Reeves JW, et al. - Given that research on the link between respiratory sinus arrhythmia (RSA) and posttraumatic stress disorder (PTSD) has mainly focused on average levels of RSA and that to maintain adaptive cardiac variability, rapid shifts in parasympathetic tone are necessary and parasympathetic cardiac regulation is incompletely quantified on exclusively focussing on these tonic estimates, researchers conducted this analysis addressing this limitation by examining the dynamic regulatory effect of the parasympathetic nervous system on heart rate. They assessed participants with PTSD, panic disorder (PD), comorbid PTSD and PD (PTSD + PD), and healthy controls for their epoch-to-epoch parasympathetic cardiac regulation – operationalized as the lagged relationship between RSA and heart rate across consecutive 30-s epochs – across a single night. Data gained yielded preliminary evidence indicating that participants with PTSD only and PTSD + PD display an upregulatory parasympathetic response to self-reported hyperarousal, which was reflected by increased epoch-to-epoch parasympathetic cardiac regulation.