Measurement precision of the anaerobic threshold by means of a portable calorimeter

Abstracts

FUNDAMENTO: Muitos métodos são empregados para determinar o Limiar Anaeróbio (LAn) por meio de ergoespirômetros sofisticados. OBJETIVO: Testar a variação no LAn, detectado por modelos matemáticos e de inspeção visual, quando empregado ergoespirômetro de baixo custo e destinado à aplicação clínica. MÉTODOS: Foram voluntários para esse estudo 79 indivíduos aparentemente saudáveis; desses, 57 homens. O VO2máx e o limiar ventilatório foram determinados por calorimetria indireta de circuito aberto. O método eletroenzimático foi empregado para análise da lactacidemia e determinação direta do limiar de lactato (LL). O LAn foi determinado por dois métodos matemáticos (MM SQR e MMslope), baseados nas trocas gasosas, e pelo método de inspeção visual do log-log, para determinação do LL. Dois pesquisadores independentes determinaram o LAn através da inspeção visual de três gráficos, considerando dois métodos (LAn-a= V-slope, EqV; e LAn-b = V-slope, EqV e ExCO2). Os dados foram analisados por meio da estatística paramétrica para determinação das diferenças entre LAn-a versus ExCO2, MM SQR e MMslope; LAn-b versus MM SQR e MMslope; e LL versus LAn-a, LAN-b, MM SQR e MMslope. RESULTADOS: O MMslope foi o único método que apresentou diferença significativa entre o LAn-a e LAn-b (p=0,001), com CV% >15. O LL versus MMslope não apresentou diferença significativa (p=0,274), contudo, observou-se um elevado CV (24%). CONCLUSÃO: Conclui-se que com o equipamento de baixo custo os métodos MM SQR e LAn-a podem ser utilizados para a determinação do LAn. O método MMslope não apresentou precisão satisfatória para ser empregado com esses equipamentos.

Teste de esforço; modelo matemático; limiar ventilatório; ergoespirometria

FUNDAMENTO: Muchos métodos se emplean para que se determine el Umbral Anaerobio (UAn) por medio de ergoespirómetros sofisticados. OBJETIVO: Probar la variación en el UAn, detectado por modelos matemáticos y de inspección visual, cuando empleado ergoespirómetro de bajo costo y destinado a la aplicación clínica. MÉTODOS: Fueron voluntarios para este estudio 79 individuos aparentemente sanos; de ellos, 57 varones. El VO2máx y el umbral ventilatorio se determinaron por calorimetría indirecta de circuito abierto. El método electroenzimático se empleó para análisis de lactacidemia y determinación directa del umbral de lactato (UL). El UAn fue determinado por dos métodos matemáticos (MM SQR y MMslope), basados en los cambios gaseosos, y por el método de inspección visual del log-log, para determinación del UL. Dos investigadores independientes determinaron el UAn a través de la inspección visual de tres gráficos, teniendo en cuenta dos métodos (UAn-a= V-slope, EqV; y UAn-b = V-slope, EqV y ExCO2). Los datos se analizaron por medio de la estadística paramétrica para determinación de las diferencias entre UAn-a versus ExCO2, MM SQR y MMslope; UAn-b versus MM SQR y MMslope; y UL versus UAn-a, UAN-b, MM SQR y MMslope. RESULTADOS: El MMslope fue el único método que presentó diferencia significativa entre el UAn-a y UAn-b (p=0,001), con CV% >15. El UL versus MMslope no presentó diferencia significativa (p=0,274), con todo, se observó un elevado CV (24%). CONCLUSIÓN: Se concluyó que con el equipamiento de bajo costo los métodos MM SQR y UAn-a pueden utilizarse para la determinación del UAn. El método MMslope no presentó precisión satisfactoria para ser empleado con estos equipamientos.

Prueba de esfuerzo; modelo matemático; umbral ventilatorio; ergoespirometría

ORIGINAL ARTICLES

Measurement precision of the anaerobic threshold by means of a portable calorimeter

Fernando dos Santos Nogueira; Fernando Augusto Monteiro Sabóia Pompeu

Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ - Brazil

Mailing address

ABSTRACT

BACKGROUND: Many methods are used for determining the Anaerobic Threshold (AT) by means of sophisticated ergospirometer.

OBJECTIVE: To test the AT variation, detected by mathematical models and visual inspection, when low cost ergospirometer is used and intended for clinical application.

METHODS: Seventy nine apparently healthy subjects were volunteers in this study; from these, 57 men. The VO_2max and the ventilatory threshold were determined by indirect, open-circuit calorimetry. The electro-enzymatic method was used for analyzing the lactacidemia and direct determination of the Lactate Threshold (LT). The AT was determined by two mathematical methods (MM_RSS and MM_slope), based on the gases exchange, and by the log-log visual method, for determining the LT. Two independent investigators determined the AT through visual inspection of three graphs, considering two methods (AT_-a= V-slope, EqV; and AT_-b = V-slope, EqV and ExCO₂). The data were analyzed by means of parametric statistics for determining the differences between AT_-aversus ExCO₂, MM_RSS and MM_slope; AT-b versus MM_RSS and MM_slope; and LT versus AT_-a, AT_-b, MM_RSS and MM_slope.

RESULTS: The MM_slope was the only method that presented a significant difference between the AT_-a and AT_-b (p=0.001), with CV% >15. LT versus MM_slope did not present significant difference (p=0.274), however, it was observed a high CV (24%).

CONCLUSION: It was concluded that with the low cost equipment, the MM_RSS and AT_-a methods can be used for determining the TAn. The MM_slope method did not present satisfactory precision to be employed with this equipment.

Key words: Exercise test, mathematical model, ventilatory threshold, ergospirometry.

Introduction

The anaerobic threshold (AT) was initially proposed as tolerance index for cardiopaths¹ exercises. Currently, AT determination is of capital importance in the exercise science because it is a conditioning indicator in several groups of subjects^2-7. For the indirect and bloodless determination of the AT, gas and ventilatory exchange measures were taken during an ergometric test with increments in the overload⁸.

It is believed that the best adjust for detecting the AT is the flexion in the relation between the VO₂versus VCO₂, with b > 1.15, which is obtained by means of the intersection between two line segments^9,10. This detection is performed by means of visual inspection of dispersion diagrams, although mathematical algorithms and statistical calculations have been helping in the automatization and/or semi-automatization of this procedure^9-12.

The ergospirometers with computerized systems are broadly used in measuring the gases exchange and in the inference of the AT. Sophisticated and more expensive equipment is restricted to research labs. Simpler and cheaper ergospirometers were created for clinical and field investigation. Several studies validated the measurements of VO₂, VCO₂ and V_E by means of such clinical equipment^13-19, however a few studies determined the quality of the derived measurements such as AT²⁰.

Considering the low cost of the clinical equipment and the physiological relevance of the AT for several health professionals, the objective of this retrospective study was to analyze the accuracy of the AT detected by means of the measurements obtained by the indirect calorimetry system TEEM 100^TM Total Metabolic Analysis System (Aerosport, Inc., Ann Arbor, Mich., USA)^17-20, determining the objectivity and the accuracy of the mathematical models of Beaver et al⁹ adapted by Gaskil et al²¹ and Vieth¹⁰ and of the visual inspection methods V-slope²², V_E/ VO₂²³, ExCO₂²¹ and lactate threshold²⁴.

Methods

Subjects

The present investigation was divided into two assays consisted of two volunteer groups of both sexes, from 18 to 37 years old, apparently healthy, no smokers and no athletes, with or without experience in cycloergometer, engaged or not in aerobic training programs (tab. 1). In the first group, it was analyzed the discrepancy between the mathematical methods and the visual inspection ones to determine the AT. In another group, it was investigated the association between the ventilatory parameters and the gas exchange with the blood concentration of lactate to determine AT.

It was recommended, for the 24 hours before the test, absence of extenuating physical activities (> 5 METs) and no alcohol ingestion. It was also recommended to keep the mixed diet in the 48 hours before the test. It was also requested no ingestion of food and caffeine three hours before the effort. Each subject was informed regarding the risk associated to the adopted procedures. An informed consent was read and signed. All the procedures adopted were approved by the Local Ethics Committee on Research with human beings (Rio de Janeiro, CEP/HSE 000.021/99). This study was performed as per Declaration of Helsinki.

Maximum test on the cycloergometer

Group 1 (G-1) - The continuous and maximum, scaled effort protocol¹ was used with a mechanical cycloergometer (Monark^TM, São Paulo, SP, Brazil). The height of the saddle was adjusted for each subject, so the knee could keep an angle close to the total extension (approx. 175º). The maximum power was previously estimated for each subject, in order to make possible 10% increments of the maximum load every minute²⁵. After six minutes in rest, sitting on the cycloergometer saddle, the subjects pedalled with load for four minutes and, then, the scaled phase was started. The maximum duration of the exercise was of 10 ± 2 min. The subjects kept the fixed rhythm during the test (approx. 1.23 Hz). The rhythm was controlled by an audiovisual metronome (Wittner^â Junior Plast 826, Isny/ Allgäu, Germany).

The minute ventilation (V_E) and the oxygen and carbon oxide expired fraction were continuously measured by means of open circuit indirect calorimetry (TEEM 100^TM Total Metabolic Analysis System, Aerosport, Ann Arbor, Mich., USA)^17-20. The subjects used a nose clip and a medium flow pneumotachometer (Hans Rudolph^TM, Kansas City, MO, USA). The minute consumption oxygen (VO₂) as well as the carbon dioxide excretion, per minute (VCO₂) were presented every 20 seconds. The heart rate (HR) was monitored continuously during the test by means of telemetry (Vantage NV^TM, Polar Electro Oy, Kempele, Finland) and the perceived effort concept (PEC), in Borg scale of 6 to 20, was collected at the end of each stage.

Group 2 (G-2) - The ergospirometric protocol was the same of G-1, being added the blood lactate measurements. It was collected 25 µl of blood, by puncture of the ear lobe in hyperemia, as per the procedures described by Shephard²⁶. The collections were performed during rest, two minutes before the test and every two minutes of effort. The samples were immediately analyzed by electroenzymatic method (YSI 1500 Sport L-Lactate Analyser^TM, Yellow Springs, USA). For determining the total blood lactate, the hemolytic agent Triton X-100 (YSI #1515 Agent Cell Lysing, USA) at 0.25% was added to the buffer solution. The blood collections were performed by an experienced evaluator between the 20 and 25 final seconds of every two minutes of effort.

Controls and calibration

The metabolic analyzer, the lactate analyzer and the cycloergometer were calibrated before each test. The ergospirometer was calibrated in closed circuit, by means of a certified mixture of gases containing 17.01% of oxygen, 5.00% of carbon dioxide and balanced with nitrogen (AGA^TM, Rio de Janeiro, RJ, Brazil). The flow was calibrated using a three-liter air syringe (Hans Rudolph^TM, Kansas City, MO, USA). At the end of each test, the measurement of oxygen and carbon dioxide percentage fractions in the gas mixture used for calibration was performed. The maximum error allowed was 16.16% to 17.86% for FO₂ and 4.75% to 5.25% for FCO₂. The lactate analyzer had the calibration confirmed before the test, by means of a standard solution of 5 mmol·l^-1 (YSI #2327 Lactate Standard YSI^TM, USA) of lactate. Before each test, and at every hour of use, a new calibration was performed. The linearity of the equipment was confirmed at 15 mmol·l^-1 of lactate. Before the beginning of the experiment, the precision of the equipment was checked by means of a calibration curve with standards of 1.0; 2.5; 5.0; 7;5; 12.0; 15.0; 18.0; 24.0 and 30.0 mmol·l^-1, prepared by the dilution of standards supplied by the manufacturer (YSI #2327: 5 mmol·l^-1; YSI #2328: 15 mmol·l^-1, YSI#1530: 30 mmol·l^-1 Lactate Standard YSI^TM, USA). The association between the measured and the expected values in the calibration curve was r=0.999, y=0.9436x + 0.33011 and EPE=0,20 mmol.l^-1. The cycloergonometer was calibrated by means of a 3 kg-ballast.

The tests were considered as maximum when at least three of the following criteria were observed²⁷: a) plateau in VO₂ (increase < 150 ml.min^-1 or 2 ml.kg^-1.min^-1); b) respiratory exchange ratio (RER) > 1.15; c) 90% of FC_max forecast by the age (220 - age); d) perceived effort concept > 19 (6-20); e) blood lactate concentration > 8 mmol·l^-1; and f) maximum voluntary fatigue with inability of keeping the pre-established rhythm. The VO_2max was determined as being the highest value found at the end of the test.

Methods for detecting AT by visual inspection

Three methods for detecting the anaerobic threshold for visual inspection were used:

Ventilatory equivalent method (EqV)²³ - Moment when there is an increase of the ventilatory equivalent for oxygen consumption (V_E/VO₂) without the concurrent increase in the ventilatory equivalent for carbon dioxide excretion (V_E/VCO₂).

Carbon dioxide excess method (ExCO₂)²¹ - With the increase of the exercise intensity, it is observed an excess of carbon dioxide production calculated as ((VCO₂²/VO₂)-VCO₂).

Simplified V-slope method (V-slope)²² - In the Cartesian coordinates graph, having in the abscissa axis the consumption of oxygen per minute (VO₂) and in the ordinate, the excretion of carbon dioxide per minute (VCO₂), it was observed the moment when the points surpass the line parallel to the right angle bisectrix.

For each individual, the three methods of AT determination were analyzed visually by two experienced investigators.

AT analysis by mathematical method

Two functions of linear regression were described for the test from the relation between VO₂versus VCO₂, as follows:

where (x₀,y₀) represents the coordinate of critical point (AT).

On the first interaction, the observations x₁, x₂ and x₃ were included, estimating the parameters of the first regression line. The remaining observations n-3, x₄,...x_n were used to adjust the second regression line. In the following interaction, the first regression line was adjusted with the observations x₁,...x₄ and the parameters of the second regression line were based on the observations n-4. At each interaction, an additional observation of the second part of the data was transferred to the first part and second regression line was adjusted with the remaining observations.

Vieth's mathematical method¹⁰(MM_RSS) - The estimate of the parameters was based on the least square methods. For the inference of the inflexion point, the residual sum of squares (RSS) was calculated for each regression line.

The RSS is used as a criterium to determine the best adjust. The optimum adjusts of a₁,b₁,a₂,b₂ and x₀ are the values of the minimum RSS for the two lines.

Beaver's⁹ Modified mathematical method²¹ of V-slope (MM_slope) - The intersection between the two regression lines was employed for determining the AT. The solution was accepted when it is observed an increase in the inferior segment slope to the superior equal to one. This method, which originally employed an analysis of the collected gases at each respiratory incursion, was modified for the use of the incursions average at every 20 seconds²¹.

Determination of lactate threshold

The results obtained by the blood lactate analysis were determined individually through the log-log method, described by Beaver et al²⁴. The transformation of the data for the logarithmic method was performed for locating the inflexion point or the lactate threshold, which was understood as a crossing point between the two formed lines.

Statistical analysis

The statistical treatment was performed by means of the Statistical Package for the Social Sciences^TM (SPSS, USA), SigmaPlot^TM (Systat Software Inc., Alemanha) and Microsoft Excel^TM applications for Windows XP^TM (Microsoft, USA). The descriptive statistics was used, with average ± standard deviation (SD). It was used the grand average of the results obtained from the EqV and V-slope methods, by two evaluators, designating AT_-a. The grand average of the results obtained from the EqV, V-slope and ExCO₂ methods, used by two evaluators designated AT_-b. The values measured from AT_-a versus ExCO₂, MM_RSS, MM_slope and AT_-bversus MM_RSS and MM_slope, for G-1 and; LT versus AT_-a, AT_-b, MM_RSS and MM_slope, for G-2, were compared by means of variance analysis (ANOVA) with a factor and post-hoc test of Tukey-HSD. The limits of agreement of Bland-Altman²⁸ were employed. The association degree between the methods was determined by means of an intra class correlation coefficient (ICC). The error was also observed by measurement of technical error ( s = D.P._dif ÷ ) and of the variation coefficient (VC). It was also employed the above statistical treatment to evaluate the results obtained through two evaluators. It was compared the indices obtained of AT_-a and AT_-bversus Evaluator 1 and Evaluator 2 by means of ANOVA with two factors and post-hoc test of Tukey-HSD. All the statistical tests were performed at the significance level < 0.05.

Results

The characteristics of the subjects are in table 1. Table 2 presents the average and SD for AT_-a, AT-_b, ExCO₂, MM_RSS, MM_slope and LT, including comparative statistics between AT_-a ExCO₂, MM_RSS, MM_slope and AT_-bversus MM_RSS, MM_slope for G-1 and LT versus AT_-a, AT_-b, MM_RSS and MM_slope for G-2. It is noted that only MM_slope presents significant difference, when compared to AT_-a and AT-_b (p = 0.001) and CV was above 18% in the two detection forms of AT. It was not observed a significant difference between AT_-aversus ExCO₂ and MM_RSS and AT_-bversus MM_RSS. In G-2, the visual inspection methods presented good correlation, as well as the MM_RSS, when compared to LT.

The dispersion diagrams of the left hand side of the figures 1 and 2 presented the relation between AT_-aversus ExCO₂, MM_RSS, MM_slope and AT_-bversus MM_RSS, MM_slope, respectively. In figure 3, there is a relation between LT versus AT_-a, AT-_b, MM_RSS and MM_slope.. It is noted the proximity between the identity line and the trend lines for AT_-aversus ExCO₂ and MM_RSS and AT_-bversus MM_RSS and LT versus AT_-a, AT_-b and MM_RSS. The right hand side data in these figures refer to the limits of agreement of Bland-Altman²⁷. Table 2 summarizes the values found for each analysis.

There was no significant difference for determining AT by means of visual inspection between the evaluators (p=0.757) and methods (p=0.700), and also there was no interaction between the evaluators versus methods (p=0.876). The evaluators presented for AT_-a: limits of agreement = 0.02 ± 0.29 l.min^-1, ICC = 0.92, s = 0.11 l.min^-1 and CV = 7%; and for AT_-b: limits of agreement = 0.01 ± 0.24 l.min^-1, ICC = 0.95, s = 0.09 l.min^-1 and CV = 6%.

Table 3 presents the reliability indices obtained from the previous studies of clinical equipment or for the lab use. High ICC indices were observed between the pieces of equipment for clinical use, with exception of MetaMax II. Similar indices are found in lab use equipment.

Discussion

The ergospirometric test and the AT analysis allow the integration study between the pulmonary, cardiovascular and muscleskeletal systems^29,30. There are cases where this is the only means to understand the physiopathological mechanisms as in severe pulmonary vascular disease without right hypertension, in the open oval foramen with development of left-right shunt, during the exercise, in effort dyspnea, in effort hypoxemia, among others³¹. Its application in cardiopaths and pneumopaths groups is advantageous before the invasive or high cost procedures^30,31. The present study proposal was to test the accuracy of the ergospirometric equipment that allows measurements with quality for the clinical application. Comparing the mathematical methods proposed by Beaver et al⁹ and Vieth¹⁰, with the V-Slope²², Ventilatory Equivalent for VO₂²³, CO₂ Excess²¹ and Lactate Treshold²⁴ methods for determining AT.

The sample of this study was consisted of non-athlete, healthy subjects, with or without experience in cycloergometer, engaged or not in aerobic training programs. Hereditary characteristics and the involvement in exercise programs can explain the large dispersion of VO_2max values. Even considering the experience of the individuals with the cycloergometer, this equipment can overload the inferior limbs, causing early fatigue^32,33. The fatigue of the inferior limbs may result in low and relatively high AT (%)³¹. The AT can, also, certainly be super or subestimated in response to the overload increments⁹. The AT (%) found for the subjects during the progressive test was close to what was expected for non-obese, physically active and apparently healthy individuals³¹. Billat³⁴ demonstrated that the highly trained subjects are able to use more intensively the oxidative routes, therefore with lower lactate accumulation, with intensities of up to 90% of its VO_2max. Our results allowed to estimate the AT by the AT_-a, AT_-b, MM_RSS, MM_slope and LT methods, in percentage of VO_2max and, were close to the expected results for a test performed in cycloergometer³¹.

Gaskill et al²¹ studies the association between the several visual inspection methods and suggested that there are benefits in the analysis with the combination of the methods: V-slope, ExCO₂ and EqV. Another study²⁰, using the same equipment adopted in this study, also suggested the use of combined analysis of gas exchange and ventilatory parameters in order to reduce the methodological error in determining the AT. The present investigation decided to make the comparisons with combined data (AT_-a and AT_-b) and tested only the ExCO₂ method separately. The statistical analysis showed a good association between the ExCO₂ and the AT_-a methods; however the increase of more than a detection method of AT did not reduce significantly the CV% from the one observed for MM_RSS, MM_slope and LT (tab. 2).

The data presented in table 2 and figures 1, 2 and 3 suggest a satisfactory approximation between the AT_-a and AT_-b methods. These methods presented an excellent correlation with MM_RSS and LT. A reasonable correlation was found between LT and MM_slope. Caiozzo et al²³ presented the EqV as the best isolated method for determining the AT (r = 0.93). According to these authors, the V-slope method does not produce a good estimate of AT. However, Gaskill et al²¹ did not find a significant difference between the V-slope and EqV methods, when compared with the LT (± 5 ml·min^-1) indicating good agreement between the methods. Similar findings related by Santos et al³⁵ and by Granja Filho et al²⁰.

Beaver et al⁹ proposed a method that eliminates the points that produce a regression line slope lower to 0.6 and values above the respiratory compensation point. The present investigation, based on a previous study²¹, modified the original method so all the data could be used in the linear adjust. Such adaptation was necessary due to the temporal resolution of the measurements by means of the equipment used in this study. Kelly³⁶ observed that the results obtained by the original mathematical method of Beaver et al⁹ presented a major estimate standard error, when compared to MM_RSS. Santos et al³⁵ found for this method, percentage indices of 83% of VO_2max. In our study, we observed high intensities of AT³¹ by means of MM_slope, indicating a possible method inaccuracy.

There are few studies related to the reliability of the metabolic analysis system of low cost, The table 3 presents some indicators for clinical and scientific use equipment. The equipment used in this study presents a test-retest reliability of 5.5% for VO_2max, according to Granja Filho et al²⁰. Melanson et al¹⁷ found ICC of 0.96 for VO₂ and 0.86 for V_E. On the same study¹⁷, the equipment was validated comparing it with a computerized reference system, and Pearson's correlation coefficients of 0.91 to 0.97 were found for V_E and 0.88 to 0.97 for VO₂. Wideman et al¹⁹ found differences of 2% to 11% for VO₂ and 5% to 17% for VCO₂ in several intensities. An average difference of 3.9% for VO₂ was found by Novitsky et al¹⁸ when compared to Sensormedics 2900^TM. The indices presented in table 3 related to the validity and reliability of VO₂, VCO₂ and V_E of TEEM 100, and other clinical equipment were found a little under the ones observed for the most sophisticated equipment for the scientific investigations.

Considering the AT importance as exercise tolerance índex and conditioning indicator in several groups of individuals^1-7, we conclude that MM_RSS, as well as the visual inspection methods of AT_-a and AT_-b are satisfactorily accurate and objective for the determination of that metabolic reference. The inclusion of another visual inspection method (ExCO₂) did not reduce significantly the error. We also conclude that MM_slope did not present significant accuracy for determining the AT in the equipment adopted in this study³⁸.

Acknowledgements

The authors of the present study would like to thank "Associação dos Amigos do Centro de Estudos e Aperfeiçoamento do Hospital dos Servidores do Estado do Rio de Janeiro", especially Dr. Aluysio S. Aderaldo Jr. for their important contribution in carrying out this study and their colleagues Gilberto Sabóia Pompeu Neto, Michelle F. S. Porto Nogueira and Lucenildo Cerqueira. This study was supported by FAPERJ and MCT/CNPq.

Potential Conflict of Interest

No potential conflict of interest relevant to this article was reported.

Sources of Funding

This study was funded by CAPES, MCT/CNPq and FAPERJ.

Study Association

This article is part of the thesis of master submitted by Fernando dos Santos Nogueira, from Universidade Federal do Rio de Janeiro.

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Publication Dates

• Publication in this collection
  16 July 2010
• Date of issue
  Sept 2010

History

• Accepted
  02 Mar 2010
• Reviewed
  12 Dec 2009
• Received
  15 July 2009