Chapter 4

Pneumothorax

Nils Petter Oveland

Stavanger University Hospital, Dept of Anaesthesiology and Intensive Care, Stavanger, Norway.

Correspondence: Nils Petter Oveland, Gneisveien 1 C, 4027 Stavanger, Norway. E-mail: nils.petter.oveland@me.com

The topic of TUS detection of pneumothorax (PTX) can be elucidated by two clinical scenarios.

In the first scenario, you are an air ambulance doctor on call at a level 2 trauma centre. During your shift, you are dispatched to a hunting accident where a 14-year-old boy has been shot in the chest and face. When you arrive at the scene 15 min later, the patient is lying on his back on the ground complaining of general chest pain and facial discomfort around his right eye. The clinical examination, including auscultation, shows no respiratory and circulatory instability, but his chest and face have multiple gunshot wounds from the metal pellets. On the way to the hospital, you perform an in-flight TUS examination, which shows no sign of blood in the pericardium, abdomen or thorax. However, you suspect a small PTX in the left lung apex because the normal sliding movement between the two pleural layers is absent and the tip of the lung reappears on the US screen with each breath (figure 1a). Although these US signs of abnormal lung sliding and a visible lung point are subtle, your suspicion is passed on to the trauma surgeon in the emergency department. The supine CXR shows the metal pellets but no PTX (figure 1b). Subsequent CT scans confirm the chest injury caused by the gunshot: a small 5 mm-thick apical PTX (figure 1c). No chest tube is inserted, and the patient is brought to the operating room and intubated prior to eye surgery. The patient is closely observed for any clinical deterioration and signs of tension PTX during surgery.

Figure 1. Case scenarios. a) US image of a miniscule apical pneumothorax (PTX) caused by penetrating trauma. The tip of the lung (thick arrow) has detached from the hyperechoic pleural line (thin arrow) as air interposes the parietal and visceral pleural layers (i.e. the definition of a PTX). This lung point moves to and fro along the pleural line, in a manner synchronous with respiration (visible only on video clips). b) CXR showing the multiple metal pellets from the gunshot. The PTX is not visible. c) CT confirming the small amount of intrapleural air displayed as a black pocket in the left lung apex (black arrow). The white arrow shows the lung point. Reproduced and modified from [1] with kind permission from N.P. Oveland.

In the second scenario, you are dispatched to a motorcycle accident where a 55-year-old male biker has gone off the road and hit a tree at high speed. When you arrive, the paramedics have placed the patient on a backboard, secured his neck with a collar and given him high-flow oxygen through a mask. He is anxious and complains of chest pain and dyspnoea. The clinical examination shows no chest instability, but the respiratory rate is clearly elevated (40 breaths min⁻¹) with symmetrical breath sounds. Peripheral pulses are present and the heart rate is 136 beats min⁻¹ with no signs of external bleeding or long-bone fractures. You perform a rapid sequence intubation, initiate positive-pressure ventilation using a portable respirator and transport the patient by helicopter to a level 2 trauma centre 15 min away. The trauma team there is notified. The initial assessment and supine CXR obtained in the emergency department do not show any thoracic injuries. The CT scan detects unstable L2 and C7 vertebral-body fractures, multiple small rib fractures, a sternum fracture and bilateral small PTXs in the anterior chest. The surgeon on call wants the patient transferred by helicopter to a level 1 trauma centre 70 min away because the vertebral fractures are unstable. You express concern for having a patient in a helicopter with bilateral PTX undergoing positive-pressure ventilation, and advocate the insertion of chest tubes, but the surgeon disagrees. Because lung auscultation in flight is impossible due to noise, you use the portable US machine to repeatedly scan the anterior chest of the patient every 10 min. The PTX is outlined on the patient’s chest with a pen by marking the US lung point sign (i.e. the edge of the PTX where the lung is still in contact with the interior chest wall). The patient is transported to the level 1 trauma centre without any progression of the PTX on either side.

These scenarios show how clinicians are challenged with a diagnostic dilemma when encountering trauma patients, because a physical examination combined with an anteroposterior supine CXR is insufficient to diagnose PTX. Emergent CT scans of the chest can detect all PTX but have disadvantages, such as exposure to radiation. In addition, in settings such as pre-hospital, battlefield and remote areas, clinicians do not have access to these diagnostic modalities. Thus, an accurate mobile method for diagnosing PTX is needed. TUS meets these requirements and can be performed in almost any clinical setting. First, it is a noninvasive, radiation-free, rapid and repeatable bedside diagnostic test that has been shown to be more sensitive than, and equally specific as, supine CXR. Second, TUS can also assess PTX progression, known to be an independent factor of a patient’s later need for chest-tube insertion. This is potentially helpful in real clinical settings, as it may enable clinicians to use US to make treatment decisions.

Pneumothorax

Thoracic trauma

Trauma is one of the leading causes of death worldwide (5.1 million deaths in 2010) and can be characterised as a global epidemic because it accounts for one in every 10 deaths [2]. Patients with multiple blunt injuries are far more common in civilian practice, but both penetrating and blunt trauma present significant challenges to national healthcare systems and necessitate a policy action to prevent them [3]. Chest injury is the direct cause of death in 25% of blunt trauma victims and a contributing factor in up to another 50% of trauma deaths [4], which can be explained by the large number of motor vehicle crashes and falls [2, 5]. One serious consequence of chest trauma is PTX, a potential life-threatening condition that sometimes requires immediate treatment to prevent patient death [6]. Although the origin of PTX can be spontaneous, it is trauma patients in particular who present with a respiratory instability that needs immediate diagnosis and treatment. Therefore, this chapter focuses on the basic clinical and diagnostic aspects of traumatic PTX.

Cardiothoracic anatomy

The thoracic cage is a bony structure that connects the posterior vertebral column and anterior sternum via the osteocartilaginous ribs. The thorax contains the heart, lungs, great vessels and oesophagus (figure 2), and is demarcated by the diaphragm inferiorly and structures of the neck and lung apices superiorly. The aorta, pulmonary artery, pulmonary veins and vena cava occupy the mediastinum, where they connect to the base of the heart. The heart is contained within the fibrous pericardium, and its apex projects into the left thoracic cavity [7]. In the right and left hemithorax, the pleural sac surrounds each lung: the parietal layer covers the inside of the chest wall and the visceral layer encloses the lung parenchyma. During ventilation, the visceral pleura is brought into contact with the parietal pleura, thereby reducing the pleural cavity to a closed, separate space. This space normally contains only a capillary layer of serous fluid that lubricates the apposed surfaces and reduces friction between the parietal and visceral pleura [8]. The dynamic to-and-fro sliding motion between the layers during respiration is the basis of TUS detection of PTX [9].

Figure 2. Illustration of a pneumothorax (PTX) in the right lung and detection by an US probe. PTX is caused by one of the following mechanisms: 1) air leaking from the airways (proximal or distal conduits) or the alveolar space, 2) air communication between the pleural space and the atmosphere or 3) the presence of gas-producing organisms within the pleura. Reproduced and modified from [10] with permission.

Definition

PTX is a common clinical condition where air is present in the pleural space (figure 2). When air separates the parietal and visceral pleural layers, the negative intrapleural pressure that keeps the lungs distended is disrupted, leading to a collapse of the elastic lung parenchyma and expansion of the thoracic cage [11].

Classification

There are different classification categories for PTX (table 1). According to aetiology, PTXs are classified as spontaneous or traumatic. A PTX is classified as primary spontaneous if no obvious precipitating factor is present and as secondary spontaneous if the patient has an underlying disease (e.g. COPD, cystic fibrosis). Traumatic PTX is either iatrogenic (i.e. caused by transthoracic or transbronchial biopsy, central venous catheterisation, pleural biopsy, thoracentesis) or noniatrogenic following a blunt or penetrating chest injury [11]. A PTX can have a wide continuum of severity, ranging from simple asymptomatic PTX caused by disruption of the pleural space to tension PTX associated with increased intrathoracic pressure [7]. The increasing use of CT to investigate thoracoabdominal trauma allows a more precise and three-dimensional evaluation of the thorax. Intrapleural air not seen on supine CXR is often seen on CT, and this entity is defined as occult PTX (figure 3) [12].

Figure 3. Occult pneumothorax after blunt chest trauma. The large amount of intrapleural air anterior to the right lung was not visible on the supine CXR but was subsequently diagnosed on the CT scan images. Reproduced and modified from [1] with kind permission from N.P. Oveland.

Epidemiology

In addition to rib fractures and pulmonary contusions, PTX is the most common chest injury found in patients with blunt trauma [13, 14]. In a population-based study from Italy, PTX was present in one in every five severely traumatised patients (i.e. an incidence of 81 per 1 million citizens per year) [6]. These results are consistent with a study of chest injuries from the regional trauma centre in Trondheim, Norway, where the most common thoracic injury was rib fracture (55%) and the most common internal thoracic injury was PTX (24%) [15]. A high incidence of extrathoracic injuries such as head trauma and musculoskeletal injuries in the extremities has also been associated with major blunt trauma [7, 15], and only a minority of patients with PTX (5%) have isolated chest injuries [4]. Furthermore, PTX is associated with greater on-scene physiological instability (i.e. low blood pressure and low oxygen saturation) compared with other chest injuries of similar severity [6]. These findings draw attention to the association between PTX and tension physiology, as well as the potential benefits of decompression procedures.

Tension PTX

If air is allowed to enter (from the lung or through the chest wall) but not exit the pleural space via a “one-way valve”-like opening, tension PTX will quickly develop. The increasing pressure deranges the cardiorespiratory capacity of the patient and makes this an insidious and life-threatening event. This condition is most often seen in pre-hospital, emergency department, trauma unit and critical care settings [16]. The instability of trauma patients with PTX [6] may be explained by the progressive accumulation of air within the pleural space, which exerts mechanical pressure on internal structures. The affected lung may then collapse, with a possible shift of the organs to the contralateral thoracic side. Such a mediastinal shift severely impairs the circulation by pinching the central venous return to the heart (figure 4) and reduces ventilation by compressing the remaining lung. Furthermore, the resulting hypotension and hypoxaemia can lead to cardiac arrest, at which point diagnostic and treatment delays are highly lethal [7, 16, 17]. Thus, PTX is a notable cause of preventable death (i.e. in ∼16% of pre-hospital trauma cases) [6, 18, 19] where simple field-friendly interventions may be lifesaving [20].

Figure 4. Tension pneumothorax on the left side. The expanding intrapleural air mass pushes the heart over the midline into the contralateral right hemithorax, impairing the lung and heart capacity. Image provided by N.P. Oveland.

Diagnostic evaluation of PTX

Clinical examination

The physical examination of the chest includes inspection (looking for signs of injury, symmetry of the thorax, abnormal breathing movements and distended neck veins), palpation (searching for s.c. emphysema, localised chest pain, crepitation, instability of the thorax and tracheal deviation), percussion (tympani or dullness) and finally auscultation (presence of breathing sounds and lateralisation) [20]. Clinically, PTX is characterised by mild to severe signs and symptoms of chest pain (due to rib fractures), dyspnoea, tachypnoea, cyanosis, tachycardia, hypotension, contralateral tracheal deviation and ipsilateral lung hyperresonance with diminished or absent breathing sounds (figure 5) [7]. To palpate for s.c. emphysema, crepitation from rib fractures, pain and thoracic cage instability combined with measurements of pulse rate, blood pressure and arterial oxygen saturation measurements are helpful [6, 21]. In particular, s.c. emphysema is a strong clinical predictor for concurrent PTX, although this sign is often absent despite injury (i.e. not found in 85% of patients with an actual PTX [22]). Various percussion techniques have also been used to diagnose PTX in mechanically ventilated patients in the intensive care unit [23]. The most reliable part of the physical examination is auscultation of the lungs, because clinical suspicion of PTX is reinforced by diminished or absent breath sounds on the affected side [21, 24]. The diagnostic accuracy improves further if the patient complains of respiratory distress and presents an increased respiratory rate [20]; however, auscultation is often difficult due to environmental noise (e.g. during air transport or in the emergency department [25]) and because unequal breath sounds can be caused by either air (PTX) or blood (haemothorax) within the pleural cavity. Moreover, the physical examination can be misleading, and diagnoses based solely on auscultation fail to detect up to 20–30% of PTX [4, 26]. Although a large PTX can sometimes be identified [26], others may be overlooked but can become clinically relevant [12, 27]. Furthermore, the fact that a high incidence of extrathoracic injuries is associated with blunt trauma makes the diagnosis of PTX even more challenging. Therefore, diagnostic adjuncts are needed as an extension of the physical examination.

Figure 5. Clinical manifestations of pneumothorax. Reproduced and modified from [1] with kind permission from N.P. Oveland.

Release of air

In some trauma patients, the expedited insertion of a chest tube is necessary. If there is release of air, a characteristic hiss can be heard and used to confirm the correct diagnosis of PTX. However, this invasive procedure is only to be used in physiologically nonpermissive patients who are too unstable to await further diagnostic imaging.

Chest radiography

The portable anteroposterior CXR is routinely obtained as the first radiograph in most trauma patients, and through the use of modern digital machines, these pictures are readily available for the medical team [28]. However, both lung fields must be examined carefully for the presence of PTX signs, which include a readily apparent visceral pleural line, depressed diaphragm and the deep sulcus sign (i.e. enlargement of the costophrenic angle) [29]. Furthermore, s.c. air in the soft tissue should not be overlooked, because this is a pathognomonic sign of lung and airway leakage [22]. Injured patients are often confined to the supine position (strapped to backboards) for neuroaxial protection, and the location of a PTX within the chest is directly associated with the force of gravity, the elastic recoil of the lung and the attachment of the lung to the hilar structures. The intrapleural air therefore collects anteromedially in the least-dependent pleural space [30–32]. Air trapped anteriorly and medially to the lung is particularly difficult to detect and quantify on supine CXRs, and even large air pockets may be overlooked at hospitalisation (figure 3) [33]. In one large prospective observational study of multiply injured patients, the incidence of occult PTX was 76% when the supine CXR was interpreted by the trauma team in the emergency department [22]. Most studies refer to board-certified radiologist dictations when reporting the proportions of PTX that are occult (30–55%) [27, 32, 34–37], although the time required to request and obtain the result of a CXR can be as long as 20 min [37]. Therefore, it is the initial interpretations made by the trauma team, and not the delayed dictations by the radiologist, that result in treatment decisions. The conventional supine anteroposterior CXR remains the most available but least sensitive of all plain radiographic techniques for diagnosing PTX [33] and evaluating its extension [38].

Computed tomography

A modern-generation CT scanner is an extremely valuable diagnostic tool with a high sensitivity for blunt aortic injury, great vessel injury, thoracic fractures, pulmonary contusions, haemothorax and PTX. From a multislice CT scan of the chest, coronal (figure 1), sagittal and transverse (figures 2 and 3) reconstructions of the thorax are possible, and CT imaging can therefore detect all types of PTX and provide a precise evaluation of intrapleural air [3, 28]. In fact, the awareness of occult PTXs began when intrapleural air was incidentally detected on the upper and lower parts of head and abdominal CT scans, respectively [39, 40]. At present, CT imaging is the reference diagnostic standard to safely rule out or diagnose PTX [28], although it remains disputed which patients should receive thoracic CT after blunt trauma. The value of diagnosing all occult thoracic injuries via a “whole-body” trauma scan is uncertain, because few of these additional abnormalities have demonstrated clinical importance [41]. Thus, the pendulum may have swung too far, and an emergent CT scan of the chest after obtaining a negative CXR should not be performed routinely, because this type of examination comes with a substantial cost. The radiation hazard is eminent [42–44], and the need for patient transportation to the radiology department and the time required to obtain such images can result in delayed diagnosis [37]. Indeed, timing is often critical for physiologically nonpermissive trauma patients, because CT scanning prolongs the time to lifesaving resuscitation (hence, the expression “tunnel of death”).

TUS to detect PTX

A point-of-care diagnostic test

An accurate method for diagnosing PTXs is needed. Over the last two decades, PoCUS has evolved from a modality used by only a select group of medical specialities to its current position encompassing a wide range of operator backgrounds and rapidly expanding clinical applications [45]. This has in large part been due to the increasing portability and image quality of machines combined with decreased cost [46]. Physicians from different specialities have shown that they can quickly become skilled in US for diagnostic and procedural applications relevant to their medical field. TUS is a true point-of-care diagnostic test that can be performed in almost any clinical setting. It is noninvasive, radiation free, rapid and repeatable at the bedside, and has been shown to be more sensitive than, and equally specific as, supine CXR to detect PTX [36, 37, 47–52].

Equipment

The current generation of machines are impressively lightweight and utilise sophisticated processors and imaging software to provide high-quality images, including multiple modes (e.g. brightness or B-mode, motion or M-mode, Doppler). They are often laptop sized (∼3–5 kg) or even truly pocket sized (∼0.1–0.5 kg), and importantly are designed for PoCUS use [46]. Sonographic diagnosis of PTX can be done using different machines with a variety of probes. The preferred transducers for imaging superficial structures (e.g. ribs, pleura) are linear (∼5–14 MHz), but microconvex or convex transducers (∼4–8 MHz) working at lower frequencies may also be used for imaging both superficial and deep structures [9, 53]. Both transducers fit between the ribs and can be “pushed” under the back of the supine patient to examine the posterior chest. Even a larger, low-frequency, curvilinear (∼2–5 MHz) probe may be used to look between multiple ribs. The conventional B-mode is adequate, but occasionally the M-mode setting can be helpful to distinguish movement and standstill at the pleural line [53].

The four sonographic signs of PTX

The paradox is that US imaging poorly visualises the lung parenchyma. Therefore, the conceptual basis for the US diagnosis of PTX is dynamic signs that originate from the pleural line (i.e. the adjacent two visceral and parietal pleural layers) [9, 54]. It is sufficient to combine movements and sonographic artefacts from the pleural line and study them in B-mode and M-mode to rapidly rule out or confirm PTX. The primary dynamic features are the lung sliding sign [55], B-line reverberation artefacts [56] and the pathognomonic lung point sign [57], all of which are synchronous with respiration, and finally lung pulsation transmitted from heartbeats [58].

Lung sliding

In B-mode, lung sliding is a dynamic sign that is visible as a slight and bright sparkling/twinkling/glittering/shimmering “to-and-fro” horizontal movement at the pleural line. The relative movement is between the lung’s surface covered by the visceral pleura towards the chest wall covered on the inside by the motionless parietal pleura. Lung sliding is more evident during active ventilation; it can be difficult to see with small tidal volumes, and may even be absent in apnoea.

Sometimes it is necessary to use M-mode scanning to evaluate subtle movements of the pleural line. If lung sliding is present, the US image has a granular appearance under the pleural line (resembling sand) and horizontal lines above the pleural line (resembling the horizon), and therefore this type of image is called the seashore pattern. Straight horizontal lines, called the stratosphere pattern, throughout the image indicate the lack of sliding (i.e. possible PTX) with air separating the pleural layers.

B-lines

In the normal lung, air inside the alveolar spaces hinders visualisation of the lung parenchyma, but if the interlobular septa start to fill up with fluid, this traps the US beam between elements with opposite acoustic impedance. Using B-mode, this creates a multiple reflection artefact call B-lines (i.e. a reverberation artefact) that has the visual appearance of echogenic lines or rays extending from the parietal pleura to the end of the screen without fading. The B-line artefact is dynamic, as it moves synchronously with respiration.

Lung point

In B-mode, this pathognomonic US sign of PTX is found by placing the US probe over the area where the collapsed lung is still in contact with the inside of the chest wall, as shown in figure 2. This location is called the lung point and is visible as two distinct patterns on the US screen: one suggestive of no lung sliding and/or B-lines where the parietal layers are separated by air (i.e. PTX), and one with normal lung sliding and/or B-lines where the pleural layers are still in contact. The lung point sign is dynamic, as lung sliding and/or B-lines intermittently replace motionless pleura during respiration.

In M-mode, the lung point appears as an interchanging seashore pattern and stratosphere pattern.

Lung pulse

In the absence of lung sliding, the heart may push the motionless lung and transmit vertical movements to the pleural line. In B-mode, these movements can be seen as small lung pulses synchronous with the cardiac rhythm. The heart beats transmitted through the lung can also be seen in M-mode as pulsatile seashore patterns on a motionless stratosphere background.

The scanning technique

TUS detection of PTX may appear complex due to the need to recognise or exclude these four signs, but the use of a step-by-step approach (figure 6) and a flow chart for decision making (figure 7) makes the procedure straightforward. Another simplifying factor is that intrapleural air tends to accumulate in the least-dependent parts of the chest due to gravity, and with the patient in the supine position this corresponds to the anterior and lateral parts of the chest on both sides [33, 59]. These anatomical locations are almost always accessible and easy to scan in both pre- and in-hospital patients, regardless of clinical condition and body habitus.

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