Table 1.

PICOS.

Parameter	Inclusion criteria	Exclusion criteria
Population	Healthy humans. Expert or naïve for breathing techniques	Young (<18 years) and/or old (>65 years) subjects. The population comprised any chronic or acute pathology
Intervention	Any technique of breath control that directly slows the breath down to 10 breaths per minute.	Breathing paced at a frequency higher than 10 b/min. Techniques that do not comprise an active modulation of breathing. Mixed techniques which includes psycho-physical practices other than breath regulation (e.g., meditation, visualization, yoga postures). Protocols which includes active emotional induction (e.g., fear, anger or stress induction)
Comparison	Comparison techniques (e.g., spontaneous breathing) or control groups (active interventions, no-intervention)
Outcomes	Physiological outcome related to cardio-respiratory system or central nervous system (i.e., EEG, fMRI, HRV, RSA, and Cardio-Respiratory Synchronization), together with a psychological/behavioral outcome (assessed with a psychometric quantitative approach) Measured during slow breathing techniques (state effect), immediately afterwards (state effect), or after a long-term intervention (trait effect)	Physiological parameter of no interest. Measured only a physiological or a psychological/behavioral parameter alone
Study design	Within subjects, cross sectional, randomized controlled, longitudinal, pre-post	Case reports. Lack of rigorous description of the experimental set-up and methodology, impeding replicability