Blood pressure is a measurement of the force exerted by circulating blood against the walls of the arteries. This measurement is expressed as two numbers: systolic pressure, the higher number representing the pressure when the heart beats, and diastolic pressure, the lower number reflecting the pressure when the heart rests between beats. Tracking these numbers is necessary because consistently high blood pressure (hypertension) is often called the “silent killer,” rarely showing obvious signs until significant damage occurs. The long-term strain from high blood pressure can lead to serious conditions, including heart attack, stroke, and kidney disease. Due to the importance of frequent monitoring, there is growing interest in new technologies that can estimate blood pressure without the use of a traditional cuff.
Cuffless Devices and Wearable Technology
New wearable devices like smartwatches, rings, and specialized patches are moving beyond basic heart rate tracking to estimate blood pressure using sophisticated sensor technology. These devices rely on physiological signals that correlate with blood pressure changes, offering the potential for continuous, less intrusive monitoring.
One of the main concepts utilized is Pulse Transit Time (PTT), which is the time it takes for a pressure wave to travel between two points in the arterial system, typically from the heart to a peripheral location like the wrist or finger. Blood pressure is inversely related to PTT; as blood pressure rises, the arteries stiffen and the pulse wave travels faster, resulting in a shorter PTT. To measure this, devices use an electrocardiogram (ECG) to detect the heart’s electrical signal and a photoplethysmography (PPG) sensor to detect the arrival of the pulse wave. The PPG sensor works by shining light onto the skin and measuring the changes in light absorption that occur as blood volume fluctuates with each heartbeat, providing the necessary timing data to calculate PTT.
Another technological approach for cuffless estimation is arterial tonometry, which involves pressing a sensor against an artery, often the radial artery in the wrist, to flatten it against a bone. This non-invasive method measures the pressure required to flatten the artery, which reflects the internal blood pressure. PTT remains a widely explored technique for wearable devices due to the ease of incorporating PPG sensors into small form factors like smartwatches.
Estimating Blood Pressure Through Physical Signs
Approximating blood pressure without any device is limited to recognizing severe symptoms that suggest a person’s pressure is dangerously high or low. Extremely high blood pressure, known as a hypertensive crisis, can manifest with symptoms such as a severe headache, dizziness, blurred vision, or a pounding sensation in the chest, neck, or ears. However, most people with mild to moderate high blood pressure experience no symptoms at all.
Conversely, symptoms of very low blood pressure (hypotension) include lightheadedness, dizziness, fainting, nausea, and a rapid or weak pulse, resulting from insufficient blood flow to the brain and other organs. Checking the pulse rate and strength manually can provide a crude correlation, as a weak, rapid pulse might suggest shock or severe hypotension. These physical signs are indicators of a potential medical emergency and are not a substitute for an actual blood pressure measurement. Relying on symptoms alone is unreliable and dangerous, as it only identifies extreme pressure fluctuations, leaving the vast majority of hypertension cases undetected.
Why Cuff Calibration Remains Essential
Despite the advances in wearable technology, the traditional oscillometric cuff remains the gold standard for non-invasive blood pressure measurement in clinical and home settings. The cuff-based method directly measures the pressure required to stop and then allow blood flow in the artery, providing a highly accurate, absolute pressure reading. Cuffless devices, particularly those using PTT, do not measure absolute pressure directly; they measure a surrogate, like pulse wave speed, which must be converted into a blood pressure number using a complex algorithm.
This conversion process requires initial calibration against a known, accurate reading from a standard cuff-based device. Because the relationship between PTT and blood pressure is influenced by individual factors like arterial stiffness and vessel size, this calibration accounts for inter-individual differences. Furthermore, the accuracy of the cuffless devices can drift over time due to changes in a person’s physiology or even sensor contact pressure, necessitating periodic recalibration to a traditional cuff. While cuffless technology is useful for tracking trends and providing continuous data, it cannot replace the diagnostic reliability of a properly validated and regularly calibrated cuff device.