Extreme Hypoxia Causing Brady-Arrythmias During Apnea in Elite Breath-Hold Divers

Abstract

Introduction: The cardiac electrical conduction system is very sensitive to hypoglycemia and hypoxia, and the consequence may be brady-arrythmias. Weddell seals endure brady-arrythmias during their dives when desaturating to 3.2 kPa and elite breath-hold-divers (BHD), who share metabolic and cardiovascular adaptions including bradycardia with diving mammals, endure similar desaturation during maximum apnea. We hypothesized that hypoxia causes brady-arrythmias during maximum apnea in elite BHD. Hence, this study aimed to define the arterial blood glucose (Glu), peripheral saturation (SAT), heart rhythm (HR), and mean arterial blood pressure (MAP) of elite BHD during maximum apneas. Methods: HR was monitored with Direct-Current-Pads/ECG-lead-II and MAP and Glu from a radial arterial-catheter in nine BHD performing an immersed and head-down maximal static pool apnea after three warm-up apneas. SAT was monitored with a sensor on the neck of the subjects. On a separate day, a 12-lead-ECG-monitored maximum static apnea was repeated dry (n = 6). Results: During pool apnea of maximum duration (385 ± 70 s), SAT decreased from 99.6 ± 0.5 to 58.5 ± 5.5% (∼PaO₂ 4.8 ± 1.5 kPa, P < 0.001), while Glu increased from 5.8 ± 0.2 to 6.2 ± 0.2 mmol/l (P = 0.009). MAP increased from 103 ± 4 to 155 ± 6 mm Hg (P < 0.005). HR decreased to 46 ± 10 from 86 ± 14 beats/minute (P < 0.001). HR and MAP were unchanged after 3-4 min of apnea. During dry apnea (378 ± 31 s), HR decreased from 55 ± 4 to 40 ± 3 beats/minute (P = 0.031). Atrioventricular dissociation and junctional rhythm were observed both during pool and dry apneas. Conclusion: Our findings contrast with previous studies concluding that Glu decreases during apnea diving. We conclude during maximum apnea in elite BHD that (1) the diving reflex is maximized after 3-4 min, (2) increasing Glu may indicate lactate metabolism in accordance with our previous results, and (3) extreme hypoxia rather than hypoglycemia causes brady-arrythmias in elite BHD similar to diving mammals.

Keywords: apnea and face immersion; atrioventricular block; brady-arrythmia; bradycardia; free-diving; hypoxia induced factor-1 (HIF-1); invasive blood pressure; junctional rhythm.

FIGURE 1

Peripheral measured saturation (sensor placed at neck, n = 9) during pool apnea decreased from 99.6 ± 0.5% every minute until termination of breath hold to 58.5 ± 5.5% (#P = 0.004, *P < 0.001 compared to baseline).

FIGURE 2

Heart rate during maximum pool apnea. Heart rate decreased from 86 ± 14 beats per minute (bpm) to 46 ± 10 bpm after the first 4 min of apnea compared to baseline and stabilized until termination of breath hold (*P < 0.001 compared to baseline, n = 9).

FIGURE 3

ECG during maximum pool apnea in two subjects. (A) Nodal rhythm during maximum dry apnea. Recorded at 25 mm/s. (B) Atrioventricular dissociation during maximum pool apnea. Recorded at 25 mm/s.

FIGURE 4

Invasively measured systolic blood pressure (Sys) during pool apnea. Systolic blood pressure increased every minute from 157 ± 7 before apnea (baseline) to a maximum of 239 ± 15 mm Hg after 4 min of apnea (*P < 0.001 compared to baseline, n = 9).

FIGURE 5

Invasively measured diastolic blood pressure (Dia) during pool apnea. Diastolic blood pressure increased every minute from 76 ± 3 to a maximum of 113 ± 5 mm Hg after 3 min of apnea compared to rest and remained constant hereafter until termination of breath hold (*P < 0.001 compared to baseline, n = 9).

FIGURE 6

Invasively measured mean arterial blood pressure (MAP) during pool apnea. After 2 min of breath hold mean arterial blood pressure increased every minute compared to rest from 103 ± 4 to a maximum of 155 ± 6 after 3 min of apnea (*P < 0.001 compared to baseline, n = 9).

FIGURE 7

ECG at rest and ECG during maximum dry apnea in two subjects. (A) Sinus rhythm at rest (top) and nodal rhythm during maximum dry apnea (bottom). Recorded at 25 mm/s. Continues on next page. (B) Sinus rhythm at rest (top) and second-degree atrioventricular dissociation during maximum dry apnea (bottom). Recorded at 25 mm/s.

FIGURE 8

Hypoxia-inducible-factor-1-alpha (HIF1α) expression normalized to α-tubulin shows no difference between breath-hold divers (BHD, n = 8) and controls subjects (n = 6).