Hyperoxic Training Increases Work Capacity After Maximal Training at Moderate Altitude

Hyperoxic Training Increases Work Capacity After Maximal Training at Moderate Altitude* Thomas W Chick, M.D., F.C.C.P.; Daniel M. Stark; and Glen H. Murata, M.D. High-intensity training may be difficult to sustain due to limitations in systemic oxygen transport, particularly at high altitudes. The purpose of this study was to examine the effects of a high-intensity training protocol using hyperoxic gas breathing in athletes "maximally trained" at an altitude of 1,600 m. Five subjects underwent progressive cycle training until they reached a plateau of aerobic capacity, maximal workload, and endurance time at 85 percent maximal workload. Significant decreases (2 to 6 percent) in arterial oxygen saturation were found after the 85 percent maximal workload tests. Training intensity was then increased to 95 percent maximal workload while the subjects breathed a gas mixture containing at least 70 percent oxygen. After 6 weeks of hyperoxic training,

he response to aerobic training depends on the T intensity, frequency, and duration of the training sessions. Several lines of evidence point to intensity as the most important determinant of this response . 1-3 Theoretically, as long as training intensity increases, exercise capacity should increase. However, previous studies have shown that high-intensity training programs cannot be sustained for prolonged periods without the development of chronic fatigue. 4 •5 Because arterial oxygen desaturation has been demonstrated during high-intensity exercise, 6 "maximal training" could be enhanced by interventions that increase oxygen transport. For instance, Buick7 showed that induced erythrocythemia allowed athletes to increase their aerobic capacity, an effect that persisted after their hematocrits had returned to normal. Arterial desaturation may be a particularly important problem for persons involved in high-intensity exercise at higher altitudes. The purpose of this study was to determine if a high-intensity training protocol using hyperoxic gas breathing improves exercise performance in athletes "maximally trained" at an altitude of 1,600 m . Unlike previous reports, we studied our subjects over a prolonged baseline period to assure that they had reached their peak performance. 4 •5 We were then able to increase their training intensity to a level that could not be sustained under baseline conditions by having subjects breathe a gas mixture containing 70 percent oxygen. *From the Pulmonary Section and Ambulatory Care Service, Veterans Affairs Medical Center, and the Department of Medicine, University of New Mexk" School of Medicine, Albuquerque. Manuscript received January 25, 1993; revision accepted April28.

exercise parameters were compared with the plateau values obtained during the baseline training period. Total time during maximal cycle testing increased from 19.1 to 19.6 min (p=0.015), heart rate at 85 percent maximal workload decreased from 168 to 163 bpm (p=0.047), and endurance time at 85 percent maximal workload increased from 6.2 to 8.2 min (p=0.012). There was a trend toward improvement of maximal workload. We conclude that hyperoxic training increases work capacity after attainment of"maximal training" at moderate altitude. (Chest 1993; 104:1759-62) ET85 =endurance time at 85 percent maximal workload; Tmax=maximal cycle time; Vo1 max=maximal oxygen uptake; W85HR =heart rate at 85 percent maximal workload; WLm = maximum workload

METHODS

Subjt•cts

The experimental proto<.,>l was approved by the Human Research and Review Committee of the University of New Mexi<.-o and the Research Committee of the Albuquerque Veterans Administration Medical Center. Forty-fimr subjects volunteered for the study and underwent initial testing. Nine failed to return for training or repeated testing. Thirty-five subjects trained for periods ranging from 18 days to 313 days. Five subjects completed the full proh><.·ol. The remaining individuals dropped out during the baseline tr.rining phase because of lack of time, loss of motivation, change of residen~-e . or injury. A sixth subject withdrew due to injury after completing 16 hyperoxic training sessions; his data have been excluded from analysis. Testing Procedures

Testing was perlimned at entry into the study and after every eight training sessions and ~'Onsisted of maximal exercise tests and measurements of enduran~-e time at 85 percent maximal workload (ET85). Maximal exercise tests were done using an electrically braked cycle ergometer (E . Jaeger, Rockfi>rd, Ill). After a 3-min period of unloaded pedaling, workload was increased by 20 or 25 W/min (depending on subject size) and exercise continued until exhaustion (defined as the inability to maintain a pedaling frequency >50/min). During these tests, subjects wore a noseclip and breathed into mouthpiece ~"nnected to a metabolic cart (Incarepulmobil, E . Jaeger, Rockford, Ill). Oxygen mnsumption and carbon dioxide production were measured every 15 s. The oxygen and carbon dioxide analyzers were ealibmted before each test with a standard calibration gas mixture; the pneumotachograph was calibrated with a 1-L syringe. After each test, gas analyzer drift was measured and did not exceed 0 .3 percent fi1r either gas analyzer fi>r any test. Maximal workload (WLm) was the highest workload that the subject could maintain fi>r >30 s. Maximal cycle time (Tmax) was the total duration of the test from onset of unloaded pedaling to the time of exhaustion. Maximal oxygen uptake (Vo2 max) was defined as the average of the highest two Vo2 values ra•>rded during the maximal workload. \Ve also measured heart rate at 85 peR-ent maximal workload (W85HR). CHEST I 104 I 6 I DECEMBER, 1993

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Table 1- Demographic and lraining Data

The ET85 was determined 24 to 48 h after the graded-cycle maximal test. The subject initially pedaled at a workload corresponding to 50 per<:ent maximal workload for 3 min and the workload was promptly adjusted to the value <.'Orresponding to 85 percent maximal workload determined at the previous graded exercise test. The ET85 was recorded as the time to exhaustion. Heart rate was monitored electmcardiographically and arterial oxygen saturation (SaO.) was monitored by pulse oximetry (POET, Criticare Systems, Inc, Milwaukee).

li26IF 2/43/F 3142/M

Baseline Training Protocol

5127/M

Cycle tmining was carried out in the exercise laboratory. Training frequency was at least four times per week. Each session consisted of eight 5-min sets of2 min pedaling at 50 percent maximal workload and 3 min pedaling at 85 percent maximal workload. Repeated testing was done after every eight training sessions. Training intensity was adjusted for improvement in work capacity in order to maintain the same relative intensity throughout this phase of the study. A plateau of training effect was defined as three consecutive tests in which maximal cycle time did not increase >2 percent and ET85 did not increase >8 percent over the best of the previous tests. We selected these criteria on the basis of observations made in 10 subjects whose training volume remained constant for a period of at least 12 weeks and who were using the same training protocol employed in this study. In this group, the average coefficient of variation for Tmax was 1.3 percent and for ET85 it was 12.4 percent. Hyperoric

'~raining

Protocol

Hyperoxic training workload was determined during the baseline period by having the subject attempt to complete eight 5-min sets at 95 percent maximal workload. None of the subjects was able to <:omplete more than 10 min of this protocol. Hyperoxia was then achieved by inspiration of>70 percent oxygen through a face mask; hypemxic gas was delivered by adding oxygen from cylinders and wall outlet sources to a 120-cm length of 4-cm internal diameter plastic tubing <:70 percent. Hypemxic training was continued at the higher training intensity and the same duration and frequency as during baseline training. Repeated determinations of ET85 during hyperoxic training were made at the workload corresponding to the peak 85 percent maximal workload during baseline training (fixed intensity). All exercise testing pr
All <.~mtinuous variables are expressed as mean ± standard deviation (SD). Differences in exercise parameters for the last three baseline studies were examined by analysis of variance for repeated measures. For each parameter, the mean of the three baseline values was compared with the value obtained after hyperoxic training. These differences were analyzed by the paired Students t test; p values less than 0.05 were <.'Onsidered significant.

Subject Age, yr/ Sex

4125/F

Height, em

Weight, kg

Vo,max, mllminlkg

Training Status

173 178 185 155 178

65.5 73.2 73.6 59.5 71.4

50.2 45.6 60.5 62.5 61.7

AE* R14 S5C50 R25C20

1760

79 203 244 53

83

*Indicates the time required to reach a plateau of training effect.

t Prestudy training status is reported in miles/week unless otherwise specified: C=cycling, R = running, S = swimming. *Aerobic exercise 30 min, 4 times a wk .

these variables during the plateau period. Pulse oximetry revealed a resting Sa02 of 95.8 ± 1.3 percent. During the ET85 tests, significant reductions in the Sa02 were found at the end of exercise (mean decrement=4.40± 1.67 percent; p=0.004). Table 3 compares exercise capacity attained with baseline training and with hyperoxic training. Six weeks of hyperoxic training resulted in significant improvement for Tmax (mean increment 0.46 ± 0 .25 min; p=0.015) and ET85 (mean increment, 2.0± 1.0 min; p =0 .012) and a significant decrease for W85HR (mean decrement -5.0±3.9 bpm; p=0.047). WLm tended to improve with hyperoxic training (by 6.0±6.3 W; P=0.10) but no changes were found for Vo2 max. DISCUSSION

Enhanced training intensity using hyperoxic gas breathing resulted in significant improvement in the capacity for high-intensity exercise. The duration of hyperoxic training required to achieve this effect was approximately 6 weeks. Increasing the duration of the daily training produces inconsistent changes in the exercise performance of well-conditioned athletes. A previous study of swimmers examined the response to increasing the duration of training sessions without changing their intensity.s- 10 This approach did not improve the performance of swimmers over that achieved with conventional methods. However, several adaptive changes were found, including an increase in the citrate synthase content of skeletal muscle, an Table 2-Eurcise lbrameten During "Maximal" Baseline Training

RESULTS

All subjects were engaged in some training before entry into the study (Table 1). Subjects reached a plateau in exercise performance after 53 to 244 days of supervised baseline training. Baseline training resulted in stable values for maximal workload, Tmax, Vo2 max, W85HR, and ET85 over a period of6 weeks (Table 2). No significant changes were found for any of

R20t

Days to Plateau•

Maximal workload, W Tmax, min Vo,max, ml!minlkg W85HR, bpm ET85, min

N1*

N2

N3

306±45 19.0±2.1 56.1±7.7 167± 18 5 .73±0.77

307±48 19.1 ± 1.9 55.1 ±8.0 170± 18 6.57± 1.72

311±52 19.3±2.0 55.3±8.6 168±20 6.17±1.26

*N1 , N2, and N3 refer to the final measurements taken at the plat~au of baseline training. Other abbreviations expanded in text. Hyperoxic Training Increases Won< Capacity (Chick, Starlc, Murata)

Table 3-Comparison of Mean Exercise Parameters During the Baseline Plateau (Nmean) With Those Achieved After 6 Weeks of Hyperoric Training (HO.)*

Wlm

Tmax Subject

2 3 4 5 Mean SD

ET85

W85HR

Vo,max

Nmean

HO,

Nmean

HO,

Nmean

HO,

Nmean

HO,

Nmean

HO,

20.8 20.6 20.3 16.5 17.5 19. 1 2.0

21.5 21.1 20.7 16.6 18.0 19.6 2.2

270 270 353 280 367 308 45

285 270 360 280 375 314 49

48.8 44.9 60.4 61.2 62.2 55.5 8.0

51.6 46.0 61.0 59.0 68.0 57.1 8.5

174 160 143 194 171 168 19

172 155 141 190 159 163 19

6.8 7.7 6.1 5.1 5.1 6.2

10.1 10.5 8.1 6.1 6.2 8.2 2.1

l.l

*Abbreviations expanded in text.

increase in plasma volume, and a decrease in plasma lactate and heart rate at submaximal workloads. On the other hand, increased training intensity has been shown to improve several aspects of exercise performance.5·11 These studies are difficult to interpret because it is uncertain that the subjects had attained a stable maximal level of performance before the intervention was used . We designed a training protocol that circumvented several of the methodologic problems of previous studies. We had our subjects train in the laboratory under supervised conditions. We also showed that our subjects had achieved a plateau in exercise performance under baseline conditions, since none demonstrated a change in several exercise parameters over a period of at least 6 weeks. Finally, our subjects were unable to cycle at the higher workload chosen for hyperoxic training without supplemental oxygen, so that a voluntary increase in training intensity was not possible. H yperoxic training clearly improved maximal cycle time, heart rate at 85 percent maximal workload, and endurance time at 85 percent maximal workload over the highest values obtained during prolonged baseline training. Studies have shown that increasing blood oxygen content increases exercise capacity. Thus, hyperoxia (Pa02 of 294 mm Hg) delayed the development of fatigue in isolated contracting dog muscle. 12 Idstrom et al 13 altered oxygen delivery to contracting rat hind limb muscle by using perfusate equilibrated with 20 percent, 40 percent, and 95 percent oxygen. Increased oxygen delivery resulted in higher ratio of phosphocreatine to inorganic phosphorus, higher pH, lower lactate, and higher glycogen content in muscle. Improvement in aerobic capacity with hyperoxia has been demonstrated in many studies. In human subjects, Linnarsson et al' 4 showed that hyperbaric hyperoxia (1.4 atmosphere, inspiratory 0 2 tension of 212 mm Hg) resulted in a 10 percent increase in Vo2 max and reduced intramuscular concentration of lactic acid at maximal exercise compared with values obtained during exercise at l atmosphere. Ekblom et al' 5 showed that breathing50 percent oxygen resulted

in a 12.6 percent increase in Vo2 max compared with normoxia. At maximal treadmill workload, endurance time increased from 5.9 min to 9.9 min. This effect was confirmed by Adams and Welch, 16 who showed that breathing 60 percent oxygen increased endurance time at 90 percent maximal workload from 12.5 min to 15.8 min. Recently, Powers et al 17 reported that mild hyperoxia (inspired 0 2 fraction of0.26) increased Vo2 max in highly trained athletes whose Sa02 decreased to <92 percent at maximal normoxic exercise. No effect was seen in athletes who did not develop hypoxemia. Hogan et al 18 showed that inspiration of 60 percent oxygen resulted in greater maximal workload in 4 of 6 subjects and reduced plasma lactate values at workloads greater than 120 W. The increased Vo2 max with hyperoxia could not be attributed to central or peripheral hemodynamic effects. Ekblom et al 15 and Hughes et al 19 found that hyperoxia does not alter maximal cardiac output. Increasing the oxygen content of blood also decreases blood flow to the exercising limb, resulting in constant values for peripheral oxygen delivery. ro Hyperoxia is also associated with enhanced submaximal endurance capacity and reduced rate of lactate accumulation in muscle and blood. Howley et al21 studied the effects of breathing 60 percent oxygen during 40 min of constant-load cycling at 67 percent Vo2 max. Heart rate, ventilation, respiratory exchange ratio, and plasma concentrations of lactic acid and catecholamines were all lower than during normoxia. Graham et al 22 showed that after 30 min of sub maximal exercise, hyperoxia reduced muscle lactate accumulation compared with values obtained under normoxic conditions. The mechanism of increased exercise capacity after hyperoxic training cannot be determined from our data. Four of five subjects had an improvement in Vo2 max, but because of the small number of subjects, these changes were not statistically significant. The reduction of submaximal heart rate suggests that a central adaptation may be involved. If it is assumed that the cardiac output-Vo 2 relationship was unchanged, stroke volume was increased at high exercise CHEST I 104 I 6 I DECEMBER, 1993

1761

intensities after hyperoxic training. Possible causes for this adaptation include increased cardiac hypertrophy or increased ventricular filling due to expanded plasma volume . 10 Our study shows that, at moderate altitude (1,600 m), heavy exercise results in a significant reduction in Sa02 , presumably due to reduced alveolar oxygen tension and diffusion limitation to oxygen transfer. It is possible that these changes would not have occurred at sea level. Future studies should be done to determine ifhyperoxic training is effective at lower altitudes. In addition, only our most motivated subjects were able to achieve a plateau in exercise performance during the baseline training period. Our findings should therefore be confirmed in a larger number of individuals. In conclusion, hyperoxic training was successful in achieving sustained increase in training intensity after establishment of a plateau baseline training. Moreover, this increase in training intensity resulted in greater work capacity (increased maximal cycle time and endurance time at high intensity) and lower heart rate at submaximal exercise. ACKNOWLEDGMENT: The authors wish to thank Mrs. Angela M. Padilla-Casados li>r her substantive contributions to the writing and editing of this manuscript.

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REFERENCES

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Hyperoxic Training Increases WOOl Capacity (Chick, Starl<, Murata)