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A physiology-based perspective on high-flow nasal cannula oxygen delivery in
the critically ill patient
Gustavo A Cortes-Puentes, MD, Richard A Oeckler MD, PhD
Correspondence to
Richard A Oeckler
Email:
Oeckler.richard@mayo.edu
+ Author Affiliation - Author Affiliation
aAndres Yepes-Hurtado and Khalid Monzer were fellows in Pulmonary and Critical Care Medicine at Texas Tech University Health Sciences Center, Lubbock, TX.
bSabry Omar was a resident in Internal Medicine at TTUHSC in Lubbock, TX.
cFaculty members in Pulmonary and Critical Care Medicine at TTUHSC in Lubbock, TX.
SWRCCC 2016;4(16):
doi: 10.12746/swrccc2016.0415.195
...................................................................................................................................................................................................................................................................................................................................
The heterogeneous lung injury pattern seen in hypoxic respiratory failure due to
the acute respiratory distress syndrome (ARDS) is both a cause and effect of
altered pulmonary mechanics and gas exchange.1 In an ideal world, an
appropriately timed, non-invasive oxygen delivery method, such as non-invasive
positive airway pressure ventilation (NIV) or high-flow nasal cannula (HFNC),
would not only compensate for these deficits, but also mitigate the negative and
additive effect of air hunger upon respiratory drive and the risk for
ventilator-induced lung injury (VILI) in the already compromised respiratory
system1,2 Low-tidal volume ventilation is a cornerstone of a lung
protective ventilation strategy precisely for these reasons, and has been shown
to reduce mortality. Although not established for spontaneously breathing
patients, the available literature3,4 supports a conservative tidal
volume strategy, even for patients without ARDS3, and especially for
those who are young and more likely to generate large tidal volume (VT).4
Yet with HFNC clinicians lose the opportunity to estimate or control tidal
volume, thus surrendering a key parameter for targeting lung strain and stress,
minimizing cycling frequency of shear forces, and preventing VILI.
In contrast to passive mechanical ventilation, spontaneous breathing necessarily
requires the development of negative pleural pressure (PPL).5
Thus for any given tidal volume, transpulmonary pressure (PTP;
defined as alveolar minus pleural pressure) will be larger. Theoretically, this
increased distending pressure could facilitate the recruitment of dependent lung
units throughout the tidal cycle3, improving compliance and reducing
work of breathing. This would seem to argue for spontaneous breathing as a
potential recruitment tool, allowing a larger number of functional lung units to
be exposed to a given VT, and therefore against the potential harm of
high VT during spontaneous breathing, as may occur under HFNC.
The delivery of uncontrolled and disproportioned VT relative to the
heterogeneous “baby lung” coincides with large local changes in transpulmonary
pressure and harmful lung strains1 compounded by interdependence.6
Very often, clinicians face the dilemma of whether to tolerate high VT
while the patient’s work of breathing remains increased in the absence of
positive pressure NIV. Recently Protti and Gattinoni et al have linked high
strain rates with an increased risk of pulmonary edema by augmented lung
viscoelastic behavior (parenchymal energy dissipation), and posit that this
might also explain why large strains injure healthy lungs.7 In
principle, these findings suggest that selecting strain and strain rates that
produce small dynamic true driving
pressure8 changes (tidal
changes in PTP) is not feasible when using HFNC.
A salient study regarding the use of HFNC in acute hypoxic respiratory failure
reported a significant difference in favor of oxygen delivery by HFNC in 90-day
mortality; yet when compared to standard oxygen delivery or NIV, the use of HFNC
did not result in a significantly different intubation rate.9This may
in part be due to a lack of criteria or guidelines for the determination of
treatment failure, and lack of clear recommendations for when to escalate
therapy to endotracheal intubation, heavy sedation, and paralysis to take
control of work of breathing and oxygen demand. Furthermore, the ability of HFNC
to augment work of breathing and O2 delivery is presumed to be at least
partially mediated by positive end-expiratory pressure (PEEP), yet the ability
for HFNC to generate PEEP at the level of the alveolus remains poorly understood
and highly controversial. For instance Parke et al. found a positive correlation
(~10L/min = ~1.2 cmH2O) between HFNC flow rate and nasopharyngeal
PEEP10 , but patients receiving enough flow (60L/min) to generate the
equivalent of 5cm H2O or more by NIV under this hypothesis in reality had
significantly lower PaO2 than the NIV group for a given FiO2.11 It is
also rare, at least at our institution, to see chin straps to prevent flow (and
pressure loss) through the oropharynx employed on a regular basis. In total, the
effects of HFNC on alveolar PEEP are likely variable at best. We do know,
however, that distally measured airway pressures within closed circuits of
mechanically ventilated patients may correlate poorly with actual lung stress
under commonly encountered clinical scenarios (e.g., intra-abdominal
hypertension, asymmetric lung injury 12). Thereby, nasopharyngeal
PEEP levels generated by HFNC most likely cannot compensate under these
conditions, especially with a variably open and closed circuit interface, i.e.
the patient’s oropharynx. Although the severity of lung injury may be the major
predictor of success for HFNC and/or NIV strategies4,9 , other
parameters such as body habitus (e.g., severe obesity) and reduced chest wall
compliance (e.g., intra-abdominal hypertension), should be factored when
deciding between transitory oxygen delivery via HFNC vs. early appropriate
intubation.
In conclusion, HFNC is an attractive option for oxygen delivery in the patient
with non-hypercapnic hypoxemic respiratory failure. Although the mechanism is
elusive, improvements in work of breathing, oxygenation, and outcome reported in
highly selected patient populations warrant further investigation. In
appropriate patients treated with HFNC, we recommend close observation with
pre-determined criteria for therapeutic failure and escalation to minimize
driving pressure, assure adequate oxygenation, and prevent VILI.
Key words- inferior vena cava, volume status, acute kidney injury, ultrasonography
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M, McKibben A, et al. Recruitment and derecruitment during acute respiratory
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FD, Barbas
CS, et al. Association between tidal volume size, duration of ventilation,
and sedation needs in patients without acute respiratory distress syndrome: an
individual patient data meta-analysis.
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KE, Adams
AB, et al. Value and limitations of transpulmonary pressure calculations
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S, Thille
AW, et al. High-flow nasal cannula oxygen in respiratory failure.
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JC, Adams
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...................................................................................................................................................................................................................................................................................................................................
Received: 09/23/2016
Accepted: 09/26/2016
Reviewers: Vaqar Ahmed MD
Published electronically: 10/15/2016
Conflict of Interest Disclosures: none
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