THE ANESTHETIC ZONE IN NITROUS-OXID 
ANESTHESIA. 
BY 
JOHN D. THOMAS, D.D.S., PHILADELPHIA, PA. 
(Reprinted from the Dental Cosmos for June, 1893.) THE ANESTHETIC ZONE IN NITROUS-OXID ANESTHESIA. 
JOHN D. THOMAS, D.D.S., Philadelphia, Pa. 
Ever since nitrous oxid was introduced as an anesthetic, its physio- 
logical action has been the subject of study and experiment to ascer- 
tain, if possible, how its narcotic effects are produced. After calling 
out many and various opinions, the controversy seems to have evolved 
two settled theories. One, that narcosis is produced as the result of 
depriving the blood of its necessary supply of oxygen, causing true 
asphyxia ; the other, while admitting that nitrous-oxid inhalation pro- 
duces symptoms of asphyxia, affirms that the gas possesses purely an- 
esthetic properties as well. Practical experience seems to demonstrate 
that the latter theory is the correct one. So firm is the conviction of 
many as to the existence of the factor of asphyxia in nitrous-oxid 
anesthesia, that several attempts have been made by different experi- 
menters to combine nitrous oxid with various percentages of oxygen, 
with the hope of counteracting the asphyxiating effects, believing that 
a desirable anesthetic for prolonged operations could be so obtained. 
Thus far these experiments have not given the result they seemed to 
promise, if we except the method of M. Paul Bert, which, in a measure, 
fulfills the requirements, but the apparatus appears to be too compli- 
cated to warrant its extensive use. 
In the Dental Cosmos for May, 1893, Dr. H. C. Wood, who has 
devoted much time to the study of anesthetic agents and their physio- 
logical action, gives the results of five experiments with nitrous oxid 
upon dogs, from which he draws two conclusions. First, that they 
are “ a confirmation of the view that nitrous oxid produces anesthesia 
by cutting off the supply of oxygen;” second, ‘‘that a mixture of 
nitrous oxid with oxygen does not seem available as a practical anes- 
thetic.” Experience seems to confirm the latter of the two conclu- 
sions as probably correct. 
The laws of dosage must be taken into consideration in the admin- 
istration of anesthetics, the same as with other drugs, and while the 
mixture of three or even five per cent, of oxygen would perhaps be 
perfectly compatible with children as well as with some adults, it will 
not successfully produce anesthesia with all. 
Resistance to the anesthetic action of nitrous oxid varies much 
in different individuals. A man whose system would resist the intox- 
icating effects of an inordinate amount of alcohol will exhibit the same 
tendency with all anesthetics, so that such an individual cannot be 
brought readily under the influence of nitrous oxid if it be diluted in 
bulk with air or oxygen, while one of opposite temperament might 
easily succumb to a moderate amount of such a dilution with air or 
oxygen. 2 
Again, the rapid action of the gas and its transitory effects confine 
the anesthetic stage to such narrow limits that it requires upon the 
part of the administrator greater experience than can usually be 
acquired by the regular surgeon to avoid, as Dr. Wood says, “the 
danger of passing from anesthesia to sensibility, or from partial to 
complete asphyxia.” So it would appear, so far as generally under- 
stood and administered, that nitrous oxid is not applicable for pro- 
longed anesthesia or promiscuous use in general surgery. 
The research of Dr. Wood has experimentally shown the existence 
of the anesthetic zone in nitrous-oxid anesthesia, and the conditions 
which limit its duration and extent. He, however, entertains doubts 
of its practical utility in surgical operations, because of its evanescent 
character and the narrow limits which separate it on the one hand from 
asphyxia, and on the other hand from a sudden return of sensibility. 
It is the purpose of this communication to show how in actual daily 
practice the zone of pure anesthetic action is attained and made use of, 
and how it may be prolonged and the anesthesia controlled in opera- 
tions, other than the extraction of teeth, which require more time for 
their proper performance. 
The existence of a zone of true anesthesia uncomplicated by 
asphyxia during the exhibition of nitrous oxid cannot be doubted by 
any practitioner or competent observer who has practically studied its 
action upon the human subject. That the evanescent action of the 
gas, due to its rapid diffusibility, confines the zone of true anesthesia 
to very narrow limits is likewise true, so that under the conditions 
which ordinarily govern the administration of nitrous oxid this factor 
in the physiological action of the gas may be, and probably is, gener- 
ally overlooked. The importance of recognizing the zone of true anes- 
thesia and taking advantage of it in nitrous-oxid administration 
requires no argument to defend it, for therein lies the question not 
only of its safe use, but of the successful production of the true anes- 
thetic state which is necessary for its efficient use in minor surgery. 
The anesthetic zone can be preserved from three and four up to 
eight, nine, and ten minutes, as has been frequently done by the writer, 
by using the nose of the patient as a valve to admit atmospheric air in 
sufficient quantities to relieve and avoid the accompanying asphyxia, 
and no better proof could be offered of the legitimate anesthetic prop- 
erties of nitrous oxid than is given by cases so treated. To combine 
atmospheric air with the gas in bulk is not practically applicable, but 
to admit a small amount during the inhalation, as the disagreeable 
appearances of the accompanying asphyxia occur, will mitigate the 
asphyxiating effects in all cases, while in a majority of them they will 
be entirely obliterated, except perhaps a very slight discoloration of 
the lips, which no one could claim to be sufficient to produce narcosis 
by asphyxia. By experience one can detect the stage which marks the 
anesthetic zone, and, using the nose as a valve to admit atmospheric 
air as required, can produce complete and profound narcosis, free from 
asphyxia, the patient meanwhile remaining as calm and placid as when 
under the action of any other anesthetic. 
The evidences of perfect nitrous-oxid anesthesia, independent of 
the asphyxiating influence, are so abundant that to one in practice the 
first conclusion of Dr. Wood, that nitrous oxid produces anesthesia 
by cutting off the supply of oxygen, which he believes to be confirmed 
by his later experiments, seems to be open to question. 3 
If we accept the symptoms of asphyxia to be discoloration, short- 
ness of breath, violent efforts at expiration and expiratory convulsions, 
and finally exhaustion, and a feeling of impending suffocation, as is the 
case when nitrogen is inhaled, and compare them with the symptoms 
produced by nitrous oxid, they will be found to be greatly dissimilar 
in many respects. 
The inhalation of pure nitrous oxid will produce discoloration to 
some extent, which may be much or little according to the tempera- 
ment of the subject ; but there is no oppression as of impending suffo- 
cation, no violent efforts at breathing nor shortness of breath, 
while on the contrary there is at the beginning a feeling of stimulation 
and exhilaration, quickly followed by complete narcosis, from which 
the patient awakens only with recollections of a pleasurable sensation, 
with no succeeding languor or lassitude or even giddiness, and the 
whole space of time occupied is, in a majority of cases, less than two 
minutes. There may be accelerated heart-beat at the preliminary 
stage, but this is clearly due to cerebral excitement, caused by anticipa- 
tion and nervous fear of taking an anesthetic. This rapid heart- 
action usually subsides to about the normal pulsation when uncon- 
sciousness takes place, and very seldom will fall below normal, except 
in cases of cardiac weakness. 
In Dr. Wood’s experiments it appears that to produce narcosis in 
the dog by nitrous oxid required two minutes and thirteen seconds, 
while by mechanical asphyxia the same was produced in two minutes 
and nine seconds, four seconds sooner. 
Nitrous oxid will produce narcosis in man in from thirty to fifty 
seconds, a period of time much shorter, I understand, than is required 
by nitrogen. 
The length of time required to produce narcosis by mechanical 
asphyxia, by experiment upon a human being, would be difficult to 
ascertain, but some time ago there was a performance given at a 
theater in Philadelphia by a man under the sobriquet of “ The Man- 
Fish.” He exhibited his specialty under water in a glass tank, and if 
my memory serves me he remained under water for the remarkable 
space of three minutes and nine seconds, and showed none of the 
symptoms of approaching asphyxia other than considerable discolo- 
ration. Taking that exhibition as a criterion, there could be no par- 
allel between the two in regard to time. 
We do have, however, in isolated cases, coincident effects of as- 
phyxia which are, I think, to be ascribed solely to the want of oxida- 
tion when the pure gas is given. These consist of cases of constric- 
tion of the larynx or glottis, jactitations, opisthotinus, and muscular 
convulsions, but hundreds and thousands of people take the gas with- 
out giving the slightest indication of any of these, and they can be 
avoided almost universally by the judicious admission of atmospheric 
air at the proper time, as before alluded to. 
There can be no question that the inhalation of pure nitrous oxid 
does carry with it some symptoms of asphyxia, the oxygen supply 
being cut off, especially when large quantities are given for the pur- 
pose of producing a profound anesthetic impression ; but the very 
rapid production of anesthesia indicates that the nitrous-oxid narcosis 
takes place much in advance of the effects of the asphyxiating ac- 
companiment, except in very rare instances. 4 
Here it would be well to consider what kind of people are likely to 
readily succumb to asphyxia. 
In a conglomerate population such as we Americans are, we have 
people from the pale and exceedingly anemic to those of full habit and 
florid complexion, and the symptoms of asphyxia as produced by 
the inhalation of pure undiluted nitrous oxid will vary just as much as 
does the degree of blood-color in each individual between the extremes 
of the whole line. In the very anemic, heart-failure could be pro- 
duced before any of the symptoms of approaching asphyxia would 
present themselves. In a healthy subject of brunette complexion and 
bilio-sanguinary temperament, perfect unconsciousness can be pro- 
duced by pure nitrous oxid with absolutely none of the symptoms of 
asphyxia appearing, other than a slight discoloration of the lips, and 
this can be completely relieved by the admission of a proper amount of 
air so that it would be unnoticeable to the inexperienced eye. This 
class of people no doubt would be slow to succumb to suffocation 
under any circumstances, yet very easily come under the anesthetic 
influence of the gas. 
Those of light complexion, with rosy cheeks and bright red lips, 
and those of the full habit and deeply florid complexion, will show 
to a greater or less extent the asphyxiating appearance, and some 
occasionally to a marked degree, when the pure gas is given. I have, 
however, on several occasions seen this condition extend to severe 
glottis constriction and convulsed muscles under the pure gas, which 
showed no such tendency by taking advantage of the anesthetic zone 
and admitting air as before indicated, when they have acted in a per- 
fectly satisfactory manner during the narcosis. 
The blood in these florid people seems to require the full amount 
of oxygen to relieve their systems of the accumulating carbonic oxid, 
and they are more difficult to handle, but they are not beyond the 
possibility of accomplishment with exceptional regularity. 
There are some cases which from appearances seem to indicate that 
their blood is constantly in a semi-deoxidized condition ; they not 
only show a dark red color, but their lips are as blue from apparent 
improper oxidation of their blood as most people show when well 
under the effects of the gas. Such cases are usually those whose 
respiration seems decidedly weak. They never breathe to the full 
capacity of their lungs, and their respiratory motion is sometimes as 
low as twelve or fourteen to the minute. These will show the asphyx- 
iating effects more rapidly than the anesthesia. They will occasion- 
ally exhibit approaching suspension of the respiratory effort before 
the upper brain could possibly be affected by the anesthetic action of 
gas, and require the greatest caution in the administration. 
Viewing all the varying symptoms of nitrous-oxid narcosis from a 
practical standpoint, it is clearly evident that the inhalation of the gas 
produces a dual effect,—viz, that it produces perfect anesthesia inde- 
pendently of any other influence ; that its administration carries with 
it accompanying symptoms of asphyxia, which appear in degree only 
as the blood of the individual demands a full or partial supply of oxy- 
gen during the short space of time required for the inhalations, and 
this can be relieved by discovering the anesthetic zone and admitting 
atmospheric air in such quantities as judgment dictates ; and that the 
asphyxia narcosis will dominate the purely anesthetic effects of the 5 
gas only in such cases as are intolerant of the slightest contraction of 
their full supply of oxygen. 
From a personal experience of considerably over one hundred 
thousand administrations, I am impressed with the conviction that 
death from nitrous oxid will be produced by strictly physiological 
effects in but two ways. Other forms of death would seem to be 
purely coincident or accidental, several of which might happen. A 
tooth or some foreign body could get into the larynx, or a patient 
might be strangled by constriction of the throat or swallowing of the 
tongue ; in either case it would be an accident if the operator failed to 
get the tongue forward and the obstacles removed, or neglected to 
perform tracheotomy in time. Coincident with the inhalation one 
might, immediately before or after, be stricken with any one of the 
many diseases which carry people off, hrthe car, on the street, or else- 
where. This is liable to happen at any time, and could easily be con- 
sequent upon preliminary suffering or the fear and excitement which 
nearly always accompany the anticipation of taking an anesthetic and 
undergoing a surgical operation, no matter how slight. 
But cases of death which could be legitimately laid to the effects of 
the gas, or rather the want of oxidation which accompanies its inhal- 
ation, comprise those at the two extremes of the line of temperament 
which I have mentioned,—the markedly anemic and the very florid or 
deep-red-blooded people,—the anemic young girls who are suffering 
from the suppressed or retarded functions of womanhood, the marked 
pallor of the morphia-taker, and the waxy complexion of Bright’s 
disease. The blood of these is so deficient in red corpuscles that the 
lack of oxygen will be impressed upon the nerves of the heart almost 
simultaneously with the first breath of the gas, and syncope follow 
as rapidly as will the suspension of breathing in those cases which 
mark the other extreme,—viz, the very florid and slow breather ; in the 
one heart-stoppage would be the cause, in the other suspension of 
respiration, in which the heart would continue to pulsate for perhaps 
several minutes after. In each of these conditions the result would 
evidently take place before the upper brain could be affected to anes- 
thesia. These two comprise the dangerous cases from the physio- 
logical effects of nitrous-oxid inhalation, and both of them would 
appear to be from the asphyxiating accompaniment, for they will 
readily respond to atmospheric admission. 
I would here strongly emphasize the condemnation, which on 
another occasion I iterated, respecting the use of the face-piece in 
connection with the inhaling apparatus, which idea has been imported 
from abroad and is used extensively in this country. 
The effects of nitrous oxid as they develop are shown most clearly 
through the mucous surfaces of the lips, so that the administrator can 
tell at once how much the want of oxygen is affecting the blood, and he 
can relieve it accordingly. In the event of approaching danger the 
signs would be seen in the lips, either by too much discoloration in 
the florid or plethoric habit, or excessive pallor in the anemic, in time 
to avert disaster ; but with the face covered this most reliable index 
is lost to view, under which circumstances the first indication to 
apprise the operator of approaching danger would be cessation of 
respiration, which might occur too late for resuscitation.