A system of practical medicine. By American authors. Vol. 3

by Unknown

In the writer's view, the explanation is to be found in the changed conditions of the circulation, which result first from the increased pressure upon the surface, and then from the sudden removal of the pressure. While the subject is in the caisson the blood is driven from the peripheral vessels toward the interior of the body, where the pressure is less than at the surface.⁷ It is also forced from the more compressible tissues into the solid and resisting organs, such as the liver and kidneys; and lastly, it flows toward bony cavities, for the reason that their walls resist the effect of direct pressure, and equilibrium of pressure can be restored within them only by an afflux of blood. Thus the distribution of the blood is everywhere changed, and the size of the vessels is no longer determined by the muscular action of their walls, but by the amount of blood forced into them, the vital action which should regulate the circulation being entirely overpowered and set at naught by an overwhelming physical force operating from without. The vessels become merely passive tubes, distended in some places where they are protected from pressure, and compressed in others where the tissues about them are compressible. By this transfer of blood from one part to another the equilibrium of pressure is restored and the circulation goes on, though without any regard to the physiological demands of the different organs. There is no stasis anywhere so long as pressure and counter-pressure are equal, thus allowing fair play for the action of the heart.

⁷ This is shown by the marked pallor of the skin and the shrunken and wrinkled appearance of the hands.

If, now, the external pressure is suddenly removed, what will be the result? Vessels which have been compressed and almost emptied of blood will now offer new channels through which the blood can rush, and vessels overcrowded with blood, with their walls paralyzed by over-distension, will have the current within them slowed almost or quite to the point of stopping. The vessels of the brain and spinal cord, being within bony walls, where the direct pressure of the condensed air could not affect them, will be found the most distended and the most helpless to relieve themselves. They will get little aid from the vis a tergo of the circulation, for the blood will find easier courses by other ways, vascular tension being almost nil and the vaso-motor system out of use.

The longer the sojourn in the caisson has been, the more entirely passive the vessels will have become, and the longer will be the time they will require to resume their normal condition. At some points the circulation will be greatly slowed or entirely interrupted, and nerve-elements lying beyond and deprived of their blood-supply will express their want by pain or paralysis. Areas of stasis once formed will be likely to extend, and may thus affect nerve-elements which at first escaped. This would explain those cases in which the attack is deferred until some time after leaving the caisson.

It is readily conceivable that in persons beginning work when the pressure is slight and continuing day by day, as the pressure slowly increases the vessels should acquire the power of adaptation to the variations in the amount of their contents, since this is only an extension of the physiological principle which we see exemplified in all organs having an intermittent function.

The influence of the trophic system of nerves also, as the connecting link between central nerve-lesions and peripheral vascular disturbances, must not be forgotten in this connection. Suspension of function in trophic cells, either in the cerebral cortex or in the anterior horns of the cord, could easily be brought about by the action of the mechanical causes already described, and would result in areas of vaso-motor paralysis and consequent congestion at the termination of the corresponding nerve-fibres. The proneness of the large joints, and especially the knees, to be attacked is suggestive, in view of the like circumstance in chronic degeneration of the cord.

CAUSES.—The one essential cause without which the disease can never be developed is transition to the normal atmospheric pressure after a prolonged sojourn in a highly-condensed atmosphere. Hence we have to consider two elements, pressure and time, the danger in these cases being as the degree of pressure to which the person has been exposed multiplied by the duration of the exposure.

But inasmuch as a prolonged sojourn in the caisson does not in every case produce the disease (many of the men employed escaping it entirely), it follows that there must be concurrent causes which determine its development.

The first of these is a special predisposition. This is occasionally strongly marked, some persons being affected by a short exposure to a low pressure from which there would generally be experienced no inconvenience whatever.

Perhaps the most frequent exciting cause of the caisson disease is too rapid locking out. Indeed, it is altogether probable that if sufficient time were allowed for passing through the lock the disease would never occur. But what is sufficient time for one is too short for another; and all that can be done is to fix upon a duration for the process which shall be proportioned to the pressure, and as great as is consistent with the circumstances, and then to see that the rule is rigidly observed. At least five minutes should always be allowed for each additional atmosphere of pressure.