Six-Minute Walk Test: A Study to Evaluate Normal Values in Female Saudi Arabia Population

Background: Different ethnic populations have normal values for reference. These values are vital to a precise interpretation of the 6MWT in clinical practice. Objective: To evaluate normal six-minute walk distance among young Saudi female population. Design: Cross sectional, Prospective. Setting: Academic set up. Patients and Methods: Cross sectional study conducted on female volunteers aged 18-30 years old. 6MWD was done in a 30 meters long corridor. Vital signs were measured before and after the test. MET was calculated from the distance walked. Main Outcome Measure(s): The primary outcome measure was the 6MWD. Results: Sixty-nine female volunteers participated in the study. Mean age was 23.00 (1.67) (years) and BMI was 22.67(4.10) kg/ m2. 6MWT mean distance covered was 403.43 (91.62) meters. The Metabolic Equivalent and Maximum Heart Rate (%mHR) achieved were 2.81(0.76) ml/kg/min and 50.41(6.7) %, respectively. Height was found to be positively correlated with 6MWD. The 6MWD was found to be in agreement with the reference equation for Saudi population (P <0.05). There is a significant difference in heart and respiratory rates, p<0.000, CI {-14.16(-4.54)}, {-5.45(-2.63) respectively. Conclusion: Height was found to be positively correlated with 6MWD. A large multicenter study is required to confirm the present data and to include a larger age scale.


Introduction
Six-Minute Walk Test (6MWT) is a simple method of evaluating exercise capacity of an individual. Assessment of functional capacity of an individual without any equipment or additional training is a unique to 6MWT [1]. Furthermore, it examines global and integrated responses of pulmonary and cardiovascular systems, systemic circulation, peripheral circulation, blood, neuromuscular units, and muscle metabolism [2]. However, it does not provide parameters about each organ systems compared to other cardiopulmonary exercise tests [1]. 6MWT was found to be more suitable in patients with respiratory disorders compared to 12 min walking [3]. As most of activities of daily living are done in submaximal level of effort makes 6 MWT a superior assessment of daily activities of living.

Materials and Methods
A prospective cross-sectional study performed over one year, 2017-2018. Participants were recruited through convenient sampling. Ethical approval was obtained from IRB of Scientific Research Unit (SRU) at Medical College. All participants signed a written consent form. Anthropodermic data including height, weight, and age as well as participants' medical history were collected before the 6MWT. 6MWT procedures were explained to the participants and any questions concerning procedures were answered. Height was measured using a standard gauge calibrated as per manufacturer's standards without shoes by standing on height measurement scale stadiometer. Commercially available electronic weighing machine (TCS-200-RT, China) calibrated as per manufacturer's standards were used for the weight measurement and BMI was calculated as Weight/ Height 2 . BMI was categorized according to the World Health Organization definition: BMI <18.5 kg/m 2 -Underweight, BMI 18.5-24.9 kg/ m 2 -Normal weight, BMI 25.0-29.9 kg/m 2 -Pre-obesity, BMI 30.0-34.9 kg/m 2 -Obesity class I, BMI 35.0-39.9 kg/m 2 -Obesity class II, BMI Above 40 kg/m 2 -Obesity class III. Inclusion criteria included absence of illness, age > 18 years, and willingness to participate. Exclusion criteria included presence of any chronic diseases, cardiac disorders, respiratory disorders and pregnancy.

Six-Minute Walk Test
6MWT was performed according to ATS guidelines [1]. Walking corridor was 30 meters long. Turnaround point was marked with orange cones and chairs were placed at small interval distance in walkway. Participants were informed to avoid strenuous exercise two hours before test. Only a light meal was allowed before test. Participants were instructed to walk as long as they can during six minutes time limit. Vital signs, oxygen saturation and Borg Dyspnea Scale of were measured prior to starting test. Participants were informed to have a rest and sit on nearby chair if they experience any signs and symptoms such as dizziness, fatigue, leg cramps, chest pain or dyspnea and continue walking after feeling better. Time and distance were recorded as well as encouraging participant by saying "you are doing well; you have 3 min to complete the test". Distance covered in 6 minutes was calculated and documented. After completion of 6 MWT, vital signs and Borg Dyspnea Scale were measured again. Metabolic Equivalent (MET) was calculated using conversion tables for estimating MET based on distance walked. Sample size calculated with 95% Confidence Interval was 119.

Statistical Analysis
Data was analyzed using SPSS software version 25.0 (SPSS, Inc., Chicago, IL). Descriptive data is expressed as a mean ± Standard Deviation (SD). Paired t test was performed for prepost variables. Variables such as height, weight, age, BMI, oxygen saturation, heart rate and respiratory rate were analyzed with Pearson's correlation. Regression was performed for 6 MWD, age, weight, height, and BMI. P ≤ 0.05 was considered statistically significant.

Results
Sixty-nine females were recruited. Mean age was 23.00 (1.67) (years) and BMI was 22.67 (4.10) (kg/m 2 ). 6MWT mean distance covered was 403.43± 91.62 meters. MET was 2.81(0.76) ml/kg/ min. Table 1 presents the characteristics of the participants. Table  2 presents 6MWT values before and after. Data shows that there is a significant difference in heart rate and respiratory rates, p<0.000, CI {-14.16-(-4.54)}, {-5.45-(-2.63) respectively. No statistically significant difference in systolic blood pressure, diastolic blood pressure, and SpO 2 was observed. Figure 1 illustrate the significant difference in the vital signs before and after the 6MWT. Percentage of Maximum Heart Rate (%mHR) achieved was 50.41(6.7)%. Table 3 presents Pearson correlation for variables collected during study with 6MWT distance. Figure 2 represents the relationship between 6MWT and other variables. Height and SpO 2 correlated well with 6MWT with significant statistical difference. 6MWD was calculated using reference equation by Alameri, H et al and is compared with measured 6MWD of present participants as seen in Table 4. No significant difference between the two studies was observed.

Discussion
Present study is considered one of a few studies that have evaluated 6MWT in female Saudi Arabia population with age group 18-30 years. Current study shows that 6MWD among this population is 403.43 (91.62) meters. This is shorter than other normal reference value of different ethnic population [5,[24][25][26]. Furthermore, findings show that age is negatively correlated with 6MWD but is not statistically significant. Small sample size could be a contributing factor. On the other hand, height is statistically significant and correlated well with 6MWD. In contrast, there was a negative correlation between BMI and 6MWD. Moreover, there was a significant difference between the HR and RR before and after the 6 MWT. This could be attributed to cardiopulmonary effects of the 6MWT. Study result is on agreement with previously published similar investigations. 6 [4]. However, in comparison to other ethnic groups such as Caucasian and Asian population, present 6MWD is less than other ethnic groups. Thus, present data strongly suggest that using Caucasian or Asian population reference values would lead to an overestimation among female Saudi Arabia population with age group 18-30 years [27].
Differences may be caused by differences in anthropometric factors; demographic data; race and the level of physical activity. Low physical activity or physical inactivity has been reported in literature in oil producing Arab countries such as Bahrain, Kuwait, Oman, Qatar, Saudi Arabia, and the United Arab Emirates [28]. which accounts for increased mortality and morbidity. In Saudi Arabia, physical activity is found to be 6.1% in male and 1.9% in female; respectively [29]. Our participants reached a maximum percentage HR (%maxHR) of 50.41± 6.7% of their maximum predicted HR. Chetta A et al reported 67±10% of their maximal predicted heart rate in their study6 which is higher than our observation. Participants in Alameri et al study reached %maxHR of 47% which is lesser than our observation. Participants studied by Zou et al reached an average %maxHR of 59% which is close to our findings. Limitations of this study include: first; we did not include individuals more than the age of 30 so results cannot be generalized for age group other than 18-30 years. Second, small sample size. Third; present investigation was conducted in a female college cohort group. Therefore, male participants were not recruited.

Conclusion
6MWD among female Saudi Arabia with ages between 18-30 years is 403.43 (91.62) meters. Height was found to be positively correlated with 6MWD. A large multicentre study is required to confirm the present data and to include a larger age scale.