Physiologic Range of Myocardial Mechano-Energetic Efficiency among Healthy Subjects: Impact of Gender and Age

Background: Myocardial mechano-energetic efficiency (MEE) is the capability of the left ventricle (LV) to convert the chemical energy obtained from the cardiac oxidative metabolism into mechanical work. The aim of present study was to establish normal non-invasive MEE and MEEi reference values. Methods: In total, 1168 healthy subjects underwent physical examinations, clinical assessment, and standardized transthoracic echocardiographic (TTE) examination. MEE was obtained by TTE as the ratio between stroke volume (SV) and heart rate (HR): MEE = SV/HR [HR expressed in seconds (HR/60)]. Because MEE is highly related to left ventricular mass (LVM), MEE was then divided by LVM with the purpose of obtaining an estimate of energetic expenditure per unit of myocardial mass (i.e., indexed MEE, MEEi, mL/s/g). Results: The mean values of MEE and MEEi in the overall population were 61.09 ± 18.19 mL/s; 0.45 ± 0.14, respectively. In a multivariable analysis, gender, body surface area (BSA), diastolic blood pressure, left atrial volume indexed to BSA, E/e’ and tricuspid annular plane systolic excursion (TAPSE) were the independent variables associated with MEE, while age, gender, BSA and TAPSE were the independent variables associated with MEEi. Conclusions: The knowledge of age- and gender-based MEE and MEEi normal values may improve the global assessment of LV cardiac mechanics and serve as a reference to identify phenotypes at high risk of cardiovascular events.


Introduction
Myocardial mechano-energetic efficiency (MEE) is defined as the capability of the left ventricle (LV) to convert the chemical energy obtained from the cardiac oxidative metabolism into mechanical work [1]. It has been hypothesized that the increased energy expenditure relative to work contributes to disease progression [1]. In fact, in the case of pathophysiological states, such as heart failure, MEE is reduced [1]. However, although MEE can be quantified by dual-sided heart catheterization and selective catheterization of the coronary sinus, its widespread clinical applications have been limited by the need of invasive measurements requiring complex calculations [1]. In this regard, the availability of a surrogate measure of MEE based on a non-invasive echocardiographic approach allows more extensive clinical applications [2][3][4][5][6][7][8]. Indeed, strong evidence exists supporting the role of non-invasively derived indexed MEE (MEEi) in predicting cardiovascular (CV) clinical outcomes, with altered values associated with CV risk factors/disease such as insulin resistance and diabetes, hypertension, obesity, heart failure with preserved ejection fraction (HFpEF) [2][3][4][5][6][7]. Therefore, the aim of the present study was to explore the full range of MEE and MEEi values (as calculated by standard transthoracic Doppler echocardiography (TTE)) in a large cohort of healthy subjects, evaluating clinical and echocardiographic correlates.

Study Population
The study population consisted of 1168 healthy subjects (volunteers or subjects undergoing work ability assessment (mean age 43.4 ± 14.0); 123 (45.7%) men) referred to the echocardiographic laboratories of the Cardiology Division, "Cava de' Tirreni-Amalfi Coast", Heart Department, University Hospital of Salerno, Italy, and the Department of Cardiology and Emergency Medicine of San Antonio Hospital, San Daniele del Friuli, Udine, Italy [9,10]. The participants underwent full screening for CV disease including a questionnaire on medical history, use of medications, CV risk factors and lifestyle habits (alcohol intake, smoking, physical activity). Physical examinations (height, weight, heart rate (HR) and blood pressure (BP)) and clinical assessments were conducted according to standardized protocols by trained and certified staff. Body surface area (BSA) was calculated according to the DuBois formula (0.20247 × height (m) 0.725 × weight (kg) 0.425) [9]. Three BP measurements were obtained from the right arm by a sphygmomanometer, and the results were averaged to determine systolic and diastolic BP. Pulse pressure (PP) was calculated as systolic BP (SBP)-diastolic BP (DBP). The study was approved by the institution's ethics board, and informed consent was obtained from all participants [9,10].

Echocardiography
A TTE with continuous ECG recording was performed with commercially available equipment on all subjects (Aloka α10-Aloka, Tokyo, Japan; Vivid 7-GE Healthcare, Milwaukee, WI, USA), according to the American Society of Echocardiography/European Association of Cardiovascular Imaging Guidelines, as previously described [11][12][13]. All studies were reviewed and analyzed off-line by two certified independent cardiologists, expert in TTE (F.F. and O.V.). Specific average measurements were taken of the 5 cardiac cycles. The left ventricular mass (LVM) (LVM indexed to body surface area (LVMI)) was estimated by left ventricular internal diameter (LVID), interventricular septum (IVS) and inferolateral wall thickness (PWT) at end-diastole from the parasternal approach, carefully obtained perpendicular to the left ventricular (LV) long axis: LVM = 0.8 × 1.04 × [(IVS + LVID + PWT) 3 − LVID 3 ] + 0.6 g. The left ventricular outflow tract (LVOT) diameter was measured in the zoom mode from the parasternal long axis view using standard electronic calipers in mid-systole, between the hinge points of the aortic valve leaflets from inner edge to inner edge. The LV stroke volume (SV) was calculated as the product of LVOT area and LVOT velocity time integral (VTI), obtained by a pulsed wave Doppler: SV = π × (LVOT/2) 2 × LVOT VTI [14].

Myocardial Mechano-Energetic Efficiency Measurements
MEE can be defined as the ratio between the external systolic work and the total amount of energy produced by cardiomyocytes, estimated by the rate pressure product, which is an indirect measure of MVO2 [2][3][4][5]. LV MEE was estimated as the ratio between SW and MVO2 [2][3][4][5], SW as the product SBP × SV (mmHg × mL), and MVO2 using the "double product" (DP) of SBP in mmHg × HR, as the time of cardiac cycle (CC) by the following formula: CC = HR/60 in seconds (HR/60). Thus, MEE (mL/s) was calculated as follow: SBP × SV/SBP × HR = SV/HR, where HR was expressed in seconds (HR/60) [15].
As MEE is highly related to LVM, MEE was divided by LVM to obtain an estimate of energetic expenditure per unit of myocardial mass (i.e., indexed MEE, MEEi, mL/s/g) [3].

Statistical Methods
Continuous variables are expressed as means and standard deviations (SD). The data were tested for normality through the Shapiro-Wilk test. The Wilcoxon rank-sum test or t-test was used, as required, for comparisons of continuous variables between groups. Categorical variables are expressed as percentages and were compared using the chi-square test or the Fisher's exact test. To compare more than two groups, the Kruskal-Wallis test was used. A two-tailed p-value less than 0.05 was considered significant. Holm's correction was used for multiple hypothesis correction, if necessary. Spearman's rank correlation was carried out for continuous variables to assess univariate associations. The variables were selected according to their clinical relevance. Multivariable linear regression analysis, including all variables that showed a significant correlation from the univariate analysis, was constructed to assess the independent associations of these variables with MEE and MEEi. Inter-observer agreement was tested with two independent observers remeasuring echocardiographic parameters in 20 randomly selected cases. Intra-observer variability was considered in 20 randomly selected cases by repeating the measurements on 2 occasions. The inter-and intra-observer variability were examined using both paired t tests and intraclass correlation coefficients (ICCs). An ICC >0.9 indicated excellent agreement. A statistical analysis was performed using R software (version 3.6.1, Vienna, Austria) [16].

Results
The demographic data of the study population are reported in Table 1. Compared with men, women had a lower weight, lower BSA and body mass index (BMI), lower BP and higher HR. The main echocardiographic parameters are reported in Table 2 [9].   The mean values of MEE and MEEi in the overall population were 61.09 ± 18.19 mL/s and 0.45 ± 0.14, respectively. The upper and lower limits (CI 95%) of normal MEE and MEEi were 62.13-60.05 and 0.46-0.44, respectively (Table 3).

Inter-and Intraobserver Variability
The overall inter-and intra-observer agreement rates were similar, showing nonsignificant variability. The interobserver variability analysis revealed an ICCs of 0.96 (95% Confidence Interval: 0.94 to 0.98). The intraobserver agreement rates were 0.97 (95% Confidence Interval: 0.96 to 0.99).

Discussion
MEE provides useful information about LV cardiac mechanics, representing the capability of the LV to transform the chemical energy obtained from oxidative metabolism into mechanical work [1]. It appears an attractive simple tool that may improve CV risk stratification [17].

Previous Studies
Several investigators reported the MEE and MEEi values of subjects with CV risk factors, including arterial hypertension, obesity, diabetes, hyperlipemia and former or current smoking habits (Table 7).    [3]. BMI, body mass index; CV, cardiovascular; CVRF, cardiovascular risk factors; EF, ejection fraction; IFG, impaired glucose tolerance; MEE, myocardial mechanical efficiency; MEEi, indexed myocardial mechanical efficiency; NGT, normal glucose tolerance. * Arterial hypertension was defined as office BP values at least 140 (SBP) and/or at least 90 mmHg (DBP) or when participants were taking antihypertensive medications. Obesity was defined as a BMI of at least 30 kg/m 2 . Diabetes was defined as fasting plasma glucose >125 mg/dL or current antidiabetic treatment. ** Hypertension was considered present if the 24 h ambulatory BP was elevated or if the participants reported the use of antihypertensive medications. Obesity was defined as BMI ≥30.0 kg/m2. Diabetes mellitus was considered present if fasting blood glucose ≥7 mmol/L, 2 h blood glucose ≥11.1 mmol/L after a 75 g oral glucose test, or glycated hemoglobin A1c ≥6.5%. *** Obesity was defined as body mass index ≥30 kg/m 2 . Hypertension was defined as history of hypertension or current antihypertensive treatment or elevated blood pressure at the baseline clinical visit. **** Cardio-metabolic risk factors included family history of diabetes, dysglycemia, hypertension, dyslipidemia, and overweight/obesity. Individuals were classified as having normal glucose tolerance (NGT) when fasting plasma glucose was < 100 mg/dL and 2 h post-load glucose was <140 mg/dL; isolated impaired fasting glucose (IFG) when fasting plasma glucose was 100-125 mg/dL and 2 h post-load glucose was <140; impaired glucose tolerance (IGT) when 2 h post-load glucose was 140-199 mg/dL in accordance with the ADA criteria. Individuals with NGT were further subdivided into two groups (NGT 1 h-low and NGT 1 h-high) using the 1 h plasma glucose cut-off of 155 mg/dL.  [2]. The mean MEE values were not significantly different in normotensive controls vs. hypertensive patients (86.1 ± 25.7 vs. 85.4 ± 22.6 mL/s, p value ≤ 0.78). As a note, hypertensive patients with low LV mechanical efficiency (MEE values below the 90th percentile of the normal distribution) had much higher HR, systolic BPs, and pulse pressure than those exhibiting normal LV mechanical efficiency (all p < 0.001). Low MEE was also associated with inappropriately high LV mass (p < 0.0001).
Furthermore, among 12,353 hypertensive patients (F/M = 5429/7008; mean age = 52.4 ± 12.5 years), De Simone et al. showed that reduced MEE was associated with altered metabolic profile, LVH, and concentric LV geometry and independently predicted hard CV events, reducing the statistical impact of LVH [3].
The fat-associated CV dysfunction (FATCOR) study explored the association of MEEi with LV systolic circumferential and longitudinal myocardial function in 480 subjects with increased body mass index (BMI), without known CV disease (mean age 47 ± 9 years, 61% women, 63% obese, 74% with hypertension). Patients with lower MEEi values were more frequently men with obesity, hypertension, dyslipidemia, and a higher insulin resistance index (all p for trend < 0.05) [4]. The lower MEEi quartile (< 0.41 mL/s per g) was associated with lower circumferential and longitudinal LV myocardial function assessed by midwall fractional shortening (MFS) and global longitudinal strain (GLS), independent of cardiometabolic factors [4].
Interestingly, Losi et al., among 1912 unselected participants of a population-based cohort of American Indians with normal baseline EF, demonstrated that the lowest MEEi quartile (i.e., ≤0.34 mL × s −1 × g −1 ) predicted incident heart failure, after adjustment for LVH, prolonged relaxation and associated CV risk factors, including hypertension, obesity, diabetes, and smoking habits [5].

Uniqueness of the Present Study
To the best of our knowledge, this is the largest study that (a) comprehensively assessed the full range of MEE and MEEi values in a large cohort of 1168 healthy individuals stratified by age and gender; (b) demonstrated that the MEE values were higher in males than in females, whereas the MEEi values were significantly higher in females; (c) showed that the MEEi values were reduced in older age, while MEE was not significantly correlated with age; (d) revealed gender, BSA, DBP, LAVI, E/e' and TAPSE as independent variables associated with MEE, and age, gender, BSA and TAPSE as independent variables associated with MEEi.
In the present study, the normal MEE values measured by the Doppler method were significantly lower than the values reported by De Simone et al. (61.0 ± 18.1 vs. 86.1± 25.7, respectively) in an older population (mean age = 45.4 ± 15.6 vs. 35.3 ± 11.9 years, respectively). In the latter study, SV was calculated as the difference between 2 D volumes (SV = LV end-diastolic volume-LV end-systolic volume), using the biplane method of disc summation (modified Simpson's rule). This difference may also be consistent with the hypothesis that SV and CO are significantly lower if calculated by 2D rather than by Doppler and 3D methods (difference of 26 ± 0.4% of the measured 2D value) [19]. Thus, different methods (2D, Doppler and 3D) for calculating SV and thus the MEE and MEEi values should not be used interchangeably.
There are well-known gender differences in LV structure and function, including larger volumes, greater LVM, and higher CO and SV in men compared with women [9,19,20]. Similarly, the MEE values are significantly higher in males than in females. Thus, there is a need to adapt the reference value to the gender. In the present study, the lower limits of normal MEE and MEEi values were 65.5 mL/s and 0.41 mL/s/g in males and 53.6 mL/s and 0.45 mL/s/g in females (p value). These differences may be attributable to gender-related differences in biometrics characteristics. In this regard, a significant and independent association between the MEE and MEEi values and BSA was found.
Furthermore, our findings suggest a physiological impact of the aging process on MEE and MEEi (more evident), highlighting the need to adapt the normal reference values to age. This is consistent with previous results demonstrating that, in both genders, CO and SV tended to decrease with age [21]. Of note, the reduction in MEEi was related to an increase in LVM associated with aging. In contrast, as previously described, other LV function parameters such as LVEF and GLS in healthy subjects did not decrease significantly with older age, [9,22,23].
On the other hand, the significant and independent association of MEE and MEEi values with TAPSE may be indicative of the ventricular interdependence concept (the function of one ventricle is altered by changes in the filling of the other ventricle) [9].

Limitations of the Present Study
First, the study was limited to Caucasian healthy subjects. Thus, in the present study, the clinical relevance of MEE and MEEi for different races and pathologic states was not investigated. Secondly, additional echocardiographic techniques such as strain imaging and 3D echocardiography (3DE) were not performed.

Conclusions and Future Directions
We reported the physiologic ranges of MEE and MEEi measured by TTE in a large population of healthy subjects. MEE was significantly correlated with gender (higher values in males than in females) and BSA. Instead, MEEi was significantly decreased in older age and correlated significantly with gender (higher values in females than in males) and BSA. The knowledge of age-, BSA-and gender-based normal MEE and MEEi values may serve as reference to identify phenotypes at high risk of CV events. Institutional Review Board Statement: The study protocol followed the principles of the Declaration of Helsinki, and each participating center obtained approval from its local research ethics committee. Clinical trials Gov Identifier: NCT03041337.
Informed Consent Statement: Informed consent from each subject was obtained.

Data Availability Statement:
The data that support the findings of this review are openly available in the References section.