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Blood pressure measurement

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Training in the measurement of blood pressure has been lacking in both the basic and continuing education of nurses and doctors. Many also appear to think they do not need to update their knowledge through further training and assessment.

VOL: 97, ISSUE: 04, PAGE NO: 33

Christine Feather, SRN, Dip Comm Studies, is district nurse and hypertension coordinator, Barkerend Health Centre, Bradford

 

But recent research by Meert (2000) showed an alarming deficit in knowledge on the measurement of blood pressure and the adequate maintenance of equipment, leading to the widespread misdiagnosis of hypertension.

Despite evidence of a need to standardise technique in blood pressure management, the literature in support of this seems to target only a limited audience, in particular practitioners with an interest in hypertension (O’Brien et al, 1995). For this reason, it has had little impact on practice.

Problems associated with the equipment used to measure blood pressure are widespread: it has been reported that half of all hospital mercury sphygmomanometers are defective and doctors and nurses working in the community often find there are no systems in place to maintain their equipment. This is extremely worrying as the first link in the chain of hypertension control is accurate equipment and technique.

Audit

An audit was carried out in Bradford to establish:

  • - The type of equipment being used;
  • - The availability of different cuff sizes;
  • - The repair/calibration status of the equipment;
  • - Who, in each team, was responsible for ensuring that the equipment was maintained.

A total of 204 audit forms were sent to nursing teams in Bradford, including:

  • - District nurses;
  • - Mental health services;
  • - Learning difficulties nurses;
  • - General practices;
  • - Nursing homes;
  • - Community hospitals.

The equpment in use included:

  • - Mercury sphygmomanometers;
  • - Aneroid sphygmomanometers;
  • - Digital devices;
  • - 24-hour ambulatory devices.

Most teams did not have access to different sizes of cuffs. At interview, 34% of staff said the calibration of their equipment had been checked in the past year. This may be an overestimation as records show that the systems in place for repair and recalibration had not been used to that extent. No one had been given specific responsibility to ensure the equipment was maintained.

Equipment standards

The equipment used to measure blood pressure needs to be validated, repaired and calibrated to acceptable standards, as defined by the manufacturer and outlined by the Medical Devices Agency.

These guidelines state that aneroid and mercury sphygmomanometers that have not been checked represent a hazard and that their calibration should be checked every six months. There is no means of checking the calibration of digital equipment, so many manufacturers recommend that they are returned every two years for recalibration or discarded.

If nurses are to improve the management of hypertension and therefore coronary heart disease and stroke, it is essential that they monitor blood pressure accurately. Decisions arising from these measurements may be crucial to the management of patient care in the short term. More importantly perhaps, the level of pressure recorded may influence patients’ quality of life for the rest of their lives.

Whatever the circumstances or the device used to measure blood pressure, certain principles must be adhered to when performing and interpreting blood pressure measurement if it is to be used in the assessment of overall cardiovascular risk (O’Brien et al, 1995).

Measuring correctly

Nurses must be competent in performing the measurement technique because the observer has long been recognised as one of the main sources of error. Observer error can be divided into three categories (Rose, 1965):

  • - Systematic error: the values a person produces are always too low or too high. This may be caused by lack of concentration, hearing impairment or confusion of auditory and visual clues. The most important factor is failure to interpret Korotkoff’s sounds (the pulse wave heard through a stethoscope) accurately, especially for diastolic pressure;
  • - Terminal digit preference: the observer rounds off the pressure reading, often to the nearest 10mmHg, which has serious implications for decisions on diagnosis and treatment;
  • - Observer prejudice or bias. This observer may adjust the reading to meet his or her preconceived notion of what the pressure should be. Observer prejudice is a serious source of inaccuracy because the error cannot be demonstrated.

The observer must also be aware of other factors that can affect blood pressure measurement. These include: exercise, respiration, food, tobacco, alcohol, emotion, pain and bladder distension. Blood pressure is also affected by a person’s age and race and may exhibit a circadian variation - it is usually at its lowest during sleep.

Measurement principles

The British Hypertension Society’s guidelines for measuring blood pressure (Ramsey, 1999) can be summarised as:

  • - Follow the British Hypertension Society guidelines on technique;
  • - Use a device with validated accuracy that is properly maintained and calibrated;
  • - Measure blood pressure routinely - standing blood pressure in patients who are elderly or have diabetes;
  • - Remove tight clothing, support the arm at heart level and ensure that the patient is relaxed;
  • - Use a cuff that is an appropriate size;
  • - Lower the mercury slowly, by 2mm a second;
  • - Read the blood pressure to the nearest 2mmHg;
  • - Measure diastolic pressure at the disappearance of sounds (Korotkoff five);
  • - Take two measurements during each visit.

Acting on the audit

After the audit, local action was taken to improve the measurement of blood pressure and the Bradford Crossroads Group was formed. Its aim is to assess the measuring equipment used in primary care settings. Current projects include:

  • - An audit of the accuracy of the calibration of aneroid sphygmomanometers used in clinical settings. This involves nurses employed in both primary and secondary care. The project requires all devices to be logged, inspected for serviceability and correct functioning, and given a full calibration check of their measuring system against a certified manometer. More than 50% have been condemned.
  • - A clinical evaluation of automated non-invasive blood pressure measuring devices for home use and in GP and hospital clinics. This project is designed to look at electronic measuring devices that have satisfied one of the published protocols for clinical accuracy and determine what users think of them. Four general practices, the gynaecology preassessment unit at Bradford Royal Infirmary and the preoperative assessment unit at St Luke’s Hospital, Bradford, were chosen as pilot sites. The results will be published later this year.

Conclusion

All nurses who measure blood pressure have a duty of care to ensure that they have the knowledge and expertise to carry out the procedure correctly. They must also be certain that the equipment they use is fit for the purpose.

Good management is required to introduce systems that enable staff to purchase and maintain accurate equipment.

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