Muscle oxygenation and local blood volume difference between intercostal and deltoid during dry static apnea in breath-hold divers

During either a static or dynamic apnea, oxygen is delivered to vital organs, i.e., the brain and heart, and there is a compensatory reduction of oxygen consumption in peripheral tissues. Additionally, maximal apnea is characterised by the initial easy-going phase and subsequent struggle phase in which involuntary breathing movements appear. The aim of this study was to compare the oxygenation and local blood volume of one active (external intercostal) and one non-active (deltoid) muscle during maximal dry static apneas in breath-hold divers while supine. Thirteen breath-hold divers performed 3 preparatory apneas followed by 3 maximal apneas with 5 min of supine rest between each apnea. During all apneas (duration, 115–323 s; IBM, 7–35) muscle oxygenation and muscle blood volume change were measured via NIRS. The variables quantified were muscle oxygen saturation (SmO₂) and total hemoglobin (tHb). We found that the decline in oxygen saturation was similar in external intercostals and deltoids, even though their physiological role during a breath-hold is different. However, the external intercostals re-saturated at a significantly higher rate following a maximal apnea than the deltoid muscle (p = 0.02). Also, during the apneas, there was a significantly different response between muscles, where external intercostals had an increase and deltoid a decrease in tHb (p = 0.01). These findings indicate that despite respiratory muscle activity resulting from IBMs during breath-holds external intercostal muscle re-oxygenation occurs faster than peripheral muscles which may allow for a faster return to normal breathing.