Hook Breathing Facilitates SaO2 Recovery After Deep Dives in Freedivers With Slow Recovery

To facilitate recovery from hypoxia, many freedivers use a breathing method called "hook breathing" (HB) after diving, involving an interrupted exhale to build up intrapulmonary pressure. Some divers experience a delay in recovery of arterial oxygen saturation (SaO(2)) after diving, interpreted as symptoms of mild pulmonary edema, and facilitated recovery may be especially important in this group to avoid hypoxic "blackout." We examined the influence of HB on recovery of SaO(2) in freedivers with slow recovery (SR) and fast recovery (FR) of SaO(2) after deep "free immersion" (FIM) apnea dives to 30 m depth. Twenty-two male freedivers, with a mean (SD) personal best in the discipline FIM of 57(26) m, performed two 30 m deep dives, one followed by HB and one using normal breathing (NB) during recovery, at different days and weighted order. SaO(2) and heart rate (HR) were measured via pulse oximetry during recovery. The SR group (n = 5) had a faster SaO(2) recovery using HB, while the FR group (n = 17) showed no difference between breathing techniques. At 105 s, the SR group reached a mean (SD) SaO(2) of 95(5)% using HB, while using NB, their SaO(2) was 87(5)% (p < 0.05), and 105-120 s after surfacing SaO(2) was higher with HB (p < 0.05). In SR subjects, the average time needed to reach 95% SaO(2) with HB was 60 s, while it was 120 s at NB (p < 0.05). HR was similar in the SR group, while it was initially elevated at HB in the FR group (p < 0.05). We conclude that HB efficiently increases SaO(2) recovery in SR individuals, but not in the FR group. The proposed mechanism is that increased pulmonary pressure with HB will reverse any pulmonary edema and facilitate oxygen uptake in divers with delayed recovery.