CORRESPONDENCE: LETTER TO THE EDITOR
Have We Really Come to Understand the Relationship Between the Left Ventricular Outflow Tract Gradient and Left Ventricular Emptying in Hypertrophic Cardiomyopathy?
Joseph P. Murgo, MD, MMM*
* University of Texas Health Science Center at San Antonio, Department of Medicine (Cardiology), Mail Code 7872, 7703 Floyd Curl Drive, San Antonio, Texas 78229-3900 (Email: murgo{at}uthscsa.edu).
In the recent review of hypertrophic cardiomyopathy (HCM) by Maron et al. (1), historical controversies relating to the relationship between left ventricular outflow tract (LVOT) pressure gradients and left ventricular (LV) ejection dynamics are revisited. The crux of these controversies is an assumption that the very presence of such gradients negatively impacts LV emptying (outflow) in obstructive HCM.
Maron et al. (1) point to earlier studies in which LV emptying and aortic volumetric flow were shown to be rapid and fairly complete by midsystole, whether a gradient was present or not (2–5). Furthermore, even in the same patient, LV emptying was shown to be faster in cardiac cycles in which gradients were present compared with those in which they were absent (3,6). The investigators of those studies concluded that an LVOT gradient was not associated with an impediment to LV emptying. Maron et al. (1) state that these conclusions were subsequently proven to be invalid, but cite a study that never investigated LV emptying or aortic flow in volumetric terms (7).
Unfortunately, much of this lack of agreement derives from the fact that we often use, in discussing hemodynamics, colloquial terms that have no rigorous mathematical or physical definitions in the field of physics or its subdiscipline of fluid dynamics. For example, the terms obstruct or impede are not found in the physical sciences. Surely, one can find them defined in dictionaries: "to block or fill (a passage) with obstacles ..." or "to impede, retard, or interfere with; hinder (sic) progress... ." (8). However, such terms are subjective and are not defined by quantitative physical measurements.
Another major problem in these debates has been the difficulty that most clinical cardiologists have in understanding the physical relationships between pressure gradients and flow. We were all trained in the resistance concept of Poiseuille's law, in which forward flow is always associated with a positive pressure gradient. However, this concept is only a small part of the physics of pressure and flow, in which the additional impact of inertial and convective acceleration components plays a major role and the significance of pressure gradients is more completely understood (9).
So, what does the term outflow obstruction mean? Where has it ever been actually defined so that all participants in these debates are operating from the same platform? Unfortunately, no rigorous definition exists. This has resulted in confusion, semantic differences, and much misunderstanding.
If one examines the issue of obstruction from a muscle mechanics viewpoint, the presence of marked LVOT gradients must result in inappropriate endocardial wall stress, which can lead to exacerbation of the diastolic abnormalities that underlie many of the congestive symptoms in HCM. However, from a pump function standpoint, where LV volume or aortic volumetric flow is actually measured as a function of time through systole, there is no evidence that outflow is compromised as a result of an LVOT gradient.
Such an understanding does not imply that elimination of LVOT gradients is not potentially beneficial. Rather, one hopes that when one does recommend an intervention to eliminate such gradients, one understands that that intervention is not designed to improve ejection itself.
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1. Maron BJ, Maron MS, Wigle ED, Braunwald E. The 50-year history, controversy, and clinical implications of left ventricular outflow tract obstruction in hypertrophic cardiomyopathy J Am Coll Cardiol 2009;54:191-200.[Abstract/Free Full Text]2. Criley JM, Lewis KB, White RI, Ross RS. Pressure gradients without obstruction: a new concept of "hypertrophic subaortic stenosis." Circulation 1964;32:881-887.[Web of Science] 3. Wilson WS, Criley JM, Ross RS. Dynamics of left ventricular emptying in hypertrophic subaortic stenosis: a cineangiographic and hemodynamic study Am Heart J 1967;73:4-16.[CrossRef][Web of Science][Medline] 4. Murgo JP, Alter BR, Dorethy JF, et al. Dynamics of left ventricular ejection in obstructive and non-obstructive cardiomyopathy J Clin Invest 1980;66:1369-1382.[Web of Science][Medline] 5. Murgo JP. Does outflow obstruction exist in hypertrophic cardiomyopathy? N Engl J Med 1982;307:1008-1009.[Web of Science][Medline] 6. Murgo JP, Miller JW. Hemodynamic, angiographic and echocardiographic evidence against impeded ejection in hypertrophic cardiomyopathyIn: Goodwin JF, editor. Heart Muscle Disease. Lancaster, England: MTP Press; 1985. pp. 187-211. 7. Jenni R, Ruffmann K, Vieli A, et al. Dynamics of aortic flow in hypertrophic cardiomyopathy Eur Heart J 1985;6:391-398.[Abstract/Free Full Text] 8. The American Heritage Dictionary of the English Language4th edition. Boston, MA: Houghton Mifflin; 2009. 9. Murgo JP. Systolic ejection murmurs in the era of modern cardiology—what do we really know? J Am Coll Cardiol 1998;32:1596-1602.[Abstract/Free Full Text]
Related Article
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Reply
- Barry J. Maron, Martin S. Maron, E. Douglas Wigle, and Eugene Braunwald
J. Am. Coll. Cardiol. 2010 55: 608-609.
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