Which ultrasound transducer type is best for diagnosing pneumothorax?

In the critically ill and injured patient, an accurate physical examination is essential in the care of the patient. However, auscultating for breath sounds in a respiratory distressed patient is often difficult or even impossible, especially in a noisy accident scene, or patient compartment of a (moving) ambulance or helicopter.

The sensitivity of auscultation for the diagnosis of hemothorax, hemopneumothorax, and pneumothorax is only 58–66% [1–3]. Unilateral decreased or absent breath sounds are often interpreted as a pneumothorax. However, conditions such as splinting from rib pain, lung contusion, atelectasis, pneumonia, pleural effusion, and tumor growth may account for the same abnormal auscultation.

Lung ultrasonography (US) for the diagnosis of pneumothorax was first described in 1986 [4]. It may rule-in pneumothorax with a sensitivity ranging from 81 to 98% and rule it out with a specificity approaching 99–100% [5–7]. Additionally, pleural effusion, lung contusion, and atelectasis may be detected [7]. It has even been suggested that US might 1 day replace the stethoscope [8, 9].

US is feasible in the prehospital setting including inside ground ambulances and a helicopter emergency medical service (HEMS) [10–12]. Similar to most diagnostic and therapeutic procedures, US requires training and regular practice. Time pressure and limited working space are additional challenges [12]. To facilitate the best possible images, it is important that optimally set-up US equipment is readily available. In an optimal configuration, the most suitable transducer is connected to the US machine.

Lung US can be performed with high-frequency linear-array, curved-array, or phased-array transducers. However, it is not known which one is preferable and provides the best images.

We hypothesized that a linear-array transducer is the optimal transducer for the appreciation of the pleural line for diagnosing pneumothorax. The aim of the study is to compare three transducer types on diagnostic accuracy, speed of the diagnosis, and image quality in the assessment for pneumothorax.

We performed a double-blinded, cross-sectional, observational study to compare three types of ultrasound transducers for the diagnosis of two conditions: normal ventilation, and pneumothorax. Ethical approval was obtained from the institutional ethics review board of the Radboud university medical center, Nijmegen. Written informed consent was asked and obtained from every patient and from every observer.

At the preoperative outpatient evaluation clinic of the Radboud university medical center, Nijmegen, the Netherlands, from September to October 2017, we recruited a consecutive series of eleven eligible patients that were scheduled for video-assisted thoracoscopic surgery (VATS) for pulmonary, mainly neoplastic, pathology. The inclusion criteria were a body mass index < 30 kg m⁻² and the absence of pathology of the chest wall, visceral or parietal pleura.

Lung US is a valuable test for the detection or exclusion of a pneumothorax [13, 14]. A US transducer is positioned on the chest wall perpendicular to two adjacent ribs. Between the acoustic shadows of two ribs, a hyperechoic line is visible representing the interface of the parietal and visceral pleura. With normal ventilation, lung sliding is observed as a to-and-fro movement at the pleural line as a result of the sliding of the visceral pleura against the inner chest wall. B-lines may be observed as hyperechoic lines radiating down from the pleural line. Their presence excludes pneumothorax (at the transducer position). Horizontal repetitions of the pleural line appearing below at multiples of the skin-pleural line distance are called A-lines. Their appearance is more prominent in the presence of a pneumothorax when B-lines are absent and no longer obscuring the A-lines.

We used a portable X-Porte ultrasound system (Fujifilm SonoSite Inc., Bothell, WA, USA) equipped with three transducers: a high-frequency linear-array 15–6 MHz (HFL50xp), a curved-array abdominal 5–2 MHz (C60xp) and a phased-array cardiac 5–1 MHz (P21xp) transducer. The footprints of the transducers are 5 cm, 6 cm, and 2.1 cm, respectively.

For the VATS procedure, isolated ventilation of the dependent lung via a double-lumen endotracheal tube was necessary. First, all patients underwent general anesthesia, were intubated and ventilated, and placed in a lateral decubitus position. The ventilator was set to deliver a tidal volume of 5 ml kg⁻¹ at a rate of 20 min⁻¹. The anesthesiologist verified the position and depth of the double-lumen endotracheal tube with fiberoptic bronchoscopy.

Second, the linear array, curved-array, and phased-array transducer were positioned over the fourth or fifth intercostal space at the axillary line in a craniocaudal orientation. 15-second ultrasound video clips were recorded of normal ventilation at a respiratory rate of 20 min⁻¹. The zone of interest was the pleural line with its two adjacent ribs. A typical clip was framed as shown in Fig. 1.

Third, after the chest was prepped and draped, ventilation of the non-dependent lung was interrupted while the surgeon opened the chest, introduced the videoscope and visually confirmed the collapse of the lung. Thereafter, the surgeon recorded three similar 15-s video clips of the established pneumothorax with the three transducers wrapped in sterile transducer covers (Fig. 2). Hence, six clips were recorded in every patient. The time interval between the induction of the pneumothorax (reference test) and the performance of the three ultrasound video clips was no longer than 2 min. No adverse events occurred.

We cropped the video clips using iMovie for OS X, version 10.1.8 (Apple Inc., Cupertino, CA, USA). After we cropped and removed the text from the captured video clips, it was now no longer possible for the observers and the researchers to reliably recognize the transducer type by the image shape (rectangular or sector-shaped). An uncropped still image of the video clip and its cropped version are displayed in Fig. 3.

The Nijmegen physician-staffed HEMS carries a portable US machine since 2006. All HEMS physicians were trained in lung US either at the introduction of the US machine or at the start of their employment. They use lung US regularly in their prehospital practice.

We recruited all 13 HEMS physicians (except the author, RK) and two anesthesiology residents with extensive experience in lung ultrasonography as observers to assess a randomized set of 66 15-s ultrasound clips. We used PotPlayer for Windows, version 1.7 (Kakao Corp., Jeju, South Korea) to separately randomize and playback the cropped set of clips for each observer (Fig. 4). Before the observers assessed the set of video clips, they were informed about how we acquired the clips and about the two possible conditions (normal ventilation and pneumothorax). Due to the cropping and randomization, the observers were blinded for the diagnosis and for the transducer type.

The observers were requested to pause the playback themselves when they were certain about the diagnosis based on the presence or lack of lung sliding, A-line sign, or B-lines. The equally blinded researcher recorded the elapsed time (s), the observer’s diagnosis and perceived image quality rated on a 1–5 Likert scale (1, very poor; 5, very good). For every observer, their experience (years) with lung US and preferred transducer type for lung US was recorded.

In this study, we found no difference in the diagnostic performance of the linear-array, curved-array, and phased-array transducer. The diagnostic performance was very good: sensitivity, specificity, PPV, and NPV were all between 96.4 and 98.8%. The observers needed an additional .51 s to reach a final diagnosis with the phased-array transducer compared to the linear-array transducer. A final diagnosis was reached much faster when lung sliding was present regardless of the transducer type.

The image quality scored by the observers on a 1–5 scale was significantly different between all three transducers; the linear-array transducer achieved the best scores, the phased-array transducer the worst. Moreover, the image quality was too bad to reach a diagnosis in eight phased-array transducer clips and two linear-array transducer clips.

These findings suggest that the actual diagnostic performance of the three transducers for pneumothorax is comparable. However, these experienced observers perceived the best image quality and needed the least amount of time when they judged the linear-array transducer clips. Based on these findings, the linear-array transducer might qualify as the preferred transducer for lung ultrasonography. However, the transducer choice may depend on more important factors such as the intended gamut of indications US is used for and whether the machine will be equipped with one or more transducers. In a single transducer setup, the best choice is probably a curved-array or a phased-array transducer to be able to evaluate both the abdomen and pericardium, in addition to the chest.

To our knowledge, there are no studies that have compared US transducers for diagnosing pneumothorax in a similar study design.

In a study with a comparable design, but not focused on pneumothorax, the authors compared a 10–5 MHz and a 14–5 MHz linear-array transducer for a wide array of emergency department point-of-care ultrasound indications [15]. However, lung ultrasonography was discussed only briefly. Overall, their observers most frequently preferred the 10–5 MHz transducer over the 14–5 MHz transducer.

In another study, the investigators compared a 5–10 MHz linear-array and a 1–5 MHz phased-array sector transducer in 55 patients scheduled for a thoracic-computed tomography (CT) scan [16]. The authors evaluated the diagnostic performance for pneumothorax, pleural effusion, consolidation, and interstitial syndrome. In six patients with a pneumothorax, confirmed with CT, they found that the linear-array transducer performed best with a sensitivity and specificity of 83% and 100%, respectively. The phased-array transducer showed a sensitivity and specificity of 67% and 100%. Sensitivity of both auscultation and chest radiography was only 50%. In our study, the gold standard was a thoracoscopically induced and confirmed pneumothorax. Because we assessed 495 ultrasound clips showing pneumothorax, the diagnostic performance we found is more reliable.

We hypothesized that the linear-array transducer would have the best diagnostic performance. This study, however, showed no difference in diagnostic accuracy between the transducers.

The Nijmegen HEMS introduced prehospital ultrasonography to the Netherlands in 2006 and used a phased-array transducer ever since. Only years later, a linear-array transducer was added. A curved-array transducer has never been used. This history might explain the transducer preferences of the observers and the high and equal diagnostic performance between transducers.

Although diagnostic performance is equal, we recommend the linear-array transducer for (prehospital) lung ultrasonography. The diagnosis is made faster and with a better image quality. These are important advantages in the dynamic prehospital environment, HEMS physicians encounter challenges such as time pressure, limited working space, residual clothing, defibrillator pads, and Velcro^® straps. Most importantly, the interpretation of US images may be hampered by sunlight or precipitation. When the HEMS physicians have the best possible image quality, they can better deal with these factors and do the best possible for our patients.

Furthermore, when the linear-array transducer is installed as the default transducer, it may have additional advantages. It is the preferred transducer for vascular access and assessment of the airway and endotracheal tube position [15]. These matters often take precedence over detailed assessment of breathing, although it may be of vital importance to be informed about a significant pneumothorax before airway management is commenced.

In addition, the linear-array transducer is superior for ultrasound-guided regional anesthesia (UGRA) in severely injured or trapped extremities and for optic nerve sheath diameter (ONSD) measurements in traumatic brain injury (TBI) patients [17]. For abdominal ultrasound and echocardiography, however, the phased-array or curved-array transducer is still invaluable.

The observers were able to successfully assess the video clips of normal ventilation and pneumothorax without having access to the US machine or the patient. This situation is comparable to a telemedicine setup in which the US operator could be at a different physical location than the observer of the images. Therefore, we agree that lung ultrasound can be successfully used in telemedicine setups [18].

In conclusion, we found no difference in the effectiveness of detecting or excluding a pneumothorax between a high-frequency linear-array ultrasound transducer, a curved-array, and a phased-array transducer. Besides many indications for which it is essential, the linear-array transducer produces the best image quality in lung ultrasonography. Based only on image quality, the linear-array transducer might qualify as the preferred transducer for lung ultrasonography and the preferred default in our prehospital setting.