Articles in PresS. J Appl Physiol (April 30, 2015). doi:10.1152/japplphysiol.00092.2015
1
Females have a blunted cardiovascular response to 1-year of intensive supervised
2
endurance training
3 4
Erin J. Howden, PhD1,2, Merja Perhonen, MD1, PhD, Ronald M. Peshock, MD2, Rong
5
Zhang, PhD1,2, Armin Arbab-Zadeh, MD1, Beverley Adams-Huet2, MS, Benjamin D.
6
Levine1,2
7
1
Institute for Exercise and Environmental Medicine, Dallas, TX, USA
8
2
University of Texas Southwestern Medical Center, Dallas, TX, USA
9
Running title: Sex differences in response to endurance training
10
Keywords: cardiac magnetic resonance imaging; exercise training; gender
11
Address for Correspondence:
12
Benjamin D. Levine, MD
13
Institute for Exercise and Environmental Medicine
14
7232 Greenville Ave, Suite 435, Dallas, TX, 75321
15
Telephone (214) 345-4605
16
Fax (214) 345-4618
17
Email: [email protected]
18
Total word count: 7739
19
Abstract: 249
20 21 22 23 1 Copyright © 2015 by the American Physiological Society.
24
ABSTRACT
25
Cross-sectional studies in athletes suggest that endurance training augments
26
cardiovascular structure and function with apparently different phenotypes in athletic
27
males and females. It is unclear whether the longitudinal response to endurance training
28
leads to similar cardiovascular adaptations between sexes. We sought to determine
29
whether males and females demonstrate similar cardiovascular adaptations to 1-year of
30
endurance training, matched for training volume and intensity. Twelve previously
31
sedentary males (26±7, n=7) and females (31±6, n=5) completed 1-year of progressive
32
endurance training. All participants underwent a battery of tests every 3 months to
33
determine VO2 max and LV function and morphology (cardiac MRI). Pulmonary artery
34
catheterization was performed before and after 1-year of training, and pressure/volume
35
and Starling curves were constructed during decreases (lower body negative pressure)
36
and increases (saline infusion) in cardiac volume. Males progressively increased
37
VO2max, LV mass and mean wall thickness, before reaching a plateau from month 9 to
38
12 of training. In contrast, despite exactly the same training, the response in females was
39
markedly blunted, with VO2max, LV mass and mean wall thickness plateauing after only
40
3 months of training. The response of LV end-diastolic volume was not influenced by sex
41
(males +20% and females+18%). After training Starling curves were shifted upward and
42
left; but the effect was greatest in males (interaction P=0.06). We demonstrate for the
43
first time clear sex differences in response to 1-year of matched endurance training, such
44
that the development of ventricular hypertrophy and increase in VO2max in females is
45
markedly blunted compared to males.
46
2
47
Abbreviation list: LV, left ventricle; EDV, end diastolic volume; PCWP, pulmonary
48
capillary wedge pressure; Qc, cardiac output; QI, cardiac output indexed to body surface
49
area; TRIMP, training impulse; SV, stroke volume; SI, stroke volume indexed to body
50
surface area; HR, heart rate; MWT, mean wall thickness; RV, right ventricle, TMP,
51
transmural pressure; arterio-venous oxygen difference, a-VO2diff
52
3
53
Regular exercise training leads to changes in the structure and function of cardiac and
54
skeletal muscle, which is fundamentally influenced by the type of exercise performed
55
(endurance versus strength training) (36). Physiological skeletal muscle hypertrophy is
56
associated with strength training, whereas endurance training is a more potent stimulus to
57
induce physiological cardiac hypertrophy. Clear sex differences are observed between
58
strength trained athletes, such that males demonstrate a greater magnitude of skeletal
59
muscle mass when compared to females(1), primarily due to the profound anabolic effect
60
of testosterone on protein synthesis within the skeletal muscle(9).
61 62
It is less clear whether the cardiovascular response to endurance training is also
63
influenced by sex. Cross-sectional studies of endurance athletes suggest that females have
64
a lower maximal oxygen uptake (VO2 max) and cardiac output (Qc), and smaller left
65
ventricular (LV) dimensions and wall thickness when compared to similar male athletes
66
(12, 34, 40). Confounding the interpretation of sex differences is the effect that body size
67
has on cardiovascular anatomic and physiologic variables(11). Typically studies have
68
scaled cardiovascular variables to body size or lean body mass, which eliminates some,
69
but not all of the differences between sexes.
70 71
Cross-sectional studies are also limited by the different sports played by male and female
72
athletes and different training performed, which may alter the magnitude of the effect on
73
the cardiovascular adaptations observed. Longitudinal studies comparing the
74
cardiovascular response in males and females to a comparable endurance training
75
program is lacking.
4
76
We have recently demonstrated in previously sedentary young subjects that 1-year of
77
intensive endurance training increases cardiac mass to levels similar to elite endurance
78
athletes, results in favorable cardiac remodeling and improves LV compliance (3). In this
79
secondary analysis of the same study, we sought to determine whether males and females
80
demonstrate similar cardiovascular adaptations to 1-year of endurance training, matched
81
for training volume and intensity. We hypothesized that the response to training would
82
be similar in males and females, evidence by a similar change in VO2max, cardiac
83
morphology and function in response to training. To test this hypothesis we prescribed a
84
progressive training plan, identical between males and females, based on a strategy
85
derived from optimal training by competitive athletes; this study design allowed us to
86
collect time-dependent measures of metabolic and LV structure parameters in response to
87
1-year of training.
5
88
Methods
89
Subjects.
90
This study is a secondary analysis of 12 healthy previously sedentary males (n=7) and
91
females (n=5), who completed 1-year of intensive endurance training. The primary
92
findings from this study have been published recently.(3) One subject completed all the
93
initial testing but became pregnant in the second quarter of training and was excluded
94
from the study. Participants were not eligible for the study if they had engaged in any
95
regular endurance training defined as >30min/day more than three times per week. All
96
participants were carefully screened and underwent a physical examination, ECG and
97
echocardiogram. None of the subjects smoked, used recreational drugs, or had significant
98
chronic medical problems. All female subjects were eumenorrheic when they
99
commenced training and remained so for the duration of the study.
100 101
Ethical approval.
102
All subjects signed an informed consent, which was approved by the Institutional Review
103
Boards of the University of Texas Southwestern Medical Center at Dallas and Texas
104
Health Resources Presbyterian Hospital of Dallas.
105 106
Exercise Training.
107
Endurance training was designed to enable subjects to complete a marathon at the end of
108
the 1-year period. All subjects performed an incremental treadmill test for determination
109
of VO2max, at baseline and every 3 months to document exercise performance. A
110
detailed description of the exercise intervention has been previously described (3, 18).
6
111
Training zones for optimal training prescription were individualized by heart rate (HR)
112
response to exercise. “Maximal steady-state” was first estimated from the ventilatory and
113
lactate threshold according to standard criteria(18). The HR at maximal steady-state and
114
max HR were then used to calculate training zones for each subject; five training zones
115
were determined for individualized training prescription: zone 1 = recovery, zone 2= base
116
pace, zone 3= maximal steady state or “threshold”, zone 4= race pace/critical power, and
117
zone 5= intervals. During the early phase of the training program, the subjects trained 3-
118
4x/week for 30-45 minutes/session at base pace by brisk walking, slow jogging,
119
swimming or cycling. As subjects became fitter, the duration of the base training sessions
120
were prolonged, including the addition of one long run per week, which was performed at
121
the lower end of base pace heart rate range. In addition, during the second and third
122
quarters of the training program, sessions of increased intensity (maximal steady state
123
and interval sessions) were added first once, then twice and occasionally 3x/week.
124
Interval sessions were followed the next day by recovery session to maximize
125
performance gains. By the end of the year-long training program, subjects were
126
exercising for 7-9 hours/week, including long runs of up to 3 hours, plus regular interval
127
sessions on the track and races. The purpose of this template was to maximize training
128
efficiency and to provide a periodization of the training program. This strategy of varying
129
intensity and duration of training session within any given week (microcycles) applying
130
periods of increasing stress followed by recover from month to month (mesocycle) with
131
an ultimate goal of a specific competition (macrocycle) is a classic training routine used
132
by competitive athletes, and is widely considered the optimal approach to training(37).
133
Although the majority of subjects chose to complete a marathon at completion of the
7
134
intervention, one female subject completed a 100 mile endurance and one male subject
135
completed a Olympic distance triathlon. We followed the principles of training
136
specificity; thus the subjects completing the marathon performed predominantly running
137
and the cyclist cycled, while the triathlete performed a combination of swimming, cycling
138
and running.
139 140
Heart rate was recorded during each training session using a heart rate monitor provided
141
to each participant. This approach was an important aspect of the study and gave us the
142
ability to match and quantify training load throughout the intervention. To quantify the
143
training stimulus, we calculated the training impulse (“TRIMP”)(5). This method
144
multiplies the duration of a training session by the average heart rate achieved during that
145
session, weighted for exercise intensity as a function of heart rate reserve. Using this
146
method, exercise sessions of longer duration and/or greater intensity such as interval
147
workouts, are assigned relatively higher TRIMP values (higher heart rate, and higher
148
weighting factor), than sessions of shorter duration and/or lower intensity. The average
149
TRIMP for the male and female subjects is presented in Figure 1. Note the oscillations in
150
TRIMP score throughout the training period, i.e. micro- and macrocycles of training in
151
Figure 1 and the remarkably similar training load between sexes.
152 153
Study protocol
154
All measurements were performed before training (baseline), and repeated after 3, 6, 9
155
and 12 months of training. Pressure/volume studies were completed pre- and post-
156
training.
8
157
Exercise testing.
158
A modified Astrand-Saltin incremental treadmill protocol (individualized treadmill speed,
159
with changes in grade of 2% every 2 minutes) was used to determine maximal exercise
160
capacity. Measures of ventilatory gas exchange were made by use of the Douglas bag
161
technique. Gas fractions were analyzed by mass spectrometry (Marquette MGA 1100),
162
and ventilatory volumes was measured by a dry-gas meter (Collins). Maximum oxygen
163
uptake (VO2max) was defined as the highest oxygen uptake measured from ≥40 s
164
Douglas bag. In nearly all cases, a plateau in VO2 was observed with increasing work rate,
165
confirming the achievement of VO2 max. Cardiac output (Qc) was measured with the
166
acetylene rebreathing method, which has been previously validated in this laboratory (19).
167
Heart rate was monitored continuously via ECG (Polar). Systemic arterio-venous oxygen
168
difference (a-VO2diff) was calculated from the Fick equation (a-VO2diff = VO2/Qc),
169
while stroke volume was calculated as Qc/HR. Stroke volume and Qc were scaled
170
relative to baseline body surface area (stroke index [SI] and cardiac index [QI]) by
171
clinical convention to reduce the confounding effect of body size and composition (11).
172 173
Body composition.
174
Body density and composition were determined by underwater weighing with correction
175
for residual lung volume(41). Each participant performed at least three adequate
176
measurements defined as a definite plateau in underwater weight, and the mean value was
177
calculated.
178 179
9
180
Plasma, blood volume and hematocrit
181
At baseline and at each quarter testing session, all subjects underwent measurement of
182
plasma volume using Evans Blue dye indicator dilution technique. The methods from this
183
technique have been published(14). Briefly individuals rested in the supine position for at
184
least 30 minutes, after which a known quantity of Evan blue dye was injected through a
185
peripheral intravenous catheter, and venous blood was drawn at 10, 20, and 30 min after
186
injection for the measurement of absorbance at 620 and 740 nm via spectrophotometry
187
(DU 600, Beckman). Hematocrit was measured via microcapillary centrifuge, and blood
188
volume was estimated by dividing plasma volume by 1-hematocrit using appropriate
189
corrections for trapped plasma and peripheral sampling. To reduce the confounding effect
190
of body size and composition on blood volume (11), absolute values were scaled relative
191
to total body mass (ml/kg) and FFM (ml/kgFFM).
192 193
Cardiac MRI.
194
Cardiac morphometric parameters were assessed by cMRI 1.5 tesla Phillips NT MRI
195
Scanner (Phillips Medical Systems, Best, The Netherlands). Short-axis, gradient echo,
196
cine MRI sequences with a temporal resolution of 39 ms were obtained to calculate
197
ventricular volumes as previously described(17). Ventricular mass was computed as the
198
difference between epicardial and endocardial areas multiplied by the density of heart
199
muscle, 1.05 g/ml(21). The Simpson’s rule technique was used to measure LV mass,
200
which has been demonstrated to be accurate and reproducible in our lab(21) and by
201
others(15).
202
10
203
For LV volumes, the endocardial border of each slice was identified manually at end-
204
diastole and end-systole and volumes were counted by summation(31). Mean wall
205
thickness (MWT) for the entire LV included the papillary muscle was calculated as
206
previously described(30). For each short-axis slice the epicardial area (LV chamber plus
207
myocardial wall) and endocardial area (chamber area) were determined using software on
208
the imaging device. The “average” radius for each area was calculated by approximating
209
the cross-section as a circle and using the equation for the area of a circle (Area=πr2). To
210
reduce the confounding effects of body size and lean mass on LV and RV mass, measures
211
were scaled to fat free mass (7).
212 213
Cardiac Catheterization and Experimental Protocol.
214
Cardiac catheterization was performed at baseline and after the completion of the training
215
period. Participants were studied in the resting supine position. A 6 French balloon-tipped
216
fluid-filled catheter (Swan-Ganz, Baxter) was placed using fluoroscopic guidance
217
through an antecubitital vein into the pulmonary artery. All intracardiac pressures were
218
referenced to atmospheric pressure with the pressure transducer (Transpac IV, Abbott)
219
zero reading set at 5 cm below the sternal angle. The wedge position of the catheter tip
220
was confirmed using fluoroscopy, as well as the presence of an appropriate PCWP
221
waveform. The mean PCWP was determined visually at end expiration and was used as
222
an estimation of LV end-diastolic pressure.
223 224
Qc was measured and SV, calculated from Qc/HR measured during rebreathing. Left
225
ventricular end-diastolic volume (LVEDV) was measured with two-dimensional
11
226
echocardiography using standard views and formulas as described by the American
227
Society of Echocardiography(27). Images were obtained with an annular phased-array
228
transducer using a frequency of 2.5 to 3.5 MHZ (Interspec Apogee CX) and stored on
229
VCR tapes for off-line analysis by a skilled technician. For calculation of LVEDV for
230
each subject, either modified Simpson’s rule method, the area length method, or the
231
bullet model (cylinder hemiellipsoid) was chosen on the basis of which views provided
232
the most optimal endocardial definition(39). The same formula was used for each
233
individual subject throughout the study.
234 235
Testing protocol. Resting supine measures (baseline 1) were collected after ensuring
236
hemodynamic stability (~20 min of quiet rest), and then cardiac filling was decreased
237
using lower body negative pressure (LBNP) as previously described(26). Measurements
238
of PCWP, Qc (and therefore SV), LVEDV, heart rate, and blood pressure were made
239
after 5 minutes each of -15 mmHg and -30 mmHg LBNP. The LBNP was then released.
240
After repeat baseline measurements to confirm a return to hemodynamic steady state
241
(usually 20-30 min), cardiac filling was increased by rapid (200 ml/min) infusion of
242
warm (37ºC), isotonic saline. Measurements were repeated after 15 ml/kg and 30 ml/kg
243
had been infused.
244 245
Data were used to construct Frank-Starling (SV/PCWP) and pressure/volume
246
(PCWP/LVEDV) curves. For the purpose of the present study, we characterized two
247
explicitly different but related mechanical properties of the heart during diastole: a) static
248
stiffness or overall chamber stiffness referred to as the stiffness constant S, of the
12
249
logarithmic equation describing the pressure/volume curve (see below); and b)
250
distensibility, which is used to mean the absolute LVEDV at a given distending pressure,
251
independent of dP/dV, or S.
252
To characterize the LV pressure/volume relation, we modeled the data in the present
253
experiment according to the equation described by Nikolic et al.,(28) : P= -S ln[(Vmax-V)/Vm-V0)]
254 255
Where P= PCWP, V=LVEDV, V0 is equilibrium volume or the volume at which P=0,
256
Vmax is the maximal volume obtained by this chamber, and S is a stiffness constant that
257
describes the shape of the curve. In addition, pressure/volume curves were also calculated
258
using the difference between PCWP and RA pressure (PCWP-RA) as an index of
259
transmural filling pressure(8), in order to assess the contribution of pericardial constraint.
260 261
Statistics.
262
Continuous data are expressed as mean ± SD except for figures, in which SEM is used.
263
Baseline sex differences were compared with Student’s t-test. Continuous variables
264
measured over the 12 month study duration were analyzed longitudinally using linear
265
mixed effects model repeated measures analysis. The repeated measures model had five
266
repeated measurements (time points at baseline, 3, 6, 9, and 12 months) and the study
267
participant was modeled as a random effect. The covariance structure was selected based
268
on Akaike’s Information Criteria and model parsimony. Our analysis was unadjusted and
269
the analysis was by available data (last observation was not varied forward). To test our
270
primary hypothesis whether previously sedentary healthy young males and females differ
271
in response to 1-year of endurance training, the sex × time interaction was used, with a P
13
272
value of
1
Females have a blunted cardiovascular response to 1-year of intensive supervised
2
endurance training
3 4
Erin J. Howden, PhD1,2, Merja Perhonen, MD1, PhD, Ronald M. Peshock, MD2, Rong
5
Zhang, PhD1,2, Armin Arbab-Zadeh, MD1, Beverley Adams-Huet2, MS, Benjamin D.
6
Levine1,2
7
1
Institute for Exercise and Environmental Medicine, Dallas, TX, USA
8
2
University of Texas Southwestern Medical Center, Dallas, TX, USA
9
Running title: Sex differences in response to endurance training
10
Keywords: cardiac magnetic resonance imaging; exercise training; gender
11
Address for Correspondence:
12
Benjamin D. Levine, MD
13
Institute for Exercise and Environmental Medicine
14
7232 Greenville Ave, Suite 435, Dallas, TX, 75321
15
Telephone (214) 345-4605
16
Fax (214) 345-4618
17
Email: [email protected]
18
Total word count: 7739
19
Abstract: 249
20 21 22 23 1 Copyright © 2015 by the American Physiological Society.
24
ABSTRACT
25
Cross-sectional studies in athletes suggest that endurance training augments
26
cardiovascular structure and function with apparently different phenotypes in athletic
27
males and females. It is unclear whether the longitudinal response to endurance training
28
leads to similar cardiovascular adaptations between sexes. We sought to determine
29
whether males and females demonstrate similar cardiovascular adaptations to 1-year of
30
endurance training, matched for training volume and intensity. Twelve previously
31
sedentary males (26±7, n=7) and females (31±6, n=5) completed 1-year of progressive
32
endurance training. All participants underwent a battery of tests every 3 months to
33
determine VO2 max and LV function and morphology (cardiac MRI). Pulmonary artery
34
catheterization was performed before and after 1-year of training, and pressure/volume
35
and Starling curves were constructed during decreases (lower body negative pressure)
36
and increases (saline infusion) in cardiac volume. Males progressively increased
37
VO2max, LV mass and mean wall thickness, before reaching a plateau from month 9 to
38
12 of training. In contrast, despite exactly the same training, the response in females was
39
markedly blunted, with VO2max, LV mass and mean wall thickness plateauing after only
40
3 months of training. The response of LV end-diastolic volume was not influenced by sex
41
(males +20% and females+18%). After training Starling curves were shifted upward and
42
left; but the effect was greatest in males (interaction P=0.06). We demonstrate for the
43
first time clear sex differences in response to 1-year of matched endurance training, such
44
that the development of ventricular hypertrophy and increase in VO2max in females is
45
markedly blunted compared to males.
46
2
47
Abbreviation list: LV, left ventricle; EDV, end diastolic volume; PCWP, pulmonary
48
capillary wedge pressure; Qc, cardiac output; QI, cardiac output indexed to body surface
49
area; TRIMP, training impulse; SV, stroke volume; SI, stroke volume indexed to body
50
surface area; HR, heart rate; MWT, mean wall thickness; RV, right ventricle, TMP,
51
transmural pressure; arterio-venous oxygen difference, a-VO2diff
52
3
53
Regular exercise training leads to changes in the structure and function of cardiac and
54
skeletal muscle, which is fundamentally influenced by the type of exercise performed
55
(endurance versus strength training) (36). Physiological skeletal muscle hypertrophy is
56
associated with strength training, whereas endurance training is a more potent stimulus to
57
induce physiological cardiac hypertrophy. Clear sex differences are observed between
58
strength trained athletes, such that males demonstrate a greater magnitude of skeletal
59
muscle mass when compared to females(1), primarily due to the profound anabolic effect
60
of testosterone on protein synthesis within the skeletal muscle(9).
61 62
It is less clear whether the cardiovascular response to endurance training is also
63
influenced by sex. Cross-sectional studies of endurance athletes suggest that females have
64
a lower maximal oxygen uptake (VO2 max) and cardiac output (Qc), and smaller left
65
ventricular (LV) dimensions and wall thickness when compared to similar male athletes
66
(12, 34, 40). Confounding the interpretation of sex differences is the effect that body size
67
has on cardiovascular anatomic and physiologic variables(11). Typically studies have
68
scaled cardiovascular variables to body size or lean body mass, which eliminates some,
69
but not all of the differences between sexes.
70 71
Cross-sectional studies are also limited by the different sports played by male and female
72
athletes and different training performed, which may alter the magnitude of the effect on
73
the cardiovascular adaptations observed. Longitudinal studies comparing the
74
cardiovascular response in males and females to a comparable endurance training
75
program is lacking.
4
76
We have recently demonstrated in previously sedentary young subjects that 1-year of
77
intensive endurance training increases cardiac mass to levels similar to elite endurance
78
athletes, results in favorable cardiac remodeling and improves LV compliance (3). In this
79
secondary analysis of the same study, we sought to determine whether males and females
80
demonstrate similar cardiovascular adaptations to 1-year of endurance training, matched
81
for training volume and intensity. We hypothesized that the response to training would
82
be similar in males and females, evidence by a similar change in VO2max, cardiac
83
morphology and function in response to training. To test this hypothesis we prescribed a
84
progressive training plan, identical between males and females, based on a strategy
85
derived from optimal training by competitive athletes; this study design allowed us to
86
collect time-dependent measures of metabolic and LV structure parameters in response to
87
1-year of training.
5
88
Methods
89
Subjects.
90
This study is a secondary analysis of 12 healthy previously sedentary males (n=7) and
91
females (n=5), who completed 1-year of intensive endurance training. The primary
92
findings from this study have been published recently.(3) One subject completed all the
93
initial testing but became pregnant in the second quarter of training and was excluded
94
from the study. Participants were not eligible for the study if they had engaged in any
95
regular endurance training defined as >30min/day more than three times per week. All
96
participants were carefully screened and underwent a physical examination, ECG and
97
echocardiogram. None of the subjects smoked, used recreational drugs, or had significant
98
chronic medical problems. All female subjects were eumenorrheic when they
99
commenced training and remained so for the duration of the study.
100 101
Ethical approval.
102
All subjects signed an informed consent, which was approved by the Institutional Review
103
Boards of the University of Texas Southwestern Medical Center at Dallas and Texas
104
Health Resources Presbyterian Hospital of Dallas.
105 106
Exercise Training.
107
Endurance training was designed to enable subjects to complete a marathon at the end of
108
the 1-year period. All subjects performed an incremental treadmill test for determination
109
of VO2max, at baseline and every 3 months to document exercise performance. A
110
detailed description of the exercise intervention has been previously described (3, 18).
6
111
Training zones for optimal training prescription were individualized by heart rate (HR)
112
response to exercise. “Maximal steady-state” was first estimated from the ventilatory and
113
lactate threshold according to standard criteria(18). The HR at maximal steady-state and
114
max HR were then used to calculate training zones for each subject; five training zones
115
were determined for individualized training prescription: zone 1 = recovery, zone 2= base
116
pace, zone 3= maximal steady state or “threshold”, zone 4= race pace/critical power, and
117
zone 5= intervals. During the early phase of the training program, the subjects trained 3-
118
4x/week for 30-45 minutes/session at base pace by brisk walking, slow jogging,
119
swimming or cycling. As subjects became fitter, the duration of the base training sessions
120
were prolonged, including the addition of one long run per week, which was performed at
121
the lower end of base pace heart rate range. In addition, during the second and third
122
quarters of the training program, sessions of increased intensity (maximal steady state
123
and interval sessions) were added first once, then twice and occasionally 3x/week.
124
Interval sessions were followed the next day by recovery session to maximize
125
performance gains. By the end of the year-long training program, subjects were
126
exercising for 7-9 hours/week, including long runs of up to 3 hours, plus regular interval
127
sessions on the track and races. The purpose of this template was to maximize training
128
efficiency and to provide a periodization of the training program. This strategy of varying
129
intensity and duration of training session within any given week (microcycles) applying
130
periods of increasing stress followed by recover from month to month (mesocycle) with
131
an ultimate goal of a specific competition (macrocycle) is a classic training routine used
132
by competitive athletes, and is widely considered the optimal approach to training(37).
133
Although the majority of subjects chose to complete a marathon at completion of the
7
134
intervention, one female subject completed a 100 mile endurance and one male subject
135
completed a Olympic distance triathlon. We followed the principles of training
136
specificity; thus the subjects completing the marathon performed predominantly running
137
and the cyclist cycled, while the triathlete performed a combination of swimming, cycling
138
and running.
139 140
Heart rate was recorded during each training session using a heart rate monitor provided
141
to each participant. This approach was an important aspect of the study and gave us the
142
ability to match and quantify training load throughout the intervention. To quantify the
143
training stimulus, we calculated the training impulse (“TRIMP”)(5). This method
144
multiplies the duration of a training session by the average heart rate achieved during that
145
session, weighted for exercise intensity as a function of heart rate reserve. Using this
146
method, exercise sessions of longer duration and/or greater intensity such as interval
147
workouts, are assigned relatively higher TRIMP values (higher heart rate, and higher
148
weighting factor), than sessions of shorter duration and/or lower intensity. The average
149
TRIMP for the male and female subjects is presented in Figure 1. Note the oscillations in
150
TRIMP score throughout the training period, i.e. micro- and macrocycles of training in
151
Figure 1 and the remarkably similar training load between sexes.
152 153
Study protocol
154
All measurements were performed before training (baseline), and repeated after 3, 6, 9
155
and 12 months of training. Pressure/volume studies were completed pre- and post-
156
training.
8
157
Exercise testing.
158
A modified Astrand-Saltin incremental treadmill protocol (individualized treadmill speed,
159
with changes in grade of 2% every 2 minutes) was used to determine maximal exercise
160
capacity. Measures of ventilatory gas exchange were made by use of the Douglas bag
161
technique. Gas fractions were analyzed by mass spectrometry (Marquette MGA 1100),
162
and ventilatory volumes was measured by a dry-gas meter (Collins). Maximum oxygen
163
uptake (VO2max) was defined as the highest oxygen uptake measured from ≥40 s
164
Douglas bag. In nearly all cases, a plateau in VO2 was observed with increasing work rate,
165
confirming the achievement of VO2 max. Cardiac output (Qc) was measured with the
166
acetylene rebreathing method, which has been previously validated in this laboratory (19).
167
Heart rate was monitored continuously via ECG (Polar). Systemic arterio-venous oxygen
168
difference (a-VO2diff) was calculated from the Fick equation (a-VO2diff = VO2/Qc),
169
while stroke volume was calculated as Qc/HR. Stroke volume and Qc were scaled
170
relative to baseline body surface area (stroke index [SI] and cardiac index [QI]) by
171
clinical convention to reduce the confounding effect of body size and composition (11).
172 173
Body composition.
174
Body density and composition were determined by underwater weighing with correction
175
for residual lung volume(41). Each participant performed at least three adequate
176
measurements defined as a definite plateau in underwater weight, and the mean value was
177
calculated.
178 179
9
180
Plasma, blood volume and hematocrit
181
At baseline and at each quarter testing session, all subjects underwent measurement of
182
plasma volume using Evans Blue dye indicator dilution technique. The methods from this
183
technique have been published(14). Briefly individuals rested in the supine position for at
184
least 30 minutes, after which a known quantity of Evan blue dye was injected through a
185
peripheral intravenous catheter, and venous blood was drawn at 10, 20, and 30 min after
186
injection for the measurement of absorbance at 620 and 740 nm via spectrophotometry
187
(DU 600, Beckman). Hematocrit was measured via microcapillary centrifuge, and blood
188
volume was estimated by dividing plasma volume by 1-hematocrit using appropriate
189
corrections for trapped plasma and peripheral sampling. To reduce the confounding effect
190
of body size and composition on blood volume (11), absolute values were scaled relative
191
to total body mass (ml/kg) and FFM (ml/kgFFM).
192 193
Cardiac MRI.
194
Cardiac morphometric parameters were assessed by cMRI 1.5 tesla Phillips NT MRI
195
Scanner (Phillips Medical Systems, Best, The Netherlands). Short-axis, gradient echo,
196
cine MRI sequences with a temporal resolution of 39 ms were obtained to calculate
197
ventricular volumes as previously described(17). Ventricular mass was computed as the
198
difference between epicardial and endocardial areas multiplied by the density of heart
199
muscle, 1.05 g/ml(21). The Simpson’s rule technique was used to measure LV mass,
200
which has been demonstrated to be accurate and reproducible in our lab(21) and by
201
others(15).
202
10
203
For LV volumes, the endocardial border of each slice was identified manually at end-
204
diastole and end-systole and volumes were counted by summation(31). Mean wall
205
thickness (MWT) for the entire LV included the papillary muscle was calculated as
206
previously described(30). For each short-axis slice the epicardial area (LV chamber plus
207
myocardial wall) and endocardial area (chamber area) were determined using software on
208
the imaging device. The “average” radius for each area was calculated by approximating
209
the cross-section as a circle and using the equation for the area of a circle (Area=πr2). To
210
reduce the confounding effects of body size and lean mass on LV and RV mass, measures
211
were scaled to fat free mass (7).
212 213
Cardiac Catheterization and Experimental Protocol.
214
Cardiac catheterization was performed at baseline and after the completion of the training
215
period. Participants were studied in the resting supine position. A 6 French balloon-tipped
216
fluid-filled catheter (Swan-Ganz, Baxter) was placed using fluoroscopic guidance
217
through an antecubitital vein into the pulmonary artery. All intracardiac pressures were
218
referenced to atmospheric pressure with the pressure transducer (Transpac IV, Abbott)
219
zero reading set at 5 cm below the sternal angle. The wedge position of the catheter tip
220
was confirmed using fluoroscopy, as well as the presence of an appropriate PCWP
221
waveform. The mean PCWP was determined visually at end expiration and was used as
222
an estimation of LV end-diastolic pressure.
223 224
Qc was measured and SV, calculated from Qc/HR measured during rebreathing. Left
225
ventricular end-diastolic volume (LVEDV) was measured with two-dimensional
11
226
echocardiography using standard views and formulas as described by the American
227
Society of Echocardiography(27). Images were obtained with an annular phased-array
228
transducer using a frequency of 2.5 to 3.5 MHZ (Interspec Apogee CX) and stored on
229
VCR tapes for off-line analysis by a skilled technician. For calculation of LVEDV for
230
each subject, either modified Simpson’s rule method, the area length method, or the
231
bullet model (cylinder hemiellipsoid) was chosen on the basis of which views provided
232
the most optimal endocardial definition(39). The same formula was used for each
233
individual subject throughout the study.
234 235
Testing protocol. Resting supine measures (baseline 1) were collected after ensuring
236
hemodynamic stability (~20 min of quiet rest), and then cardiac filling was decreased
237
using lower body negative pressure (LBNP) as previously described(26). Measurements
238
of PCWP, Qc (and therefore SV), LVEDV, heart rate, and blood pressure were made
239
after 5 minutes each of -15 mmHg and -30 mmHg LBNP. The LBNP was then released.
240
After repeat baseline measurements to confirm a return to hemodynamic steady state
241
(usually 20-30 min), cardiac filling was increased by rapid (200 ml/min) infusion of
242
warm (37ºC), isotonic saline. Measurements were repeated after 15 ml/kg and 30 ml/kg
243
had been infused.
244 245
Data were used to construct Frank-Starling (SV/PCWP) and pressure/volume
246
(PCWP/LVEDV) curves. For the purpose of the present study, we characterized two
247
explicitly different but related mechanical properties of the heart during diastole: a) static
248
stiffness or overall chamber stiffness referred to as the stiffness constant S, of the
12
249
logarithmic equation describing the pressure/volume curve (see below); and b)
250
distensibility, which is used to mean the absolute LVEDV at a given distending pressure,
251
independent of dP/dV, or S.
252
To characterize the LV pressure/volume relation, we modeled the data in the present
253
experiment according to the equation described by Nikolic et al.,(28) : P= -S ln[(Vmax-V)/Vm-V0)]
254 255
Where P= PCWP, V=LVEDV, V0 is equilibrium volume or the volume at which P=0,
256
Vmax is the maximal volume obtained by this chamber, and S is a stiffness constant that
257
describes the shape of the curve. In addition, pressure/volume curves were also calculated
258
using the difference between PCWP and RA pressure (PCWP-RA) as an index of
259
transmural filling pressure(8), in order to assess the contribution of pericardial constraint.
260 261
Statistics.
262
Continuous data are expressed as mean ± SD except for figures, in which SEM is used.
263
Baseline sex differences were compared with Student’s t-test. Continuous variables
264
measured over the 12 month study duration were analyzed longitudinally using linear
265
mixed effects model repeated measures analysis. The repeated measures model had five
266
repeated measurements (time points at baseline, 3, 6, 9, and 12 months) and the study
267
participant was modeled as a random effect. The covariance structure was selected based
268
on Akaike’s Information Criteria and model parsimony. Our analysis was unadjusted and
269
the analysis was by available data (last observation was not varied forward). To test our
270
primary hypothesis whether previously sedentary healthy young males and females differ
271
in response to 1-year of endurance training, the sex × time interaction was used, with a P
13
272
value of
