A statistical assessment of the dependability of transcutaneous tissue oxygen tension measurements

Abstract

Transcutaneous oxygen tension measurements ( P tcO 2) were obtained for subjects in two groups: peripheral vascular patients ( N = 15) and disease-free controls ( N = 9). Readings were taken in each of three locations, arm, knee and foot, in supine and erect positions, at fixed and random times, on each of 3 consecutive days. The dependability (the proportion of true variance in total variance) of P tcO 2 measurements was evaluated using a generalizability model. The preliminary generalizability study analysis indicated that day-to-day variation of P tcO 2 level was larger than within-day variation. Therefore, in order to decrease the measurement error variance one would preferably increase the number of days for measurements ( n d) rather than increase the number of measurements within each day ( n t). A decision study analysis was also performed which resulted in estimates of error variance and two interdependent dependability measures: dependability coefficients (DCs) and signal to noise ratios ( S N s). P tcO 2 dependability values were generally different for the two groups. Cases had high DCs and S N s (DC ⩾ 0.9, S N ⩾ 9 ) in all location-position combinations except for arm measurements. On the other hand, controls had low and unacceptable DCs and S/Ns (DC < 0.8, S N < 4 ) in all location-position combinations. Cases and controls had generally lower dependability values when P tcO 2 ratios were analyzed. The only two ratio-position categories for which dependability values were in the acceptable range (DC ⩾ 0.8, S N ⩾ 4 ) were foot/arm erect and foot/arm supine for the cases. We recommend the following measurement frequencies for peripheral vascular patients: for the knee supine, knee erect, and foot supine positions, n d = 2 and n t = 1; for the foot erect position, n d = 2 and n t = 2. We conclude that measurement of P tcO 2 was dependable for peripheral vascular patients when readings were taken at the knee or foot and for foot/arm ratios. Controls had generally low P tcO 2 dependability values. P tcO 2 is therefore not a dependable method of noninvasive vascular assessment for healthy individuals.