DAT Practice Test 15

Measuring the Pressure of the Blood

¹Today a blood pressure reading takes about a minute and requires little more than an inflatable cuff, a gauge and, in many clinics, a stethoscope. The measurement is so routine that it is easy to forget how long it took to develop, and how strange the first attempt to make it would look to a modern observer.

²That attempt was made in the eighteenth century by Stephen Hales, an English clergyman with a passion for experiment. In a now-famous procedure, Hales tied down a horse, opened an artery in its leg, and inserted a brass tube connected to a tall glass column. Blood rushed up the column to a height of more than eight feet and then rose and fell slightly with each heartbeat. Hales had shown that blood in the arteries is under considerable pressure and that this pressure pulses with the heart. He also noticed that the level fell as the animal lost blood, an early hint that pressure depends on the volume of fluid in the circulation.

³Hales’s method, however, was useless for medicine. It required cutting into an artery, and a column of blood several feet tall could hardly be carried to a bedside. For more than a century, physicians who wanted to judge the force of the circulation had to rely on feeling the pulse with their fingertips, a skill that some practiced with great subtlety but that yielded no numbers that one doctor could compare with another’s.

⁴Two developments in the nineteenth century prepared the way for a practical instrument. The first was the replacement of the blood column with mercury. Because mercury is about thirteen times denser than blood, the same pressure supports a column only about one-thirteenth as tall, a few inches rather than several feet. This is why blood pressure is still reported in millimeters of mercury. The second development was the idea of measuring pressure indirectly, from outside the body, by finding how much external pressure was needed to stop the flow of blood in an artery. Several inventors built devices along these lines, but most were cumbersome or unreliable.

⁵The design that endured, introduced in the 1890s by an Italian physician, used an inflatable rubber cuff wrapped around the upper arm and connected to a mercury gauge. The examiner inflated the cuff until the pulse at the wrist disappeared and then slowly let air out. The pressure at which the pulse returned marked the systolic pressure, the peak pressure produced as the heart contracts. The device was simple and safe, and it spread quickly. Yet it had a significant limitation: feeling for the pulse could reveal only the systolic value. The lower, diastolic pressure, which persists in the arteries while the heart relaxes and refills, could not be detected this way.

⁶The missing piece came in 1905 from Nikolai Korotkoff, a Russian army surgeon. Korotkoff placed a stethoscope over the artery just below the cuff and listened as the cuff deflated. When the cuff pressure fell just below the systolic pressure, he heard a tapping sound with each heartbeat as blood began to spurt through the partly compressed artery. As the pressure fell further, the sounds changed in character, and at a certain point they disappeared entirely, because blood was again flowing smoothly through a fully open vessel. The pressure at which the sounds vanished corresponded to the diastolic pressure.

⁷Korotkoff’s report was remarkably brief, and it was met with some skepticism; critics questioned whether sounds produced under the cuff could reliably reflect what was happening inside the artery. Over the following years, however, comparisons with direct arterial measurements supported his method, and the auscultatory technique, as it came to be called, became the standard approach for most of the twentieth century. The sounds he described still bear his name.

⁸The two numbers that resulted, systolic over diastolic, made it possible to study blood pressure in large populations, and those studies eventually revealed that persistently high pressure is a major cause of strokes and heart attacks. In recent decades, automated cuffs that detect pulsations electronically have replaced listening in many settings, but they rest on the same principle of balancing an external pressure against the pressure within the artery, the principle first glimpsed in a column of blood rising above a horse.