Applanation tonometry: the PulsePen is based on the applanation tonometry principle. According to this method, the sensor is placed over the skin where the artery is found, applying a moderate pressure so that the artery comes slightly compressed (applanation tonometry) with a balance of the circumferential forces inside the vessel. In this way, the sensor records the pressure in the middle of the compressed artery. Pressure Waveform Analysis and non-invasive acquisition of Central Blood Pressure The PulsePen is an accurate instrument for estimating central aortic pressure and pressure waveforms employing applanation tonometry. Carotid pressure curves recorded with the PulsePen and with an intra-arterial catheter show that the pressure waves recorded at the ascending aorta level can be compared to those measured in the common carotid artery. This site is ideal for the acquisition of central blood pressure with non-invasive methods and the analysis of all its components: - measurement of peak values of Systolic Blood Pressure, Telesystolic Pressure and Telediastolic Pressure at the central level - decomposition of pressure wave, analysis of systole and diastole and definition of Mean Diastolic and Systolic Blood Pressure - definition of the Augmentation Index, related to the level and the early rise time of the reflection wave Pulse Wave Velocity The measurement of the Pulse Wave Velocity (PWV) is a simple and rapid way to assess the compliance of great arteries. Pulse wave flows along the arteries at a speed rate that varies according to the wall elasticity: less elastic the wall, the higher the propagation rate. It's important to point out that PWV is referred to the propagation velocity of the pressure waves and not to the blood flow. The great arteries help in dampening stroke volume pulsatility and in turning the discontinuous flow of the cardiac pump into a continuous precapillary flow. After the stroke volume and with the aortic valves closed, a portion of the blood is still "stuck" in the aortic wall and in the great arteries and is later freed in the diastolic phase (Windkessel effect). This process helps to keep appropriate pressure levels during diastole. This phenomenon is made possible by the viscoelastic properties of the great arteries. Ageing, atherosclerosis, or arterial hypertension alter all anatomical, structural, and functional features of the arteries causing an increased wall rigidity and a loss in arterial compliance. In other terms, differential pressure increases producing higher systolic pressure on one hand, and lower diastolic pressure on the other. The PWV is defined as the distance between the measuring sites divided by the transit time of the related pulse waves. The PulsePen can automatically estimate the transit time in two ways: - using two tonometer probes and capturing the two pulse waves at both sites. - using one tonometer probe and the ECG unit with two measurements in rapid succession. The operator starts positioning the tonometer sensor at the common carotid artery, the central detection site, while simultaneously performing ECG. Then, the same procedure must be followed for the peripheral artery. Transit time was defined as the difference between the delay of the distal pulse wave to the R wave belonging to the ECG qRs complex and the delay of the proximal pulse wave to R wave belonging to the ECG qRs complex. The pulse wave delay can be determined by calculating the time elapsed from the peak of the R wave and the "foot" of the pressure pulse wave The concepts here expressed in the synthetic form are developed in detail in the book "Pulse Waves - How Vascular Hemodynamics Affects Blood Pressure" by Paolo Salvi, Springer Ed.