D’Souza JM, et al. - Among 49 adults with substance use disorders and at least four symptoms of posttraumatic stress disorder (PTSD), researchers examined the potential value of high frequency-heart rate variability (HF-HRV) as a biomarker of a common underlying process by testing if the severity of substance use and PTSD symptoms could be related to HF-HRV. Findings revealed the significant correlation of low resting HF-HRV with drug and alcohol symptom severity but not with PTSD symptom severity after controlling for age, indicating a higher sensitivity of HF-HRV to the severity of drug and alcohol use rather than PTSD. In PTSD populations, HF-HRV may primarily index comorbid substance use disorder symptoms and may objectively measure the severity of substance use.