Impaired cardiac mechanical synchrony revealed with increased myocardial work in women with advanced age

Background: Whether the synchronous nature of the myocardium is sex-dependent or affected by the aging process remains unknown. This study aimed to determine the influence of sex and age on cardiac mechanical synchrony during controlled hemodynamic stress. Methods: Transthoracic speckle-tracking echocardiography analyses and central hemodynamics were assessed at rest and during moderate- to high-intensity exercise in healthy young ( < 45 yr) and older ( ≥ 45 yr) women ( n = 32) and men ( n = 34) matched by age, physical activity and exercise capacity. Left ventricular mechanical dyssynchrony (LVMD) was determined as the time to peak standard deviation (TPSD) of longitudinal and transverse strain and strain rates (LSR, TSR). Results: Physical activity, aerobic capacity, heart rate, blood pressure and LVMD at rest were similar between women and men in each age group ( P > 0.05). The rate pressure product, an index of myocardial work, did not differ between sex and age groups at rest and during exercise at a given percentage of peak heart rate ( P > 0.05). A consistent age effect was observed for transverse LVMD ( P -for-age ≤ 0.011). Specifically, older women presented with marked increments ( ≥ 42 %) in TSR TPSD at all exercise levels compared with younger women ( P ≤ 0.005). Sex per se did not generally affect LVMD. Conclusion: A prevailing impairment of cardiac mechanical synchrony in the transverse axis of the left ventricle is revealed during conditions of elevated hemodynamic stress in women with advanced age.


Introduction
With the increasing emphasis on understanding sex differences in human physiology, revived interest is focused on the heart (Diaz-Canestro and Montero, 2020;Regitz-Zagrosek and Kararigas, 2017). To date, multiple structural and functional dimorphisms have been attributed to the 'female' heart (Regitz-Zagrosek and Kararigas, 2017). In essence, compared with men, women are prone to thickened cardiac walls without internal volume expansion, known as concentric hypertrophy as well as attenuated relaxation in the most muscular cardiac chamber, i.e., the left ventricle (LV), which is embedded in a relatively smaller heart that must develop similar pressures to sustain blood supply to body tissues (Chung et al., 2006;Hayward et al., 2001;Regitz-Zagrosek and Kararigas, 2017). The LV in women is thus persistently exposed to increased hemodynamic stress per unit of muscle (myocardial) fiber. Female-specific LV adaptations are also evident when long-term hemodynamic stimuli, such as those induced by exercise training, are controlled (Diaz-Canestro and Montero, 2020), implying the presence of constitutional differences, markedly apparent with advance age (Diaz-Canestro and Montero, 2020;Dworatzek et al., 2014).
LV wall alterations underpin prevalent cardiac diseases in older women and independently predict hard clinical outcomes in the general population (Biering-Sorensen et al., 2017;Donal et al., 2008;Modin et al., 2018;Morris et al., 2012;Phan et al., 2010;Santos et al., 2014). Herein, the partial loss of mechanical synchrony among LV myocardial fibers, as detected via speckle-tracking imaging (STI) in the absence of electrical alterations, is increasingly recognized as a salient cardiovascular risk factor compromising the efficiency of cardiac contraction (Biering-Sorensen et al., 2017;Modin et al., 2018;Morris et al., 2012;Phan et al., 2010;Santos et al., 2014;Shah and Solomon, 2016). In this regard, the Copenhagen City Heart Study, which followed 1138 individuals from the general population for 11 years, found that LV mechanical dyssynchrony (LVMD) independently predicts cardiovascular death (4 % increment in mortality per 10 ms increase in LVMD) after adjustment by traditional mortality risk factors (Modin et al., 2018). Furthermore, LVMD adds incremental prognostic power over established risk charts (e.g., SCORE and the American Heart Association Pooled Cohort Equation) in the prediction of cardiovascular mortality (Modin et al., 2018). However, to date, whether clinically relevant alterations in the highly synchronous nature of the myocardium syncytium are manifest in healthy humans according to constitutional factors (sex, age) is unknown. Namely, previous investigations have reported sex-and age-related differences in cardiac phenotypic variables plausibly associated with LVMD (Chung et al., 2006;Hayward et al., 2001;Regitz-Zagrosek and Kararigas, 2017), yet the specific impact of sex and age on LVMD remains to be elucidated, notably under non-resting conditions in which cardiac alterations are magnified (Lee et al., 2010;Tan et al., 2013).
The purpose of this study was to determine the impact of sex and age on LVMD at rest and during intensity-controlled exercise in healthy young and older women and men matched by potential confounding factors. We hypothesized that the combination of female sex and older age will evidence alterations in LVMD, notably at high exercise intensities in that myocardial work is maximal and potential dysfunctional traits are enhanced (Lee et al., 2010;Tan et al., 2013). In addition, this study explored the relationship of LVMD with major cardiac structural, volumetric and functional variables.

Study participants
Sixty-six adult women and men matched by age (young (<45 yr) and older (≥45 yr) groups) and physical activity levels were recruited via online and printed advertisements in the city of Calgary. Moderate-tovigorous physical activity (MVPA) levels were assessed as previously detailed (Montero et al., 2016). Inclusion criteria comprised healthy status, absence of current medical symptoms or medication limiting incremental exercise testing, and no history of cardiac, pulmonary or neuromuscular diseases. Healthy status was determined via health/ clinical questionnaires as well as echocardiographic screening at rest. Presence of any abnormality in resting echocardiography or ECG invalidated the healthy status. Only individuals with complete cardiac strain analyses at all assessment points, as detailed in the 'Measurements' section, were included. The study was approved by the Conjoint Health Research Ethics Board (REB18-1654) of the University of Calgary and conducted in accordance with the declaration of Helsinki. All participants provided informed oral and written consents before starting the measurements.

Study protocol
Participants were required to report once to our laboratory for testing in addition to a voluntary familiarization visit. All individuals were instructed to avoid strenuous exercise, alcohol and caffeine as well as to record fluid intake and maintain their usual daily dietary habits from 24 h prior to testing. All measurements were performed after a fasting period (≥5 h) in a quiet room with controlled temperature between 22 and 23 • C.

Stress echocardiography and central hemodynamics
Two-dimensional apical cine-loops were recorded via highresolution ultrasound with a 3.5 MHz transducer (Mindray Medical M9, USA) at supine rest and during predetermined levels of incremental exercise relative to peak heart rate (HR peak ) (80 and 100 % HR peak ) as well as at a fixed submaximal absolute workload (100 W). Exercise measurements were performed in a supine cycle ergometer designed to facilitate the precision of echocardiography and hemodynamic measurements and the attainment of peak aerobic capacity in 7-10 min via 10-30 W increments, as previously detailed (Diaz-Canestro et al., 2022;. Following the American Society of Echocardiography and the European Association of Cardiovascular Imaging recommendations, cardiac chamber quantification was completed offline using the modified Simpson method (biplane method of disks) by tracing the endocardial border of the LV at end-diastole and end-systole at a target frame rate of 30 Hz (range 25-40 Hz) to optimize spatial resolution (Lang et al., 2015;Pellikka et al., 2007). Diastolic function was assessed via transmitral inflow velocities determined by pulsedwave Doppler, with the sample volume placed between the mitral leaflet tips. The peak inflow velocities during early (E) and late (A) diastole were assessed, and the E/A velocity ratio was calculated. Myocardial tissue e ′ and a ′ velocities were measured via tissue Doppler imaging, and the E/e ′ ratio was calculated. The recommended Cube algorithm was used to estimate left ventricular mass (LV mass ) (Eq. (1)) (Lang et al., 2015). Left ventricular relative wall thickness (LVRWT) was determined according to Eq. (2). In addition, systolic blood pressure (SBP), diastolic blood pressure (DBP) and mean blood pressure (MBP) were continuously assessed non-invasively via the volume-clamp method and finger plethysmography adjusted to the heart level at a sampling frequency of 200 Hz (Finometer PRO, Finapres Medical Systems, Netherlands), with data exported into a pre-established acquisition software (Labchart 7, AD Instruments, UK). Stroke volume (SV) was calculated as left ventricular end-diastolic volume (LVEDV) minus left ventricular end-systolic volume (LVESV), while the product of SV and HR provided cardiac output (Q). Total peripheral resistance (TPR) was determined as the ratio of MBP and Q. Finally, the rate pressure product (RPP), an index of myocardial work, was obtained via the product of SBP and HR. The reproducibility of key echocardiographic and hemodynamic measurements (intraobserver coefficient of variation (CV)) during exercise in our laboratory is ≤6 % for LV volumes and ≤3 % for blood pressures. (1)
According to a power analysis of previous findings (Donal et al., 2008), a sample size of 11 individuals per group provides ≥85 % power to detect clinically relevant differences in LVMD with a two-sided α of 5 %.
Pairwise comparisons among sex and age groups were performed via independent sample t-tests. In addition, linear regression analyses were implemented to assess the relationship of LVMD (independent variable) with (i) LV volumes (LVEDV, LVESV, SV) and ejection fraction (dependent variables) at rest and during exercise, and (ii) LV mass and LV diastolic function (dependent variables) at rest, in women and men irrespective of age status. The Pearson correlation coefficient (r) and root mean square error (RMSE) were calculated. A two-tailed P-value <0.05 was considered significant. All data are reported as mean (±SD).

General characteristics
Main characteristics and resting echocardiography variables are presented in Table 1. All individuals were non-smokers and non-obese (body mass index <30 kg⋅m − 2 ). Physical activity, aerobic capacity, heart rate and blood pressure were similar between women and men in each age group (P ≥ 0.211). Moreover, age did not significantly differ between sexes in young (P = 0.089) and older (P = 0.251) individuals. Young and older women presented smaller anthropometric indices (height, weight, body surface area) compared with men (P ≤ 0.010). MBP was increased in older individuals (P ≤ 0.010), irrespective of sex. Older age as well as female sex in older individuals were associated with increased TPR (P ≤ 0.041). With respect to cardiac variables at rest, LVRWT was elevated in older compared with younger women (P = 0.019), denoting a shift towards LV concentric hypertrophy with advanced age. Likewise, all measures of LV diastolic function, except the E/e ′ ratio, were reduced in older compared with younger individuals in both sexes (P ≤ 0.005). Fig. 1 displays HR, SBP and RPP (HR × SBP) at rest and during exercise. Relative HR (80 and 100 % HR peak ) during exercise was lower in older compared with younger women and men (P < 0.001). At a given absolute workload (100 W), HR was higher in women compared with men in both age groups (P ≤ 0.035). No sex differences at rest and during exercise were present in SBP (P ≥ 0.400), which was consistently augmented in older relative to younger individuals irrespective of sex (P ≤ 0.021). RPP was higher in older compared with younger men at 100 W (P = 0.031) and in older men relative to age-matched women at 100 W (P = 0.038). In contrast, RPP did not differ between sex and age groups at rest and during exercise at intensities relative to HR peak .

Fig. 2.
Longitudinal left ventricular mechanical dyssynchrony (LVMD) at rest and during exercise in young and older women and men. Measurements during exercise were performed at a given absolute workload (100 W) and at relative intensities (80 and 100 % of HR peak ). Data are expressed as mean ± SEM. Myocardial segments: 1. Basal anterolateral/2. Basal inferoseptal/3. Mid anterolateral/4. Mid inferoseptal/5. Apical lateral/6. Apical septal/7. Apex. HR, heart rate; HR peak , peak heart rate; LS, longitudinal strain; LSR, longitudinal strain rate; TPSD, time to peak standard deviation. *, P < 0.05 compared with age-matched women within a specific level of stress. Fig. 3. Transverse left ventricular mechanical dyssynchrony (LVMD) at rest and during exercise in young and older women and men. Measurements during exercise were performed at a given absolute workload (100 W) and at relative intensities (80 and 100 % of HR peak ). Data are expressed as mean ± SEM. Myocardial segments: 1. Basal anterolateral/2. Basal inferoseptal/3. Mid anterolateral/4. Mid inferoseptal/5. Apical lateral/6. Apical septal/7. Apex. HR, heart rate; HR peak , peak heart rate; TPSD, time to peak standard deviation; TS, transverse strain, TSR, transverse strain rate. † , P < 0.05 compared with sex-matched young individuals within a specific level of stress.

Left ventricular mechanical dyssynchrony (LVMD)
Myocardial strain in the LV at rest and during exercise in the longitudinal and transverse axes is presented in Figs. 2 and 3. With respect to longitudinal LVMD, at rest, an age effect was observed for LS TPSD, which was increased in older compared with younger individuals (effect size (ES) = 9824.8, F-value = 6.425, P for age = 0.014). LS TPSD was augmented with older age during exercise at 100 % HR peak (ES = 3855.8, F-value = 7.598, P for age = 0.008). No age effect in LS TPSD was noted in women or men separately (Fig. 2). Regarding the specific effect of sex, no differences were found at rest and only LS TPSD and LSR TPSD during exercise inconsistently differed between sexes (Fig. 2). As regards transverse LVMD, TS TPSD was increased during exercise at 100 W and 100 % HR peak in older compared with younger individuals (ES ≥ 3155.1, F-value ≥6.175, P for age ≤ 0.016). A prevailing age effect was observed for TSR TPSD, which was elevated at all exercise levels in older individuals (ES ≥ 2812.9, F-value ≥6.860, P for age ≤ 0.011). Notably, older women showed increased TSR TPSD at all exercise levels compared with younger women (P ≤ 0.005), an effect that was less pronounced in men (P ≥ 0.053) (Fig. 3). Sex-specific differences were not present in transverse LVMD (ES ≤ 4.6, F-value ≤0.011, P ≥ 0.916).

Discussion
This study determined the impact of sex and age on the mechanical synchrony of the myocardium at rest and during moderate-to highintensity exercise in healthy individuals. The major findings are: (i) sex per se has only a minor influence on LVMD at rest and during exercise; (ii) LVMD in the longitudinal and prominently in transverse axis are augmented during exercise with older age; (iii) transverse LVMD during exercise is markedly increased with older age in women. These findings provide evidence of the effect of aging on LVMD in the absence of common pathophysiological alterations.
Sex differences in cardiac structure and function are being increasingly recognized over the last two decades (Chung et al., 2006;Diaz-Canestro and Montero, 2019, 2020Dworatzek et al., 2014;Hayward et al., 2001;Regitz-Zagrosek and Kararigas, 2017). According to the established effect of sex on major phenotypic traits such as cardiac cavity dimensions, myocardial wall thickness and relaxation properties (Chung et al., 2006;Hayward et al., 2001;Regitz-Zagrosek and Kararigas, 2017), corresponding divergences in the inner mechanical synchrony of the LV were hypothesized, at least during conditions of elevated hemodynamic stress. Under these conditions, e.g., via dobutamine administration or exercise stimuli, differences in LVMD are magnified (Lee et al., 2010;Tan et al., 2013). Namely, failure in the coordination of contraction and relaxation between myocardial fibers is amplified with conditions involving increased metabolic demand, when efficient cardiac function is more needed. However, the current study indicates that LVMD is not predominantly influenced by sex in both the longitudinal and transverse axes from rest up to peak incremental exercise (Figs. 2 and 3). Consistent with these results, previous normative data available at rest did not unveil sex differences in LVMD in healthy individuals with advanced age (Cheng et al., 2013). To our knowledge, this is the first study to determine the impact of sex on cardiac mechanical dyssynchrony under nonresting conditions. As a distinctive feature of cross-sectional studies, the control of potential confounding factors between groups is essential. In this regard, emphasis was made to match women and men by fundamental constitutional and lifestyle factors modulating the cardiac phenotype (Lundby et al., 2017), which resulted in the balance of age, physical activity, heart rate, blood pressure, aerobic capacity and myocardial work between sexes (Table 1, Fig. 1). Such a comprehensive match may have contributed to the absence of substantial sex differences in LVMD for a given age status (young, older) during high hemodynamic stress (Armstrong et al., 2016).
In contrast to the limited influence of sex on LVMD, a prevailing impact was revealed for age. Namely, older age impaired myocardial mechanical synchrony, notably in the LV transverse axis during exercise, from moderate to peak intensities (Fig. 3). The transverse axis refers to the horizontal dimension of the LV wall in the apical view, which is equivalent to the radial axis in the parasternal short-axis view. In plain words, the shortest linear path to traverse the LV wall represents the transverse and radial axes. As the ventricle contracts, the myocardium thickens along these axes. As the ventricle contracts, the myocardium thickens in the transverse and radial axes. In parallel, the contracting myocardium shortens in the longitudinal and circumferential axes. In this regard, the strain associated with myocardial thickening is superior to that linked with myocardial shortening in predicting ejection fraction and hard clinical outcomes after cardiac resynchronization therapy (CRT) (Delgado et al., 2008;Suffoletto et al., 2006;Tanaka et al., 2010). In particular, in the absence of high transverse or radial LVMD (≥130 ms) at baseline, 53 % of patients subjected to CRT experienced a serious adverse event (death, heart transplant, implantation of LV assist device) over a 3.5 yr follow-up (Tanaka et al., 2010). Conversely, these adverse events only occurred in 12 % of CRT patients with high transverse or radial LVMD (Tanaka et al., 2010). The mechanical synchrony in the remaining LV axes (longitudinal, circumferential) did not predict CRT outcomes (Tanaka et al., 2010). Hence, the dyssynchrony of myocardial thickening is a crucial modifiable variable with major impact on cardiac function, morbidity and mortality. Given that a fraction of older individuals (11 %) in the present study demonstrated high levels of transverse LVMD (≥130 ms), its potential improvement could be conceived to entail benefits in healthy individuals. Of note, the herein obtained resting data for transverse LVMD in older individuals fall close to the 50th percentile of the population (Cheng et al., 2013). Further research is needed to determine whether the impact of aging on transverse LVMD is amenable to modification via lifestyle or low-risk pharmacological interventions in the high end of the spectrum (>90th percentile for transverse LVMD) of the healthy population.
The aging process is inherently associated with a progressive stiffening of the cardiovascular system (Camici et al., 2015;Montero et al., 2015;Pierce et al., 2022). Such a phenotypic modification strongly contributes to age-related alterations in cardiac function, e.g., impaired LV relaxation (Table 1) (Westermann et al., 2008). While both sexes are affected, large population studies indicate that women with advanced age present with magnified stiffening and LV diastolic dysfunction compared with age-matched men (Okura et al., 2009;Redfield et al., 2005). Hence, the aging female heart requires higher filling pressures, which contribute to a pathophysiological remodeling characterized by LV concentric hypertrophy (Regitz-Zagrosek and Kararigas, 2017). This propensity was detected in our study sample, with older women, but not men, presenting with augmented LVWRT, a marker of LV concentric hypertrophy, compared with younger women. Age-related LV functional and structural alterations at rest were accompanied by marked increments in transverse LVMD at all exercise intensities in women with older age (Fig. 3). Specifically, the dyssynchrony of transverse myocardial strain rate (TSR) was augmented by 42 to 63 % during exercise in older relative to young women. Likewise, consistent negative relationships were exclusively found in women between TS TPSD and measures of LV diastolic function, while TSR TPSD was positively associated with LV mass . Consequently, LV relaxation in women could be, at least in part, a function of the synchrony of transverse LVMD, which in turn might be partly determined by sex-specific structural dimorphisms. Collectively considered, the impaired coordination of transverse myocardial deformation (thickening) per unit of time might be a novel and functionally relevant maladaptation to aging in the female heart, as revealed in conditions of high hemodynamic stress.

Limitations
Cause-and-effect relationships cannot be established in crosssectional analyses (Table 2). Lifelong longitudinal studies, while seldom implemented due to logistical and technical challenges, may eventually provide a stronger level of evidence with respect to effects of aging on LVMD.

Conclusion
The present study demonstrates age-related impairments in the intrinsic mechanical synchrony of myocardial fibers under elevated hemodynamic stress. Notably, older age markedly increases LVMD in myocardial thickening during exercise in women, which parallels the age-and female-related shift towards disproportionately thickened LV walls. Nonetheless, sex is not a predominant determinant of LVMD. Novel cardiac insight is provided contributing to the understanding of deeply ingrained myocardial wall modifications potentially leading into established pathophysiological alterations of the aging heart.

Funding
This work was funded by the Swiss National Science Foundation

Availability of supporting data
All data relevant to this study are presented in the manuscript.

Ethics approval
The study was approved by the Conjoint Health Research Ethics Board (REB18-1654) of the University of Calgary and conducted in accordance with the declaration of Helsinki.

Declaration of competing interest
The authors declare no conflict of interest.

Data availability
Data will be made available on request.

Table 2
Schematic display of the study protocol. Subsequent steps (chronological order) Participant arrives at the laboratory after the fasting period Completion of health and clinical questionnaires (20 min) Resting supine for stabilization of cardiac and hemodynamic variables (20 min) Echocardiographic screening at rest (15 min) Incremental exercise test including echocardiography recording (cine loops) and continuous BP monitoring (10 min) Recovery from incremental exercise test (10 min) Storage of echocardiographic and hemodynamic data (20 min) Offline analyses (>3 h per participant) BP, blood pressure measured via the volume-clamp method and finger plethysmography adjusted to the heart level.