Predisposing disorders include lesions that may result in local compliance changes, gas trapping or airway obstruction. These include sub-pleural blebs of the type associated with spontaneous pneumothorax, asthma, sarcoidosis, cysts and bullae, tumors, pleural adhesions, pulmonary fibrosis, infection and inflammation.
Precipitating factors include inadequate exhalation or outright breath-holding during ascent (often in association with panic), rapid ascent, faulty breathing apparatus or water inhalation.
Although many cases of PBT may be caused by voluntary breath-holding during ascent or by the pathological lesions mentioned earlier, it is clear that these risk factors are not present in all cases. About half the submarine escape ascent trainees who develop PBT have been observed to carry out correct exhalation techniques. These divers were also passed as medically fit before the dive and exhibited none of the contributory pathological features afterward. A frequent finding with some of these subjects is a reduction of compliance at maximum inspiratory pressures, i.e. the lungs are less distensible (stiffer) and are exposed to more stress than normal diver’s lungs, when distended. Brooks and colleagues6 demonstrated that a lower than predicted forced vital capacity (FVC) was associated with PBT in submarine escape trainees, and this finding further supports the suggestion that reduced pulmonary compliance is a predisposing factor. Interestingly, many medical standards refer to the requirement for the ratio of forced expiratory volume in 1 second (FEV1) to FVC (FEV1/FVC ratio) to be greater than 75 to 80 per cent of predicted levels, yet this spirometric parameter has not been shown to be causally related to PBT in trainees who have no evidence of lung disease.