Preoxygenation in Anesthesia for Beginners

Preoxygenation in anesthesia refers to the process of giving oxygen through a face mask just before the anesthesia is induced. This is done both for intravenous and mask induction. Preoxygenation is performed routinely to achieve denitrogenation in the lungs. This provides extra security against desaturation. 

Let’s jump to find out more about this.

By Dr. Rajeev Iyer, MBBS, MD, FASA; Board Certified Anesthesiologist

Associate Professor of Anesthesiology,

The University of Pennsylvania, Perelman School of Medicine, Philadelphia, USA

How is preoxygenation done?

Preoxygenation is done with a face mask attached to a breathing circuit. The breathing circuit is attached to the anesthesia workstation. The anesthesia workstation will receive oxygen supply from a central pipeline or from an oxygen cylinder. This mask is tightly held on the face to administer 100% oxygen. This amount of oxygen displaces the nitrogen present in the lungs.

Here is the sequence of oxygen flow 

1. Oxygen pipeline (this can also be an oxygen cylinder) – High pressure system
2. Anesthesia workstation – Intermediate pressure system
3. Oxygen flowmeter
4. Breathing circuit – Low pressure system
5. Anesthesia face mask
6. Lungs

Why is preoxygenation important before intubation?

The lungs are normally filled with room air. Room air has mostly nitrogen. Here are the constituents of room air.

Preoxygenation is important before intubation because it displaces most of the nitrogen in the lungs and is replaced with oxygen. This extra oxygen provides extra oxygen flow in the body. During the placement of a breathing tube, there is an interruption of oxygen flow into the lungs and body cells. Hence, this extra oxygen buffer with preoxygenation prolongs the time that is required to desaturate. This reduces the stress placed on the body with low oxygen and makes intubation much safer.

Why do we preoxygenate for 3 minutes?

Preoxygenation for 3 minutes increases the total body oxygen stores 4 times compared to a person breathing room air.

In general, there are different ways to achieve preoxygenation

• Tidal volume breathing of 100% oxygen for 3 minutes to 5 minutes
• Four deep vital capacity breaths of oxygen taken within 30 seconds
• Eight deep vital capacity breaths of oxygen are taken within 1 minute.
• Transnasal rapid insufflation ventilator exchagne (THRIVE). This is used in difficult airway situations. 

The commonly used technique is a 3-minute tidal volume breathing which resulted in average arterial oxygen saturation of 369 mm Hg according to this study. All these three techniques have been shown to achieve similar peak arterial oxygen saturation. Maximum preoxygenation is achieved when all the tissue compartments including alveoli and blood are filled with oxygen.

In my experience, the technique used depends on the time available to induce anesthesia and the patients underlying health conditions. Extremes of age like neonates and the elderly may require modifications of this.

How long does it take for oxygen levels to drop?

It takes 283 seconds for the oxygen level to drop from 100% to 95% after preoxygenation in the supine position and 386 seconds with preoxygenation with a 20-degree head up. This is according to a study from NHS, UK. Hence it is recommended to do preoxygenation in the head-up position. In patients with a spine injury, the reverse Trendelenburg position should be used.

Preoxygenation with 100% oxygen for 5 minutes increased the duration of apnea until oxygen saturation reached 90% to 10 minutes. This was increased to 12 minutes when preoxygenation was done with PEEP according to this study from Switzerland.

Preoxygenation protects against desaturation during apnea and hypoventilation. It increases the duration until a patient has a fall in oxygen saturation thus giving time to place an endotracheal tube.

What is adequate preoxygenation?

The adequacy of preoxygenation can be measured by a few indices

• The rise in alveolar oxygen fraction
• Reduction in alveolar nitrogen fraction
• Increase in arterial oxygen stress

Practically, none of these are routinely available in the operating room. Hence, the adequacy of preoxygenation is ensured clinically by following the steps of preoxygenation outlined below.

What are the steps of preoxygenation?

• Explain to the patient what is being done
• Turn on 100% oxygen
• The flow rate will depend on the patient

My rule of thumb is flow = minute ventilation which is 100 to 150 ml/kg/min. If you have a 50 kg patient, the flow rate will be 5 to 7.5 Liters/minute

• Place head up by 20 to 45 degrees (assuming no contraindication)
• Place the face mask on the face. Be cautious of pressure on the nose and eyes
• Hold a tight seal. This is important to prevent dilution with room air which will reduce the efficacy of preoxygenation
• Ensure the breathing bag is moving fully during inhalation and exhalation
• Instruct the patient on tidal breathing vs. deep breathing
• Note the time

Why do you preoxygenate before suctioning? 

Suctioning sucks out oxygen from the lungs and can even precipitate the collapse of the alveoli. This predisposes to immediate desaturation and also to bradycardia. Preoxygenation with 100% oxygen prior to suctioning increases the duration of apnea before which desaturation occurs.

Here are some tips for suctioning –

• Preoxygenate with 100% oxygen for a minute or two
• Lubricate suction catheter with jelly for suctioning endotracheal tube
• Insert suction catheter gently
• Do not connect to suction while inserting the catheter
• Limit suction to less than 10 seconds
• Connect to suction
• Gently pull out in a rolling motion
• Monitor vitals while suctioning
• If on a ventilator, ensure settings are back to before (transient 100% oxygen may be required)
• If suctioning the nose, be gentle as the nose can bleed easily

What is the purpose of denitrogenation? 

Oxygen is weakly soluble in blood. This causes a low reservoir of oxygen in the blood compared to the lungs. Denitrogenation with 100% oxygen washes out the nitrogen from the lungs and the blood. This maximizes the storage of oxygen in the lungs and bloodstream. The denitrogenation with preoxygenation should ideally replace the lung’s functional residual capacity with oxygen and reach end-tidal oxygen above 90%.

Practically, the patient should be advised to take maximum inspiration and expiration during preoxygenation. When tidal breathing preoxygenation is done, the breathing should be maximized at the end of the 3 or 5-minute mark.

Where is preoxygenation a must?

Preoxygenation should be done on all patients regardless of age and underlying complexity. In my experience, I have stayed away from preoxygenation when the patient is extremely phobic of the face mask and refuses to have a mask. Even in these situations, I have placed the face mask as soon as the patient is induced.

When an anesthesia mask is placed, the concentration of oxygen delivered is between 50-100%. However, there are three situations where providing 100% oxygen for preoxygenation to achieve maximum denitrogenation is important. These include:

• Difficult airway
• Increased oxygen demand
  • Fever
  • Sepsis
  • Pregnancy
• Reduced oxygen reserve or functional residual capacity (FRC)
  •  Overweight
  • Pregnancy
  • Pulmonary pathology
  • Full stomach
  • Ascites

How does obesity affect the respiratory system?

Obesity generally defined as a body mass index (BMI) ≥ 30 kg/ m2, is associated with changes in both the upper and the lower airways. Obesity causes changes in the lungs which predispose them to desaturation much sooner than non-obese patients. This has been shown in this study from the Cleveland Clinic in otherwise healthy obese patients having a routine procedure like gastrointestinal endoscopy.

The following changes can be seen in pulmonary mechanics

• Low upper airway muscle tone
• Obstructive sleep apnea
• Low oxygen reserve
• Reduced lung compliance
• Decreased lung volumes
• Alveolar collapse
• Intrapulmonary shunting
• Ventilation-perfusion mismatch

Can preoxygenation fail?

Approximately 11.5% of patients who are edentulous have a beard, facial anomalies, burns on the face or neck, and who have nasogastric tube are difficult to preoxygenate.

Preoxygenation can also be difficult or not possible to do in the following circumstances:

• Patient refusal
• Claustrophobia
• High anxiety
• Not enough time for preoxygenation (stat emergency)
• Low oxygen flows
• Leaks in the breathing system
• Lack of a tight mask seal
• Insufficient time
• Pacifier