Deep diving mammals: Dive behavior and circulatory adjustments contribute to bends avoidance

A mathematical model was created that predicted blood and tissue N-2 tension (P-N2) during breath-hold diving. Measured muscle PN2 from the bottlenose dolphin after diving repeatedly to 100 m (Tursiops truncatus [Ridgway and Howard, 1979, Science, 4423, 1182-1183]) was compared with predictions from the model. Lung collapse was modelled as a 100% pulmonary shunt which yielded tissue P-N2 similar to those reported for the dolphin. On the other hand, predicted muscle P-N2 for an animal with a dive response, reducing cardiac output by 66% from surface values (20.5 to 6.81(.)min(-1)), also agreed well with observed values in the absence of lung collapse. In fact, modelling indicated that both cardiovascular adjustments and dive behaviour are important in reducing N-2 uptake during diving and enhancing safe transfer of tissue and blood N-2 back to the lung immediately before coming to the surface. In particular, diving bradycardia during the descent and bottom phase together with a reduced ascent rate and increase in heart rate reduced mixed venous P-N2 upon return to the surface by as much as 45%. This has important implications as small reductions in inert gas load (similar to 5%) can substantially reduce decompression sickness (DCS) risk by as much as 50% (Fahlman et al., 2001, J. Appl. Physiol. 91, 2720-2729). (c) 2005 Elsevier B.V. All rights reserved.