June, 1951]
V.
R.,
J.
C.
D.
AND
V.
I.
99
An isolated perfused heart which is devoid of the extrinsic nervous mechanism
was a tropinised by applying a few drops of a 0.5 per cent solution of atropine.
Such a heart continued to respond to gigantin in the usual way indicating clearly
that the action of the drug is completely independant of the nervous mechanism.
Changes in rhythm and conductivity—When the heart was perfused with 1 in
100,000 to 1 in 1 million dilution of giantin in the ringer, slowing was the first symptom
noticed ; more rarely acceleration or no change. A definite change in the normal
rhythm was observed between the 4th and the 6th minute of perfusion and it con-
tinued till the 7th or 8th minutes. During this period premature beats or extra-
systole of the ventricle were observed, (Plate Xb). The appearance of extra-systole
was the first indication of the beginning of irregularity of the frog's heart under
the influence of the drug.
The earlier stages of the irregularity were manifested by the frequent inter-
missions of the ventricle, by the setting in of heart block. The ventricle responded
only for every 2 to 4 auricular systole. The auriculo-ventricular interval was
prolonged in successive beats until an intermission occurred and it appeared as
though the auricular beat followed the ventricular one and the normal pause occurred
in between the auricular and ventricular systole. The block was transient and
intermittent, but towards the later stages of the irregularity of the heart it developed
more frequently and in a complete form, the ventricle responding for every 6 or 7
auricular beats. Finally the ventricle was arrested but the auricle continued to beat
for a varying period of 3 to 7 minutes. (Plate VII, fig. 2).
The ventricle however was responding to mechanical stimuli and contracted
when pinched with a fine forceps or when a light pin prick was applied, showing
that the heart muscle was still alive.
Sino-auricular blocks giving rise to auricular intermissions and pauses of the
same character as those seen more frequently in the ventricle have not infrequently
been met with in the above experiments.
Similar changes in the rhythm and conductivity had also been observed in frog's
heart in situ. Slowing was the only symptom manifested when 0.01 mg. of gigantin
was administered to pithed frogs. The heart usually recovered in about two minutes;
but on repeating the same dose heart block was produced with the sinus, auricle and
the ventricle maintaining their independent rhythms. For every two or three
auricular systole there was a single ventricular response. The heart block usually
disappeared, with the intermisions becoming less and less frequent and in about
ten minutes the heart returned to normal. When the dose was repeated for the
third time, complete block was produced, first at the auriculo-ventricular connecting
fibres then at the sino-auricular fibres, and was manifested by the arrest of the
ventricle first then the auricle and finally the sinus venosus.
The arrest of the heart therefore appears to be due to the failure of efficient
stimuli reaching the ventricle due to the impairment of conduction and not due to
the death of heart muscle, for the ventricle still responds to mechanical stimuli.
Changes in existability—A frog was arranged as for a myocardiogram. The
ventricle was stimulated by a pair of pin electrodes fixed to the ventricle, by single
induction shocks. The minimum strength of current to produce a break-shock extra