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3. Blood pressure measurement 

In order to obtain accurate data with the indirect blood pressure methods that are used in surveys, it is important to follow the prescribed procedures precisely. Otherwise, correlation with the true blood pressure no longer holds. Therefore, the instructions given in the following sections should be incorporated into Manuals of Operation in their entirety.

In daily life blood pressure changes from instant to instant and is influenced by many physiological and environmental factors. To obtain reproducible results, it is important to control these factors as much as possible and otherwise document them in the blood pressure data recording forms (see Appendix 3.1). The items that need to be recorded before beginning the measurement are: time of the day, room temperature, arm circumference and cuff width used. Also, if deviations from the measurement protocol are unavoidable, e.g. a person has lost his/her right arm and the measurement had to be taken from the left arm or if a person for some reason is not able to sit and measurement had to be taken in recumbent position, they have to be documented together with the blood pressure data.

3.1 Equipment

For survey  blood pressure measurements the following equipment is required (See Picture 3.1.):

  • simple mercury sphygmomanometer,
  • stethoscope,
  • cuffs,
  • non-elastic measuring tape.
photo:Blood pressure measurement equipment

Picture 3.1. Blood pressure measurement equipment


The simple mercury sphygmomanometer is recommended because there are no reliable automated devices on the market. This may change when the accuracy of future automated devices is found to be sufficient in validation against the simple mercury sphygmomanometer.

The bell of the stethoscope should be used because it gives clearer sounds than the diaphragm.

A set of 3-4 cuffs with different size should be available and special attention should be paid to the use of proper cuff width in relation to the size of the arm.

A measuring tape is used to measure arm circumference before selecting the proper cuff width.

3.2 Measurement procedures

Preparation for measurement

Before the blood pressure measurement begins the following conditions should be met:

  1. Subjects should abstain from eating, drinking (anything else than water), smoking and taking drugs that affect the blood pressure one hour before measurement. 
  2. Because a full bladder affects the blood pressure it should have been emptied.
  3. Painful procedures and exercise should not have occurred within one hour. 
  4. Subject should have been sitting quietly for about 5 minutes.
  5. Subject should have removed outer garments and all other tight clothes. The sleeve of shirts, blouses, etc. should have been rolled up so that the upper right arm is bare. The remaining garments should not be constrictive and the blood pressure cuff should not be placed over the garment.
  6. Blood pressure should be measured in a quiet room with comfortable temperature. The room temperature should have been recorded.
  7. The time of day should have been recorded.
  8. The blood pressure measurer should be identified on the blood pressure data recording form.

Position of the subject

Measurements should be taken in sitting position so that the arm and back are supported. Subject's feet should be resting firmly on the floor, not dangling. If the subject's feet do not reach the floor, a platform should be used to support them. 

Position of the arm

The measurements should be made on the right arm whenever possible. 

The subject's arm should be resting on the desk so that the antecubital fossa (a triangular cavity of the elbow joint that contains a tendon of the biceps, the median nerve, and the brachial artery) is at the level of the heart and palm is facing up. To achieve this position, either the chair should be adjusted or the arm on the desk should be raised, e.g. by using a pillow (see Picture 3.2). The subject must always feel comfortable.

photo

Picture 3.2. Position of the arm and placement of the cuff

Selection of the cuff

The greatest circumference of the upper arm is measured, with the arm relaxed and in the normal blood pressure measurement position (antecubital fossa at the level of the heart), using a non-elastic tape (see Picture 3.3). The measurement should be read to the nearest centimeter. This reading should be recorded in the data form.  photo

Picture 3.3. Measurement of the arm circumference

Select the correct cuff for the arm circumference and record the size of the selected cuff in the blood pressure recording data form. The instructions for deriving rules to select the proper cuff size for each arm circumference are given in Appendix 3.2.

The cuff should be placed on the right arm so that its bottom edge is 2-3 cm above the antecubital fossa, allowing sufficient room for the bell of the stethoscope. The top edge of the cuff should not be restricted by clothing. (See Picture 3.2)

Number of measurements

Three measurements should be taken one minute apart. If three measurements are not feasible, two will suffice with a certain loss in data stability. 

Procedure of the pulse rate and blood pressure measurement

  1. The radial pulse is palpated and the pulse rate is counted for 30 seconds, measured by a digital wrist watch or one with second hand. (See Picture 3.4)

     photo
    Picture 3.4. Measurement of the pulse rate
  2. Record 30-second pulse count and whether pulse was regular.
  3. The manometer should be placed so that the scale is at eye level, and the column perfectly vertical. The subject should not be able to see the column of the manometer. (See Picture 3.5)

     photo
    Picture 3.5. Placement of the manometer
  4. Determining the peak inflation level:
    1. The mercury column has to be at 0 level.
    2. The subject's radial pulse is again palpated.
    3. The cuff is inflated and the level of the top of the meniscus of the mercury column is noted at the point when the radial pulse disappears. The cuff is immediately deflated by completely opening the valve.
    4. The peak inflation level is determined by adding 30 mm to the pressure where the radial pulse disappeared.
  5. Venous blood pool in the forearm is normalized by waiting at least 30 seconds or by raising the arm for 5-6 seconds. 
  6. The brachial pulse is located and the bell of the stethoscope is placed immediately below the cuff at the point of maximal pulsation. If it is not possible to feel the brachial pulse, the bell of the stethoscope should be placed over the area of the upper arm immediately inside the biceps muscle tendon. The bell should not touch the cuff, rubber or clothing. (See Picture 3.6)

    photo
    Picture 3.6. Placement of the bell
  7. The cuff is rapidly inflated to the peak inflation level and then deflated at a rate of 2 mmHg per second.
  8. The pressure should be reduced steadily at this rate until the occurrence of the systolic level at the first appearance of a clear, repetitive tapping sound (Korotkoff Phase 1) and diastolic level at disappearance of repetitive sounds (Phase 5) have been observed. Then the cuff should be rapidly deflated by fully opening the valve of the inflation bulb. Note: There may be a brief period (auscultatory gap) between systolic and diastolic pressure, when no Korotkoff sounds are heard. Therefore, the 2mmHg/second deflation should be continued until the diastolic blood pressure is definitely established. If Korotkoff sounds persist until the cuff is completely deflated, a diastolic blood pressure of 0 should be recorded.
  9. The measurements should be recorded to the nearest 2 mmHg. If the top of the meniscus falls half way between two markings, the marking immediately above is chosen. The subject is not told the blood pressure values at this point.
  10. After one minute of wait to allow redistribution of blood in the forearm a second measurement is made by repeating steps 7 to 9. The subject should not change position during the wait.
  11. After another one minute a third measurement is made by repeating steps 7 to 9. 
  12. The subject may now be told the measurement values.

3.3 Selection and training of the measurers

When recruiting the measurers one should remember:

After persons have been recruited as candidates for blood pressure measurer, they have to pass a hearing test administered by an audiometrist, i.e. no loss of hearing in either ear. 

Following the successful hearing test all candidates have to undergo thorough training covering theory and practice of indirect blood pressure measurements.

During the theoretical lectures the blood pressure measurement protocol is reviewed and discussed in detail. Possible problems during field operation are examined and solutions analyzed. Also, the quality control measures during the survey are presented, e.g. monitoring for terminal digit preference.

The practical training includes

Before being accepted as blood pressure measurers, the candidates have to pass a certification test that could be based on similar techniques as the training methods, but now a predefined minimal percentage of correct measurements has to be achieved for successful certification.

3.4 Quality control

3.4.1 During the survey

Quality control during the survey includes two parts, the checking of equipment and performance monitoring of the blood pressure measurers.

Measurers should check every day before the first blood pressure measurement are made that the mercury column of the sphygmomanometer is at zero, that the mercury column falls smoothly when the cuff is deflated, and that the column latches properly into vertical position. Any equipment failing these tests has to be replaced. The results of checking should be recorded in a log book.

It is important to continuously monitor the performance of blood pressure measurers to avoid an accumulation of data that will have to be discarded because of unreliability. Monitoring every blood pressure measurement onsite is not possible but there are several simple indicators that can be calculated regularly for monitoring purpose. For monitoring to be effective it is desirable that measurements from the field are reviewed regularly, preferably daily.

For each measurer the following information should be checked regularly during the survey:

  1. Availability of data for selected cuff width, measured arm circumference, room temperature and time of the day of the blood pressure measurement. This will detect if some measurer is omitting some parts of the protocol.
  2. Distribution of terminal digits for systolic and diastolic measurements separately. This will detect if
    1. some measurer tends to prefer some digits over others (for example zero preference), indicating unreliable detection of Korotkoff sounds; 
    2. some measurers use odd digits that, by protocol, should not be used.
  3. The proportions of identical readings for the first and second measurements, the second and third measurements, and for all three measurements of systolic and diastolic measurements separately. This will detect if a measurer is actually taking three measurements (identity should be rare).
  4. Monitor that daily/hourly work load does not exceed agreed limits.  
  5. Means and standard deviations of the systolic and diastolic blood pressure measurements. This will detect if some measurer produces systematically lower or higher readings than the average of the team.
  6. Cross-tabulation between used cuff width and measured arm circumference. This will detected compliance with the protocol.

If some problems are detected they need to be immediately discussed with the individual measurer and corrective action taken. Just letting the measurer know that he/she has problems with the measurement procedures may suffice. Otherwise, the measurer should be retrained and re-certificated or dismissed.

During extended surveys, a refresher session for all blood measurers every three months is a desirable practice.

The room temperature should be monitored during the survey on a regular basis and adjusted when needed.

External auditors should make surprise visits to the examining sites and observe measurers' performance by documenting step-by-step compliance with the protocol. Auditors should also act as guest subjects and participate actively in all steps of blood pressure measurement.

3.4.2 After the survey

After the survey, it is important to assess and document the overall quality of blood pressure measurements. This information can be used to verify that results presented in publications are accurate and comparable with other studies. In addition, the information will also be useful for planning of future surveys and for designing the training of the future blood pressure measurers. 

The retrospective quality assessment report for blood pressure measurements no longer focuses on the data of individual measurers, but instead concentrates on the pool of all measurements. The report should include the following information:

  1. Item response rates for blood pressure measurement.
  2. Availability of data on:
    1. used cuff width;
    2. measured arm circumference;
    3. room temperature;
    4. time of the day of the blood pressure measurement.
  3. Proportion of incomplete measurements.
  4. Proportion of identical measurements for systolic and diastolic measurements separately.
  5. Difference between two sequential measurements for systolic and diastolic measurements separately.
  6. Proportion of odd-valued readings for systolic and diastolic measurements separately.
  7. Distributions of terminal digits for systolic and diastolic measurements separately.
  8. Cross-tabulation between cuff widths and arm circumferences.
  9. Mean and standard deviation of the room temperature.

 


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