Assessment of global left ventricular (LV)

Transcription

1 Annals of Cardiac Anaesthesia 2006; 9: Martin et al. TOE LV Systolic Function 157 Assessment of Left Ventricular Global Systolic Function By Transoesophageal Echocardiography Martin J. London, MD Professor of Clinical Anesthesia University of California, San Francisco TUTORIAL Assessment of global left ventricular (LV) systolic function is the most basic component of any comprehensive echo examination performed in any clinical setting. It should always be the first step even when one is focused on an obvious indication for the examination (eg. a known valvular abnormality), since knowing the end result of the heart s ability to generate a stroke volume and resultant cardiac output is always the most important endpoint. Its direct or easily measured although less specific surrogates (eg. ejection fraction, qualitative contractility) are commonly used by surgeons and cardiologists in stratifying patients for surgery either positively [coronary artery bypass grafting (CABG) may be more beneficial than percutaneous intervention (PCI) in patients with 3 vessel disease and moderately impaired ejection fraction (EF)] or negatively (eg. aortic valve replacement (AVR) in a patient with severe aortic stenosis (AS) with a markedly depressed EF). We certainly use preoperative information to make important decisions regarding monitoring [arterial line, pulmonary artery catheter (PAC) transoesophageal echocardiography (TOE)] and management (etomidate vs. propofol, setting up inotropes, etc.). Proper evaluation of systolic function includes consideration of anatomical and qualitative/ quantitative assessments of both 2D and Doppler imaging. Although the latter has predominantly focused on intravascular velocities, attention is now being paid to measurement of parameters of ventricular (wall) motion/velocity (either global or regional) with Doppler tissue imaging (DTI) allowing meaurement of new basic (S, E, A velocities) and derived parameters (stress and strain rates). Address for Correspondence: Martin J, London, Professor of Clinical Anesthesia, University of California, San Francisco Annals of Cardiac Anaesthesia 2006; 9: Key words: Transoesophageal echocardiography, Cardiac surgery, Left ventricular function It is probably fairly obvious but you need to appreciate a variety of anatomical abnormalities that clearly are associated with abnormal systolic function (eg. increased ventricular volumes, wall thickness, consistency and texture of the wall as may be with infiltrative or other cardiomyopathies, aneurysms of the ventricular wall with possible thrombus and areas of low flow evidenced by spontaneous echo contrast, valvular abnormalities causing systolic dysfunction, etc.) as well as make some quantitative assessment of them. An area that most anaesthesiologists tend to have substantially less familiarity with (for obvious practical reasons) is in the dynamic nature of changes in ventricular shape and function ( remodeling ) that occur following myocardial infarction, with treatment of congestive heart failure (CHF) or progression of valvular lesions. The regional and global expansion that occurs after acute myocardial infarction (MI) has substantial implications on outcome and perhaps on intraoperative management for coronary artery bypass grafting depending on how it is treated medically and surgically. The abnormal spherical shape that occurs after acute MI substantially influences ejection velocity, stroke volume and reduces the normal amount of diastolic suction leading to an abnormal pump and sump. We are all well aware of the basic determinants of stroke volume (preload, afterload and contractility) and cardiac output (stroke volume x heart rate). You have already been introduced to the critical fundamental concept of use of the integrated velocity time integral (VTI) of aortic (for the LV) or pulmonary artery [for the right ventricle (RV)] outflow multiplied by the outflow tract diameter as a measure of stroke volume (and since that diameter rarely changes significantly, it generally can be ignored anyway leaving changes in VTI alone to track changes in stroke volume although one has to be careful since minor changes 157

2 158 Martin et al. TOE LV Systolic Function Annals of Cardiac Anaesthesia 2006; 9: in diameter are amplified by the square of the radius). This is usually obtained with TOE from the deep transgastric view (Figures. 1A and 1B) or Fig. 1A. Deep transgastric view showing left atrium (LA), left ventricle (LV) and proximal portion of ascending aorta (AO). This view allows an excellent alignment of the ultrasound beam and the blood flow, and is the best view for Doppler measurement of cardiac output. Fig. 1B. Pulsed wave Doppler flow across the aorta (deep transgastric view) for measuring the velocity time integral and stroke volume. from a modified transgastric image but could also be obtained using a non-imaging continuous wave (CW) suprasternal notch probe (rarely done perioperatively but such probes are present on nearly every cardiology system). This is also the basis for noninvasive cardiac output measurement using oesophageal Doppler probes and this can also be tracked using pulsed wave (PW) on the descending aorta via TOE (somewhat controversial however). Of these variables, the physiological definition and clinical monitoring of preload is relatively straightforward by echo (determining area using various linear dimensions and transforming it to volume using a variety of formulae). The unique ability of TOE to easily acquire this and the much closer correlation of LV volume with cardiac output than with LV pressure is a major advantage of TOE over PAC. 1 Determination of afterload and contractility is more controversial. Afterload can be assessed by measurement of wall stress (see below) but is so complex as to preclude routine clinical application. However, we are all well aware of its importance in the matching of cardiac output with the peripheral circulation, which forms the basis for use of vasodilator therapy for CHF and treatment of aortic regurgitation (AR) or mitral regurgitation (MR) (although the simply measured but imprecise systemic vascular resistance is nearly always substituted). Given the more obvious nature of contractility (eg. we think we can see it ) and its strong literature association with outcome (eg. impaired survival with reduction in its most common and easily measured but imperfect surrogate EF), there is considerably more information on how to assess it with echo. Diastolic function is also a crucial determinant of cardiac function but as of yet, despite a variety of measurable Doppler indices, quantitative echo measurements have little direct clinical application (outside of extreme situations such as cardiac tamponade and severe restrictive states). 2 Trying to precisely isolate systolic from diastolic function is not entirely rational but in this tutorial we focus on the former. We will make brief mention of at least one new combined index of both systolic and diastolic function that has gotten recent attention in the literature (the Myocardial Performance or Tsai Index) that I don t think will see much use in the operating room (OR) but that you should at least have some passing familiarity with for examination purposes (possibly). 3 It is also obvious that the LV doesn t exist in isolation so the effects of the left atrium and its reservoir and booster pump functions (which are getting considerably more literature attention recently), as well as the well described interaction of the RV with the LV 158

3 Annals of Cardiac Anaesthesia 2006; 9: Martin et al. TOE LV Systolic Function 159 via the septum (with under-filling of LV in the setting of acute RV failure as might occur with RV infarction or severe pulmonary hypertension) are of considerable importance. The standard approach to learning, clinical application and testing is to first consider the load dependent indices of systolic function (fractional shortening, fractional area change, ejection fraction, cardiac output and dp/dt) followed by the more complex and less clinically applicable load independent indices (rate corrected mean velocity of fibre shortening, circumferential wall stress, meridional wall stress, and even pressure volume or area loops). Measurement of LV mass, although not a measure of LV function, is a critical moderator or end result of abnormal LV function and can significantly influence outcome in both CHF and coronary artery disease (CAD) so it is now commonly reported on transthoracic echocardiography (TTE) and TOE examinations. Tissue Doppler techniques are seeing increasing clinical use. The older concept that descent of the mitral annulus (originally termed descent of the base but this has been supplanted by the anatomically more precise term mitral annular excursion ) along the longitudinal axis of the LV from a relatively immobile apex is a sensitive measure of function (and outcome), has recently been reinvigorated by routine measurement of mitral annular velocities with DTI (noted above) although this technique is not yet commonly performed perioperatively. 4-6 Recent physiology studies have used 4-D echo to quantitate its contribution and clarify older controversies related to LV filling. We do know that the majority of muscle fibres in the LV are oriented circumferentially and thus play a larger role than the longitudinally oriented fibres found in the subendocardial and subepicardial free wall and papillary muscles. 7 The normal circumferential shortening in the minor axis is 25 40%. Without the longitudinal component of shortening, it is estimated that the EF would be < 30%. Of note, although minor axis shortening is directly influenced by long axis shortening, the converse is not true. Early cine studies demonstrated shortening of 10 12% with ejection as the minor axis decreased by 25%. A recent 4-D TOE study calculated that the annular excursion volume represented 19% of the total LV stroke volume. 8 The correlation with stroke volume (SV) was significant if not somewhat modest (r = 0.73) and was also related to body size but not LVEF or heart rate. Peak mitral annular area occurred during mid-diastole and 90% of its reduction in area occurred before the onset of LV systole. In order to assess the most basic clinical measures of LV function (the load dependent EF paradigm indices), one must be able to accurately determine LV areas and then appreciate the optimal methods to mathematically transform them to volumes. In this regard, there is a considerable amount of information out there, much of which is more of historical interest since the formulas were derived or applied using technology available at the time prior to our contemporary systems with high resolution imaging and extensive on-board computing power. Thus, for those of us motivated enough not just to eyeball ejection fraction, we can spend a few minutes tracing LV endocardial borders to obtain the relatively robust (primarily with regards to its relative independence from the impact of regional wall motion abnormalities on the assumption of a symmetrically shaped ventricle required by the area length method) Simpson s Method of Discs (MOD) determination of LV volume or if we have the right equipment and optimal images, use automated boundary detection based methods (ABD, colour kinesis, etc) to automatically monitor fractional area change. Few of us will ever use the prolate ellipsoid or area length formulas for volume measurements. It is likely that real time 3D imaging will advance this topic dramatically in the coming years. The literature suggests that these systems will be more accurate with considerably less variation between repeated measurements by the same or different sonographers which plague our current 2D methods. Given the importance of this topic and the close linkage of clinically used techniques to technical evolution of ultrasound equipment, the American Society of Echocardiography (ASE) has previously 159

4 160 Martin et al. TOE LV Systolic Function Annals of Cardiac Anaesthesia 2006; 9: and continues to establish standards for how best to measure key variables. The seminal consensus document Recommendations for Quantitation of the LV by 2D Echo first published in 1989 formed the basis for much of our current clinical practice 9. It has very recently been updated and extensively revised. 10,11 A very recent document on the use of echo in clinical trials also provides guidance (as well as including detailed information on many echo measurements) and is likewise essential reading. 12 A variety of reviews and textbook chapters are also relevant. 2,13,14 Determination of LV Volumes and Ejection Fraction Assumption of symmetrical geometry and contraction are required for application of single dimensions to determine ventricular volumes. Use of biplane methods (eg. 2 roughly orthogonal imaging planes) is more accurate in the abnormal ventricle with wall motion abnormalities or abnormal shape (eg. usually spherical due to remodeling from CHF or MI). 12,15,16 TOE derived measurements have been said to underestimate LV volumes relative to either TTE or angiography although a more recent simultaneous TOE and TTE study disputes this. The most commonly cited factor for these discrepancies is foreshortening of the LV apex with TOE yielding a shorter LV long axis dimension (note: long axis dimension is assumed to start at apex and run to midplane mitral valve). With TTE, the sonographer is able to move the transducer over a wide area to find the true apex (greatly assisted by a thin patient), while with TOE, this is not possible. The differences are usually minor, particularly for our perioperative applications. When using the biplane Simpson s MOD you should not average both views if the long axis dimensions vary by more than 20% between the 2 imaging planes (although its certainly not something you want to take the time to measure!). One tip for recognition of the true apex is that it should really be nearly immobile and if it isn t you are likely imaging foreshortened myocardium. An obvious limitation of any technique using long axis dimensions is that longitudinal scanning results in much poorer endocardial resolution than axial (short axis) planes and tracing of these images can involve a fair amount of artistic license. The ASE guidelines urge against this and use of intracavitary contrast agents can greatly assist in accurate measurements although this is rarely done in the OR for cost and logistical reasons. Nonetheless, longitudinal methods are clearly preferred. The new ASE guidelines present detailed information on normal ranges of LV diastolic diameters and diastolic/systolic volumes (calculated). 10,11 These vary by gender and are optimally compared using indexing to body surface area. It is recommended that the length and minor diameters be obtained from the midoesophageal 2 chamber view imaged at approximately degrees or in the transgastric 2 chamber view imaged at approximately degrees with the caveat that miximal obtainable LV size is obtained by adjusting medial lateral rotation. Measurement of LV wall thickness is recommended in the midseptum and the posterior wall in the transgastric view at the papillary muscle level at approximately 0 to 30 degrees. Note that the posterior wall designation is somewhat inconsistent with newer recommendations that the term posterior no longer be used for segmental analysis and thus this segment is more correctly termed infero-lateral in the new 17 segment model which includes the apical cap imaged in nuclear studies, which is of no practical relevance to echocardiographic analysis as this segment does not move as noted above. 18 The prolate ellipse method (or cube formula) is quite simple in that it states that volume approximates the internal diameter (short axis at the tips of the mitral valve by TTE) by D to the third power. This formula has been shown to overestimate LV volumes as in larger ventricles the LV dilates to a more spherical shape (eg. along the short axis dimension). The more commonly talked about area length formula assumes a bullet shaped LV and was developed for angiography. It is said to be preferable to Simpson s method, if only one apical view is available. The formula is V = 0.85 (Area of the LV short axis squared)/long axis length. Area can be traced manually using onboard software or it can more be determined using the short axis diameter 160

5 Annals of Cardiac Anaesthesia 2006; 9: Martin et al. TOE LV Systolic Function 161 determined at the level of the papillary muscles (TGSAX, Figure 2). Acquisition of a true long axis is of course critical. Fig. 2. Transgastric short axis view of the left ventricle at the level of papillary muscles used to calculate end-diastolic area. As noted above Simpson s MOD is clearly the most common used method and is automated on all echo machines. It is based on the assumption that the ventricle can be modeled as a stack of circular discs that can vary slightly in diameter based on the shape of the ventricle. The discs are automatically determined based on perpendicular radii from the long axis dimension (which is actually determined from tracing the endocardial borders at end systole and diastole). Usually 20 discs are calculated, although it is not known precisely how many discs are optimal. Likewise, a flaw in the assumption that this technique is better than single length methods for patients with abnormal anatomy or contraction, is that the discs must be modeled as perfectly circular, which may not be the case anatomically. Nonetheless, this method is the current gold standard and will be for sometime. There are other methods that have been evaluated (particularly use of several different short axis measurements) but none are commonly used. Once you have made the appropriate measurements in both end-systole and enddiastole, it is very simple mathematics to determine the ejection fraction paradigm indices listed below: 1. M-mode echo: end-diastolic internal diameter end-systolic internal diameter/end-diastolic internal diameter x 100%. This is termed Fractional Shortening (FS), mean 33% (range 28 41%) and is usually made with TTE in a parasternal plane. It is easy to do with TOE using the TGSAX view from inferior to anterior wall if you are interested. 2. 2D echo: end-diastolic area (EDA) end-systolic area (ESA)/EDA area x 100%. This is termed Fractional Area Change (FAC), mean 57% (range 37 76%). Most commonly done in the TGSAX view. The lack of contribution of the apex, which is commonly abnormal in patients with severe CAD is an obvious limitation. 3. 2D echo: end-diastolic volume end-systolic volume/end-diastolic volume x 100%. This is EF, mean 62% (range 45 90%). Volume is measured using one or more long axis dimensions using Simpson s method of summated discs. Usually EDA and ESA are estimated qualitatively as the largest and smallest areas during contraction respectively. For EDA, the first frame at or before systolic coaptation of the mitral valve or the first frame on the ECG QRS complex (most common and easy to do assuming you use ECG leads); for ESA the frame preceding early diastolic mitral opening is mentioned, but in fact, the smallest cavitary size is easiest (note there is no precise ECG event associated with end systole other than it usually falls within the latter portion of the T wave). The ASE recommends that when obtaining any of these measurements that the LV be displayed on the screen in its most magnified setting possible. They discuss the fact that there is a trade-off here, since a smaller image on the screen will likely have higher resolution for tracing of the endocardial borders, but the smaller dimensions would be more likely to subject to measurement errors. They also note (as do numerous articles) that reproducibility of EF determinations has at very best, 7% error and often much worse. Clinicians should realize that for most epidemiological and risk assessment purposes that EF s are usually grouped into wide categories (eg. < 30%, 30 50%, > 50%) due to measurement errors and different types of technology used for them. A hotly debated topic in the echo literature is how closely one can eyeball EF and whether quantitative 161

6 162 Martin et al. TOE LV Systolic Function Annals of Cardiac Anaesthesia 2006; 9: measurements are required repeatedly. A recent meta-analysis reported that neither Simpson s method, use of wall motion score index nor subjective assessment either over or underestimate EF relative to radionuclide or contrast ventriculography. 19 An accompanying editorial by Dr. Nelson Schiller, strongly disputes this contention. 20 Few of us will make these measurements in the OR on a routine basis and a large extent these issues are much more important for longterm assessment of medical therapies for heart failure where small errors can have serious consequences. But if you never make these measurements you are deluding yourself into thinking you accurately communicate your findings consistently to your colleagues. This lack of precision is similar to our cardiac surgical colleagues most of whom believe that they can easily palpate the aorta outside to detect aortic plaque inside! Cheung et. al. have found that TOE measured reduction in LVEDA is a sensitive measure of reduction in blood volume, decreasing linearly with deficits in the range of %, even in patients with LV wall motion abnormalities. 21 Measurement of LV mass is not performed in the OR, but is important clinically and the ASE has specifics in their guideline documents. It is possible to obtain it using area determined by the area length method and measurement of thickness of the septal and posterior walls. The general principle is to subtract the LV cavity volume from the volume enclosed by the LV muscle shell and multiply this by myocardial density of 1.04 g/ml. Measurement of wall stress as a reflection of afterload is not specifically covered by the ASE given its considerable physiological complexity (including the obvious fact that afterload varies continuously during systole). Long axis (meridional) or short axis (circumferential) stress can be measured. Given the capability of magnetic resonance imaging, there is considerable interest developing in precise measurements of these forces but it will be many years before they become clinically applicable. References 1. Kumar A, Anel R, Bunnell E, et al. Pulmonary artery occlusion pressure and central venous pressure fail to predict ventricular filling volume, cardiac performance, or the response to volume infusion in normal subjects. Crit Care Med 2004; 32: London MJ. Ventricular Function, Clinical Transesophageal Echocardiography, 2nd Edition. Edited by Konstadt S, Shernan S, Oka Y. Philadelphia, Lippincott, Williams and Wilkins, 2003; pp Lax JA, Bermann AM, Cianciulli TF, et al. Estimation of the ejection fraction in patients with myocardial infarction obtained from the combined index of systolic and diastolic left ventricular function: a new method. J Am Soc Echocardiogr 2000; 13: Trambaiolo P, Tonti G, Salustri A, et al. New insights into regional systolic and diastolic left ventricular function with tissue Doppler echocardiography: from qualitative analysis to a quantitative approach. J Am Soc Echocardiogr 2001; 14: Simmons LA, Weidemann F, Sutherland GR, et al. Doppler tissue velocity, strain, and strain rate imaging with transesophageal echocardiography in the operating room: a feasibility study. J Am Soc Echocardiogr 2002; 15: Skulstad H, Andersen K, Edvardsen T, et al. Detection of ischemia and new insight into left ventricular physiology by strain Doppler and tissue velocity imaging: assessment during coronary bypass operation of the beating heart. J Am Soc Echocardiogr 2004; 17: Henein MY, Gibson DG: Normal long axis function. Heart 1999; 81: Carlhall C, Wigstrom L, Heiberg E, et al. Contribution of mitral annular excursion and shape dynamics to total left ventricular volume change. Am J Physiol Heart Circ Physiol 2004; 287: H Schiller NB, Shah PM, Crawford M, et al. Recommendations for quantitation of the left ventricle by two-dimensional echocardiography. American Society of Echocardiography Committee on Standards, Subcommittee on Quantitation of Two-Dimensional Echocardiograms. J Am Soc Echocardiogr 1989; 2: Lang RM, Bierig M, Devereux RB, et al. Recommendations for chamber quantification. Eur J Echocardiogr 2006; 7: Lang RM, Bierig M, Devereux RB, et al. Recommendations for chamber quantification: a report from the American Society of Echocardiography s Guidelines and Standards Committee and the Chamber Quantification Writing Group, developed in conjunction with the European Association of Echocardiography, a branch of the European Society of Cardiology. J Am Soc Echocardiogr 2005; 18: Gottdiener JS, Bednarz J, Devereux R, et al. American Society of Echocardiography recommendations for use 162

7 Annals of Cardiac Anaesthesia 2006; 9: Martin et al. TOE LV Systolic Function 163 of echocardiography in clinical trials. J Am Soc Echocardiogr 2004; 17: Marwick TH. Techniques for comprehensive two dimensional echocardiographic assessment of left ventricular systolic function. Heart 2003; 89 Suppl 3: iii Walton JS, Reeves ST, Dorman BH. Ventricular systolic performance and pathology, A practical approach to transesophageal echocardiography. Edited by Perrino AC, Reeves ST. Philadephia, Lippincott, 2003, pp St John Sutton M, Otterstat JE, Plappert T, et al. Quantitation of left ventricular volumes and ejection fraction in post-infarction patients from biplane and single plane two-dimensional echocardiograms. A prospective longitudinal study of 371 patients. Eur Heart J 1998; 19: Otterstad JE, St John Sutton M, Froland G, et al. Are changes in left ventricular volume as measured with the biplane Simpson s method predominantly related to changes in its area or long axis in the prognostic evaluation of remodelling following a myocardial infarction? Eur J Echocardiogr 2001; 2: Colombo PC, Municino A, Brofferio A, et al. Crosssectional multiplane transesophageal echocardiographic measurements: comparison with standard transthoracic values obtained in the same setting. Echocardiography 2002; 19: Cerqueira MD, Weissman NJ, Dilsizian V, et al. Standardized myocardial segmentation and nomenclature for tomographic imaging of the heart: A statement for healthcare professionals from the Cardiac Imaging Committee of the Council on Clinical Cardiology of the American Heart Association. J Am Soc Echocardiogr 2002; 15: McGowan JH, Cleland JG. Reliability of reporting left ventricular systolic function by echocardiography: a systematic review of 3 methods. Am Heart J 2003; 146: Schiller NB. Ejection fraction by echocardiography: the full monty or just a peep show? Am Heart J 2003; 146: Cheung AT, Savino JS, Weiss SJ, et al. Echocardiographic and hemodynamic indexes of left ventricular preload in patients with normal and abnormal ventricular function. Anesthesiology 1994; 81: