Evaluation of the Penàz servo-plethysmo-manometer for the continuous, non-invasive measurement of finger blood pressure

Abstract

Blood pressure in the finger was measured by a servo-plethysmomanometer constructed after the design of Penàz, which uses the principle of the unloaded arterial wall. The device contains a photoelectric plethysmograph mounted in an inflatable cuff and an electro-pneumatic transducer to control air pressure in the cuff via a servosystem. Comparison of simultaneous measurements of intra-arterial pressure in the brachial artery was performed on 33 patients suspected of having hypertension. In 12 patients evaluation of the technique could not be carried out due to technical failures or distorted blood pressure wave forms. Results of the remaining 21 patients show a mean underestimation of intra-arterial blood pressure by finger cuff blood pressure of 0.8 kPa (6 mm Hg), both for systolic and diastolic levels. The scatter range of the difference is from 1.9 to -3.5 kPa for systolic and 0.1 to -2.5 kPa for diastolic values. It appears that, although not all technical problems are solved, the Penàz servo-plethysmo-manometer is potentially an elegant method by which to arrive at the fully calibrated wave form of blood pressure in a finger in a non-invasive and continuous fashion.

Accuracy and precision of blood pressure determination with the Finapres: an overview using re-sampling statistics

“…This is surprising as it is generally accepted that amplification of the systolic BP occurs distally, due to a combination of physiological events including waveform dispersion and reflection. 11,12,33 We have previously noted 23 that the bulk of validation studies on the Finapres suggested that the systolic and diastolic pressures were underrecorded compared with that directly assessed; 3,14,18,24,26,28 physiologically one would expect an accurate peripheral device to record a higher systolic but a lower diastolic pressure compared with that recorded at a more proximal recording site. However, the extent to which the systolic pressure is amplified may be variable, depending both on biological (age and extent of arterial pathology) and technical factors.…”

“…The BP, meas-ured at the digit using servo-plethysmomanometry, employs the volume clamp technique. 2,3 The finger artery is unloaded such that its arterial volume is held constant (or clamped) at any given instant. A servo-controller compares the arterial diameter with a reference value (the set point) that has been chosen to ensure a near-zero transmural digital artery presure.…”

“…Re-sample 3: re-sampling of eight robust studies and 186 patients: Of the various studies reported, only nine documented the frequency response and damping ratio of the IABP system; 3,[8][9][10]16,17,22,23 one was an exercise study and was therefore excluded. 27 A total of 186 patient-grouped data, with average IAPFinapres bias and s.d.…”

“…The technique uses a small cuff to apply a volume clamp to the digit. 2,3 The total finger volume under an unloading cuff is determined by infra-red plethysmography and despite the changing arterial pressure, this volume is maintained at a set level by modulating the cuff pressure using a high speed electropneumatic servo-system. The continuously changing cuff pressure is measured electronically, and the signal displayed as the arterial pressure.…”

“…12 Therefore any periheral BP method such as the Finapres requires rigorous validation to understand its strengths and limitations. The accuracy of the Finapres compared with intra-arterial reference pressure has now been assessed in many studies [3][4][5][6][7][8][9][10][13][14][15][16][17][18][19][20][21][22][23] without any clear consensus emerging on the average performance of the method; precision data has been infrequently reported. 20,24 This paper is a synthesis of these literature data; a novel statistical method of re-sampling allowed a global overview of the performance of the system to be derived.…”