Effect of Acute Altitude Exposure on Anaerobic Threshold Assessed by a Novel Electrocardiogram-Based Method

Study population and setting

In a cross-sectional study setting, a total of 14 healthy recreational athletes (4 females, 10 males) with a mean age of 32 years (Range: 24–56 years) performed an exercise test at high altitude (Zugspitze at the Environmental Research Station Schneefernhaus, short Umweltforschungsstation Schneefernerhaus [UFS]): 2,650 m/715 mbar) and low altitude (Munich, Germany: 512 m/949 mbar). The mean body mass index was 23.21 ± 2.74 k g × m − 2 . The exercise was performed on a cycling ergometer following a step protocol. The exercise was initiated with 40 W of workload in women and 60 W in men and was increased by 20 W every 3 minutes. Capillary blood samples were taken at rest and at the end of each step to measure lactate concentration. The test was performed within 1 hour after arrival at the UFS to avoid adaptation. If none of the formal termination criteria were met, exercise was continued until physical exhaustion.

Assessment of ATdT°

To measure dT, a high-resolution (2,048 Hz) digital ECG with orthogonal Frank-lead configuration was used. The ECG recording was started at rest 15 minutes before the beginning of the exercise and lasted until 15 minutes after the completion of the test. Following adjustments were made to reduce ECG alterations: cleaning of the skin surface; ECG cables were taped to the patient's body and secured by a net bandage to reduce movement of the electrodes. ECG records were analyzed using the SMARTlab software. Briefly, the program uses the spatiotemporal information of each T-wave and translates the information into a single vector T0, which defines the main direction of the T-wave in space. The angle dT between two successive vectors in time is the degree of dT. At rest, dT is underlying typical low-frequency oscillations (<0.1 Hz) (Rizas et al., 2016; Rizas et al., 2014). For the correct capturing of the T-waves, preestablished algorithms were used (Laguna et al., 1994; Pan and Tompkins, 1985). For further information on the method, see Rizas et al. (2016) and Rizas et al. (2014).

During exercise, dT increases, falls, and at the end increases again. Inside this typical three-phased scheme, the AT (ATdT°) is defined by the moment of maximal discordance between the heart rate and dT, which is converted into power output (W), assuming a linear increase of exercise increments (Hamm et al., 2019).

Assessment of lactate levels and thresholds

Blood samples were taken from the participants' earlobe to measure lactate concentrations (mmol/l). Sampling was accomplished before the examination and at the end of each step of the exercise.

The LT was measured using routine methods, according to Mader and Dickhuth. Mader defined LT as fixed at 4 mmol/l, while Dickhuth defined it as 1.5 mmol/l above the lactate equivalent (Faude et al., 2009).

Statistical analyses

Statistical analyses were conducted using SPSS Statistics 29 (IBM Cooperation). Mean values and standard deviations are presented for descriptive analyses. For the comparison of ATdT-levels (low altitude vs high altitude), t-test was calculated according to the distribution of data (Kolmogorov—Smirnoff Test). A p-value <0.05 was considered statistically significant.

ATdT results

First, the mean ATdT° at baseline was 159.80 ± 52.21 W. At high altitudes, ATdT° decreased significantly (134.66 ± 34.91 W; p < 0.001) (Fig. 1), which translates into a reduction of 15.73%. Corresponding to that, the time to reach ATdT° decreased by 21.03% (baseline: 21.4 ± 7.6 minutes vs. high altitude 16.9 ± 4.8 minutes) (Table 1).

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FIG. 1.

Box plot diagram of ATdT° at baseline vs ATdT° at height. ATdT°, anaerobic threshold trough dT; LT, lactate threshold.

Rizas et al. demonstrated that the dT, which is dependent on ATdT°, reflects the activity of the sympathetic nervous system on the ventricular myocardium (Rizas et al., 2016; Rizas et al., 2014). At altitude, the sympathetic branch of the autonomic nervous system adapts (Kanai et al., 2001; Yamamoto et al., 1996), and thereby might impact the ATdT°. However, our findings suggest that ATdT° deviates from the usual pattern observed in the sympathetic branch as it exhibits a decrease rather than an increase.

Further, exposure to high altitudes is known to increase heart rate and breathing rate, which may have the potential to distort our results (Schüttler et al., 2021b). Nevertheless, dT and therefore, ATdT° is independent of heart rate and breathing, which should resolve the issue (Rizas et al., 2016; Rizas et al., 2014).

Second, we analyzed the effect of altitude on LT_Dickhuth and LT_Mader to compare our ATdT° results with well-established methods. After acute exposure to high-altitude, LT_Mader decreased by 17.37% and LT_Dickhuth by 13.13% (Table 1). These results are in line with the observations of Weckbach et al., who described similar values in a study on the effect of acute high-altitude exposure on the LT, using the methods by Dickhuth and Mader to assess AT (Weckbach et al., 2019).

Figure 2 shows a significant correlation at high altitudes between LT_Dickhuth and ATdT° (r = 0.969; p < 0.001) and between LT_Mader and ATdT° (r = 0.942; p < 0.001). They also correlated at baseline: between LT_Dickhuth and ATdT° (r = 0.979; p < 0.001) and between LT_Mader and ATdT° (r = 0.943; p < 0.001). These findings are confirmed by recent studies that also show highly significant correlations of ATdT° with LTs (Hamm et al., 2019; Schüttler et al., 2021a). The validation study by Schüttler et al. described notable correlations between ATdT° and standard methods (LT_Mader, LT_Dickhuth) in 65 team sports athletes (Schüttler et al., 2021a). These LT methods are perhaps some of the most common ways to assess physical performance. However, direct measurement of AT using blood lactate levels presents several factors that could affect its accuracy, including the sampling location, sweat interference, and ambient temperature (Mann et al., 2013). In contrast, our ECG-based method is straightforward to use, repeatable, and noninvasive.

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FIG. 2.

Pearson's correlation coefficient test for power at low altitude and high altitude between ATdT° and LT_Dickhuth and ATdT° and LT_Mader. (a) ATdT° and LT_Dickhuth at baseline. (b) ATdT° and LT_Mader at baseline. (c) ATdT° and LT_Dickhuth at height. (d) ATdT° and LT_Mader at height.

Although the exact mechanisms of ATdT° remain unclear, in conclusion, the presented results support ATdT° to be a reliable und valid measure of AT under different conditions.

Nevertheless, it is important to note that recent studies have falsified the original idea about the mechanisms of AT conceived by Wasserman et al. (Brooks, 2018). There is no evidence supporting the idea that increasing lactate is the result of muscle anoxia. Today, the lactate shuttle is considered the correct paradigm for describing lactate metabolism (Poole et al., 2021). However, the conception, that increasing lactate during physical activity is a sign of poor capacity to mediate metabolic burden, has gained acceptance over time. Therefore, the assessment of AT, such as through the use of ATdT°, remains crucial in the fields of medicine and sports (Poole et al., 2021).

Limitations

As the sample size is rather small and only includes four women, the generalizability of the results is restricted. Nevertheless, all shown findings are consistent with the current research on the topic. Our study is intended to lay a foundation for further, larger-scale research. Additionally, we investigated the effect only at one specific height. The effects of higher altitude might be different, and we cannot draw any conclusions about the variability of ATdT° at different heights. Therefore, further studies with larger cohorts are necessary to confirm our results and to strengthen the assessment of ATdT° as a new standard method.