E. DONG, G. C. M. WIEDERHOLD 
1093 
eters entered by keyboard in the laboratory 
(program lines 41 to 45) containing zeroes or 
one-volt values v^hich represented the digital 
to analog output buffer (program lines 25 to 
30). This array is transferred to (line 31) the 
1800, and clocked out at 2 msec intervals after 
receipt of a trigger. 
The first protocol that we carried out v^as a 
simple repetitive stimulation of the vagus nerve. 
The stimulation was carried out for 2 minutes. 
The stimulation consists of a train of bursts. 
Each burst consists of 1 to 5 stimuli each sep- 
arated by 10 msec. The period between bursts 
varied from 250 to 2,000 msec. Figure 3 sum- 
marizes the data from two dogs. The horizontal 
axis gives the stimulus period and the vertical 
axis is the mean heart period. Each curve has a 
different number of pulses in the burst. The low- 
est curves represent the heart period response, 
where there is one pulse in each burst. As the 
period between stimulation decreases, the heart 
period increases. Note in both animals, there is 
a slight change in the configuration of the curve 
when the stimulation is about 500 msec. In the 
past, to achieve greater cardiac intervals, the 
frequency of the stimulation was increased. 
We decided to increase vagal stimulation by 
varying the number of pulses in each burst, in 
addition to changing the base period. It is 
pertinent to remark again that this was 
achieved simply by typewriter entry. As the 
number of stimuli in each stimulation burst in- 
creased from 2 to 5, we noted that the mean 
heart period varied considerably from previous 
predictions. As the stimulation period decreases, 
the cardiac period in general increases. How- 
ever, there are zones in which the cardiac pe- 
riod is the same period as the forcing function. 
Indeed, during these zones, the cardiac period 
decreases with the decreasing stimulation in- 
terval. As the number of stimulations in each 
burst increases, the length of the locking in- 
Sti mulus-sfimulus interval In msac 
Figure 3. — Stimulation schematic. 
