Vital Signs: Blood Pressure Measurement


Studies  addressing  the  measurement  of blood pressure with a sphygmomanometer have  focused on issues such as the  accuracy of indirect blood pressure, palpation versus  auscultation  cuff  size,  position  of arm during measurements and health care workers technique. 

Direct versus Indirect
Several studies have compared direct (intra-arterial)  and  indirect  (auscultation) measurements  of  blood  pressure.  There appears  to  be  little  significant  difference in  systolic  pressures  measured  by  either method,  with  differences  ranging  from  3 mmHg in two studies to 12mmHg in a third.
Differences in diastolic blood pressure are greater, and  are  influenced  by  the  refer- ence point that is used. When the phase V Korotkoff's sound is used (disappearance of the sound), both methods provide similar pressures. However when the phase IV Korotkoff's sound (muffling), is used, auscultated measurements are significantly greater  than  intra-arterial  pressures  (see table  one).  A  study  in  children  reported the use of either auscultation or palpation overestimated systolic pressure. See table two for current recommended practice for the measurement of blood pressure.

Palpation versus Auscultation
A comparison between systolic blood pressure  measurements  taken  by  auscultation and  palpation  found  both  were  within  8 mmHg. While palpation has been commonly limited to the measurement of systolic blood pressure,  one  study  reported  that  diastolic pressures could be accurately palpated using the brachial artery to identify the sharp phase  IV  Korotkoff's  sound,  However,  the value  of  this  technique  in  clinical  practice, and its accuracy when used by health care workers, has yet to be demonstrated.



Cuff Size
The length and width of the inflatable cuff (bladder)   that   is   used   during   the measurement of blood pressure may be a source of error. Much of the research has focused on  cuff  width,  (the  dimension across along the bladder) as the potential source of this error. The standard width of currently available cuffs is approximately 12cm, with both larger and smaller sizes also available. Studies have shown that the use  of  a cuff  that  is  too  narrow  results in an overestimation of blood pressure, and a cuff that is too wide underestimates blood pressure.  Length of cuff  appears  to  have little influence on accuracy.

For  obese  people  it  has  been  suggested that  large  cuffs  (15cm  width)  will  be required    when    the    person's    arm circumferences is between 33 -35cm, and a thigh cuff (18cm width) may be needed if  the  arm  circumferences  is  greater  than 41cm.  However,  difficulties  in  applying thigh cuffs to large arms have been reported. Cuff width may also be important when  measuring   blood   pressure   in neonates  and  a  cuff    width  equal  to approximately     50%     of     the     arm circumference has been recommended.

Arm and Body Position
Comparisons of blood pressures measured in  the  sitting  person  with  their  arm  supported  horizontally  or  with  the  arm  resting  at  their  side,  have  found  an average difference in systolic pressure of 11mmHg and diastolic pressure of 12mmHg. When the  arm  was  placed  above  or  below  the level  of  the  heart,  blood  pressure  measurements   changed   by   as   much   as 20mmHg. As a result  of  this, it  has been recommended  that  blood  pressures  be taken in the sitting position with arm supported horizontally at approximately heart level.

Bell versus Diaphragm
The accuracy of blood pressures measured with  the  bell  or  the  diaphragm  of  the stethoscope  have been investigated. One study  found  the  bell  of  the  stethoscope resulted  in  higher  readings  than  those taken using the  diaphragm. These  results were  supported  by  another  study,  with researchers recommending the use of the bell for all blood pressure measurements. 


Health Care Workers Technique
The  technique  used by  health care  workers  to  measure  blood  pressure  has  been shown to differ from recommended practice.  Using  the  American  Heart  Association Guidelines as the standard, one  study found that 57% of nursing students failed to  comply  with  these  guidelines  in  areas such  as  cuff  placement,  estimation  of systolic pressure by palpation, calculation of  proper  inflation  pressure,  and  proper stethoscope  placement.  Another  study  of 172  health  care  workers  concluded  that nurses  and  physicians  evaluated  blood pressure in an inadequate, incorrect and inaccurate way, and that only 3% of general  practitioners  and  2%  of  nurses  obtained reliable  results. Two studies evaluating  the  impact  of  education  programs on  blood  pressure  measurement,  found they increased agreement between the different  blood  pressure  readings  and  also significantly reduced differences in operator technique.

Limitations
A  descriptive  study  of  blood  pressures in critically  ill  patients  who  had  suffered  a cardiac arrest highlighted some limitations to these measurements. Of the 15 patients investigated,  5  patients  had  adequate intra-arterial   blood   pressures,   but unreadable  cuff  pressures.  Four  patients had  cuff  pressures  approaching  normal, but had an inadequate cardiac output. This study suggests that indirect blood pressure measurements  do  not  always  accurately reflect haemodynamic status of critically ill people.



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