By Richard Kallet, MS, RRT, FAARC, FCCM
Director of Quality Assurance, Respiratory Care Services, San Francisco General Hospital
Mr. Kallet reports no financial relationships relevant to this field of study.
SYNOPSIS: Use of apneic oxygenation with a high-flow nasal cannula during endotracheal intubation for acute respiratory failure is no better at preventing arterial oxygen desaturation than usual care using pre-oxygenation.
SOURCE: Semler MW, et al. Randomized trial of apneic oxygenation during endotracheal intubation of the critically ill. Am J Respir Crit Care Med 2016;193:273-280.
In this prospective trial, 150 medical ICU patients were randomized to receive oxygen therapy with a high-flow nasal cannula (HFNC) at 15 L/min during endotracheal intubation or usual care (no supplemental oxygen during laryngoscopy). All other decisions regarding endotracheal intubation (e.g., pre-oxygenation strategy, patient positioning, etc.) were based on clinician discretion. Primary exclusion criteria were emergency intubations that prevented randomization (i.e., cardiac or respiratory arrest) or need for fiberoptic/video-assisted endotracheal intubation. The primary endpoint was lowest pulse oximetry saturation (SpO2) between induction and 2 minutes following successful intubation. Secondary outcomes were any incidence of hypoxemia (SpO2 < 90%), severe hypoxemia (< 80%), and any decrement in SpO2 > 3% from the start of induction. Baseline demographics and patient characteristics were not different between groups. Most patients (66%) had sepsis and 57% were intubated for hypoxemia and/or hypercapnia. The median number of intubation attempts was 1, with an initial success rate of 67% in each group. Endotracheal intubation duration was not different between the groups (2.2 vs 2.5 minutes for HFNC and usual care, respectively).
All patients received singular or multiple forms of pre-oxygenation. For the HFNC and usual care treatment arms, the median SpO2 prior to induction was 99% and 98%. There was no statistically significant difference between HFNC and usual care with regard to median lowest SpO2 (92% vs 90%, respectively), incidence of hypoxemia (44.7% vs 47.2%, respectively), incidence of severe hypoxemia (16% vs 25%, respectively) or incidence of an SpO2 decrement > 3% (54% vs 56%, respectively). Moreover, even when controlling for risk factors associated with hypoxemia (e.g., high body mass index, higher oxygen requirements or lower SpO2 prior to intubation, difficult or prolonged intubation), use of HFNC still provided no benefit compared to usual care.
COMMENTARY
This is the second prospective, randomized, controlled trial demonstrating that use of HFNC during endotracheal intubation for acute respiratory failure in the ICU setting does not offer any benefit compared to pre-oxygenation alone. Vourc’h et al recently compared HFNC at 60 L/min during endotracheal intubation to pre-oxygenation using a face mask at 15 L/min in patients whose acute respiratory failure was primarily due to pneumonia and acute respiratory distress syndrome (ARDS).1 In that study, pre-oxygenation was protocolized using each assigned technique for 4 minutes prior to induction. The median SpO2 for the HFNC and face mask treatment arms was 92% vs 89%, respectively (P = 0.2). For the 25% of study patients whose pre-oxygenation period was extended beyond 4 minutes, SpO2 was 89% vs 91%, respectively (P = 0.8).
Apneic oxygenation occurs because mass flow of oxygen from the upper airway toward the alveoli is sustained via continuous oxygen uptake by the blood with the simultaneous, rapid loss of a carbon dioxide diffusion gradient into the alveoli. This lowers intra-alveolar pressure, creating an intrapulmonary pressure gradient (estimated to reach -20 cm H2O) favoring mass flow.2
With appropriate pre-oxygenation, healthy, supine individuals have a functional residual capacity (FRC) of ~1.9-2.3 L that, together with a fully saturated blood volume, yields a total body oxygen reservoir of ~2.8-3.2 L. With normal oxygen consumption during apnea (250 mL/min), it would take approximately 11-13 minutes to reach complete depletion. In contrast, patients with acute respiratory failure are at higher risk for hypoxemia because of increased oxygen consumption, decreased FRC, and severe pulmonary oxygenation dysfunction. The mean FRC in these patients is ~1.8 L and ~0.5 L in severe ARDS. The effect of these factors was apparent in both ICU studies, as the incidence of severe hypoxemia (SpO2 < 80%) during intubation was 16-25% and 22-26%.2
A study examining intubation time in more than 600 patients in both controlled (general anesthesia) and pre-hospital settings found the median time for non-difficult airways was < 1 minute (89% of all intubations).3 In the minority of difficult airway cases, the median intubation time was 2.5-8.3 minutes. In two ICU trials (n = 269), the median intubation duration was 1-2.5 minutes, and in one trial, the reported incidence of difficult intubation was 4%.2
These studies imply that pulmonary oxygenation dysfunction, endotracheal intubation duration, and adequate pre-oxygenation are the most important determinants of procedural hypoxemia. Therefore, to guarantee that FRC contains 100% O2, patients should be pre-oxygenated for approximately 4 minutes using a tight seal, form-fitting mask connected to a device with sufficient oxygen flow (2-3 times/minute ventilation or > 15 L/min) and a large reservoir bag (e.g., 1 L modified Jackson-Rees circuit). Apneic oxygenation with HFNC during intubation is likely to be beneficial only when confronted with a difficult airway, particularly in someone with low FRC (e.g., severe ARDS, morbid obesity).
REFERENCES
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Vourc’h M, et al. High-flow nasal cannula oxygen during endotracheal intubation in hypoxemic patients: A randomized controlled clinical trial. Intensive Care Med 2015;41:1538-1548.
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Fruman MJ, et al. Apneic oxygenation in man. Anesthesiology 1959;20:789-798.
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Adnet F, et al. The intubation difficulty scale. Anesthesiology 1997;87:1290-1297.
