Medical Management and Therapy of Bronchopleural Fistulas in the Mechanically Ventilated Patient: CHEST TUBE MANAGEMENT

9 May

Patients with chest trauma, adult respiratory dis­tress syndrome (ARDS)-related barotrauma, and pa­tients undergoing invasive chest procedures, including thoracotomy and central line placement, are general categories of patients in whom the potential for the development of a BPF exists and who are fre­quently encountered by the critical care specialist. A chest tube placed to manage a BPF in these patients can be both helpful and detrimental and may play a role far more important than that of a passive conduit (Table 1). Air leaks in these settings range from <1 to 16 L/min and a chest tube capable of permitting prompt and efficient drainage of this level of air flow is necessary. Gas moving through a tube does so in laminar fashion and is governed by Poiseuilles Law. In the clinical situation, the gas moving through a chest tube is likely moist and therefore subject to turbulent flow and governed by the Fanning equation. Therefore, both the length (1) and, more importantly, the radius (r) are important when choosing a chest tube and connecting tubing to adequately evacuate a BPF (flow varies exponentially to the fifth power of the radius of the tube). The smallest internal diameter that will allow a maximum flow rate of 15.1 L/min at —10 cm H20 suction is 6 mm, with a No. 32 F chest tube having an internal diameter of 9 mm. Hence, a chest tube with an adequate diameter to convey the poten­tially large air flow of a BPF must be taken into consideration when managing a BPF. A chest tube too small in diameter can lead to lung collapse and tension pneumothorax in the setting of a mobile mediastinum.

Not only can the chest tube be used to drain the pleural accumulation of air due to a BPF, but it can also be used to limit the air leak in certain circum­stances. Past management of a BPF in mechanically ventilated patients on positive end-expiratory pressure (PEEP) has included the application of intrapleural pressure equivalent to the level of PEEP during the expiratory phase of ventilation. With positive intra­pleural pressure applied through the chest tube, an air leak persists during the inspiratory phase of venti­lation but decreases during the expiratory phase of ventilation and allows maintenance of PEEP. This application of intrapleural positive pressure may facil­itate resolution of the BPF and allow maintenance of PEEP in patients in whom it is a requisite for oxygenation. A similar use of the chest tube to control leakage of air through a BPF involves synchronized closure of the chest tube during the inspiratory phase of mechanical ventilation. It has been suggested that a combined use of the above two methods may be applicable when there is significant BPF air leak during both the inspiratory and expiratory phase of mechanical ventilation, and it has been used suc­cessfully in two patients. These techniques pose potential hazards, including developing increased pneumothorax and tension pneumothorax. Con­sequently, close monitoring of the patient is required when such chest tube manipulations are used.
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Table 1—Chest Tube in Bronchopleural Fistulas (BPF)
• Large diameter chest tube for
• High flow BPF
• Drainage of infected pleural space
• Therapy (ventilated patient)
• Positive intrapleural pressure (expiratory phase)
• Chest tube occlusion during inspiratory phase
• Combination of above
• Application of sclerosing agent to pleural space
• Negative effects of chest tube (ventilated patient)
• Loss of tidal volume
• Altered partial pressure of C02 and 02
• Ventilator cycling

The chest tube may also be used as a vehicle to apply various sclerosing agents to the pleural space to promote pleural symphysis. Numerous reports using different sclerosing agents in the setting of recurrent and spontaneous pneumothorax have appeared. The combined use of a thoracoscope and sclerosis has recently been reported to be advantageous in the management of spontaneous pneumothoraces. Ap­plication of a sclerosing agent in the setting of a persistent air leak (in effect a BPF) in patients with a spontaneous pneumothorax and in animal studies resulted in control of the air leak and resolution of the pneumothorax. Whether sclerosing agents in the set­ting of other BPFs can also be successful remains unproven.

In addition to its beneficial effects, the chest tube may also be associated with adverse effects in patients with BPF. The gas escaping through the chest tube in patients with a BPF represents part of the minute ventilation delivered to the ventilated patient and makes maintenance of an effective tidal volume diffi­cult. Maintaining a specific level of ventilation, and hence Pco2 control, is not affected solely by the amount of gas (tidal volume) escaping through the fistula. The escaping gas does not passively flow from the airways into the BPF, but it is involved in physiologic gas exchange. Approximately 25 per­cent of the minute ventilation has been found to escape via a BPF in a group of patients with ARDS with more than 20 percent of the C02 excretion occurring by this route in about half of the patients. The involvement of a BPF in active gas exchange is complex with proposed mechanisms of C02 exchange, including drainage of gas from alveoli in the area of the BPF and removal of gas from more remote alveolar areas by pressure gradients created by the BPF.

Carbon dioxide excretion and lost minute ventila­tion have also been demonstrated, but to a lesser degree, in a small group of primarily trauma victims with BPF. The trauma group demonstrated variable C02 excretion and loss of minute ventilation that appeared temporally dynamic and dependent on the level of chest tube suction. The difference in C02 excretion between patients with ARDS and patients with trauma with BPF was attributed to the use of different ventilators and variability in lung compli­ance. The difference may also depend on the path­ogenesis of the BPF, further making prediction of the level of C02 excretion via a BPF more difficult. In addition, a BPF not only affects COz excretion but also 02 utilization. The presence of a BPF generally decreases the utilization of inspired oxygen before it escapes through the fistula. This relationship, however, is variable and requires consideration in patients with oxygenation problems in the setting of a BPF.
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The chest tube in the setting of a BPF may have other detrimental effects. The negative pressure ap­plied to the chest tube may be transmitted beyond the pleural space and into the airways creating inap­propriate cycling of the ventilator. The increased flow through a BPF that can occur with increased negative chest tube pressure may interfere with closure and healing of the fistula site. Hence, some authors advocate the use of the least amount of chest tube suction as possible in patients with BPF. Low levels of chest tube suction may, however, predispose to increasing pneumothorax or tension pneumothorax. Finally, as with any foreign body, the chest tube is a potential source of infection both at the insertion site and within the pleural space.