Monitoring cardiac output from the radial artery pressure waveform

We would like to raise a number of points concerning de Wilde and colleagues' paper [1] comparing the radial and femoral artery for measurement of cardiac output using the PiCCO system (Pulsion Medical Systems, Munich, Germany). Their study involved collecting radial and femoral artery pressures traces onto computer and then playing back the data through the PiCCO device. The radial artery pressure was recorded from a 3.5-cm catheter, whereas a 4 F 16-cm PiCCO catheter was used for the femoral artery waveform. The calibration factor obtained from the femoral catheter by arterial thermodilution was then used to calculate cardiac output from the radial artery waveform. Bland-Altman analysis of radial vs femoral artery-derived cardiac output yielded acceptable bias and a precision of − 0.01 l.min−1 and 0.61 l.min−1, respectively. We believe that their conclusion that radial and femoral artery pressure waveforms are interchangeable for cardiac output determination using the PiCCO system fails to appreciate the fundamental issue of calibration. To determine cardiac output via pulse contour analysis, it is first measured by transpulmonary arterial thermodilution using a modified Stewart-Hamilton equation to obtain a value for aortic impedance. Previously, we have shown that to achieve successful calibration requires the thermistor-tipped arterial catheter to be sited centrally [2]. We compared thermodilution measurements of cardiac output from a 50-cm radial artery catheter using the PiCCO system with a pulmonary artery catheter. Although the catheter tip was likely to lie within either the distal subclavian or proximal brachial artery, we did not use the brachial route as stated by de Wilde and colleagues. In addition, we were unable to measure cardiac output and hence reliably calibrate the device for pulse contour analysis when the radial catheter was withdrawn by more than 5 cm despite using iced injectate to improve the signal to noise ratio. Pulsion Medical Systems also recommend that the device is calibrated at least once every 24 h to maintain acceptable accuracy. We believe, therefore, that the authors' study has limited practical application as it is impossible accurately to measure cardiac output by pulse contour analysis using the PiCCO system via a short radial catheter without first inserting a centrally sited thermistor-tipped catheter. Only in the unlikely situation of the failure of the dedicated arterial catheter following successful calibration could a radial catheter be used; and then it could only be used for the short-term.