A mathematical study is performed to assess how the arterial pressure-volume (P-V) relationship, blood pressure pulse amplitude and shape affect the results of non-invasive oscillometric finger mean blood pressure estimation by the maximum oscillation criterion (MOC). The exponential models for a relaxed finger artery and for a partly contracted artery are studied. A new modification of the error equation is suggested. This equation and the results of simulation demonstrate that the value of pressure estimated by the MOC does not exactly agree with the value of the true mean blood pressure (the latter being defined as pressure corresponding to maximum arterial compliance). The error depends on the arterial pressure pulse amplitude, as well as on the difference between the arterial pressure pulse shape index and the arterial P-V curve shape index. In the case of contracted finger arteries, the MOC can give an overestimation of up to 19 mmHg, the pressure pulse shape index being 0.21 and the pulse amplitude 60 mmHg. In the case of relaxed arteries, the error is less evident.

The volumetric pulses in the finger PPG signal appear as an information source for several indirect measurement methods. In this study we modelled transformation of pressure pulses into volume pulses in a wide transmural pressure range. The noninvasive finger arterial pressure and photoplethysmographic (PPG) signals were simultaneously registered in 13 healthy subjects while the pressure in the PPG cuff ramped up and down. The nonlinearity of the pressure-volume (P-V) relationship was modelled by an asymmetric function, consisting of two arctangents, each for a different pressure region. The time dependency was described by the first order lag. The disturbing effect of slow creeps in the PPG signal was suppressed by an equal filtering of the measured and model-predicted signal. The differences between the two estimates of the subject's P-V relationship for the increasing and decreasing cuff pressure were small thus showing the repeatability of this method, which can be used for the characterization of individual finger arterial behaviour as well as its changes.

The present study compares two different methods for non-invasive beat-to-beat finger arterial blood pressure monitoring. The measurements using the volume clamp method (FINAPRES, Ohmeda, USA) were compared with measurements applying the differential oscillometric method (UT9201 device, University of Tartu). 13 healthy volunteers were studied at rest, during head-up tilt (HUT) and during deep breathing (DB) with a fixed rate of 6 breaths/min. Blood pressure was recorded from adjacent fingers of the right hand. 150 pairs of mean blood pressure values in each subject were included for statistical evaluation. No systematic differences between the two methods were found. The difference between values (FINAPRES-UT9201) at rest was -1.1 mmHg (SD 5.5), during HUT 0.5 mmHg (SD 6.9) and during DB -3.6 mmHg (SD 7.7). The insignificant differences between the results obtained by the two independent methods allow us to conclude that both methods give reliable data on finger mean arterial pressure.

Oscillometric readings of systolic and diastolic pressures are strongly influenced by pulse pressure and the shape of the artery-cuff pressure/volume curve, whereas those of the mean arterial pressure are affected by pulse pressure and both the shape of the artery-cuff pressure/volume curve and the shape of the arterial pulse.

Non-invasive beat-to-beat mean arterial pressure (MAP) in finger arteries recorded by the differential oscillometric device was compared with MAP recorded invasively from A. radialis in 22 patients after cardiac surgery. Based on all 132 paired measurements, the MAP values measured at the radial artery were 2.7 ± 4.9 mmHg higher than those measured on fingers. Among 22 patients there were 8 patients receiving inotropic support, their difference being 2.1 ± 5.6 mmHg. The present study revealed that the mean discrepancy between the invasive radial pressure and finger pressure was small; however, patient data sets showed marked variability in average pressure differences when examined individually.