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Barrie, In reply to: On Wed, 29 May 1996, Barrie Kovish wrote: > >Peter also wrote: > > > >3. The MOD of the unit is the MOD of the gas (nitrox) supply, using a > >maximal pPO2 of 1.4 ATA. > > Interesting Drager seems to have no problem with exceeding the MOD of the > supply gas for both their military and recreation breathers. I have US Navy > documentation recommending both exceeding the MOD of the supply gas and not > exceeding the MOD of the supply gas. The more conservative limit is the > newest. They either wised up to the fact that we are "at peace" or had some bad experiences. I don't think you can predict accuarately without a pPO2 meter. > The cool thing is if you have a Bridge II and are running 8 l/m and 50% > you can use the Bridge idiot mode for both O2 and deco calculations. Sounds way cool - how does this work? > I wonder what happens with the Drager LAR V. I would expect the front > mounted counter lung has a lot more than 3 cms of water PEEP. That's why the LAR V is known as the "lung-buster" This is true of any system with the counter lung below the diver, on the front of the chest. It's much harder to breathe out, easier to breath in. The same is true for back mounted systems if you roll over on your back or do wheelies. > Do you have references to the lung toxicity effects of elevated PEEP? To the best of my knowledge, PEEP has been studied only in the context of patients on ventilators, already exposed to high FiO2 and usually with existing lung disease. In such patients PEEP improves oxygenation by preventing alveolar collapse at end-expiration, unlikely to be of value in divers. In patients PEEP is uses to *lessen* the risk of oxygen toxicity by allowing a lower FI O2 to be given and is used in cases where high concentrations of O2 (FI O2 > 0.5!) are required for a prolonged period. So if you deliver the same FI O2 with and without PEEP, my bet is that you deliver more O2 with PEEP than without. If CNS toxicity is realted to PaO2, then you get a higher PaO2 for the same pPO2 (or FI O2 at the same depth) with PEEP. Hence my conclusion that PEEP enhances O2 toxicity. Also, high levels of PEEP decrease cardiac output and increase the incidence of barotrauma (e.g., pneumothorax, pneumomediastinum, and subcutaneous emphysema). This is of interest to divers. Very high levels of PEEP can actually decrease oxygenation of arterial blood by overdistending open alveoli, thereby compressing the capillaries surrounding these alveoli and shunting more blood to the alveoli that remain closed. These are some refs that deal with principles and patients on ventilators; but an SCR *is* a ventilator. Pepe PE, Marini JJ: Occult positive end-expiratory pressure in mechanically ventilated patients with airflow obstruction: the auto-PEEP effect. Am Rev Respir Dis 126:166, 1982 Marini JJ: Strategies to minimize breathing effort during mechanical ventilation. Crit Care Clin 6:635, 1990 Marcy TW, Marini JJ: Inverse ratio ventilation in ARDS: rationale and implementation. Chest 100:494, 1991 slainte -ph
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