There is a regular sequence in the phenomena of dyspnœa leading up to the final stage termed “asphyxia.” If the trachea be suddenly blocked, so that no air can pass, the respiratory movements at once become deeper and more rapid. This condition is termed “hyperpnœa.” In a comparatively few seconds the system appears, as it were, to find out that inspiration is not needed. Expiratory efforts begin to preponderate. They increase in violence. All accessory muscles are brought into play. The cry for air is heard even by muscles which cannot help. Muscles of the limbs contract, although their contraction has no effect upon the capacity of the chest. Every expiratory effort is accompanied by convulsions of a flexor type. At the end of two minutes there is usually a sudden change. Attempts at expiration cease. Slow, deep, infrequent inspirations take their place, accompanied by convulsions of extensor muscles. Pupils are widely dilated, mouth open, head thrown back. The subject is absolutely insensitive to every kind of stimulus. The pulse shows a high arterial tension. The beating of the heart is slow and strong. In about four minutes from the time at which the windpipe was blocked respiratory movements cease. The arterial tension falls. The heart’s action grows rapidly weaker, although for two or three minutes longer it may still continue to flicker. Recovery is possible until it finally gives up. After death the right side of the heart is found gorged with blood, the left side empty, showing that the heart had been unable to force the blood through the capillaries of the lungs.
Under all ordinary conditions the sequence of phenomena of asphyxia is the same—a stage of exaggerated breathing (hyperpnœa), a stage marked by the co-operation of muscles which are not called into action in tranquil breathing (dyspnœa), followed by the condition of asphyxia properly so termed. An animal whose supply of fresh air is cut off passes through these three stages, whether it be enclosed in a small space or in a very large one. It must, however, be noted that in asphyxia several factors combine in varying degrees. Carbonic acid is in excess in the blood, oxygen deficient. The nervous mechanism which regulates respiratory movements is thrown out of gear. Motor and inhibitory impulses are in conflict. It is important, if these complex phenomena are to be analysed, that one factor only should be altered at any given time. For example, carbonic acid may be allowed to increase in the air while a constant oxygen tension is maintained. Under these circumstances the dyspnœic contractions are much less marked. No convulsions follow. The paralysing action of carbonic acid predominates. Anæsthesia passes into complete unconsciousness. Death is tranquil. And this, speaking generally, is what happens in disease of the lungs. Asphyxia comes on slowly. The supply of oxygen is undiminished, but carbonic acid accumulates in the blood, acting as a narcotic poison which lowers the excitability of the nervous system, suspends consciousness, and slowly brings the vital activities to a standstill.
In cases of drowning, when the lungs are filled with water, the resistance to the passage of blood through their capillary vessels is greater than it is when they are still filled with air. The heart is sooner beaten in its effort to drive the blood through them. Usually it stops in about four minutes. Yet it is difficult to say for how long after a person has been immersed in water it may be still possible to resuscitate him. Reports vary, owing in large measure to uncertainty as to the exact time at which the immersed person sank and his lungs filled with water. It is a wise precept that artificial respiration should be tried in every case, without waiting a single instant to ascertain whether the heart still beats. The first thing to do is to empty the chest of water. Then place the subject on his back. Kneel on the ground behind his head. Grasp an arm just below the elbow, in each hand. Draw the arms up above the patient’s head, so that the pectoral and other muscles drag on the ribs, enlarging the chest; then lower them, and press them into the sides. This must be done with the natural rhythm of respiration, and not more frequently than twenty times in a minute. It is well if an assistant draws the tongue forward, to give free admission to air. Presumably the slight exchange of air brought about by mechanical expansion and compression of the chest favours the passage of blood through the capillaries of the lungs; but the real object of artificial respiration is to stretch the endings of the vagus nerve, and in this way to originate impulses which will call the respiratory centre into action. Perhaps it may not be superfluous to point out that the failure of the pulse must not be taken as an indication that the heart has ceased to beat. Owing to the obstruction to the circulation through the lungs, the left side of the heart is almost empty. Very little blood is pumped into the aorta. None reaches the wrist.
Exchange of Gases in the Lungs.—In the lungs each red corpuscle takes from the air a charge of oxygen which it carries to the tissues. In the tissues the plasma of the blood receives carbonic acid, which escapes from it when it reaches the lungs. Water dissolves oxygen and carbonic acid. Towards animals and plants which live in it, water plays the same rôle as the atmosphere towards dwellers on land. The quantity of a gas which will dissolve in water is proportional to the pressure to which it is subjected. If water were the circulating fluid, some oxygen would enter it in the lungs; some carbonic acid would be taken up in the tissues and liberated in the lungs. But it is clear that the small quantity of fluid which the vascular system will hold would be incapable of serving as an efficient medium of exchange between the tissues and the lungs. When a given quantity of venous blood is agitated with air, five times as much oxygen is taken up as the blood could carry if the gas were simply dissolved. Both oxygen and carbonic acid are held by the blood in chemical combination.
The condition in which oxygen is carried was discovered in 1864 ([cf. p. 68]). From all time it had been noticed that the blood which flows from a vein is darker and of a more purple tint than the blood which spurts out of a cut artery. Shortly before the date mentioned above, the spectroscope had begun to be used to distinguish more accurately than the eye can do the groups of rays which a coloured solution transmits. The colour of a ray of light depends upon its wave-length. The light of the sun, when its rays are sorted by a prism, according to their wave-lengths, shows all colours from the long waves of red to the short rays of violet, with certain gaps. At intervals where rays are missing, the spectrum exhibits dark bands—Fraunhofer’s lines. The colour of a solution is measured by placing a flat-sided vessel containing it in the course of a beam of the sun’s light, on its way to a prism. When the rays are spread out, it is observed that certain groups have been absorbed by the coloured fluid. The colour of the solution is due to the rays which it transmits. It had been pointed out in 1862 that blood diluted with water absorbs parts of each end of the spectrum, and also two groups of rays lying between the fixed bands of Fraunhofer which spectroscopists had labelled D and E. Stokes observed that this is true only of arterial blood. Venous blood absorbs a broad band in this part of the spectrum in place of the two narrow bands. He showed that, “like indigo, it is capable of existing in two states of oxidation, distinguishable by a difference of colour and a fundamental difference in the action on the spectrum. It may be made to pass from the more to the less oxidized condition by the action of suitable reducing agents, and recovers its oxygen by absorption from the air.” The reducing agents of which Stokes made use were alkaline solutions of ferrous sulphate or of stannous chloride containing some citric or tartaric acid. These sub-salts of iron and tin very rapidly absorb oxygen from the air or from any chemical substance which parts with it readily. With these solutions Stokes replaced the tissues. He abstracted the oxygen from the oxyhæmoglobin; then, shaking the solution of reduced hæmoglobin with air, he reproduced the action which occurs in the lungs.
If the hand be held between a spectroscope and the source of light, in such a position that the beam passes through the thin tissue of two fingers where they are in contact, the spectrum of oxyhæmoglobin is obtained. If now the circulation through the fingers be impeded by putting strong indiarubber bands round them, the blood becomes venous, and the two narrow bands of oxyhæmoglobin give place to the broad band of reduced hæmoglobin.
Although very soluble, hæmoglobin may be obtained in crystals, the form of which varies in different animals. When obtained from human blood, the crystals are rhombic prisms; from the guinea-pig, tetrahedra; from the squirrel, hexagonal plates. Yet it is unlikely that the hæmoglobin of one animal differs chemically from that of another in any proper sense of the term. Probably the form of the crystals depends upon the amount of water of crystallization. The apparent polymorphism of hæmoglobin may be associated with the great size of its molecules ([cf. p. 66]).
Even when in the crystalline form, hæmoglobin can take up oxygen; but the difficulties which attend its purification and crystallization render somewhat uncertain the amount of oxygen which a gramme of crystallized hæmoglobin can absorb. In solution, 1 gramme can take up 1·34 cubic centimetres. The whole of the hæmoglobin of the body would, therefore, if it were all in the oxidized condition, hold about 4 grammes of oxygen.
It is not with oxygen alone that hæmoglobin can combine. It can absorb the same volume of carbonic oxide or of nitric oxide gas. Both of these gases it holds more firmly than oxygen. Neither carbonic oxide-hæmoglobin nor nitric oxide-hæmoglobin is of any use to the tissues. If the blood becomes charged with the fumes of carbonic oxide (CO) given off by a coke-fire, this gas proves extremely poisonous. The blood does not lose it in its circuit through the body, nor is it exchanged for oxygen in the lungs.
The instability of the compound of hæmoglobin and oxygen is shown under the air-pump. The pressure of air in the open equals 760 millimetres of mercury. When the pressure falls to about 250 millimetres, the oxygen is rapidly given off. This is a matter of considerable interest in its bearing upon the question of the height to which it is possible for a human being to ascend. An animal placed in a chamber from which the air is pumped dies when the pressure falls to 250 millimetres of mercury. It has been ascertained that a man under the same circumstances can bear with impunity a reduction to 300 millimetres. How much lower must the pressure fall before it proves fatal? Of three aeronauts who ascended in the balloon Zenith to a height of 8,600 metres (26,500 feet), two died. The third, Tissandier, became unconscious, but recovered during the descent. The pressure of the atmosphere at such a height is 260 millimetres. The greatest mountain heights yet attained are 23,100 feet (Aconcagua, in the Southern Andes), reached by Fitzgerald, and 23,400 feet (Trisul, in the Garhwal Himalayas), reached by Dr. Longstaff and his companions. The pressure at this height was 320 millimetres. From these facts it is clear that mountaineers have just about reached the limit; but since they have not as yet mounted to a height at which the barometric pressure is less than 300 millimetres, it is possible that slightly higher mountains are still waiting to be conquered. At 23,000 feet the oxygen contained in arterial blood does not exceed 10 volumes per cent. ([cf. p. 190]). It is therefore about half the normal amount. Hence the breathlessness and sense of feebleness experienced by climbers. The least exertion leads to the consumption of all the circulating oxygen. But since the effects of want of oxygen are felt at altitudes much lower than those to which reference has been made, it is clear that the question cannot be regarded as simply one of physics. The nervous system suffers when an attempt is made to do work with a deficient oxygen-supply. Violent headache and nausea attack most persons long before a level is reached at which the combination of hæmoglobin with oxygen ceases to be possible. The occurrence of this “mountain sickness” reminds us that we must not take for granted that the nervous system will continue to do its work right up to the altitude at which oxyhæmoglobin is dissociated. Still, the figures show that, apart from these nervous symptoms, which disappear after a time, no serious disturbance occurs even though the atmospheric pressure be but little higher than the absolute minimum at which hæmoglobin combines with oxygen.