[fpernett]

Hi Sebastian
I believe is important to define what really is "lung squeeze". The proper term should be something like pulmonary edema of immersion. And pulmonary edema means that some fluid is going away from blood vessels into pulmonary interstitium an even to alveolar space. The main factor for this fluid to move are hydrostatic pressures gradient, the higher the pressure gradient more fluid will move away from vessels. The pulmonary vessels can withstand a lot of blood volume without significant increase in pressure, and that's why I don't believe that over-hydration (in fact, is quite difficult to be over-hydrated during immersion) is a factor in pulmonary edema (if your heart is working fine).
How can we increase the hydrostatic pressure gradient?
-Increasing intra-arterial hydrostatic pressure: This is affected by several things
1) Lung Volume: At Total pulmonary Capacity (TPC) there is High resistance in pulmonary vasculature, and the same is for Residual Volume (RV). The Lowest resistance is at Functional Residual Capacity (and advantage of FRC diving). After a full pack you are over your TPC and the resistance of the vessels is higher than normal, and the flow is minimal. When the dive start lung volume decrease rapidly and resistance goes down, with increasing flow that can produce shear stress on endothelial cells. So, I think rapid changes in lung volumes is a main factor
2) Interstitial pressure: Contractions produce high negative pressures in pleural space, that is transmitted to interstitial tissue, increasing the hydrostatic pressure. This is also an explanation for contraction of intercostal muscles (due to stress or fear) this reduces flexibility of the chest and increase the negative pressure in pleural space.
3) Exercise: This is not a problem only for freedivers, SCUBA and swimmers also are affected by pulmonary edema. The exercise induces high flow in pulmonary vessels and increase pulmonary pressure with increasing work for the right side of the heart. In the last part of a deep and long dive you are probably very hypoxic and due to the pulmonary hypoxic vasoconstriction the resistance is very high, and with higher flow than usual make the apneist prone to the pulmonary edema.
Also, with all of this conditions it's possible a rupture (microscopic) of pulmonary vessels that affect the permeability and produce more edema.

That's why I believe that training in a conservative way is the best way to avoid it. And agree with your statements, and will include Confidence, that make you go more relaxed and without fear.

It's quite difficult to know where the blood comes from, but with X-ray and Thoracic CT of the few reported events in medical literature the more severe forms are from alveolar damage. Spitting blood is not always the same as pulmonary edema, you can spit blood from sinuses if equalization was too hard, or even throat specially with heavy contractions. Shortness of breath and rapid and shallow breathing indicate a more serious conditions.
My brother had in 2004 a severe pulmonary edema, with fluid in his pleural and pericardic space, high fever and low levels of Oxygen in blood, he took oxygen for 12 hours.
There is a lot of room for investigation to make things clear