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
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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.
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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.
vi
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
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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
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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 wholebody 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
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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-bybreath VO2 data within box. B) 30-second technique defined by linear
regression equation fit to breath-by-breath VO2 data within box. C) 5minute 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
x
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
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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.
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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,
13
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
14
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
15
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.
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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
17
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 wbVO2; 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.
18
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 S430, Van Nuys, CA) using a metabolic gas analyzer system (AEI Technologies, Model S3A 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
19
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.
20
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
21
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.
22