Effects of hyperventilation on oxygenation, apnea breaking points, diving response, and spleen contraction during serial static apneas

All participants completed the apnea protocol successfully. The studied variables changed equally between sexes in response to apnea and hyperventilation, and therefore, men and women were analyzed as one single group.

Apnea duration

Mean (SD) apnea duration was longer in HV, at 133 ± 6 s, compared with 111 ± 12 s in NB (p < 0.001, \(_^\) = 0.61, 90% CI [0.30–0.73], \(_^\) = 0.05; Table 1, Fig. 3). There was an increase in duration across apnea A1–A5 in both conditions (p < 0.001, \(_^\) = 0.70, 90% CI [0.50–0.78], \(_^\) = 0.06; Table 1, Fig. 3).

Table 1 Apnea duration between conditions and within apnea seriesFig. 3figure 3

Mean apnea duration for apnea 1–5 (A1–A5), with SD bars, for the two conditions: normal breathing (NB) and hyperventilation (HV). *p < 0.05, **p < 0.01 and ***p < 0.001 compared with A1 in the same condition (n = 18)

End-tidal carbon dioxide

At the start of each apnea in HV, the mean PETCO2 was 17.4 ± 3.4 mmHg (2.3 ± 0.5 kPa) which was lower than the 29.0 ± 4.0 mmHg (3.9 ± 0.5 kPa) in NB (p < 0.001, \(_^\) = 0.93, 90% CI [0.86–0.96], \(_^\) = 0.76; Fig. 4a, Fig. 6). In both conditions, PETCO2 at the start was lower in the last apnea compared with the first (p = 0.001, \(_^\) = 0.53, 90% CI [0.32–0.61], \(_^\) = 0.11; Fig. 4a).

Fig. 4figure 4

End-tidal carbon dioxide pressures (PETCO2) at a the start of each apnea 1–5 and b at the end of apnea 1–5 in the two breathing conditions. End-tidal oxygen pressures (PETO2) at c the start of each apnea 1–5 and d at the end of apnea 1–5 in the two conditions. *p < 0.05, **p < 0.01, and ***p < 0.001 between conditions; †p < 0.05, ††p < 0.01, and †††p < 0.001 compared with A1 in the same condition (n = 13)

At the end of apneas in HV, the mean PETCO2 was lower in HV with 38.5 ± 5.2 mmHg (5.1 ± 0.7 kPa), compared to after NB with 40.3 ± 5.5 mmHg (5.4 ± 0.7 kPa, p = 0.005, \(_^\) = 0.49, 90% CI [0.12–0.67], \(_^\) = 0.03; Fig. 4b, Fig. 6). PETCO2 at the end of apnea was similar across both series (p = 0.272, \(_^\) = 0.10, 90% CI [0.00–0.25], \(_^\) = 0.006; Fig. 4b).

Involuntary breathing movements

Of the 18 participants, 7 (39%) had IBM in A1–A5 in both conditions, allowing comparison of easy phase duration. The start of the struggle phase was delayed in HV, at a mean of 112 ± 49 s, compared with 89 ± 42 s in NB (p = 0.003, \(_^\) = 0.79, 90% CI [0.32–0.88], \(_^\) = 0.08; Fig. 5). There was no difference across the series in any condition (p = 0.446, \(_^\) = 0.34, 90% CI [0.02–0.46], \(_^\) = 0.04; Fig. 5).

Fig. 5figure 5

Duration of the easy (a) and struggle (b) phases for apneas 1–5 (A1–A5) after normal breathing (NB) and hyperventilation (HV) for the seven subjects with involuntary breathing movements (IBM) in all apneas. ***p < 0.001, **p < 0.01, and *p < 0.05 between conditions

End-tidal oxygen

At the start of each apnea, PETO2 was higher in HV with 137.2 ± 7.4 mmHg (18.0 ± 1.0 kPa) compared with 121.6 ± 11.1 mmHg (16.2 ± 1.5 kPa) in NB (p < 0.001, \(_^\) = 0.917, 90% CI [0.80–0.94], \(_^\) = 0.43; Fig. 4c, Fig. 6). There was no difference across A1–A5 in any condition (p = 0.094, \(_^\) = 0.25, 90% CI [0.05–0.35], \(_^\) = 0.009; Fig. 4c).

Fig. 6figure 6

O2–CO2 diagram showing individual alveolar values during NB (a) and HV (b). Circles represent values at the start of apnea. Triangles represent values at the end of apnea. Filled symbols indicate NB (a) and empty symbols indicate HV (b). Colors show apnea series. Horizontal dashed lines show the range of normal PACO2. Black continuous lines represent the conventional apnea breaking point (Agostoni 1963) (n = 13)

At the end of each apnea, PETO2 was lower in HV with a mean of 83.4 ± 17.3 mmHg (11.1 ± 2.3 kPa), compared with 89.4 ± 15.6 mmHg (11.9 ± 2.1 kPa) in NB (p = 0.005, \(_^\) = 0.498, 90% CI [0.12–0.68], \(_^\) = 0.04; Fig. 4d, Fig. 6). The PETO2 at the end of apnea was also decreasing across the series in HV, being higher in A1 than in A5 (p < 0.001, \(_^\) = 0.426, 90% CI [0.16–0.55], \(_^\) = 0.03; Fig. 4d).

Peripheral oxygen saturation

Figure 7 shows a representative recording of SpO2 across the NB and HV series from one subject, where the HV series results in progressive desaturation.

Fig. 7figure 7

SpO2 recording in one participant during series of five dives after normal breathing (NB) and hyperventilation (HV)

Mean baseline SpO2 was similar between conditions and apnea series (p = 0.421, \(_^\) = 0.04, 90% CI [0.00–0.24], \(_^\) = 0.01; Fig. 8a). SpO2 nadir did not change across the apnea series in NB, (p = 0.576, \(_^\) = 0.03, 90% CI [0.00–0.14], \(_^\) = 0.10, Fig. 8b, Fig. 9a), but it decreased during the apnea series in HV (Fig. 8b, Fig. 9b) with the lowest value in A5 (p < 0.001, \(_^\) = 0.48, 90% CI [0.25–0.61], \(_^\) = 0.17, Fig. 9b). Mean SpO2 nadir was lower in HV with 90.6 ± 5.6%, compared with NB with 93.6 ± 4.9% (p < 0.001, \(_^\) = 0.644, 90% CI [0.35–0.76], \(_^\) = 0.10; Fig. 8b). In 4 participants, the SpO2 nadir resulting from the fifth apnea during HV was ≤ 75%. There was no difference between NB and HV when comparing the SpO2 values at the same time point as the average in NB at the end of apnea was 96.4 ± 3.5% and in HV was 96.0 ± 3.5% (p = 0.25, \(_^\) = 0.08, 90% CI [0.00–0.23], \(_^\) = 0.003).

Fig. 8figure 8

Mean baseline pre-apnea SpO2 from A1–A5 (a) and at post-apnea nadir (b). *p < 0.05, **p < 0.01, ***p < 0.001 compared with A1 in the same condition; †p < 0.05, ††p < 0.01, †††p < 0.001 compared with HV (n = 18)

Fig. 9figure 9

Mean (SE) SpO2 in NB (a) and HV (b) series during A1–A5. The grey boxes indicate apnea time. Negative time values before apnea time indicate breathing just before apneas. Positive values after apnea time indicate recovery time directly after apneas and partly overlap with pre-apnea values of the subsequent apnea. The gap in the middle of apnea time has variable duration, as apnea duration was different between participants. *p < 0.05, **p < 0.01, ***p < 0.001 indicates significance compared with A1 in the same condition; †††p < 0.001 indicates significance compared with HV (n = 18)

Diving response

The HR at baseline was similar in both conditions, with a mean of 68 ± 12 bpm in NB and 68 ± 9 bpm in HV (p = 0.923, \(_^\) = 0.001, 90% CI [0.00–0.04], \(_^\) = 0.0001; Fig. 10a). Recovery HR between apneas was also similar within series (p = 0.795, \(_^\) = 0.02, 90% CI [0.00–0.15], \(_^\) = 0.004; Fig. 10a).

Fig. 10figure 10

Heart rate average for baseline (a), HRpeak (b), and during apnea (c). ***p < 0.001 between normal (NB) and hyperventilation (HV) conditions; †††p < 0.001, ††p < 0.01, and †p < 0.05 compared with A1 in the same condition

Mean HRpeak after apnea onset was higher in HV with 80 ± 15 bpm compared with 66 ± 13 bpm in NB (p < 0.001, \(_^\) = 0.76, 90% CI [0.54–0.84], \(_^\) = 0.23; Figs. 10b, 11b). During the NB series, HRpeak fell from 71 ± 14 bpm in A1 to 62 ± 11 bpm in A5 (p < 0.001, \(_^\) = 0.39, 90% CI [0.21–0.48], \(_^\) = 0.07; Figs. 10b, 11a). Across the HV series, HRpeak fell from A1, at 87 ± 17 bpm, to A5 at 75 ± 13 bpm, (p < 0.001, \(_^\) = 0.36, 90% CI [0.18–0.45], \(_^\) = 0.07; Figs. 10b, 11b).

Fig. 11figure 11

Mean (SE) heart rate during apnea 1–5 at Normal (a) and HV (b). The grey boxes indicate apnea time. Negative time values before apnea time indicate breathing. Positive values after apnea time indicate recovery time and overlap with the previous values of the subsequent apnea. The gap in the middle of apnea time has variable duration, as apnea time was different between participants (n = 18)

The mean HR during apnea in NB was 54 ± 7 bpm and in HV was 55 ± 8 bpm, hence no differences (p = 0.205, \(_^\) = 0.09, 90% CI [0.00–0.32], \(_^\) = 0.01; Fig. 10c). Neither were there any differences in mean HR during apnea across the series (p = 0.232, \(_^\) = 0.08, 90% CI [0.00–0.19], \(_^\) = 0.01; Fig. 10c). The HR reduction in percent from baseline was also the same at 19% in NB and 18% in HV (p = 0.694, \(_^\) = 0.09, 90% CI [0.00–0.17], \(_^\) = 0.003).

There was no difference in the SkBF reduction between NB with − 10.0 ± 17.3% and HV with − 8.1 ± 18.3% (p = 0.10, \(_^\) = 0.10, 90% CI [0.00–0.33], \(_^\) = 0.01; Fig. 12a). Neither were any differences observed between series (p = 0.716, \(_^\) = 0.02, 90% CI [0.00–0.10], \(_^\) = 0.01; Fig. 12a).

Fig. 12figure 12

SkBF reduction in percentage from baseline in both conditions from apnea 1 to apnea 5 (a) and recordings from one participant in the fifth apnea comparing both conditions (b). PU, perfusion units. (n = 18)

Spleen contraction

Mean (SD) baseline spleen volume before the apnea series was similar at 197 ± 54 mL in NB and 194 ± 56 mL in HV (p = 0.276; Fig. 13). The spleen contracted during all apneas in both NB and HV series and increased in size during breathing intervals, although not reaching baseline values. The average volume during recovery after every apnea was still 11% lower than baseline in NB, and 8% lower in HV, from A2 to A5 (p = 0.012; Fig. 13).

Fig. 13figure 13

Mean spleen volume during the NB and HV apnea series. The grey boxes indicate apneas. Negative values before A1 indicate baseline measurements 5 min before starting. Positive values after every apnea indicate recovery time in minutes. The brackets indicate that an average period was compared. ***p < 0.001 compared with baseline; ###p < 0.001 compared with apnea values

There were no differences in mean splenic volume reductions during apnea between conditions or within series. During apneas, mean spleen volume was 131 ± 40 mL in NB and 126 ± 43 mL in HV series (p = 0.348), reflecting reductions from baseline by 33% and 35%, respectively (p < 0.001; Fig. 13). The mean spleen volume during the 2 min of recovery was similar at 176 ± 46 mL in NB and 179 ± 57 mL in HV series, and after 5 min recovery spleen volume was restored to normal in both series (Fig. 13).

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