Hi,
A few points if I may:
You can 'exhale' diving for spearfishing (active) but there are some important considerations which require the diver to minimize activity, especially during the descent and, subsequently, move across and then up with a 'stroke-and-glide' technique.
The workshop which I had the pleasure of running last weekend clearly indicated to just about everyone participating that dive duration on 'exhale' was essentially similar to their ability on 'inhale' and, moreover, after just two days experience. I expect the same thing this upcoming weekend, just like in every previous clinic.
To answer Brett's question about the rate of adaptation there are a few things to consider.
Firstly, to be able to tolerate the large negative lung pressures it is fundamentally important to realize that it does take time. Similarly, one can't be expected to climb, say, Mt. Everest, without prior and long-term acclimatization. Failure to appreciate this will in all probability result in a 'lung squeeze' with pulmonary oedema and haemoptysis (bleeding). This in itself can result in serious health complications and even death.
So, is a 10-20m dive on 'exhale' deep? It's all relative, but if you compare what has been achieved on 'inhale' the answer is... not really. 10-20m is, for example, approximately equivalent to diving between 30m and 50m; both of which can be achieved comportably and without lung squeeze with practice. Nevertheless, many divers have experienced problems even at such relatively shallow depths.
Recalling way back then, it probably took me about a year of constant and consistant practice, in digestible sized pieces, to be able to reach the stage whereby I essentially no longer felt any pressure on the chest, irrespective of how deep I dove. One can understand this if one considers Boyle's exponential pressure-volume law. For our purpose, what it implies is that volume changes, and hence the feeling of pressure on the chest, become effectively quite small as one dives deeper.
Why some of bleed whilst others do not is easily explainable. As some of you may know the pulmonary capillaries are quite compliant to distension, as they become engorged the deeper one dive. Further increases in ambient (water) pressure may lead to intolerable pressure difference across the alveolar-capillary membrane, resulting in trauma as stress failure occurs. Typically, when large convective forces (cardiac output) are required to supply oxygen to the working muscles it is advantageous to have a very thin alveolar-capillary membrane for efficient gas-exchange. It is therefore, not a surprise to hear that race horses which have some of the highest aerobic scopes (VO2max) frequently bleed after racing. The bottom line is that although an elevated scope for aerobic exercise permits efficient gas-exchange it is also associated with thin and fragile alveolar-capillary mebrane walls. What this means is that there is, unfortunately, a decreased tolerance of the membrane wall to elevated pressures. This effect is magnified with exercise in cold and elevated ambient pressures, e.g., swimming to depth. The consumption of excessive amount of water befroe diving or aspirin will also potentiate this problem.
Can lung re-modelling occur such that the alveolar-capillary mebrane walls become reinforced (thicker?) and so that one may perhaps be better able to tolearte large hydrostatic pressures? According to researchers, and I refer you to West et al, repeated stress can and will invoke such changes. In my own case, we have found that as I've become able to tolerate greater and greater pressures my VO2 max has, for example, correspondingly decreased. This is not a real surprise as deep diving mammals have an aerobic scope about a third of those of similar sized terrestrial mammals. What this means is that improving your VO2max is not necessarilly associated with improved pressure tolerance. I would suggest, it's actually a disadvantage for exhale diving. Of course this is not to say that improved pressure tolerance works against the ability to undertake frequent diving bouts (serial diving).
Apologies for the long-winded discourse.
From sunny Perth
Seb