Handbook of Anæsthetics.
Every anæsthetic drug has certain pharmacological peculiarities of its own, but all have much in common, and it is to these common features we shall first direct our attention.
Reaching the blood stream by absorption from the lung alveoli, the drug enters into loose combination with the red blood corpuscles; a small proportion only is carried in the plasma. Within the corpuscles it must of necessity displace a certain proportion of the oxygen normally carried: this factor is of great importance only in the case of nitrous oxide gas, which readily displaces the larger part of the normal oxygen content. In the case of other anæsthetics, the same process occurs; but to a less extent. Detailed figures of the extent to which the blood gases are altered in various stages of chloroform anæsthesia will be found in Appendix III.
The actions of individual drugs upon the circulatory, respiratory, and excretory systems differ so considerably that a small section has been devoted to this subject in each of the chapters devoted to nitrous oxide, ether, and chloroform respectively. One feature is, however, dependent upon the state of anæsthesia rather than the action of the particular drug, and that is a certain slight fall of blood pressure. This phenomenon is seen even in natural sleep, and is presumably due simply to lack of normal stimuli such as tactile, visual, and auditory impressions which in the ordinary circumstances of life, help to maintain the tone of the vasomotor system. That such a fall is due to the state of anæsthesia admits of little doubt, but the fact is not always easy to demonstrate since each of the drugs themselves have a marked influence upon the B.P., which masks the pure effect of the anæsthetic sleep.
It is in this system, of course, that we look for the characteristic action of anæsthetics, since if we had a choice, it is the brain only which we should desire to influence by our drug. It used to be said that anæsthetics paralyse the brain from above downwards, but that is only approximately true. More correctly we may say that the more highly developed parts of the brain are earliest affected, and that those portions, such as the vital medullary centres, which man shares in common with his humbler zoological relatives, maintain their activity until the last. Moreover, it must be remembered that before any brain centre succumbs, it passes through a preliminary stage of excitement, varying in intensity with varying drugs and also with different types of patients. Those who are accustomed to administer to their nervous centres repeated large doses of such nerve poisons as alcohol and tobacco, may show very evident signs of this preliminary cerebral irritation during the process of induction of anæsthesia; so do also the unhappy possessors of nervous systems deranged from other causes such as epilepsy.
The first centres to be attacked are those of thought and perception. The patient is incapable of coherent reasoning, and loses touch to some extent with impressions from the outside world. Muscular sense and co-ordination next become affected. Although still able to move the limbs or the head, movements are incoherent, and if at this stage the patient were put upon his feet, he would stagger as he does in alcoholic intoxication. By this time sensation, both tactile and special, begins to be affected. The patient is no longer cognisant of pain,—if cut he would at any rate not have a remembrance of pain. The special senses are at this stage also lost, one of the last to go being the auditory sense, a point which is sometimes forgotten by those inclined to talk while anæsthesia is being induced. Muscle tone is the next function to be lost, and at this stage all movements on the part of the patient should cease except those of respiration. The reflexes disappear at varying stages: the spinal reflexes, e.g. the knee-jerks, disappear fairly early, probably before muscle tone is entirely abolished, but certain other reflexes persist to a later stage. Those which are of most interest to the anæsthetist are the conjunctival, corneal, and pupillary reflexes of which he will find full details in Chapter V.
Lastly the vital medullary centres, respiratory, vasomotor, and cardiac are overcome, and at this stage we have passed beyond the stage of a proper anæsthesia into that of over-dosage. In passing it may be observed that the level at which one endeavours to work is that indicated by the loss of muscle tone and of some of the reflexes and the full activity of the medullary centres, and that an anæsthetic is good or bad according as it gives a wide or narrow margin between these two events.
Upon the peripheral nerves, anæsthetics have much less effect than on the central nervous system. Faradisation of a motor nerve will in the deepest anæsthesia still cause immediate contraction of the muscles supplied by it, showing that the conductivity of the nerve is unaffected. Of far more importance, however, is the fact that the sensory nerves are not paralysed. That pain is not felt by the patient is due simply to the loss of function of the cerebral sensory centres; injury to the nerve still causes an impulse to be transmitted to the brain. Since no operative procedure can be carried out without more or less trauma (injury) to sensory nerves, we may picture the brain of the patient undergoing a surgical operation while under a general anæsthetic, as being constantly bombarded by sensory stimuli, which though not consciously appreciated by the sleeping patient, are yet capable of producing reflex effects of a definite character, the importance of which to the work of the surgeon and anæsthetist it is difficult to exaggerate, and of which a condensed account will be found in the succeeding chapter.