No Desat: Maximally Aggressive Preoxygenation

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.