Popular Science Monthly/Volume 11/July 1877/Atmospheric Pressure and Life
|ATMOSPHERIC PRESSURE AND LIFE.|
By Dr. PAUL BERT,
PROFESSOR IN THE PARIS FACULTY OF SCIENCES.
THE great influence that may be exerted upon living beings by atmospheric pressure is now questioned by none, and there is even a disposition to exaggerate its importance. If the barometric column rises or falls a few millimetres, nervous people affected with the asthma perceive phenomena, whether of a beneficial or of a noxious kind, which they do not hesitate to attribute to the weight or to the lightness of the atmosphere. But if this were the only cause of their sensations, then they should experience the same symptoms whenever they subject themselves to equal variations of pressure, as in passing from the level of the sea to a point only a few feet above it, or vice versa.
Rarefied Air.—As every one knows, in proportion as we ascend from the sea-level, the barometric pressure diminishes at the rate of about one centimetre per 100 metres of vertical ascent. And this diminution is progressive: suppose that at the sea-level the pressure is 76 centimetres, then it will be 66 centimetres at the height of 1,123 metres (summit of Vesuvius), 56 centimetres at 2,432 metres (pass of the Great St. Bernard), 46 at 3,998 metres (Mont Pelvoux), 39 at 5,920 metres (the height of the highest pass of the Himalaya is 5,835 metres). The greatest height attained by man was reached by Glaisher in a balloon—8,840 metres, pressure 24. 76 centimetres—and by the brothers Schlagintweit on foot, in the Himalaya, 6,882 metres, pressure 32 centimetres. The highest mountain on the globe, Gaurisankar, measures precisely 8,840 metres—the elevation at which Mr. Glaisher fell fainting to the floor of his car.
Such modifications of pressure cannot be endured with impunity by the human organism. Though life in moderately elevated regions, as the Jura and Auvergne, seems to be so beneficial to those who dwell there constantly, that multitudes come thither from afar in pursuit of health; and though in regions situated at a greater altitude, as the admirable plateau on which the city of Mexico stands, the sum of the climatic conditions seems to oifer hygienic advantages: still all are agreed that at very great elevations there always supervene, with more or less intensity according to persons and circumstances, certain characteristic perturbations and discomforts described by travelers in the Alps, the Pyrenees, the Andes, and the Himalaya.
These are, first, a sense of fatigue out of proportion to the amount of walking or of work performed. The legs appear to become leaden, and one feels a weakness in the knees. Then the breath becomes short, diflicult, labored; the pulse is quickened; the heart-beats occur isolatedly, and reverberate in the head. Next come singing in the ears, dimness of sight, and vertigo. The general sense of malaise, the feebleness, become such that the traveler must rest, else he will fall to the ground. Simultaneously there occur other symptoms having their seat in the digestive organs, such as nausea and vomiting. These various symptoms, taken together, constitute mountain-sickness (mal des montagnes), which bears a resemblance to sea-sickness.
When they first appear, a few moments' rest suftices to banish them; this instantaneous restoration of strength and vigor sharply distinguishes mountain-sickness from ordinary fatigue. But at greater elevations, where graver symptoms appear, such as bleeding from the nose or from the lungs, repose cannot bring back the condition of perfect health, though it always afibrds some relief. Travelers agree in saying that a person on horseback suffers far less than one on foot. On the high plains of the northern Himalaya, a rather brisk pace in walking, the ascent of a hill however low, the carrying of a moderately heavy load, sufiice to exhaust one's strength, to cause him to faint, and in some cases even to produce death.
This is the reason why aëronauts are attacked much later than those who ascend the mountain-side. Ever since the day when Montgolfier, realizing the immemorial aspirations of the human race, gave to man the means of overcoming the gravity which ties him to the earth, many a bold aeronaut has gone above the clouds. Only after they have reached the height of 6,000 metres do they usually experience symptoms resembling those of mountain-sickness.
But on land these symptoms make their appearance at elevations far lower, and differing according to locality. In the Alps, definite symptoms first appear at 3,000 metres; in the Bolivian and Peruvian Andes, at 4,000 metres; higher still, in the equatorial Cordillera and on the Himalaya. In general, the elevation at which they first appear depends upon that of the line of everlasting snow, the lower limit of mountain-sickness being situated a little above the snow-line. The influence of the temperature is very evident. As for anomalies special to circumscribed localities or to individuals, the consideration of them would take us beyond the bounds we have set for ourselves here.
These grave and curious symptoms have been explained in many different ways by travelers, physicians, and experimenters. As for the native mountaineers, they solve the problem of their origin by referring them either to supernatural intervention or to the influence of noxious efliuvia. In the Ancles these efiluvia are reputed to be of an antimonial nature; in the Himalaya the cause is supposed to be vegetal poisons given forth from flowers, mosses, etc. These hypotheses need not detain us.
Among the many theories more or less tenable a priori, but none of which will stand the test of experiment, there is one which is almost universally accepted, and which reckons De Saussure among its distinguished supporters. It is known that the atmospheric pressure on a square centimetre of surface is 1.03 kilogramme. If we multiply this by the number of square centimetres of surface in a man's body, the product is something enormous. Take an average case, a pressure of say 15,000 kilogrammes. We are in equilibrium with this great pressure, they say; lessen the pressure, and the result is like the application of an immense cupping-glass over the entire surface of the body. The heart's action is now no longer sufficiently counterbalanced, and hence congestion and hseraoi-rhage of the mucous membranes and of the skin, engorgement of the blood-vessels of the face, cerebral troubles, and the rest.
It is amazing to find a theory so plainly at variance with elementary physical laws accepted by eminent men. What would be the result if we had to bear upon the surface of the body a pressure of 15,000 kilogrammes, and if every variation of the barometer added or subtracted from this sum one or two hundred kilogrammes?
Another theory, first offered by De Saussure, is far more worthy of attention. "On the top of Mont Blanc (4,810 metres)," says he, "the air is nearly one-half less heavy than at the sea-level; hence it results that if, in a given time, we pass through our lungs a given volume of air, that volume will represent only about one-half the weight of the same volume of the air to which we are accustomed. Hence there must result insufficient respiration, or, more accurately speaking, insufficient absorption of oxygen." "The quickening of the respiration, which tends to offset the evil, is insufficient," says Martins, "for it would have to be twice as frequent, and have double amplitude, in order to compensate the diminution in the quantity of air inspired." Finally, Dr. Jourdanet adds that, "the pressure being reduced, the oxygen must be dissolved in the blood in a less proportion:" hence a pathological state analogous to anæmia, and which he calls anoxy-hœmia.
These ideas have been met with many objections. In reply to De Saussure it was said that the atmosphere, even at half-pressure, contains a great deal more oxygen than is needed for respiration; and in reply to Jourdanet that, according to the researches of Fernet, oxygen in the blood being in the state of combination, and not of solution, its quantity does not depend on barometric pressure.
My own experiments show that De Saussure and Jourdanet are right. They further prove the sagacity of Jourdanet in recognizing in the inhabitants of the Anahuac plateau the injurious influence of low pressure which, though not perceptible in the state of health, reveals itself on the slightest attack of disease. I need not detail here the long series of experiments which have led me to conclude that the symptoms following diminished pressure, whether slowly or rapidly applied, are simply the result of a diminution of the oxygen in the blood; in a word, that they are nothing but a sort of asphyxia in the midst of the "pure and invigorating mountain-air."
Still I may repeat here an experiment which can be performed wherever we have a pneumatic apparatus; this experiment clearly proves that the lessening of the barometric pressure is of no account, mechanically, in the production of the phenomena. These are the result rather of chemico-physical action, the blood not being sufficiently charged with oxygen.
We place a sparrow in the pneumatic bell-glass A (Fig. 1), which communicates with the manometric tube C E, The pressure is gradually lessened by means of the tube B. When the manometer shows only 30 centimetres' pressure in the bell-glass, the bird gives pretty serious evidence of suffering; at 20 centimetres it totters, reels, and falls upon its side; at 18 centimetres it struggles violently, and would die in a few seconds, were I to leave it in this situation. So I quickly place at a an indicator, to show the height attained by the mercurial column, and, opening the cock D, I introduce into the bell-glass not air, but oxygen from the India-rubber bag O. At once the bird becomes himself again. I let it breathe a little while, and again I diminish the pressure as before. But now we reach 30 centimetres, 25 centimetres, without difficulty; not till we reach 20 centimetres does the bird appear to show some little signs of discomfort; we reach 13 centimetres, a', a pressure much less than before, and yet the life of the bird is plainly not at all endangered. If I were to admit oxygen once again, I might diminish the pressure still more.
Hence it appears that it is not the lowering of mechanical pressure that produces the symptoms, but the low tension of the oxygen of the
dilated air, which low tension prevents the oxygen from entering the blood in sufficient quantity.
This experiment I have made not only with sparrows, but also on my own person; and in the latter case the results are quite as striking as in the former, and I dare affirm, without vanity, no less interesting.
By the kindness and liberality of Dr. Jourdanet, I have been enabled to set up in the physiological laboratory of the Sorbonne great apparatus, by the aid of which I have studied the effects of compressed and dilated air. The dilated air-chamber consists of two cylinders of riveted sheet-iron, from which the air is gradually exhausted by means of a steam-pump (Fig. 2).
This apparatus I have entered, taking with me a large India-rubber bag filled with oxygen. As the pump began to work, I experienced all the well-known symptoms of mountain-sickness, viz., quickening of respiration and pulse, which was considerably augmented by the least movement; sense of loathing, nausea, sensorial and intellectual perturbation. I felt indifferent to everything and incapable of action. On one occasion, having counted my pulse-beats for one-third of a minute, I tried to multiply the number of beats by three, but could not do it, and so was obliged to write on my bit of paper, "It is too difficult." But all these symptoms disappeared as by enchantment so soon as I respired some of the oxygen in the bag; returning, however, when I again breathed the air of the cylinder.
Fig. 3 gives the details of one of these experiments. In this figure the times of the different stages of the experiment are given at the foot: just above this is seen the curve which represents the rate of pulsation; and above this another curve, showing the barometric pressure in centimetres: the figures in the left-hand margin represent the changes of pressure and pulsation. It will be seen that, as the pressure is diminished, the pulse is accelerated. Thus, the pressure being 42 centimetres (answering to the elevation of Mont Blanc), the pulse-rate, which at the beginning of the experiment was 60, rose to 84. At this moment I took two or three inhalations of oxygen, and at once the pulse fell to 71. Then I omitted the oxygen, and moved a little, when the pulse rose to 100, falling again to 70 when I returned to the oxygen. Ten times during an interval of one hour and twenty minutes, and with the pressure ranging between 40 and 50 centimetres, I at will produced these sudden oscillations, causing my pulse instantly
to rise or fall 20 beats. I may add that this is an experiment which I do not mean to repeat, having suffered during that evening slight congestive symptoms, which I attributed to these sudden changes of cerebral circulation.
On the other hand, those experiments in which oxygen is respired continuously are not followed by any injurious effects. Fig. 4 states the results of that one of my experiments in which I reached the lowest degree of pressure. My pulse had grown more frequent, having risen from 60 to 85; the pressure was then only 40 centimetres. I now began to inhale oxygen from the bag, and at once the pulse fell to 65, at which point it stood during the remainder of the experiment, and at last even fell to 48. In the mean while the pressure had fallen to 246 millimetres. This is exactly the pressure on the highest
summit of the Himalaya—the same degree of pressure which was so near proving fatal to Glaisher and Coxwell; I reached this point without the slightest sense of discomfort, or, to speak more accurately, the unpleasant sensations I felt at the beginning had entirely disappeared, A bird in the cylinder with me was leaning on one side, and very sick. It was my wish to continue the experiment till the bird died, but the steam-pump, conspiring, as I suspect, with the people who were watching me through glass peep-holes, would not work, and so I had to return to normal pressure, I placed, for a moment, the oxygen-tube under the beak of the bird, and at once he recovered.
Two other persons have, like myself, entered these cylinders, experiencing the same symptoms, and deriving the same benefit from the use of oxygen: these are Messrs. Crocé-Spinelli and Sivel. Crocé, who was very sensitive to reduced pressure, had turned black at the lips and on the ears, and could hardly see his paper, when he decided to have recourse to the oxygen. The effect was instantaneous, both upon him, who at once was able to write, and upon me, who observed with some anxiety the purple color of his ear, and was about to let in air.
Fresh from these experiments, Crocé-Spinelli and Sivel made their ascension of March 22, 1874, during which they rose to the height of 7,500 metres (a pressure of 30 centimetres). The faintness, the disordered vision, and the nausea, disappeared every time they "drank a little oxygen," as Sivel would say.
On the 15th of April, 1875, they made another ascension, in company with Gaston Tissandier. I was not then in Paris, and hence could not, as on the former occasion, superintend the making of the oxygen-bags. I would surely have made them larger, but probably I should no more than any of you have dreamed of what was the true cause of the catastrophe which followed. The oxygen-tube hung at a certain height above their heads, and, knowing that they had but very little of that gaseous cordial, they economized it against the moment when they should be more seriously attacked by sickness. But, when they wanted to take hold of the tube and to apply it to their mouths, their arms were paralyzed.
This terrible occurrence ought to be a lesson of prudence, but it must not serve as a pretext for discouragement. Crocé-Spinelli and Sivel died at 8,600 metres, with a pressure no greater than that reached by me without the slightest shadow of unfavorable symptoms, and it will be easy to devise measures which will insure the aeronaut against an attack of sudden paralysis. As for the value of ascensions to great heights, I am surprised to see it questioned by eminent men. What could be a more curious object of inquiry, from the point of view of the meteorologist, than that aërial zone, 10 or 12 kilometres in depth, in which are developed rain, hail, and snow storms? It is not wise to assign limits either to human activity or to the usefulness of scientific researches.
But to return to the theory of the symptoms produced by decompression. The experiments made in the cylinders demonstrate, beyond the possibility of doubt, that these symptoms depend solely on the tension of the oxygen in the air respired; an aëronaut breathing common air (21 per cent. oxygen) at one-half common atmospheric pressure is precisely in the same condition as a man who, at normal pressure, breathes air that holds only one-half the normal proportion of oxygen. Consequently, he is subject to conditions of insufficient oxidation, and threatened with asphyxia. Hence his rapid respiration, in the effort to introduce into the blood the oxygen which is lacking; hence, too, the accelerated palpitation of the heart, and nervous and muscular debility.
But if the traveler whose blood is thus impoverished keeps perfectly still, he will not suffer much, for it needs very little oxygen to support the body in the state of immobility. But if he stirs about, if he tries to lift the weight of his body by climbing, then he has need of more oxygen than is contained in the blood; and, as this is not to be had, the symptoms of mountain-sickness make their appearance, and the only hope of relief is in repose. This is the reason why aeronauts, who perform no work, experience "balloon-sickness" at a far greater elevation than mountain-climbers experience the symptoms to which they are liable.
The cooler the air, the earlier the appearance of the symptoms. When it is warm, the traveler needs only a small quantity of oxygen to keep up the bodily temperature. But, when the air is cold, the loss of bodily heat increases, and hence the need of a more intense calorific oxygenation. But how can this be attained if the blood does not contain enough oxygen? This is the reason why, as I have already stated, mountain-sickness makes its appearance much earlier in the Alps than in the Himalaya.
Compressed Air.—For thirty years, physicians, following the footsteps of Junod, Pravaz, and Tabarié, have made use of compressed air in the treatment of sundry diseases, and they have produced remarkable results in cases of anæmia, passive hæmorrhage, chronic bronchitis, and emphysematous asthma. This I merely note in passing. Among the physiological phenomena, all observers have noted a diminution in the number of the heart-beats and of the respirations, and an increased amplitude of the latter. Physicians commonly employ a pressure of only one-third or one-half of an atmosphere, while I have specially studied pressures of several atmospheres.
These great pressures have been employed in various industries for a few years past, but more especially in submarine diving and in sinking piers for bridges.
In submarine diving, the diver incloses his head in a metal helmet with glass eye-pieces. Into this helmet, by means of a pump, compressed air is driven with force sufficient to expel it again through special orifices. Thus there is established an equality of pressure between the water around the diver and the air he breathes, and this is the conditio sine qua non of his being able to live beneath the water. Lead-weighted shoes and a water-proof dress complete the outfit. Messrs. Rouquayrol and Denayrouse have made the diver independent of the lighter or vessel from which, prior to their improvements, he could not wander away, and this they have done by strapping upon his back a compressed-air receiver which works very ingeniously. Divers who fish in this way for coral, pearls, sponges, etc., descend to the depth of forty metres, and the air they breathe is under a pressure of five atmospheres.
The apparatus used in erecting bridge-piers is a great improvement on the old diving-bell. The discovery of the principle involved in these is due to M. Triger, who, in 1841, applied it to the construction of mine-galleries under the Loire. Nothing can be simpler than this principle: it is employed by children when they amuse themselves by blowing into a half-submerged tube, and causing the air to issue in bubbles. The apparatus, reduced to its simplest expression, may be described as follows: A tube of the length proposed for the pier is let down to the bottom of the river. It is capped with a chamber, into which is forced compressed air. This air expels the water from the lower end of the tube, and passes out just as in the child's play. The workmen can then, by means of a system of doors, as seen in Fig. 5, descend to the bottom, and there dig for the foundation of the pier. As the soil is removed, the tube descends by its own weight; it is lengthened by the addition of successive sections, till the solid rock is reached. The cylinder is now filled with béton and the pier is complete.
In these apparatus workmen have also been subjected to pressures as high as five atmospheres.
Now, both among divers and workmen in these tubes, symptoms have been noted often so serious as to terminate fatally. First there is an intolerable itching, called by the workmen "the fleas" (puces); then violent pains in the muscles and joints which have done most work; paralytic symptoms, particularly in the lower limbs, which often are persistent and fatal; finally, sudden death. Of 160 men employed on the foundations of the St. Louis (Missouri) Bridge, thirty were seriously attacked, and twelve died.
You are welcome to all the hypotheses invented by the fertile minds of physicians to explain these redoubtable troubles. Quite as a matter of course, we find here, first of all, the mechanical explanation: "When a man enters the tube," says one author, "he is flattened out!" (aplati). Very likely, indeed, if we admit that at the pressure of three atmospheres 4,500 kilometres more weighs upon our body. But, happily, we are saved from this fate by elementary physics.
The workmen in the tube at Kehl had a saying, as is usual among workmen everywhere; it is one full of acuteness and depth: "You pay when you leave." It is decompression and not compression that does the mischief.
But how does decompression act? Very simply indeed. Here in this glass jar is a rat subjected to ten atmospheres. I now turn a cock, and in a moment bring the animal back to normal pressure; he turns around two or three times and drops dead. Were I to make an autopsy now, I should find the heart and the great vessels full of gas; so great is the amount, that once I drew off fifty cubic centimetres of
gas from the vessels of a cat decompressed in this way. This gas is nitrogen with a little carbonic acid.
The process is as follows: The animal by breathing compressed air charges its blood with air in the proportions indicated by physical law; on the normal pressure being restord, the gases with which it was supersaturated pass into the free state. It is like drawing the cork of a bottle of beer. The oxygen combines on the spot, but the nitrogen is at once set free, and carries with it carbonic acid in becoming disengaged. Death is explained by the arrest of the circulation.
But it must not be supposed that the action of compressed air is harmless. If we subject a sparrow to a pressure of twenty atmospheres, it will, after a few minutes, be seized with tremors, increasing to most violent convulsions—convulsions stronger than those of tetanus or of strychnine-poisoning—and the bird soon dies. These terrible symptoms are not the result of compression, as I have been able to prove by two experiments. In the first place, they can be produced at the pressure of five atmospheres, provided pure oxygen be used instead of air, which latter has no special effect at this pressure. Secondly, they do not make their appearance if the air subjected to twenty atmospheres' pressure is very poor in oxygen.
Thus it is the oxygen that is to blame. Oxygen at too high a degree of tension destroys animal life. Long I hesitated to characterize as a poison the "nursing father" of everything that lives, but there was no help for it. Oxygen, which gives us life, slays also, when administered in too strong a dose. I have had to study thoroughly this paradoxal poison to determine the different effects of varying doses, and its action upon our tissues.
Here a new surprise awaited me. Having seen a sparrow killed by oxygen, I supposed that this agent must have accelerated organic combustion, thus consuming all the material which goes to maintain the animal heat. But great was my astonishment when the thermometer indicated in animals laboring under strong convulsions a fall of several degrees in the temperature. The analysis of other phenomena confirmed this first observation, and led me to the strange conclusion that oxygen in excess kills by interfering with, arresting, the intra-organic oxydation.
The effects of this powerful agent begin to be distinctly felt at the pressure of about five atmospheres. Perhaps they might be noticed at a lower pressure, and I am inclined to attribute to this cause the unfavorable symptoms presented by workmen who have spent several months in compressed air; but this is a complex problem. In any case, if the necessities of industry subject men to pressures higher than six atmospheres, they will be in danger not only at the instant of decompression, but even from the effects of the compression.
Oxygen at a high tension kills not only the higher animals: it acts alike on vertebrates and invertebrates, animals aerial and aquatic, plants and animals, big and little, even microscopic organisms. Its effects upon the latter are highly interesting.
From the admirable researches of Pasteur it appears that the phenomena of fermentation are of two kinds. One set of phenomena are correlated with the development of microscopic organisms; this is fermentation proper. The other set are dependent upon the action of amorphous soluble substances; this is diastasic fermentation. Now, oxygen in a state of tension arrests the former class of phenomena, but in the latter class it is without any action.
Thus we can entirely prevent the fermentation of must, the souring of wine, the putrefaction of meat, etc., by means of oxygen in the state of tension. This work once done, we may restore the normal pressure, and then, provided germs from without are excluded, no true fermentation will occur.
I once hoped to be able in this way to preserve meats, eggs, etc., but this was an illusion. The substances do not become putrid, but, in consequence of a pseudo-fermentation, which sours them, they acquire a disagreeable taste, which takes from the process all its industrial value.
At the close of this long discourse I would call attention simply to one consideration. Atmospheric pressure acts a far more important part in the life-conditions of organisms than is commonly supposed. If we go back to the primordial geological ages, we may regard as highly probable the hypothesis that the pressure was then much higher than now. This must be taken into account when we investigate the origin of life. But, if we look to the future, it is plain that the pressure will go on steadily diminishing, just as the amount of water on the earth's surface diminishes; and hence all living things are doomed—after the lapse of countless ages, it is true—to perish by asphyxia from the lessening of the atmospheric pressure. Hence the limits of life upon the globe are fixed on the one hand by an excess, and on the other by a lack, of pressure.—Revue Scientifique.
- Translated from the French by J. Fitzgerald, A.M.