Limitations of blood pressure monitors
Mercury manometers are the gold standard for measuring blood pressure. It works by listening to the blood flow through the vein. When cuff of a blood pressure monitor is inflated and the artery is completely closed, there is no sound. As the pressure is slowly released and blood starts rushing into the artery again, a pounding sound is created. When the pressure is further reduced, the pounding sound stops completely. The two pressure measured when the sound is heard and disappeared correspond to the systolic and diastolic blood pressure respectively.
This auscultation method requires a trained personnel and is used mostly when precision is required. Oscillometric detection is the most commonly used at home and in other environment. It measures the mean arterial pressure (MAP) and derive the readings. There are certain situations where this method does not produce accurate result.
Position: The cuff and the monitor should be at the same level of the heart; otherwise the reading has to be adjusted due to the difference in height. This is especially true for wrist cuff blood pressure monitors.
Patient Movement: Measurements will be unreliable or cannot be performed if the patient is moving, shivering or having convulsions. These motions may interfere with the detection of the arterial pressure pulses. In addition, the measurement time will be prolonged.
Cardiac Arrhythmia: Measurements will be unreliable and may not be possible due to irregular heartbeats caused by cardiac arrhythmia.
Rapid pressure change: If the arterial pulse pressure changes rapidly during measurement, the blood pressure monitor would not be able to obtain a good reading.
Severe Shock: When the patient is in severe shock or having hypothermia, blood flow would be reduced resulting in weak pulses. The weak signal may lead to inaccurate readings.
Heart rate: If the heart beats too fast (>240bpm) or too slow (<40bpm), measurement would be difficult.
Fat: If there is a thick layer of fat underneath the skin, fat has a damping effect on the signal, which may be too weak when it reaches the sensor in the cuff.