Anxiety Sensitivity as a Mediator of Internet-Based Cognitive Behavioral Therapy for Panic Disorder: A Randomized Controlled Trial with Minimal Therapist Contact

Background: Internet-based cognitive behavioral therapy (iCBT) is efficacious for panic disorder (PD), yet the mechanisms of change remain underspecified. Anxiety sensitivity (AS) is theoretically central to PD maintenance, but its role as a mediator has not been formally tested in Spanish-speaking populations using minimal-contact formats. This study evaluates the efficacy of the "Free from Anxiety" iCBT program and examines AS as a mediator of clinical outcomes. Methods: In a randomized controlled trial, 95 adults meeting DSM-IV-TR criteria for PD were assigned to an 8-week iCBT program with optional email support (n = 49) or a waiting-list control (n = 46). Primary outcome was PD severity (PDSS); secondary outcomes included anxiety sensitivity (ASI-3), general anxiety (BAI), and depression (BDI-II). Mediation was assessed via Baron and Kenny's framework with bootstrapping (5,000 resamples) to estimate the indirect effect of ASI-3 change on PDSS reduction. Results: The treatment group showed significant improvements across all measures compared to controls (PDSS: d = 0.76, 95% CI [0.10, 1.42]; mean d = 1.30). Mediation analysis confirmed that ASI-3 change partially mediated the treatment effect on PDSS (indirect effect = 1.85, 95% CI [0.36, 3.70]), accounting for 27.4% of the total effect. The direct effect remained significant (b = 4.89, p < .001). Intent-to-treat (ITT) analyses supported robustness (d = 0.47 to 1.47). Gains were maintained at 6-month follow-up (d = 1.19 to 1.26). Conclusions: iCBT reduces anxiety sensitivity as a partial mechanism of change, aligning with cognitive models of panic. These findings support Free from Anxiety as an evidence-based, viable first-step intervention for Spanish-speaking clinical populations within stepped-care pathways.