Measurement of critical lower limb tissue hypoxia by coupling chemical and optical techniques

It has been a long-term goal to develop non-invasive methods that can detect critical levels of tissue hypoxia to help in the management of chronic lower limb ischaemia. In the present study, skeletal muscle oxygenation was measured using a new Clark-type TCPO₂ [transcutaneous PO₂ (partial pressure of O₂)]/PCO₂ (partial pressure of CO₂) monitoring system and optical NIRS (near-infrared spectroscopy) at graded levels of hypoxaemia using a rabbit model (n=6). The TCPO₂/PCO₂ probe was placed on the shaved hindlimb to record SPO₂ (skin PO₂) and SPCO₂ (skin PCO₂) continuously. A pair of NIRS probes were placed on the limb to monitor HbO₂ (oxyhaemoglobin) and Hb (deoxyhaemoglobin). Graded hypoxaemia was achieved by stepwise reductions of FiO₂ (fraction of inspired O₂) from 30% to 6%. Animals were allowed to recover after each episode of hypoxia at an FiO₂ of 30% as indicated by normalized arterial blood PO₂. There was a significant (P<0.05) decrease in SPO₂ with all grades of hypoxaemia and no significant changes in SPCO₂. There was a significant (P<0.05) increase in muscle Hb with all grades of hypoxaemia and a significant (P<0.05) decrease in HbO₂ when FiO₂ was below 15%. A significant correlation was found between the SPO₂ and HbO₂ (r=0.92, P<0.001) and both were significantly correlated with arterial blood PO₂ (P<0.001). The new TCPO₂/PCO₂ system, in addition to its application for the assessment of conditions such as chronic venous insufficiency where alteration in skin oxygenation occurs solely, also has potential in conditions such as peripheral vascular disease where both skin and muscle oxygenation may be affected.