Blood Pressure Units: mmHg Explained for Patients

When your doctor or nurse takes your blood pressure, you hear numbers like "120 over 80." But what do these numbers actually mean, and why is blood pressure measured in such an unusual unit? This comprehensive guide explains everything patients need to know about blood pressure units, the history behind mmHg, and how to interpret your readings.

What is mmHg?

mmHg stands for "millimeters of mercury" - a unit of pressure that measures how high a column of mercury rises in a tube. One mmHg equals the pressure exerted by a 1-millimeter-tall column of mercury at 0 degrees Celsius under standard gravitational acceleration.

In more technical terms:

• 1 mmHg = 133.322 pascals (Pa)
• 1 mmHg = 0.133322 kilopascals (kPa)
• 760 mmHg = 1 atmosphere (standard atmospheric pressure at sea level)

Despite living in an age of digital technology and SI units, we continue using this seemingly archaic measurement for blood pressure. Understanding why requires a look back at medical history.

The History of Mercury Sphygmomanometers

The Birth of Blood Pressure Measurement

The story of blood pressure measurement begins in the 18th century. In 1733, English clergyman Stephen Hales made the first recorded blood pressure measurement by inserting a glass tube directly into a horse artery and observing how high the blood rose.

However, the modern non-invasive method we use today was developed much later:

1881 - Samuel Siegfried Karl Ritter von Basch created the first sphygmomanometer that did not require cutting into the patient.

1896 - Italian physician Scipione Riva-Rocci introduced the mercury sphygmomanometer with an inflatable cuff - essentially the same design principle used for over a century.

1905 - Russian surgeon Nikolai Korotkoff developed the auscultatory method, using a stethoscope to listen for specific sounds (now called Korotkoff sounds) that indicate systolic and diastolic pressure.

Why Mercury Was Used

Mercury sphygmomanometers became the gold standard for several reasons:

1. Density: Mercury is extremely dense (13.6 times denser than water), meaning a relatively short column can measure significant pressures
2. Visibility: The silvery liquid is easy to see against a graduated scale
3. Accuracy: Mercury does not compress, providing consistent and reliable readings
4. Calibration: Mercury devices rarely need recalibration if properly maintained

The mercury column rose and fell in direct proportion💡 Definition:A fraction or ratio expressed as a number out of 100, denoted by the % symbol. to the pressure in the cuff, making measurements straightforward and reproducible.

The Transition Away from Mercury

Due to environmental and safety concerns about mercury toxicity, many countries have phased out mercury sphygmomanometers. The European Union banned mercury devices in healthcare settings in 2009. Today, most clinical settings use:

• Aneroid sphygmomanometers (mechanical dial gauges)
• Automated oscillometric devices (digital monitors)

Despite this transition, we retain mmHg as the measurement unit because:

• Decades of clinical research and treatment guidelines are based on mmHg values
• Changing units would cause confusion and potential medical errors
• International standardization ensures consistent communication between healthcare providers

Understanding Systolic and Diastolic Readings

Blood pressure readings consist of two numbers, such as 120/80 mmHg (spoken as "120 over 80"):

Systolic Pressure (Top Number)

The systolic pressure is the higher number and represents the pressure in your arteries when your heart beats and pumps blood out. This is the maximum pressure your blood vessels experience.

Think of it as the "push" phase - when your heart contracts (systole), it forces blood through your arteries, creating a pressure wave.

Diastolic Pressure (Bottom Number)

The diastolic pressure is the lower number and represents the pressure in your arteries between heartbeats, when your heart is resting and refilling with blood. This is the minimum pressure in your vessels.

This is the "rest" phase - when your heart relaxes (diastole), the pressure in your arteries drops but does not reach zero because the arterial walls maintain some tension.

Why Both Numbers Matter

Both readings provide important health information:

• Systolic pressure indicates how hard your heart is working and the condition of your large arteries
• Diastolic pressure reflects the resistance in your blood vessels and the health of your peripheral arteries

Either number being too high can indicate cardiovascular risk💡 Definition:Risk is the chance of losing money on an investment, which helps you assess potential returns., though research suggests systolic pressure becomes increasingly important as we age.

Blood Pressure Categories and Ranges

The American Heart Association and other medical organizations define blood pressure categories as follows:

What These Categories Mean

Normal: Your blood pressure is in a healthy range. Maintain healthy habits to preserve this.

Elevated: You are at risk of developing high blood pressure. Lifestyle changes like diet and exercise can help prevent progression.

Stage 1 Hypertension: Your doctor may recommend lifestyle changes and possibly medication, depending on your overall cardiovascular risk.

Stage 2 Hypertension: Typically requires medication along with lifestyle modifications.

Hypertensive Crisis: Seek immediate medical attention if your readings consistently exceed 180/120 mmHg, especially if you experience symptoms like chest pain, shortness of breath, or vision changes.

Converting Between mmHg and kPa

While most of the world uses mmHg for blood pressure, some countries and scientific contexts prefer kilopascals (kPa), the SI unit for pressure.

The Conversion Formula

mmHg to kPa: Divide by 7.5 (or multiply by 0.133322) kPa to mmHg: Multiply by 7.5 (or divide by 0.133322)

Quick Conversion Examples

Converting Your Full Reading

To convert a blood pressure reading like 120/80 mmHg to kPa:

• Systolic: 120 ÷ 7.5 = 16.0 kPa
• Diastolic: 80 ÷ 7.5 = 10.7 kPa
• Result: 16.0/10.7 kPa

Why Some Countries Use kPa

Several countries, particularly in Europe, have adopted kPa for blood pressure measurement as part of broader metrication efforts:

Countries Using kPa

• France
• Portugal
• Some Scandinavian countries
• Parts of South America

Reasons for Using kPa

1. SI Standardization: kPa is part of the International System of Units, promoting scientific consistency
2. Environmental Concerns: Moving away from mercury-associated units
3. Decimal System: Easier integration with other metric measurements

Why Most Countries Stick with mmHg

1. Historical Data: Decades of clinical trials and guidelines use mmHg
2. International Communication: Most medical literature uses mmHg
3. Familiarity: Healthcare providers and patients recognize mmHg values
4. Precision: mmHg provides finer gradations for clinical decision-making

Tips for Accurate Blood Pressure Measurement

Getting accurate readings is crucial for proper diagnosis and treatment. Follow these guidelines:

Before Measurement

• Avoid caffeine, smoking, and exercise for at least 30 minutes before measuring
• Empty your bladder - a full bladder can raise readings
• Rest for 5 minutes in a quiet environment before taking measurements
• Avoid talking during the measurement

During Measurement

• Sit correctly: Back supported, feet flat on the floor, legs uncrossed
• Position your arm💡 Definition:An Adjustable Rate Mortgage (ARM) offers lower initial rates that can change over time, making homeownership more affordable.: Supported at heart level on a table or armrest
• Use proper cuff size: The cuff bladder should cover 80% of your upper arm circumference
• Place the cuff correctly: Lower edge should be 2-3 cm above the elbow crease

Best Practices

• Take multiple readings: Measure 2-3 times, 1-2 minutes apart, and average the results
• Measure at the same time daily: Blood pressure naturally fluctuates throughout the day
• Keep a log: Record your readings with date, time, and any relevant notes
• Use a validated device: Ensure your home monitor is approved by organizations like the British Hypertension Society or AAMI

Factors That Can Affect Readings

• White coat syndrome (anxiety in medical settings)
• Stress and anxiety
• Temperature extremes
• Recent meals
• Arm position (too high or too low)
• Talking during measurement
• Unsupported back or feet

Quick Reference Table

Here is a comprehensive reference combining blood pressure categories with kPa equivalents:

Conclusion

Understanding blood pressure units helps you become a more informed patient. While mmHg may seem like an outdated measurement from the era of mercury instruments, it remains the international standard for communicating blood pressure values. Whether your reading is reported in mmHg or kPa, what matters most is knowing your numbers, understanding what they mean, and working with your healthcare provider to maintain healthy blood pressure levels.

Regular monitoring, accurate measurement technique, and awareness of your target ranges are key to cardiovascular health. If you have concerns about your blood pressure readings, always consult with a qualified healthcare professional who can provide personalized guidance based on your complete medical history.