A reply

We thank Drs Orme and Pigott for their comments. They questioned our conclusion that radial and femoral artery pressure waveforms are interchangeable because it fails to take into account the fundamental issue of calibration. This is only partially correct. Calibration by transpulmonary thermodilution with detection of the dilution curve in the radial artery leads to an overestimation of cardiac output [1]. This overestimation is not related to a poor signal-to-noise ratio, which might otherwise be compensated for by using iced injectate. It is related to loss of indicator during its transport from injection to detection site. In their letter, Orme and Pigott conclude that after changing from the femoral to the radial pressure site, the pulse contour method could be used for a maximum of 24 h because Pulsion Medical Systems recommend calibrating the PiCCO device at least once every 24 h. In our opinion, they have concentrated too much on the use of monitoring absolute cardiac output over longer time periods and therefore the weakness in the pulse contour method. They have ignored the ability of this technique in monitoring changes in cardiac output due to interventions or treatments over short time periods (such as hours) as well as its ability to monitor changes in the patient's filling status by determining stroke volume variation or pulse pressure variation. A further reason to undertake our study was our curiosity about whether the shape of the pressure wave form influences the results of pulse contour analyses. The pulse contour method used by Pulsion can be subdivided into two parts. The first part is related to the integration of the area under the systolic part of the pressure curve. This process filters out the shape of the curve. The second part is related to the shape of the pressure wave by multiplication of the arterial compliance with the first derivative of the arterial pressure. These two parts must be added to compute cardiac output. Although the shapes of the femoral and radial artery differ, the calculated cardiac output does not. We showed that the more frequently available radial artery pressure is interchangeable with the femoral artery pressure. Both sites can be used to determine cardiac output estimates of equal quality. We hope this finding will result in more widespread use of this device and further work on its calibration by methods other than transpulmonary femoral thermodilution.