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Journal of Obstrectic Anaesthesia and Critical Care
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 Table of Contents  
Year : 2023  |  Volume : 13  |  Issue : 1  |  Page : 35-39

Evaluation of high flow nasal oxygenation as a technique for preoxygenation in full term pregnant women

Department of Anaesthesia, ABVIMS and Dr. Ram Manohar Lohia Hospital, Delhi, India

Date of Submission24-Mar-2022
Date of Acceptance21-May-2022
Date of Web Publication09-Mar-2023

Correspondence Address:
Dr. Namita Saraswat
Anaesthesia and FIPM, Associate Professor of Anaesthesiology, ABVIMS and Dr. Ram Manohar Lohia Hospital, Delhi
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/JOACC.JOACC_24_22

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Background and Aims: Obstetric airway guidelines recommend preoxygenation before the induction of general anesthesia to achieve an end tidal oxygen concentration of ≥90%. Recently, high flow nasal oxygenation (HFNO) has been evaluated as a technique for delivering a high concentration of oxygen using high flow rates to patients. We evaluated the use of HFNO as a technique for preoxygenation in full-term pregnant women. Methods: A cross-sectional observational study was conducted on 100 term pregnant women. They underwent preoxygenation using HFNO for 4 minutes (30 L/min for 30 secs followed by 50 L/min for 210 secs) and end tidal oxygen concentration (ETO2) was measured at the end of preoxygenation. The primary outcome was the percentage of women who achieved an expired oxygen concentration of ≥90% for the first expired breath. The secondary outcome was the acceptability and comfort of HFNO as compared to facemask preoxygenation using a 4-point Likert scale. Results: The percentage of women who achieved expired oxygen concentration of ≥90% after 4 minutes of HFNO preoxygenation was 32% [95% confidence interval (CI):22.7-41.3%] with the mean end tidal oxygen (SD) being 86.67 (3.4). 71% [mean (SD): 2.94 (0.92)] found nasal cannula and 56% [mean (SD): 2.67 (1.21)] found facemask comfortable and acceptable for preoxygenation (P value, 0.05). Conclusion: Although HFNO is a comfortable technique, when used for preoxygenation for 4 minutes, it did not achieve an acceptable level of preoxygenation (ETO2 ≥90% in 95% of individuals). Therefore, it is an inadequate technique for preoxygenation in term pregnant women.

Keywords: End tidal oxygen concentration, high flow nasal oxygenation, preoxygenation

How to cite this article:
Ajeetha P G, Kachru N, Saraswat N. Evaluation of high flow nasal oxygenation as a technique for preoxygenation in full term pregnant women. J Obstet Anaesth Crit Care 2023;13:35-9

How to cite this URL:
Ajeetha P G, Kachru N, Saraswat N. Evaluation of high flow nasal oxygenation as a technique for preoxygenation in full term pregnant women. J Obstet Anaesth Crit Care [serial online] 2023 [cited 2023 Mar 25];13:35-9. Available from: https://www.joacc.com/text.asp?2023/13/1/35/371306

  Introduction Top

Airway management in the obstetric population is very challenging. The physiological and anatomical changes during pregnancy lead to an increase in oxygen requirement and the decreased Functional Residual Capacity (FRC) makes them prone to rapid desaturation during periods of apnoea. This can be prevented by adequate preoxygenation.[1],[2],[3]

The latest guidelines by the Obstetric Anesthetists' Association and Difficult Airway Society recommend that the aim of preoxygenation in pregnant women before general anesthesia is to achieve an expired oxygen concentration value of ≥90% in 95% of the individuals, to be considered adequate preoxygenation. (Ref) Preoxygenation in the obstetric population is, as a standard, done by normal tidal volume breathing with 100% oxygen for a minimum of 2 minutes using a tight-fitting face mask connected to the anesthesia circuit.[1],[4],[5]

Recently, the use of high flow humidified nasal oxygenation (HFNO) is becoming increasingly popular and has been proposed as a technique for preoxygenation and apnoeic oxygenation in operating rooms. It delivers a high concentration of humidified oxygen (FiO2 OF 0.95-1), up to 70 liters per minute through the nasal cannulas. HFNO facilitates carbon dioxide clearance, reduces dead space, provides an anatomical oxygen reservoir within the nasopharynx and oropharynx, provides positive pressure, and reduces work of breathing. HFNO also provides hands-free control to the anesthetist and less discomfort than a face mask to the patient.[6],[7],[8]

There is a paucity of data in the literature regarding the use of HFNO as a technique for preoxygenation, especially in pregnant women. Early studies have demonstrated that in pregnant women, preoxygenation using HFNO for 3 minutes was not effective in achieving the target end point.[9]

Most anesthetists preoxygenate patients for a period of 3 minutes. However, there are no studies that have assessed whether a small increase in the duration of preoxygenation, using HFNO, will be able to achieve the target end point of preoxygenation i.e., an expired oxygen concentration ≥90% in pregnant women. Hence, we conducted this study to determine whether oxygenation for 4 minutes using high flow nasal oxygenation is an effective method of preoxygenation in pregnant women.

  Methods Top

We conducted a cross-sectional observational study among term pregnant women in our hospital during the period November 2019-March 2020 after obtaining approval from the Institutional Ethical Committee and Institutional review board. We enrolled 100 Healthy pregnant women in our study (ASA Physical status 2) with a gestational period ≥37 weeks. Women with eclampsia and preeclampsia, BMI >35 kg/m2, any severe nasal pathology, fetal distress, and those in labor, were excluded from the study.

After taking written informed consent, term pregnant women were taken to the operation theatre and made to lie supine with a right lateral pelvic wedge to minimize aortocaval compression. Baseline values of maternal heart rate, blood pressure, oxygen saturation (SpO2), and fetal heart rate were measured while breathing room air (FiO2 = 21%). A tight-fitting face mask was selected by connecting to the anesthesia circuit and anesthesia machine and a good seal and ideal fitting were determined by observing the capnography trace. The facemask was then removed. Preoxygenation with HFNO was done through a nasal cannula and high-flow oxygen delivery system (Fisher and Paykel Healthcare, AIRVOTM 2). Inspired fraction of oxygen (FiO2) was set at maximum and temperature at 37◦C.

Preoxygenation was done at an initial flow rate of 30 L/min for 30 sec and 50 L/min for the next 210 sec. Patients were instructed to keep their mouths closed and take slow deep breaths, as deep as possible. At the end of 4 minutes, they were asked to hold their breath in inspiration and the nasal cannula was rapidly removed and the tight-fitting face mask was applied and connected to 100% inspired oxygen at a 6 L/min flow rate. They were asked to exhale and the expired oxygen concentration was measured by the analyzer on the anesthetic machine. Fetal heart rate and peak oxygen saturation values were recorded at the end of the procedure. Comfort levels of the standard face mask and the high flow nasal cannula were assessed using a 4-point Likert scale and the women were asked which of the two they would prefer regarding comfort.

In the absence of any previous study with a duration of oxygenation of 4 minutes, assuming the proportion of patients with ≥90% expired oxygen concentration as 50%; and taking this value as reference, the minimum required sample size with a 10% margin of error and 5% level of significance was 97 patients. It was rounded off and the total sample size taken was 100. Presentation of the categorical variables was done in the form of numbers and percentages (%). Quantitative data were presented as the mean ± SD and as median with 25th and 75th percentiles (interquartile range). Pearson correlation coefficient was used to correlate body mass index and expired oxygen concentration at 1st breath. Paired 't' test was used to compare comfort and acceptability between HFNO and face mask quantitatively and hemodynamic parameters across follow up and the Chi square test was used to compare comfort and acceptability between HFNO and face mask qualitatively. For statistical significance, P value of less than 0.05 was considered statistically significant.

  Results Top

100.term pregnant women were recruited and completed the study. There were no complications recorded.

The mean age, weight, and BMI of the study subjects are depicted in [Table 1]. The gestational age of subjects ranged from 37-40 weeks [Table 1].
Table 1: Participant characteristics

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Hemodynamic parameters, respiratory rate, SpO2, and fetal heart rate measured at baseline and after 4 minutes of preoxygenation are shown in [Table 2]. There was no statistically significant change seen in these parameters.
Table 2: Hemodynamic parameters

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Expired oxygen concentration after 4 minutes of preoxygenation using HFNO of the study subjects is described in [Table 3] and [Figure 1].
Figure 1: Expired oxygen concentration after 4 minutes of preoxygenation using HFNO is presented as box whisker plots. The line in the middle of the box is the median (87) and the whisker extends to a minimum and maximum values (78-91). The box extends from the 25th to 75th percentile (84-90)

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Table 3: Expired oxygen concentration (ET02) after 4 minutes of HFNO

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Only 32 [32.00% (95% CI-22.70% to 41.30%)] women achieved the recommended expired oxygen concentration of ≥90% with mean (SD) EtO2 86.67 (3.4) and a range of 78-91.

Comfort and acceptability scores of facemasks and HFNO using the Likert scale are described in [Figure 2]. About 71% of the study group found HFNO comfortable with Likert scale values of mean ± SD score of 2.94 ± 0.92 and median of 3, whereas 56% of the study population found the facemask comfortable (mean ± SD score of 2.67 ± 1.21 and median of 3) (P < 0.05%).
Figure 2: Comfortability and acceptability for HFNO and face mask of study subjects using Likert scale presented as box whiskers plot

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  Discussion Top

This study evaluated the use of HFNO for 4 minutes, as a technique for preoxygenation in 100 term pregnant women. The key findings of our study were that after 4 minutes of preoxygenation, HFNO does not achieve an acceptable level of preoxygenation (≥90% in 95% of individuals). In our study, only 32% of the study population achieved an expired oxygen concentration (ETO2) of 90% or greater with HFNO, 67% of the individuals achieved an expired oxygen concentration of 80-90% while 1% achieved an expired oxygen concentration of less than 80%. Therefore, our results suggested that using HFNO at a rate of 30 L/min for 30 sec followed by 50 L/min for 210 sec does not allow adequate preoxygenation in term pregnant women.

Our results are consistent with previous studies, Tan et al.[9] studied preoxygenation for 3 minutes using HFNO in pregnant women. In their study, they used preoxygenation with HFNO at 30 L/min for 30 sec and then at 50 L/min for the next 150 secs. They found that only 60% of their study population achieved the target end point of expired oxygen concentration of 90% or greater. Although the percentage of patients who achieved the desired ETO2 level is higher in their study, it is still below the acceptable target level of 95%, and they too have suggested that HFNO is inadequate for preoxygenation in pregnant women.

Another study by Shippam et al.[10] also found results similar to our study. They compared preoxygenation for 3 minutes using HFNO and standard facemask in 40 pregnant women. In their study, they used a flow rate of up to 70 L/min and measured the highest value of ETO2 achieved after 3 minutes of preoxygenation. Even though they used a higher flow rate, they found that only 47% of their population were able to achieve the target end point of ETO2 of 90% or greater with HFNO, whereas 85% of the patients in the facemask group achieved the target end point.

The standard technique of preoxygenation in obstetric patients consists of preoxygenation for 3 minutes of tidal volume breathing. In term pregnant women, functional residual capacity (FRC) decreases by 20% and minute ventilation increases. This should result in faster denitrogenation. However, in the supine position, the closing capacity exceeds FRC. This results in small airway closure during tidal breathing, thus limiting oxygen access to some parts of the lungs. This might lead to an inability to achieve the desired ETO2 after 3 minutes and a long time may be required for preoxygenation in the obstetric population as compared to the non-obstetric population.[3]

Chiron et al.,[4] in their study of preoxygenation in the pregnant population, also found that only 76% of the women were able to achieve an ETO2 ≥90% while using the standard 3 minutes of tidal volume breathing technique even when using a tight-fitting facemask for preoxygenation. Since 24% of the women in their study were not able to achieve the target endpoint, it was suggested that not all pregnant women could achieve the target ETO2 >90% after 3 minutes of preoxygenation. The study by Tan et al.[9] also suggested that with HFNO, a longer period of preoxygenation might be required to achieve adequate end tidal oxygen levels. Together these results suggested re-examining the standard practice of 3 minutes of preoxygenation. Hence, in our study, we increased the duration of preoxygenation using HFNO to 4 minutes to determine whether increasing the duration would have any beneficial effect on the achieved end tidal oxygen levels.

In 2020, Au et al.[11] conducted a study to determine the effective time interval required for preoxygenation using HFNO or facemask in pregnant women. They found the effective time interval to achieve an ETO2 >90% in 90% of pregnant women using facemask was 3.6 min. But they could not reach target ETO2 values with HFNO even after 8 minutes of preoxygenation. In their study, none of the patients reached the target value in the HFNO group after 3 minutes, while after 4 minutes, 67% of the patients were able to achieve the target end point. There was no improvement in expired oxygen concentration beyond 4 minutes.

Similarly, in our study, we too could not achieve the target end point (ETO2 >90% in 95% of individuals) after 4 minutes of preoxygenation using HFNO. Even though neither study could achieve the desired end tidal oxygen value in >95% of patients, as compared to their study, an even lesser proportion of our study population was able to achieve the target endpoint. This could be because we used lower flow rates of 50 L/minute as compared to 70 L/min in theirs. Another possible reason for the lower results in our study could be because difference in the technique of measurement of ETO2 levels. At the end of 4 minutes, they used a nose clip to ensure that patients were holding their breath before changing to facemask to prevent air dilution, while we didn't. Some of our patients might not have been able to hold their breath which would have affected the readings.[9],[10],[11]

Shippam et al. also used HFNO for preoxygenation in pregnant women at flowrates up to 70 L/min, but they were also not able to achieve the target endpoint even with higher flow rates. However, in the nonpregnant population, higher flow rates of 70 L/min have also been used by Pillai et al.[12] and Mir et al.[13] and to achieve equivalent blood gas results compared with the facemask preoxygenation. In our study, the maximum flow rate of oxygen used via the HFNO was 50 L/min for 210 secs keeping in mind the comfort of our study population.

The proposed reason for the failure of HFNO to achieve the target ETO2 is air entrainment which occurs during breathing through a nasal cannula and thus there is an inability to maintain the desired FiO2. This is more so in patients who do not keep their mouths closed during preoxygenation. Also, as recently studied by Chanques et al.,[14] if the mouth is opened during the use of high flow oxygen therapy delivery devices, it was found that the level of positive airway pressure generated decreased and hence oxygenation decreased. In some patients, the inspiratory flow rate can also affect the level of oxygenation achieved. A higher inspiratory flow rate compared to the HFNO gas flow can lead to greater air entrainment.

Apart from air entrainment, another proposed reason for not achieving the target end tidal oxygen values using HFNO is the physiological and anatomical changes that are seen during pregnancy such as mucosal engorgement, pharyngeal and laryngeal edema.[15] These factors may decrease the delivery of oxygen and prevent the generation of continuous positive airway pressure in pregnant women by changing their aerodynamics as compared to the nonpregnant population. Hence, HFNO does not increase oxygen reserve in these parturients.

Studies in nonpregnant individuals have found variability in terms of acceptability and comfort levels of HFNO as compared to the face mask.[12],[16] In our study, a greater proportion of parturients were comfortable using the HFNO (71/100) than the face mask (56/100). These results are similar to previous studies by Tan et al.[9] In their study, they also found that more patients preferred nasal cannula as compared to face masks for oxygenation. This can be explained because of the physiological changes to the nasal and pharyngeal mucosa and the feeling of suffocation felt while using a tight face mask. In contrast, the study by Shippam et al.[10] found that there was no significant difference in the comfort or acceptability between the facemask and nasal cannulae.

There were several limitations to our study. We used HFNO up to a maximum flow rate of 50 L/min. Therefore, it cannot be generalized to other flow rates and breathing methods while using HFNO for preoxygenation. Further studies are needed to predict whether higher flow rates and time intervals while using HFNO will be able to achieve the target end point of end tidal oxygen.

Secondly, our study population comprised term pregnant women who were not in labor, were non-obese, and with no complications such as pre-eclampsia or those undergoing emergency surgeries. In these situations, oxygen consumption would further increase because of physical and emotional stress and levels of preoxygenation achieved in these patients may be even less adequate. A coincidental finding in our study was the association of BMI with expired oxygen concentration. Although we did not include obese patients in our study, however, with an increase in BMI, we found that the achieved ETO2 was steadily lower. Thus, this is important when we investigate preoxygenation in obese parturients.

Although ETO2 is the recommended parameter to assess the adequacy of preoxygenation, expired nitrogen, PaO2 or newer parameters such as the oxygen reserve index sensor may be considered in future clinical trials.

HFNO works by maintaining oxygenation during the apnoeic period and when used for apnoeic oxygenation in other populations, has been found to be beneficial in prolonging the apnoea time. Further studies are needed to examine the role of HFNO in apnoeic oxygenation in pregnant women.

Hence, we conclude that the HFNO is a comfortable technique for preoxygenation in pregnant women but it does not achieve a clinically acceptable level of preoxygenation (ETO2 >90% in 95% of pregnant women). Therefore, it cannot be used as a technique for preoxygenation in full term pregnant women.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

  References Top

Mushambi MC, Kinsella SM, Popat M, Swales H, Ramaswamy KK, Winton AL, et al. Obstetric anaesthetists' association and difficult airway society guidelines for the management of difficult and failed tracheal intubation in obstetrics. Anaesthesia 2015;70:1286-306.  Back to cited text no. 1
Nimmagadda U, Salem MR, Crystal GJ. Preoxygenation: Physiologic basis, benefits, and potential risks. Anesth Analg 2017;124:507-17.  Back to cited text no. 2
Russell GN, Smith CL, Snowdon SL, Bryson TH. Pre-oxygenation and the parturient patient. Anaesthesia 1987;42:346-51.  Back to cited text no. 3
Chiron B, Laffon M, Ferrandiere M, Pittet JF, Marret H, Mercier C. Standard preoxygenation technique versus two rapid techniques in pregnant patients. Int J Obstet Anesth 2004;13:11-4.  Back to cited text no. 4
McClelland SH, Bogod DG, Hardman JG. Pre-oxygenation in pregnancy: An investigation using physiological modelling. Anaesthesia 2008;63:259-63.  Back to cited text no. 5
Renda T, Corrado A, Iskandar G, Pelaia G, Abdalla K, Navalesi P. High-flow nasal oxygen therapy in intensive care and anaesthesia. Br J Anaesth 2018;120:18-27.  Back to cited text no. 6
Jeremy Cooper MB, Ehrenwerth J. Safe use of high-flow nasal 6 oxygen (HFNO) with special reference to difficult airway 7 management and fire risk. Circulation 2018;33:122,210.  Back to cited text no. 7
Bonnet N, Martin O, Boubaya M, Levy V, Ebstein N, Karoubi P, et al. High flow nasal oxygen therapy to avoid invasive mechanical ventilation in SARS-CoV-2 pneumonia: a retrospective study. Ann Intensive Care 2021;11:37.  Back to cited text no. 8
Tan PC, Millay OJ, Leeton L, Dennis AT. High-flow humidified nasal preoxygenation in pregnant women: A prospective observational study. Br J Anaesth 2019;122:86-91.  Back to cited text no. 9
Shippam W, Preston R, Douglas J, Taylor J, Albert A, Chau A. High-flow nasal oxygen vs. standard flow-rate facemask pre-oxygenation in pregnant patients: A randomised physiological study. Anaesthesia 2019;74:450-6.  Back to cited text no. 10
Au K, Shippam W, Taylor J, Albert A, Chau A. Determining the effective pre-oxygenation interval in obstetric patients using high-flow nasal oxygen and standard flow rate facemask: A biased-coin up–down sequential allocation trial. Anaesthesia 2020;75:609-16.  Back to cited text no. 11
Pillai A, Daga V, Lewis J, Mahmoud M, Mushambi M, Bogod D. High-flow humidified nasal oxygenation vs. standard face mask oxygenation. Anaesthesia 2016;71:1280-3.  Back to cited text no. 12
Mir F, Patel A, Iqbal R, Cecconi M, Nouraei SA. A randomised controlled trial comparing transnasal humidified rapid insufflation ventilatory exchange (THRIVE) pre-oxygenation with facemask pre-oxygenation in patients undergoing rapid sequence induction of anaesthesia. Anaesthesia 2017;72:439-43.  Back to cited text no. 13
Chanques G, Riboulet F, Molinari N, Carr J, Jung B, Prades A, et al. Comparison of three high flow oxygen therapy delivery devices: A clinical physiological cross-over study. Minerva Anestesiol 2013;79:1344-55.  Back to cited text no. 14
Dunietz GL, Chervin RD, O'Brien LM. Sleep-disordered breathing during pregnancy: Future implications for cardiovascular health. Obstet Gynecol Surv 2014;69:164-76.  Back to cited text no. 15
Hanouz JL, Lhermitte D, Gérard JL, Fischer MO. Comparison of pre-oxygenation using spontaneous breathing through face mask and high-flow nasal oxygen: A randomised controlled crossover study in healthy volunteers. Eur J Anaesthesiol 2019;36:335-41.  Back to cited text no. 16


  [Figure 1], [Figure 2]

  [Table 1], [Table 2], [Table 3]


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