#### Calibration

The method of arterial applanation tonometry is based on applying light pressure from outside, on a superficial Artery, so that it slightly flattens going, in turn, to press against the rigid or semi-rigid (bone, muscle, ...) underlying structure, if existing: in a situation of ideal balance (left image) the arterial internal pressure (Pi) is the same of the external one (Pe).
In the real case, the presence of the skin must be considered, with superficial layers interposed between the sensor and the Artery: in such case (right image) the internal arterial pressure (Pi) differs from the external one (Pe). Indeed firstly, to flatten the Artery you must apply extra pressure to deform the skin from outside, secondly, the pulse pressure inside the Artery arrives outside attenuated by the skin.
Mechanical properties of the skin, as elasticity, thickness, consistency, ..., vary from patient to patient, with age, etc.. : for this reason, a transcutaneous applanation tonometer cannot provide the exact values for the Systolic and Diastolic blood pressure while preserving an identical morphology of the pressure curve.
These values can be deduced applying the established concept that the mean arterial pressure remains unchanged in the arterial tree from the Aorta to the peripheral Arteries and that the difference between central and peripheral Diastolic blood pressure values is insignificant (generally between 0.2 and 0.6 mmHg in the Brachial Artery referred to the ascending Aorta). On the other hand, the Systolic blood pressure value increases from the Aorta to the peripheral Arteries due to reflected waves (amplification phenomenon).
Simultaneously with the recording of transcutaneous tonometry, the value of Systolic and Diastolic blood pressure in the Brachial Artery is measured by a traditional, validated sphygmomanometer. Then, mean arterial pressure is calculated from Diastolic and Systolic Brachial blood pressure values.
As mean arterial and Diastolic pressures are equal in the center and at the periphery, the difference between the mean and Diastolic pressures will be constant as well. The mean arterial pressure value of the central sphygmic wave is defined by the integral of the pressure wave curve. Now, we have the value in mmHg of the difference between mean and Diastolic pressures and the bits corresponding to this value. At this point, it's possible to find the value in mmHg of central Systolic arterial pressure solving the equation.