This wildly inaccurate device, the Gaertner Tonometer, was devised to estimate the force needed to arrest pulsation in a finger encircled by a compressing ring. This force was expressed by the height of a column of mercury.
Contrary to popular belief, it was not designed to measure blood pressure, but rather permitted one to know what pressure applied to the second joint of the finger, would prevent the blood from entering the finger, as shown by the sensation and change of color which followed the return of the blood. Gaertner declared this to be the mean arterial pressure (MAP), which is of course completely inaccurate.
Mean arterial pressure is defined as the average pressure in a patient’s arteries during one cardiac cycle. It is a better indicator of perfusion to vital organs than simple systolic blood pressure. True MAP can be determined in a highly complicated and invasive test, but can also be calculated using a relatively simple formula using the systolic and diastolic (SBP and DBP of the patient.
To calculate a mean arterial pressure, double the diastolic blood pressure and add the sum to the systolic blood pressure. Then divide by 3. For example, if a patient’s blood pressure is 83 mm Hg/50 mm Hg, his MAP would be 61 mm Hg.
Here are the steps for this calculation:
MAP = (SBP + (2 x DBP))/3
MAP = (83 + (2 x 50))/3
MAP = (83 + 100)/3
MAP = 183/3
MAP = 61 mmHg
Or you could just use the Pulse Pressure:
To calculate Pulse Pressure (PP) simply subtract the diastolic from the systolic pressure. Then to get the MAP, simply divide the PP by 3, then add the DBP:
MAP = (SBP – DBP)/3 + DBP
MAP = (83-50)/3 + 50
MAP = 33/3 + 50
MAP = 11 + 50
MAP = 61 mm Hg
There are several clinical situations in which it is especially important to monitor mean arterial pressure. In patients with sepsis, vasopressors are often titrated based on the MAP. For sepsis patients it is recommended that mean arterial pressure (MAP) be maintained ≥65 mm Hg. Also, in patients with head injury or stroke, treatment may be dependent on the patient’s MAP.
Made in 1894 in Vienna, this stunning example of medical quackery was invented by Gustav Gaertner, an Austrian pathologist. Gaertner earned his doctorate in 1879 from the University of Vienna, where he conducted scientific studies of kidney functions, experiments dealing with electrical skin resistance and research involving innervation of vessels of the brain. He was one of the early protagonists of the use of cocaine as an anaesthetic during eye surgeries.
The Gaertner Tonometer is kind of a useless device, but it is still a beautiful example of human ingenuity, and does demonstrate the inventiveness of the medical fraternity at the turn of the century.
Discover the Gaertner Tonometer, and other stunning medical devices, on www.bloodpressurehistory.com