Download Body height and arterial pressure in seated and supine young males

Am J Physiol Regul Integr Comp Physiol 309: R1172–R1177, 2015.
First published August 19, 2015; doi:10.1152/ajpregu.00524.2014.
Body height and arterial pressure in seated and supine young males during
⫹2 G centrifugation
Sine K. Arvedsen,1 Ola Eiken,2 Roger Kölegård,2 Lonnie G. Petersen,1 Peter Norsk,1,3 and
Morten Damgaard1,4
1
Department of Biomedical Sciences, Faculty of Health Sciences, University of Copenhagen, Copenhagen, Denmark;
Department of Environmental Physiology, Swedish Aerospace Physiology Centre, Royal Institute of Technology, Stockholm,
Sweden; and 3Division of Space Life Sciences, Universities Space Research Association and Biomedical Research and
Environmental Sciences Division, NASA Johnson Space Center, Houston, Texas; and 4Department of Clinical Physiology and
Nuclear Medicine, Centre for Functional Imaging and Research, Hvidovre Hospital, Hvidovre, Denmark
2
Submitted 22 December 2014; accepted in final form 17 August 2015
cardiovascular system; hydrostatic pressure; body height; human
centrifuge
supine to upright, gravity
causes a redistribution of blood from the upper to the lower
parts of the body. This induces an abrupt fall in central venous
pressure that through the Frank-Starling mechanism decreases
cardiac stroke volume (SV) and subsequently arterial pulse
pressure (APP). To ensure a sufficient venous return and
maintenance of adequate mean arterial pressure (MAP), baroreflexes originating from sensors in the carotid sinuses, aorta,
and cardiac chambers induce contraction of the veins and the
arterial resistance vessels, as well as an increase in heart rate
UPON A CHANGE IN POSTURE FROM
Address for reprint requests and other correspondence: S. K. Arvedsen,
Dept. of Biomedical Sciences, Faculty of Health Sciences, Univ. of
Copenhagen, Blegdamsvej 3b, DK-2200 Copenhagen, Denmark (e-mail:
[email protected]).
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(HR) and cardiac contractility through the autonomic nervous
system (3, 27).
Hydrostatic pressure gradients in upright individuals reduce
venous return and unload the carotid baroreceptors. It is,
therefore, possible that the hydrostatic pressure gradient from
the heart to the head in upright subjects is a determinant of
arterial pressure at heart level. Tall individuals might, therefore, be endowed with a higher arterial pressure than short
individuals through inhibition of the carotid baroreceptors. An
animal that illustrates this idea is the giraffe. It has a long
vertical distance between the heart and the brain and probably,
therefore, possesses the highest arterial pressure of all living
species, with MAP at heart level exceeding 200 mmHg (4, 13,
14, 18). In a recent study on humans, we found that 24-h
ambulatory arterial pressures in young males increased with
body height by 2/2 mmHg [systolic (SAP)/diastolic (DAP)
arterial pressure] per 15 cm increase in body height (1).
Furthermore, we found that the increases in SV and APP and
decrease in HR in response to a moderate antiorthostatic
posture change were significantly more pronounced in tall than
in short individuals. This indicates that body height affects
regulation of arterial pressures.
In a human-use centrifuge, it is possible to increase the
gravito-inertial (G) stress in the head-to-seat direction (⫹Gz)
and, thus, augment intravascular hydrostatic pressure gradients. Arterial pressure at heart level typically increases in
response to slightly elevated ⫹Gz-stress in seated humans,
probably because of hydrostatic inhibition of the carotid baroreceptors. Hence, we tested the hypothesis that during ⫹2 Gz
centrifugation, MAP at heart level increases more in tall than in
short males because of the larger hydrostatic pressure difference between heart and head.
METHODS
Subjects. Nine short (168 –171 cm) and 10 tall (194 –203 cm) males
participated in the study (Table 1). All were nonsmokers, had no
history of medical diseases, and were healthy, as indicated by a
physical examination, a resting arterial pressure below 140/90 mmHg,
and values within the normal range for body mass index, blood
hemoglobin concentration, and plasma concentrations of glucose,
cholesterol, liver enzymes, and creatinine. None of the subjects took
any medication.
Ethics. Written consent was obtained after the subjects were informed, orally and in writing, about the study protocol and experimental procedures. The protocol was approved by the Regional
Human Ethics Committee in Stockholm (no: 2011/2012-31/1) and
was in compliance with the Helsinki-II declaration.
0363-6119/15 Copyright © 2015 the American Physiological Society
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Arvedsen SK, Eiken O, Kölegård R, Petersen LG, Norsk P,
Damgaard M. Body height and arterial pressure in seated and supine
young males during ⫹2 G centrifugation. Am J Physiol Regul Integr
Comp Physiol 309: R1172–R1177, 2015. First published August 19,
2015; doi:10.1152/ajpregu.00524.2014.—It is known that arterial
pressure correlates positively with body height in males, and it has
been suggested that this is due to the increasing vertical hydrostatic
gradient from the heart to the carotid baroreceptors. Therefore, we
tested the hypothesis that a higher gravito-inertial stress induced by
the use of a human centrifuge would increase mean arterial pressure
(MAP) more in tall than in short males in the seated position. In short
(162–171 cm; n ⫽ 8) and tall (194 –203 cm; n ⫽ 10) healthy males
(18 – 41 yr), brachial arterial pressure, heart rate (HR), and cardiac
output were measured during ⫹2G centrifugation, while they were
seated upright with the legs kept horizontal (⫹2Gz). In a separate
experiment, the same measurements were done with the subjects
supine (⫹2Gx). During ⫹2Gz MAP increased in the short (22 ⫾ 2
mmHg, P ⬍ 0.0001) and tall (23 ⫾ 2 mmHg, P ⬍ 0.0001) males, with
no significant difference between the groups. HR increased more (P ⬍
0.05) in the tall than in the short group (14 ⫾ 2 vs. 7 ⫾ 2 bpm). Stroke
volume (SV) decreased in the short group (26 ⫾ 4 ml, P ⫽ 0.001) and
more so in the tall group (39 ⫾ 5 ml, P ⬍ 0.0001; short vs. tall, P ⫽
0.047). During ⫹2Gx, systolic arterial pressure increased (P ⬍ 0.001)
and SV (P ⫽ 0.012) decreased in the tall group only. In conclusion,
during ⫹2Gz, MAP increased in both short and tall males, with no
difference between the groups. However, in the tall group, HR
increased more during ⫹2Gz, which could be caused by a larger
hydrostatic pressure gradient from heart to head, leading to greater
inhibition of the carotid baroreceptors.
BODY HEIGHT AND ARTERIAL PRESSURE DURING ⫹2 G
Table 1. Baseline data for the eighteen subjects
participating in the study
Height, cm
Weight, kg
Age, yr
BMI, kg/m2
Short (n ⫽ 8)
Tall (n ⫽ 10)
167 (162–171)
69 (58–82)
24 (19–29)
23 (19–25)
198* (194–203)
88* (79–104)
23 (18–41)
25* (22–29)
The values are means (range). BMI, body mass index. *Statistically significant difference (P ⬍ 0.05) between the groups by unpaired t-test.
Fig. 1. Schematic depiction of the ⫹Gz level along the upper body of a tall
subject (upper body 1.00 m) seated in the gondola. The gravito-inertial load
was ⫹2 Gz at the pivot point, roughly corresponding to the vertical level of the
heart. In the supine position, the G load was adjusted so that ⫹2 Gx level
corresponded with vertical level of the heart.
Prior to the experiment, the rebreathing maneuver for measurement
of cardiac output (CO) was rehearsed thoroughly inside the gondola in
both the seated and supine position.
Each experiment was performed, with the subject once in the seated
and once in the supine position in a randomized balanced order. In the
seated position, the legs were kept horizontal. The reason for this was
twofold 1) our group has previously used this position extensively and
documented that the changes in cardiovascular variables are pronounced and reproducible (19 –22); 2) the local reflexes in the legs
(myogenic and/or veno-arteriolar reflex responses, etc.) are not, or to
a negligible extent, activated when the lower legs are kept horizontal.
In this way, the baroreflex responses (arterial and/or cardiopulmonary)
are the dominating causes for the observed changes in central hemodynamic changes (15, 28). In the gondola, the subjects were restrained
by a five-point safety belt. At 1 G (without rotation), baseline
measurements were collected including CO, brachial arterial pressure,
and HR at 8 and 2 min before initiation of centrifugation. Subsequently, the G load was increased by 0.5 G/s to ⫹2 G and maintained
at this plateau for 10 min, during which time, measurements of CO,
arterial pressure, and HR were recorded after 3 (⫹2 G3min) and 8 min
(⫹2 G8min), respectively.
After changing posture, the subject rested for ⬃10 min, and the
experiment was repeated. Each subject was instructed to keep his head
still during acceleration and deceleration to avoid motion sickness.
Cardiac output. CO was determined by a foreign gas rebreathing
technique, whereby a gas mixture consisting of 0.5% sulfur hexafluoride (SF6), 0.1% nitrous oxide (N2O), and 28% oxygen (O2) in
nitrogen (N2) was inhaled from and exhaled to a rubber bag over ⬃15
s at a rate of 20 min⫺1, while the nose was occluded with a clip. The
volume of rebreathing gas was determined as 30% of the subject’s
vital lung capacity (2) (Table 1). During rebreathing, gas was continuously sampled at the mouthpiece for estimations of end-expiratory
concentrations of N2O (blood-soluble) and SF6 (blood-insoluble) with
an infrared photoacustic gas analyzer (Innocor, Innovision A/S,
Odense, Denmark). From the disappearance rate of N2O, pulmonary
blood flow and, thus, CO were calculated (7, 12). SF6 was used to
correct for inadequate mixing of air in the lungs with the gas mixture
from the bag and for changes in the distribution space of N2O.
The equipment was fastened perpendicular to the wall of the
gondola and operated by the subjects. The mouthpiece was suspended
from the gondola wall by a string so that the subjects only needed to
maneuver the mouthpiece. The string length and, thereby, the position
of the mouthpiece was adjusted for each subject both in the seated and
supine position.
Anthropometric measures. Body height was determined with a
Leicester height-measuring scale. To estimate arterial pressure at the
level of the carotid sinus and at the level of the aortic valves, we used
anthropometric data obtained in a recent publication (1) from 75
subjects with a body height varying from 147 to 206 cm. In these
subjects, the distance from the carotid sinus to the aortic valves
(CS-AV) was determined using an ultrasound technique (1). By
simple measurements using a ruler on the subject in a seated position,
we, furthermore, determined the distance from the carotid sinus to the
top of the head (CS-TH) and the distance from the middle of the
brachial arterial pressure cuff to the top of the head. From these
values, the distance MidCuff-CS was calculated as the difference of
MidCuff-TH ⫺ CS-TH, whereas the distance MidCuff-AV was calculated as MidCuff-CS ⫺ CS-AV. Thereafter, by simple linear
regression, we expressed the correlations MidCuff-CS ⫽ 4.9 ⫹
0.16 ⫻ body height and MidCuff-AV ⫽ 1.2 ⫹ 0.07 ⫻ body height.
These equations were used in the present investigation to calculate
the distances MidCuff-CS and MidCuff-AV of the subjects and,
thereby, estimate the pressure at the level of the aortic valves and the
carotid sinus.
Heart rate. HR was measured from a precordial ECG lead (AS2,
Datex-Engström, Helsinki, Finland).
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Equipment and measurements. The experiments were conducted in
the human-use centrifuge (ASEA, Stockholm, Sweden) at the Royal
Institute of Technology, Stockholm, Sweden. The rotational radius to
the center of the centrifuge gondola is 7.25 m. The gondola was
tangentially pivoted, so that the gondola floor remained perpendicular
to the G-vector at all times. Each subject was tested in seated and
supine positions. In the seated position, they leaned against a vertical
backrest and kept their legs horizontal, so that the G vector was
aligned with the upper body (⫹Gz), whereas in the supine position,
the G vector was directed front-to-back (⫹Gx). The gravitational
gradient of a seated person in a long-armed centrifuge is very small
(Fig. 1). The difference in ⫹Gz at the carotid sinus level between a
short and a tall male is, therefore, negligible. In the supine position,
the gradient is near zero.
Multiple slip rings at the center of rotation allowed for audiovisual
monitoring, power supply, and transmission of physiological signals
between the gondola and the control room. An electrocardiogram
(ECG) was recorded from a precordial lead using a monitoring system
(type AS2; Datex, Helsinki, Finland). The G force was determined
using an accelerometer positioned at heart level, both when the subject
was in the seated and the supine position. Continuous signals were
recorded at 20 Hz per channel in a digital data acquisition system
(Biopac, Goleta, CA). The operator monitored and instructed the
subject continuously via voice communication and video camera.
Protocol. All subjects were instructed not to eat for at least 2 h
before the experiment, so that they were neither hungry nor full during
the experiment. They were, furthermore, instructed to avoid nicotine
and caffeine during the 4 h prior to the experiment.
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BODY HEIGHT AND ARTERIAL PRESSURE DURING ⫹2 G
RESULTS
Ten tall and nine short subjects completed the experiment
(Table 1). One short subject was excluded after the experiment,
as he reported afterward that he had felt motion sick during the
session.
Seated. In response to ⫹2 Gz, MAP increased in both short
(22 ⫾ 2 mmHg, P ⬍ 0.0001) and tall (23 ⫾ 2 mmHg, P ⬍
0.0001) individuals (Fig. 2A) with no significant difference
between the groups. HR increased in both the short (P ⫽
0.012) and the tall group (P ⬍ 0.0001, Fig. 2B), with a
significant interaction (height·G level), indicating (P ⫽ 0.020)
that the increase from ⫹1 Gz to ⫹2 Gz3min was greatest in the
tall group (tall: 14 ⫾ 2 vs. short: 7 ⫾ 2 bpm, P ⫽ 0.015).
SV (Fig. 2C) decreased in the short group (26 ⫾ 4 ml, P ⫽
0.001) yet more so in the tall group (39 ⫾ 5 ml, P ⬍ 0.0001;
short vs. tall: P ⫽ 0.047) when exposed to ⫹2 Gz. Also CO
decreased in both the short (P ⫽ 0.015) and tall group (P ⫽
0.001) (Table 2). TPR increased in both groups (P ⬍ 0.001,
Table 2).
APP did not change significantly during centrifugation in
either group (short, P ⫽ 0.231; tall, P ⫽ 0.080). Although the
test for interaction indicated that the two groups responded
differently (P ⫽ 0.027) to the ⫹G exposure, the change from
⫹1 Gz to ⫹2 Gz3min (3 ⫾ 4 vs. ⫺5 ⫾ 2 mmHg, P ⫽ 0.125) and
⫹2 G3min to ⫹2 G8min (1 ⫾ 5 vs. ⫺2 ⫾ 2 mmHg, P ⫽ 0.473)
was not significantly different in the two groups (Table 2).
Supine. In response to ⫹2 Gx, MAP and SAP increased in
the tall group only (P ⫽ 0.005, Fig. 2A and P ⬍ 0.001, Table
2, respectively). HR increased significantly in the short (P ⫽
0.009) and tall (P ⫽ 0.049, Fig. 2B) group. SV decreased (P ⫽
0.012, Fig. 2C) in the tall group only. TPR increased in both
the short (P ⫽ 0.006) and tall group (P ⬍ 0.001, Table 2).
Fig. 2. A: mean arterial pressure (MAP) before the onset of centrifugation (⫹1
G), after 3 min (⫹2 G3min), and after 8 min of centrifugation at ⫹2 G (⫹2
G8min). Short group n ⫽ 8 except at ⫹2 Gz8min and at ⫹2 Gx3min (n ⫽ 7). Tall
group n ⫽ 10, except at ⫹2 Gx8min (n ⫽ 9). Œ denotes short group in seated
position, while denotes short group in supine position. ⌬ denotes tall group
in seated position, while ⵜ denotes tall group in supine position. B: heart rate
(HR) before the onset of centrifugation (⫹1 G), after 3 min, (⫹2 G3min), and
after 8 min of centrifugation at ⫹2 G (⫹2 G8min). Short group: n ⫽ 8. Tall
group: n ⫽ 10. C: stroke volume (SV) before the onset of centrifugation (⫹1
G), after 3 (⫹2 G3min), and after 8 min of centrifugation at ⫹2 G (⫹2 G8min).
Short group: n ⫽ 8. Tall group: n ⫽ 10, except at ⫹2 Gz8min and at ⫹2 Gx8min
(n ⫽ 9). Values are expressed as means ⫾ SE. *Statistically significant (P ⬍
0.05) difference from ⫹1 G determined by a post hoc multiple-range test
(Scheffé). #Statistically significant difference between the short and tall group
in the change from ⫹1 Gz to ⫹2 Gz3min by an unpaired t-test.
DISCUSSION
The present results show that during ⫹2 Gz centrifugation,
MAP at the heart level increases in seated males with no
significant difference between short and tall individuals. However, during the initial 3 min of ⫹2 Gz, HR increases significantly more in the tall than in the short group (⫹2 Gz3min),
indicating an augmented baroreflex-controlled HR response in
the tall subjects. Presumably, the greater G-induced HR response in the tall subjects is attributable to a combination of a
larger inhibition of the 1) cardiopulmonary baroreflexes caused
by a more pronounced decrease in cardiac preload, and 2)
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Arterial pressure. Brachial arterial pressure (SAP, MAP, DAP) was
measured in the upper right arm with an oscillometric equipment
(Omron M4-I). During all measurements, care was taken to ensure
that the midpoint of the arm cuff was located at mid-heart level.
Data analysis. Offline data analysis was performed with an Acknowledge 3.90 BioPac digital data handling system (BioPac, Goleta,
CA). HR (ECG) was registered over a period of 30 s before each
rebreathing maneuver. HR used for calculation of SV was read and
manually recorded from the ECG output during the rebreathing
maneuver. SV was calculated by dividing CO by HR, and total
peripheral resistance (TPR) by dividing MAP by CO.
Statistical analyses. For each subject, cardiovascular variables
were averaged during the two baseline measurements in each posture.
A mixed-model ANOVA for repeated measures with height (tall and
short) as between-subject factor and G-level as within subject factor in
both the seated position (⫹1 Gz, ⫹2 Gz3min, and ⫹2 Gz8min) and
supine position (⫹1 Gx, ⫹2 Gx3min, and ⫹2 Gx8min) was used to test
for intragroup changes over time. Differences between mean values at
⫹1 Gx and ⫹2 Gx3min and ⫹2 Gx8min were evaluated by a post hoc
multiple-range test (Scheffé). If a different response to changes in G
level between the short and tall group was indicated as a significant
interaction (height·G level), an unpaired t-test was used to evaluate
whether these differences (⫹1 Gz to ⫹2 Gz3min and ⫹2 Gz3min to ⫹2
Gz8min) were significantly different. Baseline data (Table 1) are
presented as means and range, whereas all other data are presented as
means ⫾ SE. In all tests, P ⬍ 0.05 was chosen as the level of
significance. The statistical analyses were performed using SPSS for
Windows (IBM SPSS Statistics 19).
BODY HEIGHT AND ARTERIAL PRESSURE DURING ⫹2 G
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Table 2. Cardiovascular parameters for the short and tall group in the seated position at ⫹1 Gz, ⫹2 Gz3min, and ⫹2 Gz8min
and in the supine position at ⫹1 Gx, ⫹Gx3min, and ⫹2 Gx8min
⫹1 G
Seated
SAP, mmHg
DAP, mmHg
APP, mmHg
CO, l/min
TPR, mmHg·min⫺1·l
DAP, mmHg
APP, mmHg
CO, l/min
TPR, mmHg·min⫺1·l
⫹2 G8min
Short
Tall
Short
Tall
Short
Tall
Short
Tall
Short
Tall
125 ⫾ 3
134 ⫾ 4
80 ⫾ 2
76 ⫾ 3
45 ⫾ 3
58 ⫾ 4
6.4 ⫾ 0.4
7.9 ⫾ 0.2
15.3 ⫾ 0.9
12.2 ⫾ 0.3
144 ⫾ 4
152 ⫾ 6*
97 ⫾ 5*
98 ⫾ 4*
48 ⫾ 5
54 ⫾ 4
5.6 ⫾ 0.3
6.9 ⫾ 0.2*
20.6 ⫾ 1.3*
17.1 ⫾ 0.6*
150 ⫾ 4* (n ⫽ 7)
153 ⫾ 4*
101 ⫾ 2* (n ⫽ 7)
101 ⫾ 4*
49 ⫾ 4 (n ⫽ 7)
52 ⫾ 3
5.2 ⫾ 0.2
6.1 ⫾ 0.3* (n ⫽ 9)
22.4 ⫾ 0.7* (n ⫽ 7)
19.7 ⫾ 0.8*¤ (n ⫽ 9)
Short
Tall
Short
Tall
Short
Tall
Short
Tall
Short
Tall
123 ⫾ 5
133 ⫾ 4
71 ⫾ 4
66 ⫾ 3
52 ⫾ 4
67 ⫾ 5
6.5 ⫾ 0.4
9.0 ⫾ 0.6
13.7 ⫾ 0.5
10.1 ⫾ 0.5
128 ⫾ 3 (n ⫽ 7)
143 ⫾ 4*
77 ⫾ 2 (n ⫽ 7)
71 ⫾ 3
51 ⫾ 4 (n ⫽ 7)
72 ⫾ 3
6.2 ⫾ 0.4
8.3 ⫾ 0.5
15.2 ⫾ 0.8 (n ⫽ 7)
11.8 ⫾ 0.6
128 ⫾ 3
142 ⫾ 4* (n ⫽ 9)
75 ⫾ 2
69 ⫾ 2 (n ⫽ 9)
52 ⫾ 3
73 ⫾ 4 (n ⫽ 9)
6.0 ⫾ 0.3
7.7 ⫾ 0.3 (n ⫽ 9)
15.8 ⫾ 0.7
12.5 ⫾ 0.5* (n ⫽ 9)
Data are presented as means ⫾ SE for the tall group (n ⫽ 10) and the short group (n ⫽ 8). SAP, systolic arterial pressure; DAP, diastolic arterial pressure;
APP, arterial pulse pressure; CO, cardiac output; TPR, total peripheral resistance. Statistically significant (P ⬍ 0.05) difference from ⫹1 G. ¤Statistically
significant (P ⬍ 0.05) difference from ⫹2 G3min determined by a post hoc multiple-range-test (Scheffé).
carotid baroreflexes because of the larger hydrostatic pressure
gradients. This is, however, speculative but could be an explanation for our findings.
Seated Centrifugation
⫹Gz-induced MAP increase. The mechanisms for the increase of some 20 mmHg in MAP in both groups of males
merit comments. First, this magnitude of increase shows that
arterial pressure regulation is sensitive to hydrostatic pressure
gradients in the cardiovascular system. Second, the increase in
the arterial pressure (Fig. 2A and Table 2) must primarily have
been accomplished by systemic peripheral vasoconstriction,
because CO was unchanged or fell in both groups during
centrifugation. Third, the decrease in SV elicited a reflex
increase in HR to protect CO from decreasing; nevertheless,
CO decreased during ⫹2 Gz. Fourth, despite the unchanged
APP, which reflects an unchanged pulsation in the arterial tree,
MAP increased by some 20 mmHg at the midbrachial level in
both groups. This indicates that the baroreflexes involved in the
increase of TPR and HR, must predominantly be 1) the carotid
caused by the decrease in MAP at this hydrostatic level, and 2)
the cardiopulmonary caused by the decrease in preload indicated by the decrease in SV.
We assume that the baroreceptors in the aorta were not or
only modestly affected by ⫹2 Gz, because the mean body
height of the short (167 ⫾ 1 cm) and tall group (198 ⫾ 1 cm)
correspond to an estimated difference in MAP between midbrachial (cuff) to the level of the aortic valves of 19 and 22
mmHg, respectively. This is very similar to the increase in
midbrachial MAP during the ⫹2 Gz that we observed. Thus,
with very small changes in APP and MAP, the aortic baroreceptors were probably not inhibited, rendering them less important in initiating systemic arterial vasoconstriction and
tachycardia. The increase in HR and TPR were, therefore, most
likely initiated primarily by the carotid and/or cardiopulmonary
reflexes.
HR response. HR increased more and faster in the tall vs. the
short subjects during ⫹2 Gz stress (Fig. 2B). The mechanism
for this may be a more pronounced inhibition of the carotid
baroreflexes caused by the hydrostatically lower arterial pressure in the carotid sinus in the tall subjects. This is in accordance with our previous observation that the HR response
during moderate ⫹1 G upper body posture change is more
pronounced in the tall compared with short individuals (1). In
previous studies from our laboratory, we have demonstrated
that sudden pressure changes in the carotid sinus (8 –10) within
seconds initiate reflex changes in HR by modulating the vagal
input to the heart.
Systemic arterial vasoconstriction. During ⫹2 Gz, MAP
increased within the same magnitude in the two groups, indicating a minor importance of the hydrostatic gradient from the
heart level to the baroreceptors in the regulation of MAP. In
contrast, the results confirm our previous suggestion, that the
higher ambulatory MAP in tall males is not caused by hydrostatics and carotid baroreflexes, but rather by a larger SV and
thus CO (1). Even though the hydrostatic pressure changes
must have been augmented in the tall compared with the short
subjects, this was apparently not enough to induce a greater
systemic arteriolar constriction. From previous data (1), it can
be estimated that the difference in body height between the tall
and short group corresponds to a 5-cm difference in MidCuffCS. Because we observed the same MAP at heart level in the
two groups during centrifugation (Fig. 2A), the arterial pressure
in the carotid sinus must have been about 7 mmHg lower at ⫹2
Gz (2 ⫻ 5 cm because of the ⫹2 Gz) in the tall subjects.
Apparently, this drop in carotid sinus pressure in the tall
subjects did not induce an increase in the systemic arterial
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Supine
SAP, mmHg
⫹2 G3min
BODY HEIGHT AND ARTERIAL PRESSURE DURING ⫹2 G
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Despite the abolishment of the hydrostatic pressure gradient,
the hemodynamic response differed between short and tall
subjects when exposed to ⫹2 Gx. In future studies, sizedependent differences in the compression of the thorax in
response to ⫹Gx exposure should be evaluated by esophageal
or central venous pressure and/or by estimation of left ventricular diameter.
Conclusion. The hypothesis that MAP at midbrachial heart
level would increase more in the tall than in the short males
was not confirmed. However, HR increased faster in tall than
short males during ⫹2 Gz centrifugation, which might be
attributed to the greater hydrostatic heart to brain distance and,
hence, lower MAP in the carotid sinus. Our findings indicate
that body height is a factor for the increase in HR to protect CO
from falling due to a decrease in cardiac preload during
orthostatic challenges. Peripheral arteriolar constriction seems
relatively independent of body height as the increase in TPR
was not different between the short and tall group.
The differences in arterial pressure and SV between short
and tall during the ⫹2 Gx load may reflect a more pronounced
compression of the thorax in the front to back direction in the
tall males. This needs to be further investigated.
Perspectives and Significance
REFERENCES
The present findings suggest that acute baroreflex setting of
systemic vascular resistance is probably independent of body
height, whereas the setting of HR is not. It would be of interest
to investigate the combined effect of age and body height on
HR and arterial pressure regulation to understand the implications of gravity for the development of hypertension and other
negative impacts on health.
During ⫹2 Gz, MAP (at heart level) increased to approximately similar levels in tall and short males. As the hydrostatic
gradient from the heart to the head was greater, MAP was
considerably lower at the carotid sinus level in tall individuals.
Hence, it is conceivable that the cerebral perfusion pressure
could be lower in the tall subjects. Notably, although the
cerebral vasculature regulates flow across a wide range of
perfusion pressures, the autoregulatory capacity is exceeded
once local arterial pressure drops below 60 –70 mmHg (30). It
has been shown that the capacity to withstand increasing ⫹Gz
load (G level tolerance) is negatively correlated to body height
and, in particular, to the distance between the heart and the eye
(17). Therefore, it would be interesting to investigate whether
the blood flow to the brain is different in tall vs. short males
during high-level and/or long-duration ⫹Gz loads, and whether
the autoregulatory capacity is related to body height.
GRANTS
The study was funded by the European Space Agency, ESA (grant/contract
no. ESA-CORA-GBF-2010-300).
DISCLOSURES
No conflicts of interest, financial or otherwise, are declared by the authors.
AUTHOR CONTRIBUTIONS
Author contributions: S.K.A., O.E., R.K., L.G.P., P.N., and M.D. conception and design of research; S.K.A., O.E., and R.K. performed experiments;
S.K.A. analyzed data; S.K.A., P.N., and M.D. interpreted results of experiments; S.K.A. prepared figures; S.K.A. drafted manuscript; S.K.A., O.E., R.K.,
L.G.P., P.N., and M.D. edited and revised manuscript; S.K.A., O.E., R.K.,
L.G.P., P.N., and M.D. approved final version of manuscript.
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