DISCUSSION

Our findings confirm that distinct strain curve patterns correspond to different cardiomyopathic phenotypes, and the strain-volume approach extends concepts previously described in humans. In the hypertrophic phenotypes, strain reduction occurred without marked LV dilatation, resulting in curves closer to those of the normal controls. These observations support the concept that strain curve analysis captures the interplay between function and remodeling in feline cardiomyopathy. This study employed a multi-chamber approach, providing a more comprehensive assessment of myocardial and atrial function than prior studies that focused on individual chambers. The role of LA strain is particularly noteworthy. Conventional indices, such as the LA volume index, provide information on chronic remodeling but are less responsive to dynamic changes in diastolic function [16]. LARS may offer additional functional information beyond LA/AO, consistent with findings from human and canine studies, in which atrial strain may improve the assessment of diastolic dysfunction in felines [17].

Global longitudinal strain

A previous study showed that LVGLS values in control and preclinical HCM cats are comparable to our findings [30]. Longitudinal strain (LS) mapping through a bull’s-eye plot enables the quantification of regional myocardial deformation and visual assessment of spatial heterogeneity in LV contractility (Figures 1 A–H). Results provide a more detailed evaluation of both the location and extent of myocardial dysfunction, enabling identification of regional abnormalities that may be missed by global indices such as FS. In the present analysis, the bull’s-eye map demonstrated segmental variations in LS, with reduced deformation predominantly within the septal and inferior regions, while the lateral and apical segments retained near-normal values. This finding of regional inhomogeneity of strain is particularly relevant in the context of HCM, where FS often remains normal or supranormal because of concentric remodeling and reduced cavity size. In these cases, FS or EF alone underestimates systolic dysfunction, as geometric alterations mask impaired myocardial shortening. In contrast, GLS and regional LS patterns can be used to sensitively detect subtle abnormalities in contractility.

Figure 1: (A-D) Left ventricular images of a healthy cat and (E-H) a cat with HCM An image of the LV through the apical long-axis. The LV is traced, and speckles are placed throughout the myocardium. The movements of these speckles are tracked throughout the cardiac cycle. (A, C) Each colored line shows the amount of strain of a left ventricular segment. (B) The integrated strain is incorporated into a 17-segment model of the LV, representing global strain. (C) The strain curve. (D) The myocardial work. Figure E–H. Representative images of LS analysis in a cat with hypertrophic phenotype. (E) Apical long-axis view with segmental LS tracking. (F) Polar (bull’s-eye) projection of regional LS values showing segmental differences in deformation. (G) Strain–time curves from multiple segments, illustrating asynchronous contraction. (H) Quantitative strain dispersion map highlighting non-uniform myocardial function across LV regions.

The current strain pattern may indicate segmental hypertrophy with nonuniform wall thickening. Regional strain reduction has been associated with myofiber disarray and interstitial fibrosis, providing additional diagnostic insight beyond conventional echocardiographic measures. Therefore, incorporating LS bull’s-eye mapping into routine echocardiographic analysis may enhance the differentiation of hypertrophic phenotypes and may help to improve the recognition of early myocardial dysfunction in cats.

Some previous studies reported preserved longitudinal systolic strain and strain rate (SR), with only diastolic abnormalities in early and late diastolic SR (E and A SR), whereas others reported significant reductions in both the systolic and diastolic deformation indices. These discrepancies are likely attributable to differences in study design, sample size, degree of hypertrophy, and methods used for segmental and global strain analysis. In one earlier study, no change in longitudinal systolic strain (S) or the SR was observed, although a reduction in diastolic SR was documented [25]. That cohort consisted of untreated cats with relatively mild hypertrophy and limited heterogeneity in disease severity, and segmental strain data were not provided. The relatively small sample size and uniformity of the study population likely limited its statistical power and generalizability. Conversely, another investigation demonstrated decreased circumferential and longitudinal systolic strain in asymptomatic cats with HCM compared with healthy controls, but these differences were less evident in cats showing mild clinical signs such as difficulty breathing and tachypnea [3]. However, subgroup analyses with few animals per category may have further reduced the robustness of their findings.

The results of the present study show that cats with asymptomatic HCM exhibit significantly reduced circumferential and longitudinal deformation when compared with the healthy control Group, despite preserved FS. These alterations suggest the presence of subclinical myocardial dysfunction, which is consistent with early contractile impairment detectable by STE before conventional echocardiographic indices decrease. Interestingly, our data revealed preserved circumferential strain in Group 3 (Table 2), in agreement with previous studies reporting that circumferential deformation may be maintained until more advanced stages of hypertrophy. One recent investigation demonstrated preserved endocardial circumferential strain but reduced epicardial strain in cats with dynamic left ventricular outflow tract obstruction, resulting in an increased epicardial-to-endocardial strain ratio [4–6]. These findings imply a layer-specific pattern of deformation, in which subendocardial fibers remain functional for longer, whereas subepicardial dysfunction may contribute to disease progression.

The circumferential and radial strain patterns observed in this study (Figures 2A and 2B) illustrate synchronous contraction in healthy cats and reduced strain amplitude with delayed peak timing in HCM cats, indicating heterogeneous regional deformation. These results emphasize the importance of incorporating layer-specific and regional strain analyses to fully characterize myocardial mechanics in feline HCM. Overall, circumferential strain assessment complements longitudinal measurements, providing a multidimensional understanding of myocardial dysfunction in feline cardiomyopathy. The integration of strain imaging across myocardial layers and orientations may enhance the early detection and staging of HCM, supporting improved risk stratification and therapeutic monitoring in clinical practice.

Figure 2: Mid-ventricular short-axis view depicting circumferential strain analysis in cats. (A) Representative example from a healthy cat showing homogeneous segmental contraction and uniform peak strain. (B) A cat with hypertrophic cardiomyopathy demonstrates reduced strain amplitude and temporal dispersion across myocardial segments, indicating early mechanical dyssynchrony.

LA strain

Previous studies in human cardiology have shown that both an increased LA volume index and a reduced LARS are independently associated with the LV mass index. However, a previous study stated that they are not always directly linked to myocardial fibrosis quantified by cardiac magnetic resonance imaging (CMR). In the present feline cohort, which included cats diagnosed with HCM, we observed similar trends: LARS correlated closely with conventional echocardiographic indices of LV diastolic dysfunction, particularly the mitral inflow E/A ratio and tricuspid regurgitation (TR) velocity (Figures 3A–F). Cats with lower LARS values demonstrated greater LV wall thickness, increased fibrosis extent, and reduced LV LS, indicating that impaired atrial compliance accompanies ventricular hypertrophy and stiffness. Notably, a reduced LARS-defined diastolic dysfunction grade was also associated with resting LV outflow tract obstruction, highlighting the interaction between abnormal diastolic filling and dynamic obstruction in feline HCM.

The relationship between LARS and other diastolic markers persisted even when adjusted for potential confounders such as arrhythmia or the risk of sudden cardiac death. However, no outcome data, such as congestive heart failure (CHF), arterial thromboembolism, or survival, were assessed in this study. These findings suggest that LARS may provide independent prognostic information beyond traditional echocardiographic parameters. The progressive reduction in LARS may therefore serve as an early marker of disease severity and a predictor of HCM-related morbidity, including hospitalization for CHF.

The incorporation of LARS into echocardiographic assessment provides a quantitative and reproducible tool for evaluating diastolic performance. In addition to LV and RV strain analyses, LARS adds substantial value in identifying early hemodynamic changes, stratifying the risk of affected cats, and guiding clinical management. As in human cardiology, this parameter represents an important advancement in the noninvasive evaluation of LV filling pressures and diastolic function in feline HCM. Collectively, these results reinforce the concept that strain-based imaging, particularly when combining LVGLS and LARS, enhances the diagnostic precision of echocardio-graphy and provides superior insight into the pathophysiology and progression of feline cardiomyopathy.

Figure 3: Representative echocardiographic assessment of left atrial (LA) strain in cats. (A–C) Echocardiographic images obtained from a healthy cat (Group 1). (A) Pulsed-wave Doppler mitral inflow profile. (B) Left atrial reservoir strain (LARS) curve. (C) Apical four-chamber view showing the color-coded region of interest used for two-dimensional speckle-tracking analysis of LA strain. (D–F) Representative echocardiographic findings from a cat with hypertrophic cardiomyopathy (Group 2). (D) Pulsed-wave Doppler mitral inflow demonstrating a reduced E/A ratio (0.71), consistent with impaired left ventricular relaxation (diastolic dysfunction). (E) LARS curve demonstrating reduced peak reservoir strain (16%), indicative of impaired left atrial compliance. (F) Two-dimensional speckle-tracking image illustrating the color-coded tracking of the LA wall (blue) used for LARS measurement.

Right ventricular longitudinal strain (RVLS)

RVLS, derived from STE, has emerged as a reliable and reproducible index of RV systolic function. It enables the detection of subtle functional changes that often precede alterations in conventional parameters such as fractional area change or tricuspid annular plane systolic excursion (TAPSE). In both human and veterinary cardiology, the RVLS provides additional prognostic information and has become an important complement to the standard echocardiographic assessment of biventricular function. In this study, the RVLS was quantified from the RV-focused apical four-chamber view (Figures 4A–D), focusing specifically on the free-wall segments. The strain–time curves demonstrated segmental variability, suggesting early RV involvement in HCM. Results emphasize that ventricular interdependence in feline HCM extends beyond the LV, as increased LV wall stiffness and diastolic dysfunction can impose secondary strain on RV performance through interventricular coupling and shared myocardial fibers. In human cardiology, an absolute RVLS value greater than -20% is generally considered normal, with less negative values indicating impaired systolic function [31]. Comparable strain ranges have been reported in healthy cats, although species-specific reference intervals remain under development [32]. Reduced RV strain has been associated with various pathological states, including pulmonary hypertension, congenital heart disease, cardiomyopathies, and systemic RV [33].

Human studies have shown that RV strain has independent prognostic significance in predicting adverse cardiac events, particularly in patients with cardiomyopathy or pulmonary hypertension [34]. RVLS may remain preserved in early-stage feline HCM due to the relatively low prevalence of secondary pulmonary hypertension and limited changes in right ventricular afterload. The incorporation of RVLS into feline echocardiographic protocols could enhance early recognition of right-sided dysfunction in conditions such as restrictive or HCM.

The integration of RVLS with LV and LA strain analyses provides a more comprehensive understanding of biventricular–atrial mechanics and allows for earlier detection of global myocardial dysfunction. This approach, using strain echocardiography, may improve disease staging and monitoring of therapeutic outcomes in cats with cardiomyopathy [35], thereby establishing strain echocardiography as a valuable adjunctive marker in routine cardiac evaluation. Clinical application of strain echocardiography in cats is growing. Our results extend these findings by integrating volumetric analysis to provide a comprehensive assessment of cardiac mechanics that may aid in early diagnosis, prognosis, and treatment monitoring.

Figure 4: (A) A six-segment model of the endocardial borders in an RV for the free wall and the ventricular septum. (B) The global (dotted line) and free-wall RV (dashed line) LS curves of a cat in Group 1. Figure C-D. Right ventricular longitudinal strain analysis in a cat with hypertrophic cardiomyopathy. (C) RV-focused apical four-chamber view showing segmentation of the right ventricular free wall for strain analysis. (D) Strain-time curves depicting longitudinal deformation of individual RV segments. The basal segment demonstrates markedly reduced strain, indicating early regional RV dysfunction in Group 3.

Figure 5: Spearman correlation heatmap between strain parameters and conventional echocardiographic indices in cats. Positive correlations are shown in blue, and negative correlations in red, with color intensity corresponding to correlation strength. Left ventricular global longitudinal strain (LVGLS) correlated moderately with radial strain and fractional shortening, while left atrial reservoir strain (LARS) demonstrated inverse associations with indices of wall thickness (IVSs and LVPWs). Weak relationships were observed between strain and the mitral inflow E/A ratio, indicating relative independence of strain from conventional diastolic measurements.

Limitations

A limitation of the present study is that it was conducted at a single center with a relatively small sample size. Some cats were excluded todue of suboptimal image quality and elevated heart rates, reflecting the technical challenges associated with feline patients. Additionally, threshold values for feline LALS remain to be standardized across platforms and populations. Larger, multicenter studies are warranted to validate diagnostic cut-offs and prognostic implications. Another limitation of this study is that circulating cardiac biomarkers, such as N-terminal pro-B-type natriuretic peptide (NT-proBNP), were not evaluated. Because myocardial deformation parameters and cardiac biomarkers may reflect different pathophysiological processes, combining STE with biomarker assessment may improve the detection of early myocardial dysfunction in cats.