A 47-year-old man with COVID-19 pneumonia complicated by severe acute respiratory distress syndrome (ARDS) suddenly desaturates. The ventilator pressure alarm fires and further investigation reveals elevation of both the peak and plateau pressure. Physical exam reveals decreased breath sounds on the right side with associated lack of lack of lung sliding on ultrasound. Additionally, there is no lung pulse or vertical artifact present. Chest x-ray confirms the presence of a large right-sided pneumothorax. A 14-French pigtail catheter is placed to suction with evacuation of the pleural air. However, a large continuous air leak is present and the patient remains hypoxemic. What are you concerned about and what is the next step in management?   

Diagnosis of bronchopleural fistula  

A bronchopleural fistula (BPF) occurs when a communication forms between the pleural cavity and the bronchus causing a continuous air leak. BPF is most commonly observed in the post-operative setting following pneumonectomy (4.5-20%) or lobectomy (0.5-1%). While a continuous air leak can also arise from the lung parenchyma, this is not a true BPF, although it is often described with this terminology. The most common clinical settings for continuous alveolar-pleural air leaks are pulmonary trauma, spontaneous pneumothorax, ARDS or pneumonia. BPF is associated with increased mortality, particularly in patients that are mechanically ventilated for a prolonged duration or those that have a large air leak > 500 mL per breath.

The diagnosis of BPF is not always obvious and can manifest as subtle hypoxemia due to loss of PEEP or tidal volume or the presence of pneumomediastinum or subcutaneous emphysema. For patients on positive pressure ventilation, BPF typically leads to pneumothorax, necessitating drainage with tube thoracostomy. After evacuation of the pleural space, a continuous air leak must be present for greater than 24 hours to be considered a BPF. Ventilator waveforms enable rapid diagnosis of air leaks in mechanical ventilated patients where the expiratory limb of the volume tracing fails to return to the zero baseline (figure 1). It is important to distinguish an air leak in the pleural cavity from elsewhere in the ventilator circuit or endotracheal tube cuff. Localization of the leak necessitates close examination of both the patient and the junctions of the ventilatory circuit. Large air leaks can also lead to auto-trigger of the ventilator and high respiratory rates that must be identified by the clinician at the bedside after evaluating the frequency of the patient’s spontaneous muscular effort on physical exam.

 Figure 1. The volume versus time scalar (bottom) does not return to the zero baseline during expiration. Additionally, there is a 352 cc differential between the inhaled and exhaled tidal volume. 

Mechanical ventilation strategies to mitigate air leak

The ultimate goal in management of a BP fistula is to minimize the pressure and flow across the defect in the visceral pleura to allow time for healing. Flow through a BPF is determined by several variables, including the size of the injury, airway resistance and peak pressure as well as the alveolar distending pressure, or transpulmonary pressure. To calculate the transpulmonary pressure, both the pressure inside the alveolus (i.e., airway circuit pressure measured by the ventilatorr) and outside the alveolus or BPF (pleural pressure) must be known. When an esophageal pressure monitor is used to estimate esophageal pressure (Pes) as a surrogate for pleural pressure, the transpulmonary pressure (TPP), or the alveolar distending pressure, can be calculated by subtracting the pleural pressure (Ppl) from the airway circuit pressure. However, in the absence of an esophageal pressure monitor to directly measure pleural pressure, controlling peak inspiratory pressure through reduction of tidal volume and PEEP can reduce air leak through a BPF. Other strategies include lowering the inspiratory time and reducing the respiratory rate to lessen mechanical power or energy applied to the injured lung. This necessitates a strategy of permissive hypercapnia due to the reduction in minute ventilation. 

Independent lung ventilation is an advanced strategy that permits separate ventilation of the healthy lung in patients with large proximal defects preventing adequate oxygen and ventilation due to the size of the air leak. Independent lung ventilation requires placement of a double-lumen endotracheal tube to reduce the flow across the fistula in the affected lung. Double-lumen endotracheal tubes have two lumens and two cuffs (located in the trachea and bronchus). Positioning can be confirmed with bronchoscopy. This technique facilitates independent ventilation of each lung utilizing separate mechanical ventilators, either synchronized in respiratory rate via an external cable or providing asynchronous ventilation with different respiratory rates. Independent lung ventilation permits individual titration of tidal volume, inspiratory flow, PEEP and fraction of inspired oxygen to each lung, protecting the injured lung until the air leak resolves, or until the compliance and tidal volume nearly equalize. It is important to perform a best PEEP trial in the diseased lung first to avoid overdistention of the good lung and further shunting of the injured lung. Avoiding displacement of the carefully positioned endotracheal tube is crucial and frequently necessitates deep sedation and neuromuscular blockade. Additionally, prolonged use of a bronchial cuff may increase the risk of mucosal ischemia. Consultation of interventional pulmonology can be considered for possible endobronchial valve placement to seal off the injured airway while awaiting BPF closure.

Some patients with BPF may fail to ventilate or oxygenate despite these strategies of conventional mechanical ventilation. For patients with refractory hypoxemia or hypercapnia, venovenous extracorporeal membrane oxygenation (VV-ECMO) can be performed at specialized centers. Mechanical circulatory support provides additional gas exchange via an external membrane lung enabling further reduction in tidal volume and peak inspiratory pressure applied to the native injured lung.  

Returning to our patient, despite placement of an additional large-bore thoracostomy tube, the continuous air leak persisted for greater than 24 hours. A double-lumen endotracheal tube was inserted followed by synchronous independent lung ventilation, deep sedation and paralysis. After several days of independent lung ventilation, the air leak nearly resolved and the differential in lung compliance improved. Subsequently, the double-lumen endotracheal tube was exchanged for a single lumen tube and he was successfully extubated a few days later. 

References 

Sevransky JE et al. Bronchopleural fistula in the mechanically ventilated patient: A concise review. Critical Care Medicine 2021;49(2):292-301. 

Berg S et al. Independent lung ventilation: Implementation strategies and review of literature. World Journal of Critical Care Medicine 2019;8(4):49-58.