Aspects of the Use of Ultrasound in Vascular Cannulation

To the Editor Toianos et al.1 fail to mention one important step in the initial ultrasound assessment of a vascular structure, which is to confirm that the anechoic structure visualized is tubular (blood vessels are tubular) and not cystic (e.g., lymph nodes, hematomas).2 This is easily done by scanning the structure up and down and will also confirm the patency of the structure and help detect any local narrowing or thrombosis. In Section 14: Vascular Access Confirmation, the statement “ultrasound … is not a substitute for roentgenography … to identify complications such as pneumothorax or hemothorax” is questionable. Transthoracic ultrasound (as opposed to transabdominal) is at least as sensitive as chest radiography (CXR) with the patient in the lateral decubitus position3 and more sensitive than CXR with the patient in the supine4 or erect positions for the detection of pleural fluid and is able to distinguish atelectasis or consolidation from pleural fluid, which may be difficult on CXR. Transthoracic ultrasound performs better than CXR in the detection of small pneumothoraces5 and is able to rule in or rule out a pneumothorax. The presence of anterior lung sliding in the supine patient rules out a pneumothorax. In the absence of anterior lung sliding, B lines or a lung pulse sign or a tissue-like image of the lung will rule out a pneumothorax at that site.6 A lines with absent lung sliding has a 96% specificity for pneumothorax,7 which becomes 100% when a positive lung point sign is detected.5 A preprocedural scan of the anterior chest is useful to confirm the presence of anterior lung sliding where sudden loss of lung sliding during or after the procedure is highly suggestive of a pneumothorax. There are situations in which ultrasound examination of the chest will not be possible, such as the inability to gain access to the chest wall because of dressings, etc., the presence of subcutaneous emphysema of the chest wall,8 or extensive pleural calcification. Also, where there is absent lung sliding and A lines but no lung point is detectable, other diagnostic modalities will be needed such as CXR, thoracic computed tomography, or in the event of an acutely unstable patient with signs highly suggestive of a pneumothorax, needle thoracostomy.6 There are no special requirements for transthoracic ultrasound other than adequacy of depth of imaging. The pleura is a relatively superficial structure. However, to image structures deeper in the chest, the use of curvilinear, microconvex, or phased array probes with their wide far-field view and the appropriate depth of imaging predominantly related to probe frequency are necessary. Andrew Verniquet, MD, FRCPC Rafid Kakel, MD James Paton Memorial Hospital Central Health Gander, Canada [email protected]

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