Dr. Kazzi presents some clinical pearls about preoxygenation for today’s Morning Report (originally presented 4/10/2012):
Preoxygenation
Physiology of Preoxygenation:
- Preoxygenation extends the duration of safe apnea (defined as the time until a patient reaches a saturation level of 88-90%)
- Below 88% patients are on steep decline of O2 saturation curves and can quickly desaturate further.
- 450 mL of O2 are present in the lungs at room air. When preoxygenated with 100% FiO2 the amount is ~ 3,000 mL.
- Oxygen reservoir is about 1 – 1.5 L in room air patients vs. 3.5 – 4 L in optimally preoxygenated patients.
- When breathing room air, desaturation occurs in 45 – 60 seconds. Optimally preoxygenated patients have ~ 8 minutes of safe apnea.
- Preoxygenation with high FiO2 will denitrogenate the residual capacity of lungs and blood stream to increase the oxygen reservoir.
Oxygenation Tools:
a) Oxygen Reservoir Mask: Erroneously called Non-rebreather, which should have one way valves. Deliver about 60 – 70% FiO2 when at 15 L / min
b) Non-Rebreather: at 15 L/min O2 can deliver 90% FiO2
c) O2 Flow Regulators: often have calibrated limit of 15 L / min but can frequently give higher uncalibrated amounts
d) Bag – Valve Mask: unless patient is spontaneously breathing or PPV by provider is given, these will not provide high enough FiO2
Time Period:
On high FiO2 of > 90%
3 minutes of tidal volume breathing
8 vital capacity breaths in cooperative patients
Positive Pressure Ventilation:
If patient has not achieved saturation > 93 – 95% before tracheal intubation, there is a higher likelihood of desaturation during their apneic and tracheal intubation phases. If > 95% saturation not achieved after 3 minutes of high FiO2 –> exhibiting shunt physiology (alveoli are perfused but no ventilated because of pulmonary edema, pneumonia etc.) and PPV likely needed.
PPV can be achieved with PEEP valves attached to BVM masks or by CPAP masks hooked up to regular ventilators or NIPPV machines
Positioning:
- Posterior lung prone to atelectatic collapse in supine position.
- Head elevated position of 20 degrees or higher during preoxygenation increases time to desaturation
- Higher head elevations should be considered in obese patients
- Reverse trendelenberg (head of stretcher 30 degrees higher than foot) is useful in immobilized patients
- External Auditory meatus at level of sternal notch. Often requires head elevation and padding under the shoulders.
Time to Desaturation:
Be wary of pulse oximeter lag time which can be 30 – 60 seconds as peripheral circulation lags central circulation.
1 minute in healthy room air patient and 8 minutes in optimally preoxygenated healthy patient. One study on 100% FiO2:
Healthy – 8 minutes Moderately Ill – 5 minutes Obese – 2.7 minutes
Shunting, anemia, poor cardiac output, high metabolic demands, volume depletion will all decrease safe apnea time.
Apneic Oxygenation:
- Alveoli continue to take up O2 despite lack of lung movement. Exists a difference in O2 and CO2 movement across alveolar membrane due to gas solubility differences as well as O2 affinity for Hemoglobin. Given this uptake of O2 there is a pressure differential generated and gas will flow from pharynx to alveoli.
- Extends the duration of safe apnea.
- Does not affect carbon dioxide levels.
- Flow rate of 15 LPM with Nasal cannula will allow 100% FiO2. An open mouth did not affect this. Nasal trumpet can be used if any concern for nasal tissue redundancy.
Manual Ventilation Uses:
- Do not initiate laryngoscopy prior to full muscle relaxation to prevent triggering gag reflex.
- Ventilation during the apneic period is only of benefit in severely acidotic patients whom respiratory compensation is maintaining pH (salicylate toxicity)
- Oxygenation in hypoxemic patients is the other group needing manual ventilations during apneic period. PEEP valve should be used if available.
- Ventilation – deliver over 1 -2 seconds, low volume (8 cc), low rate (6-8 breaths/min)
- Caution in low output, volumes depletion, obstructive airway disease (intrinsic PEEP)
- Low risk of desaturation (> 95%) –> No ventilation needed in apneic period
- High risk ( 91 – 95% ) –> Risk / Benefit analysis
- Hypoxemic ( < 90%) –> Ventilations required
Paralytics:
- Rocuronium likely provides increased safe apnea time vs. succinylcholine
Clinical Use:
Risk Stratification into 3 Patient Profiles:
Low Risk ( 96 – 100% O2 Sat on high flow O2 )
High Risk ( 91 – 95% O2 Sat on high flow O2 )
Hypoxemic ( 90% or less on high flow O2 )
Time Periods of Intubations: Preoxygenation (3 min) Muscle Relaxation (~60s) Apneic Period (~ 30 sec)
(1) Preoxygenation (need 2 O2 regulators ideally. Consider O2 tank if not available)
Position of patient: Semi-recumbent position (~20 degrees) or reverse Trendelenberg. Ear to sternal notch using head elevation or padding.
Nasal Cannula in Nares: not yet hooked up to oxygen
Non-Rebreather Mask at highest rate possible (at least 15 Lpm) for about 3 minutes
If not saturating > 90% remove face mask and switch to CPAP by using the ventilator, NIPPV device or BVM with PEEP valve. Titrate between 5 – 15 PEEP to achieve O2 sat > 98%. Consider these steps in 91 – 95%
Attach BVM to O2 regulator and set to max flow. If patient needed CPAP then attach a PEEP valve and set to same level
(2) Apneic Period
Administer sedative and paralytic.
Detach face mask from O2 regulator and then attach Nasal Cannula on 15 Lpm
Perform jaw thrust to maintain pharyngeal patency
If hypoxemic then consider performing 4 -6 breaths with BVM
Nasal cannula remains on during entire airway management
Source:
Preoxygenation and prevention of desaturation during emergency airway management. Weingart , Levitan Ann Emerg Med March 2012
Thanks Kazzi! We’d love to hear your thoughts on preoxygenation, so leave any comments below.
Jay Khadpe MD
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