Department of Physiology, University of Tartu, Tartu, Estonia Abstract— We studied how interactions between the arterial pressure pulse and mechanical characteristics of the arterial wall and occluding cuff can modulate the characteristic ratios used for oscillometric estimation of systolic and diastolic blood pressures (k syst and k diast , respectively). Using an integrated artery–cuff pressure/volume model with different arterial pressure pulses as input signals we obtained the oscillation envelopes and calculated characteristic ratios. For the tested range of affecting factors, k syst varied from 0.41 to 0.81 and k diast from 0.56 to 0.90. This gives evidence that oscillometric estimation may lead to substantial inaccuracies if fixed charac- teristic ratios are used. Errors can be reduced by considering changes in the pulse pressure amplitude and in the symmetry index of the artery-cuff pressure/volume relationship. Keywords— Oscillometric blood pressure, characteristic ra- tio, accuracy of measurement, modelling. I. II. INTRODUCTION Latest practice guidelines of the European Society of Hypertension [1] recommend oscillometric monitoring as a useful adjunct to conventional office blood pressure (BP) measurement. The accuracy of measurements and ability of physicians to interpret results have been noted as the key issues of this methodology. Oscillometric BP is typically determined from the enve- lope of successive oscillometric pulse amplitudes obtained from the occlusive cuff during its inflation or deflation. The highest point of the envelope curve is generally regarded as the mean arterial pressure (MAP) [2]. A basic criterion which has been used to estimate systolic and diastolic blood pressures (SBP and DBP, respectively) is the height-based methodology [3]. In the height-based approach the systolic and diastolic pressures are determined using special frac- tions of the maximum oscillation amplitude [4]. These frac- tions are known as characteristic ratios (k syst and k diast , re- spectively). A large variability in the results of oscillometric meas- urements is quite typical and can be explained by a number of different factors able to affect oscillometric estimation. Theoretical and experimental work has demonstrated that among the factors which can influence the oscillometric BP measurement are pulse pressure (PP), shape of the arterial pressure/volume (P/V) relationship, heart rate, cuff size and compliance [5–8]. Changes in these factors can have sig- nificant effects on the characteristic ratios and, subse- quently, on the accuracy of estimation of SBP and DBP on the basis of fixed ratios. The aim of this paper is to investigate by means of com- puter simulation how the interaction of the arterial pressure pulse with mechanical characteristics of the arterial wall and occluding cuff, can affect the values of characteristic ratios used for oscillometric estimation of SBP and DBP. METHODS In the present study, we used an asymmetric arctangent model of the arterial P/V relationship, which was initially used by us to model the finger arterial wall mechanics. This model was identified by photoplethysmographic recording of pulsations from finger arteries [9–10]. Considering the fact that in conventional oscillometric monitors, volume oscillations are picked up from the occluding cuff rather than directly from the tissue, we modified the model to consider also cuff-related parameters (artery-to-cuff signal transfer and cuff mechanics). Subsequently, we applied an integrated artery–cuff P/V model with arterial pressure pulses as an input signal and cuff volume oscillations (like those recorded by a conventional oscillometric device) as an output signal. There was a possibility to modify the shape of the P/V relationship. Thereafter, the oscillation envelopes were drawn and characteristic ratios k syst and k diast calculated to fit the oscillometrically estimated SBP and DBP values with corresponding values of the input pressure pulses. The amplitude of the arterial pressure pulse is known as pulse pressure (PP), defined as PP=SBP–DBP. Transmural pressure (TP) can be calculated from intra-arterial pressure (IP) and cuff pressure (CP) as TP=IP–CP. The artery–cuff P/V relationship determines how arte- rial pressure pulses are converted into cuff volume pulses. We characterised the shape of the used P/V relationship by two indices. Definitions of these indices are given in Fig.1. First, similarly to arctangent steepness in Langewouters model [11] we used the steepness index P 0.5 (known as a half-maximum width of the pressure/compliance curve), indicating the rate of decrease of the maximum compliance C max to its half value. It is generally known that a stiff artery K. Dremstrup, S. Rees, M.Ø. Jensen (Eds.): 15th NBC on Biomedical Engineering & Medical Physics, IFMBE Proceedings 34, pp. 73–76, 2011. www.springerlink.com An Influence of Multiple Affecting Factors on Characteristic Ratios of Oscillometric Blood Pressure Measurement J. Talts, R. Raamat, K. Jagomägi, and J. Kivastik