The rise of at-home blood pressure monitoring has made it easier for people to track their cardiovascular health, leading to the increased popularity of convenient wrist-style monitors. These devices appeal to users because they are compact, easy to apply, and portable for travel. However, the convenience of a wrist cuff introduces questions about its accuracy and how the readings compare to the traditional upper arm measurement. The location of the measurement on the body directly influences the resulting numbers, making it important to understand the inherent differences between the wrist and the upper arm.
Understanding the Typical Measurement Discrepancy
A wrist blood pressure reading is often less reliable than one taken from the upper arm, and it can frequently register higher values. Studies comparing simultaneous measurements show that systolic pressure readings from the wrist can be elevated by an average of 5 to 10 mmHg compared to the upper arm device. This difference is highly variable from person to person and depends heavily on technique and the specific device used.
This variability is why health organizations, including the American Heart Association (AHA), generally recommend using an automatic, cuff-style monitor on the upper arm. The upper arm model, which measures pressure in the brachial artery, is considered the gold standard for at-home monitoring due to its consistent reliability. When a wrist device is used for regular tracking, the potential for significant measurement error is higher.
Anatomical and Physiological Sources of Variation
The difference in readings between the wrist and the upper arm is explained by the physics of blood circulation and the anatomy of the measurement sites. Blood pressure naturally changes as blood flows farther away from the heart and through smaller arteries. The upper arm cuff measures pressure in the brachial artery, which is a larger vessel closer to the central circulation. In contrast, the wrist cuff measures pressure in the smaller radial artery.
Measuring pressure farther down the circulatory path makes the reading more susceptible to various physiological factors. The most significant factor is hydrostatic pressure, which is the pressure exerted by a fluid due to the force of gravity. If the wrist is held below the level of the heart, gravity causes blood to pool slightly, artificially increasing the pressure reading. Conversely, holding the wrist too high can cause an artificially low reading.
Essential Technique for Accurate Wrist Measurement
The largest cause of inaccuracy in wrist monitors is incorrect user technique, which drastically increases the measurement discrepancy. To obtain the most reliable reading, the wrist cuff must be positioned exactly at heart level. This is typically accomplished by placing the hand on the opposite shoulder or resting the forearm on a table with the wrist near the chest. Failure to maintain this heart-level positioning can introduce significant error; for example, a wrist held just 30 centimeters below the heart level can inflate the reading by as much as 40 mmHg.
Before taking a measurement, a person should sit quietly for at least five minutes with their back supported and feet flat on the floor. It is also important to refrain from talking, using a phone, or crossing the legs during the measurement process. Additionally, ensuring the cuff is correctly fitted and placed directly over the radial artery on the bare skin, not over clothing, helps guarantee that the device is sensing the pulse accurately.
Practical Considerations for Monitor Selection
While the upper arm monitor is generally preferred for its accuracy, a wrist monitor can be a practical alternative in specific circumstances. For individuals with a very large upper arm circumference, where a standard or large upper arm cuff does not fit properly, a wrist device may be a necessary option. They are also useful if a medical condition like lymphedema or an injury prevents the use of an upper arm cuff.
However, these devices are less suitable for people with certain health conditions, such as severe hypertension or an irregular heartbeat, because the measurement method is less robust against these variables. Any wrist monitor chosen for regular health tracking should be validated against standardized testing protocols. It is also recommended that a person take their wrist monitor to a healthcare provider to compare its reading against the office’s professional device, confirming the margin of difference for that specific user.