EMS World

AUG 2017

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48 AUGUST 2017 | EMSWORLD.com CONTINUING EDUCATION mask ventilation works, a service CPAP does not perform. 5 BiPAP offers another pulmonary mecha- nism that CPAP already offers: positive end expiratory pressure. CPAP provides a con- stant airway pressure in the lungs during both inspiration and expiration. This con- stant baseline provides the lungs with PEEP, which increases the pulmonary reserve, or functional residual capacity. BiPAP does this through expiratory positive airway pressure. During the expiratory phase, the noninvasive ventilator lowers airway pressure to a preset EPAP, which is synonymous with the PEEP. This maintains small-airway patency, pre- vents atelectasis and increases FRC, dras- tically improving oxygenation. 6 While CPAP works to improve only oxygenation (hypox- emic respiratory failure), the use of BiPAP improves not only oxygenation but also ventilation with the use of pressure support. 12 Technical Aspects There are different types of BiPAP modes. Some models provide a time-triggered mode, where the IPAP and EPAP cycles occur at a prescribed respiratory rate. This type of BiPAP is less frequently used because it is associated with patient-ventilator dyssyn- chrony and increased respiratory distress. 7 The more commonly used mode of BiPAP is known as spontaneous timed. This mode allows the operator to set a minimal respi- ratory rate, usually around 8–12 bpm. The idea is that the patient continues to breathe spontaneously, and the IPAP and EPAP are triggered according to the patient's sponta- neous effort; however, if the patient experi- ences a period of apnea or their respiratory rate drops below the rate set on the ventila- tor, the machine will switch to a prescribed respiratory rate until the patient begins to breathe spontaneously again. 7 This approach is not designed to actu- ally ventilate the patient, but it is meant as a safety mechanism to allow for some positive pressure ventilation to occur should the patient begin deteriorating. Most mod- els have built-in alarms that alert providers that the BiPAP has switched to a prescribed respiratory rate or if no spontaneous effort is noted from the patient. This is most helpful when BiPAP is used in patients who have central sleep apnea, a form of sleep apnea in which efforts to breathe do not occur (versus obstructive sleep apnea, where the airway is compromised by decreased muscle tone or from some other obstruction). 8 The machine time-cycled IPAP and EPAP are meant to prompt the patient to begin spontaneously breathing again. In the emergency setting, noninvasive ventilation (NIV) is often used for two dis- tinct types of respiratory disorders: restric- tive lung disease and obstructive airway disease. Most of the time this is due to acute cardiogenic pulmonary edema, asthma or chronic obstructive pulmonary disease. Other conditions that benefit from the use of NIV are acute hypoxemia, acute respira- tory distress syndrome and palliative care. Settings for NIV are most often determined based upon presentation and titrated to effect. Most clinicians start at an IPAP of 10 cm H 2 O and an EPAP of 5 cm H 2 O, leav- ing a pressure support of 5 cm H 2 O. Sources recommend titrating the pressure support to meet appropriate tidal volumes of 6 ml/ kg of ideal body weight. If the presenting tidal volume is lower than predicted, it is important for the clinician to increase the pressure support to assure adequate venti- lation. Other sources seriously recommend not exceeding an IPAP of 20–25 cm H 2 O due to the risk of gastric distention. 13 It is helpful to distinguish which respira- tory process is compromised, oxygenation or ventilation. If the patient is oxygenating well with respiratory distress, increased lev- els of PEEP may not be necessary, allowing the clinician to leave EPAP at a normal level of 5 cm H 2 O. However, in some cases, such as acute pulmonary edema, increased lev- els of PEEP may be necessary to maintain alveoli patency and reduce fluid infiltration to improve oxygenation. EPAP levels of 5, 8 or 10 cm H 2 O may be necessary in the pres- ence of hypoxia. It is theorized that consis- tently high levels of intrathoracic pressure due to PEEP decrease preload on the heart, reducing the circulating volume of blood and fluid through the lungs, reducing pulmonary edema and decreasing hydrostatic pres- sures in the pulmonary vasculature. 9 Most prehospital providers are comfort- able with determining the liter flow required for oxygen devices such as a nasal cannula or nonrebreather; however, when using a noninvasive ventilator for BiPAP, oxygen is most often determined by the fraction of inspired oxygen, or FiO 2 . FiO 2 is the percent- age of oxygen inspired, room air being 21% and a nonrebreather with 15 lpm of oxygen at 100% FiO 2 (theoretically). When initiating BiPAP it is also important to determine the amount of oxygen the patient needs. The use of CPAP (PEEP) or EPAP (in the case of BiPAP) often eliminates the need for high levels of FiO 2 ; some clinicians with experi- ence using BiPAP often find the mechanism of expiratory pressure allows clinicians to supply lower levels of oxygen, sometimes Figure 1—A graphical representation of IPAP, EPAP and pressure support. Some ventilators even offer waveforms such as this on their interface so the provider can view the patient's respiratory effort. As you can see, the difference between IPAP and EPAP is pressure support, working on ventilation (carbon dioxide). It is important to note that if the EPAP is increased without increasing the IPAP, then the difference between IPAP and EPAP decreases, leading to a smaller amount of pressure support. Pressure (cmH2O) IPAP (10) Pressure Support (5) Time 10 5 EPAP (5)

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