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PRESSURE-DISTENSION RELATIONSHIP IN ARTERIES AND ARTERIOLES IN

RESPONSE TO FIVE WEEKS OF HORIZONTAL BEDREST

Running head: Sustained bedrest and arterial/arteriolar distensibility

Authors: Ola Eiken1, Roger Kölegård1 and Igor B. Mekjavic2

1 Swedish Defence Research Agency, Berzelius v. 13, Karolinska Institutet, SE -171 77

Stockholm, Sweden

2 Department of Automation Biocybernetics and Robotics, Jozef Stefan Institute, Jamova

39, SI-1000 Ljubljana, Slovenia

Correspondence: Ola Eiken, Swedish Defence Research Agency, Berzelius v. 13,

Karolinska Institutet, SE-171 77 Stockholm, Sweden

e-mail: ola.eiken@ki.se

Telephone: +46 8 524 86342

Fax: +46 8 330923

Articles in PresS. Am J Physiol Heart Circ Physiol (July 25, 2008). doi:10.1152/ajpheart.00576.2008

Copyright © 2008 by the American Physiological Society.

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Abstract

We hypothesized that exposure to prolonged recumbency (bedrest), and thus reductions of

intravascular pressure gradients, increases pressure distension in arteries/arterioles in the legs.

Ten subjects underwent 5 weeks of horizontal bedrest. Pressure distension was investigated in

arteries and arterioles prior to and after the bedrest, with the subject seated or supine in a

hyperbaric chamber with either one arm or a lower leg protruding through a hole in the

chamber door. Increased pressure in the vessels of the arm/leg was accomplished by

increasing chamber pressure. Vessel diameter and flow were measured in the brachial and

posterior tibial arteries using Doppler ultrasonography. Electrical tissue impedance was

measured in the test limb. Bedrest increased (p < 0.01) pressure distension threefold in the

tibial artery (from 8 ± 7 to 24 ± 11%) and by a third (p < 0.05) in the brachial artery (from 15

± 9% to 20 ± 10%). The pressure-induced increase in tibial artery flow was more pronounced

(p < 0.01) after (50 ± 39 ml/min) than before (13 ± 23 ml/min) bedrest, whereas the brachial

artery flow response was unaffected by bedrest. The pressure-induced decrease in tissue

impedance in the leg was more pronounced (p < 0.01) after (16 ± 7%) than before (10 ± 6%)

bedrest, whereas bedrest did not affect the impedance response in the arm. Thus, withdrawal

of the hydrostatic pressure gradients that act along the blood vessels in erect posture markedly

increases pressure distension in dependent arteries and arterioles.

Keywords: Gravity, microgravity, vascular compliance, vascular deconditioning, vascular

stiffness

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Introduction

In erect posture, blood vessels oriented longitudinally in the body are aligned with the gravity

force field, resulting in substantial hydrostatic pressure differences along the vessels. In a

previous study, investigating the in vivo distensibility of the vasculature in human limbs, we

demonstrated that both arteries and precapillary resistance vessels are more pressure resistant

in the distal portions of the legs than in the arms, suggesting that the wall stiffness of such

vessels adapt to meet the long-term demands imposed by the hydrostatic pressure acting along

the vessels (13). This finding is in keeping with the concept that long-term changes of

transmural pressure alter both the function and structure of peripheral blood vessels in humans

(for reviews see 14, 29). It also raises the question as to whether the reduced distensibility in

the vasculature of the legs is permanent or dependent on the regular increments of local

intravascular pressures, associated with habitually assuming the erect posture during normal

daily life activities. It is well established that healthy individuals exposed to prolonged

periods of recumbency (bedrest) exhibit orthostatic intolerance (for ref. see 15), as well as

reduced capacity to withstand increased gravitoinertial load in the head-to-foot direction (G

tolerance) (16). The cause of such impairments in orthostatic and G tolerance appears to be

multifactorial and includes impaired intrinsic myocardial function (22), cardiac atrophy (18)

and diminished arterial baroreflex function (8). Also, augmented pooling of blood in

dependent veins may contribute to bedrest-induced reductions in orthostatic and G tolerance

(23). Whether prolonged bedrest affects the distensibilty of dependent arteries and arterioles,

which may also affect orthostatic tolerance and most certainly G tolerance, is as yet unknown.

Accordingly, we tested the hypothesis that arterial and arteriolar distensibility in

limbs will increase in response to five weeks of prolonged bedrest, and that such increase will

be especially pronounced in leg vessels.

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Methods

Subjects

Ten healthy male subjects took part in the study. Upon enrollment in the study, their average

(range) age, body mass and height were 23 (18 - 32) yrs, 70.5 (57 - 86) kg and 1.79 (1.66 -

1.90) m, respectively.

Potential subjects were recruited by advertisement. They were not included if

they were taking any medication, had a family history of frequent deep vein thrombosis

(DVT) or showed pathological results in a blood screening test for DVT susceptibility, had a

history of vascular headaches (i.e. migraine or Hortons disease), or were unable to perform

the pre-bedrest criterion tests (see below) and/or baseline experiments.

The subjects gave their informed consent to participate and were instructed that

they were free to terminate any experiment, and also to withdraw from the study at any time.

The study protocol and experimental procedures were in accordance with the Declaration of

Helsinki and were approved by the Committee for Medical Ethics of the Ministry of health of

the Republic of Slovenia.

General study protocol

Experiments and criterion tests reported herein were performed about one week prior to

bedrest (pre), within 12 hours after the 35 days of bedrest (post) and again after four weeks of

recovery (recovery). For each subject, the order and schedule of the experiments and criterion

tests was identical prior to, and after the bedrest.

Bedrest and recovery procedures

The bedrest procedure was conducted at the Valdoltra Orthopaedic Hospital in Ankaran,

Slovenia. The subjects commenced and completed the 35-day bedrest period at predetermined

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intervals and in sequence over a 3-day period. During bedrest the subjects were accomodated

in two rooms (5 individuals in each room). Each subject remained in the horizontal position at

all times. He was allowed one pillow for the head and to occasionally lean on an elbow while

eating or being transferred to a gurney. He was allowed to move his arms in and above the

horizontal plane, whereas the legs had to be kept in the horizontal plane at all times. Muscular

exercise (e.g. static contractions using the foot board of the bed as support) was

prohibited.With the exception of alcohol-containing beverages subjects were not restricted

regarding food and drinks. Each subject was provided three meals a day; the meals were

designed by the hospital nutritionist. Hospital routines were adhered to at all times.

To ensure that subjects complied with the requirements of the bedrest protocol,

and for subject safety, video cameras in the rooms provided continuous (24 hrs per day),

surveillance of the subjects throughout the bedrest period. To minimize problems with

bedrest-induced neck/back pain and stiffness of joints each subject received physiotherapy

twice a week, and on request. The therapy was performed with the subject in a horizontal

position and consisted of massage as well as assisted (passive) stretching and assisted joint

flexions. Each subject was screened for DVT twice every week throughout the bedrest period.

Specifically, the popliteal veins were examined bilaterally using an ultrasound/Doppler

system with a 6.0-11.0 MHz linear array transducer (Aspen, Acuson, Mountain View, CA,

USA).

During the 4-week period between the post and recovery tests, each subject

resumed his normal ambulatory life style, and also participated in a supervised training

regimen consisting of 11 to 12 one-hour sessions of either cycle ergometry (n=5) or lower

body resistance training (n=5).

Criterion tests

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Certain “criterion tests” were performed to evaluate subject compliance, and hence to verify

that the present bedrest procedure brought about expected degrees of cardiovascular and

musculoskeletal deconditioning. The tests included investigations of anthropometry, isometric

muscle strength, aerobic work capacity and orthostatic response. Only the orthostatic response

tests are reported in the present paper; results from other criterion tests have been (cf 1), or

will, be reported elsewhere.

Orthostatic response was determined during a stand test performed in a

temperature-regulated environment (28 oC). Following a 10-min period of motionless rest in

the supine position, the subject was asked to rapidly stand up. He then stood with heals and

back against a wall for 10 min, or until appearance of signs/symptoms of imminent syncope.

The subject was instructed to remain still and to avoid conducting voluntary muscle

contractions during the test period. Heart rate (HR) was derived from electrocardiographic

recordings with the electrodes positioned in a 5-lead precordial arrangement using a

cardiomonitor (Physiocontrol Lifepak 8, Physio-Control corp., Redmond, WA, USA).

Systolic, diastolic and mean arterial pressures (SAP, DAP, MAP) were measured using a

volume-clamp technique (Portapres®, TNO, Amsterdam, The Netherlands) with the pressure

cuff placed around the mid-phalanx of the third finger of the right hand and the reference

pressure transducer taped to the skin at the level of the heart. The right arm was supported by

a mitella and the distal portions of the fingers were positioned at heart level.

The post-bedrest orthostatic test coincided with the completion of the 35-

bedrest period.

Vascular pressure-distension experiments

Methods and procedures. The experiments were carried out with the subject positioned in a

pressure chamber with one arm (here termed the "test arm") or the lower portion of a leg (test

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leg) extended through a hole in the chamber door. During the arm-pressure provocation the

subject was seated, whereas during the leg-pressure provocation he was in the supine position.

The test arm/leg was supported at the level of the heart by means of a stand external to the

chamber. The test limb was hermetically sealed to the door hole slightly distally of the axilla,

or proximally of the knee, by use of a short self-sealing rubber sleeve. Special harnesses were

used to stabilize the subject and to prevent involuntary muscle activity and movements in the

arm and leg as the pressure in the chamber was elevated. Room temperature was maintained

at 28 oC (range: 25-30 oC).

Diameters of the arteries in the test limb were measured using ultrasonography,

all measurements being performed by the same sonographer. Measurements were conducted

in B-mode image during end-diastole (determined from ECG recordings) as wall-to-wall

distance in the sagittal section using a 6.0-11.0 MHz linear array transducer (Aspen, Acuson,

Mountain View, CA, USA). The brachial artery diameter was measured about 5 cm proximal

to the cubital fossa, and the tibial posterior artery diameter was measured 5 cm proximal to

the ankle. To ensure that the same vessel segment was investigated in the three trials, intra-

and extravascular ”landmarks” were assigned and recorded during the sonography.

Subsequently, off-line measurements of the intima-media thickness were made for both

arteries; measurements were conducted in the B-mode sagittal-section images as the distance

between the leading edge of the lumen-intima echo and the intima-adventitia echo in the far

wall of the vessel.

Flow was estimated in the brachial and tibial arteries by simultaneous

measurements of vessel diameter and mean flow-velocity, employing an ultrasound/Doppler

technique (6.0-11.0 MHz transducer; Aspen, Acuson). Assuming a circular cross-section of

the artery, flow was calculated by multiplying vessel cross-sectional area by the time integral

of the mean flow-velocity.

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HR was derived from electrocardiographic recordings using a bipolar pre-

cordial lead. SAP and DAP were measured in the mid phalanx of the third finger inside the

chamber using the volume-clamp technique. The level of the heart was used as the point of

reference for the arterial pressure measurements. Arterial distending pressure (DP) was

calculated, and arteriolar DP approximated, by adding chamber pressure to DAP (cf. 12, 13).

Changes in forearm and lower leg volumes were estimated from impedance

plethysmographic recordings using a tetrapolar constant-current impedance system

(Minnesota Impedance Cardiograph Model 304A, Instrumentation For Medicine Inc,

Greenwhich, CT, USA) with four pairs of standard disposable pre-gelled electrodes placed on

the test limb.

Each subject rated his perceived pain in the test limb using a ratio scale (4), in

which pain can be rated from 0, no pain to 10, very, very strong (almost intolerable).

Experimental protocol. Each experiment commenced with a 10-min baseline period with

normal atmospheric pressure in the chamber. Thereafter, chamber pressure was increased

step-wise every other min, resulting in 2.0 min plateaus of negative pressure surrounding the

test limb relative to that surrounding the rest of the body. For the arm, the chamber pressure

plateaus were 15, 60, 90, 120 and 150 mmHg, and for the leg, 15, 60, 90, 150, 180, 210 and

240 mmHg above atmospheric pressure. Upon completion of the pressure provocation

protocol, chamber pressure was rapidly reduced and a 2.5-min recovery period at atmospheric

pressure ensued. Recordings of HR, SAP, DAP, vessel diameters, flow, forearm/lower leg

impedance and rating of perceived pain in the test limb were obtained during the last 0.5 min

at each pressure level.

In all subjects pressure provocations were performed in the left arm and left leg.

For the individual subject, the two trials (leg and arm) were separated by ≥ 1 h. The order of

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the trials was alternated among subjects.

Analyses

The statistical significance of differences was evaluated by repeated measures analysis of

variance, followed by Tukey’s honestly significant difference post hoc test (Statistica Statsoft,

Tulsa OK, USA).

Results

All ten subjects completed the 35-day bedrest exposure without any major adverse events.

During the first two weeks, several subjects complained of soreness/ache in the neck and/or

lower back. These problems invariably disappeared within one-to-two days either

spontaneously or after physiotherapy (see Methods). Neck/lower back problems were less

frequent during the latter 3 weeks of the bedrest.

Orthostatic responses

Prior to bedrest, all subjects could endure the 10-min stand test without any signs or

symptoms of imminent syncope. They all exhibited normal heart rate and arterial pressure

(AP) responses (∆HR (mean ± SD) upon standing = 29 ± 8 beats/min; ∆SAP = 13 ± 13

mmHg; ∆DAP = 14 ± 7 mmHg). In the post trial, only 3 subjects completed the stand test, the

remaining 7 subjects had to discontinue the provocation prematurely (p < 0.05) due to

signs/symptoms of imminent syncope; the endurance time in the stand test for these subjects

was 3.6 ± 2.1 min. The HR response was markedly exaggerated, and the AP responses were

attenuated (p < 0.001) in the post trial (∆HR = 45 ± 14 beats/min; ∆SAP = -2 ± 17 mmHg;

∆DAP = 7 ± 8 mmHg) compared to pre bedrest. After recovery, all subjects completed the

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stand test without any unwarranted symptoms and with normal HR and AP responses (∆HR =

21 ± 9 beats/min; ∆SAP = 6 ± 12 mmHg; ∆DAP = 3 ± 8 mmHg).

Vascular pressure-distension

Exposure to high intravascular pressures induced pain in both the arm and the leg. Pain

increased with increasing pressure to reach median values of about eight at the highest

chamber pressures that could be attained in the different conditions. Pain was always

considerably stronger in the arm than in the leg at any given level of increased intravascular

pressure, and the pain response was aggravated by bedrest. Since it can be assumed that such

pressure-induced pain predominantly results from distension of veins (12, 13), details of the

pain responses will be reported in a forthcoming publication concerning effects of prolonged

bedrest on the mechanical properties of limb veins.

Arterial diameters. Prior to bedrest, tibial artery diameter remained unchanged up to a DP of

about 250 mmHg (range 205 to ≥ 305 mmHg), but then increased (p < 0.05) by 8 ± 7% at the

highest DP (Fig. 1A). Bedrest increased (p < 0.001) pressure distension in the tibial artery

threefold to 24 ± 11% at the highest DP that could be attained in all three conditions (average

peak DP = 276; range = 245 - 305 mmHg). After recovery, the tibial artery showed a similar

pressure-distension response as in the pre-bedrest trial (8 ± 5%). Also the diameter of the

brachial artery was preserved at slight and moderate elevations of DP. However, in the

brachial artery, distension occurred at considerably lower DP than in the tibial artery (p <

0.001); the pressure at which brachial artery distension was initially observed in the pre-

bedrest trial ranged from 150 to 180 mmHg for the different individuals (Fig. 1B). Bedrest

increased pressure distension in the brachial artery by a third (p < 0.05) from a pre-bedrest

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value of 15 ± 9% to 20 ± 10% post bedrest at the highest DP (206 ± 18 mm Hg) that could be

attained in all three conditions; after recovery pressure distension was 13 ± 7 % at this DP.

Thickness and cross-sectional area (CSA) of the intima-media (IM) and lumen-to-IM

thickness ratio. In the tibial artery, both the IM thickness and the lumen-to-IM thickness ratio

were similar pre (0.47 ± 0.05 mm; 4.90 ± 0.80), post (0.45 ± 0.05 mm; 4.84 ± 0.72) and after

recovery (0.49 ± 0.06 mm; 4.86 ± 0.78). Likewise, IM thickness and lumen-to-IM thickness

ratio of the brachial artery were similar pre (0.49 ± 0.05 mm; 6.59 ± 0.93), post (0.47 ± 0.06

mm; 6.36 ± 1.16) and after recovery (0.48 ± 0.04 mm; 6.45 ± 1.04). The CSA of the tibial

artery IM was larger (p = 0.07) in the pre (4.11 ± 0.73 mm2) than in the post (3.68 ± 0.58

mm2) condition, whereas brachial artery IM CSA was similar pre (6.59 ± 0.93 mm2) and post

(6.36 ± 1.16 mm2). After the recovery period, IM CSA was similar to pre values both for the

tibial (4.39 ± 0.69 mm2) and brachial (6.45 ± 1.00 mm2) artery.

Arterial flow. Prior to bedrest, flow in the tibial artery remained unchanged up to a DP of

about 264 mmHg (range 238 – 298 mmHg), but then increased by 96 ± 114% (p < 0.05) (Fig.

2A). At the highest DP that could be attained in all three conditions, the pressure-induced

increase in tibial artery flow was more pronounced (p < 0.01) post (50 ± 39 ml/min) than pre

(13 ± 23 ml/min) bedrest, and than after recovery (15 ± 12 ml/min). Also in the brachial

artery, flow remained unaltered at slight to moderate DP elevations and increased (p < 0.001)

at the highest DP. However, the pressure-induced increase in flow occurred at substantially

lower DP in the brachial than in the tibial artery (p < 0.001). The brachial artery flow

response was unaffected by bedrest, with increments at the highest DP that could be attained

in all three conditions of 205 ± 129 ml/min in the pre, 194 ± 88 ml/min in the post and 207 ±

103 ml/min in the recovery trials (Fig. 2B).

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Tissue impedance. In both the forearm and the lower leg, tissue impedance decreased (p <

0.001) with increasing pressure (Fig. 3A). In the pre-bedrest trials, the pressure-induced

decrease in tissue impedance was, in relative terms, greater in the arm than in the leg at any

given level of markedly increased intravascular pressure (p < 0.05). In the leg, the pressure-

induced decrease in tissue impedance was more pronounced (p < 0.01) post (16 ± 8% at the

highest attainable chamber pressure) than pre (10 ± 6%) bedrest and than after recovery (8 ±

2%). Bedrest did not affect the impedance response in the arm, with decrements of 9 ± 4% in

the pre, 9 ± 2% in the post and 8 ± 2% in the recovery trials (Fig 3B).

Heart rate and arterial pressure.

Table 1 shows HR and AP responses during the arm- and leg-pressure provocations. It can be

seen that HR tended to increase by 5-10 beats/min during the course of a pressure

provocation, and that bedrest elevated baseline HR by about 20 beats/min, but did not affect

HR responses during the pressure provocations. Both SAP and DAP increased in response to

the arm- and leg-pressure provocations, SAP by about 15-25 mmHg and DAP by about 10

mmHg. Bedrest did not affect either baseline values, or pressure responses of SAP and DAP.

Discussion

The present study investigated long-term effects of gravity-dependent pressure components in

peripheral blood vessels on the in vivo vascular wall stiffness. Such adaptive responses to

gravity were assessed by determining the pressure-distension relationship in arteries and

arterioles of the arm and lower leg as influenced by removing, during a 5-week period, the

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gravity-dependent pressure gradients that act along longitudinally oriented vessels in erect

posture.

Arterial and arteriolar pressure distension

The results demonstrate that, in both the brachial and tibial arteries, diameter as well as flow

remained unaltered at slight to moderate elevations of the distending pressure. At high

distending pressures, vessel diameter and flow increased markedly; the distending pressure at

which these increments occurred was much higher for the tibial than the brachial artery.

Bedrest augmented the pressure-induced increments in tibial artery diameter and flow, as well

as the pressure-induced decrease in lower-leg impedance.

That distension in arteries and arterioles occurred at lower distending pressures

in the arm than in the leg confirms previous results (13), and supports the notion that in

humans regional arterial/arteriolar compliances are distributed in such a way that the

hydrostatic forces acting upon them in erect posture are partially counteracted. Since arterial

pressure remained relatively stable during the course of a pressure provocation, the two- and

threefold increments in tibial and brachial artery flow, respectively, observed at high

distending pressures, predominantly reflect precipitous decrements in local peripheral

resistance. The notion that the precapillary resistance vessels were forced open by high

distending pressure was supported by the observation that tissue impedance, and hence

presumably tissue edema (19), in the test limb changed at a much faster rate at the highest

levels of distending pressures. Thus, pressure-induced decrements in test-limb impedance can

be assumed to result from a combination of tissue edema formation and filling/distension of

veins (12, 13). Since the distensibility, and hence the rate of blood pooling, in leg and arm

veins decreases with increasing distending pressure (12, 13), the increased rate of impedance

changes observed at high distending pressures must result from increased edema formation. A

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progressively increasing rate of edema formation suggests a concomitant increase in capillary

permeability or, more likely, capillary filtration pressure. The most plausible explanation for a

sudden increase in capillary pressure is an increase in the diameter of the precapillary

resistance vessels.

That the pressure provocations consistently induced minute increments in heart

rate and minute to modest increments in arterial pressure is in keeping with previous results

(12, 13). The mechanisms underlying such cardiovascular responses are not clear but it

appears reasonable to assume that they might fully or in part be secondary to pressure-induced

limb pain.

Present results thus indicate that both peripheral arteries and arterioles distend at

high intravascular pressures, which concurs with our previous findings (12, 13). It is not

possible to conclude that the pressure-induced increases in arterial diameter represented solely

true passive distension. To some extent, the arterial diameter changes might have been

secondary to the flow increments, reflecting reduced vascular muscle tone elicited by

increased shear stress on the arterial wall (for review see 21). However, it appears unlikely

that shear-stress mediated dilatation played a key role in the present pressure-induced arterial

diameter increments since we have found that such increments remain also when the arterial

flow augmentation is abolished by blocking of flow in the downstream vasculature (R

Kölegård, O Eiken, unpublished results).

Our findings that bedrest augmented the pressure-induced increments in tibial

artery diameter and flow, as well as the pressure-induced decrease in lower-leg impedance,

suggest that removal of the hydrostatic pressure components that act along the blood vessels

in erect posture during normal daily life activities markedly increases pressure distension in

both arteries and precapillary resistance vessels. The possibility that the increased pressure

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distension in arteries/arterioles of the lower leg was attributable to removal of arterial pressure

perturbations induced by muscular exercise, or to any other general/systemic effect associated

with prolonged bedrest, rather than to the removal of the large hydrostatic pressure gradients

acting along the vessels in upright, seems unlikely. Physical training is believed to increase

arterial distensibility, at least when distensibility is not determined by measuring the pressure-

distension relationship, but merely defined as the vessel diameter excursions during a normal

pulse-pressure wave (28). Also, the vasculature in the arm was not affected to the same extent

as that in the lower leg by the present bedrest; a slight augmentation of brachial artery

pressure distension was observed post bedrest, but judging from the responses of brachial-

artery flow and forearm impedance, bedrest did not affect the distensibility of forearm

arterioles. Even though, during normal life activities, local arterial pressure in the arm vessels

commonly exceeds heart level arterial pressure the average decrease in local vascular pressure

imposed by prolonged horizontal bed rest is much larger in the vascular circuits of the legs.

These observations confirm and extend previous findings indicating that in vivo

wall stiffness in both arteries and arterioles may adapt to meet the demands imposed by the

hydrostatic pressure gradients that act along the vessels. Previous experiments have shown

that the distensibility of arteries/arterioles can be considerably reduced by pressure

habituating the vessels. A 5-week pressure-habituation regimen during which an

intermittently applied vascular distending pressure was gradually increased, resulted in

substantial attenuations of pressure-induced increments in brachial artery diameter and flow

(11). The present results suggest that even after life-long adaptation to intravascular pressure

gradients associated with erect posture, arteries and arterioles need regular exposure to such

pressure gradients to maintain their pressure resistance characteristics.

The question arises as to the mechanisms underlying the bedrest-induced

increments in arterial and arteriolar distensibilitity. It is well documented that the wall

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structure of arteries may change in response to changes in the strain of the wall (for reviews

14, 29). Indeed, it can be assumed that changes in the lumen-to-wall thickness ratio, would

affect the pressure-distension relationship of the vessels (14). Our findings that

ultrasonographic determinations of the intima-media thickness and the lumen-to-intima media

thickness ratio in the tibial artery were unaffected by bedrest do not exclude significant

atrophy of the vessel wall, since decrements in the intima-media thickness less than about 0.1

mm, corresponding to about 20% of the present thicknesses, are not necessarily detected using

this technique (24). Furthermore, the 10% reduction in post bedrest cross-sectional area

(CSA) of the tibial artery intima-media layers, supports the notion of bedrest-induced atrophy

of the walls of leg arteries. It has been suggested that changes in arterial wall thickness are

more accurately determined by measuring CSA than by measuring wall thickness (29).

Increasing the gravitational pull acting along longitudinally oriented vessels in rats, by

suspending them from their tails or hindlimbs for prolonged periods of time, may increase the

CSA in cranially located conduit and resistance arteries and arterioles and decrease the CSA

of corresponding vessels located in the caudal portion of the body (6, 10).

It is also possible that bedrest-induced reductions in neurovascular

input/sensitivity and/or in local release of vasoconstrictive substances may have contributed to

the increased distensibility of leg arteries/arterioles observed in the post-bedrest pressure

provocation. There are indications that prolonged bedrest reduces vasoconstrictor as well as

flow-mediated and nitroglycerin-mediated conduit artery dilatatory responses in humans (2,

25,). It has been shown that prolonged tail suspension in rats reduces maximal isometric

contractile tension in the abdominal aorta, irrespective of whether contractions are evoked by

receptor- or nonreceptor-mediated vasoconstrictors (20). It has been suggested that gravity-

dependent changes in vascular responsiveness observed in tail-suspended rats might, in part,

be induced by changed expressions of endothelial and neuronal nitrous oxide (NO), as well as

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of inducible NO synthase (17, 27). By contrast to the aforementioned studies, there is

evidence suggesting that extended exposures to bedrest/microgravity does not affect

vasoconstrictor capacity in humans, at least not in the arm vessels (for review see 9). Thus,

the mechanisms underlying bedrest-induced increments in vascular distensibility may be

multifactorial and need to be further investigated.

It also remains to be elucidated if, and to what extent, a bedrest-induced increase

in pressure distension of the precapillary resistance vessels of the legs contributes to the

orthostatic intolerance consistently observed after prolonged bedrest and space flights (15).

Flow resistance in the leg vasculature constitutes a key determinant for orthostatic tolerance

as well as for the capacity to withstand increased gravitoinertial load in the head-to-foot

direction (3). However, it can be argued that the present findings suggest that, in erect posture

at normal gravity, local arterial pressure in the lower portions of the legs is about 200 mmHg,

which might not be of sufficient magnitude to distend precapillary resistance vessels even

after they have been deconditioned by 5 weeks of horizontal bedrest. This does not exclude

the possibility that an even longer period of bedrest or microgravity exposure might increase

pressure distensibility in the precapillary vessels of the legs to the extent that significant

distension of dependent arteries/arterioles would occur when assuming erect posture. There

are indications that impaired arterial/arteriolar function contributes to the orthostatic

intolerance induced by bedrest or microgravity (5, 7, 25). Further, our results suggest that

even slight increments of G-load in the head-to-foot direction of less then 0.5 G will elevate

pressure in the leg vessels to levels capable of substantially distending arteries/arterioles in the

bedrest-deconditioned vasculature. Therefore, it must be considered that the well documented

reduction of G-tolerance following bedrest or space missions (16) may, to a large extent, be

attributable to increased pressure distension in the precapillary resistance vessels of the lower

body.

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Methodological consideration

Present results of bedrest-induced effects on vascular distensibility should also be viewed in

the context of subject compliance. Neither the video surveillance, nor the criterion tests gave

reason to doubt that all subjects complied with the requirements of the bedrest protocol. In

general, long-duration bedrest induces signs of orthostatic intolerance in about 50-60% of the

subjects (26). That the incidence of orthostatic intolerance varies considerably between

studies is a consequence of different methods used to provoke orthostatic stress (i.e. stand

tests, different tilt-table and lower body negative pressure protocols) and of the different

indices used to define orthostatic tolerance. The response is also dependent on time elapsing

between termination of bedrest and the orthostatic provocation (cf. 15). In the present post-

bedrest stand test, all subjects exhibited exaggerated heart rate responses and 70% of the

subjects had to discontinue the orthostatic provocation, because of signs/symptoms of

imminent syncope. Thus in the present study, bedrest induced expected degrees of

cardiovascular and, as reported previously (1), musculoskeletal deconditioning, confirming

subjects compliance with the protocol.

To increase the transmural pressure in the blood vessel of a subject’s test limb

we used a method described in detail previously (12, 13). Briefly, the subject, except for his

test limb, is enclosed in a pressure chamber. As chamber pressure is elevated, transmural

pressures in the vasculature of the unexposed test limb are increased in direct proportion to

the increase in chamber pressure.

Lumen diameters of the arteries were measured during end-diastole by use of

ultrasonography. Arterial DP was calculated by adding applied chamber pressure to DAP, and

also changes in flow were evaluated as functions of arterial DP. Since arterial flow, in the

forearm or lower leg, is mainly controlled by local vascular resistance it can be argued that

19

flow should be treated as a function of arteriolar rather than arterial DP. The average arteriolar

DP is however difficult to determine, and hence the flow patterns in the brachial and tibial

arteries were analyzed as functions of peak arteriolar DP (i.e. DP at the upstream end of the

arterioles), which, in turn, was assumed to correspond to arterial DP.

Even though blood volume was not measured in the present study it is

reasonable to assume that the 5-week bedrest period reduced circulating blood volume by

about 500-600 ml (15). There is evidence suggesting that bedrest-induced reductions in

circulating blood volume may increase baseline vascular resistance and thereby reduce the

vasoconstrictor reserve in the forearm vasculature (for review see 9). However, judging from

our observations that, under baseline conditions, arterial pressure as well as brachial and tibial

artery flow, and hence flow resistance in the vasculatures of the forearm and lower leg, were

unaffected by bedrest, it appears unlikely that a bedrest-induced blood volume reduction per

se had any major impact on the pressure-distension determinations.

Conclusion

Present results indicate that removal of the hydrostatic pressure gradients that act along the

blood vessels in erect posture markedly increases pressure distension in arteries and arterioles

of the legs, suggesting that the increased stiffness observed in dependent arteries/arterioles is

not inherent, but rather represents an acquired feature that depend on regular exposures to

such gradients.

20

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24

Legends for figures.

Fig. 1. Relative changes in lumen diameters of the posterior tibial (a) and brachial (b) arteries

as functions of the distending pressure before bedrest (Pre), immediately after bedrest (Post)

and after 4 weeks of recovery (Recovery). Note that due to slight intercondition variations in

the arterial pressure responses the distending pressures at which diameter measurements were

obtained also vary slightly between conditions. Values are means+SD. n=10.

Fig. 2. Flow in the posterior tibial (a) and brachial (b) arteries as functions of peak arteriolar

distending pressure (i.e. distending pressure at the upstream end of the arterioles) before

bedrest (Pre), immediately after bedrest (Post) and after 4 weeks of recovery (Recovery). Note

that due to slight intercondition variations in the arterial pressure responses the distending

pressures at which flow measurements were obtained also vary slightly between conditions.

Values are means+SD. n=10.

Fig. 3. Relative change in lower leg (a) and forearm (b) impedance as functions of applied

pressure before bedrest (Pre), immediately after bedrest (Post) and after 4 weeks of recovery

(Recovery). Values are means+SD. n=10.

0

20

40

60

80

100

0 50 100 150 200 250 300 350

Flow

(ml/m

in)

A

Post ( ) vs. pre ( ) p< 0.01

Post ( ) vs. recovery ( ) p< 0.01

0

50

100

150

200

250

300

0 50 100 150 200 250 300 350

Flow

(ml/m

in)

prepostrecovery

B

-5

0

5

10

15

20

25

30

0 50 100 150 200 250 300 350Distending pressure (mmHg)

∆ di

amet

er (%

)A

-5

0

5

10

15

20

25

30

0 50 100 150 200 250 300 350Distending pressure (mmHg)

∆ di

amet

er (%

)

prepostrecovery

B

Post ( ) vs. pre ( ) p< 0.001

Post ( ) vs. recovery ( ) p< 0.001

Post ( ) vs. pre ( ) p< 0.05

Post ( ) vs. recovery ( ) p< 0.05

-20

-15

-10

-5

00 30 60 90 120 150 180 210 240

Distending pressure (mmHg)∆

impe

danc

e (%

)

A

-20

-15

-10

-5

00 30 60 90 120 150 180 210 240

∆ im

peda

nce

(%)

prepostrecovery

B

Post ( ) vs. pre ( ) p< 0.01

Post ( ) vs. recovery ( ) p< 0.01

Table 1. Heart rate (HR), systolic (SAP) and diastolic (DAP) arterial pressure responses during the arm and leg pressure provocations before (pre) and after (post) bedrest and after the recovery period.

Arm pressure provocations Leg pressure provocations

0 0 vs max max 0 0 vs max max

HRPre 76 ± 16 p<0.05 81 ± 14 77 ± 15 p<0.01 91 ± 18pre vs post p<0.01 p<0.01 ns p=0.06post 95 ± 14 p=0.09 103 ± 16 82 ± 15 p<0.01 110 ± 22post vs recovery p<0.01 p<0.01 ns p<0.05recovery 78 ± 16 p<0.05 83 ± 13 74 ± 14 p<0.05 91 ± 16

SAPPre 115 ± 17 p<0.02 134 ± 21 116 ± 16 p<0.05 134 ± 19pre vs post ns ns ns nspost 111 ± 32 p<0.02 133 ± 26 116 ± 19 p<0.05 142 ± 24post vs recovery ns ns ns nsrecovery 113 ± 19 p<0.01 128 ± 29 114 ± 15 p<0.05 134 ± 17

DAPPre 62 ± 10 p<0.01 73 ± 9 60 ± 11 p<0.01 72 ± 14pre vs post ns ns ns nspost 63 ± 18 p<0.01 77 ± 13 64 ± 9 p<0.01 81 ± 15post vs recovery ns ns ns nsrecovery 54 ± 15 p<0.01 66 ± 23 51 ± 8 p<0.01 69 ± 16

Values were obtained at normal atmospheric chamber pressure (0) and at the highest chamber pressure that was attained in all three conditions (max; arm pressure provocation = 141 ± 14 mmHg and leg pressure provocation = 207 ± 22 mmHg).Values are means ± SD; n = 10.