Head&Neck Surgery Anesthesia

Apneic Oxygenation: An Overview

The use of apneic oxygenation has grown significantly in recent decades. Early studies focused on the physiological principles of gas exchange during apnea and investigated various methods to maintain oxygen saturation in the absence of ventilation. As research progressed, newer techniques were developed and tested, leading to the widespread adoption of apneic oxygenation in clinical practice.

Early Research

Early research on apneic oxygenation explored the basic mechanisms of gas exchange during apnea.

The conclusion of Frumin’s study on apneic oxygenation emphasized that it is feasible to sustain life in anesthetized patients without spontaneous or mechanical ventilation by continuously insufflating oxygen into the trachea. This approach could maintain adequate arterial oxygenation for up to 55 minutes of apnea in human subjects. The study also highlighted the importance of the proper method and careful control during the procedure to avoid complications such as hypercapnia. This finding was significant as it offered an alternative method for managing patients’ airways and oxygenation in specific clinical scenarios where conventional ventilation might be challenging or contraindicated . A key focus was understanding the rate of carbon dioxide (CO2) increase, a major limiting factor for apneic techniques. Seminal work by Eger and Severinghaus in 1961 established the foundational knowledge regarding PaCO2 rise in apneic anesthetized patients. They documented an average PaCO2 increase of 13.4 mm Hg in the first minute of apnea, followed by a consistent rise of 3.0 mm Hg/min thereafter. Subsequent studies further explored factors influencing CO2 dynamics during apnea, such as the impact of preceding hypocapnia.

Apneic Oxygenation Techniques

Early techniques to maintain oxygenation during apnea focused on delivering oxygen through various routes. These included nasopharyngeal oxygen insufflation, buccal oxygen administration, and nasal prongs attached to oxygen sources. These methods aimed to create an oxygen reservoir in the upper airway, allowing for passive diffusion of oxygen into the alveoli while the patient remained apneic. Research investigated the effectiveness of these techniques in preventing desaturation during intubation and other procedures.

Factors Affecting Effectiveness
1. Lung and Airway Pathology: Patients with lung diseases such as chronic obstructive pulmonary disease (COPD) or asthma may have impaired gas exchange, which can reduce the effectiveness of apneic oxygenation.
2. Obesity: Higher body mass index (BMI) can negatively affect lung mechanics and respiratory physiology, limiting the efficacy of apneic oxygenation due to increased oxygen consumption and decreased functional residual capacity.
3. Pulmonary Shunt: Conditions that increase pulmonary shunting, where blood bypasses oxygenated lung areas, such as pneumonia or atelectasis, can diminish the effectiveness of this technique.

High-Flow Nasal Cannula: A Paradigm Shift

The development of high-flow nasal cannula (HFNC) marked a significant advancement in apneic oxygenation. HFNC delivers heated and humidified oxygen at high flow rates, resulting in several physiological benefits:

Creation of a pharyngeal oxygen reservoir.
Flushing of anatomical dead space.
Potential generation of positive airway pressure.

THRIVE: Initial Promise and Subsequent Evaluation

Patel and Nouraei introduced the concept of Transnasal Humidified Rapid-Insufflation Ventilatory Exchange (THRIVE) in 2015. THRIVE involves the continuous delivery of high-flow, humidified oxygen via nasal cannula from preoxygenation to intubation, aiming to prolong safe apnea time in patients with difficult airways. Initial reports suggested that THRIVE might enhance CO2 clearance beyond the expected physiological rate observed in traditional apneic oxygenation.

Refining the Understanding of HFNC and CO2 Clearance

Subsequent research scrutinized the purported ventilatory effect of THRIVE. While some studies reported slightly lower CO2 accumulation rates with HFNC compared to historical controls, several randomized controlled trials refuted the notion of active CO2 elimination during apneic HFNC. Notably, a study by Riva et al. in 2022 directly compared CO2 changes during apneic oxygenation with different HFNC flow rates and found no significant difference between high, medium, low, and minimal flow groups, effectively disproving the ventilatory effect.

Current Applications and Considerations

Today, apneic oxygenation, primarily using HFNC, is a widely accepted technique in various clinical scenarios:

Airway surgery: Allows surgeons to work in an unobstructed field during short procedures.
Endoscopy: Facilitates airway examination and interventions.
Difficult airway management: Provides crucial time to secure the airway in challenging situations.
Rapid sequence intubation: Reduces the risk of hypoxemia during the apneic phase.

Despite its widespread use, it's crucial to recognize the limitations of apneic oxygenation:

CO2 Accumulation: Remains the primary drawback. Prolonged apnea leads to hypercapnia and potential respiratory acidosis, which can be detrimental in certain patient populations (e.g., those with increased intracranial pressure, severe metabolic acidosis, cardiovascular disease).
Patient Selection: Not appropriate for all patients. Factors such as obesity, upper airway obstruction, and pre-existing cardiopulmonary disease can significantly shorten safe apnea time and increase the risk of complications.
Monitoring: Continuous pulse oximetry monitoring is essential and the role transcutaneous CO2 is still debated.
Rescue Plan: A well-defined rescue plan is crucial. Be prepared for interventions like bag-valve-mask ventilation, jet ventilation, or endotracheal intubation.

Future Directions

Ongoing research continues to refine our understanding of apneic oxygenation and optimize its application. Key areas of focus include:

Improving CO2 Monitoring: Developing more reliable and accurate methods for monitoring CO2 levels during apnea, especially non-invasive techniques.
Optimizing HFNC Parameters: Investigating optimal flow rates, FiO2 settings, and humidification levels for specific patient populations and clinical scenarios.
Patient-Specific Strategies: Identifying patient characteristics that may predict safe apnea time and personalize apneic oxygenation protocols accordingly.
Exploring Adjunctive Techniques: Evaluating the potential benefits of combining apneic oxygenation with other airway management strategies, such as non-invasive ventilation.