The sphygmograph, developed by Karl von Vierordt (1818–1884) in 1854, was the first non-invasive device used to estimate blood pressure. Von Vierordt’s sphygmometer was a system of levers and weights placed to determine the amount of external pressure needed to stop blood flow in the radial artery (Fig. 5.3). However, the result was a bulky and rudimentary device, providing inaccurate measurements.

Étienne Jules Marey (1830–1904) improved von Vierordt’s sphygmograph by making it portable and available in clinical practice (Fig. 5.4). He placed, above the radial artery, a specialized instrument able to magnify arterial pulse waves and record them on paper with an attached pen (Fig. 5.5). In addition to being a capable physiologist, Marey was also a physicist of genius. He had a dynamic outlook on life, and this attitude dominated all his studies in the field of physics, anatomy, and physiology.

Photography had just developed in those years, but Marey was not satisfied with giving just a photographic likeness of reality. After all, a photograph was nothing more than a painting, even though a perfect one. He was looking for a more revolutionary invention. His interest in capturing instant of movement and dynamism of things led him to deal with this spirit photography.

Actually, Marey was also the inventor of the first portable camera, called “chronophotographic gun”, in 1882 (Figs. 5.6 and 5.7). This “chronophotographic gun” was equipped with photographic plates, circular or octagonal, placed in a small dark room. The barrel was used as a viewfinder, and the lens were placed inside it. The gun recorded 12 frames/s on the same picture, resulting in a single picture that captured several phases of the subject’s movement. In this way, Marey studied the movements of humans, horses, birds, and other animals, shooting tight movements, elusive to the human eye. In 1888, Marey built a new chronophotographic device able to shoot 60 images/s and of excellent image quality. Thus, Marey is considered to be one of the forerunners of modern cinematography nowadays.

Marey’s sphygmograph paved the way for many other devices for measuring and recording arterial pressure waveforms from the brachial or radial artery. The sphygmographs proposed by scientists and physiologists, such as F.H.H.A. Mahomed (1849–1884), J. Jaquet, R.E. Dudgeon (1820–1904), and L. Landois (1837–1902), were used to record pulse waves and, indirectly, to study blood pressure and cardiac function in clinical research and clinical practice in the last decades of the 19^th century (Figs. 5.8, 5.9, 5.10, 5.11, 5.12, and 5.13). Since then, radial pulse wave recording has been commonly used in classic cardiovascular semiological examinations.

In 1860, Marey was able to measure blood pressure by enclosing an arm in a water-filled glass chamber; water pressure would increase until it stopped the arterial circulation. This point of pressure was identified as systolic blood pressure.

Improving upon these methods, in 1881, Samuel Siegfried Karl Ritter von Basch (1837–1905) built the first sphygmomanometer (Fig. 5.14). He placed a rubber bag around a manometer bulb and filled the bag with water to restrict blood flow in the artery. Then he connected the bulb to a mercury column, which was able to translate the pressure required to completely obscure the pulse into millimeters of mercury; that peak of pressure was called systolic pressure.

The true inventor of the modern sphygmomanometer (Fig. 5.15) was Scipione Riva-Rocci (1863–1937). He placed an air cuff around the arm’s circumference. Thanks to the uniform distribution of the compression around the arm, it was possible to avoid common errors related to blood pressure measurements, which previous devices used to do. He also led to the measurement of arterial blood pressure at the level of the brachial artery, which was more accurate than the radial artery. The pressure in the cuff would increase until the radial pulse was no longer present. At this point, the pressure in the cuff was released. The pressure at which the radial pulse reappeared was recorded as systolic pressure. Riva-Rocci published the results of his studies in the Gazzetta Medica Torinese in 1896. However, the assessment of diastolic blood pressure remained elusive with the Riva-Rocci’s sphygmomanometer. The ability to measure diastolic pressure was first achieved in 1905 with the auscultatory method, proposed by Nikolai Korotkoff (1874–1920), using a sphygmomanometer and a stethoscope.

The cuff of Riva-Rocci is placed on the middle third of the upper arm; the pressure within the cuff is quickly raised up to complete cessation of circulation below the cuff. Then, letting the mercury of the manometer fall one listens to the artery just below the cuff with a children’s stethoscope. At first no sounds are heard. With the falling of the mercury in the manometer down to a certain height, the first short tones appear; their appearance indicates the passage of part of the pulse wave under the cuff. It follows that the manometric figure at which the first tone appears corresponds to the maximal pressure. With the further fall of the mercury in the manometer one hears the systolic compression murmurs, which pass again into tones (second). Finally, all sounds disappear. The time of the cessation of sounds indicates the free passage of the pulse wave; in other words, at the moment of the disappearance of the sounds the minimal blood pressure within the artery predominates over the pressure in the cuff. It follows that the manometric figures at this time correspond to the minimal blood pressure.

(Nikolai Korotkoff. 1905. Reports of the Imperial Military Medical Academy of St Petersburg)

However, the use of arterial pressure measurement in clinical practice was not universally accepted until much later. An editorial, published in 1905 in the British Medical Journal, argued that “by such methods (sphygmomanometry) we pauperize our senses and weaken clinical acuity”. In this regard, a wise judgment is attributed to Arthur Schopenhauer (1788–1860): “All truth passes through three stages. First, it is ridiculed. Second, it is violently opposed. Third, it is accepted as being self-evident”.

As a matter of fact, the introduction of the cuff sphygmomanometer caused an interruption in the study of pulse wave recording and analysis. Besides, the possibility of measuring systolic and diastolic blood pressure, i.e., the zenith and the nadir, respectively, of the pulse pressure wave, made pulse wave recording outdated, under the belief that these measurements could provide a full explanation of vascular hemodynamics, throughout the 20^th century.

In the second half of the 20^th century, blood pressure values became an object of increasing clinical interest and a target for therapeutic intervention. The operational model being used was simplified and reduced to a pump working against a peripheral vascular resistance. Although certainly oversimplified, it cannot be denied that this model helped to effectively antagonize high blood pressure, which is the most important risk factor for cardiovascular disease, achieving one of the greatest medical success of the past century, with a significant reduction in cardiovascular mortality.

When the registration of pulse waves was technically possible using invasive catheterization, pressure wave morphology was brought to the forefront once again, and physiologists and interventional cardiologists were mostly interested in aortic blood pressure measurement and pulse wave recording.

Over the past 20 years, there has been a radical change in scientific knowledge, thanks to clinical research, and this has dramatically changed the approach to the hypertensive patient. The single acquisition of blood pressure values was considered to be inadequate to rightly and entirely study patients with high blood pressure values.

Marey’s intuition regarding a dynamic outlook of blood pressure and all physical phenomena was taken up again.

The “direct method” records pulse wave in common carotid artery; it is a surrogate for ascending aortic pressure because these arterial sites are in close proximity.

It has been widely tested that the shape of the pressure wave in the ascending aorta is similar to the one recorded in the common carotid artery, so that direct application of tonometry in the carotid artery is an easy and reproducible approach to record central blood pressure. Moreover, the carotid artery is generally well accessible and superficial (Figs. 5.17 and 5.19), and good quality carotid waveforms can be easily obtained even in obese patients. This technique is reliable for routine high-throughput screening of central pressure.

This is the method used by the PulsePen^® (DiaTecne srl, Milan, Italy) and the Complior Analyse^® (Alam Medical, Vincennes, France).

The validity of transcutaneous tonometry in measuring the aortic central pressure wave is based on two principles, both widely validated and tested.

With the “indirect method”, applanation tonometry is performed in the radial artery (Fig. 5.25); using a generalized transfer function, the central pressure waveform is rebuilt, starting from the waveform recorded in the radial artery calibrated to the blood pressure values measured in brachial artery [11] (Fig. 5.26).

This indirect method is used by the SphygmoCor^® (AtCor Medical Pty. Ltd., Sydney, Australia). A similar method is used by the Omron HEM-9000AI^® (Omron Healthcare Co. Ltd., Kyoto, Japan), which uses peripheral pressure waveforms recorded by tonometry at the site of the radial artery in order to calculate central systolic blood pressure via a regression equation employing the second systolic peak as an independent variable.

Several researchers are skeptical about this indirect system, most of all in the extreme phases of life or under particular hemodynamic conditions. One wonders why people make things more difficult and analyze the radial arterial pressure waveform, when, in most cases, access to the carotid artery is much easier (measurement of central pressure wave with direct method).

The results of the Asklepios Study [12] showed the occurrence of significant differences between pulse pressure (and systolic blood pressure) values assessed in brachial artery and those obtained in radial artery, differences which were much greater than the differences between central arterial pressure and brachial arterial pressure values (Fig. 5.27). In other words, pulse pressure amplification is more marked in the brachio-radial arterial segment than in the aortic-axillo-brachial one. If a peripheral waveform is calibrated to brachial systolic and diastolic cuff pressure values, given the abovementioned differences between blood pressure parameters derived from different arterial segments, such procedure might introduce relevant errors in the estimation of central blood pressure. Thus, as a general consideration, caution is needed whenever using algorithms for central pressure estimation based on the measurement of pressure values in brachial artery to calibrate a pressure wave recorded in radial artery.

The main limitation of applanation tonometry is that it cannot provide absolute values of arterial pressure.

A tonometer is able to define pulse pressure values, but it is unable to provide accurate values of diastolic and systolic blood pressure. These are gathered starting from the concept, clearly shown by now, that mean arterial pressure remains constant from the aorta to the peripheral arteries, as does diastolic blood pressure (which tends to decrease, albeit insignificantly, i.e., by <1 mmHg, from the center to the periphery). To sum up, a calibration of tonometric pressure wave using brachial arterial pressure values is always required, in particular when central blood pressure is estimated through the analysis of more or less peripheral pulse waves.

How then are effective values of arterial pressure on the central pressure wave obtained?

Simultaneously, with the recording of transcutaneous tonometry, the value of arterial blood pressure in the brachial artery is measured by a traditional, validated sphygmomanometer. Then mean arterial pressure is calculated from diastolic and systolic blood pressure values. As mean arterial and diastolic pressures are equal in the center and at the periphery, the difference between mean and diastolic pressures will be constant as well. Subsequently, the mean blood pressure value of the central sphygmic wave is defined by the integral of the pressure waveform (Fig. 5.28).

Now, we have the value in mmHg of the difference between mean and diastolic blood pressures and the bits corresponding to this value. At this point, all we have to do is to solve the equation to find the value in mmHg of central systolic arterial pressure:

The calibration of pulse wave assures that arterial tonometry is performed accurately, as it makes tonometry free from all the variables which could alter the test, such as the pressure exerted by the operator with the probe on the skin when the pressure signal is recorded, and anatomical factors such as the depth of the artery.

Moreover, the examination of the pulse pressure waveform and the definition of pressure values occurs by analyzing the pulse pressure waveform beat to beat; in this way, all variables related to instability in the signal offset secondary to respiratory movements of the patient are removed.

To obviate the problem linked to the choice of the algorithm to define mean arterial pressure starting from the brachial artery, the PulsePen^® tonometer provides the possibility of defining the real value of mean arterial pressure starting from the integral of the tonometric pressure wave recorded in the brachial artery. Theoretically, the most reliable method to derive calibrated central blood pressure values from analysis of radial or carotid waveforms should be based on brachial pulse waveform recordings, calibrated by systolic and diastolic blood pressure measured through a validated sphygmomanometer at the same brachial artery level. Subsequently, the mean arterial pressure so calculated from the integral of the brachial pressure waveform can be used, together with the corresponding diastolic blood pressure, to calibrate pulse waveforms recorded elsewhere. Actually, with a standard procedure, the inter- and intraobserver reproducibility of pulse waveforms recorded at brachial site is weak because the brachial artery lies beneath the stiff bicipital aponeurosis; it is not supported by bone and, thus, cannot be reliably flattened under the sensor. However, the reliability of brachial pulse wave recordings may significantly increase by placing the index and middle fingers against the lateral wall of the brachial artery aimed at preventing its displacement by the probe and at allowing the adequate recording of pulse waveforms (Fig. 5.29).