Читать книгу Emergency Medical Services - Группа авторов - Страница 162

Patients who are awake, protecting their airways, have respiratory drive, and can cooperate may be given ventilatory support with noninvasive modalities that provide positive airway pressure. These include continuous positive airway pressure (CPAP), which delivers constant pressure above that of the atmosphere throughout the ventilation cycle, and bi‐level positive airway pressure (BiPAP), which delivers different pressures during the inspiratory and expiratory phases. Portable devices for the delivery of CPAP in the prehospital setting are generally of three types. Two of these require a high‐pressure (50 psi) oxygen source. One continuously delivers oxygen under pressure to a mask with a pop‐off valve that opens when the desired pressure is reached. The other uses a controller that essentially acts as a demand valve, adjusting flow to maintain the desired pressure. The third type of device uses oxygen flow from a standard regulator and a Venturi valve to create a virtual valve resulting in elevated pressure. Usually, treatment is started at 5–10 cm H₂O and increased as needed to a maximum of 20 cm H₂O. Prehospital devices generally deliver near 100% FiO₂, while more advanced devices allow the FiO₂ to be titrated. Until recently, BiPAP has been only available to EMS personnel who carry full‐function mechanical ventilators, but newer devices can now provide this modality for the field.

NIPPV has been shown to improve oxygen delivery. This is thought to be the result of the hydrostatic pressure effects of increasing the gas diffusion gradient, promoting displacement of fluid in the alveoli back into the capillary bed, and stenting open small bronchioles (which do not have cartilaginous walls), thereby increasing both the volume of air exchanged and the number of alveoli ventilated. NIPPV also decreases the work of breathing. The ultimate clinical effects are that patients will often have improved oxygenation, improved ventilation, marked improvement in respiratory distress, and significantly lower likelihood of needing intubation and mechanical ventilation [9].