Rapid habituation of the cold shock response
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KeywordsRespiratory Frequency Minute Volume Respiratory Drive Cold Water Immersion Water Exposure
Sudden immersion into cold water initiates a series of cardio-respiratory responses collectively known as the cold shock response (CSR) which may increase an individual's risk of drowning. The CSR is stimulated by a rapid fall in skin temperature and includes tachycardia, a reflex inspiratory gasp followed by uncontrollable hyperventilation. Repeated cold water immersions conducted over several days have been shown to reduce the magnitude of the CSR . This study investigated whether an habituation could be achieved in a couple of hours; it was hypothesised that following this rapid habituation the CSR would be reduced on a subsequent cold water immersion.
Nine healthy males (mean [SD] age 21 years, height 179 cm, mass 76 kg) with no previous cold water exposure, undertook two head-out immersions into stirred water at 15 °C for 5 minutes wearing swimming trunks. These immersions were undertaken one week apart at the same time of day (IMM1 and IMM7). One or two days after IMM1, participants undertook five, 3-minute, head-out immersions into 15 °C water over a period of 55 to 120 min. In between each habituation immersion they rewarmed in a bath at 38 °C for 3 minutes and the next immersion occurred approximately 10 minutes later. Heart rate (fC), respiratory frequency (fR), tidal volume (VT) and inspiratory minute volume (VI) were measured prior to and during each immersion. Data for IMM1 and IMM7 were averaged over the following time periods: 0-30, 30-60, and 60-300 s and are presented as mean[sd].
fC was reduced throughout IMM7 compared to IMM1 (0-30 s: 117 v 106 bpm; 30-60 s: 110 v 86 bpm; 60-300 s: 90 v 78 bpm; all P < 0.05). VI was attenuated in IMM7 compared to IMM1 over the first minute of immersion (0-30 s: 61.3[7.5] v 52.5[12.1] L.min-1; 30-60s: 50.8[13.5] v 40.5[13.6] L.min-1; P < 0.05) whereas fR was only reduced in the first 30 s from 37(11) to 29(9) breaths.min-1 (P < 0.05). The inspiratory gasp observed within the first 10 s of immersion was similar in IMM1 and IMM7 (2.44[0.62] v 2.71[0.64] L, p > 0.05) as was VT throughout the immersions.
Repeated immersions conducted over a short time period (1-2 hours) on the same day resulted in a decrease in fC, fR and VI during the first 30 s of immersion. This may reduce the risk of drowning by attenuating ventilation and thus the risk of water aspiration as well as lowering fC and therefore cardiac strain on immersion. No reduction in either VT or the inspiratory gasp was observed, probably because fR decreased giving a longer duration for each inspiration. As previous studies  have indicated that fR is a better indicator of respiratory drive than VT during the CSR, the current findings suggest that respiratory drive was reduced following the rapid habituation protocol.
Rapid habituation to the CSR is possible and may provide a practical and inexpensive method of protection against drowning for individuals who are deployed at short notice to situations where they are at risk of accidental cold water immersion.
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