R E V I E W Open Access
The pathophysiology of ‘happy’hypoxemia
, Eric Derom
, Eva Van Braeckel
, Pieter Depuydt
and Bart N. Lambrecht
The novel coronavirus disease 2019 (COVID-19) pandemic is a global crisis, challenging healthcare systems worldwide. Many
patients present with a remarkable disconnect in rest between profound hypoxemia yet without proportional signs of
respiratory distress (i.e. happy hypoxemia) and rapid deterioration can occur. This particular clinical presentation in COVID-19
patients contrasts with the experience of physicians usually treating critically ill patients in respiratory failure and ensuring
timely referral to the intensive care unit can, therefore, be challenging. A thorough understanding of the pathophysiological
determinants of respiratory drive and hypoxemia may promote a more complete comprehension of a patient’sclinical
presentation and management. Preserved oxygen saturation despite low partial pressure of oxygen in arterial blood samples
occur, due to leftward shift of the oxyhemoglobin dissociation curve induced by hypoxemia-driven hyperventilation as well
as possible direct viral interactions with hemoglobin. Ventilation-perfusion mismatch, ranging from shunts to alveolar dead
space ventilation, is the central hallmark and offers various therapeutic targets.
Keywords: COVID-19, SARS-CoV-2, Respiratory failure, Hypoxemia, Dyspnea, Gas exchange
Take home message
This review describes the pathophysiological abnormal-
ities in COVID-19 that might explain the disconnect be-
tween the severity of hypoxemia and the relatively mild
respiratory discomfort reported by the patients.
In early December 2019, the first cases of a pneumonia
of unknown origin were identified in Wuhan, the capital
of Hubei province in China. The pathogen responsible
for coronavirus disease 2019 (COVID-19) has been iden-
tified as a novel member of the enveloped RNA betacor-
onavirus family and named severe acute respiratory
syndrome coronavirus 2 (SARS-CoV-2), due to similar-
ities with SARS-CoV and Middle East Respiratory Syn-
drome (MERS) viruses. Although much is known about
the epidemiology and the clinical characteristics of
COVID-19, little is known about its impact on lung
pathophysiology. COVID-19 has a wide spectrum of
clinical severity, data classifies cases as mild (81%), se-
vere (14%), or critical (5%) [1–3]. Many patients present
with pronounced arterial hypoxemia yet without propor-
tional signs of respiratory distress, they not even
verbalize a sense of dyspnea [4–8]. This phenomenon is
referred as silent or ‘happy’hypoxemia. Tobin et al. re-
cently presented three cases of happy hypoxemia with
ranging between 36 and 45 mmHg in the absence
of increased alveolar ventilation (P
34 and 41 mmHg) . In patients with COVID-19, the
severity of hypoxemia is independently associated with
in-hospital mortality and can be an important predictor
that the patient is at risk of requiring admission to the
intensive care unit (ICU) [9,10]. Since correct recogni-
tion of hypoxemia has such an impact on prognosis and
timely treatment decisions, we here offer an overview of
the pathophysiological abnormalities in COVID-19 that
might explain the disconnect between hypoxemia and
patient sensation of dyspnea.
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* Correspondence: Sebastiaan.email@example.com
Department of Internal Medicine and Paediatrics, Ghent University, Corneel
Heymanslaan 10, 9000 Ghent, Belgium
Full list of author information is available at the end of the article
Dhont et al. Respiratory Research (2020) 21:198
Dyspnea as a sensation
Breathing is centrally controlled by the respiratory cen-
ter in the medulla oblongata and pons regions of the
brainstem (see Fig. 1) that control the ‘respiratory drive’
to match respiration to the metabolic demands of the
body [11,12]. The main input affecting the respiratory
drive is derived from chemical feedback among periph-
eral and central chemoreceptors. The center is, however,
also influenced by higher brain cortex, hypothalamic in-
tegrative nociception, feedback from mechanostretch re-
ceptors in muscle and lung, and metabolic rate. The
output of the respiratory center can be divided into
rhythm- (e.g. respiratory rate) and pattern generating
(e.g. depth of breathing effort) signals, and these outputs
may be controlled independently [11,13,14]. Dyspnea is
generally defined as a sensation of ‘uncomfortable, diffi-
cult, or labored’breathing and occurs, in general, when
the demand for ventilation is out of proportion to the
patient’s ability to respond. It should be distinguished
from tachypnea (rapid breathing) or hyperpnea (in-
creased ventilation). Dyspnea grading relates to whether
this feeling occurs in rest or upon exercise. This semi-
quantitative approach of scoring is best exemplified by
the frequently used modified Medical Research Council
(MRC) dyspnea scale, which categorizes dyspnea from
grade 0 (dyspnea only with strenuous exercise) to grade
4 (too dyspneic to leave house or breathless when dress-
ing) in relation to subjects of the same age [15,16].
Various sensory, pain and emotional stimuli affect the
sensation of breathing via the cerebral cortex and hypo-
thalamus [17,18]. The abnormal sense of muscle effort
is another contributor to dyspnea. Conscious awareness
of the activation of respiratory muscles is absent in
healthy breathing. However, when the respiratory mus-
cles are fatigued or weakened due to altered lung me-
chanics (e.g. decreased thoracic compliance), breathing
may be perceived as a substantial effort . Dyspnea
can also be caused by input from the mechanoreceptors
in the respiratory tract and the chest wall. Stimulation of
vagal irritant receptors (e.g. bronchoconstriction, breath-
ing through an external resistance) appears to intensify
dyspnea [12,19,20]. The contribution of metabolic rate
in modulating sense of dyspnea in critically ill patients
remains unclear, despite its well-established role during
exercise [11,21]. The best-known determinants of the
respiratory drive are the central and peripheral chemore-
ceptors. Changes in partial gas pressure of dissolved car-
bon dioxide in the blood (PaCO
) seem the most
important component, causing shifts in pH at the level
of both the peripheral and central chemoreceptors [11,
12,22]. At steady state, the arterial PaCO
by the following equation:
Fig. 1 Main inputs affecting respiratory center (RCC)
Dhont et al. Respiratory Research (2020) 21:198 Page 2 of 9
K constant (863 mmHg)
Rate of CO
Vt tidal volume
The normal response to hypercapnia (caused by increased
, hypoventilation or increased VCO
) is an increase in
respiratory drive and minute volume ventilation .
Hypoxemia itself rather plays a limited role in the sensation
of breathlessness experienced by patients with
cardiopulmonary disease on the opposite of hypercapnia that
creates per se dyspnea [12,24,25]. In healthy subjects, the
respiratory drive shifts minimally in mild hypoxemia (PaO
60–65 mmHg), such as resulting from stays at high altitude
or experimental hypoxic chambers [11,26]. Many patients
with dyspnea are not hypoxemic, while those who are,
usually experience only a slight improvement in symptoms
after hypoxemia is corrected with supplemental oxygen
therapy . When arterial P
drops below 40 mmHg,
dyspnea often occurs . Of note, the normal response to
hypoxemia is a rise in minute ventilation, primarily by
increasing tidal volume and respiratory rate. Increased
respiratory rate (tachypnea) and tidal volume (hyperpnea) -
signs of impending hypoxemic respiratory failure [11,27].
serves as one of the fundamental
regulators of cerebral blood flow. Hyperventilation causes
decreased PaCO2 which subsequently leads to arterial
vasoconstriction thus lowering cerebral blood flow and
intracranial pressure. In contrary, increase in PaCO
increased intracranial pressure ultimately leading to
deteriorating level of consciousness, altered brainstem
reflexes, and altered postural and motor responses [28,29].
At the bedside, a profound understanding of the
pathophysiological determinants of respiratory drive and
hypoxemia may promote a more complete comprehension
of a COVID-19 of a patient’s clinical presentation and timely
Happy hypoxemia in COVID-19
The disconnect between the severity of hypoxemia and
the relatively mild respiratory discomfort reported by the
COVID-19 patients contrasts with the experience of phy-
sicians usually treating critically ill patients in respiratory
failure . Guan reported dyspnea in only 18.7% of 1099
hospitalized COVID-19 patients, despite low PaO2/FiO2
ratios, abnormal CT scans (86%) and common require-
ment for supplemental oxygen (41%) . Happy or silent
hypoxemia is not exclusively seen in COVID-19, but may
also occur in patients with atelectasis, intrapulmonary
shunt (i.e. arterio-venous malformations) or right-to-left
intracardiac shunt. The adequacy of gas exchange is pri-
marily determined by the balance between pulmonary
ventilation and capillary blood flow, referred as ventila-
tion/perfusion (V/Q) matching . In the initial phase of
COVID-19, several mechanisms contribute to the devel-
opment of arterial hypoxemia (see Fig. 2), without a con-
comitant increase in work of breathing. Rapid clinical
deterioration may occur.
Changes in oxyhemoglobin dissociation curve
Oxygen saturation measured by pulse oximetry
) is often used to detect hypoxemia. However,
should be interpreted with caution in COVID-
19. The sigmoid shaped oxyhemoglobin dissociation
curve seems to shift to the left, due to induced re-
spiratory alkalosis (drop in PaCO
) because of
hypoxemia-driven tachypnea and hyperpnea. During
hypocapnic periods, the affinity of hemoglobin for
oxygen and thus oxygen saturation increases for a
given degree of PaO
, explaining why SpO
well-preserved in the face of a profoundly low PaO
[33–35]. This finding is also seen in high altitude
hypoxemia, in which hypocapnia significantly shifts
the oxygen-hemoglobin dissociation curve and im-
proves blood oxygen saturation . The alveolar gas
equation also predicts that hyperventilation and the
resulting drop in the alveolar partial pressure of
carbon dioxide leads to an increase in the alveolar
partial pressure of oxygen and ultimately lead to an
increase in SpO
There might also be a biological explanation for the
leftward shift of the curve in COVID-19. Liu et al. put
forward hypotheses about direct viral interaction with
the heme group of hemoglobin. According to this the-
ory, heme serum levels are increasing in COVID-19
along with harmful iron ions (Fe
) causing inflamma-
tion and cell death (ferroptosis). This leads to the pro-
duction of large amounts of serum ferritin to bind these
free irons in order to reduce tissue damage . In con-
should be interpreted in the light of the
presence of hyperventilation (tachypnea, low P
and, if possible, P
via arterial puncture. Measuring
the alveolar to arterial oxygen (P(A-a)O
mmHg - PaCO2/0.8 - P
at sea level) and relating this
value to age and supplemental oxygen (age/4 + 4 + 50
–0.21) in mmHg) can be insightful. This can be
performed rapidly on a smartphone app . The P(A-
gradient is increased either by V/Q mismatch or by
intrapulmonary shunting. Hypoxemia due to V/Q
Dhont et al. Respiratory Research (2020) 21:198 Page 3 of 9
mismatch can be easily corrected by supplemental oxy-
gen therapy whereas pulmonary shunts have a poor re-
sponse to oxygen therapy .
Causes of hypoxemia in COVID-19
Arterial hypoxemia early in SARS-CoV-2 infection is
primarily caused by V/Q mismatch and thus persistence
of pulmonary arterial blood flow to non-ventilated al-
veoli, reflected by a marked increase in P(A-a)O
ent. The infection leads to a modest local interstitial
edema, particularly located at the interface between lung
structures with different elastic properties, where stress
and strain are concentrated . Due to increased lung
edema (leading to ground-glass opacities and consolida-
tion on chest imaging), loss of surfactant and superim-
posed pressure, alveolar collapse ensues and a
substantial fraction of the cardiac output is perfusing
non-aerated lung tissue, resulting in intrapulmonary
shunting . As previously discussed, tidal volume in-
creases during the disease course leading to rising nega-
tive inspiratory intrathoracic pressure. The latter, in
combination with increased lung permeability due to in-
flammation, will eventually result in progressive edema,
alveolar flooding, and patient self-inflicted lung injury
(P-SILI), as first described by Barach in 1938 [11,40,
41]. Over time, the increased edema will further enhance
lung weight, alveolar collapse, and dependent atelectasis,
resulting in progressively increasing shunt fraction and
further decline of oxygenation which cannot completely
be corrected by increasing F
Loss of lung perfusion regulation
The persistence of high pulmonary blood flow to non-
aerated lung alveoli appears to be caused by the relative
failure of the hypoxic pulmonary vasoconstriction mech-
anism (constriction of small intrapulmonary arteries in re-
sponse to alveolar hypoxia) during SARS-CoV-2 infection,
as recently illustrated by Lang et al. using dual-energy CT
[42,43]. Whether the latter mechanism is only triggered
by the release of endogenous vasodilator prostaglandins,
bradykinin, and cytokines associated with the inflamma-
tory process or also by other yet undefined mechanisms
remains to be investigated [33,44,45]. Vasoplegia also
seems to be influential in the loss of lung perfusion regula-
tion, possibly induced by shear stress on the interfaces
between lung structures, as part of the P-SILI spectrum
[45–47]. Further, dysregulation of the renin-angiotensin
system (RAS) contribute to the pathophysiology of
COVID-19 [48–52]. Angiotensin-converting enzyme 2
(ACE2) is the principal functional receptor used by SARS-
CoV-2 for cell entry, implying ACE2 internalization [52–
54]. ACE2 converts angiotensin II (Ang II) to angiotensin
1–7(Ang1–7) and is also important for degrading brady-
kinin. Hence, diminished levels of ACE2 lead to an in-
crease in Ang II, mediating pulmonary vasoconstriction
through agonism at Ang II-receptor, while Ang 1–7op-
poses the actions of Ang II [50,51,54]. Recently, Liu et al.
revealed that serum Ang II levels were linearly associated
with viral load and lung injury in COVID-19 .
Endothelial injury is emerging as a central hallmark of
COVID-19 pathogenesis, and the cytopathic virus can
Fig. 2 Mechanisms of hypoxemia in COVID-19
Dhont et al. Respiratory Research (2020) 21:198 Page 4 of 9
directly infect lung capillary endothelial cells that ex-
press ACE2 [54,56]. Intravascular microthrombi are the
net result of an imbalance between procoagulant and fi-
brinolytic activity in the presence of acute inflammation
and endothelial injury [45,57–59]. The pro-coagulant
activity might result from complement system-mediated
activation of clotting, similar to some forms of throm-
botic microangiopathy (TMA), or could be due to inhib-
ition of plasminogen activation and fibrinolysis via
increased activity of plasminogen activator inhibitor
(PAI-1 and -2) which are induced as acute-phase pro-
teins under the influence of IL-6. Diffuse intravascular
coagulation (DIC) is also seen in patients with severe
COVID-19, mediated via endothelial release of tissue
factor and activation of clotting factor VII and XI. Many
patients with COVID-19 develop elevated D-dimers sug-
gesting the formation of blood clots. D-dimer levels on
admission are used to predict in-hospital mortality in
COVID-19, and DIC present much more frequently
(71%) in COVID-19 patients with a dismal prognosis,
versus only 0,6% of survivors [60–63]. Autopsy of the
lungs after severe disease showed fibrin deposition, dif-
fuse alveolar damage, vascular wall thickening, and fre-
quently occurring complement-rich microthrombi
occluding lung capillaries and larger thrombi causing
pulmonary artery thrombosis and embolism [63–65].
Hypercoagulable state leads to further deterioration in
V/Q mismatch and lung tissue damage. Moreover, co-
agulation is also modulated by activating C-reactive pro-
tein and ensuing complement activation and hepatic
synthesis of fibrinogen as an acute phase protein in
Impaired diffusion capacity
Lung diffusion capacity (DLCO) can be impaired,
although pure diffusion defects are rarely a cause for
gradient at rest [67,68]. SARS-CoV-
2 propagates within alveolar type II cells, where a large
number of viral particles will be produced and released,
followed by immune response mediated destruction of
infected cells (virus-linked pyroptosis) . Loss of al-
veolar epithelial cells and a pro-coagulant state cause the
denuded basement membrane to be covered with debris,
consisting of fibrin, dead cells, and complement activa-
tion products, collectively referred to as hyaline mem-
branes [54,69]. With incremental exercise and in the
face of absent hypoxic vasoconstriction in COVID-19, a
hyperdynamic pulmonary circulation might not allow
sufficient time for red blood cells to equilibrate their
oxygen uptake. A diffusion limitation may, therefore,
occur in COVID-19 leading to a raised P(A-a)O
ent and exercise-induced arterial hypoxemia (EIAH)
. Recently, Xiaoneng Mo et al. confirmed a decrease
in DLCO in COVID-19 patients at the time of discharge.
The prevalence of impaired diffusing-capacity was linked
to the severity of disease, respectively 30.4% in mild ill-
ness, 42.4% in pneumonia and 84.2% in severe pneumo-
nia . Long-term studies are needed to address
whether these deficits are persistent as seen in MERS
where 37% of MERS survivors still presented with an im-
pairment of DLCO .
Preservation of lung mechanics
The outline presented in the previous paragraphs largely
clarifies the dissociation between the severity of
hypoxemia in COVID-19 and relatively well-preserved
lung mechanics. Gas exchange abnormalities in some
patients with COVID-19 occur earlier than increases in
mechanical loads . During the first days of infection,
there is no increased airway resistance, and there is pre-
sumably no increased anatomical or physiological dead
space ventilation. The breathing effort also remains ra-
ther low because lung compliance is normal in many pa-
tients without pre-existing lung disease. As recently
shown by Gattinoni et al. in a cohort of 16 critically ill
patients, relatively normal values for respiratory system
compliance (50.2 ± 14.3 ml/cmH2O) went hand in hand
with a dramatically increased shunt fraction of 0.50 ±
0.11 . Such a wide discrepancy is highly unusual for
most forms of disorders that lead to acute lung injury
and ARDS [47,71]. Relatively high compliance indicates
a well-preserved lung gas volume and explains in part
the absence of dyspnea early in the course of illness [42,
47,61,72,73]. In contrast, Ziehr et al. described a low
compliance and a uniform presentation consistent with
the Berlin definition for ARDS in a cohort of COVID-19
patients [30,70]. Of note, patients on mechanical venti-
lation have the highest COVID-19 severity and thus
probably the lowest respiratory system compliance. Dys-
pnea itself may have precipitated mechanical ventilation,
and the latter may be a surrogate marker for low com-
pliance in COVID-19 . Understanding of the respira-
tory mechanics found in COVID-19 will continue to
evolve as further research is reported.
Hypoxemia-driven tachypnoea, hyperpnea and altered
oxygenation predict clinical deterioration induced by
either disease severity and/or host response and/or
suboptimal management [35,61]. As the disease
progresses, the more consolidated air spaces do not
inflate as easily at higher transpulmonary pressures. The
volume loss is proportionally greater at higher lung
volumes. This loss of volume reduces total lung
compliance and increases the work of breathing .
There is also evidence that the dynamic compliance of
the remaining ventilated lung is reduced in SARS-CoV-2
pneumonia (as seen in pneumococcal pneumonia) most
Dhont et al. Respiratory Research (2020) 21:198 Page 5 of 9
possibly by a reduction in surfactant activity, further in-
creasing the work of breathing . Physiological dead
space is also increasing due to reduced blood flow
caused by intravascular thrombi. Importantly, the anx-
iety experienced by COVID-19 patients also affects the
cortical feedback to the respiratory centers. Conse-
quently, as the disease progresses, dyspnea becomes in-
Thoughts on management
At the stage that COVID-19 patients are admitted to the
hospital with hypoxemia, viral replication is well under-
way and in addition to giving antiviral medication,
optimization of the V/Q mismatch and reduction of
cytokine storm remain the major therapeutic goals. Re-
garding perfusion, avoiding microthrombi and ongoing
fibrin deposition is one of the therapeutic strategies. It
seems prudent to use thromboprophylaxis in all
COVID-19 patients, particularly in those with high D-
dimers on admission [59,66,74]. Moore et al. recently
suggested the use of tissue plasminogen activator (tPA)
to treat ARDS in COVID-19 . In addition, tackling
the systemic prothrombotic complication using anti-
inflammatory medications (such as anti-IL6R toci-
lizumab or sarilumab, or the anti-IL6 antibody siltuxi-
mab or complement inhibiting strategies) to prevent
macro- and microthrombi represents another potential
approach and several trials are currently verifying this
hypothesis . Effective hypoxic pulmonary vasocon-
striction may be another target to improve the matching
of regional perfusion and ventilation in the lung. There
is an excessive release of inflammatory mediators that
disturbs the balance between nitric oxide (NO),
endothelin, and prostanoids in the pulmonary capillaries
, although inhaled NO has consistently failed to
show an improvement in mortality in ARDS [53–55,84].
RAS modulation (e.g. angiotensin receptor blockers,
recombinant soluble ACE2, and inhibition of the brady-
kinin system) may have a potential role in restoring lung
perfusion regulation, and trials are ongoing [50–52].
Regarding ventilation, supplemental oxygen is the first
step in facilitating oxygenation. In patients with refractory
hypoxemic respiratory failure (increasing shunt fraction),
timely but not premature intubation and invasive
ventilation support may be superior to non-invasive venti-
lation in boosting transpulmonary pressure, opening col-
lapsed alveoli, improving oxygenation, decreasing oxygen
debt, avoiding P-SILI and offering a better chance for the
lungs to heal [61,76,77]. Considering the critical cases,
most patients fulfil the Berlin criteria of ARDS, where
lung-protective ventilation, prone ventilation, effective
sedation and analgesia, and high positive end-expiratory
pressure (PEEP) are key [54,61,70,77]. COVID-19 pa-
tients are exquisitely PEEP sensitive [78–80]. Tolerance
for modest permissive hypercapnia minimizes ventilator-
induced lung injury (VILI) . Since prone position re-
cruits the dorsal lung regions and diverts blood flow to
these caudal regions, it may have particular importance in
COVID-19 when used early and in relatively long sessions
. Although further trials are needed to evaluate the im-
pact on disease severity and mortality, several authors
confirmed that awake proning can improve oxygenation
in COVID-19 [81–83].
The remarkable dissociation between profound hypoxemic
respiratory failure and a clinically ‘happy’patient is frequently
seen and should prompt physicians and health care workers
not only to rely on the patient’s apparent wellbeing but
closely monitor respiratory rate, signs of hyperventilation,
oxygen saturation and invasive measurements of hypoxemia/
hypocapnia at regular time intervals. Pulse oximetry should
be interpreted with caution, because left-sided shifting of the
oxyhemoglobin dissociation curve. The arterial hypoxemia is
induced by intrapulmonary shunting, dysregulated hypoxic
pulmonary vasoconstriction, impaired lung diffusion, and
formation of intravascular microthrombi. As in the first days
of the disease, the lung mechanics are well-preserved and
there is no increased airway resistance or dead space ventila-
tion. The respiratory center thus does not sense an uncom-
fortable sensation of breathing. However, sudden and rapid
respiratory decompensation may occur, and tachypnea and
hyperpnea might be the most important clinical warning
signs of impending respiratory failure in COVID-19 patients.
COVID-19: Coronavirus Disease 2019; SARS-CoV-2: Severe Acute Respiratory
Syndrome Coronavirus 2; MERS: Middle East Respiratory Syndrome;
ICU: Intensive Care Unit; PaCO
: Partial gas pressure of dissolved carbon
dioxide in the blood; PaO
: Partial gas pressure of dissolved oxygen in the
: Oxygen saturation measured by pulse oximetry; P(A-a)O
gradient: the alveolar to arterial oxygen gradient; VCO2: Rate of CO2
production; VE: Minute ventilation; VD: Dead space; Vt: Tidal volume; V/
Q: Ventilation/Perfusion; F
: Fraction of inspired oxygen; P-SILI: Patient self-
inflicted lung injury; VILI: Ventilator-induced lung injury; RAS: Renin-
angiotensin system; ACE2: Angiotensin-converting enzyme 2;
Ang: Angiotensin; TMA: Thrombotic microangiopathy; PAI: Plasminogen
activator inhibitor; DIC: Diffuse intravascular coagulation; DLCO: Lung
diffusion capacity; NO: Nitric oxide; PEEP: Positive end-expiratory pressure
SD, ED and BNL wrote the manuscript. BNL, EVB, PD and ED read and
corrected were needed. All authors took part in the discussion leading up to
the manuscript. The authors read and approved the final manuscript.
Availability of data and materials
Ethics approval and consent to participate
Consent for publication
Dhont et al. Respiratory Research (2020) 21:198 Page 6 of 9
The authors declare that they have no competing interests.
Department of Internal Medicine and Paediatrics, Ghent University, Corneel
Heymanslaan 10, 9000 Ghent, Belgium.
Department of Respiratory Medicine,
Ghent University Hospital, Ghent, Belgium.
Department of Intensive Care
Medicine, Ghent University Hospital, Ghent, Belgium.
VIB-UGent Center for
Inflammation Research, Ghent, Belgium.
Received: 4 June 2020 Accepted: 21 July 2020
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