RADIAL ARTERY TONOMETRY - MODERATELY ACCURATE BUT UNPREDICTABLE TECHNIQUE OF CONTINUOUS NONINVASIVE ARTERIAL-PRESSURE MEASUREMENT

Radial artery tonometry provides continuous measurement of non-invasiv e arterial pressure (CNAP) by a sensor positioned above the radial art ery. An inflatable upper arm cuff enables intermittent oscillometric c alibration. CNAP was compared with invasive radial artery pressure rec ordings from the opposite wrist in 22 high-risk surgical patients with an inter-arm oscillometric mean arterial pressure difference less tha n or equal to 10 mm Hg. Oscillometric, tonometric and invasive digital pressure values, and invasive and CNAP waveforms were obtained by the same instrument (Colin BP-508). Correlation coefficients (r) of invas ive vs oscillometric values (n = 481 pairs) were 0.83, 0.90 and 0.92, and mean absolute errors of oscillometry were 7.6, 4.7, and 2.6 mm Hg for systolic, diastolic and mean arterial pressures, respectively. Cor relation was poor for systolic (r = 0.80), diastolic (r = 0.77) and me an (r = 0.84) invasive vs CNAP values (n = 1375). Compared with oscill ometry, mean absolute errors of 15.2, 10.9 and 9.4 mm Hg for systolic, diastolic and mean CNAP, respectively, were significantly (P < 0.001) higher. Mean prediction errors of CNAP, compared with invasive values , were -5.8 (so 14.2) mm Hg for systolic, +7.2 (8.3) mm Hg for diastol ic and +3.9 (8.8) mm Hg for mean arterial pressure. Individual patient accuracy of CNAP was assessed as good (individual prediction error le ss than or equal to 5 (8) mm Hg and individual absolute error less tha n or equal to 10 mm Hg) in seven patients, as acceptable (less than or equal to 10 (12) and less than or equal to 15 mm Hg) in 11 patients, and as inadequate in four of 22 patients. Individual accuracy of oscil lometry was good or acceptable in all 22 patients. The trend in CNAP c hanges (difference between consecutive measurements) was sufficiently accurate during induction of anaesthesia, as only 47 (7.6%), 14 (2.3%) and 27 (4.4%) of 616 systolic, diastolic and mean CNAP values differe d by more than 10 mm Hg of invasive pressure trends. We conclude that: intermittent oscillometry provides accurate arterial pressure monitor ing; CNAP measurements offer a reliable trend indicator of pressure ch anges during induction of anaesthesia and may be considered an alterna tive to invasive pressure measurements, should arterial cannulation be difficult in an awake patient; and accuracy of absolute CNAP values i s only moderate and unpredictable, thus radial artery tonometry should not replace invasive monitoring in high-risk patients during major su rgical procedures.