Indications and preoperative assessment for surgery

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What the surgeon needs to know from the imager about acute myocardial infarction

Echocardiography is an important tool in acute myocardial infarction for determining infarct size and location, detecting mechanical complications (acute mitral regurgitation from papillary muscle rupture, ventricular septal defect, cardiogenic shock), and stratifying risk. Surgical intervention in acute myocardial infarction has become increasingly common; what the surgeon wants to know from the imager is the presence of specific mechanical complications, assessment of the hemodynamics and LV function, and the indications and timing of surgery. The 2003 American College of Cardiology (ACC)/Ameri-can Heart Association (AHA)/ASE guidelines [7] recommend the use of echocardiography for the assessment of mechanical complications after acute myocardial infarction as a class I indication.

Ventricular septal defect

Ventricular septal defect (VSD) is a severe complication of acute myocardial infarction with an incidence of 0.4% to 0.7% in the era of thrombolytic therapy [8]. Risk factors for VSD include age over 60 years, female sex, first myocardial infarction, and hypertension. VSD is more common with large anterior infarcts without collateral blood flow. VSD associated with inferior infarction tends to occur toward the base of the heart, making surgical access more difficult, so that the prognosis is worse for inferior than anterior infarct VSDs. The average time to development of a VSD is 5 days, and the defect may range in size from 1 to 10 mm. The size of VSD and the consequent hemo-dynamics significantly influence the mortality. Infarction VSDs are optimally treated surgically, and the definitive diagnosis must be established preoperatively. Echocardiography usually enables direct visualization of the defect and the adjacent akinesis or dyskinesis (Fig. 2). Color Doppler demonstrates the high-velocity turbulent jet traversing the septal defect. The continuous wave Doppler demonstrates a systolic gradient across the VSD. Transesophageal echocardiography may be useful in cases in which the transthoracic images are non-diagnostic especially in patients requiring mechanical ventilation support.

The surgeon also needs to know the exact location of the entrance and exit of the infarct VSD because closure of VSD is invariably achieved through the right ventricle by way of the tricuspid valve. Other prerequisites for surgical closure include the size of the shunt (Qp/Qs), VSD location, pulmonary arterial systolic pressure, ventricular function, and the degree of heart failure, all of which can be readily obtained by Doppler echocardiography.

Apical Myocardial Infarction
Fig. 2. Transthoracic apical four-chamber view of a patient who had antero septal myocardial infarction and apical ventricular septal defect shown by color flow Doppler.

Acute severe mitral regurgitation

Acute MR following acute myocardial infarction portends a poor prognosis and usually occurs in single coronary artery disease. Severe regurgitation can result from acute rupture of a papillary muscle head, which usually involves the infero-posterior papillary muscle. Acute MR may also result from acute ischemic dysfunction of the papillary muscle and adjacent LV wall without rupture. The presence of severe MR and the etiologic mechanisms must be ascertained and the severity of regurgitation quantified before corrective surgery is contemplated. Echo diagnosis may be difficult with transthoracic imaging, and often transesophageal Doppler echocar-diographic is necessary to establish the diagnosis. Severity of MR can usually be adequately semiquantitated using graded Doppler color flow mapping (0 = none/trivial, 1 = mild, 2 = moderate, 3 = severe).

Cardiogenic shock

Echocardiography can detect early infarct expansion and aneurysm formation that occurs after anterior myocardial infarctions. Infarct expansion is the stimulus for progressive LV dilatation and remodeling to heart failure. Unopposed

LV dilatation and aneurysm formation may result in cardiogenic shock, thrombus formation, stroke, and LV rupture. Before remedial surgical intervention, crucial information required by the surgeon is an accurate assessment of LV size and function, and the presence of mechanical defects (MR or VSD). Before left ventricular assist device (LVAD) placement for cardiogenic shock, significant apical thrombosis and aortic regurgitation needs to be excluded to avoid stroke risk and LVAD malfunction, respectively.

What the surgeon needs to know from the imager about indications for surgery and preoperative assessment in chronic coronary disease/post infarction

Coronary artery disease (CAD) is the major cause (70%) of heart failure [9]. Prognosis of patients who have ischemic heart failure and medical therapy alone is poor. Comparison of medical versus surgical therapy for CAD has excluded patients who have an LVEF less than 35% [10]. However, improvements in surgical techniques have shown that coronary bypass surgery may be performed with acceptable mortality, even in patients who have severe ventricular dysfunction [11]. A significant subgroup of patients who have heart failure and underlying CAD has reversible LV dysfunction following revasculariza-tion. In patients who have depressed LV function, the poorer the LV function, the greater is the likelihood of improvement post coronary artery bypass grafting (CABG) surgery. Although the relative benefit is similar to the other patients, the absolute benefit is greater because of the high-risk profile of these patients [12]. Thus, identification of subsets of patients who have CAD and LV dysfunction who benefit from revascularization is important to optimize patient outcome.

Coronary angiography is still considered the reference standard for the diagnosis of coronary artery disease. However, there are new techniques that provide diagnostic information about proximal occlusive CAD, especially the 64-slice CT coronary angiography and electron beam CT.

Contrast coronary angiography provides crucial information to the surgeon regarding the location and severity of stenoses, the number of coronary arteries involved, and global and regional left ventricular function before CABG surgery. One additional important question posed by the surgeon before CABG is the viability of the region of myocardium to be revascularized.

Viability

Myocardial viability is extremely important to establish before surgical revascularization because it predicts the clinical outcome and postop LV function. Viable myocardium is characterized by several attributes, including cell membrane integrity, intact mitochondria, preserved glucose metabolism, intact resting p erfusion, and inotro-pic reserve. The absence of myocardial shortening is not reliable as an indicator of viability because metabolic activity can be present in hibernating or stunned myocardium. There are several techniques that predict myocardial viability, some of which are impractical, unavailable, or confined to quaternary medical centers.

F-18 fluorodeoxyglucose positron emission tomography. Positron emission tomography (PET) is considered the gold standard for assessment of myocardial viability [13] by assessing the homogeneity of regional myocardial glucose metabolism. The magnitude of improvement in heart failure symptoms after revascularization in patients who have LV dysfunction correlates with the preoperative extent of F-18 fluorodeoxyglucose (FDG) "mismatch."

Dobutamine stress echocardiography. Dobut-amine augments myocardial contractility at low doses and increases heart rate and peripheral vasodilatation at higher doses. In ischemic regions, there is a biphasic response. Low doses of dobutamine augment contractility, but higher doses decrease myocardial shortening resulting in regional wall motion abnormalities. In patients who have CAD and severe LV dysfunction who demonstrated myocardial viability during dobut-amine echocardiography, revascularization improved survival compared with medical therapy [14]. Recent advances in echo technology, such as echo contrast, harmonic imaging, speckle tracking, and artificial intelligence that enhance endocardial border definition, may improve diagnostic accuracy in detecting viability.

Nuclear imaging. In myocardial single photon emission computed tomography (SPECT) with Tc-99 m, the regional functional recovery following revascularization increases in proportion to Tc-99 m sestamibi uptake. In the segments with greater than 55% maximal uptake, the likelihood of functional recovery is greater than 70%. However, there are two different conditions that may result in a mild to moderate decrease in uptake of radionuclide flow tracers: (1) chronically decreased blood flow in hibernating viable myocardium, and

(2) a nontransmural infarct supplied by either a patent or occluded proximal coronary vessel. Although revascularization will likely be beneficial in the former situation, it will not be likely to improve regional function in the latter. Thallium (Tl-201) nuclear imaging is an energy-dependent process that requires intact cell membranes. The presence of a reversible perfusion defect and/or preserved Tl-201 uptake on the 3- to 4-hour redistribution images is an important sign of regional viability. The mean sensitivity in predicting functional recovery is 86%. However, specificity is only 59%, indicating that in approximately 40% of patients who have delayed thallium-201 uptake, there was no evidence of regional functional recovery following revascularization. In a study of patients who had heart failure, Marin-Neto and colleagues [15] showed that thallium SPECT with reinjection yields information regarding regional myocardial viability that is similar to that provided by PET in patients who have severe as well as moderate LV dysfunction. However, there is discordance in over 20% of regions manifesting severe irreversible thallium defects in patients who have severely reduced LV function.

Cardiac magnetic resonance. The spatial resolution of CMR enables differentiation of subendo-cardial versus transmural myocardial viability. A new CMR method to assess myocardial viability is the detection of "delayed hyperenhancement.'' Following MRI contrast injection, T1-and T2-weighted images demonstrate decreased signal intensity in regions of myocardial scar and also in areas of resting ischemia (ie, hibernating myocardium). In delayed images, myocardial scar tissue accumulates contrast (delayed hyperenhancement), whereas resting ischemia does not. If less than 25% of the thickness of a myocardial wall demonstrates delayed hyperenhancement, wall motion will improve following revasculariza-tion. CMR hyperenhancement correlates with F-18 FDG PET data [16].

All of the four methods previously described can provide the surgeon with information about the viability of regions of myocardial ischemia that he/she planned to revascularize.

What the surgeon needs to know about ischemic MR

Chronic ischemic MR is an MR occurring more than 1 week after myocardial infarction with: (1) one or more left ventricular segmental wall motion abnormalities, (2) significant coronary disease in the territory supplying the wall motion abnormality, and (3) structurally normal mitral valve (MV) leaflets and chordae tendinae. The third criterion is particularly important because it excludes patients who have organic MR and associated coronary artery disease.

Ischemic MR occurs in approximately 20% of patients after myocardial infarction and in 56% of patients who have congestive heart failure caused by ischemic or nonischemic cardiomyopathy [17]. There is a graded independent association between the severity of ischemic MR and the development of heart failure after myocardial infarction. Even mild ischemic MR is associated with an increase in the risk of heart failure [18].

The ischemic insult results in LV remodeling toward a more spherical shape with new wall motion abnormalities. These changes lead to annular dilatation and subvalvular distortion that prevents the mitral leaflets from coapting normally during systole (Fig. 3) [19,20]. Elucidation of the pathogenesis of ischemic MR relates largely to alterations in ventricular geometry and function. This finding has led investigators to question surgical techniques that only address the mitral valve annulus [21] because annuloplasty alone fails in 20% to 30% of patients who have ichemic MR [22].

Ischemic mitral regurgitation is often clinically silent and should be systematically evaluated by imaging. Standard color Doppler imaging is a highly sensitive method to detect even mild degrees of ischemic mitral regurgitation. A unique advantage of echocardiography is that it accurately quantifies the severity of mitral regurgitation by the effective regurgitant orifice area and calculates the regurgitant volume [23]. Congestive heart failure is independently determined by larger effective regurgitant orifice area (Fig. 4) [24]. In some patients who have exertional dyspnea out of proportion to their resting systolic dysfunction or MR, exercise echocardiography can reveal the true severity of what might otherwise be considered mild MR. Peteiro and colleagues [25] demonstrated that exercise echocardiography has a higher prognostic value compared with resting echocardiography. An exercise-induced increase in effective regurgitant orifice area greater than 13 mm2 is an independent predictor of cardiac death [26].

Three-dimensional (3D) transthoracic echocar-diography (TTE) and transesophageal echocardi-ography (TEE) are new and promising tools to

Fig. 3. Schematic demonstrating a possible mechanism of mitral regurgitation. (A, B) Left ventricular dilatation due to volume overload results in the left ventricle becoming more spherical. The mitral valve ring circumference increases. The angle subtended by the papillary muscles to the mitral annulus increases, but there is no elongation of the mitral valve leaflets or chordae, which results in incomplete cusp coaptation and mitral regurgitation.

Fig. 3. Schematic demonstrating a possible mechanism of mitral regurgitation. (A, B) Left ventricular dilatation due to volume overload results in the left ventricle becoming more spherical. The mitral valve ring circumference increases. The angle subtended by the papillary muscles to the mitral annulus increases, but there is no elongation of the mitral valve leaflets or chordae, which results in incomplete cusp coaptation and mitral regurgitation.

assess the anatomy of the mitral valve (Fig. 5). New parameters can be measured using 3D: mitral valve tenting volume, the nonplanar angle (Fig. 6), annulus circumference, annulus area, and 3D commissural length. 3D has a higher sensitivity than TEE in detecting commissural and bi-leaflet defects [27]. It is also a promising method for the direct measurement of the effective regurgitant orifice area. 3D echocardiography overcomes the limitations of the proximal isovelocity surface area and is also unaffected by orifice shape [28]. Preoperative increased mitral annular diameter, tethering area, and MR grade were associated with repair failure in ischemic MR [29]. The LV may continue to remodel and dilate, rendering initial repair ineffective [30].

Detection of thrombus

TTE is the standard procedure for detection of LV thrombosis with a sensitivity ranging from 92% to 95% and a specificity of 95% assuming good image quality. However, in routine clinical cardiologic practice, poor image quality occurs in approximately 10% to 20% of patients, particularly at the apex where thrombus is most likely to form. Contrast echocardiography can be used to

Fig. 4. Effect of severity of ischemic MR, assessed by effective regurgitant orifice area (ERO) of at least 20 mm or less than 20 mm2 on survival. (From Grigioni F, Enriquez-Sarano M, Zehr KJ, Bailey KR, Tajik AJ. Ischemic mitral regurgitation: long-term outcome and prognostic implications with quantitative Doppler assessment. Circulation 2001;103:1762; with permission.)

Fig. 4. Effect of severity of ischemic MR, assessed by effective regurgitant orifice area (ERO) of at least 20 mm or less than 20 mm2 on survival. (From Grigioni F, Enriquez-Sarano M, Zehr KJ, Bailey KR, Tajik AJ. Ischemic mitral regurgitation: long-term outcome and prognostic implications with quantitative Doppler assessment. Circulation 2001;103:1762; with permission.)

MR at A2 P

Fig. 5. Mitral valve visualized by three-dimensional transesophageal echocardiography (right) and with color Doppler (left) showing the anatomy of the mitral regurgitation.

improve visualization of the blood/tissue interface and detection of the endocardial borders. TEE cannot be considered a reference technique for LV thrombosis assessment because of its limitation to adequately visualize the LV apex. Detection of thrombus is extremely important before surgical LVAD placement to avoid propagation of thrombus into the systemic circulation causing stroke.

Detection of pulmonary hypertension

The presence of pulmonary hypertension increases morbidity and mortality postoperatively in patients who have cardiomyopathies, valvular heart disease, and coronary artery disease. Pulmonary artery systolic pressure can be determined noninvasively by Doppler echocardiography using the modified Bernoulli equation. The peak velocity of tricuspid regurgitation reveals the systolic right ventricular to right atrial pressure gradient (P = 4V2), and the right atrial pressure, estimated from the respiratory change in vena caval diameter, is then added to the gradient to obtain the pulmonary artery pressure.

What the surgeon needs to know from the imager about indications for surgery and preoperative assessment in chronic valvular heart diseases

Degenerative MR

Quantitative grading of asymptomatic mitral regurgitation is a powerful predictor of the clinical outcome [31]. Patients who have an effective regurgitant orifice greater than 40 mm2 should be considered for cardiac surgery. Cardiac surgery in these patients was independently associated with improved survival (adjusted risk ratio, 0.28).

Preoperative assessment of MV anatomy is essential for planning surgical repair/replacement of degenerative MV disease with and without leaflet prolapse. Although two-dimensional TTE and TEE provide precise information regarding MV anatomy, 3D TTE and 3D TEE increases the understanding of more complex abnormalities of MV apparatus and improves individual scallop identification. Pepi and colleagues [32] demonstrated that

Anterior Leaflet Angle
Fig. 6. The nonplanar angle is the angle between the anterior leaflet plane and the posterior leaflet plane at the commissural diameter. It is proportional to the degree of flatness of the mitral annulus and the loss of the saddle shape. AC, anterior commissure; PC, posterior commissure.

3D TTE and 3D TEE are superior for accurate localization and identification of MV pathology in comparison with the two-dimensional echo methods. Real-time 3D echo allows precise localization of the diseased mitral commissures and individual scallops. Improvements in acquisition and reconstruction times will provide the surgeon with dynamic detailed continuous images that will enable more sophisticated and durable surgical repair techniques.

The ACC/AHA guidelines criteria for severe MR are the same for ischemic MR and degenerative MR and consist of a regurgitant volume of at least 60 mL per beat, a regurgitant fraction of at least 50%, and a regurgitant orifice area of at least 0.4 cm2 (Table 1).

Aortic stenosis

Aortic stenosis (AS) is the most common valvular heart disease resulting in valve replacement [33]. There is no alternative to surgery when the patient presents with symptoms of heart failure due to severe aortic stenosis (class I indication) [7]. Imaging techniques and especially echocardiogra-phy play a vital role in surgical aortic valve replacement during the preoperative, intraoperative, and postoperative periods. Echocardiography also allows identification of associated lesions, such as

Table 1

Criteria for severe valvular disease according to the American College of Cardiology/American Heart Association 2006 guidelines

Indicator Severe sortic stenosis

Jet velocity > than 4.0 m/s

Mean gradient > 40 mm Hg

Valve area index < 0.6 cm2/m2

Severe aortic regurgitation Qualitative

Angiographic grade; 3-4+

Color Doppler jet width; central jet, width > 65% LVOT Doppler vena contracta width; > 0.6 cm Quantitative (cath or echo)

Regurgitant volume; R 60 mL/beat Regurgitant fraction; R 50% Regurgitant orifice; area R 0.30 cm2 Additional essential criteria Left ventricular size; increased Severe mitral regurgitation Qualitative

Angiographic grade; 3-4+

Color Doppler jet area; large central MR jet (area > 40% of LA area) or with a wall impinging jet of any size, swirling in LA Doppler vena contracta width; > 0.7 cm Quantitative (cath or echo)

Regurgitant volume; R 60 mL/beat Regurgitant fraction; R 50% Regurgitant orifice area; R 0.40 cm2 Additional essential criteria Left atrial size; enlarged Left ventricular size; enlarged

Abbreviations: LA, left atrium; LVOT, left ventricular outflow tract.

Data from American College of Cardiology/American Heart Association Task Force on Practice Guidelines, Society of Cardiovascular Anesthesiologists, Society for Cardiovascular Angiography and Interventions, Society of Thoracic Surgeons, Bonow RO, Carabello BA, Kanu C, et al. ACC/AHA 2006 guidelines for the management of patients with valvular heart disease: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (writing committee to revise the 1998 Guidelines for the Management of Patients With Valvular Heart Disease): developed in collaboration with the Society of Cardiovascular Anesthesiologists: endorsed by the Society for Cardiovascular Angiography and Interventions and the Society of Thoracic Surgeons. Circulation 2006;114:E84-231.

the patient who has heart failure and AS does indeed have severe AS and not simply a calcified valve and heart failure due to coronary artery disease or coexistent cardiomyopathy.

Quantification of severity: three echocardiography parameters are used routinely to assess the severity of AS, blood flow velocity, pressure gradient, and valve area (see Table 1). AS is considered severe if velocity is greater than 4 m/s, gradient is greater than 40 mm Hg, and valve area is less than 1 cm2.

Parasternal Area

Fig. 7. Transthoracic parasternal long axis of (A) a patient who had a calcified aortic valve. This systolic frame shows a limited opening of the aortic valve of (B) a patient who had aortic stenosis and an aneurysmal aortic root. (C) Patient prosthesis mismatch due to replacement of too small an aortic prosthesis. There is also a prosthesis in the mitral position. AO, aortic root; LA, left atrium.

Fig. 7. Transthoracic parasternal long axis of (A) a patient who had a calcified aortic valve. This systolic frame shows a limited opening of the aortic valve of (B) a patient who had aortic stenosis and an aneurysmal aortic root. (C) Patient prosthesis mismatch due to replacement of too small an aortic prosthesis. There is also a prosthesis in the mitral position. AO, aortic root; LA, left atrium.

concomitant mitral and tricuspid valve disease, concomitant CAD, and ascending aortic aneurysm; and/or left-sided obstructive lesions, such as coarctation that are associated with bicuspid aortic stenosis that may require surgical attention.

The surgeon wants to know from the imager the pathoetiology of the AS and the size of the aortic root to avoid inserting too small a prosthesis and causing patient/prosthesis mismatch identified by elevated postoperative transvalve systolic gradients (Fig. 7). The surgeon also wants to be certain that

Aortic valve orifice area is less dependent of LV function than is the transvalvular gradient. Aortic velocity is the most reproducible and is the strongest predictor of clinical outcome [34]. Echocardiographic methods have largely replaced catheteriza-tion for the routine evaluation of AS. Cath-eterization plays a confirmatory role and for assessment of coexistent coronary artery disease.

Special situations. In patients who have low cardiac output, it is crucial to differentiate severe aortic stenosis complicated by systolic heart failure from mild to moderate aortic stenosis associated with systolic heart failure of a different etiology. In the former situation, surgery is required, whereas in the latter situation, surgery is not indicated. In selected patients who have low-flow/low-gradient AS and LV dysfunction, it is useful to measure the transvalvular pressure gradient and calculate valve area during a baseline state and during exercise or low-dose pharmaco-logic (dobutamine) stress (class IIa indication) [7]. In patients who have AS and poor LV function, it may be difficult to determine the predominant condition, and AVA may increase with increased stroke volume. Dobutamine stress echo may be used to establish whether AVA increases significantly with dobutamine; if so, severe AS is not present. If the transaortic gradient increases with increased stroke volume, the AS is severe.

TEE is important for preoperative and perioperative sizing of the aortic root for homograft placement. Sizing of the aortic root and pulmonary valve is also essential to the successful completion of the Ross procedure.

Aortic regurgitation

In the Western world, severe aortic regurgitation (AR) is usually due either to congenital diseases (bicuspid valve) or degenerative conditions (such as annuloaortic ectasia), which typically present in the fourth to sixth decades. In rare cases, aortic regurgitation is acute, caused by endocarditis or aortic dissection. Patients who have severe aortic regurgitation have higher morbidity and mortality than the general population [35]; and within 10 years of the diagnosis of severe AR, heart failure occurs in approximately half the patients, most of whom require aortic-valve replacement. The optimal treatment of chronic severe AR and heart failure is aortic valve replacement, even if the LV is severely dilated and the LVEF is low. The surgeon needs to be sure from an imaging perspective that there is severe AR and that the abnormal LV architecture and function are directly related to the AR and whether other valve pathology, CAD, or ascending aortic aneurysms need to be addressed.

Transthoracic Doppler echocardiography is the main imaging modality for diagnosing and assessing the severity of aortic regurgitation. Color flow Doppler is exquisitely sensitive for detecting even mild subclinical AR. Severe AR is defined by the ACC/AHA 2006 guidelines (see Table 1) as a regurgitant volume of at least 60 mL, a regurgitant fraction of at least 50%, a regurgitant orifice area of at least 0.3 cm2, a central jet width greater than 65% left ventricular outflow tract, and a ''vena contracta'' (width of the regurgitant flow stream at the orifice) of at least 0.6 cm with a high specificity for measurements that are at least 0.7 cm [7,36].

What the surgeon needs to know from the imager about indications for surgery and preoperative assessment in congenital heart disease

Imaging in congenital heart disease is beyond the scope of this manuscript; but it must provide the surgeon with an accurate description of in-tracardiac anatomy. This is achieved in complex cyanotic congenital heart disease by adhering to a scanning protocol that systematically determines atrial situs, atrioventricular, and ventricular arterial connections. Atrial and ventricular function and the presence of intra- or extracardiac shunts are also evaluated. This information can be obtained and analyzed by two-dimensional and real-time 3D echocardiography, CMR, and also 3D CT.

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