Cerebrovascular Reactivity during Prolonged Breath-Hold in Experienced Freedivers

BACKGROUND AND PURPOSE:

Experienced freedivers can endure prolonged breath-holds of up to eleven minutes despite severe hypoxemia and are therefore ideal subjects to study apnea-induced cerebrovascular reactivity (CVR). This multi-parametric study investigates CBF, the spatial coefficient of variation (ASL-sCoV), as a correlate of arterial transit time, and brain metabolism dynamics during prolonged apnea.

MATERIALS AND METHODS:
Fifteen male freedivers (median age: 36.0, CI 32.0–50.0, years; median previous prolonged breath-holds >2.5 mins.: 384, CI 24.0–4,536) underwent repetitive 3T pseudo-continuous arterial spin labeling and 31P-/1H-MR spectroscopy scans before, during and after a five-minute breath-hold (split into early and late phases) and gave temporally matching venous blood gas samples. CVR was temporally and regionally compared to blood gases and previous experience.

RESULTS:
ASL-sCoV decreased during the early breath-hold phase (-30.0%, P=.002), whereas CBF remained almost stable during this phase and increased in the late phase
(+51.8%; P<.0001). CVR differed between the anterior and the posterior circulation during all phases (e.g. late breath-hold: MCA 57.3±14.2 vs. PCA 42.7±10.8 mL/100 g/min.;
P=.001). There was an association between breath-hold experience and lower CBF (1,000 previous breath-holds reduced WM CBF by 0.6 mL/100 g/min.; CI 0.15 –1.1 mL/100 g/min., P=.01). While breath-hold caused peripheral lactate rise (+18.5 %) and hypoxemia (SpO2 -24.0%), cerebral lactate and ATP remained within physiological ranges, despite early signs of oxidative stress (-6.4% phosphocreatine/(ATP+ADP); P=.02).

CONCLUSIONS:

The CVR responses to prolonged apnea are complex, but guarantee the maintenance of a physiological brain metabolism in trained individuals for at least five minutes.