Article

M-mode ultrasound for the detection of pneumothorax during helicopter transport

Unlabelled imageAmerican Journal of Emergency Medicine (2012) 30, 1577-1580

Brief Report

M-mode ultrasound for the detection of pneumothorax during helicopter transport

Matthew Lyon MD, Stephen A. Shiver MD?, Perry Walton DO

Department of Emergency Medicine, Medical College of Georgia, AF 1006, Augusta, GA 30912-2800, USA

Received 7 April 2011; revised 28 July 2011; accepted 29 August 2011

Abstract

Background: The presence of the sonographic sliding lung sign (SLS) is a sensitive indicator for the absence of a pneumothorax. The addition of M-mode ultrasound (US) can be a useful adjunct in detecting the SLS.

Objective: The objective of this study is to determine the feasibility of using M-mode US in evaluating the SLS during helicopter transport.

Methods: A model simulating human lung was used during image acquisition. M-mode images of the SLS were obtained during 3 distinct phases of transport: without rotor rotation, with rotor rotation while on the ground, and at level flight. Four US-credentialed emergency physicians evaluated M-mode US tracings of the model along with examples from human lungs, both with and without pneumothorax, in random fashion.

Results: A total of 104 images were reviewed (26 images per reviewer). All of the M-mode images were correctly identified. Motion artifact was noted on the M-mode tracings taken during rotor rotation, which was greatest during level flight. The rotor artifact was not felt to affect the diagnostic utility of the M-mode US tracing.

Conclusion: M-mode US may be used successfully to detect the SLS during helicopter transport.

(C) 2012

Introduction

The use of clinician-performed ultrasound (US) has become commonplace in emergency departments (EDs) across the country. Ultrasound allows emergency physicians to quickly and accurately detect life-threatening conditions such as pneumothorax (PTX) with better sensitivity and specificity than traditional tests such as x-ray. In fact, US has been shown to have sensitivity near 100% for the detection of PTX, comparable with that of computed tomography [1].

* Corresponding author. Tel.: +1 706 721 2613.

E-mail address: [email protected] (S.A. Shiver).

Despite the widespread use of US in the hospital setting, its use in the prehospital environment represents a new frontier. Ultrasound is ideally suited for the prehospital environment, where noise can preclude the use of a stethoscope and other radiographic means of detecting PTX are not feasible. Specifically, the effective use of a stethoscope is often not possible during aeromedical transport, and the only clue to a possible PTX may be abnormal patient vital signs, physical findings such crepitus or tracheal deviation, and others [2].

A prior study has shown that prehospital critical care providers can learn to detect the sonographic sliding lung sign (SLS) with a high level of sensitivity (97%) and specificity (94%) and retain the skill over time [3]. Although a positive SLS reliably rules out the presence of

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a PTX, the sign is often subtle, and the addition of M-mode US can increase confidence in the diagnosis. However, because M-mode US represents motion over time, its utility in the prehospital setting is unknown; motion artifact from ground or air transport may alter the test or render it useless. The objective of this study is to determine the feasibility of using M-mode US in evaluating the SLS during helicopter transport.

Table 1 Distribution of acquired images

Human lung

Lung model without rotor rotation

Lung model with rotor rotation on ground

Lung model at level flight

No PTX

5

3

3

3

PTX

5

2

2

3

Methods

This was a prospective observational trial conducted at a large tertiary care medical facility with an active aeromedical critical care transport program. The trial used a model to simulate the pleural interface of the lungs. The model consisted of an air-filled intravenous pressure bag placed inside another pressure bag. The surfaces of the inner and outer bag represented the visceral and parietal pleural layer, respectively. Water between the 2 bags simulated pleural fluid, and air injected between the 2 bags simulated a PTX. The model was submerged in a water bath to allow for probe depth adjustment (Fig. 1).

A total of 16 M-mode model images were obtained in the helicopter, 9 of which simulated normal lung and 7 of which simulated PTX. Of the 16 total images, 5 were taken without rotor rotation (2 simulating PTX and 3 simulating normal lung), 5 with rotor rotation while on the ground (2 simulating PTX and 3 simulating normal lung), and 6 at level flight (3 simulating PTX and 3 simulating normal lung). A GE Logiqe with a curvilinear 1.8 to 6 MHz transducer was used to obtain all US images. In addition, 10 M-mode images of human lung (5 normal and 5 with PTX) were obtained in the ED during clinical care, not on the helicopter. The distribution of acquired images may be seen in Table 1.

Fig. 1 Lung model submerged in water bath.

Four emergency physicians (EPs) with hospital creden- tials for the performance of emergency US reviewed the images in random order. All reviewers had significant experience in emergency US (minimum of 2 years) and used US regularly in the clinical setting to detect PTX (minimum of 50 scans). No tutorial or instruction was given to the reviewers before the study, and none of the reviewers participated in image acquisition.

Results

The 4 EPs reviewed a total of 104 M-Mode images (26 images each). The M-mode images were correctly identified by all 4 EPs as human lung with a PTX, human lung without a PTX, model simulating normal lung, and model simulating PTX. Thus, the sensitivity and specificity were 100% in each clinical setting. Reviewers did note some motion artifact during rotor rotation; the normally straight horizontal lines seen on the M-mode tracing had a fine sawtooth wave pattern. This artifact was more pronounced in the trials with the simulated PTX and was greater during level flight than during rotor rotation while on the ground. ultrasound images of the lung model and human lung may be seen in Figs. 2 and 3, respectively.

Discussion

The use of US in the diagnosis of PTX was first reported in a veterinary journal in 1986 [4]. Since that time, multiple studies have investigated the use of US in the diagnosis of PTX [1,5-7]. With sonography, the 2 pleural surfaces appear as a bright interface and slide against one another with respiration. M-mode demonstrates movement at the pleural interface, thus making it a useful adjunct in the detection of the SLS.

When air is between the 2 surfaces, as with a PTX, the US waves are not able to visualize the visceral pleura deep to the air, and the SLS is not seen. The M-mode image produced in the setting of PTX was originally referred to as the “strastosphere sign”; some authors now refer to this pattern as the “barcode sign.” Chest wall soft tissue and muscle do not produce significant movement during respiration, thus yielding flat lines on M-mode seen in the near field of the US

Lung Model – No Pneumothorax Lung Model – Pneumothorax

Fig. 2 M-mode images of lung model taken at level flight.

image. In the absence of PTX, the M-mode image deep to the pleural interface appears grainy. The M-mode image produced in the setting of normal lung in M-mode is often referred to as the “seashore sign.”

Our main goal was to determine what, if any, effect the rotor would have on the M-mode tracing. Although there was some motion artifact causing the normally straight horizontal lines seen in the M-mode tracing to appear to have a fine sawtooth wave pattern, this pattern was rhythmical and easily distinguished from Lung sliding. Of note, the artifact was greater during level flight than during rotor rotation while grounded. However, none of the reviewers felt that the rotor artifact affected the Diagnostic ability of the M-mode US tracing.

Limitations

This study used a lung model to simulate the M-mode appearance of normal lung and PTX. Although the model effectively produced M-mode tracings that are characteristic of the presence and absence of PTX, it did not simulate

structures superficial to the pleural lines such as muscle, bone, and fat. Human tissue may produce different results. It is possible that the motion artifact due to the rotor rotation was exaggerated due to the construction of the model with the inner bag suspended in water. In addition, all of the EP reviewers were skilled sonographers with extensive experi- ence in using M-mode to detect the SLS; the results may have differed with less experienced reviewers.

Conclusion

M-mode US may be used successfully to detect the SLS during helicopter transport.

References

  1. Blaivas M, Lyon M, Duggal S. A prospective comparison of supine chest radiography and bedside ultrasound for the diagnosis of traumatic pneumothorax. Acad Emerg Med 2005;12(9):844-9.
  2. Holloway VJ, Harris JK. Spontaneous pneumothorax: is it under tension? Journal of Accident and Emergency Medicine 2000;17(3):222-3.

Human Lung – No Pneumothorax Human Lung – Pneumothorax

Fig. 3 M-mode images of human lung.

  1. Lyon M, Walton P, Bhalla V, et al. ultrasound detection of the sliding lung sign by Prehospital care providers. Am J Emerg Med 2011 [Epub].
  2. Goodman TR, Traill ZC, Phillips A, et al. Ultrasound detection of pneumothorax. Clin Radiol 1999;54(11):736-9.
  3. Kirkpatrick AW, Sirois M, Laupland KB, et al. Hand-held thoracic sonography for detecting post-Traumatic pneumothoraces: the Extended

focused assessment with sonography for trauma (EFAST). J Trauma 2004;57(2):288-95.

  1. Lichtenstein DA, Menu YA. Bedside ultrasound sign ruling out pneumothorax in the critically ill. Chest 1995;108(5):1345-8.
  2. Lichtenstein DA, Meziere G, Lascols N, et al. ultrasound diagnosis of Occult pneumothorax. Crit Care Med 2005;33(6):1231-8.

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