Sources of inaccuracy in the use of the Hawksley random-zero sphygmomanometer

OBJECTIVE: To identify possible causes of inaccuracy in the use of the Hawksley random-zero sphygmomanometer and methods that could reduce this. METHODS: Four Hawksley random-zero sphygmomanometers were compared with a standard sphygmomanometer under static conditions. Two methods (standard and rapid) were used to release pressure from the inflated cuff with pressures recorded by independent blinded observers. The rate at which the hand valve released pressure was analysed. The effects of varying filling times and pressures on the size of the final zero correction were investigated. RESULTS: The Hawksley devices all under-recorded pressure compared with that measured by using a standard machine. A rapid means of pressure release approximately halved this error in each case. Pressure release through the hand valve was shown to have a characteristic and prolonged exponential decay. Using low filling times and pressures reduced the observed range of zeros seen, with the production of a correlation between the size of the zero and the inflation pressure used. CONCLUSION: These findings suggest that overestimation of the final zero correction is a common and major source of error in the use of the Hawksley sphygmomanometer. A simple change in the design of the final pressure release would improve the machine's reliability in clinical usage. The machine's zero mechanism is susceptible to unintentional misuse. Such misuse could occur when the machine is used in accordance with current sphygmomanometry guidelines.