ANOXAEMIA is a general term comprising those conditions of the body under which the tissues are starved of oxygen. There are three main types : The anoxic type, in which the blood going to the tissues carries oxygen at too low a pressure and con sequently the haemoglobin is only partially charged with oxygen. (2) The anaemic type in which though the haemoglobin is fully charged there is too little of it, and hence the capacity of the blood to carry oxygen is too low. (3) The ischaemic type in which the blood is or may be normal but in which the quantity of blood running through the organ is too small.
There are some other conditions, less common, which may or may not be regarded as forms of anoxaemia. Such, for instance, is cyanide poisoning. (See TOXICOLOGY.) In this the supply of oxygen is adequate but the mechanism in the tissues for acquiring and using it is faulty.
The airman loses his faculties if he goes too high. The loss may amount merely to a loss of judgment, or a loss in manipula tive power, or it may extend to complete loss of consciousness. Because the brain symptoms are the first and the most baffling, the British air force direct not that an aviator should commence breathing oxygen when he reaches an altitude at which he judges himself to need it (by that time his judgment may be impaired) but if he is going to a high altitude he is directed to breathe oxygen from the start. (See AVIATION.) The mountaineer suffers from a somewhat different train of symptoms which comprise the condition known as "mountain sickness," or in the Andes "Seroche." In the case of a person travelling by train up to the mining districts about Cerro de Pasco, or up Pike's Peak in Colorado (both 14,00o-15,000f t. ), the symptoms usually appear some hours after arrival not im probably during the night. They are generally a selection from the following: (1) symptoms of the brain—headache, lassitude, fatigue, sleeplessness, visual and auditory impairment; depres sion; (2) cardiac symptoms—pain in the chest, palpitation, sinus arrhythmia; (3) circulatory symptoms—cold hands and feet, blue ness, throbbing; (4) respiratory symptoms—shortness of breath, periodic breathing, sighing ; (5) gastro-intestinal symptoms—nau sea, vomiting, anorexia. After a residence of two to three days these symptoms usually pass off in their acute form, but no amount of acclimatization makes the human frame at 14,000f t. as efficient as at the sea level. Exercise induces undue breath lessness and quickening of the heart; mental fatigue follows upon severe intellectual work; and the lack of oxygen makes for depression and irritability. The severest work performed at high altitudes falls far short of what could be done at sea-level.
The hurtful results of ascent to high altitudes whether on the mountain or in aircraft are governed by the condition of the blood. The processes of oxidation on which life and health de pend take place within the tissues of the actual organs concerned —the heart, the brain. the muscles, the glands, etc. Of these un doubtedly the most vulnerable is the brain. It not only carries out the processes of thought, but governs the condition of the rest of the body, e.g., the rate of the pulse, the depth of respira tion and so forth. Therefore the study of the conditions which produce trouble at high altitudes resolves itself into a study of the adequacy of the oxygen supply of the brain.
Oxygen is transported from the lungs to the brain by the blood. The haemoglobin or red pigment has the power of forming a loose compound with oxygen. Of each loogram. of haemoglobin which leave the lung, the number of grammes which transport oxygen depends upon the pressure of oxygen in the air cells of the lung, which in turn depends upon the pressure of oxygen in in the air. As this latter at any altitude is 21% of the local at mospheric pressure, it follows that the higher the subject ascends the lower will be the oxygen pressure in the air cells of his lung. At ordinary pressures there is a margin, the air pressure being more than enough to saturate his haemoglobin sufficiently. Fig. 2, A, shows the percentage of the total haemoglobin leaving the lung, which would be charged with oxygen at different altitudes if the subject made no alteration in the rate and depth of his respiration.
During the ascent of mountains a certain amount of acclima tization takes place almost from the start, the subject tends to breathe more deeply and to pass more air through the lungs in a given time; this process raises the quantity of oxygen in the air cells (alveoli) of the lung to a higher percentage than it would otherwise attain; the mountaineer at a given height, therefore, has more oxygen in his lungs that the aviator. Curve B fig. 2 shows the probable maximal degree to which the blood can be come charged with oxygen as the result of the increase of lung ventilation occurring at the altitudes indicated. These figures are the results of measurement, made by expeditions to the Andes (Cerro de Pasco) and the Alps (Monte Rosa) and Everest.
Somewhere in between curves A and B would be the oxygen in the blood of the airman according to his individual idiosyn crasy. The airman reaches a high altitude in a time measured in minutes. Experiments carried out by Schneider and his associates on 7,000 aviators for the American Government show that there is little or no increase in the amount of air passed through the lungs until oxygen pressures are reached which correspond to 4,000 feet. At higher altitudes different persons respond in dif ferent degrees. In more than 50% of all men examined the first respiratory response occurred at from 58o to 52omm. pressure and in 25% the change took place at an even lower percentage, while a few gave no evidence of an increase up to the time of uncon sciousness. Most physiologists believe that increased pulmonary ventilation is almost the only form of acclimatization of which persons exposed to high altitudes for short times only are capable. Dr. J. S. Haldane and his school regard the lungs as having in addition a special power of secreting oxygen into the blood.
The mountaineer can claim a much higher degree of acclimati zation than the aviator, several factors becoming modified in his favour : (I) His pulmonary ventilation increases as already stated. (2) The number of red corpuscles in each cubic millimetre of his blood increases almost in proportion to the altitude. The follow ing figures are given by Maj. Hingston I.M.S. (1924 Everest Ex pedition) for the natives of the Pamir plateau: Carbon monoxide is met with in many other places besides coal gas. It is a frequent source of danger in mines galleries, both of coal and of other mines. To discover the presence of the gas, ad vantage is taken of the fact that small warm-blood creatures, such as mice, rats and canaries succumb to it much more rapidly than man. If these, therefore, be taken into a suspect atmosphere they will drop while as yet man has time to escape. Indeed, it is the time factor which saves man from being asphyxiated in small concentrations of carbon monoxide. In order to lose consciousness over half of all the haemoglobin in his body must be united with The increase in the number of corpuscles is associated with a corresponding increase in the quantity of haemoglobin. At first probably the increase is wrought by some sort of emergency mechanism, such as the abstraction of some water from the blood, the contraction of the spleen and so forth. Shortly there is evi dence of increased blood formation, a type of immature cell known as the reticulated corpuscle being formed in the blood. The whole quantity of haemoglobin in the body is increased as was found by the Pike's Peak expedition of 1911. (3) Less well attested is the character of the change in the nature of the corpuscles de scribed by the 1921-22 expedition to Cerro de Pasco. They found that the haemoglobin in the red corpuscle acquired an increased chemical affinity for oxygen such as would occur if the interior of the corpuscle became more alkaline. The effect of this altera tion superimposed on the increased ventilation is shown in fig. 2 curve C.
Fig. 2 therefore represents the effects of the combined effect of the two forms of acclimatization which affect the limiting maximal quantity of oxygen which the blood in the arteries can acquire as compared with what the same blood could unite with at the sea level. This limit is perhaps not quite reached, but it is most nearly attained when the subject is at rest. The more active the exercise the greater is the discrepancy between the oxygen actually in the arterial blood, and the limiting value as shown in the figure. The handicap of exercise looms very large at the highest altitudes to which man climbs, as in Everest, for exercise is the only way of securing warmth. In the aeroplane heat is obtained by electrical appliances.
(2) Anaemic Anoxaemia.—Entailing too small a quantity of functional haemoglobin in each cubic millimetre of blood. The most obvious form is anaemia in which the actual quantity is too small (see ANAEMIA). Three important forms exist in which the haemoglobin present is temporarily put out of action. Of these the two most important are carbon monoxide (or coal gas) poison ing and methaemoglobin poisoning.
Coal gas contains percentages of carbon monoxide which range from about 7% to about 20% in Great Britain. In the United States the percentage is often greater owing to the large admixture of water gas which is legally possible. Carbon monoxide has the advantage of giving a clear hot flame and therefore is an efficient constituent whether the object of this gas is to heat mantles, to cook or to provide power. Owing to the great precautions taken by the gas companies and the general intelligence of the public accidental cases of coal-gas poisoning are singularly rare.
Very small quantities of carbon monoxide are, however, capable of producing fatal results if breathed for a long enough time. Fig. 3, based on the observations of Haldane shows the percentage of carbon monoxide in the air which if breathed indefinitely is capable of saturating the blood to any given degree. The effect on different persons varies greatly but speaking in very rough terms a saturation of 30-40% means a headache afterwards, 50-60% means unconsciousness and 75% is probably fatal.
Carbon monoxide in the blood is gradually eliminated when air free from it is breathed and eliminated at a much greater rate if pure oxygen is inhaled. Best of all is a mixture of oxygen and car bon dioxide. The latter causes panting which tends to wash out the poison from the blood.
carbon monoxide—that can be accomplished in a concentration of less than o• i % of the gas in the atmosphere but it requires the absorption of an absolute volume, about 5ooc.c. of CO, which would require perhaps Ioo minutes.
The incompletely combusted gas which comes from the exhaust of internal combustion engines has been a cause of numerous fatal ities. A 20 h.p. automobile engine is estimated as being capable of emitting a cubic foot (28 litres) of carbon monoxide per minute. "This is sufficient to render the atmosphere of a single car garage deadly within five minutes if the engine is run while the garage doors are closed." (Y. Henderson.) Methaemoglobin poisoning is another condition in which a part of the haemoglobin is thrown out of action. The simplest form is such as is produced by the inhalation of aniline volatile nitrites, nitrobenzene, etc., causing a conversion of oxyhaemoglobin into methaemoglobin (which has no respiratory value) within the blood, with injury to the corpuscles. The conversion is only tem porary if the impure air ceases to be inhaled and if the poisoning is not too severe. Some other drugs such as chlorates and bromates in addition to producing methaemoglobin in the blood, cause an actual destruction of corpuscles—a much graver condition.
(3) Ischaemic Anoxaemia.—This may be general or local. If general it may result from very different causes : (I) In cases of heart disease the heart may be unable to pump the blood round the body at the required rate ; or again back-pressure may prevent the blood circulating as it should. (2) After severe bleeding there may not be enough blood left in the body adequately to supply its needs. Under such circumstances the body makes the effort to maintain the blood supply to the brain, and for that purpose other organs are in a measure denied their share. The body is not, how ever, without resources from which to draw in case of severe haemorrhage. One store of blood is in the spleen. This organ is ordinarily distended, but when the organism makes a call for an extra supply of blood the spleen contracts and expels considerable quantities into the circulation. (3) Following on severe abdominal wounds, or surgical operations which have entailed considerable exposure of the internal organs, a condition known as surgical shock may supervene. This condition appears to be due to a decrease in the quantity of blood plasma so great that the blood no longer properly fills the vessels. The blood pressure therefore falls and the organs are starved of blood. Light was shed on the cause of surgical shock during the World War by the researches of Dale, Richards and Laidlaw, which indicate that it is due to poisoning by a particular material shed into the body by damaged tissue. The material is called "histamine" or j3-iminazolylethyla mine. This poison, among other things, makes the walls of the blood capillaries much more permeable to fluid. The plasma of the blood therefore oozes out through them.
BIBLIOGRAPHY.-J. B. S. Haldane, Respiration (New Haven, 1922) ; Bibliography.-J. B. S. Haldane, Respiration (New Haven, 1922) ; J. Barcroft, Presidential Address, Sect. I., Cardiff Meeting of the British Association (1920) ; Lessons from High Altitudes (Cambridge, 1925) C. Lovatt Evans, Recent Advances in Physiology (1926) ; Pub lications of the Medical Research Council, Great Britain, on "Surgical Shock" and the "Medical Problems of Flying"; Manual of the Medical Research Laboratory of the War Dept. (Air Service) (Washington, 1918). (J. BAR.)