Carboxyhemoglobin and depletion of blood oxygen in sleeping elephant seals

An exceptionally large blood O₂ store underlies the remarkable dive performance of elephant seals. However, elevated carboxyhemoglobin (COHb) concentrations in these seals complicate estimations of blood O₂ content and O₂ depletion rates, and may also affect monitoring of brain oxygenation in seals with new non-invasive near-infrared (NIR) recorders. Using hemoximetry analyses of blood samples during sleep apneas of juvenile northern elephant seals (Mirounga angustirostris), we constructed in vivo Hill plot equations and O₂–Hb dissociation curves (ODCs). We found: (a) COHb and methemoglobin (both of which do not bind O₂ and increase hemoglobin (Hb) affinity for O₂) comprised 8% of Hb, (b) an in vivo P₅₀ (partial pressure of O₂ at 50% Hb saturation, an index of O₂ affinity of Hb) of 27.1 mm Hg that was Hg 3.4 mm Hg less than that previously determined with an in vitro laboratory approach, and (c) when the in vivo and in vitro Hill plot equations were applied to arterial, hepatic sinus and extradural vein P_O2 profiles during sleep apneas, the resulting differences in blood O₂ content and apneic blood O₂ depletion rates were minor despite higher Hb saturations calculated with the in vivo approach. We conclude that prior blood O₂ contents and depletion rates determined with the in vitro technique during dives are accurate. The range of arterial Hb saturations calculated from the in vivo and in vitro approaches represents the most realistic Hb saturation data available for evaluation of NIR monitors of arterial Hb saturation in seals.