![]() Kelly Grogan, in The Johns Hopkins Manual of Cardiac Surgical Care (Second Edition), 2008 Pulmonary Barotraumas If the volume of air is sufficient, it can completely block the central vascular bed. If air enters the pulmonary vasculature, it can travel to the heart and embolize systemically, causing AGE. The air may dissect to the visceral pleura, causing a pneumothorax. This air can track superiorly to the neck, resulting in subcutaneous emphysema, and can dissect inferiorly and posteriorly, causing pneumoperitoneum. Air can travel along the perivascular sheaths and dissect into the mediastinum. Once the alveoli rupture, air can remain in the interstitium, causing localized pulmonary injury and alveolar hemorrhage. The actual clinical manifestations may take several forms, depending on the course that the extra-alveolar air travels. The diagnosis of PBT is based on the development of characteristic symptoms after diving. PBT has occurred from breath-holding during ascent from a depth as shallow as 4 feet of water. A pressure differential of only 80 mm Hg (alveolar air) above ambient water pressure on the chest wall, or about 3 to 4 feet of depth under water, is adequate to force air bubbles across the alveolar-capillary membrane. Thus, shallow depths are the most dangerous for breath-holding ascents. Boyle's Law dictates greater volume changes for a given change in depth near the surface than at greater depths. Significant change in barometric pressure occurs in shallow water. Currently, in the absence of breath-holding, there does not seem to be a reliable or valid method to accurately predict who is or is not at risk for PBT and subsequent AGE. 93 Tetzlaff and colleagues 94 suggest that, theoretically, divers with preexisting small lung cysts on chest CT scan may be at increased risk for PBT. 84 Scuba divers with asthma do not appear to have a greater risk for idiopathic AGE than nonasthmatic divers. There are reports of divers with AGE with no history of breath-holding ascent. This may explain the almost complete absence of case reports of AGE associated with hyperbaric chamber operations. ![]() 92 With immersion in diving, central pooling of blood causes an increase in intrapulmonary blood volume and the lungs become stiffer (less compliant) this decreased compliance of the lung may increase the risk for PBT. 90, 91 Theoretically, if there are focal areas of decreased compliance in the lungs, the adjacent areas of normal compliance would be subjected to greater forces leading to barotrauma. 89 Localized overinflation of the lung from focally increased elastic recoil may occur in divers who ascend at a proper rate. Review of pulmonary function tests in submarine-escape trainees found that a small forced vital capacity (but still within the reference range) was the only factor that correlated with a risk for PBT. In submarine-escape trainees, breath-holding did not appear to be a major factor, leading to the conclusion that, in the majority of these patients, some intrinsic abnormality of the lungs was the cause of the injury. 87, 88īreath-holding is the most common cause of PBT and AGE in sport divers. Under experimental conditions in fresh chilled human cadavers, a transpulmonic pressure (the difference between the intratracheal and the intrapleural pressures) of 95 to 110 cm H 2O is sufficient to disrupt the pulmonary parenchyma leading to extra-alveolar gas. The combination of overdistension of the alveoli and overpressurization causes the alveoli to rupture, producing a spectrum of injuries collectively referred to as PBT. If a diver does not allow the expanding gas to escape, a pressure differential develops between the intrapulmonary air space and the ambient pressure. PBT from diving results from expansion of gas trapped in the lungs during ascent. Neuman MD, FACP, FACPM, in Physiology and Medicine of Hyperbaric Oxygen Therapy, 2008 Pulmonary Barotrauma
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