Echocardiographic assessment of myocardial efficiency predicts exercise performance

Abstract

Cardiac function is a major determinant of cardiopulmonary fitness. This study aimed to determine if novel echocardiographic myocardial function and efficiency parameters at rest can predict exercise performance during different types of prolonged high‐intensity endurance exercise. Echocardiography was performed before exercise in 40 healthy (75% males) 50.3 ± 9.1‐year‐old recreational athletes. Echocardiographic parameters at rest were compared with exercise performance assessed by power output during two different exercises: A lactate threshold and cardiopulmonary exercise test (La‐CPET) and a 91‐km mountain bike sport cycling race. The La‐CPET had a median duration of 43 (40, 45) minutes and a mean power output of 2.9 ± 0.5 W/kg. The race had a median duration of 236 (214, 268) minutes and a mean power output of 2.1 ± 0.5 W/kg. There was moderate left ventricular (LV) dilatation in individuals with the highest performance. The myocardial efficiency parameter, global wasted work (GWW), was positively correlated with race duration (rho = 0.42, p = 0.008) and negatively correlated with mean power output during both the La‐CPET (rho = −0.43, p = 0.007) and the race (rho = −0.44, p = 0.005). In multivariable models, including LV volumes, left GWW remained an independent predictor of race duration (beta = 0.40, p = 0.007) and of mean power output during the La‐CPET (beta = −0.40, p = 0.006) and the race (beta = −0.43, p = 0.003). The novel echocardiographic myocardial efficiency parameter, GWW, measured at rest, is an independent predictor of prolonged high‐intensity endurance exercise performance in healthy middle‐aged athletes. These findings suggest that resting myocardial efficiency parameters may aid the identification of exercise‐induced LV dilatation.

Abbreviations

• AT
• Anaerobic threshold
• BMI
• Body mass index
• CO
• Cardiac output
• cTnI
• Cardiac Troponin I
• CVD
• Cardiovascular disease
• DBP
• Diastolic blood pressure
• E/A
• Ratio of early to late diastolic transmitral blood flow velocities
• E'
• Early diastolic myocardial tissue velocity
• EDV
• End diastolic volume
• EDVi
• End diastolic volume index
• EF
• Ejection fraction
• ESV
• End systolic volume
• ESVi
• End systolic volume index
• GCW
• Global constructive work
• GLS
• Global longitudinal strain
• GWE
• Global work efficiency
• GWI
• Global work index
• GWW
• Global wasted work
• HR
• Heart rate
• LA
• Left atrium
• La-CPET
• Lactate threshold and cardiopulmonary exercise test
• LAVi
• Left atrial volume index
• LV
• Left ventricle
• ME
• Myocardial efficiency
• MEE
• Myocardial external efficiency
• MET
• Metabolic equivalent of task
• MRI
• Magnetic resonance imaging
• PET
• Positron emission tomography
• RV
• Right ventricle
• SBP
• Systolic blood pressure
• SV
• Stroke volume
• SVi
• Stroke volume index
• Vo2 Max
• Maximal oxygen consumption

INTRODUCTION

Regular endurance exercise leads to structural and functional cardiovascular changes and increases cardiopulmonary fitness. A balanced increase in all four heart chambers, promoted by the volume loads of exercise and accompanied by increased contractile function, is the hallmark of the endurance athlete's heart (Kemi et al., , ; Paterick et al., ; Prior et al., ; Pujadas et al., ; Tokodi et al., ; Wisløff et al., ). However, at rest, conventional indices of left ventricular (LV) function may show low-normal values in athletes because of the altered geometry (Khan et al., ), and in this setting, exercise-induced cardiac remodeling can be challenging to distinguish from adverse LV remodeling. It is therefore of interest to identify additional markers of cardiac performance and physical fitness that may aid the identification of exercise-iduced LV remodeling at rest.

Myocardial efficiency (ME) is the relationship of mechanical work generated by the left (or right) ventricle to the consumed chemical energy from aerobic metabolism (Sörensen et al., ) and is the most comprehensive way to describe cardiac function. Invasive created pressure-volume loops reflect myocardial work and oxygen consumption (Suga et al., ; Takaoka et al., ); however, the invasiveness of ME-assessment have previously limited its use. Myocardial work (MW) is an emerging tool in echocardiography that incorporates left ventricular afterload into global longitudinal strain analysis, making pressure-strain loops that significantly correlate to invasive obtained myocardial work and myocardial metabolism, providing noninvasive indexes of myocardial efficiency (Clemmensen et al., ; Lustosa et al., ; Russell et al., ). Recently, MW indices have proven to be superior to traditional indexes of LV systolic function in younger athletes related to exercise capacity (Tokodi et al., ; D’Andrea et al., ) and useful as early predictors of adverse cardiac remodeling in the general population (Sahiti et al., ). Therefore, the noninvasive MW parameters serve as promising markers of cardiac performance in athletes (Marzlin et al., ) and of potential utility in the differentiation between exercise-induced and pathologic cardiac remodeling. MW assessments in athletes are novel, and there is currently limited data available, especially in the setting of slightly older exercisers, which may have pronounced cardiac remodeling. In this context, the present work aimed to determine if the novel echocardiographic myocardial function and efficiency parameters, assessed at rest, can predict physical fitness determined by power output and competitive performance in middle-aged recreational athletes.

METHODS

Study individuals were recruited among healthy, prior participants in the North Sea Race Endurance Exercise Studies (NEEDED) performed in 2013 and 2014 (Kleiven et al., ; Skadberg et al., ). This study compared echocardiographic parameters before exercise with exercise performance during two prolonged high-intensity endurance exercise types: a lactate threshold and cardiopulmonary exercise test (La-CPET) and a 91-km mountain bike race (the North Sea Race 2018) (Figure ). The La-CPET consisted of a lactate threshold test and a maximum oxygen uptake test (Supporting Information ). The primary performance assessment was to measure the power output during both exercises. To be included in the study, the participants needed bikes that could be fitted with power meters to measure power output. All participants were without any history, signs, or symptoms of cardiac disease or significant coronary artery disease (>50% stenosis) verified by coronary computer tomography angiography after the final exercise (the North Sea Race 2018). The echocardiography assessment was done 24 h before the La-CPET, and the La-CPET was done 2–3 weeks before the North Sea Race in 2018. Cardiac troponin I (cTnI) was used as a biomarker of myocardial stress (Aengevaeren et al., ). Blood samples were acquired before (baseline) and at 3 and 24 h (to assess recovery) following both exercises. Informed consent was obtained from all study subjects, and the study complied with the Declaration of Helsinki and was approved by the Regional Ethics Committee (REK nr 2018/63).

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Power output assessment

The same bikes were used for both exercises to ensure that the assessment of power output was comparable between the exercises. Therefore, at the La-CPET, the participants' racing bikes were fitted to a Cyclus2 ergometer (RBM Elektronik-Automation, Leipzig, Germany). Power output was guided via the Cyclus2 ergometer and Vuntys CPX software. Due to the large individual range of physical performance in the study cohort, the starting watt-value of the La-CPET was adapted to each individual's estimated performance level. Estimating individual physical performance levels was based on all available information, including age, sex, body weight, training, and performance levels during races and at the warm-up. Before the race, power meters (Stages Power Meters, Boulder, CO, US) were fitted to the participants' bikes and connected to the Garmin Forerunner 935 Sport Watch (Garmin, Kansas, US). All power meters were calibrated with an automatic calibration before starting. Based on previous laboratory studies, stages power meters have been shown to have sufficient quality for power output measurements during cycling across a wide range of intensities (100–1250 W) (Granier et al., ).

Echocardiographic image acquisition

GE Vivid E 95 ultrasound systems and 4V probes (GE Vingmed Ultrasound, Horten, Norway) were used for echocardiographic assessment before the La-CPET. Subjects were scanned in the left lateral decubitus position under relaxed conditions. All recordings and measurements were made according to the European Association of Cardiovascular Imaging (Badano et al., ; Lang et al., ). Comprehensive imaging protocols were applied with full coverage of both atria and ventricles, including parasternal and apical views of three cardiac cycles triggered to the QRS complex. 2D apical and 3D apical tri-plan of the four-, three-, and two-chamber views and a focused 2D apical four-chamber view of the right ventricle were obtained with care to avoid foreshortening. Appropriate frame rates (60–70 frames/second) allowed for high-quality post-processing later, including speckle-tracking strain analysis of the left and right ventricles and the left atrium. 3D volume acquisitions of the left and right ventricles were obtained from the apical four-chamber view over six cardiac cycles using multibeat full-volume acquisition during breath hold and encompassing the entire LV and RV cavity in the data sets.

Analysis of myocardial function and morphology

All echocardiographic analyses were performed offline on EchoPAC V202 (General Electric Vingmed Ultrasound AS) by a researcher blinded to clinical data and exercise information. The Devereux formula was used to determine LV mass, and LV volumes and EF were assessed using three-dimensional imaging. LV global longitudinal strain was calculated as an average strain value based on apical two-, three-, and four-chamber views at a 60–70 frames/second frame rate using automated function imaging (AFI), and the region of interest was manually corrected to ensure optimal tracking of the endocardial contour and to cover the entire thickness of the LV myocardium. Time-to-peak strain was defined as the time from the onset Q/R wave on ECG to peak negative longitudinal strain during the cardiac cycle. The right ventricular longitudinal strain was measured as the mean of the RV lateral basal, mid, and apical segments. Right ventricular fractional area change (RV FAC) was obtained from the apical four-chamber view and was calculated as the difference in the end-diastolic area and the end-systolic area divided by the end-diastolic area. Left atrial (LA) volume was measured using the biplane method and indexed to BSA. LA total emptying volume was defined as LAV max- LAV min. LA total emptying fraction (LA EF) was defined as 100 × (LAV max–LAV min)/LAV max). The left atrial strain was measured in the apical 4-chamber view and assessed as global longitudinal strain using ventricular end-diastole as the baseline. Peak 1 represents the peak strain during the reservoir phase (LA reservoir function), and peak 2 represents the peak strain during the atrial contraction phase (LA booster pump function).

Myocardial work (MW) analysis

MW calculation was performed offline using EchoPAC version 202 (General Electric Vingmed Ultrasound AS). A commercially available algorithm was used to calculate four MW parameters (see below). Peak arterial systolic pressure was measured with a brachial cuff with the study subjects sitting prior to the echocardiographic examination and was assumed to be equal to peak systolic LV pressure and uniform throughout the ventricle. A patient-specific LV pressure curve was then constructed, adjusting LV pressure curve to the duration of the isovolumic and ejection phases, defined by valvular timing events. The opening and closure of the aortic and mitral valves were determined manually in the 3- and 4-chamber apical views based on Doppler signals and visualization of valve opening and closure.

Strain and pressure data were synchronized using the R-wave on the ECG as a common time reference. Pressure strain was generated in each myocardial segment by the EchoPAC software, and global values were calculated as mean values of all segments. The global work index (GWI) parameter is the total work performed by the left ventricle using the area of the pressure-strain loop between the mitral valve closure and opening. Global constructive work (GCW) is the sum of positive work due to myocardial shortening in systole and negative work due to myocardial lengthening during isovolumetric relaxation. Global wasted work (GWW) is the sum of myocardial lengthening in systole and shortening in isovolumetric relaxation reflecting the work that does not contribute to LV ejection. Global work efficiency (GWE) is constructive work divided by the sum of constructive work and wasted work.

Statistical analyses

Normally distributed continuous variables are reported as mean ± SD, while continuous variables with markedly skewed distributions are reported as median (25th, 75th percentile). The Shapiro–Wilk test was used to test for normality. For continuous variables, The Mann–Whitney U test or a Student's t-test was used to compare groups, as appropriate, depending on the normality of the data. A two-tailed p-value <0.05 was considered significant. Spearman correlation was used to assess bivariate relations, and backward elimination analysis was used for multiple linear regression analysis. SPSS version 26.0.0.1 was used for statistical analyses.

RESULTS

Baseline findings

A total of 40 healthy, well-trained recreational athletes were included in the study (Table ). The mean age was 50.3 ± 9.1 years, and 75% were males. They reported 10.0 (6, 18) years of endurance training and a median of 12 (5.0, 20.0) endurance competitions during the past 5 years before enrollment in the present study. Mean blood pressure was 135 (±15)/83 (±10) mm Hg. BMI was 25 (23, 27) kg/m².

TABLE 1 Baseline characteristics and echocardiographic parameters (n = 40).

Characteristics
Age, years	50.3 ± 9.1
Male sex, n (%)	30 (75%)
Body mass index, kg/m2	25 (23, 27)
SBP, mmHg	135 ± 15
DBP, mmHg	83 ± 10
Heart rate (beats per minute)	57 ± 9
Former smoker, n (%)	20 (50%)
Family history of CVD, n (%)	2 (5%)
Training and competitive experience
Years of endurance training	10 (6,18)
Number of competitions in last 5 years	12.0 (5.0, 20.0)
MET hours/week	60.7 (46.8, 100.1)
Echocardiographic parameters
Left ventricle (LV)
Mass Index, 2D (g/m2)	85.0 (77.0,96.8)
End diastolic volume index, mL/m2	83.0 (69.4, 100.4)
End systolic volume index, mL/m2	36.7 ± 8.8
Ejection fraction, %	57.9 ± 3.6
Global longitudinal strain, %	−19.9 ± 2.8
Stroke volume index, mL/m2	44.8 ± 9.2
Global work index, mmHg %	2200 ± 400
Global constructive work, mmHg %	2434 ± 403
Global wasted work, mmHg %	63 (40, 133)
Global work efficiency, %	97 (94, 98)
Right ventricle (RV)
Basal diameter, mm	39.6 ± 3.6
Fractional area change, %	45 ± 5.6
3-Segment global longitudinal strain, %	−26.9 ± 3.4
Left atrium (LA)
Systolic volume, mL/m2	32.3 ± 8.9
Strain peak 1, 4C, %	30.2 (25.1, 40.5)
Strain peak 2, 4C, %	12.2 ± 4.7
Emptying volume, mL/m2	18.8 ± 5.5
Ejection fraction, %	58.4 ± 9.2

Echocardiographic parameters (Table ) showed a median ventricular mass of 85.0 (77.0, 96.8) g/m² and left atrium volume index (LAVi) of 32.3 ± 8.9 mL/m² with values in the upper range of the reference values of normal individuals (Lang et al., ). The left ventricle was mildly dilated (83.0 (69.4, 100.4) mL/m²). Conventional indices of left and right ventricular function and left atrial function were within the normal range. The myocardial work parameters, GWI, GCW, and GWE, were somewhat higher than the average/median value for healthy individuals and GWW was slightly lower (Manganaro et al., ; Olsen et al., ).

Physical performance parameters during La-CPET and the race

The mean maximal oxygen consumption (VO₂ max) was 42.2 ± 9.0 mL/kg/min (3.4 ± 0.8 L/min) with a mean power output of 2.6 ± 0.5 W/kg (206 ± 48 W) at the anaerobic threshold level. A representative example of power output measurements during the race is presented in Figure , and exercise performance parameters for the La-CPET and the race are outlined in Table . The duration of exercise, peak heart rate, and peak BP was significantly higher for the race compared with La-CPET. The peak power output achieved was higher for the race than the La-CPET, but the mean power was higher for the La-CPET. The race's total work (product of mean watt and exercise duration) and exercise duration were significantly higher than the La-CPET.

Predictors of race performance

Mean power output during both exercises was strongly correlated with race performance (race duration): La-CPET (rho: −0.843, p < 0.001) and at the race (rho: −0.944, p < 0.001). A moderate correlation was seen between VO₂ max and race performance (rho: −0.695, p < 0.001) compared with mean power output in both La-CPET and during the race. In multivariable models, mean power output during both the La-CPET and the race remained the strongest predictors of race performance in all models (Table ).

Bivariate correlations between echocardiographic parameters and exercise performance

Echocardiographic parameters with significant correlations with exercise performance parameters are presented in Table  and Figure . LV mass was significantly correlated with shorter race duration (p = 0.028). During the race, LV mass was also associated with peak (p = 0.014) and mean power (p = 0.027). Left atrial volume (LAVi) and LV end-diastolic volume (LV EDVi) were significantly correlated to all exercise performance parameters except power threshold and total work, where higher volumes were associated with better exercise capacity. Stroke volume (SV) of the left atrium and LV were correlated to all performance parameters except total work, where higher baseline SV was correlated to better exercise capacity.

TABLE 2 Bivariate correlations between baseline echocardiographic parameters and physical performance (Spearmans Rho).

			La-CPET	Race
VO2 max	Race duration (hours)	Power threshold (watt/kg)	Mean power (watt/kg)	Peak power (watt/kg)	Total work (watt/kg)	Mean power (watt/kg)	Peak power (watt/kg)	Total work (watt/kg)
LV mass	0.306	−0.347*	0.265	0.299	0.307	0.208	0.350*	0.384*	0.267
(p 0.028)	(p 0.027)	(p 0.014)
LAVi	0.543**	−0.461**	0.303	0.353*	0.376*	0.082	0.455**	0.270	0.294
(p < 0.001)	(p 0.003)	(p 0.026)	(p 0.017)	(p 0.003)
LV EDVi	0.460**	−0.505**	0.299	0.425**	0.415**	0.172	0.517**	0.450**	0.357*
(p 0.003)	(p 0.001)	(p 0.006)	(p 0.008)	(p 0.001)	(p 0.004)	(p 0.024)
LV ESVi	0.358*	−0.364*	0.218	0.302	0.305	0.221	0.389*	0.344*	0.308
(p 0.023)	(p 0.021)	(p 0.013)	(p 0.030)
GWW	−0.286	0.422*	−0.409*	−0.426**	−0.371*	−0.408**	−0.443**	−0.364*	−0.414**
(p 0.008)	(p 0.010)	(p 0.007)	(p 0.02)	(p 0.010)	(p 0.005)	(p 0.023)	(p 0.009)
GWE	0.279	−0.419*	0.367*	0.389*	0.340*	0.418**	0.422**	0.321*	0.355*
(p 0.008)	(p 0.021)	(p 0.014)	(p 0.034)	(p 0.008)	(p 0.007)	(p 0.047)	(p 0.026)
LA SV	0.448**	−0.426**	0.334*	0.354*	0.381*	0.069	0.408**	0.224	0.239
(p 0.004)	(p 0.006)	(p 0.035)	(p 0.025)	(p 0.015)	(p 0.009)
LV SV	0.478**	−0.583**	0.340*	0.478**	0.465**	0.194	0.593**	0.541**	0.409**
(p 0.002)	(p < 0.001)	(p 0.032)	(p 0.002)	(p 0.002)	(p < 0.001)	(p < 0.001)	(p 0.009)

There were no significant correlations between LA function and exercise performance. There were neither significant correlations between LV GLS and exercise performance nor between LV EF and the performance parameters. The myocardial work parameters global wasted work (GWW) and global work efficiency (GWE) were significantly correlated with all performance parameters except from VO₂ max (Table ), where higher GWW was associated with worse exercise performance and higher GWE was associated with better exercise performance. An example of this is illustrated in Figure .

Multiple linear regression models

Multiple linear regression analyses using backward elimination demonstrated that LV end-diastolic volume (LV EDVi) and global wasted work (GWW) were significant independent predictors of race performance and power output in all models (Table ). In multivariable models, including LV EDVi, left GWW remained an independent predictor of race duration (beta = 0.40, p = 0.007) and of mean power output during the La-CPET (beta = −0.40, p = 0.006) and the race (beta = −0.43, p = 0.003).

TABLE 3 Multiple linear regression (backward elimination).

Dependent variable	Independent predictors	R2 model	Standardized coefficient	p-value
La-CPET:
Vo2 max, mL/kg/min	LAVi	0.339	0.387	0.019
LV EDVi		0.275	0.09
Power threshold (watt/kg)	GWW	0.180	−0.424	0.007
Mean power (watt/kg)	LV EDVi	0.350	0.391	0.007
GWW		−0.397	0.006
Peak power (watt/kg)	LV EDVi	0.304	0.385	0.009
GWW		−0.349	0.017
Total work (minutes × watt/kg)	GWW	0.188	−0.433	0.006
Race:
Mean power (watt/kg)	LV EDVi	0.542	0.387	0.012
GWW		−0.428	0.003
LAVi		0.310	0.041
Peak power (watt/kg)	LV EDVi	0.123	0.351	0.028
Total work (minutes × watt/kg)	LV EDVi	0.394	0.398	0.003
GWW		−0.469	0.001
Race duration, hours	LV EDVi	0.305	−0.364	0.013
GWW		0.402	0.007

Baseline echocardiographic parameters as predictors of cardiac troponin response following exercise

There was a significant increase in cardiac Troponin I (cTnI) from baseline to 3 h after both the La-CPET 3.2 (1.9–6.7) ng/L–11.6 (6.4–22.5) ng/L (p < 0.001) and the race 3.7 (1.0–5.4) ng/L–77 (35.3–112.8) ng/L (p < 0.001). There was a weak association between left ventricular end-diastolic volume (LVEDVi) at baseline and cTnI 3 h following the race (rho = 0.339, p = 0.032) and LV mass at baseline and cTnI 3 h following the race (rho = 0.317, p = 0.046). Global wasted work and global work efficiency did not correlate to cTnI at baseline or 3 and 24 h following the exercises. No correlation existed between GWW or GWE and LVEDVi or between GWW or GWE and LV mass.

Reproducibility

Intra and interobserver variability analysis confirmed high reproducibility of the parameters LV GLS, GWI, GCW, GWW, and GWE (Table ).

DISCUSSION

This is the first study to suggest that echocardiographic-derived myocardial efficiency parameters at rest can predict physical fitness in recreational athletes with indices of exercise-induced LV remodeling. In the present study, the myocardial work parameter GWW was the strongest and most consistent predictor of both power output and race duration providing a measure of myocardial efficiency and a predictor of physical fitness at baseline examination. These findings suggest that GWW may be a potential tool to distinguish physiological from pathological LV remodeling.

Regular endurance exercise induces structural and functional cardiovascular adaptions that are considered physiological and ultimately enhance cardiorespiratory fitness (La Gerche et al., ). A balanced dilatation of the cardiac chambers, the hallmark of endurance athletic cardiac remodeling, enables the heart to fill and eject larger volumes during exercise and improves cardiopulmonary performance (Palermi et al., ). Because of the altered geometry, conventional indices of LV function may show low-normal values at rest (Fábián et al., ), and in this setting, exercise-induced LV dilatation can mimic pathological LV dilatation. It is, therefore, of interest to identify additional markers of physical fitness that may aid the identification of exercise-induced LV remodeling at rest.

Myocardial efficiency (MEE) is the relationship of mechanical work generated by the left (or right) ventricle to the consumed chemical energy from aerobic metabolism (Sörensen et al., ). Myocardial efficiency can be calculated invasively by LV pressure–volume loop analysis or noninvasively by using positron emission tomography (PET) and magnetic resonance imaging (MRI) (Seemann et al., ), and it is the most comprehensive way to describe cardiac function giving insights into cardiac mechanics and energetics. However, these procedures are resource- and time-consuming and involve radiation preventing them from integration into clinical practice. Russell et al. introduced a novel noninvasive method for estimating regional LV myocardial work by echocardiography (Russell et al., ). Noninvasive regional LV pressure–strain loop area has shown good agreement and correlation with invasive pressure–volume loops and regional myocardial glucose metabolism by PET and may serve as an index of both myocardial work and metabolism providing noninvasive measures of myocardial efficiency. The myocardial work parameters global work index (GWI) and global work efficiency (GWE) have recently been proven useful in an athlete's heart as they more accurately predict cardiac performance than other markers of LV systolic function (Tokodi et al., ; D’Andrea et al., ). Global wasted work (GWW), one of the MW parameters, reflects the elongation of myocytes during systole and shortening against a closed aortic valve, representing negative work that does not contribute to LV ejection. Global wasted work is a method to quantify how much work is wasted, the energy loss, and provides a measure of the overall efficacy of the LV myocardium in converting metabolic substrates into cardiac work (Russell et al., ). While several studies show reduced LV efficiency quantified by increased values of GWW in adverse cardiac remodeling (Sahiti et al., ) and electrical conduction abnormalities (Russell et al., ), no studies have previously looked at GWW in evaluating exercise-induced cardiac remodeling.

In the present analysis, the study subjects had echocardiographic findings at baseline consistent with exercise-induced cardiac remodeling; Median left ventricular volumes were mildly dilated, and left ventricular mass and atrial volume were in the upper level of the reference interval. Biventricular systolic function and myocardial work parameters were within normal ranges compared with healthy controls (Addetia et al., ; Lang et al., ; Olsen et al., ). Despite the study subjects being recreational athletes, and not elite athletes, some of them had severe exercise-induced cardiac alterations with LV volumes far above the upper reference value, which makes it difficult to separate from pathologic LV dilatation. Adverse LV eccentric remodeling is known to significantly increase GWW in the general population (Sahiti et al., ) and increased GWW is also seen in dilated cardiomyopathy (DCM) (Chan et al., ) and in DCM gene carriers (Triantafyllou et al., ). In the present study, the study subjects with exercise-induced dilated left ventricles had GWW in the lower part of the reference range (Olsen et al., ). In addition to this, GWW was an independent predictor of both power output and exercise performance (assessed as race duration). In comparison, none of the traditional LV systolic function parameters (GLS and EF) were predictors of physical fitness at rest. This makes GWW a promising marker of cardiac performance and physical fitness at baseline examination and a useful tool in the distinction between exercise-induced- and adverse cardiac remodeling. Improved efficiency of work of the endurance-trained heart could be a potentially beneficial mechanism of exercise training.

Some studies exist regarding myocardial efficiency (derived from PET or invasive assessments) and endurance-trained hearts. In rat models assessed invasively using left ventricular pressure-volume analyses, exercise-induced cardiac hypertrophy was associated with improved contractility and mechanoenergetics suggesting optimization of metabolic efficiency in the trained heart (Radovits et al., ). Wasfy et al. () assessed myocardial efficiency in endurance male athletes by PET and found preserved myocardial efficiency suggesting myocardial efficiency as a useful parameter in distinguishing between physiological and pathological remodeling. On the other hand, pathological cardiac hypertrophy was associated with decreased myocardial efficiency (Clemmensen et al., ; Timmer et al., ). Improvement in myocardial efficiency has been shown after aortic valve replacement and correlated with improved exercise capacity and maximal oxygen consumption (Güçlü et al., ), and improvement of myocardial efficiency is also seen in heart failure patients after participation in structured training programs (Stolen et al., ). Significant GWW reduction and GWE improvement have also been found following 4 weeks of isometric exercise training in hypertensive patients (O’Driscoll et al., ).

We suggest the addition of myocardial work parameters, and especially GWW, to the baseline echocardiographic examination of athletes with exercise-induced cardiac remodeling. MW parameters can be performed bedside, rapidly, and is without radiation. GWW provide an index of myocardial efficiency and has in this work proven to be a good predictor of physical fitness in exercise-induced cardiac remodeling. We suggest that increased myocardial efficiency could be one of the benefits of regular endurance exercise training. The potential mechanisms responsible for the exercise-induced increase in myocardial efficiency include a shift in substrate preference to a more energy-efficient substrate (Stolen et al., ), lower myocardial blood flow, and higher myocardial oxygen extraction, which is facilitated by longer blood mean transit time and higher coronary resistance vessel tone (Heinonen et al., ; Wasfy et al., ). Exercise-induced wall stress reduction and afterload reduction may also explain the increased myocardial efficiency (O’Driscoll et al., ). However, the exact mechanisms for increased myocardial efficiency are unknown, and further work regarding the mechanisms underpinning the relationship between myocardial efficiency and exercise performance is required.

Limitations

The major limitation of this study is the small and selected study population consisting of 40 subjects; the results from this study are, therefore, only hypothesis-generating. The present study addresses morphological and functional changes in a population different from younger athletes, and the current findings may not apply to a younger population with a higher functional capacity. Calculating the myocardial work parameters relies on global longitudinal strain measurements, systolic blood pressure, and valvular events. Small changes in the timing of valvular events could potentially lead to significant differences in work parameters, especially the global wasted work and global work efficiency parameters. When assessing and evaluating these parameters, awareness of these challenges is essential. In the present work, global wasted work and global work efficiency were used as surrogate markers of myocardial efficiency. These parameters are not equivalent to myocardial efficiency since they do not involve any measurements of oxidative metabolism. Blood pressure was measured in the sitting position, while the echocardiographic examinations were done in the supine position.

Blood pressure should have been performed in the same position as the echocardiographic examination to ensure that the blood pressure accurately correlates with the stress or afterload of the LV at the time of the images being acquired. The exercises were performed with the subjects in an upright posture, while echocardiography was performed with the subjects in a supine position. The differences in postures influence the echocardiographic measurements, and the present findings are therefore limited to assessment in a supine position.

Conclusion

The novel echocardiographic myocardial work parameter GWW, assessed at rest, was the most consistent predictor of physical fitness in recreational athletes with exercise-induced cardiac remodeling. Improved myocardial efficiency may reflect a beneficial response to exercise training. GWW may be a predictor of exercise-induced cardiac remodeling and physical fitness at baseline examination.

ACKNOWLEDGMENTS

The authors thank GE Healthcare for providing state-of-the-art echocardiography machines and post-processing software for the study, Abbott Norway for supplying ECG equipment, and Garmin Norway for providing sports watches and Stages for power meters. This work was supported by Conoco Phillips, the Simon Fougner Hartmanns Family Trust, the Western Norway Regional Health Authority, and GE Healthcare.

CONFLICT OF INTEREST STATEMENT

The authors have no disclosures to report.

References

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