First Model of Spontaneous Vagal Hyperreactivity and Its Mode of Genetic Transmission
Background— The main purpose of our study was to define an animal model of vagal hyperreactivity and its genetic transmission.
Methods and Results— We first investigated the vagal reactivity with phenylephrine in conscious rabbits. Barosensitivity and the maximal bradycardic response were measured at the upper mean blood pressure plateau. Hyperreactive (H) animals were selected and crossbred with normal (N) ones. Results showed no significant difference between calculated barosensitivity values after the different doses of phenylephrine. In contrast, an increase of the values and a great dispersion appeared 1 to 5 beats after the end of the ramp. Marked pauses (6000 to 20 000 ms) were obtained with some rabbits, which were blocked by atropine. A significant excess of hyperreactive offspring was observed in H×H crossings compared with N×N ones (39.4% male and 42.3% female offspring versus 14.4% and 4.4%, respectively). Few female offspring were hyperreactive compared with males in N×H and N×N crossings (4.1% versus 23.4% and 4.4% versus 14.4%, respectively).
Conclusions— This study describes the first model of spontaneous vagal pauses. The inheritance could be polygenic with a partial sex-limited character.
Received July 3, 2002; revision received September 3, 2002; accepted September 3, 2002.
Despite the prevalence of syncopes, significant gaps remain in our understanding of its pathophysiology and treatment. During syncopes, impairments of the vagal function may play a pivotal role.1 The barosensitivity (BRS) is genetically transmitted in normal twins,2 and there is an evidence of positive family history in vasovagal syncope.3 The main purpose of our study was to define an animal model of vagal hyperreactivity and its genetic transmission.
In the first part of the study, the vagal response was investigated in conscious rabbits. The second part aimed at selecting normal and vagal hyperreactive male and female rabbits. Finally, the vagal activity in the offspring, produced by the crossbreeding animals with and without vagal hyperreactivity, was analyzed.
In part I, we studied two parameters, the BRS and the maximal bradycardic response (R-R interval) to mean blood pressure (MBP) increases. BRS is defined as the slope (ms/mm Hg) of the regression line correlating the immediate increase in systolic blood pressure (BP) (mm Hg) and the R-R interval recorded during the injection of phenylephrine (PNE). A correlating coefficient >0.8 is required. The systolic BP of each pulse is plotted against the heart period of the succeeding cardiac beat during the increase of BP (ramp method).4
In contrast, the maximal R-R interval appears 1 to 5 beats after the end of the ramp. The ΔR-R is the maximal R-R value at the MBP plateau minus the basal R-R. We defined these maximal R-R values as vagal pauses.
In parts II and III of the study, the R-R interval value was the criterion used to allow the selection of vagal normal and hyperreactive rabbits and their offspring.
Technique: Venous and Arterial Cannulation
The rabbits (Zika strain, R. Wendling, Knoersheim, France) were restrained in a contention box. A cutaneous anesthetic cream, EMLA 5% (lidocaine 2.5 g and prilocaine 2.5 g), was applied on the skin 30 minutes before catheterism of ear artery and vein. For the continuous arterial BP monitoring, the intra-arterial catheter was connected to a pressure monitor (Statham P23Db) and the recordings were obtained on a Gould Electronic BS-272 recorder. The ECG tracings were obtained through transcutaneous patches. The experiment started 15 minutes after cannulation.
Part I: Study of BRS and Gain in Male and Female Rabbits
The 19 male rabbits (weight 2911±710 g) and 12 female rabbits (weight 3033±531 g) were 12 weeks old. BP and R-R interval responses after injections of PNE were measured. PNE was given at 10, 100, and 500 μg/kg doses at 30-minute intervals.
The great dispersion in the observed high values led us to the identification of 2 groups of rabbits fitting with a threshold of reactivity according to the following criteria. Group 1 (10 males and 10 females) showed R-R intervals 2000 ms and <4000 ms after IV injection of 100 and 500 μg/kg of PNE. In group 2 (9 males and 2 females), R-R intervals were >2000 ms and 4000 ms for the same doses of PNE.
In 5 male and 4 female rabbits (weight 3137±550 g), PNE (100 μg/kg) was injected in animals pretreated with 1 mg/kg atropine IV.
Part II: Selection of Male and Female Rabbits With Normal and Hyperreactive Vagal Responses
The response to the dose of 500 μg/kg of PNE was used to select the hyperreactive rabbits. Twelve-week-old rabbits (52 males and 61 females) were studied. The threshold of vagal hyperreactivity (H) was defined as R-R interval duration >4000 ms, whereas vagal activity was considered as normal (N) when the R-R interval was <4000 ms after injection of PNE.
Part III: Offspring
The offspring were obtained as follows: first group, male H and female H; second group, male N and female H; third group, male H and female N; and fourth group, male N and female N.
The R-R intervals were measured in 321 males and 301 females of 12-week-old offspring, products of crossbreeding vagal hyperreactive (H) with normal (N) rabbits. PNE 500 μg/kg was injected intravenously to detect vagal hyperreactivity. The threshold of vagal hyperreactivity was R-R interval >4000 ms.
Part I: Vagal Activity Study
The basal mean BP was 88.7±11 and 87.7±9.7 mm Hg and basal R-R interval was 256±20 and 256±26 ms in males and in females, respectively.
Increases in blood pressure obtained with cumulative doses of PNE were the same in males and females and in both groups. Results showed no significant difference between calculated BRS with different doses of PNE. In contrast, the analysis of ΔR-R showed an increase of the values and a great dispersion after injection of 10, 100, and 500 μg/kg of PNE (Table 1). These values were more pronounced in group 2 than in group 1. The vagal response in the second group was statistically different from the one observed in the first group with doses of 100 and 500 μg/kg of PNE.
Atropine completely blocked PNE-induced bradycardia (Figure, part B). Before atropine, R-R intervals were 1682±488 ms with 100 μg/kg of PNE. In all the animals, atropine completely blocked the increase of the R-R interval and caused ventricular tachycardia and death.
Part II: Selection of the Vagal Hyperreactive Rabbits
The results of the analysis of R-R intervals in 113 adult rabbits (52 males and 61 females) are shown in Table 2. There was a global statistical difference in R-R intervals between males and females after injection of 500 μg/kg of PNE. This difference increased when R-R intervals were longer than 4000 ms (P=0.006).
There were 20 of 52 (38.4%) males and 16 of 61 (26.2%) female rabbits that had an R-R interval >4000 ms (parental control group, Table 3), out of which 12 of 52 (23%) males and 3 of 61 (4.9%) females had an R-R interval >6000 ms. A high mortality was observed among H females at the end of the experiment (arrhythmias) as well as 2 to 5 months later, after 1 to 3 litters. All H females and their litters were explored and therefore included in the study. A total of 30% (6 of 20) of the H males and 75% (12 of 16) of the H females compared with 17.7% (8 of 45) N males versus 18.8% (6 of 32) N females died. The same mortality rates were observed in H offspring.
Part III: Study on the Offspring
Group 1 (H males and H females) and group 2 (N males and H females) were less numerous than in group 3 (H males and N females) and group 4 (N males and N females) because of the high mortality rate of hyperreactive female rabbits (Table 3). Ten pairs were made up in group 1, 7 pairs in group 2, 19 pairs in group 3, and 22 pairs in group 4.
An excess of hyperreactive offspring was seen in group 1. The ratio of H females in group 1 was much higher than the ratio in the parental control group (3 of 61 [4.9%]), especially in animals with R-R intervals >6000 ms (15 of 59 [25.4%]).
In group 2 and 4, a significant difference existed in H males when the R-R interval was >6000 ms: 14.9% versus 5.1% (P=0.03). In the same crossings, few females were found to be hyperreactive compared with males.
Marked vagal responses and even extreme bradycardias were obtained with some rabbits. Some R-R values, although unexpected, were indeed very high (20 000 ms in one rabbit). Because the maximal bradycardic responses appeared 1 to 5 beats after the maximal BP value at the upper plateau of BP and not during the ramp (Figure, part A), no correlation between the BRS and the pauses was observed in our study. In contrast, BP values did not exhibit similar dispersion, because the marked bradycardia reduced cardiac output so that BP was not able to increase. The marked pauses, revealed by extremely long-lasting R-R intervals that were observed after high doses of PNE, were sensitive to atropine. Therefore, the use of doses of PNE higher than the ones used to investigate the baroreflex seems appropriate to generate cardiac pauses related to vagal hyperreactivity that occur in patients and allows us to consider these hyperreactive rabbits as an experimental model of pauses.
Differences in the elastic modulation or stress-strain relationship of the carotid sinus wall, the afferent autonomic limb of the reflex, and central integration of the afferent signal might contribute to the differences in the heart rate responses. Whatever the impairment of the functions involved, the vagus was the final common pathway, because the bradycardic responses were always completely prevented by atropine.
In humans as well as animals, PNE can elicit arrhythmias and cardiac ischemia.5 The marked bradycardia cannot be attributed to coronary vasoconstriction, because no sign of myocardial ischemic was observed on the ECG recordings during the BP increase and the plateau of bradycardia. In contrast, in some rabbits, arrhythmias occurred later on, when the vagal protective effect was over. The high mortality rate in H genitors and offspring female might be a sex link trait or might have a genetic origin or both. Nevertheless, the etiology of the high mortality in H female rabbits remains unclear.
The difference of reactivity between males and females in parental control group may involve a sex link trait. The low rate of H females observed in the parental control group with R-R interval >6000 ms and the high mortality rate in H genitor and offspring females suggest a genetic origin.
Rabbits (Zika strain) were obtained by the breeder by crossing Blanc de Bouscat males and New Zealand females. A previous study showed that there was no difference in vagal reactivity between hybrids and genitor strains. The mode of selection of the Zika strain by the breeder, that is, the use of a small pool of males for all the females, could explain the high rates of hyperreactive male and female rabbits found in the sample of the population. This mode of selection may have generated an expression of this hyperreactive character with a frequency higher than in the whole population.
As the rate of hyperreactive offspring was higher in the H×H crossing than in the N×N group, we thought that a genetic component was likely responsible, but only 39.4% of the males and 42.3% of the females were hyperreactive in group 1. Therefore, 2 hypotheses regarding the genetic mode of transmission can be proposed: either a monofactorial process with incomplete penetrance or a polygenic heredity.
Nevertheless, concerning males, the H rate was higher when the number of H parents was higher too. An oligogenic inheritance was therefore more likely, the amount of H males being so high. A codominant mode of inheritance with incomplete penetrance is possible. The results obtained in groups 2 and 4 suggest a sex-linked trait or the existence of an X-linked component within the polygenic system or their association.
This is the first time we have identified a group of rabbits that can be considered an animal model of vagal pauses. The results suggest that inheritance of vagal hyperreactivity could be polygenic with a partial sex-limited character.
- ↵Mosqueda-Garcia R, Furlan R, Tank J, et al. The elusive pathophysiology of neurally mediated syncope. Circulation. 2000; 102: 2898–2906.
- ↵Tank J, Jordan J, Diedrich A, et al. Genetic influences on baroreflex function in normal twins. Hypertension. 2001; 37: 907–910.
- ↵Smyth HS. Phil D, Sleight P, et al. Reflex regulation of arterial pressure during sleep in man. Circ Res. 1969; 24: 109–121.
- ↵Lai YK. Adverse effect of intraoperative phenylephrine 10%: case report. Br J Ophthalmol. 1983; 73468–469.