The trajectory was directed toward the lung in six volunteers. However, if needle contact with the subclavian vein, the subclavian artery, the brachial plexus, or the first rib was recognized and needle advancement halted, then the needle would not have reached the pleura in any of them. The subclavian artery was the only obstructing structure in one volunteer, the plexus in another, and the first rib in a third subject. In the other three volunteers, a combination of obstructing structures was found: the subclavian vein and artery; both subclavian vessels and the first rib; and the subclavian artery and the plexus. Among the 14 volunteers who had the trajectory lateral to the lung, in only 4 of them did the trajectory pass the pleura by >10 mm. The depth to the pleura was <60 mm in 7 and <50 mm in 2 volunteers, with a minimum depth of 43 mm.
The first rib was contacted by the trajectory in six volunteers. This rib was mediocaudad to the trajectory in six subjects, laterocaudad in four, medial in two, lateral in one, and caudad in one.
The trajectory reached the subclavian vein, and then the subclavian artery, in three volunteers. Without prior vein contact, the subclavian artery was reached by the trajectory in two volunteers. When the trajectory did not encounter the subclavian vein or artery, it was always lateral to the vessels and usually very close to them (within 9 mm from the vein wall in 15 volunteers and within 9 mm from the arterial wall in 16 subjects).
The trajectory contacted the brachial plexus in nine subjects. In the remaining 11 volunteers, the trajectory was within 9 mm of the plexus. The depth to the plexus was 30–40 mm in 9 volunteers, 40–60 mm in 7 volunteers, and >60 mm in 2. The individual difference in depth between the brachial plexus and the pleura was 23 mm (range, 9–32 mm). Only a single volunteer had both the plexus and the pleura within the depth of 40–60 mm.
The trajectory avoided contact with all the referred structures (subclavian vessels, plexus, first rib, and pleura) in six volunteers (Table 2). It then passed the pleura by 9 mm (range, 7–15 mm).
The optimal insertion point, through which an anteroposterior trajectory contacts the mid-axis of the plexus, had a distance from Kilka’s point of 5 mm (range, 0–19 mm), according to absolute values. In five volunteers, the two points were identical. Ten subjects had the optimal insertion point lateral to Kilka’s by 8 mm (range, 2–19 mm), measured along the skin at the inferior edge of the clavicle. In the remaining 5 volunteers, the optimal insertion point was 10 mm (range, 3–12 mm) medial.
The clavicular length in the images was 179 mm (range, 155–203 mm). Only one volunteer had a clavicular length of >200 mm. In 5 women and 5 men, the clavicular length was also measured clinically as 185 mm (range, 155–220 mm). Compared with the MRI data of the same volunteers, this represented an overestimation by a median of 9 mm, with a range from 22 mm overestimation to 7 mm under-estimation. Underestimation of the length occurred in only two volunteers.
The distance between the trajectory and the pleura was shorter in women than in men (median, 1 vs 7 mm [P = 0.0499]). Correspondingly, among the six volunteers in whom the trajectory was directed at the lung, all but one were female. The trajectory’s proximity to the pleura was not correlated with any other demographic data, including the clavicular length. The pleural depth showed a positive correlation with body weight, which was significant for women (ρ = 0.82; P = 0.004), but not for men (ρ = 0.49; P = 0.150), and to body mass index (BMI), which was significant for both sexes (men: ρ = 0.64, P = 0.048; women: ρ = 0.85, P = 0.002).
The proximity of the trajectory to the brachial plexus and the position of the trajectory medial or lateral to the plexus were not significantly correlated with any demographic variable, including the clavicular length, and did not differ significantly between sexes. The depth to the plexus was significantly correlated with body weight and BMI (both ρ = 0.74), but not to age or height, nor was it significantly different between sexes. The clavicular length was positively correlated with body height (ρ = 0.65) and more weakly with body weight (ρ = 0.46) and was significantly larger in men than in women (median, 188 vs 167 mm; P = 0.0173).
This MRI study of 20 adult volunteers confirms that VIB has a risk of pneumothorax. This risk is particularly linked to women but does not correlate with other demographic data, including the clavicular length. The trajectory was directed at the lung in six subjects but did not reach the pleura without prior contact with the subclavian vein or artery, the plexus, or the first rib. If needle contact with these structures is recognized clinically, then the probability of the needle reaching the pleura should be small. To avoid a pneumothorax, it has been recommended not to advance the needle deeper than 6 cm (2). Our data indicate that it is difficult to define a safe limit, because in the range of 4–6 cm we often found either the plexus (seven subjects) or the pleura (seven subjects). Although the depths of both structures are correlated with weight and BMI, there is some uncertainty regarding which will be contacted. Regrettably, the first rib is not a reliable backstop for the needle; it obstructed the trajectory in only one third of volunteers.
Aspiration of blood, possibly from the subclavian vein or artery, is suggestive of a too medial needle position, with an increased risk of pneumothorax. This follows from the cephalolateral and posterior position of the plexus to the subclavian artery, which is cephalolateral and posterior to the subclavian vein (Fig. 3). Although such vessel punctures should be considered a specific complication, infraclavicular hematomas seldom become clinically important (1,5). Three volunteers had a trajectory contacting both the subclavian vein and artery. In clinical practice, puncture of the subclavian artery could have been avoided in these subjects by stopping the needle on recognizing the subclavian vein puncture. Two other volunteers had a trajectory reaching the subclavian artery without prior venous contact. Taken together, this may theoretically indicate a risk of subclavian vessel puncture in approximately one quarter of the volunteers.
The proximity of the trajectory to the plexus was not correlated with sex or demographic data. The trajectory approached the plexus with a median distance of 1 mm (range, 0–9 mm) and contacted the plexus in 9 volunteers. This precision compares favorably with two other infraclavicular approaches and one supraclavicular approach that our group has examined by MRI (11–13). Regarding the original infraclavicular method by Raj et al. (11) and the lateral approach to Raj’s method with the arm 90° abducted (12), the recommended trajectory’s distance from a defined midpoint of the plexus was a mean of 26 mm (range, 14–39) and 19 mm (range, 5–29 mm), respectively. The supraclavicular plumb-bob technique (13) had the mean recommended trajectory of 12 mm (range, 0–22 mm) from the closest part of the plexus. The infraclavicular, lateral, and sagittal approach (10) uses two trajectories. The primary trajectory had a median distance from the mid axis of the lateral, posterior, and medial cord of 6 mm (range, 0–25 mm), 2 mm (range, 0–17 mm), and 3 mm (range, 0–18 mm), respectively. The corresponding numbers for the secondary trajectory were 6 mm (range, 0–15 mm), 11 mm (range, 1–17 mm), and 9 mm (range, 0–18 mm). Assuming an average cord diameter of 4 mm, these data indicate that the accuracy of VIB may be comparable to that of the infraclavicular, lateral, and sagittal technique. Neuburger et al. (5) confirmed in a clinical study that VIB is precise in reaching the plexus: the first needle pass identifies the plexus in 80% of the patients with a nerve stimulator technique.
Accurate clinical determination of the insertion point (the middle point between the ventral acromial process and the jugular notch) also assumes accurate localization of the acromion, which occasionally may be difficult to palpate. This was demonstrated in our study, in which the clinically determined clavicular length often varied considerably from the MRI-measured length in the same volunteer. If this leads to an insertion site medial to Kilka’s point, the risk of subclavian vessel puncture or pneumothorax is increased.
This study was limited by simulating VIB in healthy adults only. Distances and depths may differ in patients with pulmonary or cardiac disease and certainly in children. Moreover, the number of volunteers included was small, although it was larger than in some former MRI studies (7,9,11–13). Finally, our investigation did not involve the performance of actual blocks.
There have been few clinical reports of pneumothorax with VIB (2,4,6). Mehrkens and Geiger (2) reported 4 cases (0.5%) in 800 patients, and Neuburger et al. (6) found 2 cases (0.2%) among 1100 patients. These infrequent cases are at least partly explained by the needle often contacting the plexus, the subclavian vessels, or the first rib before approaching the pleura. Additionally, one may speculate, as with any infraclavicular method, about the unknown incidence of patients with asymptomatic pneumothorax. The risk of pneumothorax associated with VIB was investigated in a previous MRI study by Neuburger et al. (7). The authors reported a remarkably longer distance between the trajectory and the pleura, with a mean of 28 mm (range, 0–41 mm) and an sd of 15 mm. Contributing to the differences in results may have been the different methods of defining Kilka’s point. They defined this point indirectly by primarily determining it clinically and then marking it with an MRI contrast marker, which was recognized in the MRIs. We defined the insertion point directly in the MRIs by using only the images. Definition of the insertion point requires measurement of the clavicular length, which we often found to be different in the same volunteer when measured clinically and by MRI.
In two clinical studies of VIB, the incidences of subclavian vein puncture were 10.3% (1), which is consistent with our study, and 30% (5). Subclavian artery puncture was not described in either series. This is remarkable, considering that it theoretically could have occurred in 2 of 20 volunteers in our study.
Our MRI data support making the first insertion at Kilka’s point. If necessary, we propose searching for the plexus by moving the insertion point laterally at intervals of 0.5 cm, as far as 2.0 cm, before insertions 0.5 and 1.0 cm medial to Kilka’s point. This represents a more lateral search than in the traditional teaching of VIB. Although aspiration of blood, possibly from the subclavian vein or artery, always suggests a more lateral reinsertion, needle contact with the first rib does not indicate whether the next insertion should be lateral or medial. Kilka’s group has always recommended maintaining the needle perpendicular to the coronal plane (2) to avoid a medial deviation of the needle, with an increased risk of pneumothorax. Our data support this caution.
In conclusion, our MRI study indicates that in performing VIB, there is a small probability that the needle may reach the pleura, particularly in women, and a high probability of reaching the subclavian vessels. Although the recommended needle direction is close to the plexus, the slightest medial deviation of the needle from the plexus risks subclavian vessel puncture or pneumothorax. Therefore, inaccurate clinical localization of the insertion point is a concern. We recommend an extended lateral search for the plexus before insertions medial to Kilka’s point.
We thank Stiftelsen Sophies Minde for grants funding the volunteers for the MRI studies and for financing dissections at the Department of Anatomy, University of Oslo.
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© 2005 International Anesthesia Research Society
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