Keywords
marathon - exercise-induced hypertension - carotid intima-media thickness
Introduction
Exercise-induced hypertension (EIH), an exaggerated systolic blood pressure (SBP)
response to exercise, is a predictor of future hypertension [1]. It is also associated with an increased risk of
stroke [2] and cardiovascular mortality [3]. Although regular aerobic exercise reduces the risk
of cardiovascular diseases (CVD), high-intensity exercises such as marathons can
cause excessive strain on the cardiovascular system. Interestingly, a recent study
showed that EIH may be associated with a 3.6-fold increased risk of hypertension in
highly trained athletes [4]. In fact, long-distance
runners with EIH have an increased risk of cardiovascular events, including the
increased incidence of coronary artery plaque [5],
elevated markers of myocardial damage [6]
[7], and increased arterial stiffness [8], compared to runners with normal blood pressure
(BP). Witham and Babbitt [9] highlight the increased
risk of cardiac events in long-distance runners and emphasize the importance of
healthcare professionals screening and educating runners on cardiac risk factors.
Notably, repeated increases in BP can cause structural changes in the blood vessels,
such as increased carotid artery intima-media thickness (IMT). Carotid artery IMT is
an important subclinical marker that can monitor the process of vascular damage
before the onset of clinical CVD, and it has high clinical utility in terms of
prevention because it can be measured non-invasively using ultrasound [10]. Furthermore, it is an independent indicator of
increased risk for cerebrovascular disease, even after accounting for other
traditional CVD risk factors [11]
[12]. Carotid IMT increases with risk factors for CVD,
such as age, hypertension, dyslipidemia, and obesity [13]. However, BP has the greatest impact on carotid IMT [14]. In contrast, it has been suggested that exercise
training and fitness can decrease carotid IMT [15]
[16]. However, most of these studies
have been conducted on general adult populations, and there is a lack of research
examining carotid IMT in long-distance runners. Gori et al. [17] suggested that prolonged, high-intensity exercise
can induce structural changes in athletes’ blood vessels, similar to athletes’
hearts; however, insufficient evidence supports this. The underlying factors and
clinical significance of these findings have been hotly debated. A study comparing
trained runners to sedentary controls found that there was no significant difference
in cIMT between the two groups [18]. Pressler et al.
[19] found that running multiple marathons did
not pose an additional risk factor for vascular impairment beyond age. However,
previous studies have shown that an increase in carotid IMT is related to a decrease
in endothelial function and compliance, which are the main factors causing increased
BP during exercise [20]. Long-distance runners with
EIH may be at higher risk, if the carotid artery is adversely affected. However,
there is insufficient evidence to support this. Therefore, this study aimed to
verify the relationship between EIH and carotid IMT in long-distance runners.
Materials and Methods
Participants
This study included 60 male runners aged 40 to 60 years with a minimum of four
years of marathon experience and participation in at least five full-course
marathons. The participants were classified into three groups based on resting
BP and maximum SBP at maximal exercise test as follows: normal BP response group
(NBPG), exercise-induced hypertension group (EIHG), and complex hypertension
group (CHG). EIH was defined as a resting SBP/DBP less than 140/90 mmHg and a
maximal SBP of 210 mmHg or higher during the maximal exercise test. Complex
hypertension was defined as a resting SBP/ DBP of 140/90 mmHg or higher and a
maximal SBP of 210 mmHg or higher during exercise. The study design is presented
in [Fig. 1]. This study was approved by the
Institutional Review Board (IRB NO: SSWUIRB 2019–017).
Fig. 1 Flow chart of the study procedure. GXT: graded exercise
testing, NBPG: normal blood pressure group, EIHG: exercise-induced
hypertension group, CHG: complex hypertension group.
Resting BP
Resting BP was measured with an automatic sphygmomanometer (Home 3MX1–1; WatchBP,
Taipei, Taiwan) after a 10-minute rest period. BP was measured twice at 3-minute
intervals. The mean value of the two measurements was used.
Carotid artery IMT measurement
Carotid artery IMT was determined using a high-resolution B-mode ultrasound
system (ACUSON X300 ultrasound imaging system, Siemens, Mountain View, CA, USA)
with an 11.4-MHz linear probe, following the guidelines of the Mannheim IMT
consensus [21]. With the subjects in a supine
position and their necks rotated to the left, longitudinal images of the common
carotid artery were acquired 10 mm below the carotid bulb to determine the
carotid artery IMT. IMT was defined as the distance between the leading edge of
the lumen-intima interface and that of the media-adventitia interface of the far
wall of the carotid artery. The mean value of thickness was defined as
IMTmean, which was automatically measured. All measurements were
taken at the end of the diastole.
Maximal exercise test
Maximal exercise test was performed using the Bruce protocol on a treadmill
(Quinton Cardiology Systems Inc., Bothell, WA, USA). Maximal oxygen uptake
(VO2max) was measured using a portable metabolic system (TrueOne 2400; Parvo
Medics, Murray, UT, USA), and breath-by-breath data were averaged over
15 seconds. Exercise BP was measured during the last minute of each 3-minute
stage using an automatic BP device (Tango, sunTECH, Wuxi, China). An integrated
headset was used by a trained researcher to reduce measurement errors and ensure
the correct identification of Korotkoff sounds during blood pressure
measurement. The maximal SBP was defined as the highest value measured during
the test. The criteria for termination and maximal effort of exercise tests
followed the guidelines of the American College of Sports Medicine [22].
Statistical analysis
Data are presented as mean±standard deviation, median
(25th–75th percentile), or number (%). Characteristics
of the three groups (NBPG, EIHG, and CHG) were compared using one-way ANOVA and
the Kruskal-Wallis test for continuous variables. Post-hoc analyses were
performed using the LSD and Mann-Whitney U test, if there was a significant
difference between groups. Furthermore, Pearson correlation analysis and
multiple linear regression analysis were conducted to analyze the association
between carotid IMT and exercise-induced hypertension. All data were analyzed
using SPSS version 26.0 (IBM Corp., Armonk, NY, USA), and statistical
significance was set at p+<+0.05.
Results
Physical characteristics
Among the 60 subjects, 14 (23.3%), 34 (58.3%), and 11 (18.3%) were the NBPG,
EIHG, and CHG, respectively. Group comparisons of the characteristics are shown
in [Table 1]. We found that the demographics,
anthropometrics, and marathon careers were similar in all three groups.
Additionally, there were no significant differences in the training volume.
Although the total exercise testing time did not significantly differ among the
three groups (p>0.05), NBPG had higher maximal oxygen uptake (VO2max) than
EIHG and CHG (NBPG vs. EIHG, p=0.022; NBPG vs. CHG, p=0.015); however, there was
no significant difference in VO2max between CHG and EIHG (p>0.050) ([Table 1]).
Table 1 Characteristics of participants.
|
NBPG (N=14, 23.3%)
|
EIHG (N=35, 58.3%)
|
CHG (N=11, 18.3%)
|
General characteristics
|
|
|
|
Age, yr
|
50.1±4.5
|
55.0±4.3
|
51.7±4.7
|
Height, cm
|
169.8±4.9
|
168.8±4.3
|
168.8±5.6
|
Weight, kg
|
65.8±5.3
|
67.3±7.1
|
67.3±6.2
|
BMI, kg·m-2
|
22.8±1.5
|
23.6±1.9
|
23.6±1.5
|
LBM, kg
|
55.4±4.0
|
55.4±4.6
|
55.1±4.8
|
Fat, %
|
16.4±2.4
|
17.6±4.0
|
18.3±2.8
|
Smoker, n (%)
|
3 (21.4%)
|
5 (14.3%)
|
3 (27.3%)
|
Alcohol, time/wk
|
2.1±2.0
|
1.9±1.6
|
2.3±2.6
|
HRrest, bpm
|
64.0±10.2
|
65.0±10.6
|
65.6±9.4
|
SBPrest, mmHg
|
113.9±8.2
|
119.5±10.0
|
143.3±6.9†§
|
DBPrest, mmHg
|
72.4±6.2
|
74.0±8.9
|
85.5±13.9†§
|
Marathon training history
|
|
|
|
Marathon careers, yr (IQR)
|
10.0(9.5–12.0)
|
8.0(6.0–11.0)
|
10.0(7.0–11.0)
|
Marathon start age, yrs
|
39.4±6.3
|
43.4±5.4†
|
42.1±5.1†
|
Marathon completed, n (IQR)
|
40.0(22.3–57.0)
|
35.0(20.0–50.0)
|
28.4(20.0–60.0)
|
Race time, min
|
203.6±16.6
|
205.3±21.4
|
197.7±14.9
|
Training volume, METs/week (IQR)
|
1440.0 (1080.0–1680.0)
|
1440.0 (1080.0–1920.0)
|
1440.0 (960.0–3840.0)
|
Training intensity Moderate, n (%)
|
13 (92.9)
|
30 (85.7)
|
8 (72.7)
|
High , n(%)
|
1 (7.1)
|
5 (14.3)
|
3 (27.3)
|
IQR: interquartile range, NBPG: normal blood pressure group, EIHG:
exercise-induced hypertension group, CHG: complex hypertension group,
HR: Heart Rate; BPM: beat per minute, SBP: systolic blood pressure, DBP:
diastolic blood pressure, †Significant difference compared to NBPG
(P+<+0.05), §: Significant difference compared to EIHG
(P+<+0.05).
Resting BP and maximal BP during maximal exercise testing
Resting SBP and DBP were the highest in CHG, and there was no significant
difference between the NBPG and EIHG (p>0.05) ([Table 1]). In addition, maximal SBP and DBP during exercise testing
were significantly higher in EIHG (SBPmax 246.01±7.1 mmHg,
DBPmax 80.1±15.5 mmHg) and CHG (SBPmax
244.9±16.8 mmHg, DBPmax 73.6±13.4 mmHg) than NBPG (NBPG vs. EIHG,
p+<+0.001; NBPG vs. CHG, p+<+0.001), and there was no significant
difference between the EIHG and CHG (p>0.05) ([Table 2]).
Table 2 Hemodynamics value response to treadmill exercise
test.
|
NBPG (N=14, 23.3%)
|
HRmax, bpm
|
168.9±14.4
|
175.2±10.5
|
SBPpeak, mmHg
|
244.9±16.8†
|
246.0±17.1†
|
DBPpeak, mmHg
|
73.6±13.4†
|
80.1±15.5†
|
SBPdiff, mmHg
|
125.0±18.7†
|
103.4±16.4†§
|
VO2max, mL/kg/min
|
46.4±4.7†
|
45.1±5.1†
|
Exercise testing time, sec
|
812.5±86.7
|
820.9±56.0
|
NBPG: normal blood pressure group, EIHG: exercise-induced hypertension
group, CHG: complex hypertension group, HR: Heart Rate, BPM: beat per
minute, SBP: systolic blood pressure, DBP: diastolic blood pressure,
diff: means the difference between peak and resting, †: Significant
difference compared to NBPG (P+<+0.05), §: Significant difference
compared to EIHG (P<0.05). SBP and DBP tested using Kruskal-Wallis
Test with Mann-Whitney U Test post hoc.
Carotid artery IMT
Carotid IMTmean was the highest in CHG (0.72±0.11 mm) than in EIHG
(0.62±0.12 mm) and NBPG (0.55±0.13 mm, CHG vs. EIHG, p=0.014; CHG vs, NBPG,
p=0.001). Additionally, the carotid IMTmean was significantly higher
in EIHG than in NBPG (p=0.029) ([Fig. 2]).
Fig. 2 Comparison of carotid IMT
mean
between
groups. IMT: intima-media thickness, NBPG: normal blood pressure group,
EIHG: exercise-induced hypertension group, CHG: complex hypertension
group, †: Significant difference compared to NBPG (P+<+0.05), §:
Significant difference compared to EIHG (P+<+0.05). Tested using the
Kruskal-Wallis test, with the Mann-Whitney U-test post hoc.
Correlation between BP and carotid artery IMT
Pearson correlation analysis showed that IMTmean was positively
correlated with both SBPrest (r=0.283, p=0.028) and SBPmax
(r=0.312, p=0.015) ([Fig. 3] and [4]). However, the results of the multiple linear
regression analysis using IMTmean as the dependent variable and
SBPrest, SBPmax, age, marathon career, and training
volume as independent variables showed that age (p=0.015) and SBPmax
(p=0.046), but not resting SBP and exercise volume, were independently
associated with IMTmean ([Table 3],
[Fig. 5]). The regression equation is as
follows: IMTmean=− 0.656+0.009*age+0.001*SBPmax.
Fig. 3 Correlation between carotid IMT
mean
and
resting SBP. IMT: intima-media thickness, SBP: systolic blood
pressure.
Fig. 4 Correlation between carotid IMT
mean
and
maximal SBP. IMT: intima-media thickness, SBP: systolic blood
pressure.
Fig. 5 Correlation between carotid IMT
mean
and
age. IMT: intima-media thickness.
Table 3 Multiple linear regression analyses for carotid
artery intima-media thickness mean.
Variables
|
ß
|
SE
|
t
|
95%CI
|
P
|
Age
|
0.009
|
0.004
|
2.516
|
0.002–0.016
|
0.015
|
BMI
|
0.008
|
0.009
|
0.868
|
− 0.011–0.027
|
0.389
|
SBPrest, mmHg
|
0.001
|
0.001
|
1.073
|
− 0.001–0.004
|
0.288
|
SBPpeak, mmHg
|
0.001
|
0.001
|
2.049
|
0.001–0.003
|
0.046
|
VO2max
|
0.002
|
0.003
|
0.477
|
− 0.005–0.008
|
0.635
|
Marathon career
|
0.007
|
0.004
|
10.724
|
− 0.001–0.015
|
0.091
|
Weekly training volume
|
0.001
|
0.001
|
0.436
|
0.001–0.001
|
0.664
|
BMI, body mass index, SBP: systolic blood pressure.
Discussion
This study investigated the relationship between the EIH and carotid artery IMT in
long-distance runners. We found that the CHG had the highest carotid IMT, followed
by the EIHG and NBPG. In the correlation analysis between carotid IMT and BP,
IMTmean showed a correlation with both resting SBP and
SBPmax. Although carotid IMT is known to be influenced most by BP
[14], the multiple regression analysis to
identify factors related to IMTmean in this study showed that only age
and SBPmax were related to IMTmean, and there was no
correlation with resting SBP. This suggests that excessive BP increase during
exercise in healthy marathon runners may increase the risk of carotid
atherosclerosis. To the best of our knowledge, our results are the first reported
regarding the carotid artery in long-distance runners with EIH. There is a
dose-response relationship between exercise volume and health benefits [23]; however, excessive exercise, such as marathon
running, can place stress on the cardiovascular system [24] and may lead to arterial stiffening [25]. Nevertheless, studies regarding the indicators of carotid
atherosclerosis in long-distance runners have reported varying results. Koutlianos
et al. [26] found that immediately after the 246 km
ultra-marathon race, there was an acute increase in arterial stiffness and vascular
resistance, but the carotid artery thickness of ultra-marathon runners was within
normal range. Kroger et al. [27] reported the
presence of atherosclerotic lesions in both the carotid and peripheral arteries of
marathon runners, while Galetta et al. [28] reported
that carotid IMT was lower in elderly long-distance runners compared to an
age-matched control group. Additionally, Heffernan et al. [29] and Taylor et al. [18] have suggested
that there is no significant difference in carotid IMT between long-distance runners
and the general population. The differences in results between these studies raise
questions regarding whether high-intensity exercise accelerates the development of
atherosclerosis and whether there are factors that cause greater vascular damage in
similar exercise conditions. Some previous studies have also suggested that
excessive exercise is not a major factor in increasing carotid IMT. In a study by
Müller et al. [30], which observed changes in carotid
IMT in 38 marathon runners over four years, carotid IMT increased by approximately
0.013±0.023 mm per year and a total of 0.05±0.09 mm over four years. This was
similar to the results of previous studies that suggested an increase in carotid IMT
of 0.01–0.03 mm per year in the general population [31]. It also showed that carotid IMT was not related to training volume
or marathon participation frequency. In this study, the training volume and marathon
experience were similar among all groups, and regression analysis showed that IMT
was not related to training indicators. Notably, the IMTmean of the
participants in this study was 0.55±0.13 mm in the NBPG, which was lower than that
of the Korean age-matched value of 0.65±0.12 mm; 0.62±0.12 mm in the EIHG, which was
similar; and 0.72±0.11 mm in the CHG, which was thicker than that of the age-matched
group [32]. These findings suggest that even
long-distance runners can have lower carotid IMT than the general population, if
they maintain a normal BP. However, if BP increases during rest or exercise, IMT may
be thicker in long-distance runners than in the general population. Therefore, it
can be considered that EIH, rather than the effects of high-intensity exercise, is a
factor that accelerates the changes in IMT of long-distance runners.
Notably, the increase in carotid IMT has been shown to be an important indicator of
elevated BP during exercise in several studies. In Kader et al.’s study [33], the group with high BP during both rest and
exercise showed decreased endothelial dilation and higher carotid IMT only at rest
compared to the group with high SBP at rest only, emphasizing that excessive SBP
during exercise is a significant risk factor for CVD. Jae et al.’s study [34] also showed a close relationship between elevated
BP during exercise and carotid artery sclerosis, with the prevalence of carotid
artery sclerosis being 1.57 times higher in the group with the highest increase in
BP during exercise. These studies suggest that an excessively high BP during
exercise is a meaningful indicator of the risk for carotid artery sclerosis and has
clinical significance [35]. According to the previous
search findings, an exaggerated blood pressure response to exercise in athletes is
not a benign phenomenon [36]. It is, therefore,
important to monitor blood pressure during exercise and intervene early to reduce
the risk of end-organ damage.
The clear mechanism by which an excessive BP increase during exercise contributes to
increased subclinical CVD risk, such as carotid IMT, has not yet been elucidated.
However, the following possible mechanisms can be considered. The first is a
decreased vascular regulation ability. For example, shear stress exerted by blood
flow on the vascular wall is the most important factor regulating vascular
remodeling [37]. Excessive BP increase due to a
disorder of decreased peripheral vascular resistance and vascular relaxation
capacity [38]
[39]
increases shear stress and local distension pressure generated by blood flow,
leading to changes in the internal structure of the arterial wall. This ultimately
causes the thickening of the carotid artery intima-media layer [40]
[41]
[42]. The second is the development of exercise-induced
hypertension and increased carotid IMT due to excessive sympathetic nervous system
activation. Abnormal regulation of the autonomic nervous system on blood vessels
might fail to reduce peripheral resistance, resulting in increased SBP during
exercise [43]. According to a study by Eryonucu et
al. [44], individuals who show EIH have increased
sympathetic nervous system activity during both rest and exercise compared to those
with normal BP responses. Moreover, animal studies have shown that prolonged
elevation of sympathetic tone leads to the proliferation of smooth muscle cells and
thickening of the vascular endothelium [45]. Another
study measuring the femoral artery also found a strong correlation between increased
sympathetic activity and IMT [46]. This current study
had some limitations. This study was a cross-sectional study; therefore, it cannot
determine the causal relationship between exercise-induced hypertension and carotid
IMT. Moreover, other clinical factors and lifestyle habits that may affect BP and
carotid IMT could not be fully controlled. Future prospective studies and
pathophysiological mechanism studies are therefore needed.
Conclusion
In this study, CIMTmean was increased in long-distance runners with EIH
compared to runners with normal BP. Age and SBPmax were also identified
as predictive factors influencing the CIMTmean in long-distance runners.
Therefore, regular health screening and management of cardiovascular risk factors,
including BP response during exercise, are necessary for safe exercise in
long-distance runners.