Comparative evaluation of transcutaneous oxygen tension versus ankle brachial index as predictors of reoperation following below-knee amputation.

OBJECTIVES: Decision making regarding level of lower extremity amputation is sometimes challenging. Selecting an appropriate anatomic level for major amputation requires consideration of tradeoffs between postoperative function and risk of wound complications that may require additional operations, including debridement and/or conversion to above-knee amputation (AKA). We evaluated the utility of common, non-invasive diagnostic tests used in clinical practice to predict need for reoperations among patients undergoing primary, elective below knee-amputations (BKA) by vascular surgeons.

METHODS: Patients undergoing elective BKA over a five-year period were identified using CPT codes. Medical records were reviewed to characterize demographics, pre-amputation testing transcutaneous oxygen tension (TcPO2 ), and ankle brachial index (ABI). Need for ipsilateral post-BKA reoperation (including BKA revision and/or conversion to AKA) regardless of indication was the primary outcome. Associations were evaluated using univariable and multivariable logistic regression models. Cut-points for TcPO2 values associated with amputation reoperation were evaluated using receiver operator characteristic (ROC) curves.

RESULTS: We identified 175 BKAs, of which 46 (26.3%) required ipsilateral reoperation (18.9% BKA revisions and 14.3% conversion to AKA). Mean age was 63.3 ± 14.8 years. Most patients were male (65.1%) and white (72.0%). Mean pre-amputation calf TcPO2 was 40.0 ± 20.5 mmHg, and mean ABI was 0.64 ± 0.45. In univariable models, post-BKA reoperation was associated with calf TcPO2 [0.97 (0.94-0.99); P = 0.013] but not ABI [0.53 (0.19-1.46); P = 0.217]. Univariable associations with reoperation were also identified for age [0.97 (0.94-0.990); P = 0.003] and diabetes [0.43 (0.21-0.87); P = 0.019]. No associations with amputation revision were identified for gender, race, end stage renal disease, or preoperative antibiotics. Calf TcPO2 remained associated with post-BKA reoperation in a multivariable model [0.97 (0.94-0.99); P = 0.022] adjusted for age [0.98 (0.94-1.01); P = 0.222] and diabetes [0.98 (0.94-1.01); P = 0.559]. ROC analysis suggested a TcPO2 ≥ 38 mmHg as an appropriate cut-point for assessing risk for BKA revision (AUC 0.682; negative predictive value 0.91).

CONCLUSIONS: Reoperation after BKA is common, and reoperation risk was associated with pre-amputation TcPO2 . For patients undergoing elective BKA, higher risk of reoperation should be discussed with patients with an ipsilateral TcPO2 < 38 mmHg.