At the level of the needle insertion or the marker (Figure 2 and Figure 3), the distances in millimeters from the skin to the interscalene groove and from the interscalene groove to the first rib were measured. After the CT study in the patient group, the local anesthetic was injected through the needle, the needle was withdrawn, and each patient was observed for 20 min prior to transfer to the operating room. The volunteers from the operating room staff were sent back to perform their daily assignments. Data are presented as mean +/- SD.
The CT examinations in 10 patients (8 men and 2 women) and 10 volunteers (6 men and 4 women) were analyzed. The mean age of the patients and volunteers was 31 +/- 8 yr (range, 19-41 yr). The mean weight and height were 56 +/- 6 kg (range, 48-65 kg) and 163 +/- 6 cm (range, 152-177 cm), respectively. The dome of the pleura was not observed at the level of the needle insertion or the marker in any study (Figure 2 and Figure 3). The distance from the level of the needle insertion or the marker to the level of the dome of the pleura was 5-15 mm with a mean of 9 +/- 4 mm in both groups. The distances from the skin to the interscalene groove and the interscalene groove to the first rib were 10-24 mm and 10-22 mm with means of 17 +/- 4 mm and 16 +/- 3 mm, respectively.
None of the patients in the study had complications or side effects. The needle did not enter the pleura or puncture the subclavian artery. Paresthesiae were elicited during needle insertion in seven cases. Surgery was performed in all patients without supplementation.
The brachial plexus is formed by the union of the ventral rami of the fifth to eighth cervical nerves and the greater part of the first thoracic nerve. The ventral rami that form the brachial plexus lie between the anterior and middle scalene muscles. The roots of the brachial plexus begin to unite at the lateral border of the anterior scalene muscle. The plexus passes through the lower part of the posterior triangle of the neck accompanied by the subclavian artery and later the axillary artery, and then passes behind the clavicle over the first rib into the axilla (Figure 5) .
Winnie  recognized the anatomical relation of the brachial plexus and the scalene muscles and was the first to describe the interscalene block, an approach to the brachial plexus with needle insertion between the anterior and middle scalene muscles at the level of cricoid cartilage. With this technique, pneumothorax is usually avoided because the needle insertion is superior to the dome of the pleura. Vongvises and Panijayanond  developed a parascalene technique of brachial plexus anesthesia at our institution in 1979 that attempted to introduce the needle lateral and superior to the dome of the pleura. The principle of the technique is to block the brachial plexus by inserting a needle perpendicular to the operating Table atthe lateral border of the anterior scalene muscle superior to the clavicle (Figure 5). The parascalene technique differs from the interscalene block in two ways. First, the level of the needle insertion appears to be approximately at the level of C-7, which is caudad to the interscalene block (C-6). Secondly, the anteroposterior direction of the needle is almost perpendicular to the needle direction of the interscalene block in order to avoid entering into subarachnoid and epidural spaces.
In the present study, the CT cuts at the level of the needle insertion (immediately lateral to the anterior scalene muscle and 2 cm superior to the clavicle) in 10 patients and 10 volunteers revealed that this level was always superior to the dome of the pleura (Figure 2 and Figure 3). This finding implies that the development of pneumothorax in the parascalene block may be minimized. The distance from the skin to the interscalene groove represents the approximate depth of the brachial plexus. Also, the distance from the interscalene groove to the first rib may represent the distance that the needle should be withdrawn to approximate the brachial plexus if the first rib is contacted during needle placement. We believe that the sample of 10 patients and 10 volunteers should be sufficient to represent the anatomic variables essential to a better understanding of the parascalene block.
In some cases, the distance from the level of the needle insertion to the dome of the pleura was only 5 mm. However, we do not recommend moving the needle entry site more cephalad because the insertion of the needle may be cephalad to the upper trunk of the brachial plexus and the first rib may not be contacted as an additional landmark (Figure 5). Since the introduction of the parascalene block in 1979, we have performed approximately 500 parascalene blocks per year using a needle entry site 2 cm (originally 1.5 cm) superior to the clavicle without encountering clinical signs and symptoms of pneumothorax.
In addition to the cephalad level of the needle entry, another advantage of our technique is the direction of the needle. With this anteroposterior direction, an injection into the epidural or subarachnoid space is usually avoided. Also, the needle is introduced at the lateral border of the anterior scalene muscle, which is lateral to the dome of the pleura. Although the pediatric parascalene technique of Dalens et al.  uses the same needle direction as ours, their entry site appears to be more cephalad and lateral. We agree that their technique has not been associated with a high complication rate, but several measurements are required to identify the skin entry site. On the other hand, the landmark for the needle entry site of the plumb-bob technique of Brown et al.  is simple, but the site is caudad to ours and may be at the level where lung and pleura can be encountered.
In conclusion, we have shown by CT study in 10 patients and 10 volunteers that the level of the needle entry site in the parascalene block is superior to the dome of the pleura, thus the development of pneumothorax is unlikely. We have also shown that the tip of the needle should be withdrawn about 1.5 cm after contact with the first rib to approximate the level of the brachial plexus.
The authors wish to thank Dr. Nopadol Wora-Urai for considerable help in reviewing the manuscript and Somsak Keangvikan and Kitti Lapanant, the radiological technicians, for technical assistance during the study.
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© 1997 International Anesthesia Research Society
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