in the previous issue of critical care hadian and colleagues compare the performances of the three best known examples of pulse contour systems - lidco plus , picco plus and flotrac - against thermodilution .
an experimental design was used based on cardiac surgery patients in the first 4 hours after surgery , in which cardiac output was managed using four different therapeutic interventions - fluid bolus , vasoconstrictor , vasodilator and inotrope - which should have provided a vigorous test of trending ability when compared with the alternative of sampling at regular time intervals .
the authors used a standard and now well - established statistical approach of bland and altman analysis , determining percentage errors   and concordance trend analysis that included an exclusion zone ( changes < 0.5 l / minute ) .
the results show that only the lidco plus system provided an acceptable level of agreement with thermodilution ( percentage limits
none of the three systems provided satisfactory trending , however , with concordance rates ( 74% , 72% and 59% ) well below the required 90 to 95% for good trending .
the reliability of the pulse contour method is known to be susceptible to changes in peripheral resistance and the study protocol involved vasoconstrictor and vasodilator drugs , which may explain these poor trending results .
this study helps to confirm what is already suspected about the reliability of pulse contour devices ; that these devices do not track changes in cardiac output reliably .
although there are many recent published studies evaluating pulse contour devices , the present one provides a very exacting examination and also compares the three main brands of pulse contour monitor .
the study could be faulted , however , because a now out of date flotrac/vigeleo software version was used .
the flotrac has been criticised for failing to compensate for low peripheral resistance states . in response ,
edwards lifesciences ( irvine , ca , usa ) produced a new third - generation software version ( for example , version 3.02 ) known as dynamic tone technology to overcome this limitation .
the authors also used continuous cardiac output readings as their reference standard if a heated wire pulmonary artery catheter was in situ .
validation should ideally be based on readings from single bolus thermodilution cardiac output measurements as these are said to be the most reliable .
there are several studies that show continuous cardiac output to be as accurate as single bolus thermodilution .
continuous cardiac output takes several minutes to stabilise and provide a valid reading , however , which makes it less reliable when measuring trends .
one must therefore be cautious interpreting data from this study , as the bland and altman analyses and the concordance analyses may use reference data that have a wider spread than usual and thus wider acceptance criteria .
the paper is made much harder to comprehend due to the many cross - comparisons , in the form of bland and altman and four - quadrant concordance plots , which show agreement between the three pulse contour methods .
these comparisons do , however , suggest that these three pulse contour methods are not interchangeable .
the reason given by the authors is the use of different proprietary algorithms , which measure different aspects of the pulse contour waveform - which is an interesting point .
furthermore , nine out of 17 patients had femoral rather than radial arterial lines inserted .
there is growing evidence that the site of puncture affects the shape of the arterial trace and thus pulse contour measurements . the more distal the puncture site , the greater the influence of acoustic reflections from vessel branching , and the extent of wave reflection also varies quite dramatically with blood vessel constriction and dilatation .
the puncture site thus seems to be an important determinant of the success of pulse contour measurements and should be paid more attention .
it is one of several recent clinical studies that show the pulse contour does not reliably reflect changes in cardiac output in haemodynamically unstable patients .
excuses can be made that the thermodilution reference method is less reliable than the quoted 20% precision error , but that is another story .
the study , however , does provide a rigorous test of the technology , does compare the performance of the three main pulse contour methods , albeit with now out of date flotrac software , and does add to the growing evidence that the pulse contour method is not the solution to providing reliable cardiac output monitoring at the bedside .
 three pulse contour systems for monitoring cardiac output - lidco plus , picco plus and flotrac - were compared in postcardiac surgery patients . none of the three methods demonstrated good trending ability according to concordance analysis . 
 pulse contour systems remain unreliable in the haemodynamically unstable patient . 
None
Competing interests