• Zoran Valic, Ivan Palada, Darija Bakovic, Maja Valic, Snjezana Mardesic-Brakus, and Zeljko Dujic.
• Department of Physiology, University of Split School of Medicine, Split, Croatia.
• Aviat Space Envir Md. 2006 Dec 1;77(12):1224-9.

IntroductionThe human diving reflex is characterized by bradycardia, decreased cardiac output, and peripheral vasoconstriction, and has an oxygen-conserving effect both at rest and during exercise. However, the resultant time course and extent of muscle desaturation is unknown.MethodsWe used near-infrared spectroscopy to continuously measure the decrease in tissue oxygen saturation (StO2) in the calf muscle during a series of breath-holds. Subjects were seven trained divers (TD) and eight untrained controls (UC). Other measured variables included arterial blood pressure, heart rate, and arterial oxygen saturation (SaO2). Each subject performed five maximal apneas during face immersion in cold water with 2-min recovery intervals between breath-holds.ResultsOn average, total apnea time for TD was significantly longer than for UC (772.6 +/- 40.9 s vs. 499.1 +/- 118.2 s, respectively). Further, TD had a more pronounced decrease in StO2 than UC (70.6 +/- 15.3% for TD vs. 87.9 +/- 6.1% UC for the fifth and longest apnea). When values for the two groups were compared at the mean breakpoint time for UC, there was no difference in StO2 and SaO2 remained at baseline. By contrast, at the same time point in all five apneas, UC experienced simultaneous, significantly larger reductions in SaO2 and StO2.DiscussionThese data indicate that TD have an attenuated diving reflex compared with UC at the same breath-hold times (the breakpoint for UC). In addition, muscle desaturation occurs earlier than arterial desaturation in both groups; the fact that this effect was less pronounced in TD suggests a training effect. This study provides further evidence for the oxygen-conserving effect of the human diving reflex in maintaining the oxygen supply of vital organs.

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