Dq wk 5/2

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Assessment Description

Mrs. P. has been in the ICU for several days, has made gradual progression, and appears to be doing well with laboratory findings and arterial blood gases indicating normal readings. The enteral feeds were held overnight for anticipation of extubation. Describe the process for weaning the patient from the ventilator and discuss when it is appropriate to remove ventilator support as the patient has improved. What are the risks to monitor for as this process is implemented for the patient? Support your summary and recommendations plan with a minimum of two APRN approved scholarly resources.

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Intubation and Airway Support

Bisan A. Salhi, MD

Todd A. Taylor, MD

Douglas S. Ander, MD

Airway management can significantly affect outcomes for hospitalized critically ill
patients. Failure to deliver adequate oxygen may cause irreversible brain damage or
preclude successful resuscitation. Options for management may range from assisted
ventilation with a bag-valve-mask (BVM) to noninvasive ventilation (NIV) support to
endotracheal intubation (Table 121-1). A successful outcome in any intubation demands
proficiency in patient assessment, knowledge of the equipment (basic and advanced),
requisite technical skills, appreciation of individual limitations, and an alternative plan to
deal with the difficult or failed airway.

TABLE 121-1 Overview of Emergency Airway Management

Technique Description Notes
Rapid Sequence
Intubation (RSI)

Defined by the
administration of a

Avoids insufflation of the stomach

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sedative and paralytic
agent to assist in
endotracheal intubation,
usually via direct

Minimizes risk of aspiration with
assisted BVM ventilation

Bag-Valve-Mask (BVM)

The ability to ventilate a
patient can be an
effective bridge prior to
intubation and is a
requirement prior to use
of any paralytic agents

Prior to ventilating with a BVM,
place an airway adjunct to
maintain patent airway and to
optimize ventilation:
• Nasopharyngeal airway if

patient’s airway (gag) reflexes

• Oropharyngeal airway if absent
airway reflexes

If patient has dentures, they
should be left in place during BVM
ventilation and removed just prior
to insertion of laryngoscope
If the operator is having problems
maintaining a seal or ventilating,
two-hand BVM should be

Endotracheal Intubation Airway control
established usually
through direct
laryngoscopy and
orotracheal intubation

Any operator attempting
intubation, particularly if using
paralytic agents, should be very
comfortable with the technique,
equipment, rescue devices, and
with other resources for
assistance, have a plan to address
any contingency

A small survey published in 2010 noted that individual hospitalists (n = 175)
performed, on average, only 10 endotracheal intubations in the previous year with a range
of 3 to 20. For those performing endotracheal intubation, it is important to maintain this
essential skill, and to be aware of their own practices and skill limitations. Depending on
their clinical environment and work setting, the expectations for different hospitalists in
advanced airway management will vary. However, all hospitalists should be versed in
initial airway management and stabilization, including effective use of oral and nasal
airway and BVM devices.

Successful intubation requires not only knowledge of the basic procedural steps, but
also knowledge of airway anatomy, landmarks, and locations of various airway structures
relative to each other.


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All indications for endotracheal intubation can be classified as (1) failure to maintain a
patent airway, (2) failure of oxygenation and/or ventilation, and (3) anticipation of a
rapidly deteriorating clinical course (Table 121-2).

TABLE 121-2 Indications for Intubation

Indication Suggestive Signs Comments
Failure to maintain a
patent airway

• Inability to phonate
• Inability to swallow or handle

• High risk of aspiration

• The presence/absence
of a gag reflex does
NOT effectively assess
airway patency

• The gag reflex is
physiologically absent
in 20% of normal adults

• Stimulation of the gag
reflex increases risk of

Failure to oxygenate or

• Unresponsiveness to noninvasive
oxygenation or ventilation

• Assess patient’s clinical
appearance including
vital signs, mentation

• Monitor oxygenation
with continuous pulse
oximetry and/or ABG

• Monitor ventilation with
capnography, ABG, or
VBG analysis.

Anticipate deterioration in
clinical condition

• Patient must be unaccompanied
for testing

• Patient unable to maintain
current work of breathing

• Likely further studies or surgery

• Consider clinical factors
such as severe
metabolic acidosis with
inadequate respiratory
weakness (impaired
maximal inspiratory
pressure); etc

A difficult airway refers to complex or challenging BVM or endotracheal intubation.
Difficult oxygenation is the inability to maintain the oxygen saturation >90% despite using
a BVM and 100% oxygen. A failed airway refers to the inability to either ventilate or
intubate a patient after three intubation attempts by the same operator. A higher rate of
poor clinical outcomes occurs when the airway is managed as an emergent (rather than
elective) procedure. In addition, an increased number of airway attempts predicts poorer

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outcomes; thus, a backup plan is necessary if initial intubation attempts are not
successfully executed. The LEMON rule is one popular rule for assessment for difficult
intubation (Table 121-3).

TABLE 121-3 Assessment for Difficult Intubation (Mnemonic: LEMON)

Look Injury, large incisors, large tongue, beard, receding
mandible, obesity, abnormal face or neck pathology
or shape

Evaluate the 3-3-2 rule Mouth opening < 3 fingers, mandible length < 3
fingers, or larynx to floor of the mandible < 2 fingers

Mallampati Class III (see base of uvula) and class IV (soft palate
is not visible)

Obstruction Any upper airway pathology that causes an
obstruction (abscess, edema, masses, epiglottitis)

Neck mobility Limited mobility of neck (eg, trauma with cervical
spine immobilization, arthritis, congenital defect)

The most important skill required for inpatient clinicians in airway management is use of
a BVM and airway adjuncts to ventilate and oxygenate the patient. BVM ventilation can
effectively maintain airway patency while an alternative plan is developed and
implemented. However, patients with a high risk of failing BVM ventilation may require
more rapid and definitive airway evaluation and management. Predictors of difficult BVM
are summarized in Table 121-4.

TABLE 121-4 Assessment for Difficulty with Bag-Valve-Mask (BVM) Ventilation
(Mnemonic: MOANS)

Mask seal Inadequate mask seal (beard, blood/emesis, facial
trauma, operator small hands)

Obesity BMI > 26 kg/m2

Age >55 y
No teeth No teeth (impairs BVM effectiveness)
Stiff ventilation Asthma, COPD, ARDS, term pregnancy

ARDS, acute respiratory distress syndrome; BMI, body mass index; BVM, Bag-valve-mask; COPD,
chronic obstructive pulmonary disease.

Rapid sequence intubation (RSI) is now the predominant and preferred method in
managing the emergent airway, precluding the apneic patient requiring a crash airway (ie,

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cardiac or respiratory arrest). Rapid sequence intubation is defined as the simultaneous
administration of a sedative and paralytic agent to assist in endotracheal intubation,
usually via direct laryngoscopy (Table 121-5). Central to the concept of RSI is the
avoidance of assisted BVM ventilation to avoid insufflation of the stomach and minimize
the risk of aspiration. Outcomes evidence supports RSI as a safe and effective technique
for emergency airway management that maximizes the patient and physician likelihood of
timely, successful airway management (Table 121-6).

TABLE 121-5 Equipment for Endotracheal Intubation

Endotracheal tubes (assortment of sizes) with stylet
Intubation blade (direct or video)
Suction with Yankuer tip
Airway adjuncts (oral and nasal)
Confirmation device (end-tidal CO2 detector)

TABLE 121-6 Procedural Steps of Rapid Sequence Intubation (RSI)

Assessment of the
airway, adequate IV
access, continuous
oxygenation monitoring,
RSI medications (sedative
and paralytic)

Laryngoscope with
functioning light & blades of
multiple sizes,
working suction, oxygen,
Medications (code cart
Backup airway devices, BVM,
Monitors (telemetry, pulse
oximetry, BP)

Medical Team
Engage team of appropriately
trained staff; backup nearby,
Utilize the assistance of
respiratory therapists early,
Call for help early

Anticipate a difficult
airway (a complex or
challenging intubation)
with a backup plan such
as fiberoptic intubation

Risk Factors
Pierre Robin syndrome, Down
syndrome, anterior epiglottis
Ludwig angina, abscess,
RA, AS, scleroderma,
temporomandibular joint

Look for injury, large incisors,
large tongue, receding
mandible, obesity, abnormal
face or neck pathology or
Evaluate the 3-3-2 rule
(LEMON rule)
Mouth opening <3 fingers
Mandible length <3 fingers

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Adenomas, goiter, lipoma,
hygroma, carcinoma tongue,
larynx, thyroid;
Facial injury, cervical spine
Obesity, acromegaly;
Active burns, inhalation injury;
Subglottic stenosis

Larynx to floor of mandible <2
Class III (see base of uvula)
Class IV (soft palate not
Any upper airway pathology
that causes an obstruction
Neck Mobility
Limited neck mobility

Assess difficulty with
bag-valve-mask (BVM)

Five Independent Risk
Factors (MOANS):

1. Inadequate mask seal
(beard, blood, emesis,
facial trauma, operator
small hands)

2. Obesity (BMI > 26

3. Age > 55 y
4. Absence of teeth
5. Stiff ventilation (asthma,

COPD, ARDS, term

Difficult oxygenation—the
inability to maintain oxygen
saturation >90% despite BVM
and 100% oxygen
RSI by trained operators
preferred, but other techniques
and backup methods should
be considered if difficulty with
BVM is predicted

Preoxygenation 100% supplemental oxygen to
induce nitrogen washout and
maximize time for intubation
without oxygen desaturation

Patients will have 7-9 min
prior to desaturation in
normal, healthy adult; less
time in ill patient with
comorbidity or critically ill

Premedication (Optional) Administration of drugs 3-5
min before induction and

To blunt effects of direct
laryngoscopy, including
bronchospasm and a strong
sympathetic response. This
step is often omitted

Sedatives for induction;
paralytics for intubation
Sedative regimen should
provide reliable amnesia;
paralytics ↓ metabolic
demands, ↓CO2
production, ↑chest

…Onset 45-60 s for 5-10 min
…Onset 45-60 s for 5-10 min
…Short-acting, allows frequent
monitoring of neurologic
Midazolam (Versed)

Side Effects
Minimal hypotension,
possible adrenal insufficiency
Hypotension, depresses
myocardial contractility;
↑triglycerides, pancreatitis
Midazolam (Versed)

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Unless contraindicated,
commonly etomidate for
sedation and
succinylcholine for

…Often used in combination
with fentanyl bolus
…Anticonvulsant properties
…Used to reduce pain from
laryngoscopy, not always
…Higher potency, faster onset,
shorter duration than

Less hypotension than
delirium, slower onset,
respiratory depression, long
half-life; contraindications:
narrow-angle glaucoma
Respiratory depression,
contraindications: end-stage
liver disease, severe
respiratory disease if not
Bradycardia, ↑ICP, histamine
release; contraindications:

…First-line for RSI outside of
…Rapid onset, short acting
…Alternative to
…Rapid onset, minimal CV

…Hyperkalemia (ESRD,
rhabdomyolysis, burns >10%
BSA, crush injury)
…Neurologic (stroke, spinal
cord injury, ALS, MS,↑ICP,
history of malignant
hyperthermia, eye injury)
…Prolonged immobility >48-72
Caution with difficult airway:
longer acting than

Proper Positioning to
Optimize Visualization of
Vocal Cords

Place a folded towel under the
occiput to raise head by ~ 3-7
This “sniffing” position lines
up the oral, pharyngeal, and
laryngeal axes, thereby
optimizing the view of the
cords during laryngoscopy

Visualize the arytenoids and
the vocal cords prior to
insertion of endotracheal tube
(ETT) by elevating epiglottis
which lies just above larynx
and vocal cords

Placement of ETT
Typically advanced to 23
cm marker at the lip of
adult male, 21 cm adult

Multiple methods to confirm
correct placement:
…Condensation of ETT
…Bilateral breath sounds
…Absence of breath sounds
over epigastrium
…End-tidal CO2 detection

Gold standard for confirming
appropriate tube placement:
use of end-tidal carbon
dioxide detection

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…CXR (ETT tube 2-3 cm above

Postintubation Care Proper stabilization of ETT to
prevent movement or
accidental dislodgment.
Patients who are medically
paralyzed require sedation
and pain control

Placement of a nasogastric or
orogastric tube can help
decompress any insufflated
air that occurred during BVM
use and will help ↓risk of
emesis and aspiration

In general, RSI is safer and more successful than awake intubation. However, RSI is not
advised if difficulty with BVM is predicted or the ability to intubate via direct laryngoscopy
is in question (eg, upper airway obstruction, stridor, angioedema, head and neck cancers).
In select patients in which awake intubation is indicated, it should be approached with
caution, and may require backup rescue airway methods and/or the involvement of
consultants (eg, anesthesiology or otolaryngology).


TABLE 121-7 Endotracheal Intubation Complications

Directly Related to Laryngoscopy Notes
Hemodynamic changes including
hypertension, hypotension,
tachycardia, and bradycardia

A pneumothorax needs to be considered in a patient
with hypoxia and hypotension and should be
evaluated for all patients with postprocedure chest

Hypoxemia Routine preoxygenation with high flow oxygen via
non-rebreather mask is standard in healthy,
nonobese adults. Consider using noninvasive
ventilation in critically ill patients with ill patient with
compromised lungs or abnormal body habitus.
Consider apneic oxygenation

Airway trauma/perforation
Laryngospasm and bronchospasm
Trauma to teeth, lips, and tongue

Proper technique is essential to avoid any local
trauma to oral anatomy and airway structures

Right mainstem bronchus

Evaluate with postprocedure chest radiography

Raised intracranial and intraocular

Unclear clinical significance

Esophageal intubation Prompt recognition of an esophageal intubation will
allow immediate removal of the ETT and
reventilation and oxygenation with a BVM prior to
reattempting intubation

Failed intubation Clinicians should assess patients for a difficult

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airway in an effort to prevent a failed intubation
attempt. When a difficult intubation is predicted,
consultation should be initiated early and backup
equipment should be readily available

Related to Endotracheal Intubation
Tension pneumothorax A pneumothorax needs to be considered in a patient

with hypoxia and hypotension and should be
evaluated for all patients with postprocedure chest

Aspiration Placement of a nasogastric (NG) or orogastric (OG)
tube following endotracheal intubation can help
decompress any insufflated air that occurred during
BVM use and will help reduce the risk of emesis and

Obstruction of endotracheal tube Suction the endotracheal tube
Accidental extubation Accidental dislodgment of the ETT should be

avoided by proper stabilization of the tube with
appropriate sedation of the patient

Various complications can occur during the course of accessing an advanced airway in a


TABLE 121-8 Contraindications to Endotracheal Intubation

Absolute Total airway obstruction

(eg, angioedema)
Total loss of facial or
oropharyngeal landmarks
(eg, blunt or penetrating
trauma to the face)

During cardiac or respiratory arrest,
oxygenation and ventilation are of
paramount importance, and therefore the
use of BVM, intubation, or both should be
attempted despite any contraindications. In
these patients it is advised to perform an
early cricothyrotomy as endotracheal
intubation will be extremely difficult

Relative Anticipated difficult

If a difficult airway is anticipated early
consultation is strongly advised. Other
options include awake intubation, video
laryngoscopy or use of the difficult airway

Contraindications to endotracheal intubation can be divided into either absolute or relative
but these need to be tailored to the specific clinical situation.

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TABLE 121-9 Noninvasive Positive Pressure Ventilation (NIPPV)

Indications Contraindications Notes
COPD (moderate
to severe
Acute CHF
Pneumonia in
some selected

Impending circulatory or
pulmonary arrest
Altered mental status
Inability to handle
Recent facial trauma or
Recent upper airway or GI
surgery (gastric
Inability to properly fit
Inability to adequately
monitor patient for

BIPAP preferred to intubation: ↓need for
intubation, ↓LOS, ↓mortality
BIPAP, CPAP ↓wall stress, ↓afterload,
↓mortality (likely due of ↓VAP)
NIPPV not shown to be helpful and may be
harmful in following situations:
Postextubation respiratory failure
(↑mortality by delaying intubation)
Failure of ABG to improve after 1 h of
therapy also highly predictive of
subsequent impending respiratory failure

In select patients, NIPPV may result in decreased need for intubation, serious
complications, decreased hospital length of stay, and/or improved likelihood of survival to
hospital discharge.

Bair AE, Filbin MR, Kulkarni RG. The failed intubation attempt in the emergency

department: analysis of prevalence, rescue techniques, and personnel. J Emerg Med.

Caplan RA, Benumof JL. Practice guidelines for management of the difficult airway: an
updated report by the American Society of Anesthesiologists Task Force on
management of the difficult airway. Anesth. 2004;101:565.

Cattano D, Paniucci E, Paolicchi A, Forfori F, Giunta F, Hagberg C. Risk factors assessment
of the difficult airway: an Italian survey of 1956 patients. Anesth Analg. 2004;99:1774-

Kheterpal S, Han R, Tremper KK, et al. Incidence and predictors of difficult and impossible
mask ventilation. Anesthesiology. 2006;105:885-891.

Masip J, Roque M, Sanchez B, Fernandez R, Subirana M, Exposito JA. Noninvasive
ventilation in acute cardiogenic pulmonary edema: systematic review and meta-
analysis. JAMA. 2005;294:3124-3130.

Pistoria M, Amin A, Dressler D, McKean S, Budnitz T. Core competencies in hospital
medicine. J Hosp Med. 2006;1(S1):87.

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Ram FSF, Picot J, Lightowler J, Wedzicha JA. Non-invasive positive pressure ventilation for
treatment of respiratory failure due to exacerbations of

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Arterial Blood Gas and Placement of A-


Joseph J. Miaskiewicz, Jr., MD

Critically ill patients require arterial blood gas (ABG) analysis to assess oxygenation and
ventilation due to limitations of noninvasive oximetry measurements. Below a pO2 of 60
mm Hg corresponding to an O2 saturation of 80%, the oxyhemoglobin saturation curve is
steep and large changes in oximetry may mean small changes in oxygenation. Below this
level oximetry may not correlate with oxygenation, and an arterial blood gas (ABG) should
be obtained (Table 122-1).

TABLE 122-1 Obtaining an Arterial Sample and Placement of an Arterial Line

  ABG A-Line
Indications In hospitalized medical patients,

an ABG is primarily obtained to
confirm the severity and likely
cause of the disturbance
• Level of oxygenation, especially

in settings when the oximeter
measurements are thought to be
unreliable or difficult to obtain

• Need for intubation: refractory
hypoxemia (pO2 < 55 on 100% O2

Usually in the ICU setting
• Frequent ABG sampling
• Continuous blood

pressure monitoring in
use of inotropic or
vasopressor agents

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NRB mask) or hypercapnic
respiratory failure (pCO2 > 55
with acidemia pH < 7.25)

• Severity metabolic acidosis and
adequacy of respiratory
compensation when ↑ work of

• Contribution of ↑pCO2 versus
other causes in somnolent

Contraindications Impaired collateral circulation
• Raynaud
• Thromboangiitis obliterans
• Cyanosis

Impaired collateral

Preparation Allen test: occlusion of the radial
and ulnar arteries by firm pressure
while the fist is clenched followed
by opening of the hand and
release of the arteries one at a time
to assess adequacy of returning
blood flow to the hand

Assess collateral
circulation with Allen test
Avoid brachial and
femoral arteries
(inadequate collateral

Technical Tips
The radial artery at the
wrist best site (near the
surface, relatively easy to
palpate, and stabilize
with good ulnar collateral

Apply local anesthetic with 1%
lidocaine in the conscious patient
Immobilize hand on a wrist board
or towel and dorsiflex wrist

Same as for ABG
If lose ability to palpate
pulse, likely arterial
spasm precluding
successful cannulation.
Wait until subsides or
choose another site
If unsuccessful, apply
pressure for several
minutes to avoid
hematoma formation
(which will make
subsequent attempts
more difficult) and
consider use of
ultrasound to visualize
Reassess perfusion of
hand after placement

Complications Transient obstruction of blood flow
may ↓ arterial flow in distal tissues
unless adequate collateral arterial
vessels available in the setting of

Remove catheter
immediately if any sign
of vascular compromise

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• Spasm
• Intraluminal clotting
• Bleeding and hematoma


Use nondominant hand

By measuring both oxygenation and ventilation ABG analysis assesses the effects of
the cardiopulmonary system in oxygen delivery. ABG analysis directly measures the pH,
pCO2, and pO2. The normal range for the pH is between 7.36 and 7.44 corresponding to a
normal range of 36 to 44 torr for the pCO2. The normal range for the pO2 is between 80
and 100 torr. However, age and the pCO2 also determine alveolar O2.

Oximetry does not measure pCO2 and does not reflect ventilation or acid-base status.
Ventilation may be defined in terms of movement of a volume of air into and out of the
lungs, removing carbon dioxide from the blood and providing oxygen. Alveolar ventilation
is defined in terms of ventilation of CO2. High oxygen saturation may be falsely reassuring
in patients whose respiratory drive is compromised by an increase of oxygenation due to
supplemental O2. Assessment of alveolar ventilation is the key to determining whether a
patient is receiving enough oxygen. A raised PaCO2 reflects reduced alveolar ventilation.
See Chapter 238 (Acid Base Disorders). An approach to interpreting arterial blood gases is
essential when caring for hospitalized patients (Table 122-3).

Respiratory failure is classified as hypoxemic respiratory failure (hypoxemia without
carbon dioxide retention [SaO2 < 95%, PaO2 < 80 on room air]) or hypercarbic respiratory
failure (pCO2 > 45 mm Hg). Calculation of the gradient between the alveolar and arterial
oxygen tensions (the A-a gradient) in respiratory failure will help to determine whether the
patient has associated lung disease or just reduced alveolar ventilation (Table 122-2). See
Chapter 138 (Acute Respiratory Failure).

TABLE 122-2 Calculation of the A-a Oxygen Gradient from the ABG

The Alveolar-Arterial Oxygen Gradient
The A-a oxygen gradient = PAO2 – PaO2
Estimated normal gradient ∼ (Age/4) + 4

The Alveolar Gas Equation
PAO2 = (FiO2 × [Patm – PH2O]) – (PaCO2/R)
• Inspired air at sea level, the FiO2 of room air = 0.21
• Atmospheric pressure, Patm = 760 mm Hg
• PH2O at 37 F = 47 mm Hg
• Respiratory quotient, R = 0.8
Hypoxemic Respiratory Failure with Normal A-a Oxygen Gradient
• Alveolar hypoventilation (oversedation, obesity hypoventilation syndrome, muscular

weakness, neurologic disease)
• High altitude (low inspired FiO2)

Hypoxemic Respiratory Failure with ↑ A-a Gradient

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• Ventilation-perfusion mismatch (pulmonary embolism, COPD, ARDS, pulmonary artery

• Right-to-left shunt (anatomic: cardiac, pulmonary AVM, hepatopulmonary syndrome;
physiologic due to fluid preventing ventilation of perfused alveoli: pneumonia,

Disorders of the lung structure reduce the efficiency of oxygen transfer and widen the A-a
gradient. The prolonged respiratory depression may lead to collapse of some areas of
lung and an increase in the A-a gradient.
Hypercarbic Respiratory Failure, Hypoxemia from Impaired Ventilation with Normal A-a
Oxygen Gradient
• Inadequate alveolar ventilation (without shunting from fluid or collapse of alveoli)
• Ventilatory pump failure (respiratory muscle weakness, neurolgic disease, thoracic cage


TABLE 122-3 Blood Gas Interpretation

Step 1: Acid-base (ventilation)
pH PaCO2 Interpretation

↓ ↑ In acute respiratory failure the change in pH will be accounted
for by the high carbon dioxide concentration.

↓ ↓ A severe metabolic acidosis or some limitation on the ability
of the respiratory system to compensate.

Normal ↑ Alveolar hypoventilation (raised PaCO2) with a normal pH
most likely a primary ventilatory change present long enough
for renal mechanisms to compensate. Increased serum
bicarbonate may also be a clue of chronic CO2 retention.
A similar picture may result from carbon dioxide retention due
to reduced ventilation compensating for a metabolic
alkalosis, although such compensation is usually only partial.

Normal ↓ A primary metabolic acidosis in which the respiratory system
has normalized the pH. Calculate the anion gap.

↑ ↓ Acute alveolar hyperventilation if the pH is appropriately
raised for the reduction in PaCO2.
Chronic alveolar hyperventilation if the pH is between 7.46
and 7.50 as the renal system seldom compensates
completely for an alkalosis.

Step 2: Oxygenation (pO2, %saturation)
pO2 PaCO2 pH  
Normal Normal ↑ A primary metabolic alkalosis

to which the ventilatory
system has not responded.

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↓ Normal ↓ Hypoxemia: when patients
with chronic CO2 retention
increase usual level of
ventilation (acute pulmonary
embolism in chronic lung

Step 3: Calculate the A-a gradient to determine whether carbon dioxide retention is
related to an intrapulmonary cause
A-a Explanation Etiology
Gradient Calculating the A-a gradient is

most useful for determining
the severity of the underlying
disorder and whether there is
a component of

Especially for hospitalized
patients who are prescribed
medications that may
suppress respiration, the A-a
gradient is used to determine
the relative contribution of
hypoventilation to hypoxia
due to underlying lung

Normal A normal A-a gradient is ∼10-
15 torr. Advancing age results
in increases of the normal A-a
gradient. A-a gradient = 2.5 +
0.21 × age in years.

The ABG abnormality is all
due to hypoventilation.

Elevated An elevated A-a gradient
represents ↑ difficulty in
getting O2 from the alveoli to
the blood.
A higher FiO2
disproportionately increases
the PAO2 more than the PaO2.

• Diseases that affect the
pulmonary interstitium
including interstitial lung
disease, pneumonia, and

• Pulmonary vascular disease:
pulmonary emboli, shunts,
pulmonary hypertension.

• Ventilation/perfusion
mismatches of large vessels
(pulmonary or tumor
emboli) and small vessels
(pulmonary hypertension,
vasculitis, interstitial lung
disease and emphysema).

• When breathing 100%
oxygen, older patients may
normally have an A-a
gradient as high as 80 torr
and younger patients as
high as 120 torr.

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Step 4: Does the result correlate with the clinical setting?
Possible Source of Error Prevention
Presence of heparin in

Express any heparin out of syringe prior to sampling

Air bubbles (resulting in
equilibrium between air
and arterial blood:
↓PaCO2, ↑PaO2

Inspect sample and remove air bubbles

Inadequate sample Obtain at least 3 mL aterial blood
Metabolically active
cellular constituents of
blood (resulting in
changing arterial gas
tensions over time)

Cool sample on ice
Analyze sample within 1 h

Sampling of venous

Pay attention to technique

Neither oximetry nor ABGs will detect the presence of a reduced O2-carrying capacity
because anemia, and carbon monoxide (CO) poisoning, and methemoglobinemia do not
affect the alveolar pO2. When there is CO poisoning, the oximeter cannot differentiate
between hemoglobin molecules with CO attached and those with O2 attached and will
report normal O2 saturation. ABGs will also report normal values because the PaO2 is a
measurement of the oxygen dissolved in the blood and not the number of O2 molecules
attached to hemoglobin molecules. In CO poisoning an elevated carboxyhemoglobin will
be required to make the diagnosis. Nonsmokers may have levels up to 3, smokers 10 to
15, and CO poisoning levels above 15. Likewise, the presence of abnormal hemoglobins,
such as sickle cell, fetal hemoglobin, and methemoglobin, will not affect the ABG results.

Oximetry may also not correlate with oxygenation with falsely low results when there is
poor blood flow and perfusion to the fingertips, vasoconstriction due to hypothermia or


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