93Peripheric Dysfunction
Carotid Baroreflex
The peripheral unit of the arterial baroreflex system consists of the baroreceptors in
the cardiovascular unit located in the arterial, venous, and ventricular walls, of
which the most investigated are those in the aorta and carotid sinuses. Their activa-
tion is prompted by the distension of the vessel wall, as a consequence of the trans-
mural pressure [ 14 ]. These mechanoreceptors are activated by the stretch, sending
signals that join the glossopharyngeal nerves to the nucleus tractus solitarius and
nucleus ambiguus before eventually being modified in the hypothalamus. The hypo-
thalamus is then responsible for the increased parasympathetic efferent activities
slowing HR and decreasing blood pressure [ 6 ].
Thus, an impaired activation of the baroreceptors elicits an increased response
from the central nervous system, with a subsequent increase in vascular tonus and
decrease in renal excretory function, generating RH [ 16 ].
Therapeutic correlation Recent studies on electrical carotid sinus stimulation with
positive results in the management of hypertension have ascertained the important
role of the carotid structures in the efficient regulation of blood pressure. The idea
behind electrical stimulation of baroreceptors or baroreflex afferent nerves is that
the stimulus is perceived as high blood pressure, and then, baroreflex efferent struc-
tures are involved to counteract the perceived blood pressure increase [ 17 ]. FDA
approved a phase II clinical trial for baroreflex activation therapy (Rheos Feasibility
Trial) to study the efficacy of the technique and investigate the safety of results [ 18 ].
Chemoreceptors
The function of chemoreceptors situated in the carotid body [ 19 ] consists in detect-
ing alterations in arterial PO 2 , PCO 2 , and pH, consequently generating respiratory,
autonomic, and cardiovascular corrections such as minute ventilation and arterial
pressure increase, in order to prevent oxygen impairment of the brain. Their input is
integrated by the pre-sympathetic neurons in the medulla and hypothalamus [ 20 ].
There is recent evidence that the sympatho-excitatory reflex response is increased in
RH, which has led to the introduction of the carotid body tone concept. It appears
that carotid body tonicity drives sympathetic vasomotor tone, while it does not
involve cardiac autonomic activity or ventilation [ 21 ].
Therapeutic correlation Currently, ablation of one of the carotid bodies is investi-
gated as a relatively safe treatment option, and it requires previous determination of
abnormal carotid body tone. However effective, this procedure still represents an
organ-specific approach, and it must be corroborated with other procedures or thera-
peutic schemes for the customized management of each individual case [ 22 ].
7 Pathophysiological Insights in Resistant Hypertension