Rapid splenic contraction and heart rate increase at the onset of apnea in human

We previously observed that the spleen contracts during maximal apnea and stays contracted for prolonged period after series of apneas. The role of central mechanisms in the initial splenic contraction and cardiovascular responses during apnea is unknown. Now, we investigated changes occurring during short apneas lasting 15 s during which arterial O 2 and CO 2 levels are unchanged. We studied changes in splenic size and blood flow, as well as central and peripheral cardiovascular parameters before, during and after a single apnea both with and without forehead cold stimulation to isolate the effect of the diving response from the apnea. Splenic artery diameter and blood velocity were not significantly changed during apnea. Spleen volume was reduced by about 20% as early as 3 seconds after the start of apnea and was quickly recovered after cessation of the attempt. Heart rate increased early during the apnea, simultaneously with spleen volume change to compensate for reductions in stroke volume and arterial pressure. Heart rate recovered towards baseline levels later in each apnea as arterial pressure was restored in concert with a rise in peripheral resistance. Splenic contraction and cardiovascular changes were not influenced by facial cold stimulation. Results indicate that sympathetic nerve activation to spleen and heart occurs during initial stage of the diving. This study was supported by Croatian Ministry of Science, Education and Sports.