Abridged from Cold-Water Immersion. Steinman A, Giesbrecht G., Wilderness Medicine .
4th edition. Auerbach P, editor. C.V. Mosby, St. Louis, 2001.
Immersion in cold water is a hazard for anyone who participates in recreational, commercial or
military activities in the oceans, lakes, and streams of all but the tropical regions of the world.
Recreational aquatic activities include swimming, fishing, sailing, power-boating, ocean kayaking,
white-water rafting, canoeing, ocean-surfing, wind-surfing, water-skiing, diving, hunting and the
use of personal water craft. In addition, use of a snowmobile, although not technically a water sport
can involve cold-water exposure due to accidental entry into lakes and streams. Commercial activities
involving water include fishing, shipping, offshore oil drilling, and diving. Military operations over
cold water include Coast Guard, Navy and Marine Corps missions; Army, Air Force and Marine Corps forces,
as well, may encounter cold-water exposure during winter operations on land.
The definition of cold water is variable. The temperature of thermally neutral water, in which heat
loss balances heat production for a nude subject at rest (i.e., not shivering), is approximately
33-35° C. Hypothermia eventually results from immersion in water below this temperature. For
practical purposes, significant risk of immersion hypothermia usually begins in water colder than
25° C. Using 25° C as the definition of cold water, the risk of immersion hypothermia in North
America is nearly universal during most of the year.
Cold water immersion is associated with two significant medical emergencies: near drowning and
Physiological Responses to Cold-Water Immersion
The primary pathophysiologic effects of hypothermia are a decrease in tissue metabolism and a gradual
inhibition of neural transmission and control. However, in the initial stages of cooling of an intact,
conscious victim, secondary responses to skin temperature cooling predominate. Sudden immersion in cold
water results in an immediate decline in skin temperature which, in turn, initiates shivering
thermogenesis with increases in metabolism (VO2), ventilation (VE), heart rate (HR), cardiac output (CO),
and mean arterial pressure (MAP). As body temperature declines and shivering ceases, VO2, HR, MAP and CO
decrease proportionally with the fall in core temperature, while hematocrit and total peripheral
resistance increase. Renal diuresis and extravascular fluid shifts can lead to a considerable loss of
intravascular volume, thus decreasing systemic perfusion.
The body's responses to cold-water immersion can be divided into three phases: 1) initial immersion and
the cold-shock response; 2) short-term immersion and loss of performance; and 3) long-term immersion and
the onset of hypothermia. Each phase is accompanied by specific survival hazards for the immersion victim
from a variety of pathophysiologic mechanisms. Deaths have occurred in all three phases of the immersion
Phase 1: Initial Immersion and the Cold Shock Response:
The cold shock response occurs within the first 1-4 minutes of cold water immersion and is dependent
on the extent and rate of skin cooling. The responses are generally those affecting the respiratory
system and those affecting the heart and the body's metabolism. Rapid skin cooling initiates an
immediate gasp response, the inability to breath-hold, and hyperventilation. The gasp response may
cause drowning if the head is submersed during the initial entry into cold water. Subsequent inability
to breath-hold may further potentiate drowning in high seas. Finally, hyperventilation causes arterial
hypocapnia, which leads to decreased brain blood flow and oxygen supply. This may lead to disorientation,
loss of consciousness and drowning.
Skin cooling also initiates peripheral vasoconstriction as well as increased cardiac output, heart rate
and arterial blood pressure. The increased workload on the heart may lead to myocardial ischemia and
arrhythmias, including ventricular fibrillation. Thus, sudden death can occur either immediately or
within a matter of minutes after immersion (i.e., due to syncope or convulsions leading to drowning,
vagal arrest of the heart, and ventricular fibrillation) in susceptible individuals.
Phase 2: Short-Term Immersion and Loss of Performance:
For those surviving the cold shock response, significant cooling of peripheral tissues, especially
in the extremities, continues with most of the effect occurring over the first 30 minutes of immersion.
This cooling has a direct deleterious effect on neuromuscular activity. This effect is especially
significant in the hands, where blood circulation is negligible, leading to finger stiffness, poor
coordination of gross and fine motor activity, and loss of power. It has been shown that this effect
is primarily due to peripheral and not central cooling. The loss of motor control makes it difficult,
if not impossible, to execute survival procedures such as grasping a rescue line or hoist, etc. Thus
the ultimate cause of death is drowning, either through a failure to initiate or maintain survival
performance (i.e., keeping afloat, swimming, grasping onto a liferaft, etc.) or excessive inhalation
of water under turbulent conditions.
These phenomena have obvious survival implications. It is, of course, advisable to avoid cold water
exposure completely. If cold-water immersion does occur however, it is best to quickly determine and
execute a plan of action: 1) try to enter the water without submersing the head; 2) escape (i.e., pull
oneself out of the water, inflate and board a liferaft); 3) minimize exposure (i.e., get as much of
one's body as possible out of the water and onto a floating object); 4) ensure flotation if one must
remain in the water (i.e., don or inflate a personal flotation device); and 5) call for assistance
(i.e. activate signaling devices). It may be difficult to execute these actions while the cold shock
responses predominate. However, once the respiratory effects are under control, immediate action should
be taken. If self-rescue is not possible, actions to minimize heat loss should be initiated by remaining
as still as possible in the Heat Escape Lessening Position (HELP), where arms are pressed against the
chest and legs are pressed together, or huddling with other survivors. Drawstrings should be tightened
in clothing to decrease the flow of cold water within clothing layers.
Phase 3: Long-term immersion and the onset of hypothermia:
Most cold-water deaths likely result from drowning during the first two phases of cold-water immersion,
as discussed above. In general, true hypothermia usually only becomes a significant contributor to death
if immersion lasts more than 30 minutes. The individual who survives the immediate and short-term phases
of cold-water immersion faces the possible onset of hypothermia as continuous heat loss from the body
eventually decreases core temperature (Tco). Many predictive models to determine the core temperature
response to cooling are based on the relationships between body composition, thermoregulatory response
(i.e., shivering thermogenesis), clothing/insulation, water temperature and sea conditions. These
variables and their impact on survival time are discussed in more detail in the Cold Water Survival
section of this chapter.
Normal body Tco fluctuates around 37° C. The clinical definition of hypothermia is a Tco of
35° C or lower; however, any exposure to cold that lowers the temperature below normal levels
results in the body becoming hypothermic. Although various temperatures and terms have been used to
classify different levels of hypothermia, the following classifications will be used here. In mild
hypothermia (Tco = 32-35° C) thermoregulatory mechanisms continue to operate fully, but ataxia,
dysarthria, apathy and even amnesia are likely. In moderate hypothermia (Tco = 28-32° C) the
effectiveness of the thermoregulatory system (i.e., shivering thermogenesis) diminishes until it fails;
there is a continued decrease in level of consciousness; and cardiac dysrhythmias may also occur.
In severe hypothermia (Tco 28° C) consciousness is lost, shivering is absent, acid-base
disturbances develop, and the heart is susceptible to ventricular fibrillation or asystole. Death from
hypothermia is generally from cardiorespiratory failure.
The rate of body core cooling during cold-water immersion depends on the following variables:
- Water temperature and sea state
- Body morphology
- Amount of the body immersed in water
Behavior (e.g. excessive movement) and posture (e.g. HELP, Huddle, etc.) of the body in
- Shivering thermogenesis
- Other non-thermal factors
Cold Water Survival
Cold water survival depends on avoidance of drowning and hypothermia and on the many factors related
to these risks.
- Ability to swim
- Ability to keep the head out of water (even without flotation aids)
- Ability to avoid panic
- Sea state
- Availability and type of personal flotation device (PFD)
- Availability of a life raft
- Availability of other floating objects to increase buoyancy (such as a capsized boat)
- Water temperature
- Physical characteristics of the survivor
Type of protective clothing worn against immersion hypothermia and initial immersion cold shock
- Behavior of the survivor in the water
Availability of signaling devices (whistles, flares, strobe lights, radios, and mirrors) and
the ability to use these devices
- Proximity of rescue personnel
Drowning is the most immediate survival problem following water entry. To maintain airway freeboard
and to avoid drowning, a survivor must possess the physical skills and psychological aptitude to combat
the effects of wave action. Although a PFD assists in maintenance of airway freeboard, waves can still
submerge a survivor's head, even in moderately calm seas. To reduce the risk of drowning in rough seas,
a survivor can increase effective airway freeboard by partially exiting the water (for example, clinging
to an overturned vessel or other debris floating in the water) or by climbing totally out of the water
into a life raft or onto a capsized vessel. In both these environments the survivor may still have to
cope with the effects of cold wind, spray, and waves.
Prehospital management of hypothermia patients, both in the field and during transportation to a site
of definitive medical care, varies with the patient's level of hypothermia, with the rescuer's level
of training, with the resuscitative equipment available, with the type of transportation, and with the
time required for delivery to definitive care. Medical personnel must exercise good clinical judgment
in balancing all these factors to select appropriate therapeutic modalities.
Rescue and Management
The primary goals in prehospital management of victims of accidental immersion hypothermia are
prevention of cardiopulmonary arrest, prevention of continued core temperature decline, moderate core
rewarming if practicable, and transportation to a site of definitive medical care. Aggressive rewarming
in the field is usually contraindicated, since the means to either diagnose or manage the many potential
complications of severe hypothermia are unavailable in this setting. In unusual circumstances, when
transportation to a site of definitive care is impossible, definitive rewarming in the field, using the
principles and techniques of management described in the following paragraphs, may be appropriate.
Retrieval of a victim from cold water immersion must be performed with caution. Sudden reduction of the
"hydrostatic squeeze" applied to tissues below the water's surface may potentiate hypotension,
especially orthostatic hypotension. Since a hypothermic patient's normal cardiovascular defenses are
impaired, the cold myocardium may be incapable of increasing cardiac output in response to a hypotensive
stimulus. A victim's vertical posture may also potentiate hypotension. Hypovolemia, secondary to
combined cold- and immersion-induced diuresis, and increased blood viscosity potentiate these effects.
Peripheral vascular resistance may also be incapable of increasing, since vasoconstriction is already
maximal because of cold stress. The net result of sudden removal of a hypothermic patient from the
water is similar to sudden deflation of antishock trousers on a patient in hypovolemic shock: abrupt
hypotension. This has been demonstrated experimentally in mildly hypothermic human volunteers, and it
has been suspected as a cause of post-rescue death in many immersion hypothermia victims. Accordingly,
rescuers should attempt to maintain hypothermic patients in a horizontal position during retrieval from
the water and aboard the rescue vehicle. If rescuers cannot recover the patient horizontally, they
should place the victim in a supine posture as quickly as possible after removal from cold water.
The patient's core temperature may continue to decline (depending on the quality of insulation provided,
the patient's endogenous heat production, active or passive manipulation of extremities, and the site
of core temperature measurement) even after he or she has been rescued, because of the physiologic
processes described earlier for "afterdrop." To diminish this effect, the patient's physical
activity must be minimized. Conscious patients should not be required to assist in their own rescue
(for example, by climbing up a scramble net or ship's ladder) or to ambulate once out of the water
(as by walking to a waiting ambulance or helicopter). Physical activity increases afterdrop, presumably
by increasing perfusion of cold muscle tissue with relatively warm blood. As this blood is cooled,
venous return (the circulatory component to afterdrop) contributes to a decline in myocardial
temperature, increasing the risk of ventricular fibrillation. Experiments on moderately hypothermic
volunteers (esophageal temperature 33° C) demonstrated a threefold greater afterdrop during
treadmill walking than while lying still. Such an exercise-induced enhancement of afterdrop could
precipitate post-rescue collapse. Throughout the rescue procedures and during subsequent management,
hypothermic patients must be handled gently. Excessive mechanical stimulation of the cold myocardium
is another suspected cause of deaths after rescue.