When BP fluctuates due to machine’s fault

Pushpa Girimaji

How accurate are the blood pressure instruments used in hospitals and clinics? I had gone for an annual health check up last week and my blood pressure was checked in three different departments and all of them recorded different readings. Is there no quality control over these instruments?

Well, some variation could be attributed to stress or exercise or some fault in the way the measurement was taken, but I will certainly not rule out the possibility of the blood pressure measuring instrument or the sphygmomanometer being defective.  

In fact a study titled “Equipment Errors: A prevalent cause for fallacies in blood pressure recording-A Point Prevalence and Estimate from an Indian Health University”, published in the Indian Journal of Community Medicine (B. Mishra, et al, Jan-March 2013) throws the spotlight on the lack of quality control and standardisation in these sphygmomanometers. The research, carried out in a rural health university in Maharashtra, chose 50 mercury sphygmomanometers in regular use in the hospital and found defects in all of them, pointing to the need for better quality control and standardisation, besides, regular verification and certification of those instruments in use. In other words, better enforcement of the Legal Metrology Act and Rules vis-à-vis sphygmomanometers.

These instruments consist of a manometer tube with a calibrated scale for measuring the pressure; the study found under-calibration of the manometer tube. To be more specific, even though the markings on the manometer tubes were from 0 mm to 300 mm, on actual measurement, it was found that 47 of them recorded a height deficit of 13 mm. This deficit can result in overestimating the BP by 5.12 mmHg (millimeter of mercury) systolic and of 3.44 mmHg diastolic in a person with normal blood pressure, the paper observed.

The study also found significant deviation from the recommended standard insofar as adequacy of the arm bladder cuff was concerned. The cuff width to length ratio in 80 per cent of the apparatus was less than 1:1.8 and this could lead to overestimation of BP by 7mm systolic and 4.3mm diastolic, the study said. In addition, 40 per cent of the instruments showed mercury manometer baseline deviation; this was more prevalent in outpatient departments and wards where the wear and tear may be more, highlighting the need for frequent verification. Emphasising the importance of accuracy in these machines, the study pointed out that a steady error of 5 mmHg can double or halve the number of patients diagnosed with hypertension.

Ironically, the Legal Metrology Act and Rules not only provide for licencing, calibration and verification of sphygmomanometers to ensure that they are manufactured to prescribed standards, but also their re-verification every two years. This not only guarantees their accuracy during use, but also ensures that faulty instruments are discarded. However, despite the importance of the accuracy of these instruments, state legal metrology departments have never been equipped to enforce the law in its letter and spirit.

In recent times, the government has launched major campaigns to attack the main causes of morbidity and mortality among citizens and I particularly refer to hypertension here. However, the success of the programme hinges on the accuracy of the blood pressure measuring instruments and, unfortunately, not enough attention has been paid to ensuring the quality and accuracy of these machines. The absence of two-yearly verification of the accuracy of these instruments, as required under the Legal Metrology Rules, also results in inaccurate measurement of blood pressure. 

How should one guard against inaccurate measurement of blood pressure?

Till such time when we can be absolutely sure of the quality and the accuracy of these apparatus sold and used in the country, consumers could do well not to bank on the reading from one device alone, but cross-check it with another instrument.

The study that I refer to above also speaks about using the correct cuff size for measuring the blood pressure. High BP may well go undetected if too large a cuff is used on a thin arm as it underestimates the pressure. Conversely, using too small a cuff on a large arm could erroneously show high BP, leading to unnecessary medication. So use of correct cuff size to match the size of the person is also important and doctors should keep this in mind while measuring blood pressure. Manufacturers, in turn, should provide at least three adult and two paediatric cuff sizes to facilitate this, says the study.