In pursuit of the unicorn.

This short review was prompted by The Physiological Society's recent online symposium on variability. It does not deal with a specific methodology, but rather with the myth that certain environmentally-induced clinical conditions can be identified, quantified, simplified and monitored with a single methodology. Although this might be possible with some clinical conditions, others resist the prevailing reductionist approach of minimizing rather than exploring variation in pathogenesis and pathology, and will not be understood fully until the variation in cause and effect are embraced. This is likely to require comprehensive methodologies and collaboration.

Basic life support on small boats at sea.

The present study examined the ability of the crew of small fast rescue boats to perform basic life support (BLS) at sea. Tests were undertaken aboard a 67m emergency response and rescue vessel (ERRV), a 9.1m (30ft) and 11.6m (38ft) fast rescue craft ("daughter craft" (DC)). It was hypothesised that the ability to perform BLS on a DC would be significantly impaired when compared with that seen on the ERRV. Nine DC crew volunteered for the study. These tests were undertaken in sea states ranging from 0.5 to 6 (13cm to 4m wave height). Wind speeds ranged between 0 and 35knots. The deterioration observed in the performance of BLS on board the DC compared to that seen on the ERRV was significant (P<0.05) and was due, in part, to a tendency to over-inflate during rescue breathing, and under-compress during BLS when on the DC. Chest compression (CC) was impaired significantly above a sea state 3 (wave height 61-92cm). It is concluded that the performance of BLS on small boats, in particular rescue breathing, is significantly adversely affected by two major factors, motion-induced interruption and early fatigue. As a consequence, the likelihood of conducting fully effective continuous BLS on a small boat in a seaway for any length of time, with a good chance of a successful outcome, is considered to be poor. However, this should not deter rescuers from attempting to make such efforts where practicable.

Permanence of the habituation of the initial responses to cold-water immersion in humans.

Sudden immersion in cold water initiates an inspiratory gasp response followed by uncontrollable hyperventilation and tachycardia. It is known that this response, termed the "cold shock" response, can be attenuated following repeated immersion. In the present investigation we examined how long this habituation lasts. Twelve healthy male volunteers participated in the experiment, they were divided into a control (C) group (n = 4), and a habituation (H) group (n = 8). In October, each subject undertook two 3-min head-out seated immersions into stirred water at 10 degrees C wearing swimming trunks. These immersions took place at the same time of day, with 4 days separating the two immersions. In the intervening period, the C group were not exposed to cold water, while the H group undertook six, 3-min head-out immersions in water at 15 degrees C. Two months (December), 4 months (February), 7 months (May) and 14 months (January) after their first immersion, all subjects undertook another 3-min head-out immersion in water at 10 degrees C. The H group showed a reduction in respiratory frequency (47 to 24 breaths x min(-1)), inspiratory minute volume (72.2 to 31.3 1 x min(-1)) and heart rate (128 to 109 beats x min(-1)) during the first 30 s of immersion on day 5 compared to day 1. Seven months later these responses were still significantly reduced compared to day 1. After 14 months, heart rate remained attenuated but respiratory frequency and inspiratory minute volume had returned towards pre-habituation levels. The responses of the C group during the first 30 s of immersion were not altered. Both groups showed an attenuation in the responses during the remaining 150 s of immersion following repeated immersions. It is concluded that repeated immersions in cold water result in a longlasting (7-14 months) reduction in the magnitude of the cold shock response. Less frequent immersions produced a decrease in the duration, but not the magnitude of the response.

Immersion deaths and deterioration in swimming performance in cold water.

BACKGROUND: General hypothermia (deep body temperature <35 degrees C) has been implicated in immersion-related deaths, but many deaths occur too quickly for it to be involved. We investigated changes in swimming capability in cold water to find out whether such changes could lead to swim failure and drowning. METHODS: Ten volunteers undertook three self-paced breaststroke swims in a variable-speed swimming flume, in water at 25 degrees C, 18 degrees C, and 10 degrees C, for a maximum of 90 min. During each swim, we measured oxygen consumption, rectal temperature, swim speed and angle, and stroke rate and length. Swim failure was defined as being unable to keep feet off the bottom of the flume. FINDINGS: All ten swimmers completed 90 min swims at 25 degrees C, eight completed swims at 18 degrees C, and five at 10 degrees C. In 10 degrees C water, one swimmer reached swim failure after 61 min and four were withdrawn before 90 min with rectal temperatures of 35 degrees C when they were close to swim failure. Swimming efficiency and length of stroke decreased more and rate of stroke and swim angle increased more in 10 degrees C water than in warmer water. These variables seemed to characterise impending swim failure. INTERPRETATION: Impaired performance and initial cardiorespiratory responses to immersion probably represent the major dangers to immersion victims. Consequently, treatment should be aimed at symptoms resulting from near-drowning rather than severe hypothermia.

Habituation of the initial responses to cold water immersion in humans: a central or peripheral mechanism?

1. The initial respiratory and cardiac responses to cold water immersion are thought to be responsible for a significant number of open water deaths each year. Previous research has demonstrated that the magnitude of these responses can be reduced by repeated immersions in cold waterwhether the site of habituation is central or peripheral. 2. Two groups of subjects undertook two 3 min head-out immersions in stirred water at 10 C of the right-hand side of the body (R). Between these two immersions (3 whole days) the control group (n = 7) were not exposed to cold water, but the habituation group (n = 8) undertook a further six 3 min head-out immersions in stirred water at 10 C of the left-hand side of the body (L). 3. Repeated L immersions reduced (P < 0.01) the heart rate, respiratory frequency and volume responses. During the second R immersion a reduction (P < 0.05) in the magnitude of the responses evoked was seen in the habituation group but not in the control group, despite both groups having identical skin temperature profiles. 4. It is concluded that the mechanisms involved in producing habituation of the initial responses are located more centrally than the peripheral receptors.

Temperature dependence of habituation of the initial responses to cold-water immersion.

The initial responses to cold-water immersion, evoked by stimulation of peripheral cold receptors, include tachycardia, a reflex inspiratory gasp and uncontrollable hyperventilation. When immersed naked, the maximum responses are initiated in water at 10 degrees C, with smaller responses being observed following immersion in water at 15 degrees C. Habituation of the initial responses can be achieved following repeated immersions, but the specificity of this response with regard to water temperature is not known. Thirteen healthy male volunteers were divided into a control (C) group (n = 5) and a habituation (H) group (n = 8). Each subject undertook two 3-min head-out immersions in water at 10 degrees C wearing swimming trunks. These immersions took place at a corresponding time of day with 4 days separating the two immersions. In the intervening period the C group were not exposed to cold water, while the H group undertook another six, 3-min, head-out immersions in water at 15 degrees C. Respiratory rate (fR), inspiratory minute volume (VI) and heart rate (fH) were measured continuously throughout each immersion. Following repeated immersions in water at 15 degrees C, the fR, VI and fH responses of the H group over the first 30 s of immersion were reduced (P < 0.01) from 33.3 breaths x min(-1), 50.5 l x min(-1) and 114 beats x min(-1) respectively, to 19.8 breaths x min(-1) 26.41 x min(-1) and 98 beats x min(-1), respectively. In water at 10 degrees C these responses were reduced (P < 0.01) from 47.3 breaths x min(-1), 67.61 x min(-1) and 128 beats x min(-1) to 24.0 breaths x min(-1), 29.5 l x min(-1) and 109 beats x min(-1), respectively over a corresponding period of immersion. Similar reductions were observed during the last 2.5 min of immersions. The initial responses of the C group were unchanged. It is concluded that habituation of the cold shock response can be achieved by immersion in warmer water than that for which protection is required. This suggests that repeated submaximal stimulation of the cutaneous cold receptors is sufficient to attenuate the responses to more maximal stimulation.