breathing and/or were having continuous haemodialysis.
One cannot, however, extrapolate these findings to patients with hemodynamic instability and low CO, patho-physiological conditions where the assessment of the precision of a method is unachievable. Moreover, it is essential to acknowledge that the precision of TPTD may be related to the experience of the practitioner and of course may be compromised in extreme situations, such as high volume haemofiltration, therapeutic hypothermia , and the presence of an intra-cardiac shunt . Finally, the impact of an increase in cold bolus volume (for example 20 ml) on the precision of the TPTD technique has not yet been determined. Therefore, until further evidence arises, three cold boluses of 15 ml each are mandatory in order to ensure precision of values when measuring CO in the critically ill patient.
extravascular lung water
global end-diastolic volume
least significance change
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