Individuals with Down Syndrome (DS) have decreased aerobic capacity (VO2peak) and blunted fitness improvements with exercise training. Both decreased capacity and limited change is likely explained by autonomic dysfunction resulting in chronotropic incompetence. Increased stroke volume could potentially compensate for blunted heart rate increases and maintain cardiac output, however it is unknown if this occurs in those with DS. Therefore, it is important to accurately measure SV and Q during exercise to further evaluate limitations to peak aerobic capacity within this population. Stroke volume (SV) and cardiac output (Q) can be non‐invasively assessed during exercise via finger photoplethysmography (PPG) however this has not been validated in DS.PURPOSEDetermine the concurrent validity of PPG, using continuous wave echocardiography (CWE) as the standard method, during exercise in adults with and without DS (control).METHODSAdults with DS (n= 6, 25 ± 2 yrs, 31.7 ± 4.6 kg/m2, 24.2 ± 4.2 ml/kg/min) and without DS (n= 5, 26 ± 4 yrs, 24.9 ± 4.8 kg/m2, 32.5 ± 6.4 ml/kg/min) performed a maximal incremental treadmill test to assess VO2peak. CWE images were captured at baseline, at each 2 minute exercise stage for 10 minutes and during recovery. PPG measurements were continuously obtained and then averaged over the last 30 seconds, matching CWE capture time points. Mean difference of combined time points between CWE and PPG was analyzed with t‐test and intraclass correlation coefficients (ICC absolute agreement) were computed between measurement methods for SV and Q. Validity based on ICC is considered poor: < 0.4, moderate: 0.5 – 0.75, or good: >0.75.RESULTSMean difference (CWE‐PPG) between measurement methods in both SV and Q in controls was significantly different from zero (SV: −13.5 ± 23.86 ml p <0.01 and Q: −1.9 ± 2.9 L/min p <0.01) while no difference were evident in those with DS (SV: 0.7 ± 31.6 ml p =0.86 and Q: −0.4 ± 3.3 L/min p =0.33) The overall ICC for SV and Q was 0.543 and 0.712, respectively. Once separated by group, the ICC for SV in the control group was 0.760 and the DS group was 0.128. ICC between Q measures in control was 0.773 and in DS was 0.646.CONCLUSIONAlthough PPG and CWE showed sufficient agreement between both SV and Q in individuals without DS, the absolute means were significantly different between methods. In individuals with DS, agreement between methods was low and moderate for SV and Q respectively, although the absolute means were not significantly different. Since the concurrent validity with CWE is not sufficient for PPG, we therefore recommend stroke volume measurement be obtained with CWE.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.