PHYSIOCAL

Defining the correct unloaded diameter of a finger artery is crucial for the accuracy of the measurement. Changes in hematocrit, stress and the tone of smooth muscle in the arterial wall will affect the unloaded diameter. Therefore, the unloaded diameter is usually not constant during a measurement and must be verified at intervals. Periods of constant cuff pressure are used to adjust the correct unloaded diameter of the finger artery based on the signal from the plethysmograph in the finger cuff.

The Physiocal (abbreviation for Physiologic Calibration) algorithm in Portapres® and Finometer®, developed by the Dutch physicist Prof. KH Wesseling, not only uses the amplitude, but also interprets the shape of the plethysmograph signal during periods of constant cuff pressure. By analyzing the plethysmograph signal at two or more pressure levels, the Physiocal algorithm, explores part of the pressure-diameter relation and is able to track the unloaded diameter of a finger artery, even if smooth muscle tone changes. The periodic interruption of a finger blood pressure measurement, with constant cuff pressure levels, is further referred to as "Physiocal".

Physiocal is the automatic algorithm that calibrates the finger arterial size at which finger cuff air pressure equals finger arterial blood pressure. Physiocal is built into Portapres and Finometer and users can determine whether they wish to use the Physiocal algorithm or switch it off during tests in which uninterrupted data-collection is required.