EMS World

JAN 2019

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22 JANUARY 2019 | EMSWORLD.com COVER REPORT: AIRWAY MANAGEMENT cardiac arrest. In an academic ED study, 2% of all RSIs had a peri-intubation cardiac arrest, and more than 80% of these were from hypoxia. 7 The Importance of First-Pass Success Much attention has been paid to achieving first-pass success when intubating. Why is this important? Because failure to achieve it is associated with an increase in adverse events, the most common of which is peri- intubation hypoxia. In a study of more than 1,800 ED intuba- tions, the rate of all adverse events was 14% with FPS but increased to 47% with two attempts and 64% with three. 8 The most common of these events was hypox- ia (9.2% seen with FPS, 38% with two attempts). The odds of having an adverse event with more than one attempt were 652% higher than with FPS. These odds were also seen in an even larger study of 2,616 patients undergoing intubation in 11 Japanese EDs, where the OR for major adverse events was 8.9 with two attempts and 13.9 with three compared with FPS. 9 In addition to increasing the odds of adverse events, multiple attempts are also less likely to succeed. There is a "plateau point" above which further attempts are statistically futile. 10 For prehospital ETI that point is 3–4, but each additional attempt comes at a clinical "cost," so the actual attempt limit likely should be lower. If peri-intubation hypoxia is common and harmful, it would be nice if it were predict- able too. Fortunately it often is. The classic oxyhemoglobin dissociation curve is a plot of different SpO 2 values at varying PaO 2 levels. 11 Figure 1 shows this is a sigmoidal curve, not a linear one. It demonstrates that the rate of desaturation is different at dif- ferent points on the curve. Above a PaO 2 of around 90, the curve is flat, at a satura- tion approaching 100%. Once PaO 2 drops below 60 (SpO 2 around 90%), small drops in PaO 2 are associated with large drops in SpO 2 . This is the steep part of the curve. This physiologic curve manifests itself in clinical practice. If your patient has an SpO 2 of 100% at the time of paralysis, they will slowly desaturate until reaching an SpO 2 of around 93%, at which point they desatu- rate progressively faster. In other words, they "fall off the curve." Of prehospital patients undergoing RSI intubation, 100% had peri-intubation hypoxia if their starting saturation was less than 93%. 12 This is an indication of how rapidly patients desaturate once they become hypoxic. In another study patients with a starting saturation between 98%–100% had a rate of peri-intubation hypoxia of only 20%. 2,12 This indicates that patients with starting saturations above 93% can tolerate a lon- ger period of apnea without desaturating, allowing more safe time for a controlled intubation attempt. So it is predictable that patients with starting saturations of less than 93% are at very high risk of peri-intubation hypoxia. 13 Preventing desaturation, then, depends at least partially on having good data on your patient's saturation. Unfortunately pulse oximetry data tends to go missing, especially during hectic intubations. Of patients with TBI undergoing RSI, 79% had at least one SpO 2 "dropout" during the intubation—there was simply no oximetry value displayed. 14 Additionally, the actual value displayed is a bit delayed. In 55% of intubations the intra-attempt SpO 2 nadir occurred after resumption of ventilations with oxygen. We can use this information to improve the safety of our intubations. First, do everything possible to assure the pulse oximetr y probe is firmly affixed to the patient somewhere other than distal to the BP cuff and is not inadvertently dislodged. Next, don't wait to abort an intubation attempt until the SpO 2 drops below 90%. We must assume that because of "pulse ox lag," the patient has already desaturated. The safe place to bail out of an attempt is when the SpO 2 hits 93%. Denitrogenation To fully maximize preoxygenation we need to not only increase the SpO 2 to above 93% but keep it there long enough to completely fill up the patient's physiologic "buffer." In most healthy patients this is accomplished by normal-tidal-volume breathing for three minutes. 15 So we need to raise the oxygen saturation above 94% and keep it there for at least three minutes. The point of this is to replace the inert gas in the lungs and blood with oxygen. Because the atmosphere contains 21% oxygen and 78% nitrogen, our lungs con- tain the same ratio. Breathing 100% oxygen will come close to completely replacing all the nitrogen in the lungs with oxygen in about three minutes. This process of gas replacement is known as denitrogenation. This term is often used synonymously with preoxygenation. Keep in mind that this three-minute guideline is based on healthy patients. We are not typically intubating healthy patients, so consider extending Figure 1. Oxyhemoglobin dissociation curve. (Infographic: Jeff rey Jarvis)

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