Habituation of the cold shock response: A systematic review and meta-analysis

Cold water immersion (CWI) evokes the life-threatening reflex cold shock response (CSR), inducing hyperventilation, increasing cardiac arrhythmias, and increasing drowning risk by impairing safety behaviour. Repeated CWI induces CSR habituation (i.e., diminishing response with same stimulus magnitude) after ∼4 immersions, with variation between studies. We quantified the magnitude and coefficient of variation (CoV) in the CSR in a systematic review and meta-analysis with search terms entered to Medline, SportDiscus, PsychINFO, Pubmed, and Cochrane Central Register. Random effects meta-analyses, including effect sizes (Cohen's d) from 17 eligible groups (k), were conducted for heart rate (HR, n = 145, k = 17), respiratory frequency (f_R, n = 73, k = 12), minute ventilation (V_e, n = 106, k = 10) and tidal volume (V_t, n = 46, k=6). All CSR variables habituated (p < 0.001) with large or moderate pooled effect sizes: ΔHR -14 (10) bt. min^-1 (d: -1.19); Δf_R -8 (7) br. min^-1 (d: -0.78); ΔV_e, -21.3 (9.8) L. min^-1 (d: -1.64); ΔV_t -0.4 (0.3) L ^-1. Variation was greatest in V_e (control vs comparator immersion: 32.5&24.7%) compared to V_t (11.8&12.1%). Repeated CWI induces CSR habituation potentially reducing drowning risk. We consider the neurophysiological and behavioural consequences.