Differential Lung Ventilation
Special Feature
Differential Lung Ventilation
By Charles G. Durbin, Jr., MD, FCCM
Advances in respiratory therapy and mechanical ventilation have led to improved outcome from a variety of lung diseases. The mortality and morbidity associated with the acute respiratory distress syndrome (ARDS) continue to challenge clinicians and have led to the application of a "lung protective strategy" for mechanical ventilation in these patients. This approach derives from the concept that ARDS consists of inhomogeneous distribution of pathology (i.e., there are normal alveoli distributed throughout the lung, with the majority of alveoli unavailable for any gas exchange). Large tidal volumes will be distributed only to the normal alveoli, these alveoli will become overdistended and suffer barotrauma or, more properly, "volutrauma."
Likewise, positive end expiratory pressure (PEEP) will only be distributed to open or normal alveoli, increasing overdistension, causing barotrauma, and redistributing blood flow to collapsed areas of lung, thus, worsening gas exchange. By using small tidal volumes and limiting plateau pressures during mechanical ventilation, it is believed that iatrogenic lung damage may be minimized and an improved patient outcome will result.
Although this approach has not been unequivocally proven to be of benefit, the concept of inhomogeneity is supported by anatomic and pathologic evidence, and centers that embrace a lung protective strategy report improved survival from ARDS. What is apparent is that ARDS is not a homogeneous disease, and that some areas are more damaged than others. Application of a therapy to the entire lung will result in a balance between desired effects in the diseased areas and undesired effects in normal areas.
In addition to ARDS, most other pulmonary disorders are not uniformly distributed. Sometimes the inhomogeneity is so great that each lung may have grossly different compliance and gas exchange properties. In cases of lobar pneumonia this is readily apparent. Following single-lung transplantation, differences in compliance and function are expected and persist indefinitely. The population of patients with unilateral lung disorders is becoming larger. When patients with different lung compliances require mechanical ventilation and PEEP, pressure and volume therapy may be inappropriately distributed (i.e., the most compliant [most normal] lung will receive most of the pressure and volume, possibly sustaining volutrauma and redistributing blood flow to the "bad" lung).
Differential lung ventilation may help prevent this maldistribution of support and treatment. While initially an attractive concept, the risks and difficulties of differential lung ventilation are significant. In this review, the indications, available options, risks, and alternatives to lung isolation and differential ventilation will be discussed.
The experience in differential lung ventilation has been driven by advances in lung and aortic surgery. More than 50 years ago, differential lung spirometry was performed using double-lumen endobronchial tubes (DLET) made of stiff rubber with two latex cuffs (the Carlin’s tube with a left bronchial lumen and the White tube with a right bronchial lumen). Oxygen consumption by each lung was measured in awake subjects to determine if the patient could survive pneumonectomy. These original tubes, still available, have carinal hooks to facilitate correct placement, are difficult to insert through the larynx, and have high-pressure cuffs that would cause severe bronchial injury if left in place for an extended period. (See Figure 1.) Modern plastics and fiberoptic bronchoscopy have improved the success of placement and reduced complications of these original DLETs.
Single-Lung Anesthesia
Complicated open surgery of structures within the thorax (e.g., lungs and their associated large airways, esophagus, the chain of sympathetic nervous system ganglia, thoracic vertebrae, thoracic lymphatic system, and the thoracic portions of the great blood vessels) is facilitated by the technique of one-lung anesthesia. (See Table 1.) Thoracoscopic, video-directed procedures are becoming more common. Single-lung anesthesia is essential for success of these "minimally invasive" procedures. One lung is selected for isolation and collapse, while the other lung is ventilated, providing gas exchange for the patient. This approach produces excellent visualization of the thoracic structures and markedly reduces movement within the operated hemithorax. During partial or total pneumonectomy deflation of the resected lung allows careful, deliberate dissection and control of vessels and bronchi prior to clamping the specimen. During vascular procedures such as aortic aneurysmectomy, deflation of the lung protects it from severe bleeding and contusion, which may occur due to systemic anticoagulation.
| Table 1 |
| Procedures Facilitated by Single-Lung Anesthesia |
Open thorax lung surgery _________________________________________________PneumonectomyClosed thorax lung surgery |
Anesthetic concerns during one-lung ventilation are many. Besides the mechanical problems of lung separation, providing adequate oxygenation and CO2 excretion as well as maintaining appropriate anesthetic depth can be a challenge. Usually CO2 excretion can be maintained with one lung simply by maintaining a relatively normal minute ventilation of the single lung. This is probably best done by increasing respiratory rate and decreasing tidal volume, thus avoiding excessively high airway pressures.
If the patient has severe chronic obstructive pulmonary disease (COPD) or active bronchoconstriction, intrinsic PEEP may develop in the ventilated lung, and a slower respiratory rate with better exhalation may be preferable, accepting a rise in PaCO2. Intrinsic or added PEEP may significantly worsen oxygenation by directing blood from the ventilated lung to the totally collapsed lung, acting like a right to left shunt.
If oxygenation is a problem during single-lung ventilation on 100% O2, PEEP applied to the ventilated lung may improve gas exchange if more collapsed alveoli are recruited than blood redirected to the collapsed lung. This balance must be empirically evaluated. Since most patients requiring thoracic procedures do not have normal cardiopulmonary function, they often do not tolerate single-lung ventilation for the extended periods necessary for completion of the surgical procedure. PEEP or CPAP on the collapsed lung may redirect blood to the ventilated lung; however, this will inflate the operated lung and make surgery difficult. Use of 100% oxygen in the lung for several minutes prior to collapse will prevent oxygen desaturation for a while during lung collapse. Periodic reinflation may be necessary to prevent severe desaturations during the procedure.
A final solution to hypoxemia during single-lung ventilation is to have the surgeon occlude the pulmonary artery of the collapsed lung. This will direct all the pulmonary blood to the ventilated lung and allow optimal oxygen exchange. A pulmonary artery balloon-tipped catheter in the collapsed lung can achieve a similar effect on balloon inflation. However, this must be placed with the lung inflated and will only restrict some of the blood flow. At the conclusion of surgery the collapsed lung is re-expanded and two-lung ventilation is re-established.
Use of Double-Lumen Endotracheal Tubes
There are several versions of the DLET on the market. The most popular has no carinal hook and is constructed of the same nonreactive materials as regular endotracheal tubes (ET). This tube without a carinal hook is termed the Robertshaw design and is produced in a right- or left-sided model. DLETs are available in a variety of sizes, 28 French (Fr), 35 Fr, 37 Fr, 39 Fr, and 41 Fr. These tubes are about 42 cm long and are produced by several different manufacturers.
The choice of size is dependent on the patient’s anatomy and is crucial since a DLET too large will not fit into the mainstem bronchus and is difficult to pass through the glottis. A DLET too small requires an inordinate amount of cuff air to obtain a seal resulting in high pressures applied to the bronchial mucosa. Also, the smaller the DLET, the higher the resistance to airflow. For example, the airflow resistance of the endobronchial side of a 39 Fr DLET is about equivalent to a 7.0 mm internal diameter (ID) single-lumen ET, while a 35 Fr DLET is about equivalent to a 6.0 ID ET. The lumens of a DLET are not perfectly round, as they are in a single-lumen ET. The tracheal lumen is D-shaped, with the flat portion of the lumen abutting the shared wall with the endobronchial lumen. This imposes signif-icant limitations for the passage of suction catheters and fiberoptic bronchoscopes.
All disposable and reusable DLETs have their length marked in centimeters to aid in correct placement. As a first approximation, the average depth of insertion for both males and females 170 cm tall is 29 cm at the teeth. For each 10-cm increase or decrease in height, the placement depth will be increased or decreased approximately 1 cm. Besides the distance markings and tube size, each DLET has a radiopaque ring marker around the endobronchial lumen lip and one proximal to the bronchial cuff as well as a line running its length to aid visualization on chest roentgenograms.
Correct placement of the bronchial lumen is essential for success of the lung separation and to minimize complications. Single-use DLETs come disassembled in sterile packages and must be assembled and tested prior to use. The components are removed from their packages and placed on a clean surface. The Cobb adapter is an adapter that permits both lumens to attach to a single ventilation source, whether ventilator or manual device. The endobronchial and tracheal cuffs are tested for leaks and symmetry after inflation. The DLET is inserted with the cuffs deflated. A soft metal bronchial lumen stylette is packaged with the DLET and should be lubricated lightly with a non-petroleum-based lubricant to allow easy removal after the tube is placed. The tip of the DLET can be shaped to facilitate seating of the tube with this stylette.
All DLETs are bulky and stiff compared to their single-lumen counterparts. Their introduction through the glottis is considerably more difficult and great care must be taken to avoid damage to the DLET or the patient during intubation. Intubation of the trachea with a DLET is usually performed only in anesthetized patients with the aid of a neuromuscular blocking agent. With the patient’s head and neck in the ideal or "sniffing" position, the patient breathes 100% oxygen for several minutes to remove nitrogen from the lungs. This period of preoxygenation (more correctly called "denitrogenation") provides a margin of safety as insertion of the DLET may take several minutes.
The natural curvature of the endobronchial tip facilitates placement in the right or left mainstem bronchus as the DLET is advanced. The curve interferes with passage through the glottis. With the glottic opening in view, the tip of the endobronchial tube is inserted between the open vocal cords with the preformed curvature directed anteriorly. As the endobronchial cuff passes through the vocal cords, the DLET is rotated approximately 90-100° to align the curved tip with the orientation of the mainstem bronchus to be intubated. At this point, some clinicians halt and remove the stylette to allow the tip more flexibility and to possibly cause less damage rubbing the tracheal wall.
Contrary to this common misconception, tracheal damage is not caused by leaving the stylette in until the DLET is in its final position in the bronchus. When further advancement of the DLET is met with resistance, it is seated in the bronchus. The tracheal cuff is inflated to form a seal and the Cobb adapter with the two ET fittings is inserted into the proximal lumens. As the lungs are inflated, the chest is assessed visually, by auscultation, and by measurement of exhaled carbon.
The final step is the fine adjustment of the DLET to enable isolation of the lungs without unintentional obstruction of the airways. The endobronchial cuff is inflated and the chest is carefully auscultated bilaterally while the tracheal and endobronchial lumens are sequentially occluded with a clamp. When the DLET is correctly placed, obvious separation of breath sounds will be readily identifiable with clamping and unclamping of each lumen. Unfortunately, even when strict criteria are applied, between 38% and 83% of DLETs placed by auscultatory means alone are malpositioned when examined using a fiberoptic bron-choscope. Final assessment of correct DLET placement by fiberoptic bronchoscopy is mandatory.
Because movement of the DLET is possible whenever movement of the patient occurs, bronchoscopy should be repeated after any patient position change. Correct DLET placement is confirmed when the blue bronchial cuff is seen protruding slightly at the carina. A pediatric fiberoptic bronchoscope is passed through the tracheal lumen, confirming bronchial cannulation. Airway secretions are difficult to remove with these small diameter scopes. If a right-sided tube is selected, the orifice of the right upper lobe can be visualized through the Murphy eye of the bronchial lumen. Due to patient variations, it may not be possible to obtain unobstructed ventilation of the right upper lobe with right-sided tubes. For this reason, a left-sided tube should always be used if possible.
Tubes often move up or down the trachea when the patient’s position or head is moved, even when the tube is firmly anchored in the mouth. Frequent re-evaluation and repeated bronchoscopy are necessary to maintain lung separation.
Lung separation may be achieved in two other ways: bronchial blockers and bronchial intubation with a single-lumen ET. Neither of these is ideal as access to and manipulation of the unitubated lung is minimal. However, in emergencies, such as massive hemoptysis, advancement of the single-lumen ET into the uninvolved lung may be life saving by preventing contamination of the "good" lung while definitive treatment is initiated.
Independent Lung Ventilation (ILV) in the ICU
DLETs may be used to allow independent lung ventilation in ICU patients with unilateral lung disease. A list of conditions treated this way is presented in Table 2. Uncontrolled bronchopleural cutaneous fistulas (BPCF), unilateral ARDS, unilateral pneumonia, unilateral air trapping, and ARDS following single-lung transplant have been successfully treated with ILV. Almost all patients will require controlled mechanical ventilation, as initiating a breath through the DLET requires significant patient work. Neuromuscular blocking agents are almost always used to facilitate CMV and prevent patient movement that may cause tube displacement.
| Table 2 |
| Conditions Amenable to Independent Lung Ventilation__________ |
Unilateral pneumonia________________________________________________________________ |
When ILV is used, two ventilators are usually needed—one for each lung. Synchronizing the ventilators can be done by connecting them with a special cable and using one ventilator as a controller of the other. Most modern ventilators permit this technique, but how it is accomplished varies by manufacturer. Since ILV is infrequently performed in most institutions, the mechanical details of ventilator preparation should be written down and practiced in advance.
It may not be necessary to synchronize the ventilators. In hemodynamically stable patients, the mediastinal shifts that may occur with unsynchronized ILV may be well tolerated, making management more simple. This is particularly true when only CPAP is needed on one lung. Most patients will need conversion to single-lumen ET prior to weaning from mechanical ventilation.
Because of the risk of bronchial mucosal damage and bronchial stenosis, ILV should be performed for as short a period as possible, usually two to three days at most; however, longer use (30 days) without tracheal complications has been reported. The difficulty in secretion removal, inability to allow spontaneous ventilation, and need for sedation and paralysis makes ILV a short-term, life-saving modality rather than a reasonable elective choice for support. However, with this modality, in some patients with rapidly reversible unilateral lung disease who cannot be adequately managed with conventional MV, improved gas exchange and improved survival can be achieved.
Correction
An error appeared in the April 1999 issue of Critical Care Alert. The reference to the first abstract and commentary on page 1 was inadvertently omitted from the end of the first paragraph. The reference to the article summarized in "Variability of arterial PO2 in critically ill patients," is Tsai Y-H, et al. Intensive Care Med 1999;25:37-43. We regret any confusion this may have caused.
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