Heart Rate Variability from Underwater Spiroergometry: How Meaningful?

Cardiovascular fitness of divers is overwhelmingly performed using bicycle ergometry. A more sport-specific alternative presents fit2dive, an underwater spiroergometry system. Purpose of this exploratory study: using fit2dive to investigate the diagnostic value of measures of heart rate variability (HRV) after markedly increasing physical load. Ten scuba divers employed the fit2dive system and increased fin-swimming speed until exhaustion. Breathing gas consumption (V̇E) and heart rate (HR) were measured. A three-lead ECG was recorded to analyze for time and frequency domain HRV-measures. V̇E increased from 16.5 ± 6.5 to 68.3 ± 26.6 L/min. HR increased from 96 ± 13 beats/min (mean ± SD) at rest to 170 ± 14 beats/min before exhaustion. Global variability (SDNN: 132 ± 42 vs. 54 ± 17 ms) decreased along with two measures of parasympathetic activity (RMSSD: 59 ± 31 vs. 24 ± 16 ms; pNN50: 22% ± 12% vs. 3% ± 3%). Measures from the frequency domain decreased [low frequency (LF): 3167 ± 2651 vs. 778 ± 705 ms²] or remained unaltered [high frequency (HF): 885 ± 652 vs. 431 ± 463 ms²]. Thus, LF/HF decreased from 4.3 ± 2.3 to 2.5 ± 1.4. The sports-specific fit2dive can help assessing diving fitness by employing HRV measures. However, this study supports the view that these measures much depend on HR. Thus, HRV measures regarding altered autonomic control during exercise will lead to serious misinterpretation: as HR increases, variability decreases.