Emergency airways commonly involve challenges of tube placement and oxygenation before and during the procedure. There are a handful of instances, however, when the issue is ventilation and, more specifically, extremes of minute ventilation. Minute ventilation is the amount of air the patient moves in one minute; it is a product of the ventilatory rate and tidal volume (minus dead-space ventilation).
Explore This IssueACEP Now: Vol 34 – No 01– January 2015
Normal minute ventilation is between 5 and 8 L per minute (Lpm). Tidal volumes of 500 to 600 mL at 12–14 breaths per minute yield minute ventilations between 6.0 and 8.4 L, for example. Minute ventilation can double with light exercise, and it can exceed 40 Lpm with heavy exercise. As defined by the alveolar gas equation, increasing ventilation rate is our body’s only innate mechanism to acutely increase oxygenation. Breathing faster and deeper, we increase the alveolar oxygen tension by decreasing the partial pressure of CO2. Therapeutically, the main method of boosting oxygenation is increasing the inspired oxygen concentration (FiO2). An additional method is to increase the barometric pressure (ie, in a hyperbaric chamber or by rapidly descending from high altitude). Adding positive end-expiratory pressure (PEEP) maintains a pressure in the alveolus throughout the ventilation cycle (and stents open the alveolus, thereby providing more surface area for oxygen absorption).
It’s useful to consider minute ventilation when assessing patients in severe distress. I now appreciate that 15 Lpm via a non-rebreather mask may not meet the minute ventilation of patients in extremis; this explains how a non-rebreather can collapse with inspiration and why many patients feel suffocated with a mask over their face. Patients also do not want to rebreathe their expired CO2, and standard emergency airway equipment, unlike the systems used in the operating room, lacks any CO2 absorption. Nasal oxygen boosts FiO2, flushes the nasopharynx, and fills the upper airway with a high concentration of oxygen available for the next breath. Combining nasal and mask oxygen increases the volume of oxygen available for the patient to inspire. Since I began using nasal oxygen routinely as part of preoxygenation, I have found fewer instances of mask or continuous positive airway pressure (CPAP) intolerance. Even 4–6 Lpm via nasal cannula, which is what I start with in patients who are not critically hypoxic, is very helpful for maximizing pulse oximetry and mask tolerance. I turn nasal cannula up to 15 Lpm after induction and throughout the intubation.