Pharmacological modelling of dissociation and psychosis: An evaluation of the Clinician Administered Dissociative States Scale and Psychotomimetic States Inventory during nitrous oxide (‘laughing gas’)-induced anomalous states

Rationale: A significant obstacle to an improved understanding of pathological dissociative and psychosis-like states is the lack of readily implemented pharmacological models of these experiences. Ketamine has dissociative and psychotomimetic effects, but can be difficult to use outside of medical and clinical-research facilities. Alternatively, nitrous oxide (N 2 O) - like ketamine, a dissociative anaesthetic and NMDAR antagonist – has numerous properties that make it an attractive alternative for modelling dissociation and psychosis. However, development and testing of such pharmacological models relies on well-characterized measurement instruments.

Objectives: To examine the factor structures of the Clinician Administered Dissociative States Scale (CADSS) and Psychotomimetic States Inventory (PSI) administered during N 2 O inhalation in healthy volunteers.

Methods: Secondary analyses of data pooled from three previous N 2 O studies with healthy volunteers.

Results: Effect sizes for N 2 O-induced dissociation and psychotomimesis were comparable to effects reported in experimental studies with sub-anesthetic ketamine in healthy volunteers. Although, like ketamine, a three-factor representation of N 2 O-induced dissociation was confirmed, a more parsimonious two-factor model might be more appropriate. Bayesian exploratory factor analysis suggested that N 2 O-induced psychosis-like symptoms were adequately represented by two negative and two positive symptom factors. Hierarchical cluster analysis indicated minimal item overlap between the CADSS and PSI.

Conclusion: N 2 O and ketamine produce psychometrically similar dissociative states, although parallels in their psychosis-like effects remain to be determined. The CADSS and PSI tap largely non-overlapping experiences under N 2 O and we propose the use of both measures (or similar instruments) to comprehensively assess anomalous subjective states produced by dissociative NMDAR antagonists.