asphyxia as the most important cause for death in drowning was still widely accepted until world war 2 , when research in the United States was initiated to see what could be done to save the lives of pilots who had been forced to land in the sea . Swann ( 1951 ) was chosen to conduct a large series of investigations and it is mainly due to his work and those who have followed that the modern view of drowning has emerged . he was able to show important differences in the mechanism of drowning in fresh and salt water , using dogs . in fresh water drowning large amounts of water entered the lungs and were absorbed with great rapidity into the circulation , giving rise within a very few minutes to rapidly fatal heart failure in ventricular fibrillation , this the result of the grossly diluted blood entering the heart muscle . the gross and rapid dilution of the blood in freshwater drowning was clearly demonstrated by Swann and at only 3 minutes after submersion the blood was found to be diluted with an equal volume of inhaled water ; it is therefore not surprising that death occurs rapidly in these circumstances . in sea water drowning Swann showed that water was rapidly withdrawn from the blood into the lungs by the inhaled sea water , concentrating the blood and giving rise to a more gradual heart failure without the ventricular fibrillation that occurred in fresh water drowning . the gross and rapid concentration of the blood in sea water drowning was well demonstrated in that after only 3 minutes &apos; submersion the blood had lost some 40 % of its water . in addition , large amounts of the salts in sea water passed in the reverse direction into the blood to cause further disorganization of the blood chemicals ; it is again not surprising that death occurs rapidly in these circumstances . Swann also showed the resuscitation was usually successful with drowned dogs when heart failure had not occurred : once heart failure and falling blood pressure had occurred survival was most unlikely , even though irregular heart beats and respirations might occur for some minutes afterwards . he was also able to show that this lethal heart failure often occurred as early as 2 minutes after complete submersion , particularly in fresh water , explaining the higher mortality in this type of drowning . the modern view . the experiments on animals suggest that the mechanism of drowning in humans would depend on whether it occurs in fresh or salt water . in fresh water drowning in humans we would expect a rapid death within a very few minutes , partly due to asphyxia , but mainly due to sudden heart failure brought about by the explosive absorption of large amounts of water into the circulation . in salt water drowning in humans we would also expect a rapid death , partly due to asphyxia and partly due to rapid concentration of the blood . in drowning in other waters the mechanism would probably depend on whether the saline concentration in the water was greater or less than in the body . it should , however , be emphasized that death often occurs within 6 minutes and almost invariably with 10 minutes of becoming totally immersed , and that many of the cases removed from the water whilst still alive are doomed to die within a few minutes from the devastating changes which have already taken place , no matter whether the water was fresh or salt . this knowledge explains the very high mortality rate in drowning . there are , however , a small number of cases which are rescued from water before large amounts of water have apparently been inhaled , due to very rapid rescue , shock , reflex inhibition of the heart or persistent spasm of the air passages , preventing or restricting inhalation of water . it is in these cases that artificial respiration would offer the greatest chance of recovery . these are presumably cases in which there has not been time for the gross disturbance of body fluid which has such grave effects in most cases of drowning . but in the vast majority of cases , drowning is not a simple asphyxia due to obstruction of the air passages and lungs by water but is a complicated process in which violent disturbances of the body fluids and chemicals make the situation so much worse for the individual concerned . this is the modern view of drowning and , although much is still not understood , it is now worth considering other important aspects , particularly the signs and symptoms , prognosis , resuscitation and prevention of drowning , as well as forensic problems relevant to dead bodies removed from water . signs and symptoms . drowning is rarely witnessed but the ordinary course of events is apparently as follows . the swimmer remains on the surface until he is exhausted and then the course in swimmers and non-swimmers is similar . the drowning person sinks and rises a number of times in the water and inhales a little water into the air passages , but this is prevented from entering his lungs by coughing and glottic spasm . ( rarely he may die at this stage from shock , reflex inhibition of the heart , from pre-existing heart disease or from almost pure asphyxia due to unrelenting glottic spasm ) . he continues to rise and sink in the water , shouts for help , coughs and chokes , but does not inhale much water into his lungs . with increasing asphyxia due to glottic spasm he loses consciousness and cough reflex , sinks and inhales large amounts of water . it is in this stage that the lethal exchanges of water occur . oxygen reserves become severely depleted within 6 minutes and within 10 minutes heart and respiration almost invariably cease . death occurs , the body sinks and remains submerged until putrefaction and gas formation bring it to the surface some days later . the symptoms of drowning vary from one case to another , from sensations of tranquillity to utmost distress . the following two cases are quoted by Polson ( 1955 ) . in the first case ( originally reported by Cullen , 1894 ) the sensations of a woman rescued from drowning at sea are particularly interesting . self-preservation was dominant in her mind at first and there was great distress as she saw others swimming away from her . she experienced only acute suffering as described in her own words : I sank and gasped involuntarily , then all other senses were overpowered by the agonizing scorching pain which followed the rush of salt water into my lungs . from that moment I was conscious only of that burning suffocation and the intense desire that others might know what had become of me . except for that one thought my brain was dulled . she complained of roaring in her ears and redness before her eyes . she was unconscious when rescued by her husband within 3 minutes of the time she first became submerged . but the experience of Admiral Beaufort , also quoted by Polson ( 1955 ) , who was partially drowned when a boy and rescued within 2 minutes , were those of painless tranquillity and thoughts of his previous life . in another case , a boy of 15 years was accidentally submerged in the River Derwent ( news chronicle , August , 1960 ) for an uncertain period . he was rescued and recovered following artificial respiration . of his experiences he stated : I was sure I was dead , I just remember sinking . whilst under the water I had a terrible dream that I was going on a train to heaven . I never expected to wake up again . it is , however , unlikely in any of these three cases who lived to tell the tale that substantial amounts of water had been inhaled . prognosis and resuscitation . the prospect of survival following drowning must , of course , depend on many factors - the fitness of the subject , the duration of immersion and the amount of water inhaled being most important . the person with heart disease may die from sudden shock the moment he falls into cold water and it is not unusual for such subjects to be found dead in their own warm domestic baths , there being no question of drowning . in fit persons the prognosis depends on the length of asphyxia and the amount of water inhaled . in general , those who have been submerged a short time stand a better chance of survival in that oxygen reserves may not have been completely exhausted and spasm or shock may have prevented or restricted the inhalation of water into the lungs . but when large amounts of water have been inhaled it is most unlikely that recovery will occur , although the heart may continue to beat ineffectually for several minutes after rescue . it should be stressed again that the time required to inhale these lethal amounts of fluid is very short indeed , especially in fresh water drowning where explosive absorption of water from the lungs into the circulation may cause fatal ventricular fibrillation in as little as 2 minutes after commencing to breathe water . the course of events in sea water drowning is almost as rapid and thus the time available for rescue and resuscitation is pitifully short in those who have passed beyond the phase of glottic spasm into the second phase in which substantial amounts of water are inhaled . the prospect of recovery for those who have probably inhaled only a little water is better but there is here no time for delay in attempting resuscitation for irrecoverable changes can occur in a few moments . there is no time to examine the victim , no time to loosen clothing or clear the airway - these matters must be left until artificial respiration by any recommended method has been commenced . in theory artificial respiration should be continued in all cases until regular spontaneous breathing has occurred or death is certain . the question asked most often is : how long should artificial respiration be continued in the absence of signs of recovery ? answers vary greatly but most would agree that 15 minutes &apos; artificial respiration should be given before an examination is made and this process repeated for at least 1 hour before attempts are finally abandoned ( Donald , 1955 ) . it is occasionally stated that successful resuscitation may take place when the drowned individual has been submerged for prolonged periods . Bates in 1938 , reporting six cases of recovery from alleged drowning with submersion up to 35 minutes , stressed the need for artificial respiration to be continued until the body had cooled substantially or the early signs of rigor mortis were present . present knowledge of the mechanism of drowning throws grave doubt on the accuracy of such prolonged periods of submersion with subsequent survival . Taylor ( 1956 ) regards recorded cases of recovery after submersion for more than 7 or 8 minutes as wholly unreliable unless this has been intermittent or incomplete as might occur in the air pockets of upturned boats . it is theoretically possible that submersion in extremely cold water might on rare occasions chill the body so rapidly that vital organs are protected from the effects of lack of oxygen ( as is now practised surgically ) , allowing survival after periods of submersion which would ordinarily be lethal ( Donald , 1955 ) . the possibility that life had been preserved by some rare chance would indicate the need for at least some attempt at resuscitation in all bodies freshly recovered from water , as is the current practice . when recovery occurs following drowning it is usually ultimately complete , without evidence of significant residual damage to the lungs ( Rushton , 1960 ) , heart or brain , though a period of observation and treatment will be required for some days to guard against complications . the individual who has survived fresh water drowning may show evidence of severe destruction of red blood cells due to excessive absorption of water , with resulting temporary kidney damage and staining of the urine by red blood pigment , as in Rath &apos;s case ( 1954 ) quoted by Bowden ( 1957 ) . there may be cardiac failure due to alteration in the blood volume brought about by the absorption or withdrawal of fluid from the circulation and gross congestion and oedema of the lungs may occur within a few hours and cause death when recovery was expected . pneumonia may also occur early due to the inhalation of substantial quantities of dirty and infected water . 