Apr 072019

A randomized trial confirmed what most intensivists have long believed and practiced: in the moments before endotracheal intubation, we should help patients achieve the highest arterial oxygen saturation possible, using bag-mask ventilation (BMV).

This seems self evident -- why wouldn't we? Bag-mask ventilation can distend the stomach with air, potentially increasing the risk for vomiting with aspiration of gastric contents. Aspiration events can obscure the visual field and worsen hypoxemia, making intubations much more difficult and dangerous.

Avoidance of aspiration is the rationale for fasting periods prior to procedural sedation, and for the adoption of rapid-sequence intubation (RSI) in the 1970s. In rapid sequence intubation, the patient is pre-oxygenated as much as possible, then given an induction agent (e.g., ketamine, etomidate, or propofol), then a neuromuscular blocker (e.g, succinylcholine or rocuronium). Paralysis occurs in about 60 seconds, at which time direct or indirect (video) laryngoscopy with endotracheal intubation are performed as quickly as possible. Intubation can often be completed within seconds, limiting the time a patient is exposed to the risks of aspiration or apnea-induced hypoxemia.

During the short interval of time between induction and the onset of paralysis during RSI, a patient can simply be oxygenated with 100% oxygen through a face mask or high-flow nasal cannula. Or, an Ambu bag can be used, squeezed every few seconds (i.e. bag mask ventilation), improving oxygen delivery and ventilation but theoretically increasing aspiration risk.

The PreVent trial, published in the New England Journal of Medicine, gives a clear signal that bag-mask ventilation up until the time of intubation is safe and effective at preventing severe hypoxemia events, without increasing the risk of aspiration.

Authors enrolled 401 patients requiring intubation in seven ICUs at U.S. academic centers; patients were randomized to receive 100% oxygen with bag-mask ventilation after induction, or oxygen alone delivered by any means other than BMV or noninvasive positive pressure ventilation. In the time leading up to induction and RSI, patients could receive any method of oxygen delivery, including bag-mask ventilation.

Patients with severe hypoxemia were excluded (these patients frequently require bag-mask ventilation to maintain barely-adequate oxygenation during RSI).

Investigators could not be blinded, and patients in the BMV group received BMV before RSI 38% of the time (compared to only 11% in the control group). Both groups had a median pre-intubation oxygen saturation of 99%, which probably concealed a higher arterial partial pressure of oxygen achieved in the BMV group.

The median nadir oxygen saturation during intubation was 96% in the bag-mask group, vs. 93% in the control group. But more importantly, 45 patients in the control group experienced severe hypoxemia, compared to only 21 in the bag-mask ventilation group.

Aspiration was noted by the (unblinded) teams among 2.5% of patients in the BMV group and 4% in the control group, with no apparent increased incidence of aspiration pneumonia on chest films in the 48 hours after intubation (16% vs 15%). Patients who received bag-mask ventilation also did not need more FiO2 or PEEP once intubated (which could have suggested aspiration events).

The idea of limiting manual ventilation in apneic patients to avoid aspiration was never guided by evidence; there was no prospective trial showing bag-mask ventilation increased aspiration events. Rather, it was dogma arising out of anesthesiology's literature and professional culture. In the operating theater, where the average patient with healthy lungs can maintain oxygen saturations during minutes of apnea after simply breathing 100% oxygen for a few minutes, there is no reason to bag-ventilate patients. One would be seeking marginal improvement in oxygenation while risking (at least theoretically) a catastrophic aspiration event.

In the ICU, the calculus changes. Most patients are already severely hypoxemic (and would be excluded from this trial), and many are severely obese, with minimal oxygen reservoirs in their compressed lungs.  As a result, the incidence of significant hypoxemia during ICU intubations has been reported as high as 40%. Most patients have comorbid cardiopulmonary or renal disease and are at increased risk for multiorgan deterioration and cardiac arrest from additional stressors like worsened hypoxemia during intubation. In other words, aspiration risk is only one of their many worries.

The takeaway: Most ICU patients about to undergo endotracheal intubation should receive careful bag-mask ventilation to optimize oxygenation throughout rapid sequence induction, until the moment laryngoscopy is attempted.

Aspiration events might be prevented by using good bag-mask ventilation technique, shown in the video below from Reuben Strayer. Pro tips:

  • Don't try to squeeze the whole Ambu bag (1,600 mL) into the patient (nor allow your assistant to do so). A 5' 9" patient should receive about 400-500 mL, or about 1/4 to 1/3 of the bag with each breath. Higher delivered volumes can distend the stomach.
  • 15 breaths per minute (a breath every 4 seconds) is a reasonable target. Squeezing 1/3 of the bag, this can achieve a minute ventilation of ~8 L/min.
  • Turn up the PEEP valve on the Ambu bag to at least 5-10 cm H2O.
  • Severely hypoxemic, decompensating patients may need higher tidal volumes, more frequent breaths, and/or higher PEEP to achieve minimally adequate oxygenation. When good oxygenation is achieved, titrate down BMV to more physiologic levels (or, intubate immediately).
  • Elevating the head of bed ("ramping" the patient) can significantly improve oxygenation by reducing atelectasis.
  • The pulse oximeter is not a realtime measure of arterial oxygen saturation; it lags behind the patient's actual arterial O2 saturation by about 30 seconds, and longer in low cardiac output states.

Source: NEJM (editorial)

Infographic: Visualmed

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Bag-mask ventilation during intubation in ICU prevents severe hypoxemia