Anatomical variation of the sympathetic trunk and aberrant rami communicantes and their clinical implications.

Surgical interventions involving the sympathetic trunk are increasingly performed to alleviate symptoms of several disorders such as hyperhidrosis. Anatomical variation has been highlighted as one of the main causes behind surgical failure and symptoms recurrence following surgeries conducted on the chain or its surroundings. This study therefore aimed to record anatomical variants within spinal segments C8-T10 of the sympathetic trunk. Thirty Thiel-embalmed cadavers were investigated bilaterally. The stellate ganglion was recorded on 29 sides. Its size was significantly greater in males and on the right side when the coalescence extended to the subsequent ganglion. The intrathoracic nerve of Kuntz was observed on 21 sides and was significantly more prevalent in males. There was a significant positive association between the presence of this nerve and the descending ramus in the first intercostal space. Aberrant rami found between spinal root C8 and the ventral ramus of the first intercostal nerve were introduced as rami communicantes superi. Aberrant rami communicantes were recorded 50 times in total, of which 70% were found in males. Descending rami showed the highest prevalence in upper intercostal levels, especially in males within the first intercostal space. Aberrant neuronal pathways in upper levels were significantly more prevalent when the stellate ganglion was present. The scientific literature has proven to be stochastic as results were significantly higher in past studies in regard to some sympathetic variants. Anatomical findings of the current study as well as the inconsistency of previous data should be acknowledged and considered for better surgical planning.

Bi-national survey of Korea and Japan related to the injection site for ultrasound-guided stellate ganglion blocks and anatomic comparisons using cadaver dissection.

The aims of this study were to investigate the current clinical practice of ultrasound (US)-guided stellate ganglion block (SGB) using a bi-national survey of Korea and Japan, and to clarify the anatomical relation of the cervical sympathetic trunk with the prevertebral fascia at the level of cervical vertebrae. The current clinical practice of US-guided SGB in Korea and Japan was investigated using an Internet survey, which received 206 (10.2%) replies from Korea and 97 (8.8%) replies from Japan. The survey questionnaire addressed the actual clinical practice for US-guided SGB, including where the tip of the injection needle is placed. Additionally, 16 half necks of 8 embalmed cadavers were used in an anatomical study. An in-plane needle approach technique and administering 5 ml of local anesthetic were preferred in both countries. However, the type of local anesthetic differed, being lidocaine in Korea and mepivacaine in Japan. The final position of the needle tip also clearly differed in an US image, being predominantly positioned above the prevertebral fascia in Korea (39.3%) and under the prevertebral fascia in Japan (59.8%). In all of the anatomic dissections, the cervical sympathetic trunk was over the prevertebral fascia at the level of the sixth vertebra and under the prevertebral fascia at the level of the seventh vertebra. These results are expected to improve the knowledge on the current clinical practice and to suggest future studies.

Location, Dissection, and Analysis of the Murine Stellate Ganglion.

The autonomic nervous system is a substantial driver of cardiac electrophysiology. Especially the role of its sympathetic branch is an ongoing matter of investigation in the pathophysiology of ventricular arrhythmias (VA). Neurons in the stellate ganglia (SG) - bilateral star-shaped structures of the sympathetic chain - are an important component of the sympathetic infrastructure. The SG are a recognized target for treatment via cardiac sympathetic denervation in patients with therapy-refractory VA. While neuronal remodeling and glial activation in the SG have been described in patients with VA, the underlying cellular and molecular processes that potentially precede the onset of arrhythmia are only insufficiently understood and should be elucidated to improve autonomic modulation. Mouse models allow us to study sympathetic neuronal remodeling, but identification of the murine SG is challenging for the inexperienced investigator. Thus, in-depth cellular and molecular biological studies of the murine SG are lacking for many common cardiac diseases. Here, we describe a basic repertoire for dissecting and studying the SG in adult mice for analyses at RNA level (RNA isolation for gene expression analyses, in situ hybridization), protein level (immunofluorescent whole mount staining), and cellular level (basic morphology, cell size measurement). We present potential solutions to overcome challenges in the preparation technique, and how to improve staining via quenching of autofluorescence. This allows for the visualization of neurons as well as glial cells via established markers in order to determine cell composition and remodeling processes. The methods presented here allow characterizing the SG to gain further information on autonomic dysfunction in mice prone to VA and can be complemented by additional techniques investigating neuronal and glial components of the autonomic nervous system in the heart.

Variations in the distance between the cricoid cartilage and targets of stellate ganglion block in neutral and extended supine positions: an ultrasonographic evaluation.

PURPOSE: Anatomic variations complicate surface landmark-guided needle placement, thereby increasing nerve blockade failure rate. However, little is understood about how anatomic distances change under different clinical conditions. As the cricoid cartilage is an easy and accurate landmark, we investigated changes in distance between the sixth or seventh cervical transverse processes (C6TP or C7TP) and the cricoid cartilage in neutral and extended supine positions. METHODS: Forty-two patients (16 men, 26 women) were included in this study. Distances between the cricoid cartilage and C6TP/C7TP were measured using ultrasonography with the patient in neutral and extended supine positions. RESULTS: C6TP and C7TP were caudally located at 6.0 ± 8.1 and 15.1 ± 7.2 mm, respectively, from the cricoid cartilage in the neutral supine position, and at 15.2 ± 8.0 and 25.3 ± 8.0 mm, respectively, in the extended supine position. In the extended supine position, the cricoid cartilage was more cephalad than C6TP and C7TP in all patients. The distance from the cricoid cartilage to C6TP was 12.1 ± 7.6 mm in men and 17.2 ± 7.7 mm in women. CONCLUSION: C6TP and C7TP are located approximately 15 and 25 mm, respectively, caudal to the cricoid cartilage in the extended supine position. Our results highlight the fact that there can be significant anatomic variation between the extended and neutral supine positions used in stellate ganglion block, which should be kept in mind when devising easily identifiable and palpable surface landmarks.

Morphological investigations of the cervicothoracic sympathetic system in the horse (Equus ferrus caballus) in Egypt / Investigaciones morfológicas del sistema simpático cervicotorácico en el caballo (Equus ferrus caballus) en Egipto

The current study was performed on twelve healthy adult horses (E. ferrus caballus) collected from Egypt were dissected to provide anatomical descriptions of bilaterally cervicothoracic sympathetic system macroscopically. On the left side, cervicothoracic sympathetic system is represented only by the caudal cervical ganglion, which presents on lateral surface of esophagus, cranial to the level of first rib. On right side, cervicothoracic sympathetic system is represented by the caudal and middle cervical ganglion. Caudal cervical ganglion was consisted of the fusion of eighth cervical and first three thoracic nerve ganglia. Caudal directed continuation branch of left ansa subclavia gave off a pericardial branch and then gave branch for ligamentum arteriosum. There are special sympathetic­parasympathetic communicating branches; on left side, there is only one branch that was present on lateral surface of esophagus, while on right side, there were four branches; two from caudal cervical ganglion and two from middle cervical ganglion. The most suitable site of ganglion blocks from both sides; needle was placed medioventrally between the articulation of first and second rib.

Doce caballos (E. ferrus caballus) adultos sanos, procedentes de Egipto, fueron disecados para realizar descripciones anatómicas macroscópicas del sistema simpático cervicotorácico bilateralmente. En el lado izquierdo, el sistema simpático cervicotorácico estuvo representado sólo por el ganglio cervical caudal, en la superficie lateral del esófago, craneal en relación a la primera costilla. En el lado derecho, el sistema simpático cervicotorácico estuvo representado por los ganglios caudal y cervical medio. El ganglio cervical caudal consistió en la fusión del octavo ganglio cervical y el primero de los tres ganglios torácicos. Se observaron ramos comunicantes entre los sistemas simpático y parasimpático; en el lado izquierdo, sólo hubo una rama presente en la superficie lateral del esófago, mientras que en el lado derecho, se observaron cuatro ramos: dos del ganglio cervical caudal y dos del ganglio cervical medio. El sitio más adecuado para la ejecución de los bloqueos ganglionares de ambos lados es a nivel medioventral, entre la articulación de la primera con la segunda costilla.

Ultrasound-guided stellate ganglion block: safety and efficacy.

Cervical sympathetic and stellate ganglion blocks (SGB) provide a valuable diagnostic and therapeutic benefit to sympathetically maintained pain syndromes in the head, neck, and upper extremity. With the ongoing efforts to improve the safety of the procedure, the techniques for SGB have evolved over time, from the use of the standard blind technique, to fluoroscopy, and recently to the ultrasound (US)-guided approach. Over the past few years, there has been a growing interest in the ultrasound-guided technique and the many advantages that it might offer. Fluoroscopy is a reliable method for identifying bony surfaces, which facilitates identifying the C6 and C7 transverse processes. However, this is only a surrogate marker for the cervical sympathetic trunk. The ideal placement of the needle tip should be anterolateral to the longus colli muscle, deep to the prevertebral fascia (to avoid spread along the carotid sheath) but superficial to the fascia investing the longus colli muscle (to avoid injecting into the muscle substance). Identifying the correct fascial plane can be achieved with ultrasound guidance, thus facilitating the caudal spread of the injectate to reach the stellate ganglion at C7-T1 level, even if the needle is placed at C6 level. This allows for a more effective and precise sympathetic block with the use of a small injectate volume. Ultrasound-guided SGB may also improve the safety of the procedure by direct visualization of vascular structures (inferior thyroidal, cervical, vertebral, and carotid arteries) and soft tissue structures (thyroid, esophagus, and nerve roots). Accordingly, the risk of vascular and soft tissue injury may be minimized.

Probabilistic mapping of the cervical sympathetic trunk ganglia.

The goal of this study was to create a heat map indicating the probabilistic location of major ganglia of the cervical sympathetic trunk (CST). Detailed dissections of human cadaveric specimens, followed by spatial registration and analysis of the cervical sympathetic ganglia in the neck and upper thorax regions (C1-T1) were performed in 104 neck specimens (both sides from 52 cadavers). Unbiased parametric mapping, visualized with a heat map, revealed a general pattern of two major ganglia located on both sides of the neck: The superior cervical ganglion (SCG) was located 80-90 mm superior to the point at which the vertebral artery entered the transverse foramen (VA-TF); the stellate ganglion (SG) was located approximately 10 mm inferior to the VA-TF in 80% of our sample, or surrounding the VA-TF in the remaining 20% of our sample. In between these ganglia, a highly variable number of smaller and less prevalent ganglia were present on either side of the neck. The middle ganglia on the right side of the neck were located closer to the SCG, possibly indicative of the middle cervical ganglion. On the left side the middle ganglia were located closer to the SG, perhaps indicative of the vertebral ganglion or the inferior cervical ganglion. Individual specimens could be classified into one of seven different patterns of cervical trunks. The results may help surgeons and anesthesiologists more accurately target and preserve these structures during medical procedures.

An anatomical investigation of the cervicothoracic ganglion.

Anatomical variability within the autonomic nervous system has long been accepted. This study evaluated the anatomical variability of the cervicothoracic ganglion (CTG) according to its form and, in addition, provided precise measurements between the CTG and the anterior tubercle of the transverse process of the sixth cervical vertebra (C6TP), the first costovertebral articulation, and the vertebral artery. Forty-two adult cadavers were dissected, 22 male and 20 females. Five main forms of CTG were documented; spindle (31.9%), dumbbell (23.2%), truncated (21.7%), perforated (14.5%), and inverted-L (8.7%). The means for length, width, and thickness of the CTG were 18.5 mm, 8.2 mm, and 4.5 mm, respectively. The dimensions were found to be slightly larger in the males than females and on the left sides as compared to the right. The mean shortest distance between the CTGs and the vertebral artery was found to be 2.8 mm, whilst the mean shortest distances to C6TP was 25.7 mm and to the first costovertebral articulation was 1.7 mm. There is great variability in the morphology of the CTG with five common forms consistently seen. The relation to the vertebral artery may influence the form of the ganglion. Two previously undocumented forms are recorded; the truncated which describes the important juxtaposition of the CTG and the vertebral artery and the perforated which describes the piercing of the ganglion itself by the artery. The findings are considered to be of clinical importance to anesthetists, surgeons, neurosurgeons, and anatomists.

Depth by ultrasound from skin surface to the C6 transverse process while applying pressure.

BACKGROUND: A stellate ganglion block is commonly performed on the anterior tubercle of sixth cervical spine's transverse process. When the procedure is performed, identifying the anatomical landmarks and confirming the depth of the needle insertion to the transverse process are essential for ensuring safety. The purpose of this study was to determine the depth of the needle insertion from the skin to the transverse process for a safe stellate ganglion block. METHODS: One hundred patients were enrolled for this study. The patients' heights, weights, and neck circumferences were measured. In the supine position, the anterior tubercle of the transverse process were palpated and pressed with the examiner's fingers. While spreading the fingers, an ultrasound probe was placed with the same strength as the fingers and the depth from the skin to the transverse process was measured. RESULTS: The mean depth from the skin to the transverse process in men was 9.5 ± 2.7 mm on the left side and 9.7 ± 2.5 mm on the right side, whereas in women, it was 8.0 ± 2.2 mm on the left side and 8.2 ± 2.0 mm on the right side. There was no significant difference between the right and left sides among or between the genders but men showed greater depths than women. Although both genders showed a correlation between body mass index and depth, only men showed a correlation between the neck circumference and depth. CONCLUSIONS: In this study, the mean depth from the skin to the transverse process did not exceed 10 mm in both genders and the maximum depth was 16.6 mm.

[Morphological features of the rat stellate ganglion during early postnatal development].

The aim of this work was to study the anatomical characteristics of the stellate ganglion (SG) and the morphometric characteristics of its neurons in rats of different age groups (newborn, 10-, 20-, 30-, 60- and 180-day-old) using anatomical and histological methods. The results obtained indicated that in rats since birth there were three variants of branch origin from the medial margin of SG. No differences were observed in these variants between right and left SG. The sizes of both SG and its neurons increased during the first two months of postnatal development. The density of neurons in SG sections decreased from the moment of birth until the six months of age. The number of SG neurons did not change significantly in the postnatal ontogenesis. Thus, SG in rats is anatomically formed by the moment of birth, while the sizes and morphometric characteristics of SG neurons become finally stabilized by the second month of age.

Anatomical study and clinical significance of the rami communicantes between cervicothoracic ganglion and brachial plexus.

The aim of this study was to provide a detailed characterization of the rami communicantes between the stellate (or cervicothoraic) ganglion (CTG) and brachial plexus (BP). Rami communicantes of 33 fixed adult cadavers were macroscopically observed, and connection between CTG and spinal nerves and branching was investigated. In all cases, except one, the hibateral medial rami communicantes was found to be positioned symmetrically between the CTG and C7, C8 spinal nerves. Gray rami communicantes arising from the CTG joined C8, C7, C6 nerve roots on 66, 63, and 6 sides, respectively, and branched from the rami communicantes to C7, C6, C5 nerve roots lying on 51, 41, and 2 sides, respectively. Forty-five sides of the branches from rami communicantes derived from CTG to C8 were observed to ascend through the transverse foramina of the C7 nerve. The branches from rami communicantes derived from CTG to C7 to the C6 nerve were observed ascending through the foramen transversarium of the six cervical vertebrae along with the vertebral artery and joining the C6 spinal nerve in 41 sides. Knowledge about the general distribution and individual variations of the rami communicantes between CTG and BP will be useful toward studies involving the inference of sympathetic nerve stimulation of the upper limbs and could be important for surgeons who perform surgical procedures in the cervical region or medical blockade of nerve fibers.

The vertebral artery is unlikely to be the sole source of vascular complications occurring during stellate ganglion block.

INTRODUCTION: Stellate ganglion block (SGB) is commonly performed for upper extremity complex regional pain syndrome and other conditions. Known complications of stellate block include Horner's syndrome, hoarseness, hematoma formation, airway compromise, immediate seizure (presumably from vertebral artery injection), and death. A previous arterial anatomy study demonstrated other vessels, eg, the ascending and deep cervical arteries, reinforcing the blood supply of the spinal cord and brain stem. The potential role of these vessels in the pathogenesis of seizures or hematoma during SGB has not been studied. METHODS: The anatomical recording log from 10 cadaver dissections and photographic records of same were reviewed to ascertain the presence of the ascending or deep cervical arteries, or other branches emanating from the thyrocervical or costocervical trunk and their relationship to the medial anterior surface of the C6 and C7 transverse processes. RESULTS: In 4 cases, as determined by the dissection log, and in 6 cases, determined by photographic images, the ascending cervical artery or a branch from the thyrocervical trunk passed over the anterior aspect of the transverse processes of C6 or C7. DISCUSSION: Arterial vessels other than the vertebral artery that also supply the anterior spinal cord and brain stem pass directly anterior to the transverse processes at the most common sites of the SGB. It is anatomically possible, therefore, that accidental injection or induced spasm of these vessels and not the vertebral arteries is responsible for some cases of seizure, hematoma, or other vascular complications during SGB.

Serial stellate ganglion blocks for intractable postherpetic itching in a pediatric patient: a case report.

BACKGROUND: While intractable itching may be rarely associated with postherpetic neuralgia, it can have catastrophic complications if present. METHOD: We highlight a severe case of postherpetic itching in a 10-year-old male with Fanconi's and aplastic anemia, refractory to conventional treatments and requiring intravenous sedation. RESULTS: Our use of 3 sequential stellate ganglion blocks with 5.5 mL of 0.25% bupivacaine provided significant improvement of the symptoms for 4 months after the last procedure. CONCLUSION: Although further evaluation is needed, we feel that novel use of sympathetic blockade may provide treatment for intractable itching. Highlighted is the possible influence of the sympathetic system in the pathophysiology of postherpetic itch. IMPLICATION: The use of serial stellate ganglion blocks may be a treatment option for patients with intractable itching and postherpertic neuralgia of the neck and arm region. This technique may lead to more permanent solutions such as pulse radiofrequency lesion or chemical neurolysis of sympathetic ganglions for postherpetic itch.

Evaluation of the longus colli muscle in relation to stellate ganglion block.

BACKGROUND AND OBJECTIVES: The longus colli (LC) muscle is an important structure of the anterior cervical spine and has a critical role in stellate ganglion block. This technique involves withdrawing the needle to locate its port for injection above the anterior surface of the LC muscle; however, its exact thickness at the C5, C6, and C7 levels has not been measured. The aim of this anatomic and magnetic resonance-supported study was to evaluate the thickness of the LC muscle at these levels from the anterior tubercle of each vertebra toward the vertebral body at 5-, 10-, and 15-mm distances to provide precise anatomic data for stellate ganglion block. METHODS: Ten cadavers, 60 vertebral body specimens, and cervical magnetic resonance imaging (MRI) scans of 40 adult patients were used for measurements. RESULTS: The main findings of this study are that the thickness of the LC muscle varies between 5.0 and 10.0 mm at C6 and C7 in cadavers and between 8.0 and 10.0 mm in MRI scans. Sex has an important role; MRI scans revealed that male patients have a considerably thicker LC muscle at each vertebral level. CONCLUSION: We found a highly variable thickness of the LC muscle in anatomic and imaging studies, which may lead to negative block results.

Low-intensity treadmill exercise-related changes in the rat stellate ganglion neurons.

Stellate ganglion (SG) represents the main sympathetic input to the heart. This study aimed at investigating physical exercise-related changes in the quantitative aspects of SG neurons in treadmill-exercised Wistar rats. By applying state-of-the-art design-based stereology, the SG volume, total number of SG neurons, mean perikaryal volume of SG neurons, and the total volume of neurons in the whole SG have been examined. Arterial pressure and heart rate were also measured at the end of the exercise period. The present study showed that a low-intensity exercise training program caused a 12% decrease in the heart rate of trained rats. In contrast, there were no effects on systolic pressure, diastolic pressure, or mean arterial pressure. As to quantitative changes related to physical exercise, the main findings were a 21% increase in the fractional volume occupied by neurons in the SG, and an 83% increase in the mean perikaryal volume of SG neurons in treadmill-trained rats, which shows a remarkable neuron hypertrophy. It seems reasonable to infer that neuron hypertrophy may have been the result of a functional overload imposed on the SG neurons by initial posttraining sympathetic activation. From the novel stereological data we provide, further investigations are needed to shed light on the mechanistic aspect of neuron hypertrophy: what role does neuron hypertrophy play? Could neuron hypertrophy be assigned to the functional overload induced by physical exercise?

The terminal insertional segments and communications of the vertebral nerve in the human cervical region.

Twenty-four cadavers (48 sides) were used to clarify the terminal insertional segment and communications of the vertebral nerve in the cervical region under a surgical microscope. After displacing the prevertebral muscles (longus colli and longus capitis) laterally, the ventral parts of the transverse foramen of vertebrae (from C2 to C6) were removed, and the insertional segment and communicates of the vertebral nerve surrounding the vertebral artery were observed. The results showed: (1) the vertebral nerve ascended along the ventral or mediodorsal vertebral artery and terminated mainly at C3 (22/36 sides) but not terminated at C4 or C5 only; (2) the superficial communicates from the cervical sympathetic trunk ran in a proximal and distal direction when the fibers entered the anterior branches of the cervical nerves. The fibers running to the proximal direction communicated with the vertebral nerve in the part of transverse foramen; (3) motor and/or sensory rami supplying the prevertebral muscles, corpus vertebrae and intervertebral discs could pass through an "arched-shaped" fiber bundle on the ventral surface of the vertebral artery. In conclusion, the vertebral nerve and the fibers surrounding the vertebral artery could be considered as a stable deep pathway of cervical sympathetic nerves. The deep pathway (vertebral nerve and its branches) with the superficial pathways (cervical sympathetic trunk and its branches) formed a sympathetic nervous "plexus" in the cervical region. This sympathetic nervous "plexus" may be involved in the effects of cervical ganglionic blockade.

Estudio topográfico y biométrico del ganglio cervicotorácico (ganglio estrellado) / Topographic and biometric study of the cervicothoracic ganglion (stellate ganglion)

El tronco simpático consiste en una serie de ganglios unidos por cordones interpuestos que se extienden a lo largo de las caras laterales de la columna vertebral, desde la base del cráneo hasta el cóccix y se divide en porciones cervical, torácica, abdominal y pélvica. Generalmente presenta de 21 a 25 ganglios de tamaños variables y recibe fibras nerviosas de la porción toracolumbar. Se analizaron 100 troncos simpáticos de cadáveres formolizados de individuos brasileños, adultos, de ambos sexos. El ganglio cervicotorácico se observó en 70 por ciento de los casos, presentándose en 75,7 por ciento de éstos constituido por la unión del ganglio cervical inferior con el primer ganglio torácico; fue fusiforme (44,2 por ciento) o irregular (44,2 por ciento). El ganglio tuvo de promedio 18,0 mm en sentido craneocaudal; 5,3 mm en sentido laterolateral y 3,7 mm en sentido anteroposterior. Se constató que su forma y localización es variable, pero con un conocimiento detallado de las relaciones topográficas y biométricas de esta estructura, se puede abordar quirúrgicamente la región de transición cervicotorácica con seguridad.

The cervical sympathetic trunk is a ganglia series joined by interganglionic segments and they are extend along of the lateral faces of vertebral column, from base of the skull until the coccyx. There are a cervical, thoracic, abdominal and pelvic portions. Generally, the sympathetic trunk consists in 21 to 25 ganglia with variable size and received nervous fibers of the thoracic-lumbar portion. We studied 100 sympathetic trunks of formaldehyde-fixed human cadavers of Brazilian individuals, adults, of both sexes. The cervicothoracic ganglion was observed in 70 percent of the cases and was formed by cervical inferior ganglion fused with the first thoracic sympathetic ganglion in75.7 percent of them; itwas spindle shaped in 44.2 percent or irregular in the samepercentage. The average of length, width and thickness werel 8.0 mm, 5.3 mm and 3.7 mm, respectively. Its shape and localization is variable but with a detailed knowledge of its topographic and biometric relationships, the surgical approach in the region of cervico-thoracic transition will be more sure.

Anatomy and physiology of the right interganglionic nerve: implications for the pathophysiology of inappropriate sinus tachycardia.

OBJECTIVE: To simulate inappropriate sinus tachycardia (IST) in experimental animals. BACKGROUND: We recently found that epinephrine injected into the anterior right ganglionated plexi (ARGP) adjacent to the sinoatrial (SA) node induced an arrhythmia simulating IST. METHODS: In 19 anesthetized dogs, via a right thoracotomy, the course of the interganglionic nerve (IGN) from the right stellate ganglion along the superior vena cava to the heart was delineated. High-frequency stimulation (HFS; 0.1 msec duration, 20 Hz, 4.5-9.3 V) was applied to IGN at the junction of innominate vein and SVC. RESULTS: HFS of the IGN significantly increased the sinus rate (SR) (baseline: 156 +/- 19 beats/minutes [bpm], 4.5 V: 191 +/- 28 bpm*, 8.0 V: 207 +/- 23 bpm*, 9.3 V: 216 +/- 18 bpm*; *P < 0.01 compared to baseline) without significant changes in A-H interval or blood pressure. P-wave morphology, ice mapping, and noncontact mapping indicated that this tachycardia was sinus tachycardia. In 8 of 19 dogs, injecting hexamethonium (5 mg), a ganglionic blocker, into the ARGP attenuated the response elicited by IGN stimulation (baseline: 160 +/- 21 bpm, 4.5 V: 172 +/- 32 bpm, 8.0 V: 197 +/- 32 bpm*, 9.3 V: 206 +/- 26 bpm*; *P < 0.05 compared to baseline). In 19 of 19 animals, after formaldehyde injection into the ARGP, SR acceleration induced by IGN stimulation was markedly attenuated (baseline: 149 +/- 17 bpm, 4.5 V: 151 +/- 21 bpm, 8.0 V: 155 +/- 23 bpm, 9.3 V: 167 +/- 24 bpm*; *P < 0.05 compared to baseline). CONCLUSIONS: HFS of the IGN caused a selective and significant acceleration of the SR. A significant portion of IGN traverses the ARGP or synapses with the autonomic ganglia in the ARGP before en route to the SA node. Dysautonomia involving the IGN and/or ARGP may play an important role in IST.