Author + information
- Michael Feneley, AM, MD⁎ ()
- ↵⁎Reprint requests and correspondence:
Dr. Michael P. Feneley, Cardiology Department, St. Vincent's Hospital, Victoria Street, Darlinghurst, NSW 2010, Australia
The term contractility refers to the intrinsic contractile capacity of the heart, so that its measurement should ideally be independent of the specific external loading conditions placed on the pump. This load insensitivity of contractility measurement has proved to be an elusive goal. For example, the most common clinical measure of contractility, the ejection fraction, accounts for cardiac preload by expressing the stroke volume ejected as a fraction of the end-diastolic volume (preload), but the ejection fraction takes no account of the afterload (resistance or impedance) against which the ventricle must pump. Because myocardial shortening and force generation are inversely related during contraction, the ejection fraction and all other measures of myocardial shortening that do not account for the generated pressure or force of contraction are inherently sensitive to changes in cardiac afterload.
In many clinical situations, the ejection fraction and other shortening indices suffice because afterload is stable, but their shortcomings become apparent when afterload changes significantly. This occurs, for example, when the ejection fraction is measured before and after mitral valve surgery for severe regurgitation. Before surgery, the low impedance to ejection into the left atrium reduces total left ventricular afterload, with the consequence that the ejection fraction significantly overestimates the intrinsic contractile capacity. When successful surgery removes the low-impedance ejection pathway, therefore, the total afterload rises dramatically, with a consequent fall in the ejection fraction, independent of the intrinsic contractility. A similar difficulty is encountered in evaluating the putative inotropic effects of drugs that also have vasoactive effects.
A better approach to contractility measurement is to measure both the volume and the pressure generated by ventricular contraction (to account for the afterload), over a wide range of volumes (to account for the preload). It is now more than a century since Ernest Starling enunciated the Law of the Heart: “the mechanical energy set free in the passage from the resting to the active state is a function of the length of the muscle fibre” (1), a principle presaged by Otto Frank. Although Starling's law is often illustrated by his famous diagram of the relationship between ventricular stroke work (the pressure-volume loop area) and the filling pressure (a relationship that is nonlinear) (1), Starling made it clear that he was using the filling pressure as a surrogate for the end-diastolic volume, and thus fiber length. Moreover, Starling later demonstrated a linear relationship between stroke work and end-diastolic volume (2), a relationship now known as the preload recruitable stroke work (PRSW) relationship (3).
What underpins Starling's law is the length dependence of activation of the contractile apparatus. As the end-diastolic fiber length (and thus cell length) increases, the number of cross-bridges cycling increases for any given amount of calcium released. This reflects the increased probability of attachment of myosin heads to actin binding sites because their proximity to binding sites and co-operativity increase with increasing cell length. In contrast, interventions that alter contractility (inotropic interventions) do so by changing the amount of calcium released for any given end-diastolic cell length or, less frequently, by increasing the sensitivity of the contractile apparatus to calcium.
The slope of the linear PRSW relationship provides a contractility index that is insensitive to preload by definition, but it is also remarkably insensitive to changes in afterload, and this is true not only of the left ventricle but also the much thinner right ventricle that normally contracts against the much lower pulmonary impedance (4). Nevertheless, a reasonable objection to any absolute claim of afterload insensitivity of the PRSW relationship is the obvious fact that stroke work is zero at both zero afterload and infinite afterload.
An alternative contractility index that was proposed to deal with these afterload extremes, at least conceptually, is the end-systolic pressure-volume relationship (ESPVR) that, as originally conceived, posited a single linear relationship that defined the contractility regardless of the afterload, so that the slope of this relationship was proposed as invariant under conditions of isovolumic contraction (zero volume ejected) and maximum volume ejection (minimal pressure generation) (5). This conceptually simple and elegant model of contractility, with its analogy of contractility to the elastance of a spring, required significant caveats to be applied, however, as its limitations were explored, with evidence of both a tendency to nonlinearity and significant afterload sensitivity under conditions of widely varying afterload and contractility (6).
The relationship between dP/dt max and end-diastolic volume is an alternative linear index of contractility that is derived from the same time-varying elastance model of contractility that underpins the ESPVR. When this relationship, the ESPVR, and the PRSW relationship were compared directly by deriving all 3 from the same pressure-volume loops, the PRSW relationship exhibited the least sensitivity to afterload at the expense of lower inotropic sensitivity (4,7).
Despite the obvious appeal of load-insensitive indices of contractility, however, their clinical application has been very limited because they require measurement of the ventricular pressure and volume over a wide range. Although noninvasive methods have been reported for recording the pressure in the ascending aorta (8) and obtaining the necessary pressure-volume data over a limited volume range, or even from a single beat (9), the great majority of contractility indices proposed for noninvasive assessment clinically have been devoid of any measurement of pressure generation, and thus ignore ventricular afterload. This inherent limitation is true of all contractility indices determined on the basis of measurement of shortening, shortening velocity, strain, and strain rate. All of these indices will be altered by changes to the impedance to contraction, independent of any true change in contractility.
This limitation must always be kept in mind when evaluating any new putative index of contractility. Ideally, the performance of all such indices should be evaluated against a well-validated contractility index that is insensitive to afterload and in relevant clinical situations in which afterload variation is an important consideration. Moreover, some account needs to be made of the incremental value to be gained from any new measurement method over existing methods.
In this issue of iJACC, Jasaityte et al. (10) propose a ventricular segmental stretch-strain relationship as a “novel noninvasive index of left ventricular inotropy.” This new index is determined on the basis of the authors' assumption that the slope of the relationship between total regional systolic shortening (%) of a ventricular segment (which they term “total systolic strain”) and the pre-systolic lengthening of the same segment during atrial contraction (which they term “pre-systolic stretch”) reflects the inotropic state. The authors go on to demonstrate a modest linear correlation between these parameters (r = 0.82), with the slope of the relationship increasing with dobutamine infusion, not changing with alterations in preload (passive leg lift), and declining with anthracycline chemotherapy.
The derivation of these segment length dynamics by tissue Doppler is novel, and the measurement is potentially of value. Nonetheless, there are some important issues to consider before this tool can be applied clinically. First, similar to the ventricular myocardium, the atrial myocardium exhibits length-dependent activation, so that increased preload to the atrium is associated with increased atrial contraction with consequent greater pre-systolic lengthening of any ventricular segment. Consequently, the increased systolic shortening of the same ventricular segment is to be expected as a direct consequence of the increased end-diastolic fiber length due to length-dependent activation. In this sense, the pre-systolic lengthening is just a surrogate measure of the preload (end-diastolic segment length). This was evident in the proportionate increase in pre-systolic lengthening and systolic shortening with increased preload, with the consequence that the slope of the relationship was not significantly altered. Nevertheless, the R2 value of 0.67 suggests that pre-systolic segment stretch is a relatively weak surrogate for end-diastolic segment length.
Second, because the stretch-strain relationship is a surrogate for the relationship between segmental shortening and end-diastolic segment length, it would be expected to correspond with results obtained for the global ejection fraction, and this was indeed the case during both dobutamine infusion and after anthracycline treatment. It should be noted, however, that neither the ejection fraction nor the stretch-strain relationship can discriminate between the inotropic effects of dobutamine and its afterload-reducing (vasodilating) effects.
As the authors acknowledge, a major limitation of their study is that they made no attempt to determine the sensitivity of their stretch-strain index to afterload. It would have been helpful also to more directly compare the stress-strain responses with the ejection fraction changes in individual subjects to gain an appreciation of the new index's incremental value and reliability as a contractility index. For example, the increase in ejection fraction with passive leg lifting would not be expected with a pure preload change, and suggests the intervention caused a small inotropic effect in subjects who did not receive autonomic blockade. The fact that the stretch-strain relationship did not mirror this change in the ejection fraction might indeed indicate it was less sensitive than the ejection fraction to subtle inotropic effects. It would also have been valuable to compare this unidimensional segmental index with the unidimensional equivalent of the ejection fraction so routinely measured in clinical echocardiography, the left ventricular fractional shortening.
The use of strain to measure shortening could be an ingenious means of providing a simple measure of contractility. However, the validation of such an approach requires some additional steps, and we should look forward to a more detailed evaluation of this new index in the near future.
Dr. Feneley has reported that he has no relationships relevant to the contents of this paper to disclose.
↵⁎ Editorials published in JACC: Cardiovascular Imaging reflect the views of the authors and do not necessarily represent the views of JACC: Cardiovascular Imaging or the American College of Cardiology.
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