One-Lung Ventilation and Challenges

Key action points in the management of a patient undergoing one-lung ventilation (OLV) include the following:

• OLV is indicated in a variety of settings (eg, hemoptysis or infection, bronchopleural injury, and unilateral lung lavage); it facilitates surgical exposure in many other settings (eg, minimally invasive lung surgery, cardiac surgery, esophageal surgery, and thoracic aortic surgery)
• OLV can be established through use of a double-lumen endotracheal tube (DLT) or a single-lumen endotracheal tube (ETT) with the use of a bronchial blocker (BB)
• Hypoxemia is a common occurrence during OLV; risk factors include surgery on the right lung, surgery in the supine position, and preexisting lung disease
• A standard algorithm can be useful in accessing and correcting hypoxema during OLV; corrective steps include recruitment and optimization of tidal volumes; application of positive end-expiratory pressure (PEEP) in the ventilated lung; application of continuous positive airway pressure (CPAP) to the operative lung; return to two-lung ventilation (TLV); and use of extracorporeal support.
• Acute lung injury (ALI) is common after OLV, but the risk can be minimized by using lung-protective strategies

OLV improves surgical exposure and operative conditions during a variety of procedures in the thorax, including lung resections, esophageal surgery, and procedures involving access to the thoracic aorta and the sympathetic chain. It is also indicated to protect the uninvolved lung in the setting of pulmonary hemorrhage or infection, during one-lung lavage, or in the setting of a bronchopleural fistula.

The current emphasis on minimally invasive surgical approaches, including a shift from open thoracic surgery to video-assisted thoracoscopic surgery (VATS) and the advent of minimally invasive cardiac surgery, has led to an increased reliance on OLV for adequate surgical exposure.

Difficulty may be encountered during the establishment of OLV, during the maintenance phase, after a return to TLV, and during the postoperative period.

Establishment of one-lung ventilation

A number of airway devices are available for establishing OLV, including left- or right-side DLTs, several varieties of BBs, and the Univent tube, a single-lumen ETT with a built-in BB.

Historically, many anesthesiologists and surgeons have stated a preference for DLTs, on the grounds that these devices yield better lung isolation and therefore improved surgical exposure. To date, no studies have proved any one device to be more effective than another. One meta-analysis demonstrated faster and more stable positioning with the use of a DLT, whereas use of BBs resulted in less postoperative sore throat and hoarseness and fewer airway injuries. ^[1]  

When a DLT is chosen, the tracheal diameter, as measured on a standard chest radiograph, can be useful for selecting the most appropriate tube size (see Table 1 below). ^[2]  Historically, it was common practice to place the largest possible DLT tube according to the patient's gender and height. However, the increasing recognition of glottic and airway trauma related to the use of a large DLT has led many anesthesiologists to prefer a smaller tube size.

Table 1. Selection of Double-Lumen Endotracheal Tube (DLT) Size According to Tracheal Diameter ^[2] (Open Table in a new window)

There are certain situations in which either a DLT or a BB may be favored. When a difficult airway is encountered, it is almost invariably easier to place a single-lumen ETT than to place a DLT. Once tracheal intubation with a single-lumen ETT is confirmed, a BB may be used to establish OLV, or a tube exchanger may be used to replace the ETT with a DLT.

In cases where postoperarative ventailation is anticipated, there may be a significant advantage to choosing an ETT with a BB rather than performing a tube exchange at the end of the case, particularly if the airway was challenging or if there is concern about glottic edema or patient hypoxia during reintubation. Similarly, in patients who have alternative airway access points (eg, nasal tube or tracheostomy), a BB may be the only feasible option.  

Although adequate lung isolation typically can be confirmed clinically by auscultating breath sounds, it is generally prudent to confirm appropriate placement of a DLT with fiberoptic bronchoscopy (FOB). FOB is required for correct positioning for all BBs.  

VATS with a native airway is gaining in popularity. During procedures using this approach—sometimes referred to as awake VATS, tubeless VATS, or nonintubated VATS (NIVATS)—the patient is sedated but breathing spontaneously. Physiologic pneumothorax and OLV are established when the surgeon enters the thoracic cavity and applies carbon dioxide insufflation. It has been proposed that the lack of positive-pressure ventilation (PPV) may decrease lung injury in these patients, but at present, the data are not sufficient to prove the superiority of VATS without tracheal intubation. ^{[3, 4]}

Maintenance phase

Although OLV is beneficial for surgical exposure, it creates a transpulmonary shunt that impairs oxygenation and can lead to hypoxemia. Hypoxemia during OLV, generally defined as a peripheral capillary oxygen saturation (SpO₂) lower than 90%, occurs in 4-10% of cases. ^[5] Several factors contribute to the degree of hypoxemia, including the following:

• Operative lung
• Positioning
• Underlying lung disease

Operative lung

As a consequence of the larger size of the right lung, the shunt fraction and associated hypoxemia are more profound when the left lung is being ventilated and the right lung is not. The side of the body on which the operation is being performed is one of the most important determinants for predicting hypoxemia during OLV. ^[6]  

Positioning

West’s lung zones define the distribution of perfusion and ventilation in different areas of the lung. In the classic West zones, the lung apex is well ventilated but relatively poorly perfused as compared with the lung bases. Gravitational forces within the pulmonary vasculature cause the basal lung segments to have significantly higher perfusion with slightly lower ventilation.

When patients are placed in the lateral decubitus position for thoracic surgery, the superior lung (ie, the operative lung) becomes West zone 1, with a decrease in perfusion. This drop in perfusion to the nonventilated lung helps decrease the shunt fraction during OLV in the lateral decubitus position. When OLV is performed with the patient in the supine position, pulmonary perfusion is nearly uniform throughout all lung segments, and the shunt fraction is increased. ^[7]

Although the lateral decubitus position is beneficial for decreasing the shunt fraction, it can contribute to compression and atelectasis in the ventilated lung. The abdominal contents shift downward, placing increased pressure on the dependent diaphragm. Table bolsters increase compression of the lateral chest wall. Mediastinal shifting due to gravitational forces compresses the dependent lung, as does thoracic insufflation during VATS.  

Underlying lung disease

As may seem intuitively obvious, poor arterial oxygenation during TLV predicts poor oxygenation during OLV. ^[2]  Obstructive lung disease, however, may offer some protection from desaturation. This effect may be related to air trapping and auto-PEEP in the ventilated lung (thereby preventing atelectasis) and in the nonventilated lung (minimizing complete collapse and thus decreasing shunt fraction). ^[8]

Hypoxic pulmonary vasoconstriction

The physiologic response to hypoxemia in the lung parenchyma, commonly referred to as hypoxic pulmonary vasoconstriction (HPV), results in contraction of vascular smooth muscle, diverting blood flow away from the hypoxic segment. Without HPV, the shunt fraction during OLV would be unacceptably high and would lead to profound hypoxemia. Although no anesthetic agents have been found to alter HPV significantly at clinically relevant doses, many nonanesthetic drugs do affect HPV, as do volume status, temperature and acid-base status.

Return to two-lung ventilation

Return to TLV alleviates the transpulmonary shunt created by OLV and rapidly improves oxygenation. However, ALI related to OLV is the leading cause of death following thoracic surgery. ^[9]  Multiple factors contribute to ALI in both the ventilated and the nonventilated lung. 

In the ventilated lung, contributors to ALI include the following:

• Volutrauma - High end-expiratory volumes due to high tidal volumes or excessive PEEP
• Barotrauma - High ventilatory pressures, commonly defined as inspiratory plateau pressures higher than 10 cm H ₂O
• Atelectotrauma - The presence of atelectasis in the ventilated lung necessitates higher ventilatory pressures and a higher fractional concentration of inhaled oxygen (FiO ₂)
• Tidal recruitment - Repetitive collapse and recruitment of atelectatic alveoli 
• Capillary hyperperfusion - Changes in pulmonary blood flow and hypervolemia contribute to high pulmonary capillary perfusion pressures and pulmonary edema
• Biotrauma - OLV, particularly at high tidal volumes, has been shown to increase circulating cytokine levels
• Oxidative injury - High FiO ₂ contributes to increases in reactive oxygen species (ROS), which have been shown to contribute to ALI after thoracic surgery, particularly in lung cancer patients

In the nonventilated lung, contributors to ALI include the following:

• Atelectasis and lung reexpansion - Lung reexpansion after dense collapse increases inflammatory markers, with the degree of inflammation being linearly related to the longevity of lung collapse; in addition, prolonged high-pressure holds at the time of reexpansion appear to be more harmful than cyclical recruitment strategies with stepwise increases in inspiratory pressure and PEEP
• Ischemia-reperfusion injury - Ischemia of the nonventilated lung contributes to inflammation, microvascular permeability, and pulmonary edema
• Damage to the glycocalyx - Damage to the glycocalyx is increasingly recognized as a significant contributor to lung injury and is an area of intensive investigation; both inflammation and ischemia-reperfusion injuery may contribute to glyocalyx damage ^[10]
• Biotrauma - Direct lung lavage demonstrates higher levels of circulating cytokines in the nonventilated lung than in the ventilated lung
• Surgical manipulation

Addressing the problem

As stated above, the primary challenges associated with OLV are as follows:

• Establishing adequate lung isolation
• Avoiding hypoxemia
• Preventing postoperative ALI

Establishing lung isolation

The choice of DLT or BB for lung isolation is made on the basis of the patient's anatomy, the findings from airway examination, the surgical procedure being performed, and the preferences of the surgeon and the anesthesiologist. As noted (see Problem), many anesthesiologists and surgeons have historically shown a strong preference for DLTs over BBs; however, with the advent of newer BBs and improved FOB, BBs are gaining popularity.  As noted above, in selective cases, lung isolation with native airway and spontaneous ventilation may also be an option.

Patient factors

For patients whose airways are known or predicted to be difficult, those who have tracheostomies, and those who are already intubated (particularly if respiratory status is tenuous), a single-lumen ETT plus a BB may be the best option for lung isolation. Likewise, if postoperative mechanical ventilation is anticipated, the use of a BB rather than a DLT obviates the need for tube exchange at the end of the procedure.  

Surgical procedure

Sleeve pneumonectomy is best performed with a contralateral DLT so as to minimize ETT interference with the surgical procedure. Because of the variable length of the right mainstem bronchus and the unpredictable location of the right-upper-lobe (RUL) bronchus, a left-side DLT is generally preferred for surgery on the RUL. For procedures that require intermittent OLV on both the right side and the left (eg, a procedure that requires access to the sympathetic chain or bilateral lung transplantion without the use of cardiopulmonary bypass [CPB]), a DLT is preferred.

For most other procedures, either a DLT or a BB may be used effectively for lung isolation. ^[11]  

Avoiding hypoxemia

As noted (see Problem), hypoxemia during OLV is generally described as an SpO₂ below 90%. Potential causes are numerous and are best addressed via a stepwise approach, as follows:

• Increase FiO ₂ while searching for the cause of hypoxemia
• Confirm circuit integrity - This may be done by manually ventilating the patient to rule out a circuit leak and by quickly inspecting all circuit connections and the breathing tube to rule out kinking
• Ensure appropriate ETT positioning - Whether a DLT or a BB is being used, FOB should be readily available throughout OLV for quickly evaluating the tracheobronchial tree and ruling out device malpositioning
• Consider the need for suctioning or bronchodilators - Changes in peak airway pressures (with volume-controlled ventilation [VCV]) or decreases in tidal volume (with pressure-controlled ventilation [PCV]) may suggest mucous plugging, whereas an upstroke in the plateau phase of the capnogram may indicate bronchospasm
• Optimize ventilation of the ventilated lung - Multiple forces (abdominal, external, and mediastinal) exert compression on the ventilated lung, rendering it prone to atectasis; lung-protective ventilation strategies with small tidal volumes (4-6 mL/kg ideal body weight), optimal PEEP and frequent recruitment maneuvers are preferred to high-tidal-volume ventilation for minimizing atelectasis ^[12] ; selecting "optimal PEEP" remains one of the largest challenges facing the anesthesiologist during OLV, because although PEEP helps prevent atelectasis in the ventilated lung, too much PEEP can compress perialveolar capillaries, shunting blood flow to the nonventilated lung and increasing ventilation-perfusion mismatch; one small study demonstrated better oxygenation with "individualized PEEP" based on a PEEP decrement trial than with a standardized 5 mm Hg PEEP setting ^[13]
• Consider CPAP to the nonventilated lung - CPAP can be provided to the nonventilated lung by connecting an additional circuit with a PEEP valve to an auxillary oxygen supply; the surgeon should be informed before CPAP is initiated because this is expected to increase the size of the nonventilated lung and may interfere with the surgical procedure, especially in the case of VATS
• Surgically reduce the shunt fraction - If lobectomy or pneumonectomy is planned, clamping the vessel feeding the segment to be resected will decrease the shunt fraction and improve oxygenation
• If all of the maneuvers listed above prove inadequate, return to TLV is indicated
• In the most extreme cases, extracorporeal support with venovenous extracorporeal membrane oxygenation (VV ECMO) or a pumpless lung assist device may be required ^{[14, 15]}

In addition, steps should be taken to avoid interfering with HPV.  A partial list of factors that affect HPV is provided in Table 2 below.  

Table 2. Common Factors That Affect Hypoxic Pulmonary Vasoconstriction (HPV) (Open Table in a new window)

Preventing acute lung injury

Lung-protective ventilation with low tidal volumes has been clearly demonstrated to minimize barotrauma, volutrauma, and the ensuing inflammatory response. Questions remain, however, about how to define "low tidal volumes," particularly in view of the finding that very low tidal volumes may contribute to atelectasis and hypoxia. ^[10]  Furthermore, there is no clear evidence to indicate whether PCV or VCV produces better outcomes, though PCV strategies usually yield lesser degrees of intertidal pressure variation, which may protect against atelectrauma.

Using the lowest FiO₂ that maintains an adequate SpO₂ (usually 92-96%) may help minimize oxygen toxicity and ROS formation. Biotrauma and inflammation are largely unavoidable, but minimizing fluid administration may help minimize pulmonary edema.  

At the time of reexpansion, the compliance of the nonventilated lung is significantly lower than that of the ventilated lung. A two-lung recruitement breath delivered at this time results in overexpansion and volutrauma and barotrauma to the ventilated lung. Accordingly, it is recommended either to exclude the ventilated lung from the respiratory circuit temporarily during reexpansion of the nonventilated lung or to use a separate circuit for lung reexpansion.  

Clinical scenario

A 69-year-old female smoker with chronic obstructive pulmonary disease (COPD) and an RUL lung nodule is being seen for VATS and RUL wedge resection. She is 163 cm tall, weighs 75 kg, and has a body mass index (BMI) of 28. On preoperative chest radiography, her tracheal diameter is measured as 16 mm; accordingly, placement of a 39-French left-side DLT is planned.

After induction of general anesthesia with fentanyl, propofol, and rocuronium, intubation is attempted with the aid of a videolaryngoscope. Although a full view of the glottic opening is obtained, passage of the DLT is met with resistance. An 8.0 ETT is placed, and the patient is ventilated while a 37-French left-side DLT is prepared. A tube exchanger is deployed, the 8.0 ETT is withdrawn, and an attempt is made to place the smaller DLT.  This too is met with resistance, and upon examination with the videolaryngoscope, it is clear the the resistance is at the level of the glottis.

Further attempts at DLT placement are aborted; an 8.0 ETT is placed, and a BB is positioned by means of FOB for lung isolation.

Resolution

This case demonstrates some of the challenges that are occasionally faced during DLT placement. Even when tracheal diameter is adequate, the glottic opening or angulation can hinder the placement of a DLT. Frequently, a jaw lift combined with gentle clockwise rotation of the DLT can help guide the tube into the trachea. However, when multiple attempts are unsuccessful, as in this case, it is prudent to consider another option, if one is available. All thoracic anesthesiologists should be comfortable with and competent in the placement of both DLTs and BBs. 

Clinical scenario

An obese 76-year-old male patient with hypertension, hyperlipidemia, sleep apnea, and a left-lower-lobe (LLL) neuroendocrine tumor is scheduled for VATS and wedge resection. He is 172 cm tall, weighs 92 kg, and has a BMI of 31.

After induction of general anesthesia and placement of a 41-French left-side DLT, the patient is placed in the right lateral decubitus position. The bronchial cuff is inflated, and the lumen is clamped to extablish lung isolation. The ventilator is set to VCV with a tidal volume of 325 mL, a respiratory rate (RR) of 16 breaths/min, an FiO₂ of 80%, and 5 cm H₂O of PEEP. An incision is made, and the surgeon confirms appropriate deflation of the left lung.

Several minutes later, a colleague offers you a break. When you return to the operating room (OR) 15 minutes later, your colleague explains that the patient's SpO₂ dropped from 98% to 90% and that in response he increased the tidal volume to 550 mL and the FiO₂ to 100%. The patient's SpO₂ is now 99%.

Resolution

Mild hypoxia under OLV is common. Although it is tempting (and sometimes successful) to increase FiO₂ and tidal volume to improve SpO₂, these maneuvers can result in volutrauma, barotrauma, and oxygen toxicity, which can contribute to postoperative ALI. The initial ventilator settings in this case were more appropriate for lung protection during OLV.

In this type of situation, rather than increase tidal volume, it is preferable to give gentle recruitment breaths. The increased abdominal compression in obese patients may also warrant a slightly higher PEEP. It is helpful to increase FiO₂ transiently while correcting problems contributing to hypoxemia, but when SpO₂ recovers, FiO₂ should be decreased whenever possible.

Clinical scenario

A 58-year-old male smoker with biopsy-proven non–small cell lung cancer (NSCLC) involving a large portion of the LLL and extending into the left mainstem bronchus is scheduled for left pneumonectomy.

After induction of general anesthesia, a 41-French right-side DLT is placed, and its position is confirmed with FOB. The patient is then placed in the right lateral decubitus position, and a left thoracotomy is performed. About 30 minutes after lung isolation is achieved, SpO₂ decreases from 96% to 89%. FiO₂, which had been set at 70%, is increased to 100%, and the circuit is quickly checked for leaks, kinks, and disconnections. Inspection of the ventilator settings and capnogram reveal no significant changes. FOB is used to confirm appropriate positioning of the DLT.

Several recruitment breaths are given, and PEEP to the ventilated lung is increased from 5 cm H₂O to 7 cm H₂O. SpO₂ temporarily increases to 94% but eventually drops again to 88%. The surgeon is alerted to the difficulty with oxygenation and states that he is nearly ready to clamp the left pulmonary artery. After the artery is clamped, SpO₂ rises to 95% and remains stable throughout the remainder of the case.   

Resolution

Anatomic differences between the right and left mainstem bronchi make a left-side DLT preferable in most cases. During left pneumonectomy, however, a left-side DLT may interfere with the resection, and a right-side DLT is preferable if the anatomy is amenable. However, when the RUL bronchus ostia is very proximal, a right-side DLT may not be a practical option.  

In this scenario, several common causes of hypoxemia during OLV—including tube kinking or dislodgment, mucous plugging, bronchospasm, and atelectasis—are ruled out, yet the patient remains hypoxic. When this situation arises, it is useful to clamp the feeding vessel to the lung segments planned for resection. As illustrated by this case, reducing the shunt fraction quickly improves SpO₂.

Clinical scenario

A 68-year-old male patient with NSCLC, rheumatoid arthritis, and ischemic cadiomyopathy undergoes apparently uneventful VATS with right lower lobe (RLL) wedge resection.

Postoperatively, the patient has a persistent air leak and respiratory distress, which eventually necessitate reintubation. After the initiation of PPV, the air leak worsens, oxygenation remains poor, and there is concern about a possible bronchopleural fistula. The thoracic surgery and intensive care unit (ICU) teams request your help in placing a BB to occlude the involved segment in the RLL.

After preoxygenation with 100% FiO₂ and sedation with midazolam and fentanyl, a fiberoptic bronchoscope and a BB are inserted into the existing ETT via a bronchoscopy adaptor. Under fiberoptic guidance, the BB is guided into the bronchus intermedius and inflated, sparing the RUL. An immediate decrease in the air leak then occurs, as seen on the air leak monitor on the chest tube.

Resolution

This case highlights the application of OLV or segmental lung isolation outside the OR. Lung isolation in the ICU is usually a last resort, in that ventilating lungs individually is cumbersome and both DLTs and BBs are more prone to malpositioning than a single-lumen ETT is. However, there are instances where lung separation is critical, including massive hemoptysis and whole-lung lavage. ^[16] Relative indications include asymmetric lung disease, single-lung transplant, and bronchopleural fistula when other efforts to optimize oxygenation and ventilation have failed.