Terrestrial and sub aquatic apneic bradycardial responses under resting and working conditions

Morphology normalised (BSA; stature) vital capacity was used to divide a sample of 13 young adult female volunteers into two groups based on lung volume. The apneic (breath-hold) heart rate response under resting and working conditions, on land and underwater, was measured. Four conditions were imposed, during which continuous monitoring of heart-rate was undertaken: resting on land, resting underwater, swimming unconstrained underwater, carrying bricks underwater. In all conditions a significant bradycardial response was exhibited (p less than 0.05), with heart rates also significantly lower for the group with larger lung volumes. No differences were found between the groups in ratings of perceived exertion during breath-hold. We contend that the lower heart rates in the group with larger lung volumes may be caused by higher negative intrathoracic pressure which, by stimulation of baroreceptors leads to a more pronounced reflex slowing of the heart rate.