CHAPTER I



THE OXYGEN COST OF VENTILATION AND ITS EFFECT ON THE VO2 PLATEAU

By

Derek W. Marks

B.S., Nutritional Science, Cal Poly San Luis Obispo, 1995

M.S., Health Promotion and Exercise Science, Cal Poly San Luis Obispo, 1998

Ph.D., Health, Physical Education, and Recreation, University of New Mexico, 2002

ABSTRACT

Because the respiratory muscles demand a significant portion of cardiac output and VO2 during high-intensity exercise, whole-body VO2 (wb-VO2) models may not accurately represent the VO2 of the exercising muscles, which could affect the ability to detect a VO2 plateau during maximal exercise.

The purposes of this study were to: a) determine the oxygen cost of ventilation (VEVO2) across a range of ventilations (VE) representing an incremental exercise test to exhaustion, b) develop a regression equation to predict the VEVO2 based on the VE data collected, c) calculate a ventilation adjusted VO2 model by subtracting the composite VEVO2 from the VO2 obtained during an incremental exercise test (VO2-VEC); d) calculate a VO2-VE by subtracting the individual VEVO2 from the wb-VO2 obtained during incremental exercise (VO2-VEI) and, e) evaluate the VO2 and VO2-VE data for the presence of a VO2 plateau.

Subjects first performed a cycling exercise test for the determination of VO2 max and exercise VE volumes using a ramp protocol. To obtain the VEVO2 data, subjects were asked to mimic, at rest, 9 different VE volumes corresponding to their exercise VE.

An equation defining the individual VO2 response to VE (VEVO2I) and a composite equation defining all of the subject’s VO2 responses to VE were calculated (VEVO2C). These were then subtracted from the wb-VO2 data in order to obtain the VO2-VEI and VO2-VEC data.

The wb-VO2, VO2-VEI, and VO2-VEC data were analyzed with three different techniques for the presence of a VO2 plateau during maximal exercise.

A repeated measures ANOVA (slope detection technique) was used determine if there was a difference between the three VO2 slopes. Significance was set at p < 0.05.

The significant findings of this study were; a) the VO2 of maximal exercise ventilation equaled 18.1% + 4.4% of wb-VO2max, b) there is an exponential relationship between increasing VE and the VEVO2 and between VEVO2 and increasing exercise intensity c) the VEVO2 can be accurately predicted among the subjects in the present study by use of the regression equation created from combining all VEVO2 trials, d) adjusting wb-VO2 for the VEVO2 significantly affects the VO2 slope during maximal exercise, and e) the presence of a plateau is greater when wb-VO2 is adjusted for the VEVO2I or the VEVO2C.

ACKNOWLEDGEMENTS

For the many hours, effort, and feedback, THANK YOU, to my Dissertation Committee, Research Technicians, and Subjects.

Dr. Robergs, I could never have made it here without your guidance. You have shown me what it is to be a mentor, teacher, and scientist. THANK YOU for your support and belief in my abilities.

SARA and CARLY you are my life, THANK YOU for the unconditional love and support throughout this journey. Mom, Chris, Heinz, and Dad, THANK YOU! You never gave-up on me.

TABLE OF CONTENTS

LIST OF FIGURES ix

LIST OF TABLES xi

SYMBOLS/ABBREVIATIONS xii

CHAPTER I: INTRODUCTION 1

Purpose of the Study 5

Hypotheses 6

Scope of the Study 7

Limitations 9

Delimitations 9

Significance of the Study 9

Definition of Terms 10

CHAPTER II: COMMENTARY 11

Title Page 12

The Relationship Between Whole-Body VO2 and Leg VO2 During Exercise 13

The Oxygen Cost of Ventilation During Exercise 15

Recommendations for Presenting and Interpreting VO2 Data During

Incremental Exercise 18

References 21

CHAPTER III: RESEARCH MANUSCRIPT 24

Title page 25

Abstract 26

Introduction 27

Experimental Procedures 31

Subjects 31

Expired Gas Analysis and Heart Rate Measurement 31

Exercise Test 32

Measurement of the VEVO2 32

Calculation of the VEVO2 and VO2-VE 33

Whole-Body VO2 and VO2-VE Slope Analyses 36

Statistical Procedures 36

Results 37

Discussion 40

References 45

CHAPTER IV: SUMMARY, CONCLUSIONS and RECOMMENDATIONS 48

Summary 48

Conclusions 52

Recommendations 52

REFERENCES 54

APPENDICES 59

Appendix A. Informed Consent 60

Appendix B. Health History Questionnaire 65

Appendix C. Data Collection Form 69

LIST OF FIGURES

CHAPTER II: COMMENTARY FIGURES

Figure 1. Comparison between whole-body VO2 (wb-VO2) and leg VO2 during 20, 40, 80, 100, and 110% of the intensity at which a whole-body VO2 plateau first occurred (WRsel). Values in parentheses are number of observations made. Data adapted from Knight et al. (12). 14

Figure 2. Steady state VEVO2 data obtained during sustained, resting hyperventilation of a representative subject in Marks et al. (13). Regression equation for subsequent determination of the breath-by-breath VEVO2 during incremental exercise to exhaustion. Data adapted from Marks et al. (13). 16

Figure 3. Oxygen cost of VE data across an incremental exercise test to exhaustion of a representative subject in Marks et al. (13). VEVO2I represents data obtained from individual VEVO2 data (i.e. Figure 2). VEVO2C represents individual data based on composite regression equation (Figure 5). Data adapted from Marks et al. (13). 17

Figure 4. Data representing wb-VO2 and ventilation adjusted VO2 (either wb-VO2 – VEVO2I or wb-VO2 – VEVO2C) during an incremental cycling test to exhaustion of a representative subject in Marks et al. (13). Data adapted from Marks et al. (13). 17

Figure 5. Prediction equation for the attainment of the oxygen cost of ventilation based on VE data. Data adapted from Marks et al. (13). 18

Figure 6. Adjusted VO2 max model that considers the O2 cost of ventilation as a significant contributor to whole-body VO2 data. The VO2 devoted to postural muscles, basal organ metabolism, and the myocardium are not known, but are estimated to require approximately 9% of whole-body VO2 max. Data adapted from Harms (7), and Marks et al. (13). 19

CHAPTER III: RESEARCH MANUSCRIPT FIGURES

Figure1. Steady state VEVO2I data with fitted curve and equation for a representative subject. 34

Figure 2. The breath-by-breath VEVO2I and VEVO2C data during an exercise test of a representative subject. Data are presented commencing at 3 min to avoid the 2 min rest and initial min of increment at 50 Watts. 35

Figure 3. The three VO2 data sets and an example of the 3 slope-detection techniques on representative subject’s data. Data are presented commencing at 3 min to avoid the 2 min rest and initial min of increment to 50 Watts. A) 1-minute technique defined by linear regression equation fit to breath-by-breath VO2 data within box. B) 30-second technique defined by linear regression equation fit to breath-by-breath VO2 data within box. C) 5-minute technique defined by change in VO2 identified by third-order polynomial slope across final 30 seconds of exercise data inside box 35

Figure 4. Composite data representing all VEVO2 mimicking trials for all subjects 39

Figure 5. The mean data for the VEVO2 per liter of VE 41

CHAPTER IV: SUMMARY, CONCLUSIONS and RECOMMENDATIONS FIGURES

Figure 1. The percent of wb-VO2 that is the VEVO2 during exercise expressed as percent of VO2 max for 12 randomly selected subjects 51

LIST OF TABLES

Table 1. Descriptive data of the subjects 31

Table 2. Ventilation volumes with corresponding VEVO2 data for the nine mimicking trials 38

Table 3. Plateau data reported as change in VO2 (L/min) from the three slope detection techniques on the three VO2 data sets 39

SYMBOLS / ABBREVIATIONS

VO2: rate of oxygen consumption.

VO2 max: maximal rate of oxygen consumption

VEVO2: rate of oxygen consumption by the respiratory muscles.

VEVO2I: oxygen consumption by the respiratory muscles as calculated by an individual’s hyperventilation data.

VEVO2C: oxygen consumption by the respiratory muscles as calculated by the VEVO2 prediction equation, which is based on all subjects’ VEVO2 data.

VE: expired ventilation.

VO2-VE: the difference between whole-body VO2 (wb-VO2) and VEVO2.

VO2-VEI: the difference between wb-VO2 and VEVO2I.

VO2-VEC: the difference between wb-VO2 and VEVO2C.

WB: the work of breathing

wb-VO2: whole-body rate of oxygen consumption.

CHAPTER I

INTRODUCTION

The measurement of maximal oxygen consumption (VO2max) is a fundamental test in exercise physiology. It is generally accepted as the best measure of the functional limit of the cardiovascular, pulmonary, and muscular systems, and is commonly used as an index of cardiorespiratory and muscular fitness (Howley, Bassett, & Welch, 1995). However, debate exists as to whether VO2max is limited by central cardiorespiratory function (maximal cardiac output, distribution of cardiac output, and arterial oxygen saturation), or by peripheral factors (oxygen extraction and utilization by muscles, mitochondrial density, and capillary density) or both (Howley, 1995; Noakes, 1997; Robergs, 2001; Wagner, 1996). The inconsistency of the presence of a plateau in VO2 as exercise reaches maximal intensities is a key issue in this debate. The presence of a plateau in VO2 provides evidence that there is an upper limit to the rate of oxygen consumption that exercising muscles can consume and thus the existence of VO2max (Robergs, 2001). Establishment of the plateau would then allow researchers to investigate its causes in order to better understand the limitations to VO2max.

The concepts of VO2max and VO2 plateau were introduced in the 1920s by the pioneering work of A. V. Hill and colleagues (Hill & Lupton, 1923). The VO2 plateau was described as the maintenance of VO2 despite an increase in oxygen demand and indicated that VO2max had been reached. Since then several investigations have examined the presence of the plateau phenomenon reporting conflicting results (Astorino, Robergs, Ghiavisand, Marks, & Burns, 2000; Duncan, Howley, & Johnson, 1997; Eldridge, Ramsey-Green, & Hossack, 1986; Mitchell, Sproule, & Chapman, 1958; Myers, Walsh, Buchanan, & Froelicher, 1989; Myers, Walsh, Sullivan, & Froelicher, 1990; Stamford, 1976; Taylor, Buskirk, & Henschel, 1955). According to Robergs (2001), the plateau phenomenon seems to be influenced by the test protocol, fitness and age of the subject, the criteria used to establish a VO2 plateau, and the data collection and analysis procedures used in the research. Despite the numerous studies examining the presence of a VO2 plateau no exact scientific criteria has been established to define it. Recently, Astorino et al. (2000) concluded that the most sensitive means for establishing the plateau is to use a VO2 sampling interval of 15 seconds or less, and to use a plateau criteria of an increase in VO2 equal to or less than 50 ml/min with increasing exercise intensity.

Over the years several models have been developed to identify the components of VO2max and to better explain the cardiovascular limitations to VO2max. The traditional Fick Model defines VO2 as the product of cardiac output and the arterial-venous oxygen difference. This model has only two components, oxygen delivery and oxygen extraction. Using this model assumes that the distribution of cardiac output does not change despite changing metabolic demands. It presents the exercising muscle as having homogenous metabolic demands, and is based on whole-body VO2 (wb-VO2) values. Newer models used for the determination of VO2 have included other potential factors that may limit VO2max, such as peripheral oxygen diffusion and hypoxemia (Robergs, 2001); however, they are also based on whole-body VO2 measurements.

The VO2 plateau is interpreted as an inability of the locomotor muscles to consume and utilize oxygen at an increasing rate despite increasing oxygen demand. When trying to detect a plateau in VO2, whole-body models cannot accurately represent the metabolic changes occurring in the locomotor muscles due to changes in the distribution of cardiac output during higher intensity exercise. As much as 85% of cardiac output can be devoted to the locomotor muscles during high intensity exercise (Harms, 2000), however, under circumstances where other muscle groups are activated a substantial portion of cardiac output can be redirected towards those muscles. This effect was demonstrated by Secher, Clausen, Klausen, Noer, and Trap-Jensen (1977), where the addition of arm exercise to steady-state leg exercise caused an increase in total cardiac output, and a decrease in blood flow to the legs by ~2 liters per minute. These findings indicate that during leg ergometry exercise the addition of work by other muscles can affect the blood flow, and therefore oxygen delivery, to the leg muscles. Using wb-VO2 models this effect would only be recognized as an increase in wb-VO2, while in actuality the VO2 of the leg muscles is not increasing at the measured rate because of the blood flow being devoted to the other muscles.

Research evidence exists that suggests the VO2 and cardiac output utilized by the respiratory muscles during high-intensity exercise may be large enough to occupy a significant portion of the whole-body VO2 (Anholm, Johnson, & Ramanathan, 1987; Harms, Babcock, McClaran, Pegelow, Nickele, Nelson, & Dempsey, 1997; Harms, Wetter, McClaran, Pegelow, Nickele, Nelson, Hanson, & Dempsey, 1998; Harms, Wetter, St. Croix, Pegelow, & Dempsey, 2000). What these researchers have demonstrated is that as exercise intensity increases so does the need to ventilate the lungs and thus the work of breathing (WB) increases. Because the distribution of cardiac output is generally proportional to the metabolic activity of the tissue, as the work of breathing increases so does the amount of blood and O2 devoted to the respiratory muscles (Harms, 2000).

A series of studies were conducted to determine whether the respiratory muscles compete with leg muscles for cardiac output during maximal exercise (Harms et al., 1997; Harms et al., 1998; Harms et al., 2000). These studies used respiratory muscle loading and unloading to increase and decrease the WB during maximal exercise, respectively. They found that respiratory muscle loading reduced leg blood flow, and leg VO2, whereas decreasing the work of the respiratory muscles during exercise had the opposite effect. Additionally, they concluded that 14-16% of total cardiac output is directed to the respiratory muscles to support their metabolic requirements under normal physiologic conditions during maximal exercise intensity. Their findings suggest that the respiratory muscles compete more effectively than limb muscles for total cardiac output during maximal exercise. They also imply that increases in wb-VO2 during high intensity exercise may be due to increases in respiratory muscle VO2 and not necessarily the VO2 of the locomotor muscles.

To quantitate the O2 cost of maximal exercise hyperpnea, Aaron, Seow, Johnson, and Dempsey (1992) had eight subjects mimic, at rest, the important mechanical components of submaximal (70%) and maximal exercise hyperpnea. They concluded that the oxygen cost of exercise hyperpnea is as high as 15% of wb-VO2 in highly trained subjects, and 8-12% in normally fit subjects during maximal exercise. These conclusions support the idea that using wb-VO2 to represent locomotor muscle VO2 may be inaccurate.

Aaron et al., (1992) investigated the relationships between VE, respiratory muscle VO2, and wb-VO2. They found that as exercise intensity increases from mild to heavy, VE rises out of proportion to wb-VO2. Another finding was that the oxygen cost per liter VE remained fairly constant between 60 and 110 liters per minute, averaging 1.5 – 2.0 mlO2/liter/minute of VE, but rose sharply to exceed 3.0 mlO2/liter/minute of VE as VE increased to above 140 liters per minute. This demonstrates that during incremental exercise the oxygen cost of ventilation increases in an exponential manor.

From the data presented above it can be concluded that the respiratory muscles demand a significant portion of cardiac output and O2 during high-intensity exercise and therefore wb-VO2 models do not accurately represent the VO2 of the locomotor muscles, especially when trying to detect the presence of a plateau in these muscles. A novel approach to address this issue would be to develop a VE-adjusted VO2. This could be done by subtracting the VEVO2 from the wb-VO2, which would result in a VO2 that would primarily represent the metabolic changes occurring in the locomotor muscles (VO2-VE). This VE-adjusted VO2 could then be examined for the presence of the plateau phenomenon.

Purpose of the Study

The purposes of this study were to: a) determine the oxygen cost of ventilation (VEVO2) across a range of VE representing an incremental exercise test to exhaustion, b) develop a regression equation to predict the VEVO2 based on the VE data collected, c) calculate a VO2-VE by subtracting the composite VEVO2 from the VO2 obtained during an incremental exercise test; d) calculate a VO2-VE by subtracting the individual VEVO2 from the whole-body VO2 obtained during an incremental exercise test and, e) objectively evaluate the VO2-VE data for the presence of a plateau in VO2. It was hypothesized that; the VEVO2 would increase in an exponential fashion as VE and exercise intensity increased; the presence of a VO2 plateau would be greater for VO2-VE compared to wb-VO2; and use of the VEVO2C prediction equation would provide similar data as the VEVO2I data.

Hypotheses

In this study the following hypotheses were tested:

Hypothesis 1: There will be a difference in the slopes of wb-VO2 and the VE-adjusted VO2 when evaluated during the final minute of exercise.

Rationale: Research evidence indicates that the relationship between increasing ventilation and the VEVO2 are exponential (Aaron et al., 1992). Therefore, subtracting the VEVO2 from wb-VO2 during maximal exercise will create a data set that is becoming smaller in an exponential fashion. Comparison of the wb-VO2 and the VO2-VE slopes will present changes in oxygen consumption that will be different.

Hypothesis 2: The presence of a plateau in VO2 during the final minute of exercise will be greater in the VE-adjusted VO2 slope when compared to the wb-VO2 slope.

Rationale: Based on the rationale for hypothesis 1, the adjustment for the VEVO2 to wb-VO2 will decrease the rate of increase in wb-VO2 and will therefore present a plateau in many subjects who did not attain one based on their wb-VO2 data.

Hypothesis 3: A prediction equation for VEVO2 using VE will be sufficiently accurate to use for all subjects in this study.

Rationale: Despite differences in fitness levels, body size, and lung capacity, the mechanical work required to ventilate a given volume of air does not differ considerably among individuals. Therefore, the VEVO2 across a range of VE will be similar across subjects.

Scope of the Study

Twenty-two moderately to highly trained, males and females, 18 to 39 years old, were recruited from the student body of the University of New Mexico and the surrounding community. Each participant gave informed consent and completed a health history questionnaire prior to testing. Participants were excluded based on age (> 39 years of age), history of disease that would significantly affect the results of the exercise tests, and unfamiliarity with cycle ergometry testing.

The variables measured in this study were VO2, VEVO2 and VE. Participants performed an incremental exercise tests to VO2max followed by ten measurements of the VEVO2 at rest. The VEVO2 measurements were made over one visit to the lab, which came at least 48 hours after the measurement of VO2max.

The measurement of expired gases for the determination of VO2 and VEVO2, were done breath-by-breath (K.L. Engineering, fast response turbine flow transducer Model S-430, Van Nuys, CA) using a metabolic gas analyzer system (AEI Technologies, Model S-3A and Model CD-3H, Pittsburg, PA) with data averaged across 7 breaths for subsequent analyses. Once the relationship between VEVO2 and VE were determined, the VEVO2 corresponding to each VE measured during the incremental exercise test was subtracted from the wb-VO2 obtained during the incremental exercise test to exhaustion. The resulting data was the VE-adjusted VO2 (VO2-VEI). This same procedure was performed using the VEVO2 prediction equation, which used all VEVO2 data across all subjects. The VEVO2 determined from this equation was also subtracted from the wb-VO2, and the resulting data (VO2-VEC) was also analyzed for the presence of the VO2 plateau.

The wb-VO2, VO2-VEI, and VO2-VEC data were analyzed by three techniques for the presence of a VO2 plateau. For all techniques, an increase in VO2 < 50 ml O2/min was considered a plateau as suggested by Astorino et al. (2000). Because no standard exists for how to analyze VO2 data for the presence of a plateau, two traditional techniques in addition to a new computer generated technique were employed. For all analyses, the breath-by-breath data was smoothed with a seven breath moving average. The first technique fit a linear regression equation to the final minute of VO2 data in all 3 VO2 data sets and used the slope identified by the equation to determine the magnitude of changes in VO2. The second technique used the exact same procedure, but only the final 30 seconds of data were analyzed. The third and final technique fit a third-order polynomial curve to the final 5 minutes of VO2 data (Prism® graphing software package, GraphPadTM Software, Inc. version 3.0, San Diego, CA), and then the final 30 seconds of the curve was analyzed for the presence of a plateau. This final approach was used for its ability to detect the multiple slope changes in the VO2 – Watts measurements. Additionally, the three VO2 data slopes within each technique were compared to each other to determine if there was a difference in their magnitude.

Limitations

This study was limited by the following conditions:

1. The measurement of VEVO2 at rest accurately reflected the VO2RM experienced during exercise.

Delimitations

This study was delimited by the following conditions:

1. Only individuals familiar with cycle ergometry and < 40 years old were used as subjects, and therefore the results may not be generalizable to individuals who are older and not familiar with cycle ergometry exercise.

2. The equipment used in this study was accurately calibrated.

Significance of the Study

The establishment of the VO2 plateau is an essential component in the determination of the limitations to VO2max. Investigations into the causes of the plateau will lead to a better understanding of what causes fatigue during incremental exercise to exhaustion. Debate still exists over the factors that determine VO2max as presented in reviews by Howley et al. (1995), Noakes (1998), Robergs (2001), and Wagner (1996). Much of this controversy stems from the inconsistency in the presence of a plateau in VO2max among subjects as exercise reaches maximal intensity. The establishment of the VO2 plateau, as proposed in this study, could allow researchers to then investigate the causes of the plateau itself in order to better understand the limitations to VO2max.

Definition of Terms

The terms in this study have been operationally defined as follows:

Locomotor muscle. The muscles used during exercise that provide work. During cycling exercise, the locomotor muscles are the leg muscles.

Oxygen cost of ventilation. The amount of oxygen consumption required to ventilate a given volume of air.

Respiratory muscle VO2. The oxygen consumption of the respiratory muscles. The respiratory muscles are not locomotor muscles, but their work increases as the need to increase ventilation increases.

VO2max. The maximal rate that an individual can consume and utilize oxygen during exercise.

VO2 plateau. The failure of oxygen consumption to increase despite increases in workload. An increase of < 50 mlO2/minute is the criteria used for identification of the plateau.

Whole-body VO2. Refers to VO2 values that represent the oxygen uptake of the entire body, as seen with indirect calorimetry.

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download