Why Blood Pressure Is Measured in mmHg Rather Than PSI

Blood pressure is a critical parameter in medicine, often measured in millimeters of mercury (mmHg). This measurement trace back to the earliest methods of measuring pressure, with the primary instrument being a mercury column. Understanding why blood pressure is measured in mmHg rather than PSI (pounds per square inch) involves delving into the origins of the measurement and the practical benefits it offers.

Where Does mmHg Come From?

The history of measuring blood pressure began with the invention of the sphygmomanometer, a device that utilizes a mercury column. The concept is simple yet profound. Imagine filling a glass tube with mercury, sealing one end, and submerging the sealed end in a bath of mercury. The atmospheric pressure will push up the mercury in the tube, creating a column. The height of this column is a direct measure of the atmospheric pressure, which was later used to measure blood pressure. This method, known as the mercury column method, established the use of mmHg as the standard unit for blood pressure measurements.

Why Not Use Pascal (Pa) for Blood Pressure Measurements?

It is theoretically possible to measure blood pressure using the standard unit Pascal (Pa). Pascal is the SI unit of pressure and is defined as one newton per square meter. However, the transition to using Pa for blood pressure measurements would have required significant changes in medical practices. The primary reason for sticking with mmHg is historical and practical.

Historical Context and Convenience

The founding fathers of medicine chose mmHg for blood pressure measurements for a simple reason of convenience. When mercury columns were standardized, their consistent and repeatable measurements became a reliable standard. Clinicians and researchers relied on the stability offered by the mercury column, establishing a widespread and unchanging method. If early medical practitioners had chosen Pascal (Pa) instead, the healthcare industry would have had to adapt to a new measurement system, which might have been cumbersome and disruptive.

Practical Benefits of mmHg

Mercury, used in these columns, is highly dense and does not evaporate easily, providing consistent and reliable measurements every time. This stability ensures that blood pressure readings are accurate and repeatable across different individuals and settings. The mercury column, though historical, is still used today as a standard for blood pressure measurements, often referred to as the "gold standard."

Conversion to Other Units

While blood pressure is most commonly reported in mmHg, it can also be converted to other units. For instance, one millimeter of mercury is approximately 133.322 Pascals. Converting mmHg to inches of mercury (inHg) is also common, with 1 mmHg being roughly equal to 0.03937 inHg (considering 1 inch 25.4 mm), as mentioned in the fluid power industry.

Conclusion

Understanding why blood pressure is measured in mmHg rather than PSI (pounds per square inch) involves a journey through the history of medical measurement and practical considerations. The use of mmHg is deeply rooted in convenience and the reliable, consistent measurements provided by the mercury column. As long as these benefits hold true, the mmHg measurement will likely remain the standard for blood pressure readings in the medical community.