Comparison of invasive and non-invasive measurement of continuous arterial pressure using the Finapres in patients undergoing spinal anaesthesia for lower segment caesarean section

We have compared arterial pressures measured by an indwelling radial cannula with those obtained non-invasively by the Finapres 2000 (Ohmeda) during spinal anaesthesia for lower segment Caesarean section. The digital outputs of both pressures were recorded using a computerized system. We studied 20 patients, yielding a total of 18,772 data points after elimination of data recorded during arterial flushing and erroneous results from each source. The data analysis demonstrated a normal distribution for differences between the two methods of measurement, and the correlations between invasive and Finapres readings for systolic, diastolic and mean pressures were 0.78, 0.72 and 0.79, respectively, indicating an overall poor reflection of intra-arterial pressure by the Finapres under these circumstances. Some patients and some periods of readings reflected a high degree of precision and little bias. However, unexplained large differences in pressure and trends of change that were out of phase over time occurred frequently. We conclude that the Finapres cannot be recommended as a monitor of arterial pressure in this group of patients in whom sudden hypotension may be a threat to maternal or fetal outcome.