Comparative Effects of Four Brief Breathwork and Mindfulness Interventions on Anxiety and Psychological Flexibility: A 4-Arm RCT

This four-arm, single-blind randomized controlled trial examines the effects of four brief daily 5-minute practices over 28 days on psychological flexibility and physiological variables in Spanish-speaking adults. Participants will be randomly assigned to one of four groups: Box Breathing, Cyclic Sighing, Mindfulness focused on exhalation with the syllable "AH", or Conscious Cyclic Sighing. Outcomes include state and trait anxiety (STAI), positive and negative affect (PANAS), psychological flexibility (MPFI-24, Psy-Flex), and physiological variables (heart rate, respiratory rate, heart rate variability). Assessments are conducted at baseline, daily during the 28-day intervention, at post-intervention (day 31), and at 2-month follow-up.

Inclusion Criteria: * Age between 18 and 65 years (inclusive) * Native Spanish speaker or fluent in Spanish (sufficient to understand audio instructions and complete psychometric questionnaires) * Stable internet access and an electronic device (smartphone, tablet, or computer) * Ability to provide informed consent and commitment to the 31-day protocol and 2-month follow-up Exclusion Criteria: * Active diagnosis of severe or chronic respiratory disease that prevents safe performance of breathing exercises (e.g., advanced COPD, severe uncontrolled asthma, cystic fibrosis) * Current diagnosis of severe psychiatric disorder (e.g., psychotic spectrum disorders, acute-phase bipolar disorder, severe major depression with suicidal risk) * Any acute medical condition contraindicated for voluntary alteration of breathing patterns