Transcutaneous oximetry: variability in normal values for the upper and lower limb

Published normal transcutaneous oxygen partial pressures (P_tcO₂) for the chest and lower limb have defined tissue hypoxia as a value of < 40 mmHg (< 30 mmHg in some patients, < 50 mmHg in others).

To determine 'normal' P_tcO₂ for the upper and lower limb in healthy, non-smoking adults using the Radiometer® TCM400 with tc Sensor E5250.

Thirty-two volunteers had transcutaneous oxygen measurements (TCOM) performed on the chest, upper and lower limbs breathing air, with leg then arm elevated and whilst breathing 100% oxygen.

Room-air P_tcO₂ (mmHg, mean (95% confidence interval)) were: chest: 53.6 (48.7-58.5); upper arm: 60.0 (56.1-64.0); forearm: 52.3 (44.8-55.8); dorsum of hand: 50.2 (46.1-54.3); thenar eminence: 70.8 (67.7-73.8); hypothenar eminence: 77.9 (75.1-80.7); lateral leg: 50.2 (46.2-54.2); lateral malleolus: 50.5 (46.6-54.3); medial malleolus: 48.9 (45.6-52.1); dorsum, between first and second toe: 53.1 (49.2-57.0); dorsum, proximal to fifth toe: 58.5 (55.0 - -62.0); plantar, 1st MTP: 73.7 (70.3-77.1). Nineteen subjects had at least one room-air P_tcO₂ below 40 mmHg (nine upper limb, 13 lower limb, four chest). Approximately 10% lower limb P_tcO₂ were < 100 mmHg on normobaric oxygen. Only one subject at one site had an upper limb P_tcO₂ < 100 mmHg breathing oxygen.

The broad dispersion in P_tcO₂ in our healthy cohort reflects the inherent biologic variability in dermal perfusion and oxygen delivery, making it difficult to define narrow, rigid 'normal' values. Thus, we cannot recommend a single P_tcO₂ value as 'normal' for the upper or lower limb. A thorough patient assessment is essential to establish appropriateness for hyperbaric oxygen therapy, with TCOM used as an aid to guide this decision and not as an absolute.