With the latest update in NICE hypertension guidelines1 and health and safety concerns with the use of mercury,2,3 electronic sphygmomanometers are now very widespread for the detection, diagnosis and monitoring of raised blood pressure. At our recent annual recalibration check, seven electronic sphygmomanometers were checked in The Project Surgery against a standard mercury column. Five were found to be within ‘acceptable’ tolerance of +/-5 mmHg of true at 100 and 200 mmHg, and two were found to be unacceptably inaccurate, one at 8% inaccurate and one 13% inaccurate and were replaced.
Given that therapeutic choices are made on very small changes on blood pressure reading, and we are invited by NICE to use unmaintained home blood pressure monitoring in diagnosis, even small systemic errors in manometer calibration can have a huge impact on individual patient care.
The only universally accurate manometer one can use to measure blood pressure is a mercury column. Until such time as gravity changes a vertical column of 100 mm of mercury will always be 100 mmHg. On our automated machines before their annual check what was said to be 100 mmHg could have been anything from 92 to 113 mmHg. How can I advise patients when the basic data is so poor? Even the so-called ‘accurate’ machines can be 5 mmHg out.
I wonder if in our rush to electronic devices we have sacrificed accuracy in favour of convenience. If one adds to this issues around monitors usually being supplied with a ‘standard’ cuff which is too small for the standard UK arm,2,3 and automated sphygmomanometers being unsuitable to detect pre-eclampsia or use in atrial fibrillation,3 is it now time for a rethink our basic surgery equipment? Is the need for a mercury spillage kit too high a price to pay for correct data?
- © British Journal of General Practice 2012