It is well established that indirect measurements of blood pressure made with a standard 13 cm-wide cuff are erroneously high for large arms and low for small arms. To correct for this error, the American Heart Association recommends adjusting cuff width to 40% of the arm's circumference.
To test the validity of this method of correction.
This study was a prospective, nonblinded, paired Student's t-test analysis.
Blood pressures in 50 subjects were measured directly by using a radial artery line and indirectly by the Korotkov method. For each subject multiple indirect measurements of blood pressure were made with the cuff width:arm circumference ratio varied from 30-55% in 5% increments. Error was defined as indirect blood pressure minus direct blood pressure.
A ratio of 40% resulted in overestimation of blood pressure for most arms and with particularly high errors for small arms. The ratio producing zero mean error for the pooled study group was 46.4 ± 0.7% (mean ± SEM). Using this ratio of 46.4%, the error varied inversely with arm circumference (P < 0.02), resulting in overestimation of systolic blood pressure for small arms and underestimation of systolic blood pressure for large arms. This error is comparable in magnitude, but opposite in sign, to that which occurs with a standard 13 cm-wide cuff for all arms. The optimum ratio was found to be closely approximated by the relationship, cuff width= 9.34 log10 arm circumference. Using this relationship, error in systolic blood pressure was insensitive to arm circumference (r=0.04, P>0.05) and near zero.
The optimum cuff width for the indirect measurement of blood pressure is not directly proportional to arm circumference, but is proportional to the logarithm of the arm's circumference.