From the Guidelines
The primary sensors for oxygen concentration in the body are the peripheral chemoreceptors located in the carotid bodies at the carotid artery bifurcation and the aortic bodies near the aortic arch, along with central chemoreceptors in the medulla oblongata of the brainstem, as stated in the BTS guideline for oxygen use in adults in healthcare and emergency settings 1. These sensors play a crucial role in detecting changes in arterial oxygen levels and triggering responses to maintain adequate oxygenation. The peripheral chemoreceptors primarily detect decreases in arterial oxygen levels (hypoxemia), while central chemoreceptors mainly respond to changes in carbon dioxide and pH.
Oxygenation Maintenance
Oxygenation in patients is maintained through several approaches depending on the severity of hypoxemia.
- For mild hypoxemia, supplemental oxygen via nasal cannula (1-6 L/min) or simple face mask (5-10 L/min) is typically used.
- For moderate hypoxemia, high-flow nasal cannula (up to 60 L/min with FiO2 up to 100%) or non-rebreather masks (10-15 L/min) may be employed.
- Severe hypoxemia often requires non-invasive positive pressure ventilation (NIPPV) such as CPAP (5-10 cmH2O) or BiPAP (inspiratory pressure 8-20 cmH2O, expiratory pressure 3-10 cmH2O).
- In critical cases, endotracheal intubation and mechanical ventilation become necessary, with settings tailored to the patient's condition.
Goal of Oxygen Therapy
The goal of oxygen therapy is to maintain arterial oxygen saturation (SpO2) between 94-98% for most patients, or 88-92% for those with chronic CO2 retention, as recommended by the BTS guideline 1. This approach works because increasing the fraction of inspired oxygen enhances the oxygen concentration gradient in the alveoli, facilitating diffusion into the bloodstream and improving tissue oxygenation. Monitoring and maintenance of target saturation are crucial, and oxygen delivery devices and flow rates should be adjusted to keep the oxygen saturation in the target range, as emphasized in the guideline 1.
Clinical Assessment and Weaning
Clinical assessment is recommended if the saturation falls by ≥3% or below the target range for the patient, and oxygen therapy should be stopped once a patient is clinically stable on low-concentration oxygen and the oxygen saturation is within the desired range on two consecutive observations, as stated in the guideline 1. Pulse oximetry must be available in all locations where emergency oxygen is used, and oxygen saturation on air should be monitored for 5 min after stopping oxygen therapy, with further monitoring at 1 hour if the saturation remains in the desired range, as recommended in the guideline 1.
Oxygen Delivery Devices
Nasal cannulae are the most common interface for oxygen delivery, but high-flow nasal cannulae, non-rebreather masks, and Venturi masks may also be used depending on the patient's condition and the severity of hypoxemia, as discussed in the guideline for home oxygen use in adults 1. The choice of oxygen delivery device and flow rate should be tailored to the individual patient's needs, with the goal of maintaining adequate oxygenation while minimizing the risk of hypercapnic respiratory failure.
From the Research
Primary Sensors for Oxygen Concentration
- The carotid body is the primary sensor for monitoring arterial blood oxygen levels and triggering reflexes to maintain homeostasis during hypoxemia 2, 3, 4.
- The carotid body senses oxygen in acute hypoxia and produces appropriate responses, such as increases in breathing, to replenish oxygen from the air 3.
Mechanism of Oxygen Sensing
- The mechanism of oxygen sensing in the carotid body involves the release of calcium ions, which ultimately leads to the activation of afferent nerves that project to the brain stem 3, 4.
- There are two main hypotheses for the response to hypoxia: the mitochondrial hypothesis and the membrane model, with evidence supporting both 3, 4.
Maintenance of Oxygenation in Patients
- In patients with severe hypoxemia, techniques such as lung recruitment maneuvers, decremental PEEP trials, prone positioning, and aerosolized pulmonary vasodilators can be used to improve oxygenation 5, 6.
- Extracorporeal membrane oxygenation can be used as a rescue therapy in cases where other treatments fail 5, 6.
- A physiopathological approach that accounts for the underlying mechanisms of hypoxemia and the physiological and clinical effects of different treatments is likely the best guide for treating severely hypoxemic patients 6.