Certain terms such as “open method,” “closed method,” etc., are used in describing different systems of anæsthetising, and it will save time later if these are now defined.
The Open Method is one in which the drug is dropped or poured upon a fabric stretched on a mask which does not lie in close apposition to the face. If the student will experiment with such a mask as Schimmelbusch’s, he will find that by no effort can he make its whole circumference touch his face at the same time. Anæsthetics vapourised from such masks must of necessity be inhaled freely diluted with fresh air. These masks are only suitable for use with chloroform.
The Perhalation Method.—This term is not used often, but it is the most strictly correct name to give to the process commonly called “open ether.” If the student will examine Bellamy Gardner’s open ether mask (Fig. 27) he will find that it is deliberately shaped to lie over its entire circumference in close apposition to the face of the average patient. In actual use it is well however to make sure of this apposition by the use of a ring of gauze as shown in Fig. 28A. Upon the mask is stretched gauze of a thickness just as great as will permit free respiration to take place through its layers. The whole bulk of the respired air must pass through the fabric, none escaping between the face and mask.
The term “Semi-Open” is applied to various methods now rarely seen. One of the best known of these was the anæsthetic cone, still used by a few for C.E. mixture (see Fig. 45).
The term “Closed Method” is applied to one in which the patient breathes in and out of a closed bag. The Clover and Ormsby inhalers are “closed” instruments. With this method the patient rapidly uses up the oxygen of the contained air, and accumulates considerable CO2; life could not be sustained for any long period of time under such a system. Oxygen must be supplied from time to time by permitting say one breath in five to be taken from the fresh air instead of from that in the bag. Alternatively, oxygen from a cylinder may be supplied by an accessory pipe into the inhaler.
This method is also referred to as the Re-Breathing Method.
The Valved Method is used only with “gas” or “gas-oxygen.” The facepiece fitting accurately, the patient draws all the volume of his inspiration from the inhaler: his expirations he propels through a valve, into the general atmosphere of the room. If nitrous oxide unmixed with oxygen is being given, the patient suffers from oxygen starvation even more rapidly and completely than in the re-breathing method. During the induction period of gas anæsthesia, such oxygen starvation is practised deliberately, and if not pushed too far is harmless. It cannot, however, be continued for more than a brief space of time. The admixture of oxygen to the vapour being breathed entirely abolishes this unfavourable feature of the valved method.
There is, however, another consequence of the use of “valves” which is unaffected by the addition of oxygen. Reference has already been made to Yandell Henderson’s acapnic theory, and if under any form of anæsthesia the patient can be reduced to a condition of CO2 starvation, it will be when the valved system of administration is in operation for a prolonged period. As a matter of experience, patients breathing “on the valves” do often exhibit shallow respirations and slight pallor which is rapidly and very strikingly remedied by turning to the rebreathing method. One can hardly doubt that the improvement is due to a gradual re-accumulation of carbon di-oxide in the blood and tissues.
Two other terms referring not to the type of inhaler but to the method of supplying the drug, are in use.
By the “Drop” Method, we mean one in which the anæsthetic is supplied in a steady series of drops. The flow may be quick or slow, but it always arrives on the mask in isolated drops of uniform size. Such a method demands more constant attention than the next to be described, but it is capable of yielding that even uniformity of vapour strength so desirable in open methods.
The Douche Method is unfortunately far more commonly used by those whose attention has never been drawn to the significance of the difference between the two. Supplies of the drug rendered, say, every twenty seconds cannot possibly give an even vapour strength.
“Single Dose” methods are of use chiefly in dental surgery. The patient is charged up with the anæsthetic, and the operator has to begin his work as soon as the mask is withdrawn from the face, ceasing as soon as the patient shows any signs of recovering consciousness of pain.
Single dose anæsthetics are in a class by themselves. In order to achieve success with them, special experience on the part of the administrator and mutual confidence between operator and anæsthetist are essential.
The period of anæsthesia available to the operator which any particular “single dose” anæsthetic may be expected to yield is obviously a matter of the first importance, and the table given in Chapter xix. will be found helpful in this connection.