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Correspondence |
St Thomas Hospital, London, UK, E-mail: nick{at}foxlinton.org
To the Editor:
A recent study concluded that cardiac output assessed by arterial pressure waveform analysis (PulseCOTM, Lidco Ltd., London, UK) was not interchangeable with thermodilution measurements, in patients undergoing off-pump coronary artery bypass grafting.1 In the introduction it was stated that PulseCOTM calculates cardiac output "from the arterial pressure ... using autocorrelation ...", and reference is attributed to a paper co-authored by me.2
In "Estimation of changes in cardiac output from the arterial blood pressure waveform in the upper limb",2 a new pulse contour algorithm was described. ‘PulseCO’ was used as an abbreviation for ‘pulse-contour cardiac output’. The new algorithm did not use autocorrelation and this word is not in our manuscript: we developed a new theoretical basis using the first harmonic of the blood pressure waveform and related this to the cardiac output using well established principles of signal analysis. The PulseCOTM product that has subsequently been developed uses a different algorithm.
Assessment of pulse contour methods is problematic. Generally, the technology is intended to track changes in cardiac output following calibration with another method. Therefore, it is the ability of the algorithm to follow within-patient change in cardiac output that should be assessed. We tested our pulse contour method by making multiple thermodilution measurements in each patient and then pooling the within-patient changes. However, Yamashita et al. pooled cardiac output results from different patients without calculating ‘within-patient changes’. This is difficult to interpret because the range of pulse contour estimations is due partly to cardiac output change and partly to different calibration cardiac outputs (as well as possible errors). Hence, the correlation coefficients may be misleadingly high. Similar errors have been made by other authors and a more detailed description of the problem with a proposed solution has previously been published.3 It would be interesting to see the data of Yamashita et al. analyzed in this way.
Another part of assessment involves the response to pharmacological agents. Some pulse contour methods have been shown to be unreliable with changes in vascular resistance, for example those caused by phenylephrine administration.4 We demonstrated that our pulse contour method could follow increases in systemic vascular resistance induced by phenylephrine as well as the large changes in hemodynamic state observed during cardiac surgery in our study.2
Footnotes
Accepted for publication April 10, 2006.
References
1 Yamashita K, Nishiyama T, Yokoyama T, Abe H, Manabe M. Cardiac output by PulseCO is not inter-changeable with thermodilution in patients undergoing OPCAB. Can J Anesth 2005; 52: 530–4.
2 Linton NW, Linton RA. Estimation of changes in cardiac output from the arterial blood pressure waveform in the upper limb. Br J Anaesth 2001; 86: 486–96.
3 Linton NW, Linton RA. Is comparison of changes in cardiac output, assessed by different methods, better than only comparing cardiac output to the reference method? Br J Anaesth 2002; 89: 336–7; author reply 337–9.
4 Rödig G, Prasser C, Keyl C, Liebold A, Hobbhahn J. Continuous cardiac output measurement: pulse contour analysis vs thermodilution technique in cardiac surgical patients. Br J Anaesth 1999; 82: 525–30.
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