Inhalation Injury

Abstract

Inhalation injury is a critical component of thermal injury that can significantly increase mortality in burn survivors. This poses significant challenges to managing these patients and profoundly impacts patient outcomes. This comprehensive literature review delves into the epidemiology, pathophysiology, diagnosis, classification, management, and outcomes of inhalation injury with burns.

Epidemiology

When a burn patient's mechanism involves flame in an enclosed space, there is a high incidence of inhalation injury. Pathophysiology involves both thermal injury and toxic inhalants resulting in damage to the respiratory tract and systemic response. This diagnosis not only results in morbidity but also can significantly increase mortality.

The modified Baux score is a calculation that incorporates burn size, age, and inhalation injury to predict prognosis. According to this score, inhalation injury is comparable to a 17% total body surface area burn or adding 17 years to the survivor's age. ¹

Classification of inhalation injury includes severity and anatomical description. The ability to stratify these injuries can result in timely interventions, including securing the airway, oxygen supplementation, and mechanical ventilation, which are crucial for patient management.

This review will highlight a transdisciplinary approach to these injuries and treatment strategies that allow optimization in a multifaceted fashion.

Classification

Main classifications address the anatomical areas affected and severity of the injury. The Abbreviated Injury Scale uses visualization by bronchoscopy. The components of this scale include the degree of erythema, edema, bronchorrhea, and amount of bronchial obstruction. At the most severe end of this scale is mucosal sloughing, necrosis, and luminal occlusion. A grade 0 to 1 injury incurs an 84% in-hospital survival while a grade 2 to 4 injury results in 57% in-hospital survival. ²

Although there is an application for airway injury within the revised Baux score, it does not play a role in the calculation for initial resuscitation. The goal is not to overresuscitate as this will result in increased edema in the tracheobronchial tree and the lung parenchyma.

Identifying the injury's location in the airway is important in understanding the insult's pathophysiology.

Pathophysiology

The pathophysiology of inhalation injury is multifactorial and complex. There is a thermal and a chemical component in inhalation injury. The context of the injury along with the environment are important in describing the likelihood of injury and the possibility of specific treatment modalities with certain inhaled toxins.

Thermal airway injuries result in proteins being denatured and direct damage to the mucosa. Inflammation and edema increase with production of fibrinous exudate that forms mucus plugs. ^{2 3} Moist air from steam and chemicals has a higher heat carrying capacity and may lead to more devastating injury. In general, the upper airway and oropharynx are the most affected by thermal injury.

Chemicals in the smoke can lead to an initial increase in bronchial microcirculation. With this increase in blood flow, there is an increase in vascular permeability to proteins and plasma. This permeability can result in transudate and shedding of the bronchial columnar epithelium which may plug the bronchial tree and cause obstruction leading to poor ventilation. Another complication of this obstruction is shunting pressures to other alveoli, therefore increasing their risk of barotrauma. With chemicals such as carbon monoxide, they tend to have deleterious effects on a patient's ability to oxygenate secondary to their increased affinity to hemoglobin. This affinity can be up to 200 times greater than oxygen and causes a left shift in the oxygen hemoglobin dissociation curve, preventing hemoglobin from releasing oxygen to the tissues. ⁴ With the inability of the cardiac muscle as well as brain tissue to receive oxygen, this leads to complications in patients that undergo this type of hypoxia. Cyanide is another chemical in fires that is known to cause complications. This chemical reversibly binds to ferric ions in the cytochrome oxidase within mitochondria, thus halting cellular respiration by preventing oxygen reduction. ⁵

Another component of airway injury secondary to burns is aspiration. Patients may have mucus in the back of their pharynx, there may be attempts at intubation, or various medications and circumstances surrounding their injury that may lead to emesis. With emesis, there is a risk that the gastric contents can be aspirated into the airway. The injury that eventually develops from this situation does not necessarily demonstrate itself on a chest X-ray within the first 24 to 48 hours. Obviously, aspirating gastric content can lead to pneumonia, one of the most common complications among burn patients. A patient who has an inhalation injury has a two- to fourfold greater risk of pneumonia than a burn patient who has no airway injury. ⁶

Any or all of these injuries can lead to an increase in pneumonia and increase in acute respiratory distress syndrome (ARDS) which significantly increase morbidity and mortality among burn patients.

Preclinical studies demonstrated the critical role of airway blood flow increases following inhalation injury. ^{7 8} It has been shown that bronchial blood flow increases up to 20% following inhalation injury, ^{8 9} resulting in airway wall edema that contributes to airway lumen narrowing in addition to airway obstruction and bronchospasm. The airway lumen narrowing is the major initial factor for acute hypoxemia after inhalation injury. It has been reported that ablation of bronchial blood flow by its ligation or the pharmaceutical agent (nebulized epinephrine) prevented increases in airway blood flow, reduced airway mucosal edema, and attenuated pulmonary edema. ^{5 10 11} A substantial portion of the bronchial circulation empties into pulmonary circulation. In normal conditions, bronchial blood flow is about 1% of cardiac output. However, a substantial increase in airway blood flow following inhalation injury leads to an increase in pulmonary congestion aggravating the parenchymal edema. However, the role of airway dysfunction, especially the impact of airway blood flow increase following inhalation injury is often neglected thus leaving this detrimental complication without treatment. It is also important to keep in mind, a fact that increases in hydrostatic pressure in airway vasculature due to increased flow lead to the extensive airway exudate which contains various inflammatory mediators as well as procoagulation factors that promote fibrin formation in the airways.

Diagnosis

The diagnosis of inhalation injury requires a systematic approach and the integration of clinical, radiological, and laboratory findings. Early signs and symptoms, such as singed nasal hair, hoarseness, and cough with carbonaceous sputum, may indicate the possibility of inhalation injury.

Imaging studies, including chest X-rays and computed tomography scans, may reveal signs of airway injury and pulmonary complications. Flexible bronchoscopy is a valuable diagnostic tool to directly visualize the airway and assess the extent of injury. Additionally, blood gas analysis, carboxyhemoglobin levels, and biomarkers such as surfactant protein-D may aid in assessing the severity and progression of inhalation injury. ¹² Despite large amount of clinical and basic science research studies being published, there is no consensus on the diagnostic criteria for inhalation injury. It differs between the burn centers and hospitals. However, the diagnosis of inhalation injury should be guided by the deep understanding of the pathophysiology of inhalation injury, and it should consider serious complications such as pneumonia. ^{5 13}

When a burn patient presents for initial assessment, a primary survey is paramount. Examining whether a patient can maintain their airway is an initial concern. The decision to secure an airway for a burn patient is based on clinical judgment and experience. There used to be an urgency to intubate patients who had burns on their face or singed hair with mild hoarseness, but these sequelae are not sensitive nor are they specific for inhalation injury. ^{11 13} If the burn patient has full-thickness burns on their face, dyspnea, or altered mental status, these would lead to a higher inclination to secure the patient's airway. Providers should have a higher suspicion for inhalation injury in circumstances such as a house fire, an enclosed space, or forest fire where they were trapped in the middle of the blaze. ¹³

Tachypnea is not an uncommon symptom in a burn patient and is expected most of the time with large burns. Dyspnea though, is not from a burn injury and should be investigated further. The goal is to provide adequate oxygenation and ventilation and support the patient with continuous monitoring.

All burn patients with suspected inhalation injury should be evaluated with flexible bronchoscopy or nasopharyngoscopy. This allows a provider to classify the injury and serves as a therapeutic intervention by clearing the soot and mucus plugs that may occur.

Additionally, arterial/venous blood gas analysis including carboxyhemoglobin and partial pressure of oxygen should be performed as early as possible. The latest technologies allowing noninvasive measurement of carboxyhemoglobin using pulse CO-oximeter can be used at the site of injury for early diagnosis of inhalation injury. ¹²

Management

Inhalation injury is a significant factor in morbidity and mortality among burn patients. Much of the management for inhalation injury is supportive and preventing further injury to the airway.

In the prehospital setting, evaluating patients who need intubation is critical. About one-third of all burn patients who are intubated end up being extubated promptly within the first 24 hours which suggests that intubation may not have been indicated. ¹⁴ Strong indications for intubation include stridor, dyspnea, upper airway trauma, altered mentation, hemodynamic instability, or full-thickness facial burns. ¹⁵ Initial title volume after intubation for these patients should be 6 to 8 mL/kg based on ideal body weight. A PaO2 range of 80 to 100 mm Hg is preferable but ranges down to 65 mm Hg are tolerable. Although there is no strong data as to a specific ventilatory mode decreasing morbidity or mortality in inhalation injury, the goal is low tidal volume ventilation.

It is also imperative that these patients have a pH higher than 7.2. Securing an endotracheal tube can be particularly challenging in a burn patient who has full-thickness burns to the face, therefore use of umbilical cotton ties or septal ties is advised. Less cuff pressure in the endotracheal tube may also be used secondary to tracheal edema. The cuff pressure should be monitored frequently. The goal of this pressure is less than 20 mm Hg to avoid damage to the mucosa and to avoid decreasing perfusion of the trachea. Tracheostomies among burn patients tend to remain controversial. ¹⁵

Prompt and safe extubation is especially important for burn patients. Guidelines for extubation include airway patency, the neurological status, their ability to take a deep breath, chest compliance, and oxygen requirements. These patients should have a leak test performed prior to extubation. Rapid shallow breathing index, negative inspiratory force, and chest X-ray should be reviewed prior to extubation. Extubating to high-flow nasal cannula or noninvasive ventilation is a good option not only for the extra positive pressure, but it also provides warm humidified air. It is crucial that these patients after extubation are given incentive spirometers, encouraged to expectorate, and continue good pulmonary hygiene.

Pharmacologic therapy used in these patients consists of nebulized agents such as mucolytics and bronchodilators. Aerosolized heparin (5,000–10,000 units in 3 mL of saline) every 4 hours, 20% N-acetylcysteine every 4 hours, and nebulized bronchodilators every 4 hours are used for up to 7 days after identification of the inhalation injury. ¹⁵ For bronchodilation, nebulized albuterol is used. However, nebulized epinephrine may be considered instead as the epinephrine can exert double effects, that is, bronchodilation via beta-2 agonist and reducing the airway blood flow and edema via its alpha-1 agonist properties. In a small randomized clinical trial (involving 16 patients; 8 in each group), nebulized epinephrine has been shown to be safe in pediatric burn patients and tended to reduce ventilator days, intensive care unit (ICU) days, and albuterol dose as well as improved physical endurance. ¹⁴ In a preclinical study, nebulized epinephrine had superior effects to the nebulized albuterol in ameliorating the severity of acute lung injury induced by inhalation injury. ¹⁶

Elevating the head of bed at least 30 degrees as aspiration precautions as well as continuing to ensure the patients are nourished are also important components of their recovery.

Burn patients who have severe ARDS are placed in a prone position and lung protective therapy. If these patients are unable to be oxygenated, medical adjuncts such as Heliox can be used as a bridge to extracorporeal membrane oxygenation (ECMO).

Airway pressure release ventilation (APRV) is a ventilation technique introduced in the 1980s. It is characterized as a time-triggered, pressure-limited, and time-cycled ventilation mode. Adopted by some burn centers for patients with inhalation injury, APRV features two levels of airway pressure over two time phases. It is often described as providing continuous positive airway pressure that intermittently cycles to a lower airway pressure. The active exhalation valve in APRV allows spontaneous breathing throughout the respiratory cycle, enabling the ventilator to achieve a higher mean airway pressure at lower tidal volumes. This facilitates optimal recruitment and maintenance of alveoli. Despite studies showing reduced ventilator days and ICU days, no significant improvement in mortality has been observed compared to conventional modes. ¹⁷ The primary advantage of APRV lies in its ability to enhance ventilation-perfusion matching, particularly when patients breathe spontaneously.

High-frequency oscillatory ventilation (HFOV), initially developed for neonates, became a rescue therapy for adults with ARDS. HFOV utilizes an oscillatory pump to create active inhalation and exhalation around a defined mean airway pressure. This approach minimizes tidal volumes, reducing volutrauma, while sustaining elevated mean airway pressures to enhance alveolar recruitment and oxygenation. However, limitations include the need for sedation or neuromuscular blockade, potential cardiac preload reduction, limited availability of oscillatory ventilators, and the risk of mean airway pressure loss with circuit disruptions. Recent meta-analyses failed to demonstrate mortality improvement in early moderate-to-severe ARDS with HFOV compared to conventional ventilation strategies. ¹⁸ Additionally, HFOV may not be beneficial in ARDS patients with inhalation injury.

Airway clearance is crucial in inhalation injury patients. Therapeutic coughing is employed to prevent airway obstruction, atelectasis, and pneumonia. Chest physiotherapy involving percussion and vibrations assists in gravity-assisted drainage of the airway. Early ambulation, nasotracheal suctioning, and bronchoscopy are additional techniques used when needed.

There should be a low threshold for repeat therapeutic bronchoscopy in these patients. Bronchoalveolar lavage does not only serve as a diagnostic adjunct for pneumonia, it also acts as a therapeutic intervention to remove mucus and other exudate that may become colonized with bacteria.

Pulmonary support after extubation should include early mobility. In patients that can tolerate it, they may be placed in a sitting position or even moved from the bed to a chair while intubated as long as safety measures are in place. ¹⁹ Nutrition, early mobility, and effective pulmonary hygiene are the three keys of early extubation and decrease morbidity and mortality from inhalation injury.

Future focus should target novel approach for management of inhalation injury-induced severe ARDS including regenerative therapy, ECMO, and lung transplantation.

Outcomes

The prognosis of inhalation injury is influenced by several factors, including the extent of the burn and inhalation injury, the presence of comorbidities, the timely initiation of treatment, and the availability of specialized burn care facilities.

Severe inhalation injury is associated with increased mortality rates and a higher risk of developing respiratory complications and organ dysfunction. Long-term outcomes may include pulmonary complications, impaired lung function, and psychological sequelae in survivors. ¹⁵

Conclusion

Inhalation injury remains a significant challenge in burn care, affecting patient outcomes and survival. A thorough understanding of its epidemiology, pathophysiology, diagnosis, classification, management, and outcomes is essential for providing timely and appropriate interventions.

Continued research efforts are necessary to identify novel therapeutic approaches, improve diagnostic tools, and enhance the overall management of patients with inhalation injury. A collaborative and evidence-based approach among clinicians and researchers is crucial to reducing the burden of this devastating condition on patients and health care systems.