Luke Holland(Supervised by Dr. Neil Maxwell)

Centre for Sports Research,Welkin Labs, Denton Road, Eastbourne, BN20 7SR

Validity of a 6 minute walk test in simulated altitude: screening hypoxic tolerance?

MethodPARTICIPANTS•Twenty nine male (n=15) and female (n=14) 2nd year sport and exercise science students and university staff, whose height and weight was 173 + 9 cms and 72 + 13 kg, volunteered for this study.

EXPERIMENTAL DESIGN AND PROCEDURES• Participants undertook four, 6 minute walk tests (6MWT), each preceded by a practice 6MWT; three 6MWTs in Eastbourne and one in Peru (Figure 1). Of the normobaric Eastbourne 6MWTs, two were treadmill-based (one in normoxia and one in hypoxia, ~3400 m) and one was outdoor-based (normoxia). The Peru 6MWT was conducted at 3395 m on the second day at altitude.

•The 6MWTs involved participants being asked to complete as much distance as possible within 6 minutes without breaking into a jog or run. The treadmill-based 6MWTs were conducted at 0% grade, with speed being increased or decreased on command by the participant.

• Distance walked during the 6MWT was recorded alongside pre and post measurements of heart rate, oxygen saturation, blood pressure, respiratory rate (RR) and ratings of perceived exertion (RPE). Derivative calculations of predicted hypoxic ventilatory response (HVR) and hypoxic cardiac response (HCR) were determined.

Results

Figure 3 – Mean (±SD) distance walked in the 6MWTs. * Denotes significant difference (P<0.05) between hypoxic indoor and all other conditions

Table 1 – Mean (±SD) physiological responses during the 6MWTs

a, b, c, d Values containing the same superscript denote conditions are not significantly different from each other (P>0.05)

Discussion• The distance walked during the indoor (treadmill-based) normoxic trial was no different to the outdoor normoxic trial, confirming results of Elazzazi et al (2012) that the 6MWT can be performed on the treadmill at sea-level. However, distance walked in the indoor (treadmill-based) hypoxic trial was significantly less than during the outdoor hypobaric trial in Peru.

• While the cardiovascular strain differed across all conditions, oxygen saturation was only different when normoxia trials were compared to either hypoxic or hypobaric trials.

•Across the other physiological and perceptual responses, the 6MWT conducted in the hypobaric trial presented the greatest strain compared to all other trials.

• Possible delimitations within this study included, a greater heat stress in the hypobaric (Ta, 24.2C; RH, 37%) compared with the indoor trials (Ta, 20.5C; RH, 34%), or the normoxic outdoor trial (Ta, 14.8C; RH, 32%) , and that the treadmill used in the hypoxic trial was a different model (and belt length) to that used in the normoxic treadmill trial which may have altered walking gait.

ReferencesBernardi, L; Schneider, A; Pomidori, L; Paolucci, E; Cogo, A (2006). Hypoxic ventilatory response in successful extreme altitude climbers. European Respiratory Journal. Volume: 27 Issue: 1 Pages: 165-171.

Burtscher M, Flatz M, Faulhaber M (2004) Prediction ofsusceptibility to acute mountain sickness by SaO2 values duringshort-term exposure to hypoxia. High Alt Med Biol 5:335–340.

Elazzazi. A, Chapman. N, Murphy. E & White. R (2012), Measurement of distance walked and physiologic responses to a 6 minute walk test on level ground and on a treadmill: A comparative study, Journal of Geriatric Physical Therapy , 35(1), pp. 2-6.

Lazio, MP; Van Roo, JD; Pesce, C ; Malik, S; Courtney, DM (2010) Postexercise Peripheral Oxygen Saturation After Completion of the 6-Minute Walk Test Predicts Successfully Reaching the Summit of Aconcagua Reply. Wilderness & environmental medicine. Volume: 22 Issue: 2 Pages: 194-195.

BackgroundA reduced partial pressure of oxygen when at altitude promotes a greater physiological challenge on humans compared with a normobaric environment (Bernardi et al 2006). Burtscher et al (2004) predicted hypoxic tolerance from passive exposure to short term hypobaric and normobaric environments, but it is likely that exercise contributes to hypoxic tolerance as well. Lazio et al (2010) found 6 minute walk tests (6MWT) performed at altitude predicted likelihood of summit success on Mount Aconcagua (6961m). It is unclear whether the physiological responses to a treadmill-based 6MWT in hypoxia (i.e. simulated altitude) represent those responses in a hypobaric environment (i.e. true altitude).

RESEARCH QUESTIONIs a treadmill-based 6MWT in hypoxia a valid protocol of hypoxic tolerance compared with a 6MWT completed in a criterion-based hypobaric environment (Cusco, Peru)?

Normoxic Indoor Hypoxic indoor Normoxic outdoor Hypobaric outdoor

Heart Rate (BPM) 126 + 29a 142 + 19b 116 + 29c 155 + 16d

Oxygen Saturation (%) 97 + 3a 84 + 5b 98 + 1a 83 + 4b

Systolic Blood Pressure (mmHg) 163 + 23a 162 + 22a 164 + 25a 179 + 27b

Diastolic Blood Pressure (mmHg) 81 + 14a 81 + 10a 90 + 21b 92 + 22b

Respiratory Rate 12 + 3a 12 + 3a 11 + 4a 19 + 5b

RPE 13 + 2a 13 + 2a 11 + 2b 15 + 2c

HVR (RR-1.%-1) N/a 2 + 2a N/a 2 + 1a

HCR (Beats.min-1.%-1) N/a 19 + 19a N/a 7 + 5b

EASTBOURNETreadmill

6MWT(Hypoxia, ~3400 m)

Treadmill6MWT

(Normoxia, sea-level)Outdoor6MWT

(Normoxia, sea-level)

Outdoor6MWT

(Hypobaric , 3395m)

PERU

Normoxic Indoor

Hypoxic indoor

Normoxic outdoor

Hypoxic outdoor

750760770780790800810

Condition

Dist

ance

(m)

Figure 2 – The 6 Minute Walk Test at Cusco Running Track, Peru.

Figure 1 – Schematic of the three 6MWTs in Eastbourne and one 6MWT in Peru.

*

ConclusionWith the exception of oxygen saturation, the data from this study does not support hypoxic tolerance derived from a 6MWT performed on a treadmill in hypoxia to represent that experienced from performing the 6MWT in a hypobaric (altitude) environment.