The N-CAT is a newly developed arterial tonometer (TBP) able to determine systolic, diastolic and mean arterial blood pressures continuously and noninvasively. The aim of this study was to evaluate the accuracy and reliability of TBP relative to directly measured invasive blood pressure (IBP) in ten haemodynamically stable postoperative cardiac patients who were in rapid atrial fibrillation (HR > or = 100 bpm). There were differences between TBP and IBP for systolic (-1.7 mmHg) and diastolic (+0.9 mmHg) values but not for the mean arterial blood pressures. The N-CAT was able to follow blood pressure changes closely and demonstrated an average systolic, diastolic and mean bias (+/-SD) of -1.71 +/- 4.6, 0.99 +/- 4.6 and 0.33 +/- 4.2 mmHg, respectively. Although these biases are within the required standards for equivalency for noninvasive blood pressure to invasively determined blood pressure, approximately 20% of the readings were > +/- 10 mmHg while only 5% were > +/- 20 mmHg. Moreover, there were occasional discrepancies of sufficient magnitude and duration which may limit the clinical usefulness of the N-CAT in patients in whom continuous and accurate blood pressure measurement is required.