Lung Isolation Anesthesia

Anatomy and Physiology

When performing lung isolation, comprehensive knowledge of normal and abnormal tracheal and bronchial anatomy is necessary. 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 to give 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. The 3 lobar bronchi are on the right (upper, middle, and lower) and 2 on the left (upper and lower). Differentiating 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 divides from the right mainstem bronchus approximately 15 to 20 mm from the carina. By comparison, the left mainstem bronchus does not divide into the upper and lower lobar segments until 45 to 50 mm beyond the carina.[1] The right upper lobar bronchus’ characteristic trifurcation can further distinguish the opening of the right upper lobe into the apical, posterior, and anterior segments, resembling a cloverleaf.[2] However, recognizing the variations of this classic description is common. Approximately 22% of individuals do not have an apical segmental bronchus of the right upper lobe, resulting in the observation of 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, typically opposite the right upper lobe bronchus. The cardiac bronchus is usually nonfunctional, petering into rudimentary bronchioles. Although generally asymptomatic, retained secretions in a cardiac bronchus may be 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, but displacement is more common. About 0.2% of the human 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 an operative lung or resulting in 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 1-lung ventilation. If a bronchial blocker is positioned in the right mainstem in a patient with an unrecognized pig bronchus, the right lung deflation is 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. Upon transitioning to 1-lung ventilation, the ventilation is exclusively directed to 1 lung, leading to a potential increase in hypoxemia due to right-to-left shunting, theoretically reaching up to 50%. However, in practice, the shunt fraction is typically less severe, thanks 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 non-operated lung is the dependent lung and receives more perfusion. In addition, the lateral decubitus position increases the elastance to 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 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, optimizing oxygenation despite the challenges posed by 1-lung ventilation.[5][6]

Indications

Surgical and non-surgical indications for lung isolation are apparent, which can be absolute or relative. One-lung ventilation can facilitate various surgical procedures involving thoracic approaches to anatomical structures. The management of severe unilateral lung disease with either differential ventilation or 1-lung ventilation can also necessitate lung isolation. Some of these disease states, such as severe unilateral infection, require a definitive anatomical separation of the lungs to prevent contamination of the non-diseased lung. Other disease states only need physiologic separation to enable 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 non-infectious secretions
• Pulmonary hemorrhage

Unilateral disease states requiring physiologic 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. 
• A double-lumen endotracheal tube through a tracheostomy or stoma may increase the risk of iatrogenic injury.
• Damage to intraluminal tumors by the placement of a double-lumen endotracheal tube can lead to potential 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 increase the chance of severe hypoxia, which, if persistent, will require reinitiating ventilation to both lungs. 

• Patients with lung disease resulting in hypoxia during 2-lung ventilation with a FiO₂ of 100% are unlikely to be able to tolerate significant periods of 1-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 be associated with an increased risk of hypoxia during 1-lung ventilation. 
• One-lung ventilation should be used with caution in patients with pulmonary hypertension. Relative hypoxia and hypercarbia 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 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 1-lung ventilation. However, surgery on the right lung is 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. This is vital for clinicians performing lung isolation to know the correct placement, positioning, and troubleshooting. Choosing the appropriate bronchoscope diameter is also critical to prevent an inappropriately sized bronchoscope from requiring removal of the entire double-lumen endotracheal tube, reintubation, or inability to advance the bronchial blockers. 

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, but because of 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 this 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 to block the lumen or ventilate bilaterally selectively. The connector piece is composed of a Y-shaped piece with 2 openings—one each for the connection to the bronchial and tracheal lumen and a common portion that fits into the ventilator circuit. The connector usually has color-coded tracheal and bronchial lumen parts. Manufacturers universally use the blue color to denote the endobronchial-lumen cuff. The cuffs are colored similarly to the connector tubes to maintain uniformity in identification. This safety mechanism ensures 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 to achieve 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. The bronchial blocker can also 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 non-infected 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. Advancing the tube into the unaffected lung may be the best choice in emergency scenarios, such as the development of unilateral pulmonary hemorrhage in a patient intubated with a single-lumen tube. 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 non-ventilated lung, and less frequent need for repositioning. In a study of patients undergoing video-assisted thoracoscopic surgery for esophagectomy, the 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 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 confirmation of tube placement after surgical positioning. Although this device is more expensive, the need for fiberoptic bronchoscopy is negated.[17][18]

The patient with 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. The majority of clinicians also prefer this technique, even for mature stomas, because of the simplicity and reduction in the opportunity for iatrogenic airway trauma. Other alternatives include introducing a single-lumen tube through the tracheostomy stoma, directing through the desired bronchus, removing the tracheostomy tube, and placing the double-lumen endotracheal tube orally.[19]

Personnel

During residency training, anesthesiologists have traditionally learned how to place double-lumen endotracheal tubes and bronchial blockers on surgical patients under the direction and supervision of experienced faculty. However, more recently, simulators have been available for training in advanced airway techniques, including fiberoptic bronchoscopy, so trainees may have the opportunity to hone their skills outside the high-stakes environment of the operating room. Current accreditation Counsel for Graduate Medical Education (ACGME) requirements include 20 non-cardiac intrathoracic cases. However, no specific requirement for lung isolation is apparent. 

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 authors 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 usually undergoing single-lung ventilation have underlying pulmonary disease. Patients should be evaluated comprehensively for their primary disease before surgical procedures. Echocardiography may be useful in patients with cor pulmonale and may provide information about baseline cardiac function and reserve. Reviewing the relevant radiological anatomy may help plan anesthetic management for 1-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 need to use higher fractions of inspired oxygen.

The presence of any bullae on the nonoperative lung may provide clues to patients more likely to develop a pneumothorax in the perioperative period. Tumors must be evaluated for paraneoplastic syndromes, as their presence may guide anesthetic management. Standardized protocol has increased adherence to lung protective strategies in 1-lung ventilation.[21][22][23] Special considerations must be placed on patient age as an independent risk factor for complication rates in patients undergoing pulmonary resection using 1-lung ventilation. Older patients have higher morbidity and mortality from pulmonary resections. Patients undergoing lung resections need additional testing to predict the risks involved in lung resection besides age. The most common tests for such an assessment are forced expiratory volume in one second (FEV1) and diffusing capacity of the lungs for carbon monoxide (DLCO). FEV1 is a predictor of postoperative complications, including death that may arise from undergoing pulmonary resection. Multiple studies have found a reduced preoperative FEV1, less than 60% predicted, to be the strongest predictor of postoperative complications. Therefore, a cutoff of 60% for FEV1 and DLCO is used to determine postoperative risk.[24][25][26] 

The current American College of Chest Physicians' guidelines do not provide numerical cutoffs for DLCO below which pulmonary resections should not be performed on patients. Instead, they emphasize the importance of determining the predicted postoperative values. Assessment of the postoperative predicted values is the deciding factor in such cases and predicts the success of single lung ventilation. 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. 
• However, patients with predicted postoperative values of FEV1 or predicted postoperative DLCO less than 60% but greater than 30% need additional testing with stair climbing or a shuttle walk test.
• If both values are less than 30%, patients should undergo cardiopulmonary exercise testing with additional measurement of the maximal oxygen consumption.

The cutoff for the stair climb test is 22 meters. Using the Incremental Shuttle Walk test, a distance greater than 400 meters denotes a maximal oxygen uptake (VO₂ max) greater than or equal to 15 mL/kg/min. Such information may help the treating clinician interpret the procedural risks and better clinical outcomes for patients undergoing one-lung ventilation.

Equipment Preparation

Proper equipment preparation is paramount for double-lumen endotracheal and bronchial blocker placement, with selections often based on patient-specific factors such as gender, height, or more precise trachea and bronchi radiologic assessments. For double-lumen endotracheal placement, various sizes must be available: single-lumen tubes for backup, a fiberoptic bronchoscope, antifog solutions, lubricating gels, and a tube exchanger. 

A lack of uniformity and objective guidelines is apparent in the selection of double-lumen sizes. Selecting the proper size reduces the incidence of complications. Tube size recommendations can be based on gender and height. A 39 Fr is recommended for men 165 cm or less, and a 41 Fr for men taller than 165 cm. A 35 Fr is recommended for women 160 cm or less, with a 37 Fr for women taller than 160 cm. Radiologic measurement of the trachea's diameter or left mainstem bronchial diameter measurement from a computer tomographic (CT) scan can also be considered when selecting a left-sided double-lumen endotracheal tube. Hannallah et al concluded that when the choice of size was based on clinical judgment, the tube was too large relative to the patient's airway.

Bronchial blocker placement requires the clinician's preferred device, a single-lumen tube of an appropriate diameter, and a fiberoptic bronchoscope (with an antifog solution and lubricating gel). When anesthesia is induced and mask ventilation is verified, paralysis is typically administered to facilitate placement. The most experienced clinician should secure the airway and achieve lung isolation when the patient is unlikely to tolerate long periods of hypoxia or hypercarbia (eg, severe pulmonary hypertension). Bronchodilators should is recommended for patients with suspected reactive airways, and soft-suction catheters should be utilized 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 intensive care unit (ICU) setting, the approach to preparation should be similar to that described for surgical procedures. However, if lung isolation is emergently required, such as in the case of significant unilateral pulmonary hemorrhage, the most accessible method to achieve lung isolation is suggested. For example, if the hemorrhaging patient is in an ICU remotely from the operating rooms, a double-lumen endotracheal tube may take too long; a single-lumen tube is advanced into the unaffected bronchus.

Technique or Treatment

Double Lumen Endotracheal Tube Placement

The most common method of this tube placement is direct laryngoscopy, followed by fiberoptic bronchoscopy to confirm positioning. Video laryngoscopy can place a double-lumen endotracheal tube into a patient with a difficult airway. Because video laryngoscopes typically leave less room in the oropharyngeal cavity to maneuver, placement may be cumbersome after viewing the vocal cords. During direct or video laryngoscopy, the tube is introduced with a rigid stylet with the endobronchial curvature facing anteriorly. When 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, respectively, and advanced until resistance is met. 

After the double lumen endotracheal lumen tube is positioned, the tracheal and bronchial cuffs should be inflated, the connector attached, and ventilation should be 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. 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 the carina might herniate, an error in placement 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. Using fiberoptic bronchoscopy, the desired bronchus is identified, 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][8]

Complications

Anatomic

Because of the 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 than bronchial blocker placement.[20][35]

Physiologic

During single-lung ventilation, ventilation to 1 of the lungs is interrupted. However, perfusion is still present in the nonventilated lung, leading to an intrapulmonary shunt in the form of wasted perfusion to the nonventilated lung. Protective mechanisms like hypoxic pulmonary vasoconstriction can counteract hypoxia to a certain degree. However, the anesthesiologist must have measures in place for hypoxemia that may arise during single-lung ventilation. Ventilation and perfusion (V/Q) matching significantly manage oxygenation in patients on single-lung ventilation. Some authors note that oxygenation is much better in the lateral decubitus position than in the supine position.[36] 

An inspired oxygen fraction (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. 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. Check the position of the double-lumen, endobronchial, or bronchial blocker. Changes in position may occur due to surgical manipulation. A repeat fiberoptic bronchoscopy through the tracheal lumen is useful 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 employed on the ventilated lung, which is in the dependent position; these are performed to overcome any atelectasis and thus help 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 2-lung ventilation. 
6. Severe hypoxemia should alert the anesthesiologist to look for causes like pneumothorax on the dependent lung. Chronic obstructive lung disease patients are more likely to experience such a complication. 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 1-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, the 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 2-lung ventilation for a given FiO₂. Because mechanical ventilation with high FiO₂ is typically used during lung isolation and 1-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 1-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 1-lung ventilation, including hypocapnia and hypothermia. Calcium channel blockers and vasodilators can also inhibit the vasoconstriction. Volatile anesthetics inhibit vasoconstriction, but this effect is minimal with modern agents and is further minimized during 1-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.

Physiologic Effects of Positioning During Lung Isolation

If lung isolation and 1-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 2-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 1-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 1 lung to the other in the intensive care 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 1-lung ventilation to minimize acute lung injury and other complications. Low tidal volumes (4 to 6 mL/kg ideal body weight) during 1-lung ventilation decrease the incidence of acute respiratory distress syndrome (ARDS), reduce pulmonary infiltration, and promote oxygenation. While PEEP can also help promote oxygenation, this must be tailored to the individual patient, as patients with chronic obstructive pulmonary disease (COPD), for example, may develop alveolar hyperinflation. Recruitment maneuvers can help promote oxygenation and prevent atelectasis during 1-lung ventilation but should be cautiously administered in patients with lung bullae or emphysema.[14] Compared to volume-controlled ventilation, pressure-controlled ventilation may be associated with higher PaO₂/FiO₂ and lower peak airway pressures; 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: 

1. Tidal volume should be initially set at 4 to 6 mL/kg based on ideal body weight (a lower dose than 6 to 8 mL/kg is recommended for mechanical ventilation using bilateral lungs). The dose can be adjusted, and airway pressures are 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 non-obese patients.
2. 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.
3. The inspiratory flow rate should be set at 60 L/min. If the patient appears to be trying to inhale more during the initiation of inspiration, the flow rate can increase. 
4. PEEP is usually set at 5 to 10 cm H₂O depending on hemodynamic status, oxygenation (FiO₂ requirement), and obstructive lung disease or auto-PEEP. 
5. An oxygenation goal of 88% to 95%.[37][38][39][40][41][42][43]

Hypoxemia during 1-lung ventilation should be approached systematically. If lung isolation and 1-lung ventilation are utilized to facilitate surgery, the surgical team should be aware of the patient's hypoxemia and a potential return to 2-lung ventilation coordinated so that instruments are withdrawn before lung re-inflation. If the patient is stable and the hypoxemia is relatively mild, a logical first step is to confirm the correct airway device placement to achieve 1-lung ventilation using fiberoptic bronchoscopy. During bronchoscopy, the patient can be evaluated for secretions, other obstructions to the airway, or potential anatomical variants, such as a pig bronchus.[4] Other methods to treat hypoxemia during 1-lung ventilation include administering bronchodilators, escalating PEEP, performing recruitment maneuvers, and administering 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 which devices are most appropriate for the clinical scenario. Establishing a plan for lung isolation in a patient with a difficult airway is essential. Emergency airway equipment, including that required for a surgical airway, should be accessible. Perioperative nurses and anesthesia technicians should ensure all necessary equipment is ready and working. 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, and 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. As a result, relatively inexperienced clinicians will perform 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 multiple clinicians, including intensivists, nurses, and respiratory therapists.  Usually, a double-lumen endotracheal tube is used to maintain lung isolation when needed, and all care team members should be familiar with the device. Because lung isolation in the ICU setting 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. Aggressive pulmonary toilet practices, including frequent suctioning of both lumens and meticulous oral care, should be performed consistently by the nursing team.  If lung isolation has been implemented due to unilateral infection, care is necessary to avoid contaminating the healthy lung with the diseased lung, including using different suction tubing for each side.  Particular attention is necessary for 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 1-lung ventilation are performed in the critical care setting, nurses are primarily responsible for monitoring these 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.