310
the vaso motor centre prior to the depression of its activity. It has already been stated that
when chloroform is given in such an amount as to cause a marked fall of pressure, there com
- m o n l y o c c u r s a t t h e s a m e t i m e a m o r e o r l e s s m a r k e d d i l a t a t i o n o f t h e h e a r t . I n t h e h e a l t h y
animal the latter change does not, in the earlier stages of a fall of blood-pressure, appear to play
nearly so important, a part in lowering the pressure as does the depression of the vaso motor
centre induced by the influence of the chloroform. The pressure is, as a rule, much more
readily lowered by the action of chloroform on the vaso motor centre than by its action on the
heart. But in the latter stages of a great fall of blood-pressure, when the anæsthetic is given
so as to produce a profound effect, the cardiac dilatation becomes a factor of prime importance,
and it becomes impossible to raise the pressure by any means affecting the arterial system alone
; t h e p r e s s u r e c a n o n l y b e r a i s e d b y r e s t o r i n g , i n s o m e m e a s u r e , t h e p u m p i n g p o w e r o f t h e h e a r t ,
or by the supplying of some mechanical substitute for this power, for example, by rhythmic
compression of the ventricles with the hand.
The relative susceptibility of the vaso motor centre and the heart to the influence of
chloroform appears to vary considerably. Commonly the vaso motor centre becomes markedly
depressed before cardiac dilatation is evidenced in any considerable degree. In some instances,
however, the cardiac dilatation begins early, before the vaso motor centre has suffered any
depression and even while it is stimulated, as sometimes occurs as a primary result of the action
of chloroform upon this centre.
Possibly a good deal depends on the state of the heart at the time. If the heart is
strong and possesses a considerable reserve of power over and above what is necessary to expel
its contents in ordinary circumstances, it can readily be conceived that a considerable amount of
chloroform might be taken without causing any evidence of failure, the reserve of power
making up for the depressing influence of the chloroform; meanwhile, the vaso motor centre
being acted upon by the chloroform, the blood-pressure would fall. On the other hand, if
the heart is, to begin with, just able to perform its function, it is intelligible that a comparative
ly slight depressing influence exerted on it by chloroform would soon cause it to exhibit signs of
dilatation. Many nervously weak hearts possibly come into this category.
Some Effects of the Falls of
Blood-pressure. — T h e f a l l o f b l o o d - p r e s s u r e i s , i n a c e r t a i n
sense, protective; it retards the continued access of the anæsthetic to the vita1 organs. I have
frequently been struck with the great resistance shown to the influence of both chloroform
and ether in animals where a very low pressure was present from arterial relaxation, due
to causes other than anæsthetics, for example v o s o m o t o r
paralysis. In such conditions large
doses of chloroform and ether can be given—mixed with the air entering the lungs—with little
o r n o e f f e c t o n t h e h e a r t a n d b l o o d - p r e s s u r e . O n t h e o t h e r h a n d , t h e f a l l o f b l o o d - p r e s s u r e
may become excessive and prove a source of great danger. Fatal failure of some of the vital
organs may be determined by the defective blood supply attendant on an insufficient pressure,
and this in all probability more readily in certain morbid conditions than in the healthy
a n i m a l .
Modes of counteracting the
F a l l o f t h e B l o o d - p r e s s u r e i n t h e c a r o t i d A r t e r y c a u s e d b y C h l o r o f o r m.—When a great fall of blood-pressure has been caused by the inhalation of chlo
-roform in the ordinary way, inversion of the animal has been found to exert a slight effect in