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Cite this article as: Kojima Y., Sendo R., Okayama N. et al. (May 18, 2022) Inhaled oxygen ratio in low and high flow devices: a simulation study. Cure 14(5): e25122. doi:10.7759/cureus.25122
Purpose: The fraction of inhaled oxygen should be measured when oxygen is given to the patient, since it represents the alveolar oxygen concentration, which is important from the point of view of respiratory physiology. Therefore, the aim of this study was to compare the proportion of inhaled oxygen obtained with different oxygen delivery devices.
Methods: A simulation model of spontaneous breathing was used. Measure the proportion of inhaled oxygen received through low and high flow nasal prongs and simple oxygen masks. After 120 s of oxygen, the fraction of inhaled air was measured every second for 30 s. Three measurements were taken for each condition.
RESULTS: Airflow decreased intratracheal inspired oxygen fraction and extraoral oxygen concentration when using a low-flow nasal cannula, suggesting that expiratory breathing occurred during rebreathing and may be associated with an increase in intratracheal inspired oxygen fraction.
Conclusion. Oxygen inhalation during exhalation can lead to an increase in oxygen concentration in the anatomical dead space, which may be associated with an increase in the proportion of oxygen inhaled. Using a high flow nasal cannula, a high percentage of inhaled oxygen can be obtained even at a flow rate of 10 L/min. When determining the optimal amount of oxygen, it is necessary to set the appropriate flow rate for the patient and specific conditions, regardless of the value of the fraction of inhaled oxygen. When using low-flow nasal prongs and simple oxygen masks in a clinical setting, it can be difficult to estimate the proportion of oxygen inhaled.
The administration of oxygen during the acute and chronic phases of respiratory failure is a common procedure in clinical medicine. Various methods of oxygen administration include cannula, nasal cannula, oxygen mask, reservoir mask, venturi mask, and high flow nasal cannula (HFNC) [1-5]. The percentage of oxygen in the inhaled air (FiO2) is the percentage of oxygen in the inhaled air that participates in alveolar gas exchange. The degree of oxygenation (P/F ratio) is the ratio of partial pressure of oxygen (PaO2) to FiO2 in arterial blood. Although the diagnostic value of the P/F ratio remains controversial, it is a widely used indicator of oxygenation in clinical practice [6-8]. Therefore, it is clinically important to know the value of FiO2 when giving oxygen to a patient.
During intubation, FiO2 can be accurately measured with an oxygen monitor that includes a ventilation circuit, while when oxygen is administered with a nasal cannula and an oxygen mask, only an “estimate” of FiO2 based on inspiratory time can be measured. This “score” is the ratio of oxygen supply to tidal volume. However, this does not take into account some factors from the point of view of the physiology of respiration. Studies have shown that FiO2 measurements can be influenced by various factors [2,3]. Although the administration of oxygen during exhalation can lead to an increase in oxygen concentration in anatomical dead spaces such as the oral cavity, pharynx and trachea, there are no reports on this issue in the current literature. However, some clinicians believe that in practice these factors are less important and that “scores” are sufficient to overcome clinical problems.
In recent years, HFNC has attracted particular attention in emergency medicine and intensive care [9]. HFNC provides a high FiO2 and oxygen flow with two main benefits – flushing of the dead space of the pharynx and reduction of nasopharyngeal resistance, which should not be overlooked when prescribing oxygen [10,11]. In addition, it may be necessary to assume that the measured FiO2 value represents the oxygen concentration in the airways or alveoli, since the oxygen concentration in the alveoli during inspiration is important in terms of the P/F ratio.
Oxygen delivery methods other than intubation are often used in routine clinical practice. Therefore, it is important to collect more data on the FiO2 measured with these oxygen delivery devices in order to prevent unnecessary overoxygenation and to gain insight into the safety of breathing during oxygenation. However, the measurement of FiO2 in the human trachea is difficult. Some researchers have tried to mimic FiO2 using spontaneous breathing models [4,12,13]. Therefore, in this study, we aimed to measure FiO2 using a simulated model of spontaneous respiration.
This is a pilot study that does not require ethical approval because it does not involve humans. To simulate spontaneous breathing, we prepared a spontaneous breathing model with reference to the model developed by Hsu et al. (Fig. 1) [12]. Ventilators and test lungs (Dual Adult TTL; Grand Rapids, MI: Michigan Instruments, Inc.) from anesthesia equipment (Fabius Plus; Lübeck, Germany: Draeger, Inc.) were prepared to mimic spontaneous breathing. The two devices are manually connected by rigid metal straps. One bellows (drive side) of the test lung is connected to the ventilator. The other bellows (passive side) of the test lung is connected to the “Oxygen Management Model”. As soon as the ventilator supplies fresh gas to test the lungs (drive side), the bellows is inflated by forcibly pulling on the other bellows (passive side). This movement inhales gas through the manikin’s trachea, thus simulating spontaneous breathing.
(a) oxygen monitor, (b) dummy, (c) test lung, (d) anesthesia device, (e) oxygen monitor, and (f) electric ventilator.
The ventilator settings were as follows: tidal volume 500 ml, respiratory rate 10 breaths/min, inspiratory to expiratory ratio (inhalation/expiration ratio) 1:2 (breathing time = 1 s). For the experiments, the compliance of the test lung was set to 0.5.
An oxygen monitor (MiniOx 3000; Pittsburgh, PA: American Medical Services Corporation) and a manikin (MW13; Kyoto, Japan: Kyoto Kagaku Co., Ltd.) were used for the oxygen management model. Pure oxygen was injected at rates of 1, 2, 3, 4 and 5 L/min and FiO2 was measured for each. For HFNC (MaxVenturi; Coleraine, Northern Ireland: Armstrong Medical), oxygen-air mixtures were administered in volumes of 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, and 60 L, and FiO2 was assessed at each case. For HFNC, experiments were carried out at 45%, 60% and 90% oxygen concentrations.
Extraoral oxygen concentration (BSM-6301; Tokyo, Japan: Nihon Kohden Co.) was measured 3 cm above the maxillary incisors with oxygen delivered through a nasal cannula (Finefit; Osaka, Japan: Japan Medicalnext Co.) (Figure 1). ) Intubation using an electric ventilator (HEF-33YR; Tokyo, Japan: Hitachi) to blow air out of the manikin’s head to eliminate expiratory back-breathing, and FiO2 was measured 2 minutes later.
After 120 seconds of exposure to oxygen, FiO2 was measured every second for 30 seconds. Ventilate the manikin and laboratory after each measurement. FiO2 was measured 3 times in each condition. The experiment began after the calibration of each measuring instrument.
Traditionally, oxygen is assessed through nasal cannulas so that FiO2 can be measured. The calculation method used in this experiment varied depending on the content of spontaneous respiration (Table 1). The scores are calculated based on the breathing conditions set in the anesthesia device (tidal volume: 500 ml, respiratory rate: 10 breaths/min, inspiratory to expiratory ratio {inhalation: exhalation ratio} = 1:2).
“Scores” are calculated for each oxygen flow rate. A nasal cannula was used to administer oxygen to the LFNC.
All analyzes were performed using Origin software (Northampton, MA: OriginLab Corporation). Results are expressed as the mean ± standard deviation (SD) of the number of tests (N) [12]. We have rounded all results to two decimal places.
To calculate the “score”, the amount of oxygen breathed into the lungs in a single breath is equal to the amount of oxygen inside the nasal cannula, and the rest is outside air. Thus, with a breath time of 2 s, the oxygen delivered by the nasal cannula in 2 s is 1000/30 ml. The dose of oxygen obtained from the outside air was 21% of the tidal volume (1000/30 ml). The final FiO2 is the amount of oxygen delivered to the tidal volume. Therefore, the FiO2 “estimate” can be calculated by dividing the total amount of oxygen consumed by the tidal volume.
Before each measurement, the intratracheal oxygen monitor was calibrated at 20.8% and the extraoral oxygen monitor was calibrated at 21%. Table 1 shows the mean FiO2 LFNC values ​​at each flow rate. These values ​​are 1.5-1.9 times higher than the “calculated” values ​​(Table 1). The concentration of oxygen outside the mouth is higher than in indoor air (21%). The average value decreased before the introduction of air flow from the electric fan. These values ​​are similar to “estimated values”. With airflow, when the oxygen concentration outside the mouth is close to room air, the FiO2 value in the trachea is higher than the “calculated value” of more than 2 L/min. With or without airflow, the FiO2 difference decreased as the flow rate increased (Figure 2).
Table 2 shows the average FiO2 values ​​at each oxygen concentration for a simple oxygen mask (Ecolite oxygen mask; Osaka, Japan: Japan Medicalnext Co., Ltd.). These values ​​increased with increasing oxygen concentration (Table 2). With the same oxygen consumption, the FiO2 of the LFNK is higher than that of a simple oxygen mask. At 1-5 L/min, the difference in FiO2 is about 11-24%.
Table 3 shows the average FiO2 values ​​for HFNC at each flow rate and oxygen concentration. These values ​​were close to the target oxygen concentration regardless of whether the flow rate was low or high (Table 3).
Intratracheal FiO2 values ​​were higher than ‘estimated’ values ​​and extraoral FiO2 values ​​were higher than room air when using the LFNC. Airflow has been found to reduce intratracheal and extraoral FiO2. These results suggest that expiratory breathing occurred during LFNC rebreathing. With or without airflow, the FiO2 difference decreases as the flow rate increases. This result suggests that another factor may be associated with elevated FiO2 in the trachea. In addition, they also indicated that oxygenation increases the oxygen concentration in the anatomical dead space, which may be due to an increase in FiO2 [2]. It is generally accepted that LFNC does not cause rebreathing on exhalation. It is expected that this may significantly affect the difference between the measured and “estimated” values ​​for nasal cannulas.
At low flow rates of 1–5 L/min, the FiO2 of the plain mask was lower than that of the nasal cannula, probably because the oxygen concentration does not increase easily when part of the mask becomes an anatomically dead zone. Oxygen flow minimizes room air dilution and stabilizes FiO2 above 5 L/min [12]. Below 5 L/min, low FiO2 values ​​occur due to dilution of room air and rebreathing of dead space [12]. In fact, the accuracy of oxygen flow meters can vary greatly. The MiniOx 3000 is used to monitor oxygen concentration, however the device does not have sufficient temporal resolution to measure changes in exhaled oxygen concentration (manufacturers specify 20 seconds to represent a 90% response). This requires an oxygen monitor with a faster time response.
In real clinical practice, the morphology of the nasal cavity, oral cavity, and pharynx varies from person to person, and the FiO2 value may differ from the results obtained in this study. In addition, the respiratory status of patients differs, and higher oxygen consumption leads to lower oxygen content in expiratory breaths. These conditions can lead to lower FiO2 values. Therefore, it is difficult to assess reliable FiO2 when using LFNK and simple oxygen masks in real clinical situations. However, this experiment suggests that the concepts of anatomical dead space and recurrent expiratory breathing may influence FiO2. Given this discovery, FiO2 can increase significantly even at low flow rates, depending on conditions rather than “estimates”.
The British Thoracic Society recommends that clinicians prescribe oxygen according to the target saturation range and monitor the patient to maintain the target saturation range [14]. Although the “calculated value” of FiO2 in this study was very low, it is possible to achieve an actual FiO2 higher than the “calculated value” depending on the patient’s condition.
When using HFNC, the FiO2 value is close to the set oxygen concentration regardless of the flow rate. The results of this study suggest that high FiO2 levels can be achieved even at a flow rate of 10 L/min. Similar studies showed no change in FiO2 between 10 and 30 L [12,15]. The high flow rate of HFNC is reported to eliminate the need to consider anatomical dead space [2,16]. Anatomical dead space can potentially be flushed out at an oxygen flow rate greater than 10 L/min. Dysart et al. It is hypothesized that the primary mechanism of action of VPT may be the flushing of the dead space of the nasopharyngeal cavity, thereby reducing the total dead space and increasing the proportion of minute ventilation (i.e., alveolar ventilation) [17].
A previous HFNC study used a catheter to measure FiO2 in the nasopharynx, but FiO2 was lower than in this experiment [15,18-20]. Ritchie et al. It has been reported that the calculated value of FiO2 approaches 0.60 as the gas flow rate increases above 30 L/min during nasal breathing [15]. In practice, HFNCs require flow rates of 10-30 L/min or higher. Due to the properties of HFNC, conditions in the nasal cavity have a significant effect, and HFNC is often activated at high flow rates. If breathing improves, a decrease in flow rate may also be required, as FiO2 may be sufficient.
These results are based on simulations and do not suggest that FiO2 results can be directly applied to real patients. However, based on these results, in the case of intubation or devices other than HFNC, FiO2 values ​​can be expected to vary significantly depending on the conditions. When administering oxygen with a LFNC or a simple oxygen mask in the clinical setting, treatment is usually assessed only by the “peripheral arterial oxygen saturation” (SpO2) value using a pulse oximeter. With the development of anemia, strict management of the patient is recommended, regardless of SpO2, PaO2 and oxygen content in arterial blood. In addition, Downes et al. and Beasley et al. It has been suggested that unstable patients may indeed be at risk due to the prophylactic use of highly concentrated oxygen therapy [21-24]. During periods of physical deterioration, patients receiving highly concentrated oxygen therapy will have high pulse oximeter readings, which may mask a gradual decrease in the P/F ratio and thus may not alert staff at the right time, leading to impending deterioration requiring mechanical intervention. support. It was previously thought that high FiO2 provides protection and safety for patients, but this theory is not applicable to the clinical setting [14].
Therefore, care should be taken even when prescribing oxygen in the perioperative period or in the early stages of respiratory failure. The results of the study show that accurate FiO2 measurements can only be obtained with intubation or HFNC. When using an LFNC or a simple oxygen mask, prophylactic oxygen should be provided to prevent mild respiratory distress. These devices may not be suitable when a critical assessment of respiratory status is required, especially when FiO2 results are critical. Even at low flow rates, FiO2 increases with oxygen flow and may mask respiratory failure. In addition, even when using SpO2 for postoperative treatment, it is desirable to have as low a flow rate as possible. This is necessary for the early detection of respiratory failure. High oxygen flow increases the risk of early detection failure. Dosage of oxygen should be determined after determining which vital signs are improved with oxygen administration. Based on the results of this study alone, it is not recommended to change the concept of oxygen management. However, we believe that the new ideas presented in this study should be considered in terms of methods used in clinical practice. In addition, when determining the amount of oxygen recommended by the guidelines, it is necessary to set the appropriate flow for the patient, regardless of the FiO2 value for routine inspiratory flow measurements.
We propose to reconsider the concept of FiO2, taking into account the scope of oxygen therapy and clinical conditions, since FiO2 is an indispensable parameter for managing oxygen administration. However, this study has several limitations. If FiO2 can be measured in the human trachea, a more accurate value can be obtained. However, it is currently difficult to perform such measurements without being invasive. Further research using non-invasive measuring devices should be carried out in the future.
In this study, we measured intratracheal FiO2 using the LFNC spontaneous breathing simulation model, simple oxygen mask, and HFNC. Management of oxygen during exhalation can lead to an increase in oxygen concentration in the anatomical dead space, which may be associated with an increase in the proportion of oxygen inhaled. With HFNC, a high proportion of inhaled oxygen can be obtained even at a flow rate of 10 l/min. When determining the optimal amount of oxygen, it is necessary to establish the appropriate flow rate for the patient and specific conditions, not dependent only on the values ​​of the fraction of oxygen inhaled. Estimating the percentage of oxygen inhaled when using a LFNC and a simple oxygen mask in a clinical setting can be challenging.
The data obtained indicate that expiratory breathing is associated with an increase in FiO2 in the trachea of ​​the LFNC. When determining the amount of oxygen recommended by the guidelines, it is necessary to set the appropriate flow for the patient, regardless of the FiO2 value measured using the traditional inspiratory flow.
Human Subjects: All authors confirmed that no humans or tissues were involved in this study. Animal Subjects: All authors confirmed that no animals or tissues were involved in this study. Conflicts of Interest: In accordance with the ICMJE Uniform Disclosure Form, all authors declare the following: Payment/Service Information: All authors declare that they did not receive financial support from any organization for the submitted work. Financial Relationships: All authors declare that they do not currently or within the past three years have financial relationships with any organization that may be interested in the submitted work. Other Relationships: All authors declare that there are no other relationships or activities that may affect the submitted work.
We would like to thank Mr. Toru Shida (IMI Co., Ltd, Kumamoto Customer Service Center, Japan) for his assistance with this study.
Kojima Y., Sendo R., Okayama N. et al. (May 18, 2022) Inhaled oxygen ratio in low and high flow devices: a simulation study. Cure 14(5): e25122. doi:10.7759/cureus.25122
© Copyright 2022 Kojima et al. This is an open access article distributed under the terms of the Creative Commons Attribution License CC-BY 4.0. Unlimited use, distribution, and reproduction in any medium is permitted, provided the original author and source are credited.
This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the author and source are credited.
(a) oxygen monitor, (b) dummy, (c) test lung, (d) anesthesia device, (e) oxygen monitor, and (f) electric ventilator.
The ventilator settings were as follows: tidal volume 500 ml, respiratory rate 10 breaths/min, inspiratory to expiratory ratio (inhalation/expiration ratio) 1:2 (breathing time = 1 s). For the experiments, the compliance of the test lung was set to 0.5.
“Scores” are calculated for each oxygen flow rate. A nasal cannula was used to administer oxygen to the LFNC.
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