Niccolai V, van Duinen MA, Griez EJ. Respiratory patterns in panic disorder reviewed: a focus on biological challenge tests. Objective: To provide a systematic review of studies investigating respiration in PD and comments on relative inconsistencies. Method: A Medline search of controlled studies focusing on pCO2, respiratory rate, tidal volume, and minute volume in PD patients was conducted for baseline/resting condition, challenge, and recovery phase. Respiratory variability and comparisons between panickers and non-panickers were also examined. Results: Lower pCO2 levels in PD subjects are a consistent finding during the baseline/resting condition, the challenge, and recovery phases. Tidal volume and minute volume are increased in PD subjects relative to controls during the baseline/resting condition. However, the most robust finding is a higher than normal respiratory variability, which appears to be a promising factor for the identification of respiratory etiopathological pathways in PD. Conclusion: Respiratory variability might be a candidate for a biological marker of PD: an abnormal breathing pattern as found in panic disorder (PD) patients compared with controls might indicate instability of the respiratory homeostasis.