Preoxygenation in Anesthesia - Objective And Techniques

Introduction:

It is essential to maintain oxygenation during intubation because a lack of control over oxygen intake can result in life-threatening complications. Anesthetic induction usually results in apnea, which occurs when tissue oxygenation is preserved by consuming the oxygen reserve and continuously administering oxygen. In most cases, stringent preoxygenation and face mask ventilation is used unless the relaxation of the muscle is adequate to allow intubation in perfect conditions. Sometimes, the anesthetist cannot maintain oxygenation due to pre-existing pulmonary disease. Although the challenges involved with ventilation and intubation vary, all patients should require preoxygenation. The main goal of preoxygenation of a patient to the most significant degree possible before the induction of general anesthesia and paralysis is to provide the maximum time during which a patient can withstand apnea and assist the anesthesia provider in dealing with a non-ventilate, non-intubate scenario.

When all the alveolar, arterial, tissue, and venous compartments are filled with oxygen, preoxygenation is at its peak. As a result, desaturation happens much faster during apnea than in healthy patients for patients with decreased oxygen absorption, increased oxygen removal, or both. Preoxygenation serves as a supportive protective cover during apnea and hypoventilation. The safe apnea time can be expanded, which refers to the period till a patient reaches a saturation limit of eighty-eight percent to ninety percent, allowing the accurate airway. If patients are desaturated below this point, their condition is on the oxyhemoglobin dissociation curve and can immediately drop to a point called the critical oxygen saturation level.

What Are the Objectives of Preoxygenation?

There are three primary goals to achieve in the emergency department:

1. The saturation of the patient should be brought as close to 100 percent as feasible.

2. Denitrogenation is the residual lung capacity or enhancing oxygen storage in the lungs.

3. The bloodstream must be oxygenated and remove nitrogen reserves to a maximum extent.

• Because oxygen is poorly soluble in blood and has comparatively low stores of oxygen relative to the lungs, denitrogenation and oxygenation of the bloodstream result in secured apnea for a limited period.

• Patients should ideally seem pre-oxygenated until they denitrogenate their lungs' residual capacity to achieve an end-tidal oxygen level of more than ninety percent.

• Three minutes of tidal-volume breathing with an elevated source of oxygen (FiO2) is a sufficient preoxygenation time for most patients.

• This tidal volume respiration technique can be enhanced by instructing the patient to exhale maximally and then inhale maximally before the three-minute mark.

• Once mask ventilation becomes challenging to manage, preoxygenation before anesthesia is essential.

• Such cases occur in patients who are highly susceptible to fast desaturation, such as obese, pregnant, febrile, or with pulmonary diseases, patients with a whole abdomen, when mask ventilation problems are anticipated, when tracheal intubation could prolong than expected, and when special intubation techniques for managing airways, such as the placement of a double lumen tube, are required.

What Are the Reasons for Pre-oxygenation Insufficiency?

1. Patient-Related Factors: If the patient is unwilling to cooperate, is fearful of the surrounding place or anxious, or is in a situation where anesthesia must be given quickly, preoxygenation may be difficult or impossible.

2. Technical Factors: Although the theoretical fraction of expired oxygen (FeO2) is ninety-five percent, ninety percent of FeO2 is commonly accepted in practice. The main reasons for achieving the above values are a lower oxygen flow, leaks, or an incomplete preoxygenation period. However, leak detection during pre-oxygenation remains a significant challenge, and roughly 11.5 percent of patients with missing teeth or beards or with anomalies in the face, burns, or the existence of a nasogastric tube, face this difficulty.

3. Positioning of Patient to Receive Preoxygenation: The patient lying flat horizontally with the face up is not considered ideal for optimal results. When placed flat, it becomes difficult to begin taking full breaths, and a more significant portion of the rear lung becomes prone to atelectatic collapse. In addition, it causes a decrease in the oxygen reservoir inside the lungs, lowering the safe apnea time. Therefore, in the head-up position, the patients should be pre-oxygenated, and reverse Trendelenburg may be indicated in the case of n immobilized patients with a spinal injury.

What Are the Preoxygenation Techniques?

• Pre-oxygenation is strongly recommended during anesthesia induction when there is a potential risk of desaturation before administering endotracheal intubation to secure the airways. Desaturation is becoming more likely in its absence.

• The device, particularly the face mask, must be customized and firmly designed for the patient. Any anatomical differences between the mask and the patient's face, such as improper mask size, beards, or mustaches, prevent perfect sealing and may fail. In addition, it must securely fit the mask to the patient's face.

• The most common methods of preoxygenation used for routine procedures are:

  • Deep Breathing: When the mask is not securely fitted, a twenty percent dilution of oxygen by surrounding air occurs, and a forty percent dilution of oxygen occurs when it is moved closer to the face. The fresh gas flow circle system with a flow rate of five liters per minute is used as a standard for comparison in anesthesia experiments that investigate the effects of various circuits. Oxygen must be pumped into the circuit and reservoir until preoxygenation.

  • Rapid Breathing at FIO2: In patients with a history of normal pulmonary function, this technique yields denitrogenation with an alveolar oxygen fraction (FAO2) of approximately ninety-five percent. From the first minute of preoxygenation, denitrogenation seems beneficial; moreover, circuit leakage negotiates the effects, as mentioned earlier, by causing a rapid decrease in FiO2. Pure oxygen breathing for an increased period of more than a minute appears to have a minimal advantage over oxygen saturation (SpO2) or alveolar denitrogenation. Still, it dramatically enhances the apnea period before arterial desaturation.

  • Four Vital Capacities Method: In instances where the patient's cooperation is compromised, the four vital capacities method is used. Followed by four power maneuvers, the period of apnea without desaturation is narrowed, particularly in comparison to spontaneous breathing. This technique's limitations are responsible for practical requirements such as the capacity of the bag, inspiratory flow rate, and inspiration of the room gas. To maximize FeO2 elevation, the vital capacity maneuver procedure should preferably proceed with forced expiration. To be effective, the inspiratory oxygen flow should be more than the maximum inspiratory flow obtained by activating the by-pass oxygen mechanism while inspiration; four to five forced oxygen breathing sessions are as successful as conventional pre-oxygenation on FeO2.

  • Transnasal Humidified Rapid Insufflation Ventilatory Exchange (Thrive): When a difficult airway in a patient is anticipated. To provide apnoeic oxygenation, oxygen is retained at a flow rate of seventy liters per minute during the process. It is preserved until induction and after the neuromuscular blockade is given.

Conclusion:

It is crucial to think about oxygenation problems because oxygen reserves are low, and the challenges of intubating the patient and ensuring appropriate ventilation are closely correlated. When oxygen reserves are depleted, the situation becomes highly crucial. Efficient technique and FeO2 monitoring can enhance pre-oxygenation performance and thus increase the margin of safety. Maximum efforts to improve outcomes and a comprehensive analysis of every patient's associated potential risk factors should be carried out. As a result, anesthesiologists must have a solid understanding of the technique and the ability to handle high-risk patients.