226 Journal ofthe AutonormcNer~ous System, 1l (1984) 226 231

Elsevier

J A N 00385

Vagal stimulation and cardiac slowing

Pamela Parker, B.G. Celler, Erica K. Potter and D.I. McCloskey School of PhvMologlv N Pharmacologl'. and School of Electrical Engineering and ('omputer Science.

Uuiuersitv of New South Wales, Kensington, Sydney (A ustraha)

(Received March 16th, 1984) (Revised version received May 21st, 1984)

(Accepted June 1st. 1984)

Key words: vagus - heart rate - pulse interval

As the frequency of stimulation of the cardiac vagus is increased up to about 15-30 Hz there is progressive slowing of the heart. In steady-state conditions, the change in heart rate becomes less, for a given increment in stimulation frequency, at higher stimulation frequencies. In 1933, Rosenblueth [10] found that the relation between heart-rate and stimulation frequency was satisfactorily described by a hyperbola. The hyperbolic form of this relation was subsequently confirmed [1,4,6], although it also has been claimed that data fit a parabolic relation [7] or single exponential expression [11]. At higher frequencies of stimulation, the cardiac re- sponse is variable and atrio-ventricular block or atrial asystole can occur. If the relation between heart-rate and stimulation frequency is truly hyperbolic, it would follow from the mathematics of the relation, (1) that the change in heart-rate from its level in the absence of vagal stimulation would also be hyperbolically related to stimulation frequency, and (2) that the relation between pulse interval (the period between beats) and stimulation frequency would be linear.

We have examined in anaesthetised dogs the steady-state relation between pulse interval and the frequency of separate and combined stimulation of left and right vagi. The simple results we report confirm and extend previous work. and are used as the basis for discussion of some conventional methods of interpreting vagal action.

Experiments were performed on 35 adult mongrel dogs of both sexes, anaesthe- tised with either alpha-chloralose (Sigma: 80 mg/kg, after thiopentone induction, 21 dogs), or with a mixture of chloralose and urethane (60 mg/kg and 600 mg/kg, respectively, 14 dogs). The animals breathed spontaneously through tracheal cannu- lae inserted low in the neck, and a femoral artery and vein were cannulated in each

Correspondence: D.I. McCloskey, School of Physiology and Pharmacology, University of New South Wales, P.O. Box 1, Kensington 2033, Sydney, Australia.

()165-1838/84/$()3.l)(1 '~: 1984 Elsevier Science Publishers B.V.

227

for measurement of arterial pressure and administration of anesthetic supplements. Arterial pressure, the electrocardiogram (ECG), heart-rate (measured beat by beat, triggered from the ECG), and pulse interval (the period between beats, also triggered from the ECG), were recorded on a pen recorder. Temperature was maintained at 37 _ I°C.

Both cervical vagus nerves were cut and cleared of coarse connective tissue for stimulation through platinum electrodes by an isolated, square wave stimulator. The nerves were immersed in a pool of warm paraffin oil to prevent drying. Stimuli were delivered for periods of 15 s (21 dogs) or 60 s (14 dogs), and the resulting changes in pulse interval or heart-rate were recorded as their means in the last 5 s of these periods. The stimuli were of supramaximal amplitude and duration (usually, 10-20 V, 0.1-1 ms); the frequencies of these stimuli were altered on successive trials. Frequencies were tested across the whole range within which there was no atrio- ventricular block, atrial asystole or appearance of ventricular escape beats. This range was usually from 0 to 15-30 Hz. At least 6 frequencies of stimulation were tested within this range for each animal. Comparisons were made of the effects of varying the stimulation frequency of right and left vagus nerves independently. In 6 dogs, the effects of separate and combined stimulation of both nerves, with both supramaximal and submaximal stimuli, were also tested. In all animals, fl-adrenergic blockade was carried out by intravenous injection of propranolol (Inderal, ICI, 1 mg/kg : [2]).

In all 35 animals, linear regression lines were fitted to the data points relating pulse interval to vagal stimulation frequency, and in all animals highly significant positive correlations were found. For 25 of the animals, the regression coefficients were equal to or greater than 0.99, and for the remainder all were greater than 0.95. This was so for points obtained from stimulation of either vagus.

Stimulation of the right vagus more effectively prolonged the pulse interval than stimulation of the left vagus in 32 of the animals: i.e. in these 32 animals, the slopes of the linear regression line fitted to the data points obtained during stimulation of the right vagus, were numerically greater for the right than left vagus. In some of these animals, the effect of left vagal stimulation on pulse interval, although significant, was weak. In the other 3 animals, the left vagus was more effective than the right. Fig. 1 shows results from one animal, showing the characteristic domi- nance of the right vagus.

In 6 animals left and right vagi were stimulated separately, then together. In these, 3 stimulation frequencies were used for each nerve, and were chosen so as to give comparable small, intermediate and large effects on pulse interval. All combina- tions of these stimulus frequencies were tested in all experiments. Fig. 2 shows the results of these experiments, plotted so as to illustrate the finding that combined stimulation always caused a prolongation of pulse interval which was less than the sum of the prolongations caused by separate stimulation.

The data illustrated in Fig. 2 are for varying frequencies of supramaximal stimuli. However, similar results were obtained when submaximal stimuli were used-- these submaximal stimuli were usually chosen to cause only about half the maximum prolongation of pulse interval at any frequency of stimulation. Furthermore, when

228

left or right vagus was split lengthwise into two large trunks, and then these trunks were stimulated separately then together, whether with supramaximal or submaximal stimuli, a similar result was found. All combinations of vagal stimulation caused less prolongation of pulse interval than the sum of the changes caused by separate stimulation.

The demonstration of very high and significant correlation coefficients for the relations between pulse interval and vagal stimulation frequency, over the range of frequencies tested, indicates that these are well described as linear. This was to be expected from the previous work, cited above, claiming to demonstrate a hyperbolic relationship between heart-rate and stimulation frequency. While the data reported here concern the relations between pulse interval and steady rates of vagal stimula- tion, other work suggests that the linearity holds also when vagal activity is rhythmically modulated by arterial pulse and respiration [e.g. refs. 5,8].

The finding indicates that there is considerable analytical value in the use of pulse interval rather than heart-rate as an indicator of vagal activity. For example, in a particular set of conditions, prolongation of the pulse interval by the vagus from, say, 600 to 800 ms would imply the same increment in the frequency of vagal firing as would prolongation from 1000 to 1200 ms. This means that the effect of vagal stimulation on pulse interval is independent of the initial interval length.

Similar arguments could not be made, however, if heart rate were used instead of pulse interval. The changes in heart rate in the examples just given are 25/min (100 to 75/min), and 10/min (60 to 50/min), respectively. Thus, a comparable fall in heart rate from 60/min would be to 35/min, which is an increase in pulse interval of over 700 ms. This would require an increment in vagal frequency of > 3.5 × the magnitude of that required to lower heart-rate by the same number of beats per minute from 100/min. This is an important consideration, particularly because some authors (e.g. see the recent authoritative review by Levy and Martin [6]L present data on vagally-mediated effects in terms only of change in heart-rate. Clearly, without knowing the initial heart rates in such circumstances, it is impossible to make inferences about changes in vagal action.

The linearity of the relation between pulse interval and stimulation frequency is difficult to account for in terms of events at the vagal nerve terminals. The cervical nerves stimulated here contain preganglionic axons, and there is a synapse at the intracardiac ganglion before the cardiac pacemaker cells. The transmission proper- ties of this synapse have not been documented. Furthermore, there is little informa- tion available on the relation between the frequency of vagal discharge and ACh concentration at the pacemaker, or between ACh concentration and its electrophysi- ological consequences [9].

The usual dominance of right over left vagus, confirmed again in the present study, is widely [4,6], if not universally [3] acknowledged~ and may well reflect the anatomical distribution of vagal nerve endings to the sino-atrial node. Both vagi slow the heart effectively, however, so there must be considerable overlap. The cervical vagal trunks stimulated here, of course, contain preganglionic fibers, so peripheral overlap between left and right vagal distributions could occur at the intracardiac ganglia, or on the pacemaker cells, or both. We are not aware of any detailed anatomical studies on these points.

229

PI

(ms)

1 1 0 0

1000-

9 0 0 -

8 0 0 -

7 0 0 -

6 0 0 -

5 0 0 -

4 0 0 -

300"

200"

100"

Right Vagus

y - 3 8 x ~" 268 (r = 0 .996)

~ / Left Vagus ~ y = 19x + 249 (r = 0 .996)

FreQuency (Hz)

Fig. 1. The characteristic linear relations between pulse interval and the frequency of supramaximal stimulation of right and left vagus for one of the dogs studied. The linear regression lines fitted to the data points are shown, and their equations are given.

/ k PI ms ( a c t u a l )

1 2 0 0

1 0 0 0

8 0 0

6 0 0

4 0 0

2 0 0

/ /

/ /

/ o

i

2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0

/k PI ms ( p r e d i c t e d )

Fig. 2. The effects of separate and combined, supramaximal stimulation of fight and left vagi in 6 dogs are shown. Each animal is represented by a different symbol. Each data point gives, by its location on the vertical axis, the steady-state increase in pulse interval which occurred when fight and left vagi were stimulated together (A PI, actual), and by its location along the horizontal axis, the sum of increases in pulse interval which occurred when right and left vagi were stimulated separately (A PI, predicted). The line of identity is shown. Note that for all combinations of right and left vagal stimulation, for all 6 dogs, the actual change in pulse interval was less than the change which would be predicted if algebraic summat ion were to occur - - th i s is readily seen in the figure by the location of all points below the line of identity.

230

A consis tent f inding in the present s tudy was the occlusion, in terms of measured effects on pulse interval , be tween stimuli arr iving along right and left vagus nerves. Hondeghem et al. [4] s tudied this mat te r also, and repor ted that right and left vagal effects were a lgebraical ly addi t ive when expressed in terms of heart-rate . This was so, however, only after various correc t ion factors were appl ied , and only over a small range of s t imula t ion frequencies. We were unable to conf i rm those f indings here. Indeed, when the results shown in Fig. 2 from our s tudy on separa te and combined vagal s t imula t ion were p lo t ted in terms of hear t - ra te ins tead of pulse interval , our result still h e l d - - t h a t is, the fall in hear t - ra te caused by any combina t ion of left and right s t imula t ion was always less than the sum of the falls caused by separa te s t imulat ion. However , a basic flaw in using hear t - ra te for such analysis is well i l lustrated when the higher range of s t imulus frequencies is used. Fo r example, in one animal , with a resting heart rate of 162 /min , a high frequency of s t imula t ion of the right vagus s lowed the heart to 7 1 / m i n (a fall of 9 1 / m i n ) , and a high(er) f requency of s t imula t ion of the left vagus s lowed it to 7 3 / m i n (a fall of 8 9 / m i n ) . If the vagal effects were p roposed to be a lgebraical ly addit ive, this would amoun t to pred ic t ing that combined s t imula t ion would slow the hear t by 91 + 89 = 1 8 0 / m i n , which is more than the initial hear t-rate! The exper imenta l result for combined s t imula t ion was a fall to 5 9 / m i n (i.e. a fall of 103 /min) .

We cannot say from the present results what the basis is of this observed occlusion between right and left vagi or, be tween subdivis ions of the same nerve. Possibly, the occlusion is s imple neural occlusion at the in t racard iac ganglia and occurs because of large over lap between d is t r ibu t ions of indiv idual efferent fibres. If this were so, however, one would expect that neural faci l i ta t ion would be seen at least on some occasions when submax ima l st imuli were used, but this was not so. It seems more l ikely that the occlusion observed at all s t imulus s t rengths tested, reflects a compe t i t ion for receptor sites, whether on the ganglia or on the pacemaker cells, by A C h released from different nerve endings.

Acknowledgements

This work was suppor t ed by the Na t iona l Hear t F o u n d a t i o n of Aust ra l ia . E.K.P. is a Pos tdoc tora l Fe l low of the Na t iona l Hea l th and Medica l Research Counci l of Aust ra l ia . We thank Diane Condie , Tricia Nichols and Yo landa H o b a n for expert assistance.

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