There blood pressure hasn’t always been an easy issue to read. For a long time it was, first of all, a force to be made visible. In the first experiments it was observed directly, connecting an artery to a glass column and looking how far the blood reached. This first invasive and cumbersome system was initially made more practical, measurable and stable thanks to the introduction of mercury, which allowed the dimensions of the column to be reduced: being much denser than blood, the same pressure could be represented by columns of a few centimetres At a certain point it becomes clear that it is not essential to enter the blood vessel: if an artery from theexternal with a finger and gradually increase the pressure, the flow stops; when we no longer apply pressure, the blood starts flowing again. This threshold is directly linked to the pressure exerted by the blood in the arteries. This is how the first non-invasive tools were born, based on palpation. Over time, attention shifts to more stable mechanical signals than simple palpation and we begin to understand and interpret the signals that allow us to recognize two distinct moments: the beginning of the flow and the return to a smooth passage. This is where the blood pressure measurement stops depending only on the fingers and becomes a reproducible physical reading.
The pressure was measured by watching the blood rise in a column
The first documented attempt to measure blood pressure had nothing “clinical” about it. In the eighteenth century, Stephen Hales inserted a tube directly into a horse’s artery and observed how high the blood rose within a vertical column. The blood stopped rising when the weight of the column of blood counterbalanced the blood pressure. It was a crude and extremely invasive measure, but revolutionary: for the first time the blood pressure became visible.
That method, however, had two enormous limits. Era invasive And it only worked in the laboratory. Furthermore, the relatively low density of the blood required a very tall column to equalize the pressure. For this reason, in the nineteenth century, Jean-Léonard Poiseuille introduced the mercury: more stable and easier to read. But above all, being much thicker than bloodthe same pressure could be measured with a much shorter and more manageable column.
Don’t worry: the mercury didn’t enter the artery. The latter was connected via a cannula to a closed system filled with liquid, which transmitted pressure up to an external mercury column. Mercury, therefore, served only as indicator: transformed the pressure into a readable height. This is how they are born millimeters of mercury (mmHg)the unit of measurement of pressure that we still use today.
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The practical twist: stop the wrist from the outside
The real change of perspective came in the mid-nineteenth century, when Karl von Vierordt demonstrated that there was no need to see blood: just stop your wrist. If sufficient pressure is applied to an artery (typically the wrist) from the outside, the flow stops, demonstrating that pressure can be estimated by applying external back pressure to the artery. And the pressure needed to do so is linked to that which the blood exerts from the inside. It’s a simple idea, but a very good one. Blood pressure is no longer measured “inside” the body, but “against” the body.
The first non-invasive sphygmomanometer
Karl Samuel Ritter von Basch works on this intuition. His instrument, introduced in the 1880s, is the first clinically usable sphygmomanometer. He doesn’t have the bracelet we know yet: he uses one small sphere filled with water resting on the radial artery of the wrist. The sphere is connected to a pressure gauge. The pressure increases until the pulse is no longer perceptiblethat is, until you can no longer feel the wave of blood pulsating in the artery under your fingers. At that moment the flow is temporarily interrupted by external pressurewhich is slowly reduced until it reappears. The value read at that moment corresponds to the systolic pressure. “Systolic” means the maximum pressure, the one exerted when the heart contracts (or to be more precise, when the left ventricle contracts). Von Basch does not measure the minimum (diastolic), but introduces the measurement of blood pressure palpation and lays the foundation for everything that comes after.
The introduction of the bracelet and the shape we know today
In 1896 Scipione Riva-Rocci introduced the decisive element: the inflatable cuff that wraps around the entire arm. The principle remains the same, but the distribution of force changes. A single point is no longer compressed, the brachial artery is compressed uniformly. The pressure gauge is mercury, inflation takes place with a pump and the reading is more stable.
Here too the measurement is palpatory: you feel the pulsation and observe when it disappears. Once again only the systolic pressure is obtained. But the tool is simple, repeatable, transportable. That’s why it spreads rapidly throughout the world.
From palpation to mechanical signals
In late nineteenth-century devices, such as the one described by Hill and Barnard, the pressure of the cuff was read via an index connected to a aneroid manometeri.e. mercury-free, based on a metal membrane that deforms with pressure.
During the slow deflation, the indicator began to oscillate as blood began to flow again in the compressed artery. According to some descriptions, the appearance of oscillations indicated systolic pressure, while the transition from large oscillations to smaller oscillations was associated with diastolic pressure, but the real distinction between these two signals will come a little later. Hill and Barnard’s approach, however, marked an important transition: the measurement no longer depended only on palpation of the pulse, but on observable mechanical signals.
The sounds of the sphygmomanometer
The final piece comes in 1905, when Nikolaj Korotkoff notices that, as the cuff deflates, bubbles appear in the artery noises, whose identification allowed him to distinguish the two blood pressure thresholds more reliably. Those sounds arise from the turbulent flow of blood that starts flowing again. The first sound you hear corresponds to systolic. Their disappearance indicates the pressure diastolic, that is, the minimum pressure between one beat and another. From that moment the sphygmomanometer becomes a complete instrument as we know it today.
