There are in asphyxia, two alterations in the blood-gasses, _i.e._ lack of oxygen and increase of CO_{2}. The action of these two conditions have been differentiated by experimental work (Starling, Kayala, Jerusalem), and one can say definitely that the excess of CO_{2} is the cause of the increased activity of the respiratory efforts, and that the remaining phenomena are due to oxygen starvation. This point is of some importance in considering anæsthetic methods in which re-breathing (breathing in and out of a bag) is practised. The use of such methods has often been thoughtlessly condemned as “poisoning the patient with his own CO_{2}.” Within the limits usually practised, a re-breathing method does not involve any such risk, provided oxygen starvation does not occur. This point is referred to again in Chapter iv. Clinical Signs of Mechanical Asphyxia in the Anæsthetised Subject. The classical signs of asphyxia above described are hardly to be expected in the operating theatre, but essentially the condition of the patient who develops respiratory obstruction while under an anæsthetic is similar to that produced experimentally in animals in the laboratory. The changes most easily observed are as follows:-- (1) _Alteration of the colour._--Cyanosis shows itself earliest in the lips, and the lobules of the ears,--later the whole face becomes dusky. (2) _Dilatation of the pupil_, which ceases to respond to the stimulus of light. (3) _The respiratory movements increase in depth and frequency._--The chest and abdominal walls heave forcibly; but (4) _The volume of air passing in and out of the glottis is diminished._--In complete obstruction, of course, none passes at all. In passing we may draw the moral that persistence of chest movements is no proof of the passage of air in and out of the chest: that can only be proved by hearing the movement of air through glottis and mouth or nose, or feeling it on the delicate skin of the back of the observer’s hand. (5) _True convulsions are not seen_, unless we may consider the jactitation of deep N_{2}O anæsthesia as such (see Chapter VII.). Nevertheless, there are obvious and most valuable signs of asphyxia to be found in the muscular system often quite early. These consist in the incidence of _muscular rigidity_, which is frequently observed first in the muscles of the abdominal wall. A surgeon performing laparatomy will notice at once the occurrence of this phenomenon, than which hardly anything can complicate and delay his task more effectively. The anæsthetist who knows his work will, upon hearing from the surgeon a complaint as to the rigidity of the abdominal wall, devote his attention first to securing a perfectly free air-way before deciding that a deeper anæsthesia is required. [Illustration: FIG. 5.--Vicious circle of asphyxia.] Prevention and Treatment of Asphyxia.