ANESTHESIA FOR THORACIC SURGERY. Dr Abdollahi. The major challenges in anesthesia for thoracic surgery are establishing: Adequate separation of the lungs, Maintaining gas exchange, Ensuring circulatory stability during one-lung anesthesia.
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ANESTHESIA FOR THORACIC SURGERY Dr Abdollahi
The major challenges in anesthesia for thoracic surgery • are establishing: • Adequate separation of the lungs, • Maintaining gas exchange, • Ensuring circulatory stability during one-lung anesthesia.
One-lung anesthesia involves lung separation and deliberate ventilation of the dependent lung by isolating its bronchus from that of the nondependent lung (the operative site) with specially designed endotracheal tubes.
In addition, thoracic surgery often involves thoracotomy incisions, which are associated with severe pain and potentially deleterious changes in cardiopulmonary • physiology after surgery. Some of these physiologic changes can be minimized by thoracic epidural analgesia for effective postoperative pain management .
Preoperative evaluation and preparation • Patients undergoing thoracic surgery are at high risk for • postoperative pulmonary complications, particularly if • coexisting chronic pulmonary disease is present. Risk • factors associated with increased pcrioperative morbidity • and mortality include: • The extent of lung resection (pneumonectomy> • lobectomy> wedge resection), • Age older than 70 years, • Inexperience of the operating surgeon
In patients with anatomically resectable lung cancer, • pulmonary function testing, lung perfusion scanning, and exercise testing to measure maximum oxygen consumption may also predict postoperative pulmonary function, as well as increased mortality .
A decrease in FEV1, to less than 70% of predicted and a reduction in diffusing capacity to less than 60% of predicted should prompt further testing with a quantitative lung perfusion scan.
If postoperative FEV1, or DLCO are less than 40% • as predicted by lung scan, an exercise study should be • obtained. A significant decrease in oxygen consumption • « 10 mL/kg/min) as measured by exercise testing predicts a postoperative mortality of 25% to 50% and should prompt discussion of alternatives to surgical resection.
DISCONTINUATION OF SMOKING • Smoking increases airway irritability and secretions, • decreases mucociliary transport, and increases the incidence of postoperative pulmonary complications. Cessation of smoking for 12 to 24 hours before surgery decreases the level of carboxyhemoglobin, shifts the oxyhemoglobin dissociation curve to the right, and increases the oxygen available to tissues.
In contrast to these short-term effects improvement in mucociliary transport and small airway function and decreases in sputum production require prolonged abstinence (8 to 12 weeks) from smoking. The incidence of postoperative pulmonary complications decreases with abstinence from cigarette smoking for more than 8 weeks in patients undergoing coronary artery bypass surgery and more than 4 weeks in patients undergoing pulmonary surgery.
Nevertheless, it is useful to encourage smoking abstinence in the perioperative period, especially because smoking shortly before surgery may be associated with an increased incidence of ST-segment depression on the electrocardiogram.
Management of Anesthesia • The five goals of anesthesia in thoracic surgery are to • (1) produce controlled levels of narcosis and analgesia, • (2) suppress cough and reflex airway activity, • (3) Minimize interference with protective reflexes such as hypoxic pulmonary vasoconstriction, • (4) maintain satisfactory blood gas exchange and cardiovascular stability, • (5) permit rapid recovery from anesthesia to avoid postoperative respiratory depression.
A practical approach is to induce general anesthesia with intravenous propofol and maintain it with a potent volatile anesthetic supplemented with intravenous opioids and controlled ventilation of the patient's lungs. Depression of airway reflexes and rapid elimination allowing for rapid recovery are important benefits of volatile anesthetics
In addition, volatile anesthetics do not seem to inhibit regional hypoxic pulmonary vasoconstriction and thus aid in the maintenance of arterial oxygenation during one-lung anesthesia
If nitrous oxide is administered, the inhaled concentration is often limited to 50% until the adequacy of oxygenation can be confirmed by pulse oximetry or measurement of Pao2 .Caution must be used in patients with increased PVR because the addition of nitrous oxide to volatile anesthetics may exacerbate increased resistance of the pulmonary vasculature.
In addition, nitrous oxide is contraindicated in situations in which it has the potential to expand within a closed air space, such as during closure of a thoracotomy after pneumonectomy when there is no thoracostomy drain.
To decrease requirements for volatile anesthetics and facilitate controlled ventilation of the lungs, a nondepolarizing neuromuscular blocking drug is usually administered; these drugs also improve surgical exposure by maximizing mechanical separation of the ribs.
Ketamine may likewise be useful for induction of anesthesia in patients undergoing emergency thoracotomy associated with hypovolemia (blunt trauma, gunshot wounds, and stab wounds).
For effective postoperative pain control, a thoracic • epidural catheter is placed preoperatively while the patient is sedated but conscious. Patients undergoing thoracotomy usually have an intra-arterial catheter in place to permit continuous monitoring of systemic blood pressure and periodic measurement of arterial blood gases and pH. A central venous catheter may be helpful for guiding intravenous fluid replacement.
Transesophageal echocardiography is also a useful intraoperative monitor for myocardial wall function, cardiac valve function, and any myocardial wall motion abnormalities that may reflect myocardial ischemia. A catheter should be inserted into the bladder of patients who are expected to undergo long operations associated with alterations in blood volume and thus the infusion of large amounts of intravenous fluids.
Separation of the Lungs (One-Lung Anesthesia) • Separation of the lungs is perhaps the most important • anesthetic procedure in patients undergoing thoracic • surgery . Separation of the lungs permits intraoperative one-lung ventilation, which greatly facilitates the surgical procedure. Double-lumen endobronchial tubes (DLTs) and bronchial blockers (BBs) with single lumen endotracheal tubes enable anatomic isolation of the lungs and facilitate lung separation.
ANATOMIC CONSIDERATIONS • The tracheobronchial anatomy should first be assessed • by reviewing preoperative radiologic studies. In addition, • bronchoscopy is helpful immediately before surgery for • detecting abnormal anatomy that may complicate lung • separation. For example, a markedly distorted carina or a proximal endobronchial tumor may necessitate fiberoptic guided endobronchial intubation.
Tracheobronchial dimensions in general are approximately 20% larger in men than women. The right main bronchus diverges from the trachea at an angle of 25 degrees, whereas the left main bronchus diverges at 45 degrees. The right main bronchus is shorter but wider than the Left.
Tracheobronchial anatomy. (Right main-stem bronchus length, 1.8 ±0.8 cm; width, 1.6 ±0.2 cm. Left mainstem bronchus length, 4.8 :I:0.8 cm; width, 1.3 :I:0.2 cm.)
Although there is variation in tracheal and bronchial width in the population, within individual patients a significant correlation between tracheal and bronchial width has been determined (bronchial diameter is predicted to be 0.68 of tracheal diameter).
Based on these dimensional relationships, a left-sided • DLT is preferred because uniform ventilation to all lobes • will most likely be achieved, and measurement of tracheal width from a posteroanterior chest roentgenogram can help select the size of a left-sided DLl
LEFT-SIDED DOUBLE-LUMEN TUBE • Placement of a left-sided DLT is the most reliable and • widely used approach for endobronchial intubation in • one-lung ventilation . Several manufacturers such as Mallinckrodt, Rusch, and Sheridan produce clear, • disposable polyvinyl chloride tubes with high-volume, low pressure tracheal and bronchial cuffs. In general, a 35- or 37-French tube can be used for most women and a 39-French tube for most men.
Insertion Technique for Placement of a Left-Sided Double-Lumen Tube • Endobronchial intubation is usually accomplished by direct laryngoscopy after induction of general anesthesia and neuromuscular blockade. The left-sided DLT tube is held so that the distal curve faces anteriorly while the proximal curve is to the right. The bronchial cuff is inserted through the vocal cords, and the stylet is removed. Next, the tube is rotated 90 degrees to the left (directing the bronchial lumen to the left main stem bronchus). The tube is advanced until moderate resistance to further passage is encountered.
Force should never be used during advancement • of the tube; resistance usually indicates impingement • within the main stem bronchus. An estimate of the • appropriate depth of placement of the DLT can be based on the patient'sheight.
The average depth of insertion referenced to the corner of the mouth is 29 cm for patients 170 cm tall, and for each 10-cm increase or decrease in height, the average depth of placement correspondingly changes by 1 cm. Correct DLT position must be confirmed by fiberoptic bronchoscopy .
Dependence on physical examination to confirm proper • position of a left-sided DLT is not reliable, with fiberoptic • assessment showing mal positioning in 20% to 48% • of placements considered to be appropriate on the basis of auscultation.
Fiberoptic Visualization of a Left-SidedDouble-Lumen Tube • A 3.6-mm fiberscope is initially passed through the tracheal lumen. Correct position of the DLT is confirmed by visualization of the carina, a nonobstructed view of the right main stem bronchus, and the blue bronchial cuff below the carina
In addition, the line encircling the tube should be visualized. This line is 4 cm from the distal lumen, and it should ideally be positioned at or slightly above the carina. Fiberoptic visualization through the bronchial lumen reveals the bronchial carina and the left lower and upper lobes .
TubeMalpositioned Left-Sided Double-Lumen • A malpositioned left-sided DLT may occur during initial • placement, after surgical positioning, or during surgery. • A mal positioned tube is usually detected by clinical signs and changes in lung mechanics. During initiation of onelung ventilation, peak inspiratory airway pressure should increase by approximately 50% when compared with two lung ventilation at the same tidal volume.
when the DLT is malpositioned, peak inspiratory airway pressure will increase by approximately 75%. Two algorithms define three types of mal positioned left-sided DLTs .
RIGHT-SIDED DOUBLE-LUMEN TUBE • The short and variable distance of the right upper lobe • orifice from the carina makes the use of a right-sided • DLT undesirable for most procedures requiring lung • separation. A small change in the position of the tube • results in inadequate lung separation or collapse of the • right upper lobe, or both.
Nevertheless, in some situations it is best to avoid intubation of the left main stem bronchus (obstructed by tumor, disrupted after trauma, distorted secondary to a thoracic aortic aneurysm). Right-sided DLTs aredesigned to incorporate a separate opening in the bronchial lumen to allow ventilation of the right upper lobe .
Confirmation of correct right-sided DLT position by physical examination alone results in a 90% chance of malposition, with most being too deep. Proper positioning of a right-sided OLT must include fiberoptic guidance.
Bronchial Blockers • Lung separation can also be effectively achieved with a • single-lumen endotracheal tube and fiberoptically guided placement of a BB.The BB technique can be useful if postoperative ventilation will be required because it eliminates the need to exchange the DLT for a single-lumen tube. Using a BB is especially helpful when managing a difficult airway.
For example, in patients requiring an awake, fiberoptic intubation where DLT placement may be impossible, use of a BB may be the only practical approach to lung separation. • Confirmation of proper BB position should include • fiberoptic bronchoscopy.
UNIVENT BRONCHIAL BLOCKER TUBE • The Univent BB tube has two compartments: a large, • main lumen for conventional air passage and a small • lumen embedded in the anterior wall of the endotracheal tube that permits passage of the movable BB .