4.5. Trans-thoracic pressures
For the dead space to completely avoid compression as in
the balloon/pipe model it must be able to withstand large transthoracic
pressure gradients (Pamb −Pint). Brown and Butler (2000)
analyzed the structural properties of the respiratory system in
breath-hold diving mammals and predicted that a trans-thoracic
difference >101 kPa would cause tissue failure. Following the logic
that upper airways are incompressible they concluded that lung
collapse is unavoidable during breath-hold dives. Our model shows
that complete alveolar collapse can be prevented with a gradient
of only 1–2 kPa if the dead space collapses along with the alveolar
space (Fig. 5). Unfortunately, this trans-thoracic pressure gradient
has not been measured.