High Flow Nasal Oxygen in Pregnant Women: Anesthesia

Anaesth Intensive Care 2018 | 46:1
P. C. F. Tan et al
High flow humidified nasal oxygen in pregnant women
P. C. F. Tan*, A. T. Dennis†
Summary
Failed airway management in the obstetric patient undergoing general anaesthesia is associated with major sequelae for the
mother and/or fetus. Effective and adequate pre-oxygenation is an important safety strategy and a recommendation in all
current major airway guidelines. Pre-oxygenation practice in the obstetric population may be suboptimal based on current
literature. Recently, clinical applications for high flow nasal oxygen, also known as transnasal humidified rapid insufflation
ventilatory exchange or THRIVE, are expanding in the non-obstetric population and may have theoretical benefits if used for
pre-oxygenation and apnoeic oxygenation in the obstetric population. We review the current literature surrounding high flow
nasal oxygen relevant to the pregnant woman. We also propose a basis for potential advantages and complications for its use
in this context.
Key Words: obstetric anaesthesia, airway management, pre-oxygenation, apnoeic oxygenation, intubation,
desaturation, nasal high flow
Difficulty in intubating the trachea or failure to oxygenate
a pregnant woman during general anaesthesia is one of the
most serious complications of obstetric anaesthesia. This
complication may result in hypoxic brain injury and fetal
and/or maternal death. Due to the known problem of the
challenging obstetric airway, the Obstetric Anaesthetists’
Association (OAA) and Difficult Airway Society (DAS) of the
United Kingdom published guidelines for management
of difficult and failed tracheal intubation in obstetrics in
November 20151. One of the key points discussed in the
guidelines is the importance of pre-oxygenation in managing
pregnant women undergoing general anaesthesia as it
increases oxygen reserve in the lungs prior to anaesthesiainduced apnoea. Also discussed for the first time in an
obstetric airway guideline was the possible use of high flow
humidified nasal oxygen for pre-oxygenation and also for
apnoeic oxygenation. High flow humidified nasal oxygen is
gaining momentum in all fields of critical care as a tool to
improve safety around airway management, especially during
emergencies, with growing applications in anaesthesia,
intensive care and emergency medicine. The objective of this
literature review is to present an overview of the physiology
underpinning high flow nasal pre-oxygenation, the changes
in the obstetric airway and maternal physiology that may
make high flow humidified nasal oxygen advantageous
during induction of general anaesthesia, and to consider the
application of this technology in the setting of term pregnant
women undergoing general anaesthesia.
* MBBS BMedSc, Anaesthetics Registrar, Royal Melbourne Hospital and Honorary
Anaesthetics Research Fellow, Royal Women’s Hospital, Melbourne, Victoria
† MBBS PhD MIPH PGDipEcho FANZCA, Staff Specialist Anaesthetist, Director of
Anaesthesia Research, Department of Anaesthesia, The Royal Women’s Hospital
and NHMRC Fellow, University of Melbourne, Departments of Pharmacology and
Obstetrics and Gynaecology, Melbourne, Victoria
Address for correspondence: Patrick Tan. Email: [email protected]
Accepted for publication on September 21, 2017
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Methods
The literature reviewed included any systematic review,
randomised controlled trial, observational study, case report
or consensus statement pertaining to pre-oxygenation,
apnoeic oxygenation and the use of high flow nasal oxygen
in airway management in the pregnant and non-pregnant
population. Obstetric airway issues were also explored in
the same methodology. Electronic search strategies included
the databases PUBMED (until May 2017), MEDLINE (until
May 2017) and GOOGLE SCHOLAR (until May 2017) using
the following key words: obstetric, parturient, pregnant, preoxygenation/preoxygenation, apnoeic/apneic oxygenation,
difficult airway, high-flow nasal oxygen, transnasal humidified
rapid insufflation ventilatory exchange.
Current guidelines
The latest airway management guidelines by the OAA
and the DAS published in Anaesthesia November 2015
recommend that an end-tidal oxygen fraction (FetO2) of ≥0.9
should be achieved prior to induction of anaesthesia. This
indicates effective lung denitrogenation. The traditional way
anaesthetists pre-oxygenate pregnant women undergoing
general anaesthesia is to use a tight-fitting facemask to
deliver 100% inspired oxygen at a flow rate that exceeds the
patient’s estimated minute ventilation. The new guideline
also states that a supplementary method, ‘nasal oxygenation’
to facilitate bulk flow of oxygen to the alveoli, could be
considered as part of:
1. Pre-oxygenation—a pre-oxygenation method and
2. apnoeic oxygenation—a technique to increase apnoeic
time.
The guidelines suggest the anaesthetist should consider
attaching nasal cannulae with 5 litres per minute oxygen
flow before starting routine pre-oxygenation to maintain
bulk flow of oxygen during intubation attempts or attach
Anaesth Intensive Care 2018 | 46:1
High flow humidified nasal oxygen in pregnant women
Figure 1: The Optiflow™ (Fisher & Paykel Healthcare, Auckland, New Zealand) high flow humidified oxygen delivery system consisting of nasal cannulae,
humidifier/heating system and circuit. The system can be used without any disruption to laryngoscopy and intubation.
nasal oxygenation after pre-oxygenation during the apnoeic
period1. There is also mention of humidified high flow nasal
oxygen, however, no data exist in pregnant women as to
its safety, efficacy or acceptability by pregnant women. It is
possible that a new pre-oxygenation technique, using high
flow humidified nasal oxygen, may have an application in
pregnant women. However, before this new pre-oxygenation
technique is considered, it should be equivalent (at least)
to the current available technique in terms of safety and
efficacy, i.e. be able to obtain an FetO2 of ≥0.9 after three
minutes of pre-oxygenation.
Currently there are no studies reporting FetO2 values
in pregnant women after pre-oxygenation with high flow
humidified nasal oxygen delivery systems and no studies
reporting the use of nasal high flow oxygen to increase
apnoea times. Therefore, it is unclear whether humidified
high flow nasal oxygen is safe and beneficial in pregnant
women.
Physiology underpinning high flow humidified nasal
oxygen
High flow humidified nasal cannula oxygen therapy is
a method of providing oxygen and continuous positive
airway pressure (CPAP) to patients whilst facilitating carbon
dioxide clearance. It consists of an oxygen inlet, an active
humidifier, a single heated circuit and nasal cannulae. The
system delivers up to 70 litres per minute flow of heated,
humidified oxygen. Due to its mechanism of action, high flow
humidified nasal cannula oxygen therapy results in washout
of anatomical dead space, a constant fraction of inspired
oxygen, good humidification and the provision of a low
level of CPAP. Work of breathing is also decreased. There is
also emerging work utilising fluid dynamic modelling which
explains the mechanism of carbon dioxide clearance during
apnoea, which was demonstrably superior to previous lowflow oxygen delivery systems2.
High flow humidified nasal oxygen in the
non-pregnant population
Concurrent development of systems such as the Optiflow™
by Fisher & Paykel Healthcare (Auckland, New Zealand) is
beginning to find utility in the non-obstetric population. Patel
et al’s THRIVE study showed a prolonged median (range)
apnoea time of 14 (5–65) minutes in 25 patients with difficult
airways2. These patients had a median (interquartile range
[IQR]) Mallampati score of 3 (2–3) and laryngoscopy grade of
3 (3–3) and none experienced peripheral oxygen saturations
(SpO2) of <90% or any clinically apparent complications
of hypercapnia2. Following that study, Mir and Patel et al
demonstrated that pre-oxygenation with high flow nasal
oxygen was not inferior to pre-oxygenation with a facemask
in patients undergoing rapid sequence induction. They
showed no significant difference in maintenance of SpO2 and
arterial blood gas profiles despite longer apnoea times in the
high flow nasal oxygen group in their randomised controlled
trial of 40 patients3. Doyle et al’s observational study targeted
patients at high risk of hypoxia during intubation. They
reported a 7% incidence of significant desaturation (drop
of SpO2 >10%) when high flow nasal pre-oxygenation and
apnoeic oxygenation was used. Participants included patients
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P. C. F. Tan et al
Anaesth Intensive Care 2018 | 46:1
Figure 2: Participant in ramped and left lateral tilted position using TroopTM (Goal Medical, LLC, Eugene, OR, USA) pillow and wedge during pre-oxygenation study
protocol. Picture reproduced with permission of the patient.
requiring intubation in the emergency department or critical
care unit and those with risk factors for hypoxia being
intubated in the operating theatre. The impact of the THRIVE
protocol was unable to be quantified however as this study
did not compare this technique to established practice or to
a pre-study incidence of significant desaturation4. MiguelMontanes et al’s prospective before–after study in 101
intensive care unit patients compared
pre-oxygenation via a non-rebreathing bag reservoir
facemask versus high flow nasal cannulae. Their results
showed a significant difference in SpO2 during intubation,
improvement in pre-oxygenation quality and reduced
episodes of severe hypoxaemia in the high flow nasal oxygen
group5. The study population comprised intensive care unit
(ICU) patients requiring intubation with a median (IQR)
Simplified Acute Physiology Score II of 44 (35–61.5) and a
corresponding mortality risk of 25% to 50%5. Another study
highlights the application of positive end-expiratory pressure
(PEEP) during induction of general anaesthesia and showed
that it prolonged safe apnoea time6. The Optiflow system
purportedly maintains 3 to 7 cmH2O of PEEP. There has also
been opinion from airway experts who believe that high flow
nasal oxygen will become part of routine practice for all adult
tracheal intubations in the near future7.
High flow humidified nasal oxygen in the pregnant
population
There is an absence of evidence surrounding the use of
high flow nasal oxygen in pregnant women. Computational
38
modelling using the Nottingham Physiology Simulator,
which models the respiratory and cardiovascular systems
accounting for the changes of pregnancy, predicts that
apnoeic oxygenation using an inspired fraction of oxygen
(FiO2) of 1.0 should prolong time to desaturation (SpO2 less
than 90%) to more than 50 minutes in the average parturient
and to more than 40 minutes in the morbidly obese
parturient (body mass index 50 kg/m2)8. In order to assess
the feasibility of undertaking a proposed larger study we have
published a small pilot study of the use of this equipment
in pregnant women9. Four pregnant women at term were
recruited with written informed consent. Participants were
placed on a hospital bed in a ramped position using a TroopTM
elevation pillow (Goal Medical, LLC, Eugene, OR, USA) with
left lateral tilt using a wedge. High flow humidified nasal
oxygen was then administered at 30 litres per minute for 30
seconds then 50 litres per minute for a further 150 seconds
and end-tidal gas sampling was performed via a facemask at
the end of the three minute period. Each woman achieved
100% SpO2 at the end of the pre-oxygenation period and
remained as such during the washout period. However, only
one woman achieved the recommended pre-oxygenation
induction of anaesthesia trigger threshold of an FetO2 ≥0.9.
All women preferred the nasal pre-oxygenation system to
the tightly fitting facemask. We anticipate that pregnant
women with elevated body mass indices may require longer
pre-oxygenation times or higher flows or both to achieve
adequate denitrogenation. Larger prospective studies are
required to delineate this relationship and so to explore these
Anaesth Intensive Care 2018 | 46:1
hypotheses, we are undertaking a larger prospective study
(ACTRN12616000531415p) to characterise the safety and
efficacy of high flow nasal oxygen as a pre-oxygenation and
apnoeic oxygenation strategy in pregnant women.
The obstetric airway and maternal physiology
The difficult obstetric airway is well described in the
anaesthesia literature. Some sources report the incidence of
difficult or failed airway to be approximately ten times that
in the general surgical population10-12. Airway management
in the parturient at term can be challenging as a result of
anatomical changes (airway oedema) and physiological
changes (decreased functional residual capacity coupled
with higher risk of aspiration) of pregnancy. Studies have
shown that airway changes can also happen acutely as
women go into labour leading to decreased upper airway
volume and increasing Mallampati scores in more than 60%
of parturients13,14. Additionally, in the instance that a term
pregnant woman requires a general anaesthetic, airway
instrumentation typically occurs in emergency situations,
requiring rapid sequence induction with the added metabolic
burden of the feto-placental unit. The combination of these
factors confers a high risk of peri-intubation hypoxia in this
population.
The more widespread use of regional anaesthesia for
obstetric procedures has had a two-pronged effect, that
is, the reduction in number of general anaesthetics (GA)
performed15-18 but also the subsequent diminishing of
training opportunities19. This becomes a major problem when
the anaesthetist is faced with the high-risk situation of an
emergency GA caesarean section. There are known major
complications of the failed airway including hypoxic brain
injury and fetal and/or maternal death. Despite the advocacy
for early use of supraglottic airway devices to facilitate
oxygenation in the failed intubation scenario, anaesthesiarelated maternal death still accounts for 1.6% of maternal
deaths in the USA, with failed intubation being
a leading cause20.
Pre-oxygenation
In Australia, recommended pre-oxygenation practice prior
to caesarean section is three minutes of tidal breathing via
a circle circuit delivering 100% oxygen or until the patient’s
FetO2 reaches 0.9. FetO2 is widely accepted as the surrogate
marker for lung denitrogenation21 and is the current accepted
standard according to obstetric airway management
guidelines1. A study of 20 pregnant women at term, using
the traditional method of pre-oxygenation, showed that
the mean time required to achieve an FetO2 of 0.9 was 107
+/-37 seconds but this was only achieved in 75% of the
participants after three minutes of tidal volume breathing
of 100% oxygen22. Porter et al reported retrospective data
showing frequent inadequate pre-oxygenation of patients
High flow humidified nasal oxygen in pregnant women
anaesthetised in their obstetric emergency theatre23. It would
seem that there is room for improvement in pre-oxygenation
practice and better prospective data in the obstetric
population to prevent undue risk to these patients. However,
any change in practice needs to be supported by safety and
efficacy data. In this area, any new pre-oxygenation technique
needs to obtain the threshold value for FetO2 of 0.9 in at
least the same percentage of the population compared with
current routine practice.
Apnoeic oxygenation
The physiological phenomenon of apnoeic oxygenation has
been known for more than a century with Volhard describing
it in 190824. The nomenclature for it has evolved over time
from ‘diffusion respiration’25 to ‘aventilatory mass flow’26 to
‘apnoeic oxygenation’27. They all refer to the physiological
generation of mass flow of gas from the pharynx to alveoli
as a consequence of the subatmospheric net pressure
in the alveoli. This pressure gradient develops from the
different rates by which oxygen in the alveoli moves into the
bloodstream (~200 to 250 ml/minute) and carbon dioxide in
the bloodstream moves into the alveoli (~8 to 20 ml/minute).
The remaining carbon dioxide is buffered in the bloodstream,
given its high water solubility. Alveoli will continue to take
up oxygen even without diaphragmatic movements or lung
expansion. In the clinical anaesthetic context, this process
could theoretically be useful in extending the ‘safe apnoea
time’ beyond which can be achieved by adequate preoxygenation alone. This time, which can be defined as the
duration between apnoea and critical oxygen desaturation
is typically six to eight minutes in a healthy, well preoxygenated non-pregnant adult. Apnoeic oxygenation can
be achieved via various routes with potentially different
levels of effectiveness and utility. For example, buccal oxygen
administration of 10 litres per minute via a modified Ring–
Adair–Elwyn (RAE) tube has been shown to increase apnoea
time 2.5-fold in obese patients whilst maintaining safe endtidal carbon dioxide levels. This oral technique allows normal
use of typical oral and nasal airway management adjuncts
and manoeuvres28.
Currently apnoeic oxygenation is not part of established
practice in obstetric anaesthesia. The current practice is to
pre-oxygenate the pregnant woman undergoing general
anaesthesia and once the FetO2 is 0.9, to induce general
anaesthesia. Tracheal intubation is performed without
additional oxygenation or ventilation. The new guidelines
advocate gentle bag–mask ventilation during the time between
ceasing pre-oxygenation and attempting to intubate the
trachea (the time it takes for the muscle relaxant to work).
This has been suggested so as to prevent the desaturation
that sometimes occurs in the setting of tracheal intubation in
pregnant women. Based on the work of Patel in non-pregnant
adults2 and Pillai’s computational modelling8, the use of
high flow humidified nasal oxygen during this time (apnoeic
39
Anaesth Intensive Care 2018 | 46:1
P. C. F. Tan et al
oxygenation) in pregnant women is likely to increase the time it
takes for desaturation to occur as the woman will be receiving
continuous oxygen. There are no clinical studies examining this.
Potential advantages of this technology in pregnant
women undergoing general anaesthesia
The application of high flow nasal oxygen in obstetric
anaesthesia seems relevant and logical. The technology may
improve safety in a population that has a higher incidence
of difficult airway, a higher metabolic rate and oxygen
consumption coupled with a lower respiratory reserve, by
minimising hypoxaemia and its consequences on mother
and fetus. As the relevant physiological changes in pregnancy
have their peak effects near term, it would seem most
pertinent to assess safety and efficacy of the use of this
equipment in this population.
If the physiology and physics of high flow humidified nasal
oxygenation as described can translate to clinical outcomes
(as it has in the non-obstetric population), then there may be
specific benefits for the obstetric population. As an adjunct
to current airway management practice, we propose that
these systems may provide a safety net in the higher-risk and
time-critical situation of a Category 1 (urgent threat to the
life or health of a woman or fetus) GA caesarean section and
alleviate some operator stress in this scenario by prolonging
safe apnoea time. As an alternative to current practice,
it has the added benefit of ‘hands-free’ pre-oxygenation
allowing anaesthetic staff to attend to other tasks in a timecritical situation. Once safety and clinical benefit can be
demonstrated, anaesthetists will have an evidence base for
its use across all gestations and indications. Applications may
expand from there.
Potential complications
As there is paucity of evidence, practitioners may
be understandably cautious in the use of high flow
humidified nasal oxygen in the obstetric population due
to the theoretical risks of epistaxis and gastric insufflation.
Epistaxis is a known but uncommon risk associated with
this technique29,30 but could theoretically be lessened by
the efficient humidification of gases through the Optiflow
system. The incidence of epistaxis may be more common in
the obstetric population due to increased nasopharyngeal
mucosal oedema and vascularity. Further study is needed.
Stolady et al in the United Kingdom have shown using a
manikin study that oesophageal pressures remain low with
the use of high flow nasal oxygen which suggests that it may
be safe to use despite the increased incidence of aspiration in
the obstetric population31.
Conclusion
This review discusses the known issue of the difficult
obstetric airway, current practice and shortcomings of pre-
40
oxygenation in the obstetric population and a potential
role for an alternative method of pre-oxygenation with the
added benefit of apnoeic oxygenation. Furthermore, in the
obstetric population, there is a risk that this technique may
be incorporated into current practice without appropriate
scientific testing for efficacy and safety. Therefore it is
important to perform an ethically approved, prospective
safety and efficacy study prior to the possibility of widespread
and uncontrolled adoption of the technique.
Acknowledgements
Dr Patrick Tan was awarded the Kevin McCaul Prize at the
2016 ASA National Scientific Congress for this review article.
We would like to acknowledge Mr Adrian Vendy from Fisher
& Paykel Healthcare for his assistance in obtaining the images
for this article.
Fisher & Paykel Healthcare provides equipment for the
purposes of ongoing research in our department.
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