Why do the readings on my mercury blood pressure instrument sometimes read very high?

Although properly designed mercurial instruments are required to provide a full pressure drop (300 mmHg to 20 mmHg with no back pressure) in 3 seconds, older instruments tend to slow down due to build up of dust or other obstructions within the system. This slow down will produce a lag - the air will release from the cuff FASTER than the mercury can drop in the column. In addition, some manufacturers whose cartridge tubes have a narrow inside bore (generally less than the mandated 3.9 mm dimension) will always suffer from some lag. This lag will generally have no impact on measurements obtained at the recommended deflation rate of 2-3mmHg/second. However, when the deflation rate is excessive, the lag can have a dramatic impact on accuracy resulting in an overestimation of pressure.