Shepherd (2011) related the stages of drowning beginning with the immersion or submersion of a person's face in water (or other liquid) (18). If the water is cooler than 20oC (68oF), it can initiate the mammalian diving reflex, an autonomic nerve response that causes involuntary breath holding. In the very young, this diving reflex is more pronounced and may extend survival. Prolonged breath holding causes a lack of oxygen (hypoxemia) and an increase of carbon dioxide (hypercapnia) in the blood. The resulting acidosis eventually forces a breath in which a very small amount of water enters the windpipe triggering a spasm of the larynx (laryngospasm) that prevents additional water to enter the lungs. Continuing immersion or submersion increases cerebral hypoxia, and the laryngospasm relaxes allowing some water to enter the lungs if the person is still struggling to breathe.

Forensic examination by Lunetta, et al, (2004) determined that 570 of 578 persons who had drowned were found to have aspirated water into their lungs (formerly termed "wet drowning"), which was indicative of an attempt to breathe during drowning (19). However, if the person was unconscious or had already died, little or no additional water would have entered the lungs (formerly termed "dry drowning"). Prolonged immersion or submersion and its corresponding aphyxia result in cardiac arrest, and this with other co-morbidities contribute significantly to drowning fatalities and the failure of resuscitation efforts for those who initially survive.

During freshwater drowning, water entering the lungs may be drawn into the pulmonary circulation diluting blood and causing as much as 75% of the body's blood to be diverted through the lungs in the body's desperate attempt to deliver oxygen to the brain. In saltwater drowning, serum proteins are drawn out of the pulmonary circulation filling the lung's alveoli, reducing respiratory compliance and inducing rapid hypoxia (18). In both situations, the lung's surface tension reducing capability (surfactant) is compromised causing atelectasis. Drowning survival is often complicated by ARDS (acute respiratory distress syndrome), which develops during and after resuscitation and is further complicated by multi-organ failure.

Warm water [20oC (68oF) or higher] is more likely to contribute to respiratory complications after resuscitation, such as pneumonia caused by inhaled waterborne debris, bacterial and fungal pathogens and chemicals in water polluted by sewage and industrial wastes (18). Even water warmer than 20oC can eventually result in hypothermia, a precursor to possible drowning, because water has the capacity to conduct heat away from the body 25 times faster than air. Chlorinated pool water (free chlorine and chloramine residuals) irritates skin, eyes and sinuses and can cause chemical pneumonitis. Pool waters much above 40oC (104oF) increase the risk of cardiac arrest and stroke, which has contributed to drowning in spas and hot tubs.

In cold water [less than 20oC (68oF)], a person may develop hypothermia if exposed for a prolonged period while struggling to remain afloat. As the body cools, blood is shunted away from the limbs and to one's core internal organs, especially the heart and brain. The resulting vasoconstriction in the extremities renders first the hands and then the arms and legs numb, and muscles are increasingly unable to function to help oneself. Prolonged immersion or submersion in cold water affects one's mental processes as well and the ability to even assist a rescuer (18).

Suominen, et al, (2002) conducted a retrospective study of 61 drowning victims aged 0.5 to 60 years admitted to hospitals and their fatal and non-fatal outcomes. Duration time of submersion comparing survivors to non-survivors (medians of 5 and 16 minutes respectively) was a significant factor but not age, water temperature or rectal temperature upon hospital admittance (20). Despite sensational news stories about very young children surviving cold water drowning due to the mammalian diving reflex and hypothermia, most heroic resuscitations end tragically in death or permanent mental and physical disability due to prolonged hypoxia. The emergency medical services' adage "not dead until warm and dead" applies to the very slow rewarming protocol for treating a cold water drowning or hypothermic patient (21).

Of the injuries related to drowning, falls are the most common, and the resulting trauma to the head, spine and limbs can exacerbate the drowning outcome. Watercraft-related injuries can produce other serious injuries, such as lacerations, puncture wounds, blunt force trauma, burns and amputations, which can reduce one's chances for survival in the water (18). Injuries from freshwater and marine life also impact drowning outcomes, and skin (snorkel) and SCUBA diving expose their devotees to drowning mishaps when judgment wanes, skill level is exceeded or equipment fails (22). Commercial fishermen, especially those working in cold waters off the coast of Alaska and similar areas, are at an elevated risk for drowning and hypothermia due to injuries sustained due to vessel and equipment problems, lax safety and sea conditions (23).

Krin (2010) and Venema, et al, (2010) noted that first responders and bystanders are often at risk of drowning when they respond on or in the water without adequate safety training and equipment (21, 24). Tidal, rip and longshore currents, undertows, floods and maelstroms await the unwary risk taker or would-be rescuer. Quickly sapped of stamina when trapped in a water hazard wielding significant oppositional forces, a person with any health problems, injuries or disabilities has little chance of survival. Elverum, et al, (2008) warned of river hazards known as hydraulics or "drowning machines" that are generated by fast flowing water over seemingly insignificant lowhead dams and spillways (25). Both victims and rescuers can become trapped and drown in the recirculating torrent of roiling water that offers no escape.