Transient effects of carotid baroreflex stimulation via the neck chamber device on central venous pressure

Abstract We examined in 11 young subjects (age 29.7±3.6 years, mean±SEM) whether carotid baroreceptor stimulation via the neck chamber device may affect central venous pressure (CVP), thus potentially involving other reflexogenic areas in the examined responses. Application of progressively greater neck chamber subatmospheric pressures caused a progressive lengthening in RR interval, which reached a peak at the maximal value of negative neck chamber pressure applied. This was accompanied by significant and progressively greater reduction in CVP values when the data were calculated considering the early changes occurring within the first 2 seconds of the stimulus. There was a weak correlation between the early changes in CVP and the RR interval responses when all stimuli were pooled together (r = 0.32, P < .05). The results of the present study suggest that the neck chamber technique employed to assess carotid baroreceptor‐heart rate sensitivity can transiently affect via the CVP reduction cardiopulmonary receptors activity, which may participate at the integrated reflex responses.

sures caused a progressive lengthening in RR interval, which reached a peak at the maximal value of negative neck chamber pressure applied. This was accompanied by significant and progressively greater reduction in CVP values when the data were calculated considering the early changes occurring within the first 2 seconds of the stimulus. There was a weak correlation between the early changes in CVP and the RR interval responses when all stimuli were pooled together (r = 0.32, P < .05). The results of the present study suggest that the neck chamber technique employed to assess carotid baroreceptor-heart rate sensitivity can transiently affect via the CVP reduction cardiopulmonary receptors activity, which may participate at the integrated reflex responses.

INTRODUCTION
Among the various techniques allowing to assess in humans carotid baroreptor modulation of sinus node activity, the approach based on the evaluation of baroreflex control of heart rate (HR) has been and it still remains one of the most common.
The subjects belonged to a group of individuals referred to the hospi- renal failure, and diabetes mellitus were also excluded by anamnestic or instrumental evaluation. 6 Echocardiographic measurements were all normal. All subjects were in sinus rhythm and none was under any cardiovascular drug treatment. They were selected on the basis of the above-mentioned criteria and in addition on the agreement to participate at the study. All gave their written informed consent to the study, the protocol of which was approved by the Ethics Committee of the two participating hospital institutions, which adopted the same recruiting and operating criteria for the present study.

Study variables
In each subject, the carotid baroreceptor-HR reflex was evaluated via the above-mentioned neck chamber technique. 1 Subatmospheric pressure within the chamber was applied in four separate steps of 10 seconds duration ranging from -7 to -40 mmHg. Each step was repeated three times to increase the reproducibility of the reflex responses and average responses were calculated for each parameter examined.
Steps were applied in a random order and separate from each other by a 2-minute interval. Before and during each stimulus, the pressure within the chamber was measured by a transducer, whereas HR was measured as RR interval using a standard EKG lead.

RESULTS
As expected, application of progressively greater neck chamber subatmospheric pressures caused a progressive lengthening in RR interval, which reached a peak at the maximal value of negative neck chamber pressure applied (Figure 1, upper and middle panels). This was accompanied by significant and progressively greater reduction in CVP values when the data were calculated considering the early changes occurring within the first 2 seconds of the stimulus applied (see original recordings in Figure 2 and average values in the lower panel of Figure 1). In contrast, the late CVP changes detected in the remain-

DISCUSSION
The main finding of the present pilot study is represented by the evidence that the neck chamber device used to assess carotid barore-

STUDY LIMITATIONS
The present pilot study has two limitations. First, the population sample of the study was small, preventing definite conclusions to be drawn.
Second, we examined patients with a first episode of syncope and, although the neurogenic nature of this episode was excluded, the present data cannot be safely extrapolated to hypertensive without the presence in their clinical history of a single episode of syncope.

CONCLUSIONS
Data collected in the present pilot study suggest that the neck chamber technique employed to assess carotid baroreceptor-HR sensitivity can transiently engage cardiopulmonary receptor activity, which may participate at the integrated reflex responses. A similar engagement has been reported also for the arterial baroreceptor testing obtained via the vasoactive drug infusion technique, which has been shown to trigger a much prolonged increase (phenylphrine) or decrease (nitroprusside) in CVP. 5 Further studies are needed to determine whether and to what extent the observed CVP changes are different as far as timecourse and/or magnitude in patients with cardiovascular disease, in which the carotid baroreflex assessment is quite frequently performed for prognostic and therapeutic evaluation. 6,10