2.5.2. Potential pre-treatment moderators2.5.2.1. High-frequency heart rate variability (HF-HRV).HF-HRV is an index of parasympathetic cardiac control (Thayer &Lane, 2000). HF-HRV was calculated for Baseline, Relaxation, and Hyperventilation 1 Recovery (see below).3 Measures of cardiovascular recovery (including HRV) after an acute stressor reflect parasympathetic reactivation (Imai et al., 1994) and are related to individual differences in emotion regulation (Tugade, Fredrickson, & Feldman Barrett, 2004).2.5.2.2. Duration of hyperventilation 2. Duration of voluntary exposure to laboratory stressors is widely used as a standardized measure of avoidance of negative or unpleasant stimuli (e.g. Arch &Craske, 2010; Cioffi & Holloway,1993). The current study measuredavoidance as the amount of time (max 180 s) participants persistedwith Hyperventilation 2 (shorter duration ¼ greater avoidance).2.5.2.3. Subjective anxiety during hyperventilation and relaxation tasks. Participant ratings of their maximum anxiety level (0-100 scale) experienced during Hyperventilation 1, Hyperventilation 2, and Relaxation were also analyzed as potential predictors or moderators of out