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Anesthetic considerations for descending thoracic aortic surgery: Part 1
REVIEW ARTICLE Kenneth J. Tuman, MD Section Editor
Anesthetic Considerations for Descending Thoracic Aortic Surgery: Part I Christopher J. O'Connor, MD, and David M. Rothenberg, MD HE ANESTHETIC management of the patient under-
T going repair of the descending thoracic aorta (DTA) reqmres not only extensive knowledge in the field of cardiovascular anesthesia but also an expertise and skill in (1) invasive and noninvasive hemodynamic monitoring, (2) managing gas exchange during one-lung ventilation, (3) regulating both proximal and distal perfusion pressures during aortic cross-clamping, including management of extracorporeal bypass and shunt techniques, (4) assessing renal and spinal cord function and the pharmacology involved in end-organ protection, and (5) monitoring and maintaining intraoperatwe hemostasis. Although the majorlty of this article focuses on intraoperative anesthetic concerns for repair of the DTA, certain aspects of caring for the patient undergoing aortic arch or ascending aortic surgery are discussed. The preoperative assessment of the patient with vascular disease, however, is not reviewed. CLASSIFICATION Disease of the DTA can be classified according to the etiology of the aneurysmal dilatation as well as the structural characteristics of the aneurysm wall. Typically, descending aortic aneurysms are secondary to atherosclerosis of the aorta, although other processes such as connective tissue disease, traumatic rupture, as well as infective and inflammatory aortitis may also produce aneurysmal changes. 1 In contrast to aneurysmal disease, aortic dissections occur when the aortic media has been split and there is extraluminal blood in the aortic wall; an aneurysm may or may not be present. 1 Two classtfications currently exist for aortic dissections, the most commonly applied being that of Debakey (Fig 1): Type I dissections extend from the ascending aorta into the descending aorta, whereas type 11 lesions are confined to the ascending aorta. Type 11I dissections begin in the descending aorta and extend from the left subclavian artery either to the diaphragm (Illa) or below it into the abdomen (1Ilb). 2 The Stanford classification of Daily designates dissections that involve the ascending aorta as type A, regardless of their site of origin, and all other dissections as type B (Fig 1). 2 These classification schemes have therapeutic implications because patients with acute distal dissections are initmlly treated medically, whereas proximal dissections are immediately repaired. Thoracoabdominal aneurysms are also divided into four groups according to Crawford's classification, which delineates both the origin
of the aneurysm and its degree of distal involvement of the abdominal aorta (Fig 2). 1 Although atherosclerosls is responsible for the majority of pure aneurysmal disease of the DTA, the underlying cause of aortic dissections involves either medial degenerative disease or hypertension. Connective tissue diseases are frequently associated with aortic dissection; in one series of patients with Marfans syndrome, more than 70% had aortic dissections. 3 Interestingly, 50% of aortic dissections in women under the age of 40 occur during the third trimester of pregnancy. 1 HEMODYNAMIC MONITORING
Arterial Blood Pressure Rapid and marked changes in arterial blood pressure may occur during thoracic aortic surgery, necessltatmg the need for continuous invasive blood pressure monitoring. Before anesthetic induction, it is imperatwe that the patient's blood pressure be assessed m each arm, because significant differences between right and left arm measurements may lead to false intraoperative interpretations and inappropriate therapeutic interventions. This variance is especially relevant in the panent with dissections involving the subclavian or renominate arteries or those with peripheral vascular disease, in whom this discrepancy may be more pronounced. 4 During repair of the DTA, placement of a proximal aortic cross-clamp between the left subclavian and common carotid arteries will eliminate a pressure recording in the left arm, thus precluding the use of the left radial or left brachial artery for monitoring proximal perfuslon pressure. The right radial or brachial artery is therefore the preferred site for monitoring, and in the setting of a left thoracotomy, with the right arm extended in the dependent position, both arteries are readily accessible. The right radial artery offers
From the Department of Anestheszology, Rush-Presbyterian-St. Luke's Medtcal Center, Chzcago, IL. Address reprint requests to Chnstopher J. O'Connor, MD, Department of Anesthestology, Rush-Presbyterian-St Luke's Medtcal Center, 1653 West Congress Pkwy, Chtcago, IL 60612. Copyright © 1995 by W.B. Saunders Company 1053-0770/95/0905-002153 00/0 Key words, anesthesta and descending thoractc aortic aneurysm, aortic dlssectton, transesophageal echocardtography, evoked potenttal monitonng
Journa/ofCardlothoraclc and VascularAnesthesta, Vol 9, No 5 (October), 1995' pp 581-588
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Type I
Type IT
Type A (proximal)
Type fir Type B (distal)
Fig 1. Classification schemes for aortic dissections. See text for discussion. (Reprinted with permission from Excerpta Medica, Inc.. 1984 )
ease of cannulatlon and a wide safety margin relative to complication rate. Slogoff et al prospectively evaluated almost 1,700 patients who received radial artery catheters for cardiovascular surgery and noted essentmlly no ischemic damage to the hand. 5 Momtorlng the braehial artery, owing to its more proximal location, will have less wave reflection than the radial artery and therefore yield a more accurate assessment of aortic pressure. As with radial artery monitoring, ischemic
? 22 Fig 2. Crawford classification of thoracoabdominal aneurysms, Type I extends from the proximal DTA to the upper abdominal aorta but terminates before the renal arteries. Type II extends below the renal arteries. Type III begins in the distal half of the DTA and extends for a variable length into the abdomen. Type IV involves most of the abdominal aorta. (Reprinted with permission. D)
comphcations of brachlal artery monitormg are also rare, given the adequate collateral circulation of the antecubital region. 6 Previous reports of peripheral ischemla during brachial artery cannulation were related to the use of the artery for diagnostic cardiac catheterization. 7,8 However, recent studies in which the brachial artery was cannulated for monitoring in cardiac surgery or in an intensive care unit failed to show any morbidity from the procedure. 9-u Monitoring blood pressure during ascending aorta repair, unlike descending aorta repair, is performed from either the left radial or left brachial artery. This relates to the surgical technique, which may require cross-clamping of the innommate artery. Finally, monitoring of the femoral artery pressure, in addition to the upper-extremity pressure, is often recommended to assess distal perfusion when a shunt, bypass, or a distal aortic clamp is used. Maintaining distal perfuslon pressure, as directed by femoral artery pressure, may aid in limiting renal and spinal cord ischemia. The femoral artery pressure monitor may also detect a malfunctioning external bypass or shunt. 12
Pulmonary Artery CatheterMomtormg The intraoperative use of a pulmonary artery catheter (PAC) for DTA surgery is necessitated by the marked hemodynamlc changes that often occur relative to aortic cross-clamping and by the degree of intraoperative blood loss. The need for expeditious and direct administration of vasoactwe drugs, as well as the measurement of cardiac output and mixed venous oxygen saturation, are other important reasons for inserting a PAC. Accurate interpretation of PAC-derived hemodynamic data, however, may be difficult during DTA surgery. When one-lung ventilation collapses the left lung during DTA surgery, a PAC placed in the left pulmonary artery may lead to pulmonary artery occlusion pressure (PAOP) tracings that grossly overestimate left ventricular end-diastolic pressure (LVEDP). Conversely, significant aortic regurgitation, as is often assooated with ascending aortic dissection, may cause premature closure of the mitral valve leading to the PAOP underestimating the LVEDP. 13 Correlation of continuous mixed venous oxygen saturation with cardiac output during aortic cross-clamping and unclamping has also been shown to be spurious. ~4 Even with the knowledge of these potential discrepancies, presently there are no studies in which the PAC has been specifically related to improved survival for DTA surgery.
Transesophageal Echocardiography Transesophageal echocardiography (TEE) is considered by some authorities to be the diagnostic procedure of choice for suspected acute aortic dissection 15 TEE has a reported sensitivity of 95% to 100% for the detection of acute aortic dissection, although its specificity is somewhat lower. Although its role in the initial evaluation of acute dissections is well accepted, TEE also has several potential applicatxons as an intraoperative monitoring technique during repair of the DTA. Assessment of LV functton. Although conventional ap-
DESCENDING THORACIC AORTIC SURGERY
proaches to the estimation of preload employ PAOPs and pulmonary artery pressures, TEE-derived analysis of LV end-diastolic dimensions is a more accurate estimation of preload and correlates closely with values obtained by radionuclide techniques. 16 In addition, analysis of enddiastolic areas more accurately approximates true enddiastolic volumes m individuals with noncompliant ventricles, such as those with LV hypertrophy, aortic stenosis, or ischemia. In these clinical scenarios, PAOP measurements poorly reflect ventricular preload. 13 As noted by Roizen et al, thoracic aortic cross-clamping produces substantial changes in ejection fraction and end-diastolic volumes. 17 TEE may thus more precisely evaluate left ventricular function after cross-clamping. In addition, ventricular short-axis views can estimate LV end-diastolic areas and preload changes during atriofemoral bypass or femoral-femoral bypass Finally, TEE can also aid in the assessment of right ventricular functxon. Myocardial tschemta monitoring. As noted by several investlgators, 17,Is myocardial ischemia is common during aortic surgery. Evidence suggests that regional wall motion abnormalities are a more sensitive and early indicator of ischemia than either electrocardiogram (ECG) changes or alterations in the pulmonary artery pressure waveforms. I6 TEE is therefore a useful monitor of early ischemla during aneurysm resection and proximal aortic cross-clamping. Evaluanon of aorttc dtssection. In patients with aortic dissections, lntraoperative TEE can confirm the preoperative diagnosis, locate entry sites in the lntimal flap (both proximal entry and distal reentry tears), define the quality of repair, and assess the adequacy of retrograde perfusion during femoral-femoral bypass.19-22 Confirmation of the preoperative diagnosis and identification of the true and false lumens, as well as locating the site of lntlmal tears, are essential components of all intraoperative TEE examinations during surgery for aortic dissections. However, defining the precise location of intireal tears and true and false lumens is most critical in ascending aortic dissections where adentfficatlon of true lumen commumcation, dissection, and intimal compression of coronary ostia provides the surgeon with important clinical information. Evaluation of the proximal DTA, the left subclavian artery, the aortic arch, and the ascending aorta ensures that the dissection is, indeed, hmited to the descending aorta. A hmitation of TEE in the setting of aortic dissection IS the inability to visualize the distal ascending aorta (caused by interference by the air filled trachea) 23and the distal extent of thoracoabdominal dissecting aneurysms, which usually requires epiaortlc or intravascular ultrasonography 19,22,24,25(Fig 3). In addition, reverberations in the aorta may m~m~c an intimal flap and are an important cause of false-positive findings in the TEE evaluation of aortic dissections. 2°,22 Differentiating between the true and false lumens may be difficult. However, typically, color Doppler and pulsed Doppler modes will demonstrate systolic forward flow with high velocities In the true lumen and slow flow or flow reversal in the false lumen (Fig 4). 22 Two-dimensional analysis often shows spontaneous echo contrast in the false
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Fig 3. Intravascular ultrasonographic images of the aorta at the level of the aortic arch (A), the middle portion of the descending thoracic aorta (B), and the proximal abdominal aorta (C) in a patient with a type I dissection An extensive dissection is seen with a circular intimal flap (F) separating the true lumen (TL) from the false lumen (FL). C denotes the catheter. The discrete signal shown adjacent to the catheter and the associated signal dropout are caused by the guide wire. (Reprinted with permission3 4)
lumen. In addition, the true lumen is smaller and tends to expand with systole and collapse during diastole. 21,26Entry tears in the intimal flap (either proximal or distal) can be located with color Doppler by detecting systolic flow from the true to the false lumen as the probe is withdrawn into the esophagus and the complete extent of the dissection is interrogated. 26 Absent flow in the false lumen indicates no
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Fig 4. Two-dimensional images and pulsed Doppler analysis by TEE of a dissection of the DTA. There is higher velocity and unidirectional blood flow in the true lumen (A), as compared with lower velocity and diastolic flow reversal in the false lumen (B}. T, true lumen. F, false lumen, (Reprinted with permission. 21)
communication or a more distal communication between the two lumens 22 or that the false lumen is filled with thrombus. Table 1 is a summary of the echocardiographic distinctions between true and false lumens using TEE. TEE evaluation of the DTA after aortic dissection repair can help to assess whether all entry sites have been closed. Persistent flow within the false lumen indicates the presence of additional unrecognized entry sites that may re-
quire further exploration and repair. 25 A recent study used TEE in the preoperative and mtraoperative assessment of 32 patients with suspected acute aortic dissection. 2° Preoperatively, TEE correctly identified the entry site in all but two patients. Visualization of the coronary arteries was possible in 25 of 28 patients, although only the left main or left anterior descending arteries could be observed. A good operative result, as judged by closure of the primary entry tear and no entry at the distal anastomosis, was noted in all patients. In addition, TEE verified retrograde flow in the true lumen after femoral cannulation for cases requiring cardiopulmonary bypass. The authors also reported intimal flaps after bypass in aortic segments that were not repaired. 2° Of interest, however, was the observation that in 35% of patients, a new entry site with flow in the false lumen could be demonstrated by TEE in these unrepaired aortic segments. The effect on prognosis of these new secondary tears has not yet been assessed; however, limited evidence as presented in this report suggests that new or persistent entry tears in unrepaired segments may not require reexploration or repair. 2° Despite this, intraoperative TEE provides the surgeon with immediate anatomic information about the completeness of the repair and the presence and precise location of these new lesions. Until adequate long-term follow-up is reported, the significance of these recently reported findings will remain unclear. In addition to the assessment of aortic repairs, TEE may also detect malperfusion from retrograde flow through the false lumen with initiation of femoral-femoral bypass during repairs of ascending or proximal descending aortic dissections. 21 One of the authors (C.J.O.) recently observed sudden death during cross-clamping of the DTA for repair of a chronic descending aortic dissection. Postmortem examination showed retrograde arch and ascending aortic dissection producing acute right and left main coronary artery occlusion. Although unavailable at the time, continuous TEE monitoring would have immediately alerted the surgical team to this operative catastrophe. Indeed, several reports have demonstrated the ability of intraoperative TEE to detect acute aortic dissection that complicated cannulation and clamping of the proximal aorta during routine cardiac s u r g e r y . 27-29 ANESTHESIA
Table 1. Aortic Dissection: Transesophageal Echocardiographic Differentiation Between True and False Lumens True Lumen Size Pulsation Spontaneous echo contrast Thrombus Locahzatlon Aortic arch flow Entry tear
False Lumen
Reduced, compressed Systohc lumen increase Rare, low
Longer than true lumen Systolic lumen decrease Dense, regular
Rare Inner curvature of aortic arch Systolic forward
Regular, progress=ve Outer curvature of aortic arch Delayed, reversed, or none Diastolic flow from false-to-true lumen
Systolic flow from true-to-false lumen
I 22 Reprinted with pe r m'sslon.
One-Lung Ventilation
Repair of the DTA requires a left thoracotomy for adequate surgical exposure. Because aneurysm resection is greatly facilitated by collapse of the left lung, double-lumen endobronehial tubes (DLT) are routinely used. Operatwe lung collapse and isolation not only improve exposure but also protect the nondependent left lung from the trauma of surgical retraction. In addition, the dependent lung is protected from blood that may enter the left mamstem bronchus because of left-lung mtrapulmonary hemorrhage, especially during procedures requiring heparinization or when the lung is adherent to the aneurysm. Most authors recommend the use of left-sided DLTs because of the potential for right upper lobe obstruction with right-sided DLTs. 3° However, special considerations apply during insertion of a left-sided DLT in the presence
DESCENDING THORACIC AORTIC SURGERY
of very large descending aneurysms, where endobronchial placement of the DLT may be technically difficult and hazardous owing to distortion or compression of the left mainstem bronchus or distal trachea by the aneurysm. 31-34 Moreover, repetitive blind attempts at DLT placement risk potential rupture of the aneurysm. Compressive phenomenona occur because of the close anatomic relationship of the aorta to the tracheobronchial tree and pulmonary artery. 33 Prolonged tracheobronchial compression caused by large thoracic aneurysms may also produce tracheomalacia, which may increase the risk of tracheobronchial disruption with DLT placement. To avoid these complications in high-risk patients (ie, those with very large aneurysms or respiratory symptoms suggesting airway compromise), DLT placement is best accomplished by initially visualizing the anatomy of the distal trachea and left mainstem bronchus with a fiberoptic bronchoscope before left endobronchial intubation. Pulsatile extrinsic compression of the left mainstem bronchus or bronchial erosion by the aneurysm contraindicates placement of a left-sided DLT. In this situation, a right-sided DLT is the appropriate choice, although a recent report documented that compression of the right mainstem bronchus can also be caused by a large thoracoabdominal aneurysm. 35Ideally, confirmation of the appropriate position of DLTs should be performed using a pediatric bronchoscope. This is especially true for right-sided DLTs where proper apposition of the slotted bronchial cuff over the right upper lobe orifice will ensure adequate ventilation of the right upper lobe. Collapse of the left lung may lead to clinically significant hypoxemia, especially m patients with preexisting pulmonary disease. Mitigating the degree of this shunt-reduced hypoxemia is the physiologic process of hypoxlc pulmonary vasoconstriction (HPV), which diverts blood flow from the nonventilated left lung to the ventilated right lung. 36 Although HPV does not appear to be affected by clinically applicable doses of inhalation and intravenous anesthetics, it is attenuated by infusions of potent vasodilators such as sodium nitroprusside and nitroglycerin, two agents commonly admlmstered during aortic surgery. 36,37 Many methods have been used in order to improve arterial oxygenation. Continuous positive alrway pressure at levels of 5 to 10 cm H20 applied to the nonventilated left lung is the simplest and most effective maneuver to improve oxygenatlon. 36 Surgery for lesions involving the ascending aorta and the aortic arch is performed through a median sternotomy and therefore requires only single-lumen endotracheal intubation to maintain ventilation and oxygenation. However, aortic arch aneurysms may also cause tracheobronchial compression, preventing even routine intubation and leading to potentially life-threatening airway obstruction. 38 Patients with this disorder commonly present with a history of respiratory distress exacerbated in the supine position and partmlly relieved on sitting or standing. Preoperative signs and symptoms suggestive of airway compromise include unilateral wheezing, cough, dyspnea, stridor, and tracheal deviatmn. In addition to aortography, computed tomography scanning, or magnetic resonance imaging, a flow volume loop study may aid in the diagnosis of such
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airway obstruction. Preoperatively, all sedation should be withheld to avoid further respiratory compromise. Femoralfemoral bypass should be performed under local anesthesia before the induction of general anesthesia, and rigid bronchoscopy should be readily available to assist, if needed, in intubating distal to the obstruction. 39,4° With huge aneurysms causing vascular or airway compression, induced hypotension has been successfully used to diminish the size of the aneurysm and allow passage of an endotracheal tube. 41 In addition to airway collapse, thoracic aneurysms may also compress the recurrent laryngeal nerve producing unilateral vocal cord paralysis, hoarseness, and a midline position of the involved cord. 3~Finally, esophageal compression with dysphagia and weight loss may also be encountered 31 and may complicate placement of a TEE probe. Resistance to probe advancement may indicate obstruction by the aneurysm. Univent tubes (Fuji Systems, Tokyo), single-lumen endotracheal tubes with a hollow moveable bronchial blocker, 3° have gained increasing popularity for lung isolation during thoracic surgery. The blocker is advanced into the respective mainstem bronchus where the blocker cuff is inflated with up to 7 mL of air, occluding the bronchus and producing lung collapse. A potential advantage of the Univent tube over DLTs for DTA repair is that it obviates the need to change from a DLT to a single-lumen endotracheal tube at the end of the case. However, a possible disadvantage is the high occlusion pressures of the blocker cuff (150 to 200 mmHg v 25 to 40 mmHg for DLT bronchial cuffs) and the higher transmural wall pressures when compared with the distal cuff of DLTs. 42 Theoretically, these higher transmural pressures may increase the risk of tracheobronchial injury or aneurysm rupture when large aneurysms compress the left malnstem bronchus. Although this complication of Univent tubes has not been observed during aneurysm resection, tracheobronchial rupture from the Univent blocker has been reported during esophageal surgery.43 When a DLT is used for lung separation, it is changed to a single-lumen tube at the end of the procedure. Frequently, however, prolonged aneurysm repair in the lateral position leads to substantial upper airway edema, and changing the DLT may be hazardous. Replacement with a single-lumen tube may be performed using direct laryngoscopy or fiberoptic guidance. If glottic visualization is inadequate, especially with a previously difficult intubation, the DLT should be pulled back into the trachea and the bronchial cuff deflated. The DLT should be left in place for 24 hours or until the edema has resolved, and the tube may then be safely changed. Anesthesia Induction and Maintenance At the present time, there are no well-controlled, randomized studies addressing whether choice of anesthesia alters outcome for DTA surgery. Decisions regarding induction and maintenance of anesthesia are therefore largely based on data taken from both the cardiac and abdominal aortic anesthesia literature. The discussion that follows focuses primarily on anesthetic choice for descending thoracic aortic
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surgery. Although caveats for anesthetic management for ascending and aortic arch repair are addressed, anesthesia for extracorporeal circulation and hypothermic circulatory arrest are beyond the scope of this review. Anesthetic induction is best performed by slowly administering a "sleep" dose of an intravenous anesthetic agent such as etomidate or midazolam followed by an intubating dose of a non-depolarizing neuromuscular agent. This initial induction technique will attenuate the risk of chest wall rigidity after high-dose opioid administration. 44Oploidinduced chest wall rigidity may cause hypercarbia and hypoxemia and trigger sympathetic nervous system-mediated tachycardia and hypertension. It is therefore recommended to establish mask ventilation and oxygenation before the administration of large doses of opioids. Intravenous opioids such as fentanyl or sufentanil may then be safely titrated to blunt the hemodynamic effects of subsequent endotracheal intubation. Laryngoscopy and endotracheal intubation often cause pronounced tachycardia and/or hypertension in the patient with myocardial or aortic disease, despite the use of high doses of opioids. 45 Irrespective of anesthetic choice, there is need to control heart rate and blood pressure not only during anesthetic induction, but during maintenance and emergence as well. This relates to the influence of heart rate on both myocardial oxygen supply (diminished diastolic filling time leading to inadequate coronary blood flow in patients with intraluminal coronary artery narrowing), and to a lesser extent on myocardial oxygen demand (increase in stroke work). The risk of sustained tachycardia is amplified during DTA surgery because of the high incidence of coronary artery disease in these patients and because of the tachycardia-induced increase in shearing force (dP/dt) within the aorta, with the subsequent risk of further aortic dissection or rupture. 46 Defining the optimal intraoperative blood pressure (mean, systolic, or diastolic) is difficult. Predictions are best ascertained by judging the patient's preoperative basehne values relative to their degree of impairment of end-organ perfusion. Patients with significant cerebrovascular, spinal cord, or renal insufficiency are dependent on maintaining an adequate mean arterml pressure. Coronary perfusion pressure is also dependent on aortic dmstohc pressure. Systolic blood pressure is governed in part by the velocity of ventricular contractions, thus influencing dP/dt and tensile strength of the aorta. 47 It ~s therefore Imperative preoperatwely to define ranges of pressures that mimmize end-organ ischemia without excessive risk to the anatomic integrity of the aorta. During induction, should tachycardia and hypertension persist despite deepening anesthesia, then pharmacologic intervention with beta-adrenergic antagonists to increase diastolic filling time and improve coronary blood flow, as well as to decrease dP/dt, may be essential. Esmolol, a short-acting, relatwely cardioselectwe beta-adrenergic antagonist, is ideally suited to treat tachycardia related to
episodic sympathetic nervous system stimulation. 48 Hypertension without associated tachycardia is best treated with intravenous nitroglycerin or sodium nitroprusside. Nitroglycerin is preferred if there are concomitant signs of myocardial ischemla, as sodium nitroprusside may decrease coronary artery blood flow to already lschemic myocardium. 49 The use of these vasodilators during aortic cross-clamping is discussed below. Based on results of large prospective studies in patients undergoing cardiac surgery, the choice of general anesthetic for maintenance is unlikely to influence the overall outcome. 5° Enflurane has been shown to increase cerebrospinal fluid (CSF) production and decrease CSF removal in dogs and theoretically should be avolded to minimize the risk of spinal cord lschemia from excessive CSF pressure. 51 The choice of other inhalation anesthetics or opiold infusions is determined by the cardaopulmonary and central nervous system effects of these agents and the physiologic responses desired. Controversy currently exists over the use of epldural anesthesia/analgesia for DTA surgery. A number of studies purport the benefits of combined general and epldural anesthesia for both abdominal aortic and thoracic surgery. These benefits not only include a decrease in postoperative pain, 52 but also decreases in adrenocortical stress response, 53 protein catabohsm, 54 hypercoagulable statey, 56 immunosuppression, 57 and pulmonary and myocardial morbidity. 55,58Presently only a single, nonrandomized, noncontrolled study appears in the hterature specifically reporting the use of thoraoc epidural anesthesia for descending thoracic and thoracoabdominal aortic aneurysms. 59 In this small study, 12 patients received epldural anesthesia, which blunted the increase in systemic vascular resistance and blood pressure associated with proximal aortic crossclamping. Unfortunately, no other risks or benefits of epidural anesthesia were examined in this study. Despite the potential benefits, many anesthesiologists and surgeons are concerned about the potential risks of epidural anesthesia during aortic surgery. Intraoperative hypotension, especially after aortic cross-clamp release, has been reported to be more pronounced in patients receiving epidural anesthesia. 6° The other major concern relates to the potential complication of epldural hematoma in patients in whom anticoagulatlon is to be initiated. However, large prospective and retrospective studies in patients receiving epidural anesthesm for major vascular surgery note the risk of hematoma to be minimal, although only low doses of heparin were used. 61,62 Most reports of epldural hematoma are anecdotal and often relate to traumatic placement rather than to an association with anticoagulation. 63 Nonetheless, until outcome studies are performed that compare combined epidural-general anesthesia versus general anesthesia alone for DTA surgery, the use of epidural anesthesia remains at the discretion of the individual anesthesiologist and surgeon.
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2 Crawford ES: The diagnosis and management of aortic dissection JAMA 264:2537-2541, 1990 3. Svensson LG, Crawford ES: Aortic dissection and aortic aneurysm surgery: Climcal observations, experimental investigations, and statistical analyses, Part II Curr Probl Surg December: 922-1011, 1992 4 Frank SM, Norris E, Crawley H, et al. Right and left arm blood pressure discrepancies in vascular surgery patients. Anesthesiology 73:A105, 1990 5 Slogoff S, Keats AS, Arlund C On the safety of radml artery cannulatlon. Anesthesiology 59:42-47, 1983 6. Bell JW: Treatment of post-catheterization arterial injuries' Use of survey plethysmography. Ann Surg 155'591-598, 1962 7 Bjork L, Enghoff E, Grevnik A, et al Local circulatory changes following brachlal artery catheterization. Vasc Dis 2:283292, 1965 8 Mortensen JD' Climcal sequelae from arterial needle puncture, cannulation and recision Clrculatmn 35:1118-1123, 1967 9. Barnes RW, Foster EJ, Janssen GA, et al: Safety of brachial arterial catheters as momtors in the mtenswe care unit-Prospectwe evaluation with the Doppler ultrasonic velocity detector Anesthesiology 44.260-264, 1976 10 Bazaral MG, Welch M, Goldmg LAR, et al' Comparison of brachial and radml arterial pressure monitoring In patients undergoing coronary artery bypass surgery Anesthesiology 73"38-45, 1990 11 Gravlee GP, Wong AB, Adkins TG, et al. A comparison of radial, brachml and aortic pressures after cardlopulmonary bypass J Cardlothorac Anesth 3:20-26, 1989 12. Kopman EA, Ferguson TB: Intraoperatwe monitoring of femoral artery pressure during replacement of aneurysm of descendlng thoracic aorta Anesth Analg 56:603-605, 1977 13 Tuman KJ, Carroll GC, Ivankovlch AD. Pitfalls in interpretation of pulmonary artery catheter data. J Cardlothoraclc Anesth 3"625-641, 1989 14 Shenaq SA, Casar G, Chelley JE, et al Continuous monitorIng of mixed venous oxygen saturation dunng aortic surgery Chest 92:796-799, 1987 15. Cigarroa JE, Isselbacher EM, DeSanctls RW, et al Diagnostic imaging m the evaluation of suspected aortic dissection N Engl J Med 328.35-43, 1993 16. Konstadt S, Reich D, Thys D, et al: Transesophageal echocardiography, in Kaplan JA (ed): Cardiac Anesthesia. New York, Saunders, 1993, pp 342-386 17. Roizen MF, Beaupre PN, Rick1 AA, et al Momtormg with two-dimensional transesophageal echocardlography. J Vasc Surg 1:300-305, 1984 18. Silversteln PR, Caldera DL, Cullen DJ, et al: Avoiding the hemodynamlc consequences of aortic cross-clamping and unclamplng. Anesthesiology 50:462-466, 1979 19 Takamoto S, Kyo S, Adachl H, et al: Decision making in the treatment of aortic dissection by transesophageal and intraoperatIve Doppler color flow mapping Echocardlography 17.308, 1992 (abstr) 20. Simon P, Owen AN, Havel M, et al: Transesophageal echocardiography in the emergency surgical management of patients with aortic dissection. J Thorac Cardiovasc Surg 103 11131118, 1992 21. Neustein SM, Lansman SL, Quintana CS, et al. Transesophageal Doppler echocardiographic monitoring for malperfusion during aortic dissection repair. Ann Thorac Surg 56.358-361, 1993 22. Erbel R. Role of transesophageal echocardlography in dissection of the aorta and evaluation of degenerative aortic disease. Cardlol Chn 11"461-472, 1993
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23. Konstadt SN, Reich DL, Quintana C, et al: The ascending aorta: How much does transesophageal echocardiography see? Anesth Analg 78'240-244, 1994 24. Weintraub AR, Schwartz SL, Pandlan NG, et al' Evaluation of aortic dissection by mtravascular ultrasonography. N Engl J Med 323:1566-1567, 1990 25 Czer LS, Siegel R J, Maurer G, et al: Transesophageal and epicardial color-flow Doppler echocardlography, m Kaplan JA (ed): Cardiac Anesthesia New York, Saunders, 1993, pp 342-386 26. Orihashl K, Sisto DA' Aorta, m Oka Y, Goldiner PL (eds): Transesophageal Echocardiography Philadelphia, Lippmcott, 1992, pp 189-227 27. Bjerke RJ Intraoperatlve diagnosis of aortic dissection using transesophageal echocardlography. J Cardiothorac Anesth 6:720-723, 1992 28 Troianos CA, Savmo JS, Weiss RL: Transesophageal echocardlographic diagnosis of aortic dissection during cardiac surgery. Anesthesiology 75'149-153, 1991 29 Katz ES, Tunlck PA, Colvln SB: Aortic dissection complicatmg cardiac surgery' Diagnosis by lntraoperatwe biplane transesophageal echocardlography J Am Soc Echocardiogr 6.217-222, 1993 30. Benumof JL' Anesthesia for Thoracic Surgery Philadelphia, Saunders, 1987, pp 223-259 31 Campos JH, Ajax TJ, Knutson RM, et al' Case conference 5-1990. J CardIothorac Anesth 4'631-645, 1990 32 Cohen JA, Demsco RA, Rlchards TS, et al: Hazardous placement of a Robertshaw-type endobronchlal tube. Anesth Analg 65:100-101, 1986 33. Morl M, Chuma R, Kiichl Y, et al: The anesthetic management of a paUent with a thoracic aortic aneurysm that caused compression of the left malnstem bronchus and the right pulmonary artery J Cardlothorac Vasc Anesth 7'579-584, 1993 34. Verdant A' Chronic traumatic aneurysm of the descending thoracic aorta with compression of the tracheobronchlal tree. Can J Surg 27 278-279, 1984 35 Gorman RB, Merrltt WT, Greenspun H, et al Aneurysmal compression of the trachea and right mamstem bronchus comphcatlng thoracoabdominal aneurysm repair, Anesthesiology 79'14241427, 1993 36 Benumof JL' One-lung ventilation and hyporac pulmonary vasoconstriction: Implications for anesthetic management Anesth Analg 64.821-833, 1985 37 Bennmof JL. Hypoxlc pulmonary vasoconstriction and sodium mtroprusslde infusion Anesthesiology 50:481-483, 1979 38 MacGillwray RG. Tracheal compression caused by aneurysms of the aortic arch Imphcation for the anesthetist Anaethesla 40.270-277, 1985 39 Charrette EJP, Wmton TL, Salerno TA: Acute respiratory Insufficiency from an aneurysm of the descending thoracic aorta. J Thorac Cardiovasc Surg 85'467-470, 1983 40. Malave G, Yerena G, Gonzalez R, et al: Method for resection and prosthetic replacement of aneurysm of aortic arch. J Thor Cardlovasc Surg 74'798-802, 1977 41. Nishiwakl K, Komatsu T. Shimada Y, et al: Severe tracheal compression caused by false aneurysm arising from the ascending aorta: Successful airway management using Induced hypotension and bronchoscopy Anesthesiology 73.1047-1049, 1990 42 Kelley JG, Gaba DM, Brodsky JB Bronchml cuff pressures of two tubes used m thoracic surgery J Cardlothorac Vasc Anesth 6"190-192, 1992 43 Schwartz DE, Yost CS, Larson MD: Pneumothorax comphcating the use of a Unlvent ® endotracheal tube Anesth Analg 76"443-445, 1993
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44. Benthuysen JL, Smith NT, Sanford TJ, et al. The physiology of alfentanil-induced rlgl&ty. Anesthesiology 64:440-446, 1986 45. Frlesen RM, Thomson IR, Husdon RJ, et al: Fentanyl oxygen anaesthesia for abdominal aortic surgery Can Anaesth Soc J 33'719-722, 1986 46. DeSanctis RW, Doroghazl RM, Austen WG, et al: Aortic dissection. N Eng J Med 317:1060-1067, 1987 47. Prokop EK, Palmer RF, Wheat MW Jr' Hydrodynamic forces m dissecting aneurysms: in-vitro studies in a tygon model and in dog aortas. Clrc Res 27'121-127, 1970 48. Girard D, Shulman BJ, Thys DM, et al. The safety and efficacy of esmolol during myocardial revasculanzatlon Anesthesiology 65:157-164, 1986 49 Mann T, Cohn PF, Holman BL, et al: Effect of nltroprusslde on regional myocardial blood flow in coronary artery disease. Circulation 57:732-738, 1978 50. Tuman KJ, McCarthy RJ, Spiess BD, et al: Does choice of anesthetic agent s~gnlficantly affect outcome after coronary artery surgery9 Anesthesiology 70:189-198, 1989 51 Artru AA: Relationship between cerebral blood volume and CSF pressure during anesthesia with halothane or enflurane in dogs. Anesthesiology 58.533-539, 1983 52 Logas WG, E1-Baz N, E1-Ganzouri A, et al: Continuous thoracic epldural analgesia for postoperative pain rehef following thoracotomy A randomized prospectwe study. Anesthesiology 67:787-791, 1987 53 Hjortso NC, Neumann P, Froslg F, et al: Effects of the extradural admmlstrat~on of local anesthetic agents and morphine on the urinary excretion of cortisol, cathecholammes, and nitrogen following abdominal surgery Br J Anestheslol 57 400-406, 1985 54. Kehlet H: The stress-response to anesthesia and surgery-
Release mechamsm and modifying factors. Clln Anaestheslol 2'315-339, 1984 55 Tuman KJ, McCarthy RJ, March RJ, et al: Effects of epldural anesthesia and analgesia on coagulation and outcome after major vascular surgery. Anesth Analg 73:696-704, 1991 56. Rosenfeld BA, Beattle C, Chrlsopherson R, et al: The effects of different anesthetic regimens on fibrinolysis and the development of postoperative arterial thrombosis. Anesthesiology 79'435-443, 1993 57. Rem J, Brandt MR, Kehlet H: Prevention of postoperative lymphopenla and granulocytosis by epldural analgesia Lancet 1:283-285, 1980 58. Yeager MP, Glass DD, Neff RK, et al' Epidural anesthesia and analgesia in high-risk surgical patients. Anesthesiology 66 729736, 1987 59. Stenseth R, Myhre HO: Anaesthesia in surgery for aneurysm of the descending thoracic or thoracoabdominal aorta Acta Chir Scand 154:147-150, 1988 60. Lun.n JK, Dannemiller FJ, Stanley TH' Car&ovascular responses to clamping of the aorta during epldural and general anesthesia Anesth Analg 58.372-376, 1979 61. Odoom JA, Sih IL: Epidural analgesia and anticoagulant therapy. Anaesthesia 38:254-259, 1983 62. Rao TLK, E1-Etr A: Antlcoagulatlon following placement of epldural and subarachnold catheters. Anesthesiology 55:618-620, 1981 63. Owens EL, Kasten GW, Hessel EA. Spinal subarachnoid hematoma after lumbar puncture and heparlnlzatlon: A case report, review of the hterature and discussion of anesthetic implications. Anesth Analg 65.1201-1207, 1986
Part 2 will appear in the December 1995 issue of THE JOURNAL.
Anesthetic Considerations for Descending Thoracic Aortic Surgery: Part I Christopher J. O'Connor, MD, and David M. Rothenberg, MD HE ANESTHETIC management of the patient under-
T going repair of the descending thoracic aorta (DTA) reqmres not only extensive knowledge in the field of cardiovascular anesthesia but also an expertise and skill in (1) invasive and noninvasive hemodynamic monitoring, (2) managing gas exchange during one-lung ventilation, (3) regulating both proximal and distal perfusion pressures during aortic cross-clamping, including management of extracorporeal bypass and shunt techniques, (4) assessing renal and spinal cord function and the pharmacology involved in end-organ protection, and (5) monitoring and maintaining intraoperatwe hemostasis. Although the majorlty of this article focuses on intraoperative anesthetic concerns for repair of the DTA, certain aspects of caring for the patient undergoing aortic arch or ascending aortic surgery are discussed. The preoperative assessment of the patient with vascular disease, however, is not reviewed. CLASSIFICATION Disease of the DTA can be classified according to the etiology of the aneurysmal dilatation as well as the structural characteristics of the aneurysm wall. Typically, descending aortic aneurysms are secondary to atherosclerosis of the aorta, although other processes such as connective tissue disease, traumatic rupture, as well as infective and inflammatory aortitis may also produce aneurysmal changes. 1 In contrast to aneurysmal disease, aortic dissections occur when the aortic media has been split and there is extraluminal blood in the aortic wall; an aneurysm may or may not be present. 1 Two classtfications currently exist for aortic dissections, the most commonly applied being that of Debakey (Fig 1): Type I dissections extend from the ascending aorta into the descending aorta, whereas type 11 lesions are confined to the ascending aorta. Type 11I dissections begin in the descending aorta and extend from the left subclavian artery either to the diaphragm (Illa) or below it into the abdomen (1Ilb). 2 The Stanford classification of Daily designates dissections that involve the ascending aorta as type A, regardless of their site of origin, and all other dissections as type B (Fig 1). 2 These classification schemes have therapeutic implications because patients with acute distal dissections are initmlly treated medically, whereas proximal dissections are immediately repaired. Thoracoabdominal aneurysms are also divided into four groups according to Crawford's classification, which delineates both the origin
of the aneurysm and its degree of distal involvement of the abdominal aorta (Fig 2). 1 Although atherosclerosls is responsible for the majority of pure aneurysmal disease of the DTA, the underlying cause of aortic dissections involves either medial degenerative disease or hypertension. Connective tissue diseases are frequently associated with aortic dissection; in one series of patients with Marfans syndrome, more than 70% had aortic dissections. 3 Interestingly, 50% of aortic dissections in women under the age of 40 occur during the third trimester of pregnancy. 1 HEMODYNAMIC MONITORING
Arterial Blood Pressure Rapid and marked changes in arterial blood pressure may occur during thoracic aortic surgery, necessltatmg the need for continuous invasive blood pressure monitoring. Before anesthetic induction, it is imperatwe that the patient's blood pressure be assessed m each arm, because significant differences between right and left arm measurements may lead to false intraoperative interpretations and inappropriate therapeutic interventions. This variance is especially relevant in the panent with dissections involving the subclavian or renominate arteries or those with peripheral vascular disease, in whom this discrepancy may be more pronounced. 4 During repair of the DTA, placement of a proximal aortic cross-clamp between the left subclavian and common carotid arteries will eliminate a pressure recording in the left arm, thus precluding the use of the left radial or left brachial artery for monitoring proximal perfuslon pressure. The right radial or brachial artery is therefore the preferred site for monitoring, and in the setting of a left thoracotomy, with the right arm extended in the dependent position, both arteries are readily accessible. The right radial artery offers
From the Department of Anestheszology, Rush-Presbyterian-St. Luke's Medtcal Center, Chzcago, IL. Address reprint requests to Chnstopher J. O'Connor, MD, Department of Anesthestology, Rush-Presbyterian-St Luke's Medtcal Center, 1653 West Congress Pkwy, Chtcago, IL 60612. Copyright © 1995 by W.B. Saunders Company 1053-0770/95/0905-002153 00/0 Key words, anesthesta and descending thoractc aortic aneurysm, aortic dlssectton, transesophageal echocardtography, evoked potenttal monitonng
Journa/ofCardlothoraclc and VascularAnesthesta, Vol 9, No 5 (October), 1995' pp 581-588
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Type I
Type IT
Type A (proximal)
Type fir Type B (distal)
Fig 1. Classification schemes for aortic dissections. See text for discussion. (Reprinted with permission from Excerpta Medica, Inc.. 1984 )
ease of cannulatlon and a wide safety margin relative to complication rate. Slogoff et al prospectively evaluated almost 1,700 patients who received radial artery catheters for cardiovascular surgery and noted essentmlly no ischemic damage to the hand. 5 Momtorlng the braehial artery, owing to its more proximal location, will have less wave reflection than the radial artery and therefore yield a more accurate assessment of aortic pressure. As with radial artery monitoring, ischemic
? 22 Fig 2. Crawford classification of thoracoabdominal aneurysms, Type I extends from the proximal DTA to the upper abdominal aorta but terminates before the renal arteries. Type II extends below the renal arteries. Type III begins in the distal half of the DTA and extends for a variable length into the abdomen. Type IV involves most of the abdominal aorta. (Reprinted with permission. D)
comphcations of brachlal artery monitormg are also rare, given the adequate collateral circulation of the antecubital region. 6 Previous reports of peripheral ischemla during brachial artery cannulation were related to the use of the artery for diagnostic cardiac catheterization. 7,8 However, recent studies in which the brachial artery was cannulated for monitoring in cardiac surgery or in an intensive care unit failed to show any morbidity from the procedure. 9-u Monitoring blood pressure during ascending aorta repair, unlike descending aorta repair, is performed from either the left radial or left brachial artery. This relates to the surgical technique, which may require cross-clamping of the innommate artery. Finally, monitoring of the femoral artery pressure, in addition to the upper-extremity pressure, is often recommended to assess distal perfusion when a shunt, bypass, or a distal aortic clamp is used. Maintaining distal perfuslon pressure, as directed by femoral artery pressure, may aid in limiting renal and spinal cord ischemia. The femoral artery pressure monitor may also detect a malfunctioning external bypass or shunt. 12
Pulmonary Artery CatheterMomtormg The intraoperative use of a pulmonary artery catheter (PAC) for DTA surgery is necessitated by the marked hemodynamlc changes that often occur relative to aortic cross-clamping and by the degree of intraoperative blood loss. The need for expeditious and direct administration of vasoactwe drugs, as well as the measurement of cardiac output and mixed venous oxygen saturation, are other important reasons for inserting a PAC. Accurate interpretation of PAC-derived hemodynamic data, however, may be difficult during DTA surgery. When one-lung ventilation collapses the left lung during DTA surgery, a PAC placed in the left pulmonary artery may lead to pulmonary artery occlusion pressure (PAOP) tracings that grossly overestimate left ventricular end-diastolic pressure (LVEDP). Conversely, significant aortic regurgitation, as is often assooated with ascending aortic dissection, may cause premature closure of the mitral valve leading to the PAOP underestimating the LVEDP. 13 Correlation of continuous mixed venous oxygen saturation with cardiac output during aortic cross-clamping and unclamping has also been shown to be spurious. ~4 Even with the knowledge of these potential discrepancies, presently there are no studies in which the PAC has been specifically related to improved survival for DTA surgery.
Transesophageal Echocardiography Transesophageal echocardiography (TEE) is considered by some authorities to be the diagnostic procedure of choice for suspected acute aortic dissection 15 TEE has a reported sensitivity of 95% to 100% for the detection of acute aortic dissection, although its specificity is somewhat lower. Although its role in the initial evaluation of acute dissections is well accepted, TEE also has several potential applicatxons as an intraoperative monitoring technique during repair of the DTA. Assessment of LV functton. Although conventional ap-
DESCENDING THORACIC AORTIC SURGERY
proaches to the estimation of preload employ PAOPs and pulmonary artery pressures, TEE-derived analysis of LV end-diastolic dimensions is a more accurate estimation of preload and correlates closely with values obtained by radionuclide techniques. 16 In addition, analysis of enddiastolic areas more accurately approximates true enddiastolic volumes m individuals with noncompliant ventricles, such as those with LV hypertrophy, aortic stenosis, or ischemia. In these clinical scenarios, PAOP measurements poorly reflect ventricular preload. 13 As noted by Roizen et al, thoracic aortic cross-clamping produces substantial changes in ejection fraction and end-diastolic volumes. 17 TEE may thus more precisely evaluate left ventricular function after cross-clamping. In addition, ventricular short-axis views can estimate LV end-diastolic areas and preload changes during atriofemoral bypass or femoral-femoral bypass Finally, TEE can also aid in the assessment of right ventricular functxon. Myocardial tschemta monitoring. As noted by several investlgators, 17,Is myocardial ischemia is common during aortic surgery. Evidence suggests that regional wall motion abnormalities are a more sensitive and early indicator of ischemia than either electrocardiogram (ECG) changes or alterations in the pulmonary artery pressure waveforms. I6 TEE is therefore a useful monitor of early ischemla during aneurysm resection and proximal aortic cross-clamping. Evaluanon of aorttc dtssection. In patients with aortic dissections, lntraoperative TEE can confirm the preoperative diagnosis, locate entry sites in the lntimal flap (both proximal entry and distal reentry tears), define the quality of repair, and assess the adequacy of retrograde perfusion during femoral-femoral bypass.19-22 Confirmation of the preoperative diagnosis and identification of the true and false lumens, as well as locating the site of lntlmal tears, are essential components of all intraoperative TEE examinations during surgery for aortic dissections. However, defining the precise location of intireal tears and true and false lumens is most critical in ascending aortic dissections where adentfficatlon of true lumen commumcation, dissection, and intimal compression of coronary ostia provides the surgeon with important clinical information. Evaluation of the proximal DTA, the left subclavian artery, the aortic arch, and the ascending aorta ensures that the dissection is, indeed, hmited to the descending aorta. A hmitation of TEE in the setting of aortic dissection IS the inability to visualize the distal ascending aorta (caused by interference by the air filled trachea) 23and the distal extent of thoracoabdominal dissecting aneurysms, which usually requires epiaortlc or intravascular ultrasonography 19,22,24,25(Fig 3). In addition, reverberations in the aorta may m~m~c an intimal flap and are an important cause of false-positive findings in the TEE evaluation of aortic dissections. 2°,22 Differentiating between the true and false lumens may be difficult. However, typically, color Doppler and pulsed Doppler modes will demonstrate systolic forward flow with high velocities In the true lumen and slow flow or flow reversal in the false lumen (Fig 4). 22 Two-dimensional analysis often shows spontaneous echo contrast in the false
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Fig 3. Intravascular ultrasonographic images of the aorta at the level of the aortic arch (A), the middle portion of the descending thoracic aorta (B), and the proximal abdominal aorta (C) in a patient with a type I dissection An extensive dissection is seen with a circular intimal flap (F) separating the true lumen (TL) from the false lumen (FL). C denotes the catheter. The discrete signal shown adjacent to the catheter and the associated signal dropout are caused by the guide wire. (Reprinted with permission3 4)
lumen. In addition, the true lumen is smaller and tends to expand with systole and collapse during diastole. 21,26Entry tears in the intimal flap (either proximal or distal) can be located with color Doppler by detecting systolic flow from the true to the false lumen as the probe is withdrawn into the esophagus and the complete extent of the dissection is interrogated. 26 Absent flow in the false lumen indicates no
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Fig 4. Two-dimensional images and pulsed Doppler analysis by TEE of a dissection of the DTA. There is higher velocity and unidirectional blood flow in the true lumen (A), as compared with lower velocity and diastolic flow reversal in the false lumen (B}. T, true lumen. F, false lumen, (Reprinted with permission. 21)
communication or a more distal communication between the two lumens 22 or that the false lumen is filled with thrombus. Table 1 is a summary of the echocardiographic distinctions between true and false lumens using TEE. TEE evaluation of the DTA after aortic dissection repair can help to assess whether all entry sites have been closed. Persistent flow within the false lumen indicates the presence of additional unrecognized entry sites that may re-
quire further exploration and repair. 25 A recent study used TEE in the preoperative and mtraoperative assessment of 32 patients with suspected acute aortic dissection. 2° Preoperatively, TEE correctly identified the entry site in all but two patients. Visualization of the coronary arteries was possible in 25 of 28 patients, although only the left main or left anterior descending arteries could be observed. A good operative result, as judged by closure of the primary entry tear and no entry at the distal anastomosis, was noted in all patients. In addition, TEE verified retrograde flow in the true lumen after femoral cannulation for cases requiring cardiopulmonary bypass. The authors also reported intimal flaps after bypass in aortic segments that were not repaired. 2° Of interest, however, was the observation that in 35% of patients, a new entry site with flow in the false lumen could be demonstrated by TEE in these unrepaired aortic segments. The effect on prognosis of these new secondary tears has not yet been assessed; however, limited evidence as presented in this report suggests that new or persistent entry tears in unrepaired segments may not require reexploration or repair. 2° Despite this, intraoperative TEE provides the surgeon with immediate anatomic information about the completeness of the repair and the presence and precise location of these new lesions. Until adequate long-term follow-up is reported, the significance of these recently reported findings will remain unclear. In addition to the assessment of aortic repairs, TEE may also detect malperfusion from retrograde flow through the false lumen with initiation of femoral-femoral bypass during repairs of ascending or proximal descending aortic dissections. 21 One of the authors (C.J.O.) recently observed sudden death during cross-clamping of the DTA for repair of a chronic descending aortic dissection. Postmortem examination showed retrograde arch and ascending aortic dissection producing acute right and left main coronary artery occlusion. Although unavailable at the time, continuous TEE monitoring would have immediately alerted the surgical team to this operative catastrophe. Indeed, several reports have demonstrated the ability of intraoperative TEE to detect acute aortic dissection that complicated cannulation and clamping of the proximal aorta during routine cardiac s u r g e r y . 27-29 ANESTHESIA
Table 1. Aortic Dissection: Transesophageal Echocardiographic Differentiation Between True and False Lumens True Lumen Size Pulsation Spontaneous echo contrast Thrombus Locahzatlon Aortic arch flow Entry tear
False Lumen
Reduced, compressed Systohc lumen increase Rare, low
Longer than true lumen Systolic lumen decrease Dense, regular
Rare Inner curvature of aortic arch Systolic forward
Regular, progress=ve Outer curvature of aortic arch Delayed, reversed, or none Diastolic flow from false-to-true lumen
Systolic flow from true-to-false lumen
I 22 Reprinted with pe r m'sslon.
One-Lung Ventilation
Repair of the DTA requires a left thoracotomy for adequate surgical exposure. Because aneurysm resection is greatly facilitated by collapse of the left lung, double-lumen endobronehial tubes (DLT) are routinely used. Operatwe lung collapse and isolation not only improve exposure but also protect the nondependent left lung from the trauma of surgical retraction. In addition, the dependent lung is protected from blood that may enter the left mamstem bronchus because of left-lung mtrapulmonary hemorrhage, especially during procedures requiring heparinization or when the lung is adherent to the aneurysm. Most authors recommend the use of left-sided DLTs because of the potential for right upper lobe obstruction with right-sided DLTs. 3° However, special considerations apply during insertion of a left-sided DLT in the presence
DESCENDING THORACIC AORTIC SURGERY
of very large descending aneurysms, where endobronchial placement of the DLT may be technically difficult and hazardous owing to distortion or compression of the left mainstem bronchus or distal trachea by the aneurysm. 31-34 Moreover, repetitive blind attempts at DLT placement risk potential rupture of the aneurysm. Compressive phenomenona occur because of the close anatomic relationship of the aorta to the tracheobronchial tree and pulmonary artery. 33 Prolonged tracheobronchial compression caused by large thoracic aneurysms may also produce tracheomalacia, which may increase the risk of tracheobronchial disruption with DLT placement. To avoid these complications in high-risk patients (ie, those with very large aneurysms or respiratory symptoms suggesting airway compromise), DLT placement is best accomplished by initially visualizing the anatomy of the distal trachea and left mainstem bronchus with a fiberoptic bronchoscope before left endobronchial intubation. Pulsatile extrinsic compression of the left mainstem bronchus or bronchial erosion by the aneurysm contraindicates placement of a left-sided DLT. In this situation, a right-sided DLT is the appropriate choice, although a recent report documented that compression of the right mainstem bronchus can also be caused by a large thoracoabdominal aneurysm. 35Ideally, confirmation of the appropriate position of DLTs should be performed using a pediatric bronchoscope. This is especially true for right-sided DLTs where proper apposition of the slotted bronchial cuff over the right upper lobe orifice will ensure adequate ventilation of the right upper lobe. Collapse of the left lung may lead to clinically significant hypoxemia, especially m patients with preexisting pulmonary disease. Mitigating the degree of this shunt-reduced hypoxemia is the physiologic process of hypoxlc pulmonary vasoconstriction (HPV), which diverts blood flow from the nonventilated left lung to the ventilated right lung. 36 Although HPV does not appear to be affected by clinically applicable doses of inhalation and intravenous anesthetics, it is attenuated by infusions of potent vasodilators such as sodium nitroprusside and nitroglycerin, two agents commonly admlmstered during aortic surgery. 36,37 Many methods have been used in order to improve arterial oxygenation. Continuous positive alrway pressure at levels of 5 to 10 cm H20 applied to the nonventilated left lung is the simplest and most effective maneuver to improve oxygenatlon. 36 Surgery for lesions involving the ascending aorta and the aortic arch is performed through a median sternotomy and therefore requires only single-lumen endotracheal intubation to maintain ventilation and oxygenation. However, aortic arch aneurysms may also cause tracheobronchial compression, preventing even routine intubation and leading to potentially life-threatening airway obstruction. 38 Patients with this disorder commonly present with a history of respiratory distress exacerbated in the supine position and partmlly relieved on sitting or standing. Preoperative signs and symptoms suggestive of airway compromise include unilateral wheezing, cough, dyspnea, stridor, and tracheal deviatmn. In addition to aortography, computed tomography scanning, or magnetic resonance imaging, a flow volume loop study may aid in the diagnosis of such
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airway obstruction. Preoperatively, all sedation should be withheld to avoid further respiratory compromise. Femoralfemoral bypass should be performed under local anesthesia before the induction of general anesthesia, and rigid bronchoscopy should be readily available to assist, if needed, in intubating distal to the obstruction. 39,4° With huge aneurysms causing vascular or airway compression, induced hypotension has been successfully used to diminish the size of the aneurysm and allow passage of an endotracheal tube. 41 In addition to airway collapse, thoracic aneurysms may also compress the recurrent laryngeal nerve producing unilateral vocal cord paralysis, hoarseness, and a midline position of the involved cord. 3~Finally, esophageal compression with dysphagia and weight loss may also be encountered 31 and may complicate placement of a TEE probe. Resistance to probe advancement may indicate obstruction by the aneurysm. Univent tubes (Fuji Systems, Tokyo), single-lumen endotracheal tubes with a hollow moveable bronchial blocker, 3° have gained increasing popularity for lung isolation during thoracic surgery. The blocker is advanced into the respective mainstem bronchus where the blocker cuff is inflated with up to 7 mL of air, occluding the bronchus and producing lung collapse. A potential advantage of the Univent tube over DLTs for DTA repair is that it obviates the need to change from a DLT to a single-lumen endotracheal tube at the end of the case. However, a possible disadvantage is the high occlusion pressures of the blocker cuff (150 to 200 mmHg v 25 to 40 mmHg for DLT bronchial cuffs) and the higher transmural wall pressures when compared with the distal cuff of DLTs. 42 Theoretically, these higher transmural pressures may increase the risk of tracheobronchial injury or aneurysm rupture when large aneurysms compress the left malnstem bronchus. Although this complication of Univent tubes has not been observed during aneurysm resection, tracheobronchial rupture from the Univent blocker has been reported during esophageal surgery.43 When a DLT is used for lung separation, it is changed to a single-lumen tube at the end of the procedure. Frequently, however, prolonged aneurysm repair in the lateral position leads to substantial upper airway edema, and changing the DLT may be hazardous. Replacement with a single-lumen tube may be performed using direct laryngoscopy or fiberoptic guidance. If glottic visualization is inadequate, especially with a previously difficult intubation, the DLT should be pulled back into the trachea and the bronchial cuff deflated. The DLT should be left in place for 24 hours or until the edema has resolved, and the tube may then be safely changed. Anesthesia Induction and Maintenance At the present time, there are no well-controlled, randomized studies addressing whether choice of anesthesia alters outcome for DTA surgery. Decisions regarding induction and maintenance of anesthesia are therefore largely based on data taken from both the cardiac and abdominal aortic anesthesia literature. The discussion that follows focuses primarily on anesthetic choice for descending thoracic aortic
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surgery. Although caveats for anesthetic management for ascending and aortic arch repair are addressed, anesthesia for extracorporeal circulation and hypothermic circulatory arrest are beyond the scope of this review. Anesthetic induction is best performed by slowly administering a "sleep" dose of an intravenous anesthetic agent such as etomidate or midazolam followed by an intubating dose of a non-depolarizing neuromuscular agent. This initial induction technique will attenuate the risk of chest wall rigidity after high-dose opioid administration. 44Oploidinduced chest wall rigidity may cause hypercarbia and hypoxemia and trigger sympathetic nervous system-mediated tachycardia and hypertension. It is therefore recommended to establish mask ventilation and oxygenation before the administration of large doses of opioids. Intravenous opioids such as fentanyl or sufentanil may then be safely titrated to blunt the hemodynamic effects of subsequent endotracheal intubation. Laryngoscopy and endotracheal intubation often cause pronounced tachycardia and/or hypertension in the patient with myocardial or aortic disease, despite the use of high doses of opioids. 45 Irrespective of anesthetic choice, there is need to control heart rate and blood pressure not only during anesthetic induction, but during maintenance and emergence as well. This relates to the influence of heart rate on both myocardial oxygen supply (diminished diastolic filling time leading to inadequate coronary blood flow in patients with intraluminal coronary artery narrowing), and to a lesser extent on myocardial oxygen demand (increase in stroke work). The risk of sustained tachycardia is amplified during DTA surgery because of the high incidence of coronary artery disease in these patients and because of the tachycardia-induced increase in shearing force (dP/dt) within the aorta, with the subsequent risk of further aortic dissection or rupture. 46 Defining the optimal intraoperative blood pressure (mean, systolic, or diastolic) is difficult. Predictions are best ascertained by judging the patient's preoperative basehne values relative to their degree of impairment of end-organ perfusion. Patients with significant cerebrovascular, spinal cord, or renal insufficiency are dependent on maintaining an adequate mean arterml pressure. Coronary perfusion pressure is also dependent on aortic dmstohc pressure. Systolic blood pressure is governed in part by the velocity of ventricular contractions, thus influencing dP/dt and tensile strength of the aorta. 47 It ~s therefore Imperative preoperatwely to define ranges of pressures that mimmize end-organ ischemia without excessive risk to the anatomic integrity of the aorta. During induction, should tachycardia and hypertension persist despite deepening anesthesia, then pharmacologic intervention with beta-adrenergic antagonists to increase diastolic filling time and improve coronary blood flow, as well as to decrease dP/dt, may be essential. Esmolol, a short-acting, relatwely cardioselectwe beta-adrenergic antagonist, is ideally suited to treat tachycardia related to
episodic sympathetic nervous system stimulation. 48 Hypertension without associated tachycardia is best treated with intravenous nitroglycerin or sodium nitroprusside. Nitroglycerin is preferred if there are concomitant signs of myocardial ischemla, as sodium nitroprusside may decrease coronary artery blood flow to already lschemic myocardium. 49 The use of these vasodilators during aortic cross-clamping is discussed below. Based on results of large prospective studies in patients undergoing cardiac surgery, the choice of general anesthetic for maintenance is unlikely to influence the overall outcome. 5° Enflurane has been shown to increase cerebrospinal fluid (CSF) production and decrease CSF removal in dogs and theoretically should be avolded to minimize the risk of spinal cord lschemia from excessive CSF pressure. 51 The choice of other inhalation anesthetics or opiold infusions is determined by the cardaopulmonary and central nervous system effects of these agents and the physiologic responses desired. Controversy currently exists over the use of epldural anesthesia/analgesia for DTA surgery. A number of studies purport the benefits of combined general and epldural anesthesia for both abdominal aortic and thoracic surgery. These benefits not only include a decrease in postoperative pain, 52 but also decreases in adrenocortical stress response, 53 protein catabohsm, 54 hypercoagulable statey, 56 immunosuppression, 57 and pulmonary and myocardial morbidity. 55,58Presently only a single, nonrandomized, noncontrolled study appears in the hterature specifically reporting the use of thoraoc epidural anesthesia for descending thoracic and thoracoabdominal aortic aneurysms. 59 In this small study, 12 patients received epldural anesthesia, which blunted the increase in systemic vascular resistance and blood pressure associated with proximal aortic crossclamping. Unfortunately, no other risks or benefits of epidural anesthesia were examined in this study. Despite the potential benefits, many anesthesiologists and surgeons are concerned about the potential risks of epidural anesthesia during aortic surgery. Intraoperative hypotension, especially after aortic cross-clamp release, has been reported to be more pronounced in patients receiving epidural anesthesia. 6° The other major concern relates to the potential complication of epldural hematoma in patients in whom anticoagulatlon is to be initiated. However, large prospective and retrospective studies in patients receiving epidural anesthesm for major vascular surgery note the risk of hematoma to be minimal, although only low doses of heparin were used. 61,62 Most reports of epldural hematoma are anecdotal and often relate to traumatic placement rather than to an association with anticoagulation. 63 Nonetheless, until outcome studies are performed that compare combined epidural-general anesthesia versus general anesthesia alone for DTA surgery, the use of epidural anesthesia remains at the discretion of the individual anesthesiologist and surgeon.
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44. Benthuysen JL, Smith NT, Sanford TJ, et al. The physiology of alfentanil-induced rlgl&ty. Anesthesiology 64:440-446, 1986 45. Frlesen RM, Thomson IR, Husdon RJ, et al: Fentanyl oxygen anaesthesia for abdominal aortic surgery Can Anaesth Soc J 33'719-722, 1986 46. DeSanctis RW, Doroghazl RM, Austen WG, et al: Aortic dissection. N Eng J Med 317:1060-1067, 1987 47. Prokop EK, Palmer RF, Wheat MW Jr' Hydrodynamic forces m dissecting aneurysms: in-vitro studies in a tygon model and in dog aortas. Clrc Res 27'121-127, 1970 48. Girard D, Shulman BJ, Thys DM, et al. The safety and efficacy of esmolol during myocardial revasculanzatlon Anesthesiology 65:157-164, 1986 49 Mann T, Cohn PF, Holman BL, et al: Effect of nltroprusslde on regional myocardial blood flow in coronary artery disease. Circulation 57:732-738, 1978 50. Tuman KJ, McCarthy RJ, Spiess BD, et al: Does choice of anesthetic agent s~gnlficantly affect outcome after coronary artery surgery9 Anesthesiology 70:189-198, 1989 51 Artru AA: Relationship between cerebral blood volume and CSF pressure during anesthesia with halothane or enflurane in dogs. Anesthesiology 58.533-539, 1983 52 Logas WG, E1-Baz N, E1-Ganzouri A, et al: Continuous thoracic epldural analgesia for postoperative pain rehef following thoracotomy A randomized prospectwe study. Anesthesiology 67:787-791, 1987 53 Hjortso NC, Neumann P, Froslg F, et al: Effects of the extradural admmlstrat~on of local anesthetic agents and morphine on the urinary excretion of cortisol, cathecholammes, and nitrogen following abdominal surgery Br J Anestheslol 57 400-406, 1985 54. Kehlet H: The stress-response to anesthesia and surgery-
Release mechamsm and modifying factors. Clln Anaestheslol 2'315-339, 1984 55 Tuman KJ, McCarthy RJ, March RJ, et al: Effects of epldural anesthesia and analgesia on coagulation and outcome after major vascular surgery. Anesth Analg 73:696-704, 1991 56. Rosenfeld BA, Beattle C, Chrlsopherson R, et al: The effects of different anesthetic regimens on fibrinolysis and the development of postoperative arterial thrombosis. Anesthesiology 79'435-443, 1993 57. Rem J, Brandt MR, Kehlet H: Prevention of postoperative lymphopenla and granulocytosis by epldural analgesia Lancet 1:283-285, 1980 58. Yeager MP, Glass DD, Neff RK, et al' Epidural anesthesia and analgesia in high-risk surgical patients. Anesthesiology 66 729736, 1987 59. Stenseth R, Myhre HO: Anaesthesia in surgery for aneurysm of the descending thoracic or thoracoabdominal aorta Acta Chir Scand 154:147-150, 1988 60. Lun.n JK, Dannemiller FJ, Stanley TH' Car&ovascular responses to clamping of the aorta during epldural and general anesthesia Anesth Analg 58.372-376, 1979 61. Odoom JA, Sih IL: Epidural analgesia and anticoagulant therapy. Anaesthesia 38:254-259, 1983 62. Rao TLK, E1-Etr A: Antlcoagulatlon following placement of epldural and subarachnold catheters. Anesthesiology 55:618-620, 1981 63. Owens EL, Kasten GW, Hessel EA. Spinal subarachnoid hematoma after lumbar puncture and heparlnlzatlon: A case report, review of the hterature and discussion of anesthetic implications. Anesth Analg 65.1201-1207, 1986
Part 2 will appear in the December 1995 issue of THE JOURNAL.