HRV	Heart Rate Variability
WHO	World Health Organisation
ANS	Autonomic Nervous System
BMA	British Medical Association
STAI	State–Trait Anxiety Inventory
AMSTAR	Assessing the Methodological Quality of Systematic Reviews
PRISMA	Preferred Reporting Items for Systematic Reviews and Meta-Analyses
PICO	Patient/Problem, Intervention, Comparison, and Outcome
MMAT	The Mixed Methods Appraisal Tool
MD	Mean Difference
CI	Confidence Interval
P	p-value