Abstract
ry efficiency (VE/VCO 2) is a strong predictor of cardiovascular diseases and defines individuals’ responses to exercise. Its characteristics among endurance athletes (EA) remain understudied. In a cohort of EA, we aimed to (1) investigate the relationship between different methods of calculation of VE/VCO 2and (2) externally validate prediction equations for VE/VCO 2.Methods:In total, 140 EA (55% males; age = 22.7±4.6 yrs; BMI = 22.6±1.7 kg·m −2 ; peak oxygen uptake = 3.86±0.82 L·min −1 ) underwent an effort-limited cycling cardiopulmonary exercise test. VE/VCO 2was first calculated to ventilatory threshold (VE/VCO 2-slope), as the lowest 30-s average (VE/VCO 2-Nadir) and from whole exercises (VE/VCO 2-Total). Twelve prediction equations for
equations for VE/VCO 2.Methods:In total, 140 EA (55% males; age = 22.7±4.6 yrs; BMI = 22.6±1.7 kg·m −2 ; peak oxygen uptake = 3.86±0.82 L·min −1 ) underwent an effort-limited cycling cardiopulmonary exercise test. VE/VCO 2was first calculated to ventilatory threshold (VE/VCO 2-slope), as the lowest 30-s average (VE/VCO 2-Nadir) and from whole exercises (VE/VCO 2-Total). Twelve prediction equations for VE/VCO 2-slope were externally validated.Results:VE/VCO 2-slope was higher in females than males (27.7±2.6 vs. 26.1±2.0,p< 0.001). Measuring methods for VE/VCO 2differed significantly in males and females. VE/VCO 2increased in EA with age independently from its type or sex (β= 0.066–0.127). Eleven equations underestimated VE/VCO 2-slope (from−0.5 to−3.6). One equation overestimated VE/VCO 2-slope (+0.2). Predicted and observed measurements differed significantly in nine models. Models explained a low amount of variance in the VE/VCO 2-slope (R 2 = 0.003–0.031).Conclusions:VE/VCO 2-slope, VE/VCO 2-Nadir, and VE/VCO 2-Total were significantly different in EA. Prediction equations for the VE/VCO 2-slope were inaccurate in EA. Physicians should be acknowledged to properly assess cardiorespiratory fitness in EA. Keywords:prediction equation; cardiac physiology; cardiopulmonary exercise testing; VE/VCO 2-slope; cardiorespiratory fitness; exercise ventilation 1. Introduction Ventilatory efficiency (VE/VCO2) describes the relationship between pulmonary ven- tilation (VE) and carbon dioxide production (VCO2) [1]. To keep acid–base balance during exercise, VE grows simultaneously with VCO2[2]. VE/VCO2merges circulatory and respi- ratory functions [1,3]. Several ways of measuring VE/VCO2have been proposed including VE/VCO2from start to first ventilatory threshold (VT1) (VE/VCO2-slope), as the minimal continuous value (VE/VCO2-Nadir) and across whole physical effort (VE/VCO2-Total) [4]. Professional and elite athletes are exposed to high training loads [5]. Thus, more endurance athletes can be referred to with suspected cardiovascular diseases (CVD) [5]. Knowledge of cardiorespiratory indicators among endurance athletes remains important [6]. Precise risk stratification is of critical importance to ensure safe physical activity because it enables medical professionals to adjust clinical management and exercise intensity [5]. An J. Clin. Med.2024,13, 490.
J. Clin. Med.2024,13, 490 2 of 13 important prognostic role in CVD has been assigned to the VE/VCO2-slope [7]. VE/VCO2- slope is elevated in pulmonary hypertension, chronic obstructive pulmonary disease, interstitial lung disease, and other diseases. The pivotal role of the VE/VCO2-slope was noted in heart failure (HF) [2,8]. Despite the VE/VCO2-slope being a submaximal parameter, its prognostic power is comparable to peak oxygen uptake (VO2peak) [1,9]. Studies on untrained populations reported lower VE/VCO2measurements in males and its increase with aging [10]. Dur- ing clinical assessment, a cutoff below 30 is considered normal, and values above 34 to 36 suggest a high risk of mortality [7,11,12]. Previous research focused mainly on univariable measurements [13,14]. VE/VCO2is a ratio of two variables, i.e., VE and VCO2. In other words, it is calculated by dividing one parameter by another. VE grows simultaneously with VCO2during continued physical effort to ensure efficient excretion of the produced metabolites [15]. The link between VE and VCO2is multifactorial. The slope continuously and stably increases from the start to VT1 [16]. Above this, VE is forced by lactate accumulation and the slope begins to steepen [10]. However, the physiology of VE/VCO2in endurance athletes is understudied. Prediction equations provide several benefits. They allow for indirect calculation and facilitate the determination of the participant’s health based on the comparison of direct measurements with predicted reference values [13]. There were some attempts to predict VE/VCO2-slope with regression models [8,10,17–20], but there is no validation of those models. Previous research suggests that the usage of unified equations might not be optimal both in untrained individuals and athletes [8,21]. Few studies evaluated VE/VCO2in athletes [22–24]. However, none of them verified the underlying dependency of different VE/VCO2measurements. Potentially, well-trained endurance athletes could maintain strenuous physical effort well beyond VT1, where VE/VCO2-slope increases nonlinearly [25]. The use of inaccurate prediction models has negative consequences. Underestimated or overestimated values may lead to incorrect monitoring of training and disregard of potential risk factors. In turn, overestimated values may unnecessarily increase awareness and prevent demanding physical activity [3,13,21]. Research suggests that in athletes, VE/VCO2could be independent of the type
effort well beyond VT1, where VE/VCO2-slope increases nonlinearly [25]. The use of inaccurate prediction models has negative consequences. Underestimated or overestimated values may lead to incorrect monitoring of training and disregard of potential risk factors. In turn, overestimated values may unnecessarily increase awareness and prevent demanding physical activity [3,13,21]. Research suggests that in athletes, VE/VCO2could be independent of the type of exercise testing, body mass, or endurance capacity [23]. Based on our recent studies on well-trained individuals [14,21], we stipulate that current prediction models do not allow for a transferable calculation of cardiorespiratory parameters from the general population. Moreover, the underlying relationship between several measuring options for VE/VCO2in athletic cohorts is still controversial. This study aimed to (1) externally validate prediction equations for the VE/VCO2-slope in the athletic population and (2) explain the relationship between the VE/VCO2-slope, VE/VCO2-Nadir, and VE/VCO2-Total. 2. Materials and Methods 2.1. Study Design This study was conducted following the guidelines of the EQUATOR Network for ob- servational studies: Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) Statement [26]. The STROBE Checklist for Cross-Sectional Studies is provided in the Supplementary Material (Table S1). Research has been reviewed and approved by the Bioethics Committee of the Medical University of Warsaw (AKBE/277/2023). Partici- pants provided their written informed consent. All study procedures were in line with the Declaration of Helsinki. Healthy endurance athletes were referred to the standardized CPET. An endurance athlete was defined as having at least four years of regular training and competitive experience (at local and international levels). Participants held a membership in a sports association or training club and were also members of elite or development national teams. The tests were carried out in the years 2022–2023 at the Institute of Sport—National Research Institute in Warsaw (https://insp.pl; accessed on 22 October 2023). To be included, all
J. Clin. Med.2024,13, 490 3 of 13 participants underwent a medical evaluation by a physician (physical examination, medical and family history) according to the routine procedures of the testing center. We applied a rigorous selection process to obtain a group free of disturbing factors to test the raw relationships of VE/VCO2and ensure safe exercise tests. The preliminary exclusion criteria were any of the following: (1) respiratory diseases, (2) CVD, (3) neurolog- ical and psychiatric conditions, (4) musculoskeletal injuries limiting performance during CPET, (5) deviations in complete blood count, and (6) being a smoker. The final selection criteria were (1) age≥18 years and (2) maximum CPET. For a visual presentation of the recruitment procedures, see Figure, and for exact definitions of exclusion criteria, see the Supplementary Material (Table S2).J. Clin. Med. 2024, 13, x FOR PEER REVIEW 3 of 14 (AKBE/277/2023). Participants provided their written informed consent. All study proce- dures were in line with the Declaration of Helsinki. Healthy endurance athletes were referred to the standardized CPET. An endurance athlete was defined as having at least four years of regular training and competitive expe- rience (at local and international levels). Participants held a membership in a sports asso- ciation or training club and were also members of elite or development national teams. The tests were carried out in the years 2022–2023 at the Institute of Sport—National Re- search Institute in Warsaw (https://insp.pl; accessed on 22 October 2023). To be included, all participants underwent a medical evaluation by a physician (physical examination, medical and family history) according to the routine procedures of the testing center. We applied a rigorous selection process to obtain a group free of disturbing factors to test the raw relationships of VE/VCO2 and ensure safe exercise tests. The preliminary exclusion criteria were any of the following: (1) respiratory diseases, (2) CVD, (3) neuro- logical and psychiatric conditions, (4) musculoskeletal injuries limiting performance dur- ing CPET, (5) deviations in complete blood count, and (6) being a smoker. The final selec- tion criteria were (1) age ≥ 18 years and (2) maximum CPET. For a visual presentation of the
The preliminary exclusion criteria were any of the following: (1) respiratory diseases, (2) CVD, (3) neuro- logical and psychiatric conditions, (4) musculoskeletal injuries limiting performance dur- ing CPET, (5) deviations in complete blood count, and (6) being a smoker. The final selec- tion criteria were (1) age ≥ 18 years and (2) maximum CPET. For a visual presentation of the recruitment procedures, see Figure 1, and for exact definitions of exclusion criteria, see the Supplementary Material (Table S2). Figure 1. Visual presentation of participant recruitment procedures. CBC, complete blood count; CPET, cardiopulmonary exercise test; RER, respiratory exchange ratio; VO2, oxygen uptake; RPE, rating of perceived exertion; HRmax, maximal heart rate. 2.2. CPET Protocol n=140 Enduranceathletes underwentCPET •Respiratory diseases •Cardiovascular diseases •Neurological or psychiatric conditions •Present musculoskeletal injury •Deviations in CBC •Being a smoker Signspresentedatmedicalevaluation Individualsreferredto Instituteof Sport-National ResearchInstitute n=77 n=63 •Age ≥18 years •Maximum CPET Maximum CPET was confirmedby: •RER ≥1.05 •Plateau in VO 2 •Volitional inability to maintain effort •RPE≥18 •HR max ≥80% of age-predicted Not fulfillingfinalinclusioncriteria Excluded Excluded Lack of any of the measured variables Missing data Excluded Figure 1.Visual presentation of participant recruitment procedures. CBC, complete blood count; CPET, cardiopulmonary exercise test; RER, respiratory exchange ratio; VO 2, oxygen uptake; RPE, rating of perceived exertion; HRmax, maximal heart rate. 2.2. CPET Protocol CPETs were conducted following the same unified protocol. All tests were performed under unified laboratory conditions. According to the reference values for CPET in en- durance athletes, the maximal effort was confirmed by the following: (1) respiratory exchange ratio (RER)≥1.05, (2) plateau in VO2(stable VO2for≥30 s), (3) volitional inabil- ity to maintain effort, (4) Borg rating of perceived exertion≥18, and (5) maximal heart rate (HR)≥80% of age-predicted [13,27]. The physiologist supervised each CPET. Participants were verbally encouraged to achieve peak performance.
J. Clin. Med.2024,13, 490 4 of 13 CPET was performed on an upright cycle ergometer (Cyclus II Ergometer, RBM, Leipzig, Germany) in a ramp protocol. The tests began with a 2–3 min warm-up in the form of light pedaling without resistance. Participants completed an incremental test starting from 55 to 70 W, gradually increasing the load by 0.17–0.28 W·s −1 . The working loads were individually adjusted in all provided ranges according to the performance capabilities of each athlete. 2.3. Study Endpoints We measured basic demographic data: sex, age, height, weight, body mass index (BMI), and exercise performance. We obtained weight with the usage of a TANITA device (TANITA Corporation, Arlington Heights, IL, USA) and height with the usage of a SECA stadiometer (SECA GmbH & Co., Hamburg, Germany). Both weight and height were measured in the morning before breakfast. HR was measured with the Polar H10 chest strap (Polar Electro Oy, Kempele, Finland), continuously synchronized with the Cortex B3 Metamax. VE, VCO2, oxygen uptake, respiratory rate, and tidal volume were measured by the Cortex B3 Metamax using the breath-by-breath method (Hans Rudolph V2 Mask, Hans Rudolph, Inc., Shawnee, KS, USA). Variables were averaged in 15-s intervals. All measurement devices were calibrated for each usage in line with the producer’s instructions. The v-slope method has been previously used to find VT1 [28]. VT1 was identified in all endurance athletes enrolled in this study. VE/VCO2-slope was defined as the linear relationship between VE and VCO2from the start to VT1, excluding the first minute of the protocol, where noise values emerged. VE/VCO2-Nadir was defined as the lowest continuous 30 s average. VE/VCO2-Total was calculated across the whole CPET protocol without the first minute of the protocol. 2.4. Sample Characteristics In total, 140 healthy, well-trained individuals fulfilled the study criteria. Sample characteristics stratified by sex are presented in Table. There were 77 (55.0%) males and 63 (45.0%) females. Participants represented the following endurance sports: 56 (40.0%) trained triathlon or cycling, 59 (42.1%) chose speedskating, and 25 (17.9%) preferred other disciplines. VO2peak was 3.21±0.48 L·min −1 for females and 4.40±0.64 L·min −1 for males.
healthy, well-trained individuals fulfilled the study criteria. Sample characteristics stratified by sex are presented in Table. There were 77 (55.0%) males and 63 (45.0%) females. Participants represented the following endurance sports: 56 (40.0%) trained triathlon or cycling, 59 (42.1%) chose speedskating, and 25 (17.9%) preferred other disciplines. VO2peak was 3.21±0.48 L·min −1 for females and 4.40±0.64 L·min −1 for males. Females noted a higher VE/VCO2-slope than males (27.7±2.6 vs. 26.1±2.0). Two (2.6%) males exceeded the cutoff of 30 both for VE/VCO2-slope and VE/VCO2-Total, while all males maintained normal VE/VCO2-Nadir. Eleven (17.5%) females exceeded the cut off of 30 both for VE/VCO2-slope and VE/VCO2-Total. Four (6.3%) of them had VE/VCO2-Nadir above 30. There were no males with any VE/VCO2> 34; however, it was present for 4 (6.3%) females in VE/VCO2-Total. 2.5. Selection of Prediction Models for Validation Prediction equations for the VE/VCO2-slope were collected based on Paap and Takken reference values for CPET [29,30] and by additional searches in 5 databases: PubMed, Web of Science, Scopus, Google Scholar, and Embase. Applied keywords were “ventilatory efficiency”, “VE/VCO2-slope”, “prediction model”, “prediction equation”, “reference val- ues”, and “linear regression”. To ensure similarity with our group, we excluded models primarily derived from pediatric or geriatric populations (<18 or >70 years old) and clinical samples with coexisting medical conditions. Finally, 12 models from 6 studies met the se- lection criteria, and their detailed description is presented in Table. All selected equations predicted the VE/VCO2-slope to the first ventilatory threshold.
J. Clin. Med.2024,13, 490 5 of 13 Table 1.Participant characteristics. Variable Total (n= 140) Sex Females (n= 63) Males ( n= 77) Age (years) 22.7±4.6 23.8 ±4.2 21.8 ±4.8 Height (cm) 174.8±9.9 166.3 ±6.2 181.6 ±6.3 Weight (kg) 69.3±10.1 61.0 ±5.5 76.1 ±7.6 BMI (kg·m −2 ) 22.6±1.7 22.1 ±1.6 23.1 ±1.7 Primary sport Speedskating 59 (42.1) 26 (41.3) 33 (42.9) Triathlon or cycling 56 (40.0) 30 (47.6) 26 (33.8) Other 25 (17.9) 7 (11.1) 18 (23.3) HR (beats·min −1 ) 190.9±8.9 191.0 ±9.1 190.8 ±8.7 VE (L·min −1 ) 154.5±34.1 127.8 ±21.1 176.3 ±26.3 VO2peak (L·min −1 ) 3.86±0.82 3.21 ±0.48 4.40 ±0.64 VCO2(L·min −1 ) 4.36±0.96 3.57 ±0.52 5.00 ±0.73 VO2peak (mL·kg −1 ·min −1 ) 55.2±8.6 52.1 ±7.0 57.8 ±9.0 RR (breaths·min −1 ) 60.0±7.6 60.2 ±6.7 59.9 ±8.3 VT (L) 2.81±0.64 2.30 ±0.32 3.22 ±0.53 RER (VO2/VCO2) 1.14±0.05 1.13 ±0.05 1.15 ±0.05 VE/VCO2-slope 26.8±2.4 27.7 ±2.6 26.1 ±2.0 VE/VCO2-Nadir 25.2±2.3 26.2 ±2.4 24.5 ±2.0 VE/VCO2-Total 28.0±2.5 28.7 ±2.7 27.3 ±2.2 O2P (VO2/HR) 20.7±4.4 17.3 ±3.0 23.5 ±3.3 Testing duration (minutes) 21.3±2.6 21.1 ±2.7 21.4 ±2.6 Workload (watts) 320.4±76.2 266.7 ±40.8 364.4 ±70.0 BMI, body mass index; HR, peak heart rate; VE, peak minute ventilation; VO2peak, peak oxygen uptake; VCO2, peak carbon dioxide output; RR, peak respiratory rate; VT, tidal volume; RER, peak respiratory exchange ratio; VE/VCO2-slope, ventilatory efficiency from start to the first ventilatory threshold; VE/VCO2-Nadir, the lowest 30-s continuous average for ventilatory efficiency; VE/VCO2-Total, ventilatory efficiency from start to peak effort; O2P, peak oxygen pulse. Data are presented as mean±standard deviation for continuous variables or number (percentage) for categorical variables. Table 2.Prediction equations selected for validation. Reference Model Testing Protocol Sample Size (Total/Males/Females) Age (Years) Males Females Salvioni et al. [8] 20.227 + 0.095·age 23.808 + 0.052 ·age • Cycling CPET; ramp protocol. • Running CPET; Bruce protocol. 1136/773/363 13–83 21.413 + 0.08·age Kleber et al. [17] 19.9 + 0.13·age 24.4 + 0.12 ·age Running CPET; modified Naughton protocol with increases in gradient and speed of 1 MET every 2 min. 101/45/56 16–75 Neder et al. [18] 21 + 0.12·age 25.2 + 0.08 ·age Cycling
+ 0.052 ·age • Cycling CPET; ramp protocol. • Running CPET; Bruce protocol. 1136/773/363 13–83 21.413 + 0.08·age Kleber et al. [17] 19.9 + 0.13·age 24.4 + 0.12 ·age Running CPET; modified Naughton protocol with increases in gradient and speed of 1 MET every 2 min. 101/45/56 16–75 Neder et al. [18] 21 + 0.12·age 25.2 + 0.08 ·age Cycling CPET; ramp protocol with increases in power of 10–25 W·min −1 in females and 15–30 W·min −1 in males. 120/60/60 20–80 Loe et al. [20] 23.897 + 0.072·age + 0.826 25.549 + 0.072·age Running CPET; ramp protocol with increase in speed of 1 km·h −1 or gradient of 2% every 2–3 min. 4631/2261/2370 20–90 Ashikaga et al. [19] 22.4 + 0.07·age 22.467 + 0.07 ·age Cycling CPET; ramp protocol with increases in power of 10 W·min −1 or 20 W·min −1 . 529/274/255 20–78 Sun et al. [10] 34.38 + 0.082·age−0.0723·height Running or cycling CPET; incremental maximal protocols with varying duration. 474/310/164 37–74 CPET, cardiopulmonary exercise test; MET, metabolic equivalent. For all models, age is expressed in years. All models apply to the ventilatory efficiency slope from the start to the first ventilatory threshold.
J. Clin. Med.2024,13, 490 6 of 13 2.6. Statistical Analysis Data distribution was examined by the Shapiro–Wilk test and quantile–quantile plots. Due to parametric distribution, continuous variables are presented as mean±standard deviation. Categorical variables are presented as numbers (percentages). In cases of missing data in any of the measured variables, participants were excluded from the analysis to ensure maximum precision. The sample was evaluated in the G*Power Software (version 3.1.9.6) [31] to obtain significance (p< 0.05) and large effect sizes for each applied statistical test. All achieved statistical powers were >0.8. Differences between VE/VCO2-slope, VE/VCO2-Nadir, and VE/VCO2-Total or ob- served and predicted VE/VCO2-slope were calculated using the Studentt-test or Wilcoxon test, as appropriate. The precision of the equations was examined by the mean absolute percentage error (MAPE) and root mean square error (RMSE). RMSE was adjusted to percentage by dividing the errors by the mean of observed VE/VCO2-slope (%RMSE). Two-way mixed effects interclass correlation coefficients (ICC3,1) with 95% confidence intervals (CI) [32,33] were calculated to test agreement between the observed and predicted VE/VCO2-slope. The relationship between observed and predicted values was visually presented by Bland–Altman plots. We regressed the predicted VE/VCO2-slope against direct measurements and presented it by the coefficient of determination (R 2 ). Data are presented following the AMAManual of Style. A two-sidedp-value < 0.05 was considered as significant. Analyses were performed in the IBM SPSS Statistical Soft- ware (version 29.0, IBM, Chicago, IL, USA). Figures were derived via GraphPad Prism (version 10.1, GraphPad Software, San Diego, CA, USA). 3. Results 3.1. Interdependency of VE/VCO2Measurements Relationships between the VE/VCO2-slope, VE/VCO2-Nadir, and VE/VCO2-Total are shown in Figure. In males, VE/VCO 2-slope (26.1±2.0) was significantly higher than VE/VCO2-Nadir (24.5±2.0,p< 0.001) and lower than VE/VCO2-Total (27.3±2.2, p< 0.001). The same relationship was observed among females, where VE/VCO2-slope (27.7±2.6) was significantly higher than VE/VCO2-Nadir (26.2±2.4,p< 0.001) and lower than VE/VCO2-Total (28.7±2.7,p= 0.043). Between sexes, female athletes observed higher VE/VCO2-slope (p< 0.001), VE/VCO2-Nadir (p< 0.001), and VE/VCO2-Total (p= 0.001). In univariable analysis, VE/VCO2increased with age: VE/VCO2-slope (β= 0.093,p= 0.27), VE/VCO2-Nadir (β= 0.127,p= 0.14), and VE/VCO2-Total (β= 0.066,p= 0.44). 3.2. Validity of VE/VCO2-Slope Predictions
was observed among females, where VE/VCO2-slope (27.7±2.6) was significantly higher than VE/VCO2-Nadir (26.2±2.4,p< 0.001) and lower than VE/VCO2-Total (28.7±2.7,p= 0.043). Between sexes, female athletes observed higher VE/VCO2-slope (p< 0.001), VE/VCO2-Nadir (p< 0.001), and VE/VCO2-Total (p= 0.001). In univariable analysis, VE/VCO2increased with age: VE/VCO2-slope (β= 0.093,p= 0.27), VE/VCO2-Nadir (β= 0.127,p= 0.14), and VE/VCO2-Total (β= 0.066,p= 0.44). 3.2. Validity of VE/VCO2-Slope Predictions The accuracy of prediction equations stratified by sex is presented in Table. Predicted and observed values differed significantly in 9 from 12 models. Models underestimated the VE/VCO2-slope from−0.5 for the Loe et al. [20]. model in females up to−3.6 for the females’ formula by Ashikaga et al. [19] and the general formula by Salvioni et al. [8]. Only the model for males by Loe et al. [20] overestimated predictions, by +0.2. RMSE ranged from 2.0 to 4.5, while MAPE varied from 6.3% to 13.0%. Alignment was poor, with all ICC3,1 far under 0.5. Equations explained a low amount of variance, with R 2 ranging between 0.003 and 0.031. The model’s agreement is visualized with Bland–Altman plots in Figure.
Description
This study investigates ventilatory efficiency in endurance athletes and validates prediction equations.