The study took place in three urban Primary Health Care centres in the Barcelonès Nord and Maresme area (Catalonia, Spain) from June to September 2008.
Patients aged 50 or over with type 2 diabetes mellitus were included in the study. The study was approved by the Ethics Committee and all patients signed the consent form. The exclusion criteria were cardiac arrhythmia, a history of PAD surgery, ankle circumference over 40 cm and refusal to sign the informed consent form.
The rationale for the selection of type 2 diabetic patients was their high prevalence of PAD and stiffness of the arteries associated with extreme ABI values .
The OMRON HEM-907 hybrid sphygmomanometer determines BP through oscilometric or auscultatory methods according to the choice of the evaluator. For de manual auscultatory BP measurement the cuff inflates automatically and deflates slowly. The pressure applied to the limb is continuously indicated and using the Doppler is possible to determine the SBP through auscultation.
The participants were examined in a relaxed atmosphere with a pleasant temperature. After 10 minute rest in supine position, SBP in the brachial artery was measured in both arms. SBP in the left and right posterior tibial and dorsalis pedis arteries was measured with the cuff placed proximal to the malleoli and the contour adjusted to the conical shape of the lower leg .
An 8-mHz Doppler device (Dopplex HNE) was used either with a mercury sphygmomanometer or the OMRON HEM-907 hybrid sphygmomanometer in the manual position. Pulse was located with the Doppler vascular probe and the cuff inflated to restrict blood flow. The examiner slowly released the pressure in the cuff to determine at what pressure blood flow was just starting, and at what pressure it was unimpeded.
In the ankle the SBP was initially measured at the dorsalis pedis pulse following the posterior tibial. If the difference was higher than 10 mmHg a new measurement was done. The new values were recorded if the difference was lower than 10 mmHg. If the difference of 10 mmHg continued a third measurement was done and recorded. The time between measurements was 3 minutes.
These measurements were carried out by two expert health workers especially trained for this study.
Firstly, one of the professionals made the measurements with one of the BP monitors randomly selected. Afterwards, a different healthcare professional blinded to the values obtained in the first examination carried out the measurements in the same patient with the other type of sphygmomanometer. The ABI of each leg was calculated separately by dividing the higher of the two ankle systolic pressures in that leg by the higher brachial systolic pressure value.
The two lower limbs of each patient were considered independent. Therefore, each patient had two posterior tibial and two dorsalis pedis BP measurements and an ABI for each leg was calculated. An ABI ≤ 0.90 was considered abnormal, whereas a value ≥ 1.40 suggested arterial calcification .
Other variables obtained during this visit were age, sex, history of arterial hypertension, weight, height, body mass index (BMI) and arm and ankle circumference.
Sample size was calculated to detect a significant kappa index of 0.25 with a bilateral alpha risk of 0.05. The estimated proportion of an abnormal ABI (≤ 0.90) in the diabetic population was 15.6%, of a normal ABI (0.91–1.39) 77% and for ABI values suggestive of calcified arteries (≥ 1.40) 7.4%.
Therefore, a sample size of 200 patients allowed to detect significant differences in the brachial SBP of 2 mmHg with a SD of 10 mmHg (α = 0.05 and β = 0.20).
For the comparison of two quantitative variables, the Intraclass Correlation Coefficient (ICC) was used to evaluate the agreement between the brachial SBP values determined with Doppler and mercury sphygmomanometer and with Doppler and the OMRON monitor.
The scale applied to evaluate the degree of agreement with this coefficient was as follows: <0.10 null agreement, from 0.10 to 0.30 bad agreement, from 0.31 to 0.50 poor agreement, from 0.51 to 0.70 moderate, from 0.71 to 0.90 good and > 0.90 very good agreement. Similarly, the ICC was used to evaluate the agreement between lower limb SBP measurements done with the mercury sphygmomanometer and the OMRON device and between the individual values of ABI determined with both methods.
The weighted kappa index (k) was used to evaluate the agreement between the qualitative ABI values determined with both methods. In this case, the scale used to assess the degree of agreement was: k ≤ 0.20 poor, k from 0.21 to 0.40 weak, from 0.41 to 0.60 moderate, from 0.61 to 0.80 good and k from 0.81 to 1 very good.
Sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), positive likelihood ratio and negative likelihood ratio of the OMRON equipment to detect an abnormal ABI (≤0.90) were calculated with the standard formulae.
The differences between the mercury sphygmomanometer and the OMRON monitor for ABI quantitative values are shown in the Bland and Altman plot. The mercury sphygmomanometer was considered to be the gold standard. All analyses were performed using Stata/SE Version 11 (StataCorp, Collage Station, TX, USA).