Theoretical Implications of Oxygenation in Limb Replant or Transplant

Prolonged ischemia adversely affects the outcomes of replant or transplant, particularly if the amputated part contains a significant amount of muscle tissue.[1] Over the last five decades, not much has changed in the way composite tissues are salvaged and preserved prior to replant or transplant. Improvement in the recovery protocol and development of new technologies for enhanced preservation has the potential to significantly improve overall outcomes. This letter discusses the implications of poor oxygenation relevant to limb replant or transplant by explaining the theoretical basis for oxygen transfer into the tissue during the ischemic period.

Once amputated (whether traumatically or surgically), a limb relies on passive diffusion for oxygen delivery. Since human-sized limbs are centimeters in thickness, it is expected that most of the tissue is poorly oxygenated during the ischemic period. This can be calculated by modeling an amputated limb as a cylindrical “bioreactor” composed of oxygen-consuming tissue ([Fig. 1]). All viable tissue requires oxygen. Tissue oxygen consumption rate (OCR) follows zero-order kinetics and depends on the temperature.[2] Obeying the principles of mass conservation, Equation 1 estimates the maximum radial penetration depth (R _max = R _L – R _A, as shown in [Fig. 1]) for oxygen diffusing into a cylindrically shaped piece of tissue:

Fig. 1 Schematic illustrating a cylindrical piece of tissue, which can be used to model oxygen delivery into an amputated limb. Lighter areas represent oxygenated tissue, whereas darkened areas represent anoxic tissue. The radius of a limb is R _L, the radius of the anoxic portion is R _A, and the maximum penetration depth is R _L – R _A. P _ext is the external oxygen partial pressure (pO₂), which, in a conventional limb salvage procedure, can be estimated to be atmospheric pO₂ at sea level (160 mm Hg).

where (αD) is tissue oxygen permeability and P _ext is the external oxygen partial pressure. OCR has been directly measured on a per gram basis for tissue preserved at 8°C and can be applied in this calculation.[3] Assuming the amputated limb is stored at 8°C and surrounded by atmospheric oxygen at sea level (160 mm Hg), R _max is approximately 700 microns. In other words, oxygen is depleted to zero less than 1 mm deep into the tissue from the surface. Using the Arrhenius equation,[2] [4] the effect of temperature on the OCR can be estimated. For example, if the remains at room temperature (22°C), the OCR is three times higher and thus the R _max is even less (375 microns). Additional implications of cooling have been discussed elsewhere.[4] These calculations suggest that standard practice may be insufficient to preserve an amputated limb with oxygen delivery by passive diffusion and cooling from the surface alone.

In conclusion, amputated human-sized limbs do not obtain adequate oxygen by passive diffusion from their surfaces alone. During the ischemic period, which can sometimes span many hours, irreversible injury may occur to the tissues and particularly muscle. Also, current salvage protocols may not cool an amputated part fast enough, further compromising oxygen delivery into the tissue.[5] Future research is needed to develop methods for enhanced oxygenation, more rapid cooling, and possibly instilling protective agents such as antioxidants to improve outcomes following limb replant or transplant. One possible solution to this problem may be the use of perfusion or persufflation preservation technologies.

• References

• 1 Kuzon Jr WM, Walker PM, Mickle DA, Harris KA, Pynn BR, McKee NH. An isolated skeletal muscle model suitable for acute ischemia studies. J Surg Res 1986; 41 (01) 24-32
• 2 Avgoustiniatos ES, Colton CK. Effect of external oxygen mass transfer resistances on viability of immunoisolated tissue. Ann N Y Acad Sci 1997; 831: 145-167
• 3 Weegman BP, Kirchner VA, Scott III WE. , et al. Continuous real-time viability assessment of kidneys based on oxygen consumption. Transplant Proc 2010; 42 (06) 2020-2023
• 4 Suszynski TM, Abbassi B. Effect of cooling on free flap ischemia. Plast Reconstr Surg 2016; 137 (06) 1069e-1070e
• 5 Weegman BP, Suszynski TM, Scott III WE. , et al. Temperature profiles of different cooling methods in porcine pancreas procurement. Xenotransplantation 2014; 21 (06) 574-581