Patients requiring endotracheal intubation in the emergency department (ED) are at a much higher risk for hypoxemia, compared to patients electively intubated in the operating room. Pulmonary disease, anemia, low respiratory drive, decreased ability to protect the airway, and severe illness causing high metabolic demand all contribute to this risk.
Maximally aggressive preoxygenation, in conjunction with other strategies to prevent desaturation, is required in all ED intubations
The goal of preoxygenation is to allow the longest apneic time possible before hypoxemia
Oxygen saturation begins to fall precipitously below 93%. It may take only a few seconds before the saturation drops below 70%, at which point the incidence of dysrhythmias, hypoxic brain injury, and death increase greatly.
While young healthy patients may remain well oxygenated for 5 minutes or more after preoxygenation, those who are acutely ill with poor lung physiology and high metabolic demands can become hypoxic in seconds regardless of preinduction oxygen saturation. The preoxygenation strategy employed should be tailored to the patient on the basis of their risk of hypoxemia.
NO DESAT means Nasal Oxygen During Efforts Securing A Tube
Strategies for oxygenating patients at both low and high risk for desaturation are discussed later
INDICATIONS
All patients requiring intubation in the ED
CONTRAINDICATIONS
None
SUPPLIES
Nasal cannula (NC)
Bag-valve mask (BVM)
Positive end-expiratory pressure (PEEP) valve
Nonrebreather (NRB) mask
Continuous positive airway pressure (CPAP) machine or ventilator
Bilevel positive airway pressure (BiPAP)/CPAP mask
General Basic Steps
Preoxygenate using NRB
Preoxygenate with head of bed elevated
Preoxygenate for at least 3 minutes or eight vital capacity breaths
Continue NC at 15 L/min for apneic oxygenation
For patients with shunt physiology, preoxygenate with positive pressure
For patients at risk for critical acidosis, continue ventilations during apneic period while waiting for full paralysis
TECHNIQUE
Place NRB mask with oxygen flow at maximal flow rate
Turn oxygen regulator to maximal flow
Turn knob counterclockwise till it will turn no further
The oxygen flow should be easily audible
This will deliver 30 to 60 L/min of oxygen and an FIO2 close to 90%
Standard NRB masks set to 15 L/min deliver only around 70% FIO2
Place an NC on all patients for preoxygenation and apneic oxygenation
Place the NC beneath the NRB mask or BiPAP/BVM at 4 to 6 L/min before induction
After induction, turn flow rate up to 15 L/min for apneic oxygenation
Before induction the NC will bring inspired FIO2 closer to 100%
During apneic period, oxygen will flow into alveoli, significantly increasing the time before the patient becomes hypoxic
During apnea, oxygen supplied by the NC flows from the nasopharynx to the alveoli because of a slightly subatmospheric pressure in the alveoli. The pressure gradient exists because oxygen moves out of the alveoli faster than CO2 moves into the alveoli.
Apneic oxygenation can be remembered with the mnemonic NO DESAT
Preoxygenate with the head of the bed elevated
Set the head of the bed to at least 20 degrees during preoxygenation
For immobilized patients, place the bed in reverse Trendelenburg to at least 20 degrees
Supine positioning leads to atelectasis, incomplete breaths, and less oxygenation, compared with upright positioning
Patients preoxygenated in head-elevated position achieve better preoxygenation and take longer to reach hypoxemia
Head-elevated positioning has the added benefit of better laryngeal exposure during laryngoscopy
Preoxygenate for at least 3 minutes
Patients should remain on NRB/NC or BiPAP/NC for a full 3 minutes
Preoxygenating for 3 minutes ensures full denitrogenation of the residual capacity of the lungs and maximal hemoglobin oxygen saturation
In cooperative patients eight vital capacity breaths can achieve similar levels of oxygen saturation and denitrogenation
PREOXYGENATION STEPS FOR PATIENTS AT HIGH RISK FOR DESATURATION
For patients not achieving 100% oxygen saturation after 3 minutes of NRB/NC or with suspected shunt physiology, preoxygenate with noninvasive ventilation or BVM with PEEP valve
Place the patient on BiPAP or CPAP with 100% FIO2 and PEEP of at least 5 for 3 minutes Or
Allow the patient to spontaneously breathe via BVM with a PEEP valve set to at least 5 for 3 minutes
Patients who do not reach 100% oxygen saturation with high FIO2 are likely to have shunt physiology, where alveoli have blood supply but are not receiving oxygen because of alveolar collapse, pulmonary edema, or pneumonia. Positive pressure will open these alveoli, allowing them to be oxygenated.
Continue ventilations after induction and administration of paralytics for patients at high risk for critical acidosis
Continue ventilations via ventilator with BiPAP mask or BVM with PEEP valve while awaiting full paralysis
Patients who fail to achieve >95% oxygen saturation despite positive-pressure preoxygenation will likely desaturate to critical hypoxemia during the 2 minutes of apneic time required for full muscle relaxation after paralytic administration
When using BVM, bag gently with breath delivered slowly over 2 seconds with low tidal volumes (6 cc/kg) and at a rate of 10 to 12 per minute
Gentle bagging should decrease risk of gastric insufflation and emesis. Inspiratory pressures <25 mm Hg are unlikely to overcome lower esophageal sphincter.
PULSE OXIMETRY LAG TIME
Be mindful that the oxygen saturation on the screen does not accurately reflect the patient’s current oxygen saturation
The pulse oximetry value on the screen lags behind the patient’s blood oxygen saturation from 30 seconds to 2 minutes
Sicker patients have longer lag times. Hypothermia, low cardiac output, and vasopressor use may all increase this lag time.
SAFETY/QUALITY TIPS
Procedural
Apneic oxygenation requires a patent nasopharyngeal passage. This can be achieved with head elevation: Ear-to-sternal notch, face parallel to ceiling positioning, jaw thrust, and nasal trumpet.
The NC for apneic oxygenation will require a third oxygen source
Before preoxygenation, discuss the concept of apneic oxygenation with your team and respiratory therapist, as the concept may be new to some and the initial impulse may be to remove the NC after induction
A pulse oxymetry probe placed on the ear or forehead may reduce lag time as the blood here is closer to the central circulation
Avoid the use of cricoid pressure. It has not been shown to prevent emesis and aspiration and may limit preoxygenation and manual ventilation by compressing the trachea. If you decide to use it, release it if there is any difficulty with glottis visualization or tube passage.
Cognitive
Patients who are hypoxemic before preoxygenation or who do not achieve a high SpO2 > 95% with NRB/NC likely have shunt physiology and will need positive pressure via CPAP/BiPAP or BVM with PEEP valve for adequate preoxygenation
In patients at high risk for desaturation, consider rocuronium over succinylcholine. Paralysis with rocuronium has been shown to lead to longer safe apneic times before hypoxemia, compared to succinylcholine. It is hypothesized that this is due to oxygen consumption from the defasciculation when using succinylcholine.
The inflection point of the O2–Hg dissociation curve, where it goes into the steep portion of the curve, is 93%. After this point the patient will desaturate rapidly.