Lung Isolation Anesthesia

Anatomy and Physiology

Comprehensive knowledge of normal and abnormal tracheal and bronchial anatomy is necessary when performing lung isolation. The trachea is a fibromuscular tube with an internal diameter ranging from 15 to 25 mm in men and 10 to 21 mm in women. The trachea is supported anteriorly by 16 to 20 incomplete cartilaginous rings and posteriorly by a strip of longitudinal smooth muscle. These landmarks aid in orientation during bronchoscopy. The trachea divides sharply at the carina, giving rise to the right and left mainstem bronchi. Because the right mainstem trajectory closely parallels the trachea, right mainstem intubation is common when an endotracheal tube is inserted blindly beyond the carina.

Beyond the mainstem bronchi, the airway divides into secondary (lobar) and tertiary (segmental) bronchial segments. There are three lobar bronchi on the right (upper, middle, and lower) and two on the left (upper and lower). Differentiating between the left and right mainstem bronchi using bronchoscopy is critical to ensure the correct placement of double-lumen endotracheal tubes or bronchial blockers. The right upper lobar bronchus branches from the right mainstem bronchus approximately 15 to 20 mm from the carina. In comparison, the left mainstem bronchus does not divide into upper and lower lobar segments until 45 to 50 mm beyond the carina.[1] The right upper lobar bronchus's characteristic trifurcation further distinguishes the opening of the right upper lobe into the apical, posterior, and anterior segments, resembling a cloverleaf.[2] However, variations from this classic description are common. Approximately 22% of individuals do not have an apical segmental bronchus in the right upper lobe, resulting in a bifurcation rather than a trifurcation on bronchoscopy.

An accessory cardiac bronchus is present in approximately 0.08% of individuals and appears as an additional branch of the right mainstem bronchus, typically opposite the right upper lobe bronchus. The cardiac bronchus is usually nonfunctional, tapering into rudimentary bronchioles. Although generally asymptomatic, retained secretions in a cardiac bronchus may become a nidus for infection in certain circumstances. Variations in bronchial anatomy can complicate lung isolation techniques, leading to disorientation during bronchoscopy or inadequate ventilation. 

Another somewhat more common abnormality that may complicate lung isolation is the presence of a tracheal bronchus. A left-sided tracheal bronchus is present in approximately 0.3% to 1% of the population, while a right-sided tracheal bronchus is present in 0.1% to 2%. Most abnormal tracheal bronchi branch within 2 cm of the carina, but some may branch 6 cm or more proximally. Tracheal bronchi may be displaced or supernumerary, with displacement being more common. About 0.2% of the population has a right upper lobe bronchus originating directly from the trachea, a configuration referred to as a pig bronchus, resembling the typical anatomical arrangement in swine. Tracheal bronchi can complicate lung isolation by preventing the total collapse of the operative lung or causing hypoxia due to inadequate ventilation of the tracheal bronchus. If a left-sided double-lumen endotracheal tube is placed in a patient with a pig bronchus for left-sided thoracic surgery, the right-sided tracheal bronchus is not ventilated either under or proximal to the tracheal cuff. Although uncommon, this could be a potential cause of unanticipated hypoxia during one-lung ventilation. If a bronchial blocker is positioned in the right mainstem in a patient with an unrecognized pig bronchus, right lung deflation may be inadequate due to continued ventilation of the right upper lobe.[3][4]

Physiologically, under normal conditions, both lungs receive air for ventilation and blood flow for perfusion, facilitated by gravitational forces. During one-lung ventilation, ventilation is exclusively directed to one lung, potentially increasing hypoxemia due to right-to-left shunting, theoretically reaching up to 50%. However, in practice, the shunt fraction is typically less severe due to mitigating factors such as patient positioning, surgical manipulation of the nonventilated lung, and the physiological response of hypoxic pulmonary vasoconstriction.

In the lateral decubitus position, common for thoracic surgeries, the nonoperated lung is the dependent lung and receives more perfusion. In addition, this position increases the elastance of the ventilated lung due to the pressure exerted by the weight of the contralateral hemithorax and the chest wall. Furthermore, surgical manipulation of the nonventilated lung reduces blood flow, directing more blood to the ventilated lung. Lastly, hypoxic pulmonary vasoconstriction is a critical regulatory mechanism, constricting blood vessels in hypoxic lung regions and redirecting blood to better-oxygenated areas, thereby optimizing oxygenation despite the challenges posed by one-lung ventilation.[5][6]

Indications

Surgical and nonsurgical indications for lung isolation can be absolute or relative. One-lung ventilation facilitates various surgical procedures involving thoracic approaches to anatomical structures. The management of severe unilateral lung disease with either differential ventilation or one-lung ventilation may also necessitate lung isolation. Some disease states, such as severe unilateral infection, require definitive anatomical separation of the lungs to prevent contamination of the non-diseased lung. Other disease states only need physiological separation to enable specific ventilatory parameters on each side.[7]

Surgical Indications for Lung Isolation and One-Lung Ventilation

Thoracic:

• Lung resection
• Video-assisted thoracoscopic surgery
• Lung transplantation
• Thoracic diaphragmatic hernia repair
• Pleurodesis or pleurectomy
• Esophagectomy

Cardiac:

• Minimally invasive cardiac surgery
• Pericardiectomy
• Surgeries involving the thoracic aorta

 Neurological:

• Thoracic sympathectomy
• Anterior approach to the thoracic spine

Pathological Indications for Lung Isolation

Unilateral disease states requiring anatomical lung isolation:

• Pathology requiring whole lung lavage (eg, pulmonary alveolar proteinosis)
• Excessive infectious or noninfectious secretions
• Pulmonary hemorrhage

Unilateral disease states requiring physiological lung isolation and differential ventilation:

• Parenchymal disease or injury
• Bronchopleural fistula
• Complications following thoracic surgery
• Unilateral bronchospasm

Contraindications

Relative contraindications to lung isolation can be categorized as factors related to the procedure and patient factors.

Factors Related to Double-Lumen Endotracheal Tube

• Placing and positioning a double-lumen endotracheal tube in a patient with a difficult airway may be challenging or impossible. Repeated forceful attempts to intubate increase the likelihood of airway injury. More significantly, loss of the airway during attempted placement in a patient with a difficult airway may contribute to dangerous levels of hypoxia. 
• Using a double-lumen endotracheal tube through a tracheostomy or stoma may increase the risk of iatrogenic injury.
• Placing a double-lumen endotracheal tube can damage intraluminal tumors, potentially leading to bleeding, tumor embolization, or occlusion.[8] 

Patient Factors

One-lung ventilation should only be used in patients who can tolerate the resulting alterations in cardiopulmonary physiology. Several patient factors, as mentioned below, can increase the risk of severe hypoxia, which, if persistent, will require reinitiating ventilation to both lungs. 

• Patients with lung disease resulting in hypoxia during two-lung ventilation with a FiO₂ of 100% are unlikely to be able to tolerate significant periods of one-lung ventilation.
• Hypoxia is more likely in patients who have had a previous contralateral lobectomy, resulting in a greater than 25% decrease in lung function on that side. 
• Morbid obesity may also increase the risk of hypoxia during one-lung ventilation. 
• One-lung ventilation should be used with caution in patients with pulmonary hypertension, as relative hypoxia and hypercarbia can exacerbate preexisting pulmonary hypertension. In severe cases, this may precipitate right heart failure and hemodynamic collapse. A recent clinical study showed that a high ratio of minute ventilation to carbon dioxide output, as seen in pulmonary hypertension and ventilation-perfusion (V/Q) mismatch, predicts hypoxia and requires risk stratification.

Surgery on the right lung, which is larger than the left, may also be associated with an increased risk of hypoxia during one-lung ventilation. However, this surgery is generally well-tolerated in patients with relatively healthy lungs, without significant or prolonged periods of hypoxia.[9][10][11]

Equipment

Depending on the clinical scenario, lung isolation is achieved using a single-lumen endotracheal tube, double-lumen endotracheal tube, or bronchial blockers. Fiberoptic bronchoscopy is typically used to guide and confirm the placement and positioning of these tubes, which is vital for clinicians performing lung isolation to know the correct placement, positioning, and troubleshooting techniques. Additionally, choosing the appropriate bronchoscope diameter is critical to prevent complications, such as the need to remove the entire double-lumen endotracheal tube, reintubation, or the inability to advance the bronchial blockers due to an inappropriately sized bronchoscope. 

Single-Lumen Endotracheal Tube 

To achieve lung isolation, a single-lumen endotracheal tube may be advanced into the left or right mainstem bronchus. This maneuver is typically easier on the right side, given that the trajectory of the right main bronchus closely parallels that of the trachea. A single-lumen endotracheal tube to achieve lung isolation is usually reserved for pediatric populations, as no double-lumen endotracheal tubes are manufactured for very small children. In emergencies, such as the sudden development of left-sided pneumothorax or pulmonary hemorrhage in a previously intubated patient, a single-lumen endotracheal tube can be advanced into the right main bronchus to achieve lung isolation.

Double-Lumen Endotracheal Tube 

A correctly positioned double-lumen endotracheal tube has the bronchial lumen in the mainstem bronchus with the bronchial cuff inflated so the carina is not herniated. The tracheal lumen should open to the opposite side, enabling selective ventilation of each lung, depending on which lumen is clamped. Both right- and left-sided double-lumen endotracheal tubes are available. However, due to the early branch point of the right upper lobe bronchus, a right-sided double-lumen endotracheal tube has a second opening on the bronchial lumen to allow ventilation of the right upper lobe.

The correct position of the bronchial cuff ensures that the bronchus is not occluded, so left-sided double-lumen endotracheal tubes are generally preferred. However, a right-sided double-lumen endotracheal tube is indicated for a left pneumonectomy or any other procedure that involves the proximal left main bronchus, which would preclude the placement of a left-sided double-lumen endotracheal tube. Both right- and left-sided tubes are available from several manufacturers, ranging in size from 26 to 41 Fr.

A connector is essential for blocking the lumen or selectively ventilating bilaterally. The connector piece is composed of a Y-shaped piece with 2 openings—1 for connection to the bronchial lumen and 1 for the tracheal lumen, with a common portion that fits into the ventilator circuit. The connector usually has color-coded parts for the tracheal and bronchial lumens. Manufacturers universally use blue to denote the endobronchial-lumen cuff. The cuffs are colored similarly to the connector tubes to maintain uniformity in identification, ensuring lung collapse and ventilation on the correct side. A fiberoptic bronchoscope is mandatory when placing a double-lumen endotracheal tube because accurate positioning is essential for achieving a good seal and lung isolation. To avoid malpositioned tubes, the tube position is checked after the patient is in the final position for the procedure.

Bronchial Blockers

Several different bronchial blockers are available. Each includes an inflatable balloon at the end of an introducer that can be advanced through a standard endotracheal tube or an endotracheal tube with a small separate lumen. The bronchial blocker can be advanced into either mainstem bronchus under fiberoptic guidance to provide lung isolation. Additionally, the bronchial blocker can be advanced into a more distal bronchus to isolate a particular lung segment.[12]

Device Selection

Several factors influence the choice of the airway device. If definitive anatomic lung isolation is required to prevent the spread of purulence from an infected to a noninfected lung, a double-lumen endotracheal tube is typically utilized as it is less likely to be dislodged when correctly positioned compared to a bronchial blocker. This also allows for suctioning or lavage of the diseased lung through the appropriate lumen, which is impossible with a bronchial blocker. In emergency scenarios, such as the development of unilateral pulmonary hemorrhage in a patient intubated with a single-lumen tube, advancing the tube into the unaffected lung may be the best choice. This approach is also utilized in small children whose airways cannot accommodate the relatively large double-lumen endotracheal tubes.[13][14]

Purported advantages of double-lumen endotracheal tubes include increased ease and speed of placement, better deflation of the nonventilated lung, and less frequent need for repositioning. In a study of patients undergoing video-assisted thoracoscopic surgery for esophagectomy, patients were randomized to either bronchial blockers or double-lumen endotracheal tubes. While the time to complete lung collapse was longer in the bronchial blocker group, no significant difference is apparent in the time required to position the devices. Additionally, postoperative hoarseness and sore throat were more frequent in the double-lumen study group. A similar study of patients undergoing video-assisted thoracoscopic surgery, randomized to an endotracheal tube with a small separate lumen containing a bronchial blocker or a double-lumen endotracheal tube, demonstrated superior and more rapid lung collapse using the bronchial blockers. In this study, surgeons were blinded to the lung isolation device utilized and unable to distinguish between them.[15][16]

A new generation of double-lumen endotracheal tubes with an integrated high-resolution camera allows real-time video guidance during placement. This device reduces intubation time and confirms tube placement after surgical positioning. Although this device is more expensive, it eliminates the need for fiberoptic bronchoscopy.[17][18]

The patient with a preexisting tracheostomy presents a challenge for lung isolation. If the tracheostomy is less than 7 days old, removal is contraindicated since the stoma may close during the procedure. In this case, a cuffed tracheostomy tube should be used as a bronchial blocker conduit. Most clinicians prefer this technique, even for mature stomas, because of its simplicity and reduced risk of iatrogenic airway trauma. Other alternatives include introducing a single-lumen tube through the tracheostomy stoma and directing it through the desired bronchus, or removing the tracheostomy tube and placing the double-lumen endotracheal tube orally.[19]

Personnel

During residency training, anesthesiologists have traditionally learned to place double-lumen endotracheal tubes and bronchial blockers on surgical patients under the direction and supervision of experienced faculty. More recently, simulators have become available for training in advanced airway techniques, including fiberoptic bronchoscopy, allowing trainees to hone their skills outside the high-stakes environment of the operating room. Current Accreditation Council for Graduate Medical Education (ACGME) requirements include 20 noncardiac intrathoracic cases, but there is no specific requirement for lung isolation. 

Because inexperienced clinicians attempting lung isolation with either a double-lumen endotracheal tube or bronchial blocker have increased rates of device malposition and complications, a deliberate practice model may be utilized to augment initial training and maintain expertise. A recent study showed that novices could acquire procedural proficiency on a mannequin with 90 minutes of training using either video didactics or a simulator. The study also found that proficiency declined 2 months after training, emphasizing the importance of continued practice in maintaining lung isolation airway skills.[13][20]

Preparation

Clinical Considerations 

Patients undergoing single-lung ventilation usually have underlying pulmonary disease and should be comprehensively evaluated for their primary condition before surgical procedures. Echocardiography may be helpful in patients with cor pulmonale and provide information about baseline cardiac function and reserve. Reviewing the relevant radiological anatomy can help plan anesthetic management for one-lung ventilation and prepare patients who require specialized airway management. Decreased baseline function due to large effusions, consolidations, and atelectasis predisposes patients to hypoxemia during the procedure, which may necessitate the use of higher fractions of inspired oxygen.

The presence of any bullae on the nonoperative lung may indicate an increased risk of developing a pneumothorax in the perioperative period. Evaluating tumors for paraneoplastic syndromes is crucial, as it can influence anesthetic management. Implementation of standardized protocols has significantly improved adherence to lung protective strategies during one-lung ventilation.[21][22][23] Special consideration should be given to patient age as an independent risk factor for complications in those undergoing pulmonary resection with one-lung ventilation. Older patients experience increased morbidity and mortality from these procedures. Additional testing is essential for patients undergoing lung resections to assess risks beyond age alone. The most common tests for this assessment are forced expiratory volume in 1 second (FEV1) and diffusing capacity of the lungs for carbon monoxide (DLCO). FEV1 serves as a predictor of postoperative complications, including mortality following pulmonary resection. Several studies indicate that a preoperative FEV1 that is less than 60% predicted is the most robust predictor of postoperative complications. Therefore, a cutoff of 60% for FEV1 and DLCO is utilized to assess postoperative risk.[24][25][26] 

The current American College of Chest Physicians' guidelines do not specify numerical cutoffs for DLCO below which pulmonary resections should be avoided in patients. Instead, they highlight the significance of calculating predicted postoperative values. Assessment of these predicted values is crucial in determining the feasibility of single-lung ventilation and predicting surgical outcomes. The interpretation of postoperative values is as follows:

• Patients with predicted postoperative values of FEV1 and DLCO greater than 60% do not need further testing to undergo pulmonary resection. 
• Patients with predicted postoperative values of FEV1 or DLCO between 30% and 60% should undergo additional testing with stair climbing or a shuttle walk test.
• Patients should undergo cardiopulmonary exercise testing with additional measurement of maximal oxygen consumption if both FEV1 and DLCO values are predicted to be less than 30%.

The cutoff for the stair climb test is 22 meters. For the Incremental Shuttle Walk test, a distance greater than 400 meters indicates a maximal oxygen uptake (VO₂ max) of 15 mL/kg/min or higher. This information can assist clinicians in assessing procedural risks and predicting better clinical outcomes for patients undergoing one-lung ventilation.

Equipment Preparation

Proper equipment preparation is crucial for the placement of double-lumen endotracheal tubes and bronchial blockers tailored to patient-specific factors such as gender, height, and detailed radiological assessments of the trachea and bronchi. For double-lumen endotracheal placement, it is essential to have a range of sizes available, single-lumen tubes as backups, a fiberoptic bronchoscope, antifog solutions, lubricating gels, and a tube exchanger. 

A lack of uniformity and objective guidelines is apparent when selecting double-lumen tube sizes, thereby reducing the incidence of complications. Tube size recommendations are often determined by gender and height. For men who are 165 cm or shorter, a 39 Fr tube is recommended, while those taller than 165 cm may require a 41 Fr tube. Similarly, women who are 160 cm or less in height typically use a 35 Fr tube, whereas those taller than 160 cm may benefit from a 37 Fr tube. Radiological measurement of the trachea's diameter or left mainstem bronchial diameter from a computed tomographic (CT) scan can aid in selecting an appropriate size for a left-sided double-lumen endotracheal tube. Hannallah et al concluded that when the choice of size was based on clinical judgment, tubes tended to be too large relative to the patient's airway.

Bronchial blocker placement requires the clinician's preferred device, a single-lumen tube of appropriate diameter, and a fiberoptic bronchoscope equipped with antifog solution and lubricating gel. Anesthesia induction and mask ventilation verification precede the administration of paralysis to aid placement. The most experienced clinician should secure the airway and achieve lung isolation, especially for patients unlikely to tolerate prolonged hypoxia or hypercarbia (eg, severe pulmonary hypertension). Bronchodilators are recommended for patients with suspected reactive airways, and soft-suction catheters should be used to clear excess secretions and promote oxygenation.[27][28][29]

Intensive Care Unit and Emergency Considerations

If lung isolation is needed urgently or electively in the ICU setting, the approach to preparation should be similar to that described for surgical procedures. However, in emergent situations such as significant unilateral pulmonary hemorrhage, the priority is to achieve lung isolation promptly and effectively. For instance, if the patient is in an ICU located far from operating rooms, using a double-lumen endotracheal tube might be too time-consuming. In such cases, a single-lumen tube can swiftly advance into the unaffected bronchus to achieve lung isolation.

Technique or Treatment

Double Lumen Endotracheal Tube Placement

The most common method of tube placement is direct laryngoscopy, followed by fiberoptic bronchoscopy to confirm positioning. Video laryngoscopy can be used to place a double-lumen endotracheal tube in a patient with a difficult airway. However, video laryngoscopes typically leave less room in the oropharyngeal cavity to maneuver, making placement cumbersome after visualizing the vocal cords. During direct or video laryngoscopy, the tube is introduced with a rigid stylet with the endobronchial curvature facing anteriorly. Once past the vocal cords, the rigid stylet should be removed. The double-lumen endotracheal tube should then be rotated 90° clockwise or counterclockwise for right- or left-sided placement, respectively, and advanced until resistance is met. 

After the double-lumen endotracheal tube is positioned, the tracheal and bronchial cuffs should be inflated, the connector attached, and ventilation initiated. Bilateral chest rise and breath sounds should be observed before checking for correct endobronchial placement. Appropriate placement can then be confirmed using auscultation of the chest during selective clamping of the bronchial and tracheal lumens. When using a left-sided tube and the tracheal lumen is clamped, the clinician should hear breath sounds only on the left side. If the bronchial lumen is clamped, the clinician should hear breath sounds only on the right side. When using a right-sided double-lumen endotracheal tube and the tracheal lumen is clamped, the clinician should hear breath sounds only on the right side. When using a right-sided double-lumen endotracheal tube and the tracheal lumen is clamped, the clinician should hear breath sounds only on the right side. If the bronchial lumen is clamped, the clinician should hear breath sounds only on the left side.

Fiberoptic bronchoscopy is typically preferred to confirm correct endobronchial placement. Examination with fiberoptic bronchoscopy can also confirm that the inflated bronchial cuff is visible but not positioned so proximally that the carina might herniate, an error that would not be detected on auscultation. Some clinicians advocate using fiberoptic bronchoscopy to guide the tube into the correct bronchus during direct laryngoscopy. However, this typically requires a second clinician.[30][31]

Another technique, particularly useful in patients with a difficult airway, is to intubate with a single-lumen tube and then use an exchanger to guide the placement. The tube exchanger should be at least 83 cm long and slender enough to accommodate a single lumen of the double-lumen endotracheal tube. Most tube exchangers have a central hollow lumen that can be connected to an oxygen source with an adaptor or used as a conduit for a fiberoptic bronchoscope.

Bronchial Blockers Placement

Available bronchial blockers are dissimilar in design and require different placement techniques. 

The different bronchial blockers and techniques for placement are:

• Wire-guided device: This involves lassoing the blocker to a fiberoptic bronchoscope, which is then directed into the desired bronchus.

• Bronchial blockers with an incorporated stylet: This is inserted through a standard single-lumen tube, allowing good control of the catheter tip, which can then be easily directed to block a selective lobar bronchus. The stylet is removed after placement, and the 1.4 mm lumen is used as a suction port for rapid deflation of lobes or oxygen insufflation to apply continuous positive airway pressure to the deflated lobes. 

• Single-lumen tube with an introducer port through which the bronchial blocker is inserted: This blocker's balloon tip is flexed to be more easily directed into the desired bronchus under fiberoptic guidance.

• Bronchial blocker with a unique bifurcated distal end with 2 separate balloons: This is designed to allow the placement of the cuffs in the left and right bronchi. The desired bronchus is identified using fiberoptic bronchoscopy, and the appropriate cuff is inflated.

Some clinicians have reported success in placing the bronchial blockers adjacent to the endotracheal tube rather than within. This technique may be advantageous in pediatric patients requiring small endotracheal tubes. Bronchial blockers are usually inserted via a single-lumen tube, but placing them through a laryngeal mask airway is an option in patients with a difficult airway.[8][32][33][34]

Complications

Anatomical Complications

Because of its larger caliber than a single-lumen tube, a double-lumen tube is more likely to cause trauma to the larynx, trachea, or bronchi. Iatrogenic tracheobronchial ruptures are rare but potentially devastating complications of double-lumen endotracheal tube placement. Women, older adults, patients of small stature, patients taking corticosteroids, and those with certain tracheal pathologies, including tracheomalacia and congenital anomalies, are more likely to experience these injuries. Postoperative hoarseness and sore throat are more common after double-lumen endotracheal tube placement than bronchial blocker placement.[20][35]

Physiological Complications

During single-lung ventilation, ventilation to 1 lung is interrupted, but perfusion continues in the nonventilated lung. This leads to an intrapulmonary shunt, resulting in wasted perfusion to the nonventilated lung. Protective mechanisms like hypoxic pulmonary vasoconstriction can counteract hypoxia to a certain degree. However, the anesthesiologist must be prepared to manage hypoxemia that may arise during single-lung ventilation. Ventilation and perfusion (V/Q) matching significantly impact oxygenation in patients on single-lung ventilation. Some authors note that oxygenation is significantly better in the lateral decubitus position than in the supine position.[36] 

The fraction of inspired oxygen (FiO₂) of 1.0 has been advocated while performing single-lung ventilation. The rationale behind using a higher FiO₂ is to have a safety margin. Higher FiO₂ also leads to vasodilatation, which may help increase the blood to the ventilated lung. However, oxygenation at FiO₂ of 1.0 can lead to atelectasis, so initiating with a FiO₂ less than 1.0 is advisable and increasing if needed.

If hypoxia develops during the performance of single-lung ventilation, the following steps must take place:

1. The position of the double-lumen, endobronchial, or bronchial blocker should be checked. Changes in position may occur due to surgical manipulation. A repeat fiberoptic bronchoscopy through the tracheal lumen is helpful in clinching the diagnosis. Additional steps involve suctioning the tube lumens to clear secretions that may contribute to hypoxia.
2. FiO₂ is increased to 1.0 to improve the amount of oxygen delivered.
3. Recruitment maneuvers are used on the ventilated lung, which is in the dependent position, to overcome any atelectasis and thus help with oxygenation. Positive end-expiratory pressure (PEEP) may be applied to this lung to eliminate atelectasis, decrease the shunt, and improve oxygenation.
4. Continuous positive airway pressure may be applied to the operative lung to decrease shunting and improve oxygenation. However, this makes the surgical procedure challenging for the surgeon and should only be an option when other measures have not improved the hypoxia.
5. If the hypoxemia is severe and does not resolve with the abovementioned steps, the next best step is to revert to two-lung ventilation. 
6. Severe hypoxemia should alert the anesthesiologist to look for causes such as pneumothorax on the dependent lung. Patients with chronic chronic obstructive pulmonary disease (COPD) are more likely to experience such a complication. The intraoperative development of pneumothorax mandates aborting the surgical procedure and the immediate insertion of a chest tube on the side of the pneumothorax. 

Using standardized protocol has increased adherence to lung-protective strategies.[23]

Clinical Significance

Hypoxic Pulmonary Vasoconstriction 

Lung isolation and one-lung ventilation result in a de facto pulmonary shunt, as blood continues to flow through the pulmonary vasculature of the nonventilated lung. An admixture of the oxygenated and deoxygenated blood from the ventilated and nonventilated lungs causes a decrease in systemic PaO₂ for a given FiO₂. This reduction in PaO₂ is mitigated by hypoxic pulmonary vasoconstriction, which is the pulmonary vasculature constriction in response to decreased oxygen tension. At its maximum, pulmonary vasoconstriction reduces perfusion of the nonventilated lung by 40% to 50%. The remaining pulmonary shunt results in a PaO₂, roughly half of that during two-lung ventilation for a given FiO₂. Because mechanical ventilation with high FiO₂ is typically used during lung isolation and one-lung ventilation, PaO₂ usually remains above 100 mm Hg, which is well tolerated systemically.

However, the full effect of hypoxic pulmonary vasoconstriction is only reached after approximately 2 hours of one-lung ventilation, such that problems with hypoxemia are more frequent and profound shortly after lung isolation is achieved. Additionally, several factors inhibit hypoxic pulmonary vasoconstriction and worsen oxygenation during lung isolation and one-lung ventilation, including hypocapnia and hypothermia. Calcium channel blockers and vasodilators can also inhibit the vasoconstriction. Although volatile anesthetics inhibit vasoconstriction, this effect is minimal with modern agents and is further minimized during one-lung ventilation because the nonventilated lung is minimally exposed. However, general anesthesia significantly decreases PaO₂ by developing atelectasis in the ventilated lung, mainly if neuromuscular blockers are utilized.

Physiological Effects of Positioning During Lung Isolation

If lung isolation and one-lung ventilation facilitate surgical exposure, the patient is usually positioned in the lateral decubitus position, with the nonventilated operative side superior to the ventilated nonoperative side. During two-lung ventilation in the lateral decubitus position, gravity results in approximately 60% of cardiac output going to the dependent lung, while the remaining 40% goes to the nondependent lung. When one-lung ventilation is initiated, this preferential perfusion of the dependent lung improves V/Q matching and oxygenation. Conversely, if lung isolation is used to prevent the spillage of secretions, purulence, or blood from one lung to the other in the ICU setting, patients are more frequently positioned either supine or with the diseased lung in the dependent position such that gravity prevents contamination of the healthy lung. This positioning worsens V/Q matching, and the resulting decrease in PaO₂ may be more exaggerated than in the operative setting.

Ventilator Management During Lung Isolation

Principles of lung-protective ventilation should be applied to lung isolation and one-lung ventilation to minimize acute lung injury and other complications. Low tidal volumes (4-6 mL/kg ideal body weight) during one-lung ventilation decrease the incidence of acute respiratory distress syndrome (ARDS), reduce pulmonary infiltration, and promote oxygenation. While PEEP can also help promote oxygenation, it must be tailored to the individual patient. For instance, patients with COPD may develop alveolar hyperinflation if PEEP is not carefully managed. Recruitment maneuvers can help promote oxygenation and prevent atelectasis during one-lung ventilation but should be cautiously administered in patients with lung bullae or emphysema.[14] 

Pressure-controlled ventilation may be associated with higher PaO₂/FiO₂ and lower peak airway pressures than volume-controlled ventilation. However, evidence for a difference in postoperative complications is lacking. Recently, lung-protective strategies recommended by the ARDSnet trials have decreased all-cause mortality. These recommendations include the following: 

• Tidal volume should be initially set at 4 to 6 mL/kg based on ideal body weight, which is lower than the 6 to 8 mL/kg recommended for mechanical ventilation using bilateral lungs. The dosage can be adjusted, and airway pressures should be monitored to identify signs of alveolar trauma or injury. This can be assessed by measuring inspiratory plateau pressures and targeting a pressure of less than 30 cm H₂O in nonobese patients.

• A respiratory rate (RR) of 16 breaths per minute is appropriate initially for most patients to achieve normocapnia. A blood gas should be sent approximately 30 minutes after the initiation of mechanical ventilation, and the RR should be adjusted based on the patient's acid-base status and PaCO₂. The RR should be increased if the PaCO₂ is significantly greater than 40 mm Hg. If the PaCO₂ is considerably lower than 40 mm Hg, then the RR should be decreased.

• The inspiratory flow rate should be set at 60 L/min. However, if the patient appears to be trying to inhale more during the initiation of inspiration, the flow rate can increase.

• PEEP is usually set at 5 to 10 cm H₂O depending on hemodynamic status, oxygenation requirements (FiO₂), and the presence of obstructive lung disease or auto-PEEP. 

• An oxygenation goal of 88% to 95%.[37][38][39][40][41][42][43]

Hypoxemia during one-lung ventilation should be managed systematically. When lung isolation and one-lung ventilation are used for surgical purposes, the surgical team must be alerted to the patient's hypoxemia. A coordinated reversion to two-lung ventilation should be planned, if necessary,  to ensure instruments are withdrawn before lung re-inflation. If the patient is stable and hypoxemia is relatively mild, the initial approach should involve confirming the correct airway device placement for effective one-lung ventilation using fiberoptic bronchoscopy. An assessment can be made for airway secretions, obstructions, or anatomical variations, such as a pig bronchus during bronchoscopy.[4] Other strategies to address hypoxemia during one-lung ventilation include administering bronchodilators, adjusting PEEP levels, performing recruitment maneuvers, and providing supplemental oxygen to the nonventilated lung.[9]

Enhancing Healthcare Team Outcomes

Effective interprofessional communication between the operative and anesthesia teams is essential. Before starting the procedure, a plan for achieving lung isolation should be established, including selecting the most appropriate devices for the clinical scenario. Establishing a plan for lung isolation in a patient with a difficult airway is particularly crucial. Emergency airway equipment, including that required for a surgical airway, should be readily accessible. Perioperative nurses and anesthesia technicians should ensure all necessary equipment is ready and functioning. This interprofessional interplay among healthcare team members can significantly improve patient outcomes. 

Conflicting evidence exists regarding whether a double-lumen endotracheal tube or bronchial blocker provides more rapid achievement of lung isolation or complete lung collapse. Equipment choice may hinge upon clinician preference and experience.[15][16][44] Larger medical centers may have dedicated thoracic anesthesia teams comprised of select clinicians who maintain their proficiency and become experts in lung isolation. In smaller centers, this is usually not the case, leading to relatively inexperienced clinicians performing lung isolation techniques sporadically. Inexperienced clinicians take 2 to 3 times longer to place these devices, and their positioning failure rates approach 40%.[45] Whether simulator training could help maintain proficiency in lung isolation techniques for clinicians who only occasionally use these skills in the clinical setting is uncertain.

Nursing, Allied Health, and Interprofessional Team Interventions

Managing lung isolation patients in the ICU requires a team of clinicians, including intensivists, nurses, and respiratory therapists. Usually, a double-lumen endotracheal tube is used to maintain lung isolation, and all care team members should be familiar with the device. Because lung isolation in the ICU is relatively rare, anesthesiology team members should be prepared to provide education and support if necessary.

If both lungs are to be ventilated but with different ventilator settings, the respiratory therapist will provide and maintain 2 separate ventilators for each side. The nursing team should consistently perform aggressive pulmonary toilet practices, including frequent suctioning of both lumens and meticulous oral care. If lung isolation is implemented due to unilateral infection, it is crucial to avoid contaminating the healthy lung with the diseased lung by using different suction tubing for each side. Particular attention is necessary during patient transport, turning, or other activities when the double-lumen endotracheal tube is at risk of dislodging from the correct position.

Nursing, Allied Health, and Interprofessional Team Monitoring

When lung isolation and one-lung ventilation are performed in the critical care setting, nurses are primarily responsible for monitoring patients for any signs indicating a problem with lung isolation. A significant change in respiratory status, such as hypoxemia, hypercarbia, or high peak airway pressures, should prompt notification of an intensivist or anesthesiologist to verify the correct device placement. Changes in the quality or quantity of suctioned secretions from each lung should also be monitored.