Extracorporeal life support (ECLS) is used in intensive care units (ICUs) as heart-lung bypass for critically ill patients to support either inadequate heart or lung function. Decisions to discontinue ECLS are typically based on clinical judgment and patient trajectory during trial-off support. We investigated an optical measurement of muscle oxygenation (MOx) as an indicator for adequacy of circulatory function during trials-off ECLS. Clinicians were queried prior to trial-off as to whether the patient was deemed: ready, might be ready, or not likely ready to discontinue ECLS. MOx was determined using an optical analysis developed in our laboratory. Optical spectra were acquired from infants using a fiber-optic probe affixed to the arm or leg. Five infants were studied during 6 trials-off ECLS. Mean initial MOx was 96.6+/-8.5% (n=6). In trials resulting in discontinuation of ECLS, MOx was > 94%. In those remaining on ECLS, MOx was lower during the trial off at all time points. Mean MOx trended lower (75.1+/-23.5%), in the first 6 minutes for those not removed from ECLS, compared with those for whom ECLS was subsequently discontinued (97.2+/-3.8%). Lactate trended higher in subjects remaining on ECLS (3.4+/-0.8) compared with those removed from support (1.7+/-0.7). Clinician predictions prior to trial-off did not correlate with ultimate decision for discontinuing ECLS. Although preliminary, we believe that MOx may be useful to assist in objectively assessing adequacy of circulatory function and may be helpful in the early determination of readiness for discontinuation of ECLS.