The Finapres® NOVA and the ADInstruments Human NIBP Nano System both use a calibration algorithm called Physiocal. This algorithm prevents drift in the measured finger pressure due to arterial contractions and relaxations in the finger's vasculature. The Physiocal defines and maintains the correct diameter (setpoint) at which the finger artery is clamped. It periodically checks the vascular physiology in the finger and the determined setpoint, and automatically adapts the setpoint when necessary. The Physiocal is essential for an accurate measurement when using the NOVA or hNIBP Nano.
At the start of a measurement, the signal needs to be calibrated because small micro vessels under the finger cuff are being emptied of blood. Since this effect is more pronounced immediately after starting a blood pressure measurement, the start of every measurement includes a 15-seconds start-up procedure to detect the setpoint. Initially, the Physiocal is repeated after every 10 heart beats.
The Physiocal procedure only occurs after a set amount of heart beats. As soon as the setpoint has become sufficiently stable, this interval automatically increases stepwise from 10 to 20, 30 etc. up to an interval of 70 beats. This normally occurs within 3-4 minutes.
The Physiocal interval and countdown is shown on this bar of the Finapres® NOVA screen or as an AutoCal Countdown Channel in LabChart when using the ADInstruments HNIBP Nano System.
If the stability of the setpoint cannot be confirmed (i.e. movement artifacts or if the change in setpoint is considered too large) the frequency of Physiocal will temporarily increase automatically. Thereby, the Physiocal frequency is an important signal quality indicator. If the setpoint cannot be confirmed during two heart beats, additional steps are added automatically to find the new setpoint. If a multi-step (>3 step) Physiocal is observed frequently, the measurement accuracy may be reduced. Two indicators of a good signal quality are if:
- The Physiocal only takes 2 or 3 steps (no multi-step Physiocal), and
- The Physiocal interval quickly increases from 10 towards every 70 beats.
Users can determine whether they wish to use the Physiocal algorithm or not. They can switch it off during tests in which uninterrupted data-collection is required either by pressing the Green Physiocal button on the Finapres® NOVA's screen (the button will turn red when the Physiocal is turned off) or by accessing the 'Human NIBP Settings' dialog in LabChart for Windows and unchecking the 'Auto calibration enabled' checkbox when using the ADInstruments HNIBP Nano System.
Please note, you will need to have the Human NIBP Device Enabler installed in LabChart to use the ADInstruments HNIBP Nano System.
For further technical assistance with this or any other issue, please contact ADInstruments Technical Support by clicking HERE.