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A pilot evaluation of a neuromuscular electrical stimulation (NMES) based methodology for the prevention of venous stasis during bed rest

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A pilot evaluation of a neuromuscular electrical stimulation (NMES) based methodology for the prevention of venous stasis during bed rest
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  Pleasecitethisarticleinpressas:BroderickBJ,etal.Apilotevaluationofaneuromuscularelectricalstimulation(NMES)basedmethodologyforthe prevention of venous stasis during bed rest. Med Eng Phys (2010), doi:10.1016/j.medengphy.2010.01.006 ARTICLE IN PRESS GModel JJBE-1628; No.of Pages7Medical Engineering & Physics xxx (2010) xxx–xxx Contents lists available at ScienceDirect MedicalEngineering&Physics  journal homepage: www.elsevier.com/locate/medengphy A pilot evaluation of a neuromuscular electrical stimulation (NMES) basedmethodology for the prevention of venous stasis during bed rest Barry J. Broderick a , b , ∗ , David E. O’Briain c , Paul P. Breen a , b , Stephen R. Kearns c , Gearóid ÓLaighin a , b a Electrical & Electronic Engineering, School of Engineering and Informatics, NUI Galway, University Road, Galway, Ireland b Bioelectronics Research Cluster, National Centre for Biomedical Engineering Science, NUI Galway, University Road, Galway, Ireland c Department of Surgery, Clinical Science Institute, University College Hospital, Galway, Ireland a r t i c l e i n f o  Article history: Received 9 July 2009Received in revised form 19 January 2010Accepted 25 January 2010 Keywords: NMESDeep vein thrombosisDVTVenous thromboembolismBed restCalf muscle pumpElectrical stimulationLower limb hemodynamicsVenous stasis a b s t r a c t Bed rest poses an increased risk factor for a potentially fatal venous thromboembolism (VTE). Lack of activation of the calf muscle pump during this resting period gives rise to venous stasis which maylead to deep vein thrombosis (DVT) development. Our aim was to investigate the effects that 4h of bedrest have on the lower limb hemodynamics of healthy subjects and to what extent electrically elicitedcontractions of the calf muscles can alleviate these effects. Outcome variables included popliteal veinblood flow and heart rate. Primary results indicated that the resting group experienced a significantdecline in popliteal venous blood flow of  ∼ 47% with ∼ 13% decrease in heart rate. The stimulated groupsmaintainedasignificantlyhighervenousbloodflowandheartrate.Volumeflowinthecontralaterallimbremained constant throughout the study and was comparable to that of the stimulated limb’s recoveryflow. The results suggest that even short periods of bed rest can significantly reduce lower limb bloodflow which could have implications for DVT development. Electrically elicited calf muscle contractionssignificantly improve lower limb blood flow and can alleviate some debilitating effects of bed rest. © 2010 IPEM. Published by Elsevier Ltd. All rights reserved. 1. Introduction Venous thromboembolism (VTE) remains a serious medicalconcern causing morbidity and mortality [1–3]. VTE includes the formationofdeepvenousthrombosis(DVT),whichistheproductof activation of coagulation in a deep vein, and pulmonary embolism(PE)whichariseswhensomeoralloftheDVTisdislodgedandtrav-els to the lungs [3]. Mortality rates associated with PE have been reportedtobeashighas17.4%[4].Approximately1in1000people will suffer a DVT in the developed world [5] and there are approx-imately 170,000 new VTE cases reported in short-stay hospitals inthe United States each year alone [6]. Post-thrombotic syndrome, whichischaracterizedbypain,swellingandvenousulceration,canoccur secondary to DVT [7,8]. This is due to venous hypertension resultingfromthedestructionofvenousvalvesorduetoabnormalmicro-circulation.Currently there are four options for prophylaxis against DVT,namely: pharmacological, compression stockings, intermittentpneumaticcompressionandneuromuscularelectricalstimulation. ∗ Corresponding author at: Electrical & Electronic Engineering, School of Engi-neering and Informatics, NUI Galway, University Road, Galway, Ireland.Tel.: +353 91 493126. E-mail address:  b.broderick2@nuigalway.ie (B.J. Broderick). Pharmacological prophylaxis (heparin, fondaparinux) has beenproventobeveryeffective,tothepointthatitisroutinelycombinedwithnon-pharmacologicalmethodsforhigherriskcases[2].How- ever, the pharmacological approach is not suitable in certain caseswhere there is an associated risk of bleeding [9] or where patientsare distressed by unpleasant subcutaneous heparin injections andrefuse treatment [10].Graduated compression stockings (GCS) are well known to beeffective against DVT development and are the most widely usedformofmechanicalprophylaxis[10,11].Themulti-factorialmecha- nismofoperationisconsideredtobeassociatedwiththereductionof the cross-sectional area of the limb [12], increase in venous flow velocity [13], reduction in venous wall distension [11] and improvement in valvular function [14]. However, there are some disadvantagesassociatedwithGCSuse,particularlyimpairmentof subcutaneoustissueoxygenationandarterialocclusion.Therefore,theproperfittingandwearingofGCSareessentialandpatientswitharterial ischemia and peripheral neuropathy should be excludedfrom their use [10,11].Intermittent pneumatic compression (IPC) devices combatvenous stasis by compressing the deep veins which, in turn, dis-places blood towards the heart. The devices typically consist of amotorized air pump that inflates a cuff that is wrapped aroundthe patient’s leg or foot. Delis et al. showed that IPC significantlyincreasesvenousvelocity,venousvolumeflowandpulsatilityindex 1350-4533/$ – see front matter © 2010 IPEM. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.medengphy.2010.01.006  Pleasecitethisarticleinpressas:BroderickBJ,etal.Apilotevaluationofaneuromuscularelectricalstimulation(NMES)basedmethodologyforthe prevention of venous stasis during bed rest. Med Eng Phys (2010), doi:10.1016/j.medengphy.2010.01.006 ARTICLE IN PRESS GModel JJBE-1628; No.of Pages7 2  B.J. Broderick et al. / Medical Engineering & Physics  xxx (2010) xxx–xxx [15].IPChasbeenshowntobemoreeffectivethanGCSinhigh-riskpatients [16] and is estimated to reduce DVT risk by 60% [17]. IPC significantly reduces venous stasis and prevents DVT. However, amajor drawback with IPC is the issue of patient compliance [18].Bockheim et al. reported an overall compliance of 48% [19] con-cluding that perhaps alternative prophylaxis should be used onnon-ICU patients who had the lowest compliance. The only twopatients who developed DVT, in a study by Froimson et al., hadlowercompliancethanthegroupaverage(<50%)[20].Otherdraw- backs with IPC include discomfort, excessive heat and sweatingunder the inflatable cuffs and peroneal nerve palsy [21–23].Neuromuscular electrical stimulation (NMES) is an alternativeform of DVT prophylaxis [21] which is often overlooked. NMES istheapplicationofanelectricalstimulustomotorpointsinthebodyusingelectrodesplacedonthesurfaceoftheskintoelicitamuscu-larcontraction.NMESappliedtothecalfmusclesfacilitatesvenousreturnbyactivatingtheskeletalmusclepump.NMESadministeredforbloodflowassistapplicationshasdemonstratedpositivehemo-dynamic responses in healthy subjects and patients suffering fromchronic venous disease [24,25].Bed rest can create an ideal situation for the development of DVT due to venous pooling or stasis; a criterion of Virchow’s triad.The aim of this study was to evaluate the effect of bed rest on thelower limb venous hemodynamics of a non-hospitalized popula-tion and to determine to what extent NMES induced calf musclecontractions alleviate these effects. In this study, NMES was facil-itated through the use of a two-channel, programmable, portablestimulatorcalledtheDuo-STIMsystem[26].TheDuo-STIMcanactas a development platform for the implementation and testing of NMES algorithms in a clinical setting. 2. Methods  2.1. Participants Ten healthy subjects were recruited for this study from theNUI Galway. Six male and four female subjects took part andhad a median age of 24 (range: 22–36 years), height 173cm(170–184cm), weight 71.9kg (70.45–91.2kg), BMI 23.8kg/m 2 (21.9–26.9kg/m 2 ). A member of the investigation team initiallyexamined the subjects to ensure they satisfied the inclusion andexclusion criteria. The inclusion criteria were healthy participantsaged between 18 and 40 years. The exclusion criteria were as fol-lows: history of heart or respiratory problems, pregnancy, currentuse of oral contraceptive pill, current smoker, history of periph-eral vascular disease or previous thromboembolic event, historyof leg fractures and/or presence of metal implants in the leg, longdistance travel within one week prior to study. All subjects gavewritten consent to take part in the study. Ethical approval for thisstudy was granted by the National University of Ireland GalwayResearch Ethics Committee.  2.2. Resting protocol Allsubjects( n =10)wereaskedtoremainlyinginahospitalbedfor the 4-h duration of the study (resting group). The study tookplace in the Clinical Skills Laboratory at NUI Galway, a simulatedhospital ward environment. The head of the bed was raised to acomfortable position to allow subjects to read or work on a laptopfor the duration of the study (Fig. 1a). Each subject wore a pair of shortsandnofootweartoalloweasyaccesstomeasurementsites.Therestingpoplitealveinbloodflowvelocity,cross-sectionalarea,volumeflowandheartrateofeachsubjectwasassessedatthestart,middle and end of the study (2h increments) from the poplitealfossa using Doppler ultrasound. Ankle thickness was measured atthe start and end of the protocol. To differentiate this resting mea-surement set from the electrical stimulation set, it was referred toas the resting group.  2.3. Electrical stimulation protocol On a separate day, subjects were asked to repeat the previousprotocol. However, this time neuromuscular electrical stimulationwas applied to the calf muscles in order to increase popliteal veinblood flow through activation of the calf muscle pump (stimula-tiongroup).Subjectsweredividedequallyintotwogroups.Forthefirst stimulation group (S1,  n =5) stimulation was applied alterna-tively to each leg with 30s rest between contractions. The secondstimulation group (S2,  n =5) had stimulation applied to one legonly with 60s rest between contractions. Stimulation amplitudeswere set by determining the maximum level tolerated for each legbefore starting the protocol. This yielded stimulation amplitudesof 29.18 ± 4.2V. Electrical stimulation was facilitated through theuseof5cm × 5cmPALSself-adhesive,hypo-allergenic,skinsurfaceelectrodes (Nidd Valley Medical Limited, England) placed over themotor points of the calf muscles.Electrical stimulation was applied using the Duo-STIM musclestimulatorwithabloodflowassistapplication[17].Thestimulator wasprogrammedtoprovideapulsewidthof350  s,aninter-pulseinterval of 100  s, a frequency of 36Hz, a contraction time of 1.2s,a ramp up time of 500ms and ramp down time of 300ms (Fig. 1b).The stimulation parameters were selected to achieve maximumblood flow while ensuring subject comfort and were based on thestimulation guidelines described by Baker et al. [27].  2.4. Duplex scanning  Duplex Doppler ultrasound was used to monitor the subjects’lower limb hemodynamics using a 4–8MHz linear transducer(LOGIQe;GEMedicalSystems).Allmeasurementswereperformedby a single examiner. Blood flow measurements were taken fromthe popliteal vein and artery at the lateral aspect of the knee(Fig.1a).Allmeasurementsweretakenfromtherightleg,exceptinthe case of group S2 where measurements were also taken in theleft leg. Three measurements were taken per parameter and theaverage of these was used for analyses. Peak venous velocity, veincross-sectional area and venous volume flow measurements wererecorded from the popliteal vein. Heart rate was measured fromthe popliteal artery. Average blood flow velocity was obtained byanalyzingtheDopplerpulsewaveformandcalculatingtheaverageof the velocity waveform over a 4-s window. Venous volume wascalculated by multiplying the average blood flow velocity by thecross-sectional area of the popliteal vein. Only baseline blood flowmeasurementswererecordedduringtherestingprotocol.Baseline(recoveryflow)andNMESbloodflowDopplermeasurementswererecorded at each time point for the stimulation group.  2.5. Other measurements Ankle and foot thickness was measured using a figure-of-eight tape measurement to quantify any ankle swelling, howeverunlikely.Ameasuringtapewaswrappedoncearoundtheankleandfoot covering the medial malleolus, Achilles tendon, lateral malle-olus and the foot arch with the zero of the tape kept between theanteriortibialtendonandthelateralmalleolus,asdescribedbyReiset al. [28]. The figure-of-eight method was chosen due to its excel-lenttest–retestreliability[28].Eachmeasurementwastakenthreetimesandthemeanwasusedforanalysis.Ankleandfootthicknessmeasurementsweretakenatthestartandendofthetrialandwereperformed on the right leg of all subjects during both the restingand electrical stimulation protocols.  Pleasecitethisarticleinpressas:BroderickBJ,etal.Apilotevaluationofaneuromuscularelectricalstimulation(NMES)basedmethodologyforthe prevention of venous stasis during bed rest. Med Eng Phys (2010), doi:10.1016/j.medengphy.2010.01.006 ARTICLE IN PRESS GModel JJBE-1628; No.of Pages7 B.J. Broderick et al. / Medical Engineering & Physics  xxx (2010) xxx–xxx 3 Fig. 1.  (a) Standard resting position adapted by subjects for the duration of the trial. Doppler measurements were taken from the popliteal vein and artery at the lateralaspect of the knee. The stimulation group used the Duo-STIM muscle stimulator unit to promote lower limb blood flow. (b) A sketch of the trapezoidal, biphasic, stimuluswaveform of one of the channels programmed into the Duo-STIM system, showing the ramp up, contraction and ramp down durations. Stimulus pulses were separated by60s of rest. An important and possible limiting factor of NMES use is theassociated onset of muscle fatigue [29,30]. Muscle fatigue can bedefined as failure to maintain the expected muscle force followingrepeated contraction/relaxation cycles [31]. Fatiguing of the calf muscles would be expected to reduce the hemodynamic perfor-manceofthecalfmusclepumpandthus,wouldbeindicativeofanoverlyintensivestimulationprotocol.Therefore,todetermineifthechosen stimulation protocol would induce muscle fatigue, musclestrengthduetoNMESwasassessedusingahand-helddynamome-ter (Lafayette Manual Muscle Test System, Lafayette InstrumentCo.) [32]. Subjects lay in the prone position with their ankle rest-ing approximately 15cm over the end of the bed. With the ankleslightly dorsiflexed, subjects manually triggered an NMES stimu-lusofthecalfmuscleswhileaninvestigatorheldthedynamometeragainsttheballofthefoot.Theresultingcontractionwasmeasuredbytheinvestigatorresistingthecontraction.Onceagain,eachmea-surement was repeated three times with the mean measurementsused for analysis and the same operator performed all measure-ments.Musclestrengthmeasurementswereperformedatthestartand end of the study on the stimulation group only.Therearetypicallytwosourcesofpossiblediscomfortassociatedwith NMES; the inadvertent activation of sensory receptors on theskinsurface[16]andmusclefatigue[33].Inordertodeterminethe participants’overallperceptionofNMES,a100mm,non-graduatedvisual analogue scale (VAS) was used. Participants were asked tomark the level of discomfort they were experiencing at each timepoint, with 0mm indicating no discomfort and 100mm indicatinganunbearablelevelofpain.Scoresof30mmorlesswereclassifiedas mild discomfort, 31–69mm as moderate discomfort and 70mmandaboveasseverediscomfort[34].Theminimumclinicallysignif-icantdifference(MCSD)inVASscoreswascalculatedasanincreasein scores between test stages of 12mm.We hypothesized that repeated electrically elicited calf musclecontractionstoinducecalfmusclepumpfunctionduringperiodsof immobility would result in an overall improvement in lower limbhemodynamics. The primary outcome variables were peak venousvelocityandvenousvolumeflowmeasurementsfromthepoplitealvein of the test and contralateral limb, and heart rate measuredfrom the popliteal artery. Secondary outcome variables includedthe ankle thickness and strength measurements of the test limb,and stimulation comfort assessment.  2.6. Statistical analysis A repeated measures ANOVA was used in all analyses (SPSS).Any violations to the assumption of sphericity were correctedusing the Huynh–Feldt correction for estimates greater than 0.75or the Greenhouse–Geisser correction for estimates less than 0.75.Relationships between variables were determined using Pearson’scorrelation coefficient. In all analyses, a  p -value of <0.05 was con-sidered statistically significant. 3. Results  3.1. Resting flow vs. recovery flow Nine subjects’ data were included in the final analysis. Time,after the start of bed rest, had no significant effect on normalizedpeak velocity measurements on the resting group. Time had a sig-nificant effect on the normalized peak velocity of the stimulationgroup,  p <0.05. Peak velocities decreased by ∼ 22% at the 2-h mark,  p <0.05, but no significant difference was observed between thestart and the 4-h mark. The normalized volume flow of the restinggroup was significantly affected by time,  p <0.01 (Fig. 2a). Volumeflow reduced by  ∼ 43% after 2h,  p <0.01, and by  ∼ 47% after 4h,  p <0.01. Time had no effect on the normalized recovery volumeflow of the stimulated group,  p =0.183.  3.2. Heart rate of the resting group vs. stimulated group Heartratewassignificantlyaffectedbytimeintherestinggroup,  p <0.001 (Fig. 2b). There was an average drop in heart rate of  ∼ 13%frombaselineatthe2-hmark,  p <0.01.Heartrateincreasedslightlyto  ∼ 12% of baseline at the 4-h mark,  p <0.01. No changes in the  Pleasecitethisarticleinpressas:BroderickBJ,etal.Apilotevaluationofaneuromuscularelectricalstimulation(NMES)basedmethodologyforthe prevention of venous stasis during bed rest. Med Eng Phys (2010), doi:10.1016/j.medengphy.2010.01.006 ARTICLE IN PRESS GModel JJBE-1628; No.of Pages7 4  B.J. Broderick et al. / Medical Engineering & Physics  xxx (2010) xxx–xxx Fig.2.  (a)Percentagechangesinnormalizedvenousvolumeflowat2hand4hofbedrestfrombaselineintheresting,stimulatedgroupandalsointhecontralaterallimb.(b)Percentage changes in heart rate at 2h and 4h of bed rest from baseline in the resting group and stimulated group. * P  <0.01 when compared to baseline. Error bars indicatestandard error. heart rate measurements of the stimulation group were observedwith respect to time. There was a significant relationship betweenvenous volume flow and heart rate in the resting group,  r  =0.99,  p <0.05.  3.3. Stimulated peak venous velocity and volume Fig. 3 shows the stimulated peak venous velocity and venousvolume flow measurements of the stimulation group compared tothe unstimulated resting group. The stimulation group was asso-ciated with significantly higher peak venous velocities than therestinggroup,  p <0.001.Thestimulationgroupexperienced ∼ 650%increase in flow velocity with respect to resting. There was nostageeffectforthestimulatedpeakvenousvelocitymeasurements,  p =0.46.The stimulation group was also associated with significantlygreatervolumeflowthantherestinggroup,  p <0.001.Thestimula-tiongroupexperienced ∼ 301%increaseinvolumeflowwithrespectto resting. There was no significant stage effect for the stimulatedvenous volume flow measurements,  p =0.348.  3.4. Effect of stimulation on the contralateral limb Background popliteal volume flow remained constant in thecontralateral limb throughout the study,  p =0.39 (Fig. 3). No sig-nificant increases in velocity or volume flow were observed in thecontralaterallimbwhenmeasuredduringcontractionsofthestim-ulated limb. No differences were detected in the recovery volumeflowofthestimulatedlimbcomparedtothebackgroundflowofthecontralateral limb,  p =0.284. Volume flow in the contralateral limbwas significantly different from the resting group’s venous flow,  p <0.05.  3.5. Ankle thickness and muscle strength Anklethicknessdidnotchangeasthestudyprogressedforeitherthe resting or stimulation group. No differences in ankle thicknesswereobservedbetweengroups.TestingthereliabilityofthemusclestrengthmeasurementsusingCronbach’salphastatisticresultedinascoreof0.933indicatingahighlevelofreliabilityoftheoperator.Musclestrengthmeasurementsofthestimulationgroupwerealsounaffected by time.  3.6. Visual analogue scale (VAS) scores VAS scores compared comfort levels between test stages foreach participant. Table 1 shows the outcome for each participantfor every stage and the difference in VAS scores between the startof the study (after NMES had commenced) and the 2-h mark andthedifferencebetweenthe2-hmarkandthe4-hmark.Categorical Fig. 3.  (a) Peak venous velocity measurements (cm/s) measured at the start and after 2h and 4h of bed rest. (b) Venous volume flow (ml/min) measured at the start andafter 2h and 4h of bed rest.  P  <0.001 when compared to resting group. Error bars indicate standard error. *An outlier at the 2-h mark having a value of 737.69ml/min.  + Anoutlier at the 4-h mark having a value of 929.81ml/min.  Pleasecitethisarticleinpressas:BroderickBJ,etal.Apilotevaluationofaneuromuscularelectricalstimulation(NMES)basedmethodologyforthe prevention of venous stasis during bed rest. Med Eng Phys (2010), doi:10.1016/j.medengphy.2010.01.006 ARTICLE IN PRESS GModel JJBE-1628; No.of Pages7 B.J. Broderick et al. / Medical Engineering & Physics  xxx (2010) xxx–xxx 5  Table 1 VAS scores (mm), pain category and the difference in scores between start of the study and the 2-h mark, and the difference between the 2-h mark and the 4-h mark.Participant VAS scores start Pain category VAS scores after 2h Pain category Difference start − 2h VAS scores after 4h Pain category Difference2h − 4h1 51 Moderate 56 Moderate 5 66 Moderate 102 17 Mild 37 Moderate 20 >MCSD 48 Moderate 113 19 Mild 16 Mild  − 3 21 Mild 54 5 Mild 14 Mild 9 14 Mild 05 7 Mild 2 Mild  − 5 3 Mild 16 29 Mild 21 Mild  − 8 15 Mild  − 67 29 Mild 35 Moderate 6 37 Moderate 28 42 Moderate 34 Moderate 8 23 Mild  − 119 53 Moderate 32 Moderate  − 21 28 Mild  − 410 25 Mild 25 Mild 0 26 Mild 1 Fig. 4.  Example of a blood flow waveform, with the vertical axis representing velocity, illustrating an NMES elicited blood flow pulse and recovery flow of the stimulatedgroup and the resting flow of the non-stimulated group. The recovery blood flow following NMES elicited blood flow remained higher than the resting group’s backgroundflow. The parallel lines indicate a break in the time axis. scores indicated mild pain in seven participants and moderate inthree. Mild pain was associated with a brief sensation of pins andneedlesduringtheonsetofstimulation.Moderatepainwasindica-tive of a strong contraction bordering on fatigue. At the 2-h mark,twoparticipant’spaincategoriesincreasedfrommildtomoderate.However,onlythesecondparticipant’sVASscoreincreasedbeyondthe MCSD with a difference from the start of 20mm. Two partic-ipants categorical scores decreased from moderate to mild at the4-h mark with a drop in VAS score of 11mm and 4mm. 4. Discussion Our data demonstrate that after 4h of bed rest, blood flow inthe popliteal vein reduced significantly by approximately 47%. Thereduction in venous flow would have little effect on the healthysubject group monitored, who have no risk factors (no evidenceof pooling was observed in any of the groups). However, thiscould have significant implications for DVT development in post-operativepatients,eventhosedischargedfromhospitalonthedayof surgery. Squizzato and Venco point out that pharmacologicalthromboprophylaxis is not recommended for this patient group[9]. The authors suggest that the anticoagulant thromboprophy-laxis would increase the risk of major bleeding and may exceedthe risk of thrombotic complications. For these reasons, the use of NMES prophylaxis may be preferable.ThefindingsofourstudysuggestthattheDuo-STIMwascapableof reversing the decline in resting popliteal blood flow due to bedrest. The stimulus pulse, delivered once every minute, caused thebetweenstimulusrecoveryvenousvolumeflowtoremainconstantthroughoutthe4-hbedrestperiodforthestimulatedgroup(Fig.4).In comparison, the resting group experienced a 47% decrease inflow. The stimulus pulses themselves elicited peak venous veloci-ties and volumes that far exceeded baseline (approximately 650%increase in peak venous velocity and approximately 301% increasein volume flow).Controversy exists over the use of peak venous velocity as anindicatorofvenousbloodflow.Despitepeakvenousvelocitybeingthe blood flow parameter typically reported in literature as a mea-sureofmechanicalDVTprophylaxisefficacy,MorrisandWoodcockfound no evidence that a higher peak velocity yields lower DVTrates[18].AccordingtoastudybyProctoretal.,itwouldappearthatvery high peak velocity inducing devices might actually increaseDVT incidences [35]. The reduction in popliteal blood flow in thepresent study was only detected by a drop in venous volume flow.Asthefocuswasonpreventionofvenousstasis,peakvenousveloc-ity measurements were of little hemodynamic importance duringthis study.Wealsoobservedthatthestimulatedgroupmaintainedahigherheart rate throughout the study. With high venous return, thecardiac muscle is stretched and therefore contracts more pow-erfully. Increased contractility with no increase in after-load willresultinincreasedstrokevolume.Theincreasedmetabolicdemandprobably leads to a more rapid decrease in aortic pressure due toblood heading to the muscles in use. This would mean the barore-ceptors are less stimulated and do not slow heart rate as much.NMES elicited calf muscle contractions would also have the effectof stimulating the sympathetic nervous system which would leadto increased heart rate and further increase in contractility of theheart muscle.The parameters chosen for stimulation (stimulation duration,one stimulus impulse per minute) resulted in some interestingobservations. Results of the muscle strength test indicated that nosubject showed signs of muscle fatigue at the end of the study.During this time frame, stimulated popliteal blood flow remainedconstant; no decrease in flow was observed. This implies that pro-longed use of electrical stimulation should be feasible without the
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