Introduction
Because of technological limitations, pulmonary oxygen uptake (Vo 2 ) and ventilation (VE ) during swimming have traditionally been determined using the Douglas bag (DB) method. Expired air has either been collected during swimming exercise (1,3,18,26,27 12,19,28,35 11,30 2 or carbon dioxide (CO2 ) fractions (30 31,33,37 30
A number of portable online metabolic carts (e.g., Oxylog by P.K. Morgan, the K2 and K4b2 by Cosmed, the Oxycon by Jaeger, and Cortex's Metamax 1, II, and 3B) have been used to assess VE , Vo 2 , and CO2 output (Vco 2 ) during terrestrial activities. The reliability of these and similar systems has typically been determined by comparing resting, submaximal, and vigorous/maximal exercise values with those obtained using the DB method (23–25,34,40 o 2 by 3–14%, Vco 2 by 3–17%, and VE by 4–8% during moderate and vigorous cycling and rowing exercise collectively (25,34,40 o 2 measured at rest and during both moderate and vigorous exercise, and 2–12% for maximal Vo 2 measures (Vo 2 max) (23,24,34,40 co 2 , <1–6% for submaximal VE , and <5% for maximal VE (23,34,40
Recent technological advances led to the development of 2 aquatic-specific metabolic cart systems. These are the Cosmed Aquatrainer, which is used in conjunction with the Cosmed K4b2 , and the Cortex MetaSwim (MS) device. Both systems can be used conventionally with a mask or in an aquatic environment through a specialized freestyle snorkel. The Aquatrainer is the more popular of the 2 aquatic-specific systems, with a number of agreement (4,15,20 31,33,37 2 assembly, the Aquatrainer has been shown to underestimate Vo 2 , Vco 2 , and VE during both submaximal and maximal cycling by 4–21% (15,20 15 4 o 2 , Vco 2 , and VE but did observe that the variation was greater during swimming compared with cycling exercise.
Collectively, these studies demonstrate that perfect agreement and repeatability between and within different open-circuit spirometry approaches does not exist. This is not surprising, given that biological and technical variability will influence the data (5,13,24 16
The aims of this study were: (a) to determine the agreement between Vo 2 , Vco 2 , and VE using the MS and DB methods during flume swimming; and (b) to assess the repeatability of these and other MS-derived measures of pulmonary gas exchange and ventilation during flume-based swimming exercise of different intensities.
Methods
Experimental Approach to the Problem
The study consisted of 2 phases that were preceded by a single familiarization session and combined incremental and supramaximal verification test (Figure 1 ) to determine swimmers Vo 2 max and gas exchange threshold (GET). Phase 1 was designed to assess the agreement between Vo 2 , VE , and Vco 2 using the MS and DB methods when swimming at different intensities. It was also used to assess within-trial and between-trial repeatability of Vo 2 , Vco 2 , and VE using the MS vs. DB methods. Phase 2, which occurred on different days to phase 1, was designed to assess the between-day variation and hence repeatability of MS derived ventilatory measures (tidal volume [VT] and breathing frequency [f r o 2 , VE , and Vco 2 , this included the end-tidal pressures of O2 and CO2 (PET O2 and PET CO2 , respectively).
Figure 1.: Schematic of the protocol. *Limit of tolerance and †order of DB and MS collections counterbalanced between participants. Duration denotes duration of the stage. Sample denotes time point (minutes) within a given stage of phase 1 that expired air was measured. DB = Douglas bag; MS = MetaSwim.
All testing was completed using the front crawl stroke and in the same swimming flume (SwimEx 600-T Therapy pool, length 4.2 m, width 2.3 m, and depth 1.5 m) housed within a climate-controlled chamber. Although swimmers could take part in both phases, only 2 swimmers completed the protocol because of the required time commitment. In addition, during each Vo 2 max test and each experimental trial of phases 1 and 2, a flow turbine meter (Model 001, Current flow meter; ValePort, Totnes, United Kingdom) was used to independently assess flume speed.
Douglas Bag and MetaSwim Overview
Briefly, DB collections were made using a modified snorkel connected through standard respiratory tubing (32 OD, Hans Rudolf, Kansas, USA) to a DB rig containing multiple 150 L bags (Cranlea, Birmingham, United Kingdom). Standardized equations (11 o 2 (standard temperature and pressure, dry, STPD), Vco 2 (STPD), and VE (body temperature and pressure, saturated with water vapor, BTPS) from the measured fractions of expired O2 and CO2 (Rapidox 3100 gas analyzer; Sensotec, Cambridge, United Kingdom), bag volume (dry gas meter; Harvard Apparatus, Holliston, MA, USA), and expired air temperature (MCP multi digital thermometer, Shanghai, China).
Breath-by-breath changes in VE , VT, f r o 2 , Vco 2, PET O2 , and PET CO2 were measured using the MS. A triple V digital flow sensor (manufacturer reported resolution of 7 ml, accuracy of ±2%) was placed at the end of the snorkel and was protected by 2 lightweight splash protectors. The snorkel contains a twin-tube and was connected to a tube-in-tube gas sample line through a hydrophobic filter (Figure 2 ). Expired O2 and CO2 were sampled at the mouth and were analyzed by an electrochemical sensor for O2 and a nondispersive infrared sensor for CO2 housed within the MS device (Figures 2 and 3 ).
Figure 2.: MetaSwim and snorkel. For clarity reasons, only one splash protector is shown.
Figure 3.: Participant swimming while instrumented with the MetaSwim.
Before testing, the gas analyzer and MS were calibrated using ambient air and gases of a known concentration in line with the manufacturer instructions, and the MS flow sensor was calibrated using a calibrated 3-L syringe supplied with the MS (Cortex, Leipzig, Germany).
Subjects
Sixteen trained club-level competitive swimmers (10 females aged 19–39 years old) volunteered for this study, which consisted of 2 phases. Mean ± SD for absolute and body mass relative maximal Vo 2 (Vo 2 max), which was measured at the start of the study during front crawl, age, body mass, and stature were 3.49 L·min−1 , 48.5 ± 10.7 ml·kg−1 ·min−1 , 22 ± 5 years, 72.0 ± 10.4 kg, and 1.75 ± 0.07 m. All participants provided fully informed written consent, and institutional ethical approval was granted and approved by the University of Portsmouth's institution review board before the study commenced.
Procedures
Familiarization and Vo 2 max Determination
Participants were first familiarized with the operation of the swimming flume and became fully accustomed to swimming in the flume wearing the relevant snorkels before any testing took place: swimmers had used the swimming flume and a snorkel before familiarization, either by participating in other swimming research studies or, in the case of the snorkel, in training. After this, participants determined a self-selected warm-up velocity that could be comfortably sustained for 10 minutes without any increase in perceived effort. This velocity (0.93 ± 0.09 m·s−1 ) was then selected as the warm-up and cool-down velocity for all subsequent phase 1 or 2 tests. The familiarization session lasted approximately 20 minutes.
The Vo 2 max test was completed in the same testing session as the familiarization session after 15-minute rest. After a 5-minute warm-up, swimmers completed a progressive intensity swimming test consisting of 2-minute stages until the limit of tolerance. At the end of each 2-minute stage, velocity was increased by 0.05–0.10 m·s−1 until the limit of tolerance (inability to maintain velocity). After this, swimmers undertook a 5-minute cool-down, followed by 10 minutes of passive seated rest on poolside. Participants then completed a supramaximal constant-velocity test to verify that their measured Vo 2 peak reflected Vo 2 max. A 3-minute warm-up preceded an individualized step transition to a work rate corresponding to 105% of the final velocity achieved during the incremental Vo 2 max test (adapted from Ref. 36 o 2 max test. Participants were required to swim at this velocity until reaching their limit of tolerance. The highest 10-second average value achieved during either the Vo 2 max or verification test was taken to represent Vo 2 max (Figure 1 ).
The GET was identified from the incremental test using the V-slope method and verified using the ventilatory equivalents for O2 and CO2 , and the end-tidal gas tension methods (7,11
Phase 1 Procedure
Nine swimmers (5 females; age: 22 ± 6 years; height: 1.77 ± 0.06 m; body mass: 77.6 ± 8.8 kg; and Vo 2 max: 48.6 ± 13.3 ml·kg−1 ·min−1 ) completed 2 variable-intensity swimming tests (barometric pressure: 764 ± 4 mm Hg; ambient temperature: 20.1 ± 0.7° C; and water temperature: 27.7 ± 0.4° C). Swimmers completed a 5-minute warm-up (velocity did not exceed velocity of stage 1), followed by 10 minutes of swimming at an intensity 15% below GET (stage 1: low) and 10 minutes of swimming at an intensity at the velocity immediately below GET (stage 2: mod) (modified from 4). Stage 1 and 2 velocities were chosen to ensure that the participants would reach a steady state in 3 minutes, so MS and DB collections could be made interchangeably during the 10-minute stage. Swimmers wore a nose clip throughout, along with the MS snorkel connected to the MS metabolic cart or a modified snorkel connected to the DB rig during the relevant part of each data collection stage.
During each 10-minute stage (low, mod), 5 minutes was designated as an MS collection phase and 5 minutes was designated as a DB collection phase. Expired air was only collected in 60-second bouts in the final 2 minutes of each 5-minute phase (minutes 3–5) per 10-minute stage. This permitted Vo 2 , Vco 2 , and VE to be calculated per 60 seconds of the 2-minute MS and DB data collection phases per stage (Figure 1 ).
After completion of stage 2, the velocity was increased (0.05–0.10 m·s−1 ) every 2 minutes until the limit of tolerance was reached (stage 3). The highest Vo 2 , VE , and Vco 2 values observed during stage 3 were recorded as peak values. Because stage 3 required non–steady-state swimming, expired air was collected continuously per 60 seconds of each 2-minute stage using only the MS in one test and DB only in the other test. The selection of either MS or DB for test 1 in participant 1 was determined using a coin-toss and then counterbalanced for all participants thereafter. In test 2, stages 1 and 2 were collected in an identical order; however, if stage 3 was collected using the DB in test 1, it was collected using the MS in test 2 and vice versa (Figure 1 ). Although the order of MS and DB collections and number of 2-minute stages were identical per participant per variable-intensity test (excluding stage 3), the order of MS and DB collections was counterbalanced between participants.
Phase 2 Procedure
Nine swimmers (6 females, age: 22 ± 7 years; height: 1.72 ± 0.07 m; body mass: 70.0 ± 13.2 kg; and Vo 2 max: 44.4 ± 7.8 ml·kg−1 ·min−1 ) completed 3 or 4, 6-minute constant-velocity swimming tests (barometric pressure: 767 ± 2 mm Hg; ambient temperature: 24.1 ± 0.7° C; and water temperature: 27.8 ± 0.1° C) on different days. The velocity of these swims was based on critical velocity. Critical velocity (VCrit: 1.08 ± 0.13 m·s−1 ) was determined separately by backward extrapolation from a 400-m (346.1 ± 48.7 seconds) and 800-m (721.7 ± 95.5 seconds) time-trial pool swim, administered in a counterbalanced order and completed on separate days after a standardized competition warm-up (22 14 38
Each 6-minute constant-velocity swimming test began with 10 minutes of seated rest. Participants were then instrumented with the MS snorkel and donned a nose clip, which they wore for the reminder of the trial. They then undertook 3 minutes of prone floating (baseline: during which a low current was switched on to aid buoyancy), followed immediately by 6 minutes of constant-velocity swimming at a pace 5% slower than critical velocity (VCrit5% slower ). After a 30-minute–seated poolside recovery, participants again floated for 3 minutes in the flume, followed immediately by 6 minutes of constant-velocity swimming at a pace 5% faster than critical velocity (VCrit5% faster ).
Statistical Analyses
All data were first assessed for normality using a Shapiro-Wilk test and were normally distributed. Vo 2 max was calculated as the mean and SD of all 16 swimmers. The agreement and within-trial DB and MS repeatability data (phase I) were based on all 9 swimmers completing phase 1. The MS repeatability data (phase 2) were based on all 9 swimmers completing phase 2.
Phase 1: Variable-Intensity Tests
Vo 2 peak, Vco 2 peak, and VE peak were compared between MS and DB (DB-MS) using limits of agreement (LoA) along with bias, random error, and 95% confidence intervals (CIs), in accordance with methods reported previously (5,9,10 t -tests (IBM SPSS, v24, α = 0.05) were used to assess for significant bias between MS and DB measurements per stage and per variable.
As heteroscedasticity was present in some stage 1 and 2 data, Vo 2 , Vco 2 , and VE were logarithmically transformed (natural log), antilogged, and displayed as ratios (5,9,10 o 2 , Vco 2 , and VE were compared between DB and MS (DB-MS) using ratio LoA, bias, random error, and 95% CI in accordance with the methods of Bland and Altman (9,10 5
To determine the within-trial repeatability for MS and DB, each 60 seconds of the 2-minute collection per stage was compared using the CV and repeatability coefficient (CR). The CV was determined by dividing the SD by the mean and multiplying by 100 (2 SD (square root of the residual mean square) by 2.77 (1.96 multiplied by the square root of 2) (5,39 r and the intraclass correlation coefficient (39 o 2 , Vco 2 , and VE data, these CR data were logarithmically transformed (natural log), antilogged, and expressed as ratio data, including the geometric mean, and displayed along with the 95% LoA (1,5,9,39
Phase 2: Swimming Above and Below VCrit
Along with measured velocity, the final minute of baseline and exercising data (VE , VT, f r o 2 , Vco 2 , PET O2 , and PET CO2 ) was averaged and compared between each of the 3–4 replicate tests. Repeatability was determined using the CR and CV as described in phase 1. As some data were heteroscedastic, all CR comparisons were made using ratio data.
Results
Vo 2 max and Vo 2 max Verification
The highest Vo 2 value determined during the Vo 2 max test was 3.46 ± 0.90 L·min−1 (48.5 ml·kg−1 ·min−1 ). The supramaximal verification test produced a Vo 2 peak of 2.05 ± 0.53 L·min−1 . In only 3 participants, Vo 2 peak was higher in the verification test (by 0.14–0.20 L·min−1 ).
Phase 1: Variable-Intensity Tests
Velocities (CV in parentheses) at stages 1, 2, and 3 were 0.98 ± 0.14 m·s−1 (5.0 ± 2.8%), 1.15 ± 0.15 m·s−1 (4.8 ± 1.8%), and 1.47 ± 0.17 m·s−1 (3.2 ± 1.9%), respectively. The CR for velocity at stages 1, 2, and 3 was 0.15 m·s−1 . The 95% lower and upper LoA were −0.04 and 0.25 m·s−1 for stage 1, 0.04 and 0.20 m·s−1 for stage 2, and −0.02 and 0.18 m·s−1 for stage 3. The GET occurred at 66 ± 7% (2.48 ± 0.63 L·min−1 ) of Vo 2 max.
Vo 2 peak (t = 1.588, p = 0.151), Vco 2 peak (t = 0.95, p = 0.37), and VE peak (t = 1.25, p = 0.25) were not statistically different between MS and DB methods. Nevertheless, there was a tendency for absolute values to be lower during MS measurements, and both bias and random error were large (Table 1 and Figure 4 ).
Table 1.: Douglas bag (DB) vs. MetaSwim (MS) limits of agreement (LoA) and precision of LoA for Vo 2 peak, VE peak, and Vco 2 peak determined during variable-intensity swimming.*
Figure 4.: Mean difference in Vo 2 peak (A), Vco 2 peak (B), and VE peak (C) between DB and MS plotted against their mean values. Heavy line = bias, Solid line = ± 1.96 SD , p = bias, r = absolute difference between DB and MS and the mean. DB = Douglas bag; MS = MetaSwim.
Bias (p > 0.05) and random error for Vo 2 , Vco 2 , and VE during low and moderate swimming velocities are presented in Table 2 . The CV and CR for Vo 2 , Vco 2 , and VE were typically as good as, if not better than, DB for within-trial MS measurements in both tests (Table 3 ).
Table 2.: Douglas bag (DB) vs. MetaSwim (MS) ratio limits of agreement (LoA) per stage and per variable-intensity test including estimated precision of the LoA.*
Table 3.: Repeatability coefficient (CR), coefficient of variation (CV), and absolute data for Douglas bags (DBs) and MetaSwim (MS) during steady-state swimming per variable-intensity test: within-equipment comparisons.*
Phase 2: Swimming Above and Below VCrit
The CR and CV for velocity, VE , VT, fr , Vo 2 , Vco 2 , PET O2 , and PET CO2 are presented in Table 4 . The repeatability of the physiological parameters was better for exercising values than baseline values during both VCrit5% slower and VCrit5% faster .
Table 4.: MetaSwim absolute, repeatability coefficient (CR), and coefficient of variation (CV) data at rest (base) and when swimming (swim) 5% below (VCrit5% slower ) and 5% faster (VCrit5% faster ) than VCrit.*†
Discussion
The aim of this study was to assess the level of agreement between MS- and DB-derived measurements of Vo 2 , Vco 2 , and VE and to determine the repeatability of VE , VT, fr , Vo 2 , Vco 2 , PET O2 , and PET CO2 measured using the MS during flume-based swimming exercise. Agreement between the MS and DB methods was poor, and that the MS typically underestimated peak and submaximal Vo 2 , Vco 2 , and VE . However, the within-trial repeatability for the MS was at least as good as, if not better than, the DB-derived values, and the test-retest variability (CV) in VE , VT, fr , Vo 2 , Vco 2 , PET O2 , and PET CO2 was consistent with that reported in the literature (23,24,34,40
When compared to the DB method, the MS underestimated Vo 2 peak by 13% (0.39 L·min−1 ), Vco 2 peak by 9% (0.26 L·min−1 ), and VE peak by 11% (9.08 L·min−1 ) (Table 1 ). This is similar to the observations of Gayda et al. (15 o 2 , Vco 2 , and VE were underestimated by 15% (0.50 L·min−1 ), 6% (0.22 L·min−1 ), and 9% (10 L·min−1 ), respectively, when using the Aquatrainer system vs. the K4b2 face mask during cycle ergometry.
The MS also tended to underestimate (bias in parentheses) submaximal Vo 2 (2–17%), Vco 2 (2–11%), and VE (0–17%). This was slightly better than the underestimation in Vo 2 (21%), Vco 2 (2–14%), and VE (18%) reported by Gayda et al. (15 20 4 2 face mask with the Aquatrainer system during cycle ergometry. Keskinen et al. (20 −1 ) for Vo 2 , 4–6% (138 ml·min−1 ) for Vco 2 , and 3–5% (3.05 L·min−1 ) for VE . Baldari et al. (4 o 2 (0.9–2.8 ml·min−1 ), Vco 2 (5.1–11.3 ml·min−1 ), and VE (0.10–.0.14 L·min−1 ). However, when only the Aquatrainer system was used during either swimming or cycle ergometry, the mean difference in Vo 2 was 3-fold higher during swimming and 2-fold higher for Vco 2 and VE (4 o 2 , Vco 2 , and VE is greater in an aquatic environment compared with a terrestrial one.
Although no statistically significant bias in peak or submaximal Vo 2 , Vco 2 , and VE was observed, a high level of random error was present, and given the small sample size, it would have been difficult to detect statistically significant bias (2 o 2 , Vco 2 , and VE mean that if the same participants were tested again, Vo 2 peak determined using the MS could be as much as 1.06 L·min−1 below or 1.84 L·min−1 above DB values (Table 1 ). Submaximal MS-derived Vo 2 may also underestimate or overestimate DB values by as much as 35% during low-intensity swimming and 78% during moderate intensity swimming because of measurement error alone (Table 2 ). This lack of agreement between DB and MS measurements is not acceptable. Although the mean difference observed across swimming intensities is consistent with that reported between DB and other metabolic carts for Vo 2 (3–14%), Vco 2 (3–17%), and VE (4–8%) (15,25,34,40 o 2 , Vco 2 , and VE during submaximal swimming was similar between MS and DB measurements for the 2 repeat tests, with the MS typically exhibiting better repeatability (Table 3 ).
Only 2 studies have examined the test-retest performance of metabolic carts, and these studies have limited the number of comparisons to only 2 (23,40 21
The repeatability of VE , VT, f r o 2 , Vco 2 , PET O2 , and PET CO2 was worse at baseline than during swimming with a CV ranging from 4 to 27% and ratio CR of ±1.09–1.75 (Table 4 ). This could reflect the manner in which these data were collected. During the 3 minutes of prone floating (baseline), the flume was switched on and a current was applied to aid buoyancy. This created a small amount of natural sway and likely increased convective heat loss because of the flowing water over the skin (29 32 o 2 (29
All physiological variables measured during swimming (VE , VT, f r o 2 , Vco 2 , PET O2 , and PET CO2 ) produced a test-retest CV <9%, 6–7% for Vo 2 specifically (Table 4 ). This is consistent with the CV (24,34 23,40 o 2 (<1–15%), Vco 2 (3–7%), and VE (<1–6%) during treadmill exercise, cycle ergometry, or rowing ergometry. These differences have been shown to be inversely related to work rate (24,30,34 f r ET O2 and PET CO2 . The 8% and 4–6% CV observed in VT and f r f r (15
Although the exercising CV data of this study are consistent with others, this does not mean that the test-retest variability is inconsequential. The LoA for all CR analyses were wide and with a ratio CR of up ±1.26 for Vo 2 and ±1.34 for VE (the worst CR observed in all parameters over both intensities), Vo 2 and VE could vary by as much as 26 and 34%, respectively, in the same participants during repeat testing. A change of at least these magnitudes would be needed in future trials to be 95% confident that a real change in, or difference between, Vo 2 and VE was evident (5,39 co 2 and f r ET O2 , and PET CO2 (Table 4 ). Whether or not the MS is capable of detecting a real change and is suitable for evaluative purposes will therefore depend on the size of the change expected or the minimum difference that is considered meaningful (16
The hydrodynamic and fluid flow differences between flume and pool swimming impact stroke characteristics. Stroke cycle duration is shorter; stroke rate is higher; and the catch and glide phases are reduced at a given velocity during flume vs. pool swimming (17 −1 during submaximal and maximal swimming in phase 1 with a CV as high as 5%. Phase 2 was slightly better with a ratio CR of ±1.09 for VCrit5% slower and ±1.13 for VCrit5% faster , and test-retest CV of <3%. This CV is worse than that reported (<1%) between target and achieved velocity when swimming at the same relative intensities (VCrit5% slower and VCrit5% faster ) in an indoor swimming pool (22
In light of this, it is possible that day-to-day repeatability would improve if data were collected in a swimming pool rather than a flume. Baseline variability could probably also be reduced by decreasing the likelihood of shivering. This could be achieved by reducing the period over which baseline data are collected if floating in water (although reducing this to less than 3 minutes is questionable), by increasing the temperature of the water, or by undertaking baseline measurements on poolside: prone floating baseline measurements were recorded in the flume to reflect the body position and environment experienced during front crawl. These recommendations require testing and data would still be subjected to the biological variability occurring between replicate tests, which can account for as much as 90% of the total variability in Vo 2 (5,13,24 E , f r ET O2 , and PET CO2 in comparison with freely breathing activities as well as other swimming strokes has not been investigated.
It should also be acknowledged that all metabolic carts can encounter errors from a linearity of sensors and a temporal mismatch between ventilation and gas fractions during breath-by-breath sampling (30
Finally, condensation of expired air inside the snorkel was frequently observed, indicating the temperature of the expired air leaving the mouth was greater than that reaching the flow sensor. The flow measured at the flow sensor would therefore not exactly equal the flow at the mouth (8 o 2 with metabolic carts comes from the measurement of ventilation (6 E and in turn Vo 2 and Vco 2.
Practical Applications
The test-retest data of the MS are consistent with other metabolic carts, suggesting similar levels of repeatability. The test-retest performance of the MS and DB method is similar, with the MS typically exhibiting smaller CV and CR values. The MS is more convenient to use than the DB method, making it appealing for practical use, and the breath-by-breath nature of data collection means the MS is more versatile. For example, as well as the traditional parameters of Vo 2 , Vco 2 , and VE that can be assessed with the DB method, pulmonary oxygen uptake kinetics, GETs, and rapid changes in ventilation and oxygen and carbon dioxide expired fractions can be examined with the MS. Although the MS can be used to assess the response of such parameters to training, the level of day-to-day variability is not inconsequential. Whether or not the MS is suitable for use as an evaluative tool will therefore depend on the size of the effect one wishes to detect.
The poor agreement and wide LoA between the MS and DB indicate that they cannot be used interchangeably during flume swimming. Biological and technical variability make perfect agreement very unlikely, and the disparity between MS- and DB-derived Vo 2 , Vco 2 , and VE values is consistent with the variability between other metabolic carts and the DB method. Given that the MS can provide a greater magnitude of physiological data, it is unlikely that the MS and DB would be used interchangeably.
Acknowledgments
The authors thank the swimmers who took part and Ms. Anne-Marie Smith.
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