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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.

We evaluated two prototype instruments that measure pulsatile blood pressure continuously and noninvasively and compared the mean arterial pressure obtained from these devices with that obtained invasively in 17 male surgical patients. Each prototype consisted of an infrared photoplethysmograph mounted inside a finger cuff. The cuff was connected to a pressure control valve, which rapidly changed the cuff pressure so as to maintain a null pressure difference across the finger arterial wall. The resultant cuff pressure rapidly tracked the pulsatile intraarterial pressure. The prototypes reproduced absolute pressure, as well as pressure changes, accurately and linearly over a wide range of mean arterial pressures (from 2 to 164 mm Hg), with an average offset error of 0.8 mm Hg (SD +/- 3.8; range, -4.6 to 7.9), a mean scatter error of 5.3 mm Hg (range, 3.6 to 8.6), a mean regression slope of 0.97 (range, 0.79 to 1.22) and a mean correlation coefficient of the regression of 0.96 (range, 0.89 to 0.98). Both prototypes worked satisfactorily on all 17 patients, but not all the time on all patients. In 7 patients, probable arterial spasm prevented measurement of finger blood pressure 12.1% of the time, or 5.4% of the time for all patients. Ninety-six percent of the lost samples occurred with prototype 2, suggesting an instrument-related cause, rather than one related to the principle itself. The prototypes were simple to use and were almost free from artifact. Continuous monitoring for up to 7 hours on a single finger caused no harm to the finger.

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