Index
Oxygen We all need it! Sometimes we even need more than usual to maintain our oxygen saturation.
In these situations, supplemental oxygen can be delivered through various oxygen delivery devices, from nose clips to invasive ventilation.
The fraction of inspired oxygen (FiO2) is the fraction of oxygen in the volume being measured.
Medical patients experiencing shortness of breath are provided oxygen-enriched air, which means a higher than an atmospheric fraction of inspired oxygen.
When supplying supplemental oxygen to your patient, two critical things to consider are the oxygen flow rate and the fraction of oxygen inspired.
The oxygen flow rate is the number that we dial on the oxygen flow meter, usually between 1-15L / min.
Natural air includes 21% oxygen, equivalent to an inspired oxygen fraction of 0.21.
Oxygen-enriched air has a higher fraction of inspired oxygen than 0.21; Up to 1.00, which means 100% oxygen.
To avoid oxygen toxicity, the fraction of inspired oxygen is generally kept below 0.5 even with mechanical ventilation.
Often used in medicine, the fraction of inspired oxygen represents the percentage of oxygen that participates in gas exchange.
If the barometric pressure changes, the fraction of inspired oxygen can remain constant, while the partial pressure of oxygen changes with the change in barometric pressure.
Oxygen monitors
Sensors that monitor the fraction of inspired oxygen are built into every modern anesthesia machine.
This is in addition to failsafe devices that detect a drop in pressure in the oxygen line.
Oxygen monitors with alarms are required to prevent the delivery of a hypoxic gas mixture. Still, they can also be configured to sound a warning for a high fraction of inspired oxygen.
A low fraction of inspired oxygen can be an unintended consequence of very low-flow anesthesia with air-oxygen mixtures in a closed-loop system.
However, there are several circumstances in which a low fraction of inspired oxygen is intentional and desirable:
- A reduced fraction of inspired oxygen may be necessary for some children with congenital heart disease to reduce oxygen saturation to balance pulmonary and systemic blood flows.
- During airway surgery to reduce the risk of airway fires.
- In infants and neonates to reduce the risk of retinopathy of prematurity.
It is not the fraction of inspired oxygen but the partial arterial pressure of oxygen (Pao2) of the child that is important concerning retinopathy of prematurity.
It is common practice to reduce the fraction of inspired oxygen if it is not safe for the neonate to the point where the oxygen saturation measured by pulse oximetry (Spo2) is between 91% and 95% to minimize the risk. Of oxygen toxicity without increasing perioperative mortality.
Medicine
In medicine, the fraction of inspired oxygen is the assumed percentage of the oxygen concentration that participates in gas exchange in the alveoli.
Applications
The inspired oxygen fraction is used in the APACHE II (Acute Physiology and Chronic Health Assessment II) disease severity classification system for intensive care unit patients.
For inspired oxygen fraction values equal to or greater than 0.5, the alveolar-arterial gradient value should be used to calculate the acute physiology and chronic health assessment II score.
Otherwise, the partial pressure of oxygen in the arterial blood (PaO2) will be sufficient.
The relationship between the partial pressure of oxygen in arterial blood and the fraction of inspired oxygen is used as an indicator of hypoxemia, according to the American-European Consensus Conference on Lung Injury.
A high fraction of inspired oxygen has altered the PaO2 / FiO2 ratio.
PaO2 / FiO2 ratio
The ratio of arterial partial pressure oxygen to the fraction of inspired oxygen, sometimes called the Carrico index, compares the level of oxygen in the blood and the concentration of oxygen that is breathed.
This helps determine the degree of any problems with how the lungs transfer oxygen to the blood.
An arterial blood sample is taken for this test.
A PaO2 / FiO2 ratio of less than or equal to 200 is required to diagnose acute respiratory distress syndrome according to the American-European Consensus Conference (AECC) criteria.
The most recent Berlin criterion defines mild acute respiratory distress syndrome (ARDS) in a proportion of <300.
A PaO2 / FiO2 index less than or equal to 250 is one of the minor criteria for severe community-acquired pneumonia (i.e., possible indication for hospital treatment).
A PaO2 / FiO2 ratio less than or equal to 333 is one of the variables in systolic blood pressure, multilobar infiltrates, albumin, respiratory rate, tachycardia, confusion, oxygen, and pH (SMART-COP) in the risk score for respiratory assistance or intensive vasopressor.
Example calculation
After drawing an arterial blood gas (ABG) from a patient, the partial pressure of oxygen in the arterial blood is 100 mmHg.
Since the patient is receiving O2 saturated air, resulting in an inspired oxygen fraction of 50% oxygen, his calculated PaO2 / FiO2 ratio would be 100 mmHg / 0.5 = 200.
Possible Limitations of the Automated Inspired Oxygen Control Fraction
The automated fraction of inspired oxygen control could lead to a reduction in caregiver attention to the patient.
For example, an automatic increase in the fraction of inspired oxygen could mask a condition (e.g., hypoventilation) that could otherwise result in hypoxemia and draw the caregiver’s attention.
To avoid this situation, the built-in warnings should alert the caregiver not only when peripheral capillary oxygen saturation (SpO2) is decreasing but also when the fraction of inspired oxygen is constantly increasing.
On the other hand, the faster automatic response could be beneficial in preventing the detrimental effects of worsened hypoxemia until appropriate corrective action is taken.
Pulse oximetry is the most common method of oxygenation monitoring in the NICU. It is the preferred choice for an automated fraction of inspired oxygen monitoring because it is non-invasive and continuously available in premature infants.
Although pulse oximetry has improved considerably over the years, its reliability in premature babies can be affected by movement, poor perfusion, and improper probe placement.
The poor reliability of peripheral capillary oxygen saturation during the automated fraction of inspired oxygen monitoring can lead to hyperoxemia if hypoxemia is erroneously detected.
This problem is not unique to the automated fraction of inspired oxygen control; It can also occur during routine care.
When certain conditions affect the reliability of peripheral capillary oxygen saturation, the clinician may choose not to consider peripheral capillary oxygen saturation readings in managing the patient.
More stringent criteria should be applied when considering the automated fraction targeting a peripheral oxygen saturation of the inspired oxygen control.
One of the most important considerations before automatic fraction-inspired oxygen control is used clinically in premature infants is that the physician is responsible for setting the target range.
This problem is particularly relevant because the optimal oxygenation range for preterm infants has not been defined.
Specific target ranges of peripheral capillary oxygen saturation may have significant physiological effects that could manifest when such fields are more effectively maintained.
Therefore, caution is advised in selecting the peripheral capillary oxygen saturation range to be targeted for an automatic fraction of the inspired oxygen controller.
Increased fraction of inspired oxygen
Increasing the fraction of inspired oxygen when there is an acute deterioration in oxygenation is undoubtedly an accepted standard practice.
The selection of a fraction of inspired oxygen high enough to achieve a minimum of appropriate partial pressure of oxygen in arterial blood (Pao2) seems trivial.
However, there are no clear guidelines on a minimum partial pressure of oxygen in arterial blood.
Typically, a threshold of 60 mm Hg is cited for partial pressure of oxygen in arterial blood (or 90% for peripheral capillary oxygen saturation). Still, much lower levels are consistent with survival depending on the specific circumstances.
For example, it was estimated that at the top of Mt.
On Everest, the partial pressure of oxygen in the arterial blood can be as low as 30 mm Hg, while the estimated Paco2 is 11 mm Hg.
Conversely, as briefly discussed above, hypoxemia during anesthesia can adversely affect outcomes.
Aiming for a supranormal partial pressure of oxygen in the arterial blood may even be beneficial.
Consequently, an increase in the fraction of inspired oxygen during general anesthesia with mechanical ventilation from the average level of 0.21 to 0.3 to 0.5 is considered standard practice by many clinicians.
An increase in the fraction of inspired oxygen more significant than the levels mentioned above may be advisable in certain situations.
In an animal model, the investigators demonstrated that in acute normovolemic hemodilution to a ‘critical’ hemoglobin concentration, an inspired oxygen fraction of 1.0 may be beneficial in terms of short-term survival.
Similar findings also occurred in severe hemorrhagic shock.
In a very different clinical setting (i.e., severe brain damage in humans), at least in short-term studies, an increase in the fraction of inspired oxygen back to 1.0 may have beneficial effects (decrease in tissue lactate).
Using a higher than average fraction of inspired oxygen values is standard practice.