Management of blunt thoracic trauma

Learning objectives.

By reading this article, you should be able to:

• •Illustrate the general principles of management in blunt thoracic trauma.
• •Describe an initial strategy for the management of immediately life-threatening injuries in blunt thoracic trauma.
• •Define an initial strategy for management of potentially life-threatening injuries in blunt thoracic trauma.
• •Explain an approach for managing cardiac arrest in the patient with blunt thoracic trauma.

Key points.

• •A structured ‘C-ABCDE’ primary survey, emphasising teamwork and bedside imaging, identifies most life-threatening thoracic injuries.
• •Life-threatening injuries, such as tension pneumothorax, can present dramatically needing urgent treatment.
• •The CT trauma survey has added value in identifying life-threatening injuries that are difficult to diagnose clinically or with bedside imaging. However, it does add some risk caused by transfer and delays in operative treatment.
• •Potentially life-threatening injuries, such as blunt cardiac injury, might be identified in the secondary survey and require careful assessment, evaluation and multidisciplinary input.
• •Priorities in cardiac arrest resulting from blunt trauma include recognition and management of reversible causes, including airway control, resuscitation with fluids and bilateral pleural decompression.

Blunt thoracic trauma is more common than penetrating trauma and represents a significant burden for trauma services.¹ This article provides an overview for clinicians caring for patients with potentially life-threatening blunt thoracic trauma.

The thoracic zone for trauma is defined as the 12 ribs bound inferiorly by the diaphragm and superiorly by the clavicles. The structures within the thoracic zone can be injured by energy transmission from external forces with resulting compression; shearing; and, less commonly, blast injuries.² Cardiorespiratory function can be compromised rapidly. Consequently, systematic evaluation, resuscitation and stabilisation are critical for an optimal outcome.

Road traffic collisions are the predominant cause of major blunt injury, exceeding contributions from falls, violence and recreational injuries. In the UK, approximately 30,000 people per year are killed or seriously injured in road traffic collisions.³ Thoracic trauma is frequently accompanied by neurological and abdominal trauma. Patients who are elderly are especially vulnerable to blunt thoracic trauma given their inherent risk for pulmonary and multisystem deterioration. In specialised centres, mortality from blunt thoracic trauma appears to be decreasing, which may reflect an overall improvement in care and multidisciplinary approach to management of blunt thoracic trauma.⁴

General principles of management

The principles for managing blunt trauma remain similar to the original approach set out in the Advanced Trauma Life Support Manual.⁵

Teamwork

The importance of leadership, delegated roles, a collaborative approach and structured communication is an essential aspect of effective trauma care.⁶ The composition of the team may differ depending upon an institution's needs, and other services might be referred to as needed. Pre-hospital triage provides invaluable information as to the severity of trauma. High-risk mechanisms of injury for blunt thoracic trauma include⁵

• •Falls from >6 m
• •Motor vehicle collisions with partial or complete ejection, intrusion into the vehicle or death of a passenger in the same compartment of the vehicle
• •Collision with a pedestrian or bicyclist
• •Motorcycle collisions>32 kph (20 mph)

Primary and secondary surveys

Control of catastrophic haemorrhage followed by systematic assessment of the airway with cervical spine control, breathing and ventilation, circulation and haemorrhage control, disability and exposure (‘C-ABCDE’) is required to identify and immediately treat life-threatening injuries. Findings from the C-ABCDE assessment are communicated to the team leader to allow integration and planning of patient care. Any change in the patient's condition should prompt a re-evaluation. Once the patient is stable, a more detailed examination incorporating both clinical and radiological assessment is conducted to identify any other significant injuries.

Disposition

Planning where the patient will be transferred to for ongoing treatment (e.g. the operating theatre or ICU) is an essential role of trauma teams.

Initial imaging

Clinical examination lacks sensitivity for identifying many life-threatening thoracic injuries.⁷ Bedside radiological assessments with chest X-ray (CXR) and extended focused assessment with sonography for trauma (E-FAST) are essential investigations for all patients sustaining major blunt trauma. In general, more definitive information can be obtained with CT imaging. However, a patient should only be transferred to the CT scanner once the patient's condition has been stabilised.

Chest X-ray

A portable, supine, anterior–posterior (AP) CXR can usually be easily obtained in most trauma bays. CXR provides valuable diagnostic information on the presence of pneumothoraces and haemothoraces. However, not all pathology is easily identified with CXR. For example, the accumulation of >200 ml of blood into the chest is typically required before a haemothorax is visible with CXR. In contrast to an erect post-anterior CXR, supine imaging can mask the presence of a haemothorax, which may appear as a veiling opacity. Furthermore, the AP orientation can falsely suggest cardiomegaly. Non-specific findings (e.g. a globular heart and widened mediastinum) on the CXR should direct further assessment (i.e. echocardiography and CT aortogram, respectively).

Extended focused assessment with sonography for trauma

A complete evaluation of E-FAST is beyond the scope of this article, and readers are referred to a recent review in this journal.⁸ The E-FAST is a sonographic assessment that adds a thoracic component to the pericardial and abdominal survey. Bilateral lower and upper thorax views are included to assess for absent pleural sliding and fluid collections within the pleural space, suggesting pneumothorax and haemothorax, respectively. Trauma ultrasound requires specific training and experience to attain a safe level of skill and diagnostic accuracy. For pneumothorax, the benefits of E-FAST of being non-invasive, rapid, highly specific (>99%) and a learnable competency are hampered by limited sensitivity (69%) and interoperator variability.⁸ It should be used as a ‘rule-in’ test, and a negative study may require repeating in a deteriorating patient or evaluating with CT imaging. The benefit of E-FAST for evaluating pericardial collections using the subcostal window by trained operators is more clear-cut with a sensitivity and specificity >90%.⁹

Computerised tomography

A CT trauma survey overcomes the limitations of CXR and E-FAST and provides a detailed radiological secondary survey in major trauma.¹⁰ Computerised tomography is increasingly used, given its value in confirming injuries and for surgical planning.¹¹ Whole-body CT protocols in trauma usually consist of non-contrast brain and cervical spine imaging, followed by a split-bolus chest, abdomen and pelvic imaging, which provides both arterial and portal venous phase imaging. These protocols allow identifying and grading of organ injury, and demonstrating any active arterial bleeding that might be present. The shortfalls of the CT trauma survey include delay to surgical control, deterioration during transfer, potential for unnecessary intervention and radiation exposure.

Management of immediately life-threatening injuries

Tension pneumothorax

In a patient that is unstable, the diagnosis of tension pneumothorax remains largely clinical. However, the typical findings of absent breath sounds on the ipsilateral side and tracheal shift towards the contralateral side are easily missed during a busy resuscitation. Low arterial oxygen saturation, tachycardia and hypotension are easily attributed to other causes. Thus, a high index of suspicion for tension pneumothorax is essential. Patients who are awake frequently present with respiratory distress and ‘air hunger’, whereas patients who are anaesthetised and ventilated mechanically typically develop hypotension and have high inflation pressures. Sonography and CXR are helpful for diagnosis. Treatment involves immediate pleural decompression ideally with finger thoracostomy within the safe zone (Fig. 1), followed by placement of an intercostal chest drain. Connection of the chest drain to an underwater-sealed system on low-pressure suction (–20 mmHg; –2.6 kPa) allows for drainage of air and blood. In the current era where immediate bedside radiography is widely available, the adage that one should never see a tension pneumothorax on a CXR is outdated.

Fig 1.

Safe zone for intercostal drain insertion is formed anteriorly by the lateral margin of pectoralis major, posteriorly by the lateral border of latissimus dorsi, inferiorly by the fifth intercostal space and superiorly by the axilla.

For small, simple pneumothoraces, there is a move towards inserting smaller intercostal drains (20 Fr) via a Seldinger technique or even conservative management without drainage. This approach is an area of controversy.¹² Our approach is to place a drain in patients who are symptomatic or have moderate or large pneumothoraces. Small pneumothoraces, which are only visible on CT but not on CXR and are not associated with any cardiorespiratory compromise, are termed ‘occult pneumothoraces’. The frequent use of CT imaging in patients with trauma means that occult pneumothoraces are commonly diagnosed. The largest pocket of the pneumothorax should be measured as a radial distance between the parietal and visceral pleurae (or mediastinum) on an axial CT. A 35 mm cut-off (’<35 mm rule’) can be used to support conservative management without chest drainage with a high likelihood (90%) of successful resolution.¹³ If the patient is mechanically ventilated, there is a small risk that a simple pneumothorax may expand, although the risk of progression to a tension pneumothorax is very low (3%).¹⁴

Massive haemothorax

A massive haemothorax can accumulate rapidly, with the rate of accumulation reflecting the type or calibre of the vessel injured. Injury to a major or arterial vessel can result in adverse cardiovascular effects in as little as 5–10 min.

The characteristic clinical findings of reduced breath sounds on auscultation and dullness to percussion may be hard to appreciate in the resuscitation room. A bedside CXR and E-FAST study can be invaluable in identifying massive haemothorax. Immediate management involves inserting a larger bore (24 Fr gauge) intercostal chest drain and appropriate resuscitation with i. v. fluids or blood.¹²

Initial drainage of >1500 ml blood or ongoing output >200 ml h⁻¹ for 2–4 h should prompt surgical exploration to be considered. Although urgent thoracotomy in the setting of blunt trauma is associated with a nearly two-fold higher mortality than that for penetrating trauma, it should be performed in the operating theatre with minimal delay.¹⁵

An undrained haemothorax can become infected, with resultant empyema. When there is a large initial output from the drain (>700 ml), a second chest drain should be considered to aid in evacuating blood from the pleural space. If clearance is not achieved by 24 h, our approach is to undertake early video-assisted thoracoscopic surgery washout, rather than persisting with additional drainage attempts.

Flail chest

A major compressive force is required to disrupt the bony protection conferred to the thoracic viscera. A flail chest segment can result if three or more ribs are broken in two or more places (Fig. 2).¹⁶ A flail chest typically leads to respiratory failure attributable to a combination of altered lung mechanics, pain with resulting hypoventilation and underlying pulmonary contusion. The pathognomonic ‘seesaw’ paradoxical motion seen with unassisted respiration can be hard to appreciate in the resuscitation room.

Fig 2.

Computerised tomography three-dimensional reconstruction of a complex flail chest injury. The white box demonstrates the region of flail in the right chest. Note that ribs 4–6 are fractured in two places. Rib fractures are also evident on the left side.

Immediate management of a flail chest involves breathing support via positive airway pressure, which provides a degree of pneumatic stabilisation of the chest. Positive airway pressure may be initially via high-flow nasal oxygen (HFNO) or non-invasive ventilation (NIV), particularly CPAP. Close observation of the patient's response to HFNO and NIV is required. If there is evidence of worsening respiratory distress, hypoxaemia or hypoventilation, the patient's trachea should be intubated and mechanical ventilation initiated using a lung-protective strategy.

Effective analgesia is essential to help prevent the need for escalating respiratory support and for the patient's comfort. Established methods of providing analgesia combine regional and systemic approaches.⁷ Non-neuraxial approaches, such as paravertebral or erector spinae blocks, are increasingly used because of their favourable safety profile with a minimised risk of critical site bleeding and infection compared with epidural analgesia. Systemic analgesia includes paracetamol, NSAIDs (if there are no renal or gastrointestinal contraindications) and opioids. An infusion of ketamine (0.1–0.15 mg kg⁻¹ h⁻¹) may be beneficial, as ketamine avoids the respiratory depression associated with opioids.¹⁷ Additional information on analgesic approaches for rib fractures is available in a recent review in this journal.¹⁸

Surgical stabilisation of rib fractures (Fig. 3) is being increasingly used as it is associated with a reduced duration of mechanical ventilation and ICU length of stay.¹⁹ Our approach is to undertake surgical fixation as early as possible, ideally within 72 h after injury. The procedure may be performed by appropriately skilled cardiothoracic, general or orthopaedic surgeons.¹⁶

Fig 3.

Chest radiograph showing sternal and bilateral rib fixation. Subcutaneous emphysema is also evident.

Cardiac tamponade

Cardiac tamponade should always be suspected in patient who develops shock after thoracic trauma. A complete Beck's triad (hypotension, muffled heart sounds and high jugular venous pressure) is unlikely to be appreciated in the resuscitation room. ECG and arterial waveform monitoring may support the diagnosis, demonstrating low QRS voltages, variably sized QRS complexes (electrical alternans) and an exaggerated decrease in systolic pressure by >10 mmHg with inspiration in a spontaneous ventilating patient (pulsus paradoxus). If cardiac tamponade is suspected, sonographic assessment—initially via the subcostal view—with a phased array transducer is mandatory (Fig. 4). A hypoechoic (black) collection around the heart should prompt further evaluation in the apical and parasternal views, which confirms the diagnosis and helps to determine the optimal site for drainage. Immediate management involves stabilisation of the patient's haemodynamics with i. v. fluid or blood transfusion and drainage of the pericardial collection. Although needle pericardiocentesis may be attempted in extremis (and may temporarily restore cardiac output), the procedure is unreliable and time-consuming. Therefore, we advocate for urgent transfer to the operating theatre to facilitate surgical drainage and repair of any cardiac injury.

Fig 4.

Subcostal image showing a four-chamber view of the heart obtained as part of an E-FAST examination. A large circumferential fluid collection can be seen around the heart, consistent with pericardial tamponade.

Management of cardiac arrest

Cardiac arrest in trauma can be defined as the absence of palpable pulses and an unrecordable BP in an unresponsive patient. The most common cardiac rhythm is pulseless electrical activity, although malignant ventricular arrhythmias can also occur, particularly in the setting of blunt trauma to the heart.²⁰

Priorities are recognition of the arrest and prompt management of potentially reversible causes. Where possible, sources of haemorrhage should be controlled and i. v. fluids and blood transfusion should be given.^21,22 The patient's airway should be secured with a tracheal tube and mechanical ventilation initiated. Both pleurae should be decompressed to exclude tension pneumothorax. Sonographic examination should be performed to exclude pericardial tamponade. If cardiac arrest is not reversed within 10 min, resuscitative thoracotomy should be considered.

Resuscitative thoracotomy is a skilled procedure, requiring shared decision-making within the trauma team. Institutional approaches vary. The rationale for resuscitative thoracotomy is to provide immediate, maximal anatomical exposure via a clam-shell incision to control haemorrhagic or obstructive shock. Thoracotomy facilitates direct assessment of a cardiac source of bleeding, evacuation of pericardial collections and access for clamping the descending thoracic aorta to support diastolic coronary perfusion and to prevent distal arterial blood loss. Resuscitative thoracotomy has a more clearly defined role in penetrating trauma; mortality is eight-fold higher when the procedure is performed for blunt thoracic trauma compared with penetrating trauma.¹⁵ In our opinion, resuscitative thoracotomy does have a role in the setting of witnessed cardiovascular collapse in blunt trauma of no longer than 10 min duration, as per the European Resuscitation Council guidelines. After the return of spontaneous circulation, massive transfusion, analgesia and anaesthesia should be provided safely. Significant morbidity is acknowledged, as are the risks to the trauma team (specifically needlestick injury). For more information on the role of resuscitative thoracotomy, readers are referred to a recent review in this journal.²³ External chest compressions are generally ineffective in cardiac arrest caused by hypovolaemia or obstructive shock and carry the risk of further visceral injury, particularly from rib fracture fragments.^24,25

Management of potentially life-threatening injuries

Pulmonary contusion

In the resuscitation phase, pulmonary contusions are not usually apparent on clinical examination or CXR. The presence of rib fractures suggests the potential for an underlying lung contusion. A CT trauma series may suggest contusion with opacification identified at sites consistent with the injury. The differential diagnosis includes pulmonary haemorrhage and aspiration.

Principles of early management involve supporting oxygenation with the provision of supplementary oxygen. High-flow nasal oxygen is increasingly used, as it allows ready titration of the fraction of inspired oxygen, humidification with improved comfort for the patient and the application of a small amount of PEEP. HFNO is contraindicated if there is concomitant neurological trauma with a depressed level of consciousness, base of skull fracture or an inability to cough. In addition to pulmonary contusion, there is an increasing acceptance of using HFNO in patients with a simple pneumothorax and in the presence of an air leak.^12,26 Respiratory care bundles are helpful in patients with contusions and commonly include patient positioning (i.e. sitting upright rather than lying in bed), early ambulation, chest physiotherapy, optimisation of analgesia and vigilance for delayed respiratory failure. Fluid overload should be avoided.

Tracheobronchial injury

Although rare, tracheobronchial injuries are associated with a high mortality. Clinical signs may not correlate with the extent of the injury. A tracheobronchial injury should be considered when a persistent air leak or subcutaneous emphysema does not improve with pleural drainage. Escaping air tracks along tissue planes manifesting as subcutaneous emphysema, pneumothorax or pneumomediastinum. The posterior membranous trachea is the most common site of disruption, and most injuries occur close to the carina, creating difficulties with airway management.²⁷ The airway must be secured distal to the injury. Management techniques include awake intubation via the open defect, bronchoscopic facilitated intubation with a smaller-than-usual sized tracheal tube or insertion of a tracheostomy below the defect. A partial airway disruption can be converted into a complete transection if excessive force is used when attempting to secure the airway. If available, cardiopulmonary bypass or venovenous extracorporeal membrane oxygenation (VV-ECMO) can be life-saving. Once the airway is secure, definitive surgical repair is required.

Bronchopleural fistula

Bronchopleural fistula is defined as a persistent air leak resulting from a connection between the pleural cavity and bronchus. The condition may only become apparent after the initiation of mechanical ventilation, with the development of subcutaneous emphysema, loss of expired tidal volume and impaired ventilation. If a chest tube is in place, there is typically persistent bubbling in the underwater seal.

The severity of the air leak determines the management strategy. A massive air leak with an inability to ventilate or oxygenate the patient requires immediate lung isolation. In an emergency, one-lung ventilation can be achieved by advancing a standard single-lumen tracheal tube into the contralateral bronchus under direct bronchoscopic guidance. More definitive lung isolation is achieved by placement of a double-lumen tracheal tube or a bronchial blocker. Once lung isolation has been achieved, multiple large-bore intercostal drains should be inserted to facilitate drainage of the pneumothorax. Difficulties with mechanical ventilation should be anticipated. Helpful strategies include using low tidal volume (4–6 ml kg⁻¹), decreased inspiratory time and ventilatory frequency, using the lowest possible PEEP and permissive hypercapnia. Efforts should be made to wean the patient to a spontaneous ventilation mode to minimise airway pressure. Suction to the underwater sealed drain(s) is best avoided to minimise the air leak. Bronchoscopy and CT imaging should be performed to identify the source of the bronchial injury. Early discussion with a thoracic surgeon is appropriate. Broad-spectrum antibiotics should be administered to help prevent pleural infection. Occasionally, VV-ECMO is required to stabilise the patient and facilitate healing of the injured airway.

Blunt cardiac injury

Blunt cardiac injury (BCI) encompasses a range of presentations, including:

• •Myocardial contusion, with regional or global cardiac dysfunction
• •Conduction abnormalities, which range from bundle branch block to rapidly fatal entities, such as commotio cordis
• •Coronary artery injury
• •Valvular injury
• •Pericardial tears
• •Cardiac rupture

Blunt cardiac injuries are under-appreciated and may account for up to 25% of trauma mortality, with many patients dying at the scene.²⁸ Mechanisms of injury include direct myocardial compression between the sternum and spine, impaired cardiac conduction secondary to impact and avulsion of the great vessels. The anteriorly positioned right ventricle and its associated structures are at the greatest risk of injury. Screening tools for BCI include a troponin concentration and an ECG, looking for ventricular ectopy, ST segment changes and bundle branch block.²⁹ Transthoracic echocardiography is a useful non-invasive screening tool. However, transoesophageal echocardiography (TOE) may be required to definitively exclude valvular injury. Oesophageal injury is an absolute contraindication to TOE.

Blunt cardiac injury should always be considered in the patient who is deteriorating despite treatment, especially if the patient has sustained an impact to the left hemithorax. Most cases can be managed conservatively (e.g. myocardial contusion), but occasionally definitive intervention is required, for instance, in the case of valvular disruption.

Vascular injury

Blunt vascular injuries are associated with high-speed deceleration forces. Shearing injury or avulsion at the ligamentum arteriosum, just distal to the left subclavian artery, is an important site for traumatic injury to the aorta (Fig. 5). Most patients with severe vascular disruption die at the scene. Clinical examination is not sensitive for vascular injury but may reveal chest contusion from steering wheel impact and differences in upper vs lower limb perfusion. Chest X-ray may demonstrate a widened mediastinum, an apical pleural cap and loss of the aorto-pulmonary window. Lack of these findings does not rule out traumatic aortic injury.

Fig 5.

Sagittal CT contrast arteriogram demonstrating a traumatic aortic injury at the level of the aortic isthmus.

The investigation of choice is a CT contrast arteriogram. However, in an unstable patient who has proceeded directly to the operating theatre without a CT, TOE can be used to assess the pericardium, aortic valve, aortic root and descending thoracic aorta. The routine use of high-resolution CT imaging after trauma has improved the detection of lower-grade blunt aortic injuries, such as intimal tears and flaps. Many of these injuries are stable, and repair can proceed in a planned fashion within a few days. Some vascular injuries can be managed conservatively. Vascular surgical review is mandatory to assess the risk of progression and to guide the urgency of intervention. Endovascular techniques are now the accepted approach to repair, as they avoid the extensive risks associated with open surgery, which include stroke, spinal cord ischaemia and death.³⁰ The goal of preoperative haemodynamic management is to reduce shear stresses within the vascular wall by maintaining the systolic BP <120 mmHg and the HR at 60–90 beats min⁻¹.^31,32

Other important vascular injuries include damage to the internal thoracic and subclavian arteries. Bleeding from an injured internal thoracic arteries is often only apparent once the patient is resuscitated, as these vessels frequently spasm immediately after the injury. Subclavian artery injury should be considered when there is a fracture involving the first, second or third rib. Both injuries are best evaluated with a contrast CT arteriogram. An endovascular approach to repair is usually undertaken.

Oesophageal injury

Oesophageal injury is extremely rare. As such, investigating potential oesophageal injuries in the acute setting is an impractical distraction. Clinical signs are non-specific (haemoptysis, haematemesis, hoarseness, odynophagia and subcutaneous emphysema) and are usually attributable to major airway or pulmonary injury. Pneumomediastinum might be seen on CXR or CT imaging but is not diagnostic. Definitive assessment with a contrast oesophageal swallow or oesophagoscopy may be done once the patient is stable. Initial management involves preventing mediastinal contamination with endoscopy-facilitated gastric tube insertion, administration of broad-spectrum antibiotics and pleural drainage. Definitive treatment may require surgical repair.³³

Diaphragmatic injury

Diaphragmatic injury is unusual with blunt thoracic trauma. Energy transfer to the abdominal cavity can increase abdominal pressure dramatically, causing diaphragmatic rupture. The most common site of injury is the left hemidiaphragm.³⁴ Clinical presentation is often non-specific. Respiratory distress from the intrathoracic herniation of abdominal contents may occur. A CXR typically demonstrates a raised hemidiaphragm or presence of bowel gas in the chest, which may be mistaken for a pneumothorax (Fig. 6). Computerised tomography imaging with multiplanar reconstruction is appropriate if the diagnosis is suspected. Further evaluation with sonography can be performed once the patient is stable. Other diagnostic methods include MRI, contrast fluoroscopy and video-assisted thoracoscopy in a spontaneously ventilating patient.³⁵ Surgical repair is appropriate in most cases.

Fig 6.

Upper abdominal radiograph demonstrating a left-sided diaphragmatic hernia with loops of bowel visible in the chest cavity.

Summary

Blunt thoracic trauma is a common injury pattern especially after high-energy impacts, such as road traffic collisions. Energy transfer from the chest wall to the underlying lungs, heart and vascular and mediastinal structures can result in a number of immediately life-threatening conditions. Imaging modalities, including bedside CXR, E-FAST, echocardiography and trauma CT series, have led to more rapid and accurate identification of other serious thoracic injuries. Because of the potential for thoracic trauma to cause airway, breathing and circulation problems, it is essential that anaesthetists and other critical care doctors appreciate the key injury patterns and management principles for those conditions.

Acknowledgements

The authors would like to thank Professor Sean Galvin for his helpful comments and sharing of clinical images for this article.

Biographies

Ravi Mistry FANZCA FCICM is a consultant intensivist at Gold Coast University Hospital. His major clinical interests are intensive care for the cardiothoracic patient and echocardiography.

James E. Moore FANZCA FCICM is a consultant intensivist and cardiothoracic anaesthetist at Wellington Regional Hospital. He is the clinical lead for New Zealand's Central Region Trauma Network, national medical advisor for St John Ambulance and is a senior research fellow at the Medical Research Institute of New Zealand. His major research interest is in trauma-induced coagulopathy.

Matrix codes: 1B04, 2A02, 3A10

Declaration of interests

The authors declare that they have no conflicts of interest.

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