You Can

1
You Cant Learn Blood Gases from a
Lecture!Lawrence Martin, M.D.Clinical
Professor of MedicineCase Western Reserve
University School of Medicine, Clevelandlarry.mar
tin_at_roadrunner.comApril 1, 2010
2
Blood Gas Interpretation means analyzing the
data to determine patients state of
Ventilation
Oxygenation
Acid-Base
You cant learn this from a lecture!
1/1/2021
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3
To interpret ABGs you need to look at all the
relevant data
4
To interpret ABGs you need to look at all the
relevant data
From the blood gas machinePaCO2PaO2pHH
CO3From the Co-oximeterSaO2CoHbMetHb
5
To interpret ABGs you need to look at all the
relevant data
From the blood gas machinePaCO2PaO2pHH
CO3From the Co-oximeterSaO2CoHbMetHb
From venous bloodNaKCl-HCO3-BUNCreatin
ineHgb
6
To interpret ABGs you need to look at all the
relevant data
From the blood gas machinePaCO2PaO2pHH
CO3From the Co-oximeterSaO2CoHbMetHb
From venous bloodNaKCl-HCO3-BUNCreatin
ineHgb
From the environmentFIO2Barometric
pressure
7
To interpret ABGs you need to look at all the
relevant data
From the blood gas machinePaCO2PaO2pHH
CO3From the Co-oximeterSaO2CoHbMetHb
From venous bloodNaKCl-HCO3-BUNCreatin
ineHgb
From the environmentFIO2Barometric
pressure
And not least, the patientMental
statusResp. rate effortVentilator settings
(if intubated)Pulmonary ABG history
8
To interpret ABGs you need to look at all the
relevant data
From the blood gas machinePaCO2PaO2pHH
CO3From the Co-oximeterSaO2CoHbMetHb
From venous bloodNaKCl-HCO3-BUNCreatin
ineHgb
From the environmentFIO2Barometric
pressure
And not least, the patientMental
statusResp. rate effortVentilator settings
(if intubated)Pulmonary ABG history
7 arterial blood gas values7 venous blood
values2 environmental valuesSeveral patient
variablesLots of variableshow to figure it all
out?
9
Give a man a fish and you feed him for a day.
Teach a man to fish and you feed him for a
lifetime. Chinese Proverb
What does this have to do with blood gas
interpretation?
10
Just this. You need a framework, a
foundation to properly learn ABG interpretation.
If I tell you how to interpret a given blood gas,
you will understand that blood gas only, and not
the next one you may encounter. Much better if I
show you an approach to interpreting all blood
gases, in all situations, so that you understand
them. But
like any other skill based on interpreting a
large number of variables (EKG, chest x-ray,
physical exam), You Cant Learn Blood Gases from
a Lecture.
11
The best way to learn ABGs is to work on blood
gas problems with some knowledge of basic
physiology, then check your work for instant
feedback. This iterative process will teach
you blood gas interpretation.
Books
Web sites
You Cant Learn Arterial Blood Gases from a
Lecture You CAN learn ABGs from selected web
sites. See list at www.lakesidepress.com/ABGi
ndex.htm
12
The Key to Blood Gas Interpretation4 Equations,
3 Physiologic Processes These 4 equations,
crucial to understanding and interpreting
arterial blood gas data, provide the basic
foundation for understanding blood gas
interpretation.

• Equation Physiologic Process
• PaCO2 equation Alveolar ventilation
• VCO2 x 0.863
• PaCO2 ---------------
• VA
• Alveolar gas equation Oxygenation
• PAO2 PIO2 - 1.2 (PaCO2)
• where PIO2 FIO2 (PB 47 mm Hg)
• Oxygen content equation Oxygenation
• CaO2 quantity O2 bound to Hb quantity
  O2 dissolved in plasmaCaO2 (Hb x 1.34 x
  SaO2) (.003 x PaO2)
• Henderson-Hasselbalch equation Acid-base
  balance HCO3-
  
• pH --------
• PaCO2

13
Start with PaCO2. PaCO2 is the center
of the blood gas universe.
pH
PaO2
PaCO2
HCO3
SaO2
14
PaCO2 equation PaCO2 reflects ratio of
metabolic CO2 production (VCO2) to alveolar
ventilation (VA) -- there is nothing clinical
in the equation except respiratory rate (f)

• VCO2 x 0.863
• PaCO2 -----------
• VA
• VCO2 CO2 production
• VA VE VD
• f (tidal volume) f (dead space volume)
• 0.863 converts units to mm Hg

Condition State of PaCO2 in
blood (PaCO2) alveolar ventilation 35 - 45 mm Hg
Eucapnia Normal ventilation gt45
mm Hg Hypercapnia Hypoventilation
lt35 mm Hg Hypocapnia
Hyperventilation
200 ml/min x 0.863 40 mmHg
-------------------------
4.3 L/min
200 ml/min x 0.863 80 mmHg
-------------------------
2.15 L/min
200 ml/min x 0.863 20 mmHg
--------------------------
8.6 L/min
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HypercapniaA serious respiratory problem

• VCO2 x 0.863
• PaCO2 -------------
• VA where VA VE VD
• f (tidal vol.) f (dead space
  vol.)
• The PaCO2 equation shows that the only
  physiologic reason for elevated PaCO2 is
  inadequate alveolar ventilation (VA) for the
  amount of CO2 production (VCO2). Since VA VE
  VD, hypercapnia can arise from insufficient VE
  (eg, drug overdose), increased VD (eg, COPD), or
  a combination.
• The PaCO2 equation also shows why PaCO2 cannot
  reliably be assessed clinically. Since you never
  know the patient's VCO2 or VA, you cannot
  determine the VCO2/VA, which is what PaCO2
  provides. (Even if tidal volume is measured, you
  cant determine the amount of air going to dead
  space.)
• There is no predictable correlation between PaCO2
  and the clinical picture. In a patient with
  possible respiratory disease, respiratory rate,
  depth, and effort cannot be reliably used to
  predict even a directional change in PaCO2. A
  patient in respiratory distress can have a high,
  normal, or low PaCO2. A patient without
  respiratory distress can have a high, normal, or
  low PaCO2.

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PaCO2 and alveolar ventilation

• A 60 yo man with severe chronic obstructive
  pulmonary disease is seen in the ED, anxious and
  tachypneic with RR30/minute. The intern (who
  didnt attend this lecture) says hes
  hyperventilating and wants to give him Xanax.
  You, being wiser (since you attended), know
  better and demand a blood gas. Blood gas shows
• PaCO2 85 mm Hg (severe hypo ventilation)
• pH 7.23
• Explain tachypnea and hypoventilation in this
  patient.

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PaCO2 and alveolar ventilation

• 60 yo man
• PaCO2 85 mm Hg
• pH 7.23
• Clinically all you know is this patients resp.
  rate, the f in the PaCO2 equation. You dont
  know his tidal volume, dead space volume or CO2
  production, i.e., you dont know the numerator or
  denominator of the equation. Thus by exam alone,
  you cannot know even if hes hypo- or hyper-
  ventilating. The blood gas shows hes
  hypoventilating. Though tachypneic, MOST OF EACH
  BREATH IS GOING TO DEAD SPACE, NOT TO FUNCTIONING
  ALVEOLI!
• There is no predictable correlation between PaCO2
  and the clinical picture. In a patient with
  possible respiratory disease, respiratory rate,
  depth, and effort cannot be reliably used to
  predict even a directional change in PaCO2.
• A patient in respiratory distress can have a
  high, normal, or low PaCO2.
• A patient without respiratory distress can have a
  high, normal, or low PaCO2.

• VCO2 x 0.863
• PaCO2 -------------
• VA
• where VA VE VD
• f (tidal volume) f (dead
  space volume)

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Hyperventilating? Hypoventilating? Normal
ventilation?

• VCO2 x 0.863
• PaCO2 -----------------
• VA
• where VA VE VD
• f (tidal volume) f (dead
  space volume)

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PaCO2 is the center of the blood gas universe.
Understanding PaCO2 facilitates understanding
oxygenation acid-base balance.
PaCO2

• Oxygenation Acid-Base
• PAO2 FIO2 (BP-47) 1.2 (PCO2) HCO3-
  pH ------------ PaCO2
• PaO2
• VCO2 x .863
• PaCO2 --------------------
• VA where VA VE
  VD
• f (tidal volume) f (dead space volume)
• Note PaCO2 is from ABGs. HCO3- is
  calculated from ABG measurement of pH and PaCO2
  or is measured in venous blood (serum) as part of
  electrolytes. When measured in venous blood it
  is variously labeled bicarbonate, HCO3- or
  CO2 (the latter NOT to be confused with PaCO2).

20
Arterial blood blood gases
Venous blood - electrolytes
21
The Key to Blood Gas Interpretation4 Equations,
3 Physiologic Processes These 4 equations,
crucial to understanding and interpreting
arterial blood gas data, provide the basic
foundation for understanding blood gas
interpretation.

• Equation Physiologic Process
• PaCO2 equation Alveolar ventilation
• VCO2 x 0.863
• PaCO2 ---------------
• VA
• Alveolar gas equation Oxygenation
• PAO2 PIO2 - 1.2 (PaCO2)
• where PIO2 FIO2 (PB 47 mm Hg)
• Oxygen content equation Oxygenation
• CaO2 quantity O2 bound to Hb quantity
  O2 dissolved in plasmaCaO2 (Hb x 1.34 x
  SaO2) (.003 x PaO2)
• Henderson-Hasselbalch equation Acid-base
  balance HCO3-
  
• pH --------
• PaCO2

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Alveolar Gas Equation

• PAO2 PIO2 - 1.2 (PaCO2)
• PAO2 is the average alveolar PO2
• PIO2 is the partial pressure of inspired oxygen
  in the trachea.
• PIO2 FIO2 (PB 47 mm Hg) 0.21 (760-47)
  150 mm Hg
• Breathing room air at sea level,
• PAO2 150 1.2 (40) 150 - 48 102 mm Hg
• Note FIO2 is fraction of inspired oxygen and PB
  is the barometric pressure. 47 mm Hg is the
  water vapor pressure at normal body temperature.
  This is the abbreviated version of the AG
  equation, suitable for clinical purposes.

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Alveolar Gas EquationPAO2 FIO2 (PB 47 mm
Hg) 1.2(PaCO2)

• In order to bring O2 into the blood, alveolar PO2
  (PAO2) has to always exceed arterial PO2 (PaO2).
  Whenever PAO2 decreases, PaO2 decreases as well.
  Thus, from the AG equation
• If FIO2 and PB are constant (i.e., constant
  PIO2), then as PaCO2 increases both PAO2 and PaO2
  will decrease hypercapnia causes hypoxemia.
• If FIO2 decreases and PB and PaCO2 are constant,
  both PAO2 and PaO2 will decrease suffocation
  causes hypoxemia.
• If PB decreases (e.g., with altitude), and PaCO2
  and
• FIO2 are constant, both PAO2 and PaO2 will
• decrease mountain climbing causes hypoxemia.

See web site Blood Gases on Mt.
Everest www.lakesidepress.com/pulmonary/MtEverest
/bloodgases.htm.
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Alveolar Gas EquationPAO2 FIO2 (PB 47 mm
Hg) 1.2(PaCO2)

• What is the alveolar PO2 (PAO2) at sea level in
  the following circumstances?
• a) FIO2 .21, PaCO2 20 mm Hg
• ANSWER PAO2 .21(713) - 1.2(20) 126 mm Hg
• b) FIO2 .21, PaCO2 60 mm Hg
• ANSWER PAO2 .21(713) - 1.2(60) 72 mm Hg
• c) FIO2 .40, PaCO2 30 mm Hg
• ANSWER PAO2 .40(713) (1.2)30 249 mm Hg
• BP 760 mm Hg

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P(A-a)O2

• P(A-a)O2 is the alveolar-arterial difference in
  partial pressure of oxygen. It is commonly
  called the A-a gradient, though it does not
  actually result from an O2 pressure gradient in
  the lungs. Instead, it results normal
  ventilation-perfusion imbalance in the lungs
  (normal venous admixture, about 3 of cardiac
  output).
• PAO2 is always calculated, based on FIO2, PaCO2
  and barometric pressure.
• PAO2 FIO2 (PB 47 mm Hg) 1.2(PaCO2)
• PaO2 is always measured, on an arterial blood
  sample in the blood gas machine.
• Normal P(A-a)O2 ranges from _at_ 5 to 25 mm Hg
  breathing room air (it increases with age and
  with FIO2). A higher than normal P(A-a)O2 means
  the lungs are not transferring oxygen properly
  from alveoli into the pulmonary capillaries.
  Except for right to left cardiac shunts, an
  elevated P(A-a)O2 signifies some sort of problem
  within the lungs that has caused
  ventilation-perfusion imbalance (increase over
  the normal venous admixture).
• Virtually all lung disease lowers PaO2
• via the mechanism of increased V-Q
• imbalance, e.g., COPD, pneumonia,
• atelectasis, pulmonary edema.

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P(A-a)O2

• Alveolar PO2 PaO2 P(A-a)O2
• FIO2 .21, PaCO2 20 mm Hg
• PAO2 .21(713) - 1.2(20) 126 mm Hg 112 mm
  Hg 14 mm Hg (nl.)
• FIO2 .21, PaCO2 60 mm Hg
• PAO2 .21(713) - 1.2(60) 72 mm Hg 62 mm
  Hg 10 mm Hg (nl.)
• FIO2 .40, PaCO2 30 mm Hg
• PAO2 .40(713) (1.2)30 249 mm Hg 129 mm
  Hg 120 mm Hg
• Always a calculation
• Always a measurement

27
P(A-a)O2 Test your understanding

• Calculate P(A-a)O2 using the alveolar gas
  equation (assume PB 760 mm Hg).
• A 28 yo woman with PaCO2 30 mm Hg, PaO2 88 mm Hg,
  FIO2 0.21.
• PAO2 .21 (760 - 47) - 1.2(30) 150 - 36 114
  mm Hg P(A-a)O2 114 - 88 26 mm Hg
• The P(A-a)O is elevated. This means the PaO2 is
  lower than expected for this degree of
  hyperventilation, indicating a gas-exchange
  problem. The patient was diagnosed with
  pulmonary embolism.
• b) A 22 yo anxious man with PaCO2 15 mm Hg, PaO2
  120 mm Hg, FIO2 0.21.
• PAO2 .21(760 47) - 1.2(15) 150 - 18 132
  mm Hg P(A-a)O2 132 - 120 12 mm Hg
• The patient had anxiety-hyperventilation
  syndrome. Hyperventilation can easily raise PaO2
  gt 100 mm Hg when the lungs are normal, as in this
  case.
• c) A 54 yo woman with PaCO2 75 mm Hg, PaO2 95
  mm Hg, FIO2 0.28.
• PAO2 .28(760 - 47) - 1.2(75) 200 - 90 110
  mm Hg P(A-a)O2 110 - 95 15 mm Hg
• Despite severe hypoventilation, there is no
  evidence here for lung disease. Hypercapnia
• is most likely a result of disease elsewhere in
  the respiratory system, either the central
  nervous system or chest bellows.

28
The Key to Blood Gas Interpretation4 Equations,
3 Physiologic Processes These 4 equations,
crucial to understanding and interpreting
arterial blood gas data, provide the basic
foundation for understanding blood gas
interpretation.

• Equation Physiologic Process
• PaCO2 equation Alveolar ventilation
• VCO2 x 0.863
• PaCO2 ---------------
• VA
• Alveolar gas equation Oxygenation
• PAO2 PIO2 - 1.2 (PaCO2)
• where PIO2 FIO2 (PB 47 mm Hg)
• Oxygen content equation Oxygenation
• CaO2 quantity O2 bound to Hb quantity
  O2 dissolved in plasmaCaO2 (Hb x 1.34 x
  SaO2) (.003 x PaO2)
• Henderson-Hasselbalch equation Acid-base
  balance HCO3-
  
• pH --------
• PaCO2

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SaO2 and oxygen content

• Tissues need a requisite amount of oxygen
  molecules for metabolism. Neither the PaO2 nor
  the SaO2 tells how much oxygen is in the blood.
  How much is provided by the oxygen content, CaO2
  (units ml O2/dl). CaO2 is calculated asCaO2
  quantity O2 bound quantity O2
  dissolved to hemoglobin in
  plasma
• CaO2 (Hb x 1.34 x SaO2) (.003 x
  PaO2)
• CaO2 15 x 1.34 x .98 (.003 x
  100)
• CaO2 19.7 0.3
  20 ml O2/dl blood
• Hb hemoglobin in gm 1.34 ml O2 that can be
  bound to each gm of Hb SaO2 is percent
  saturation of hemoglobin with oxygen .003 is
  solubility coefficient of oxygen in plasma .003
  ml dissolved O2/mm Hg PO2.

30
Oxygen dissociation curve SaO2 vs. PaO2 Also
shown are CaO2 vs. PaO2 for two different
hemoglobin contents 15 gm and 10 gm. CaO2
units are ml O2/dl. P50 is the PaO2 at which
SaO2 50.

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How much oxygen is in the blood?PaO2 vs. SaO2
vs. CaO2

• OXYGEN PRESSURE PaO2
• Since PaO2 reflects only free oxygen molecules
  dissolved in
• plasma and not those bound to hemoglobin, PaO2
  cannot tell us
• how much oxygen is in the blood for that you
  need to know
• how much oxygen is also bound to hemoglobin,
  information
• given by the SaO2 and hemoglobin content.
• OXYGEN SATURATION SaO2
• The percentage of all the available heme binding
  sites saturated with oxygen is the hemoglobin
  oxygen saturation (in arterial blood, the SaO2).
  Note that SaO2 alone doesnt reveal how much
  oxygen is in the blood for that we also need to
  know the hemoglobin content.
• OXYGEN CONTENT CaO2
• Tissues need a requisite amount of O2 molecules
  for metabolism. Neither the PaO2 nor the SaO2
  provide information on the number of oxygen
  molecules, i.e., how much oxygen is in the blood.
  (Neither PaO2 nor SaO2 have units that denote
  any quantity.) Only CaO2 (units ml O2/dl) tells
  how much oxygen is in the blood this is because
  CaO2 is the only value that incorporates the
  hemoglobin content. Oxygen content can be
  measured directly or calculated by the oxygen
  content equation
• CaO2 (Hb x 1.34 x SaO2) (.003 x PaO2)

See The Differences Between PaO2, SaO2 and
Oxygen Content www.lakesidepress.com/pulmonary/AB
G/PO2.htm
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SaO2 and CaO2
Which patient, (a) or (b), is more
hypoxemic? (a) Hb 15 gm, PaO2 65 mm Hg,
SaO288 (b) Hb 10 gm , PaO2 100 mm Hg,
SaO298
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SaO2 and CaO2
Which patient, (a) or (b), is more
hypoxemic? (a) Hb 15 gm, PaO2 65 mm Hg,
SaO288 (b) Hb 10 gm , PaO2 100 mm Hg,
SaO298

• ANSWER (b)
• (a) CaO2 .88 x 15 x 1.34 17.6 ml O2/dl
  dissolved O2
• (b) CaO2 .98 x 10 x 1.34 13.1 ml O2/dl
  dissolved O2
• Oxygen content determines hypoxemia. Patient (b)
  has a much lower oxygen content and so is more
  hypoxemic than (a). Note that PaO2 is not a
  significant factor in determining oxygen content
  and (for this question) can be ignored. Note
  that the low PaO2 in patient (a) means there is
  an oxygen transfer problem from air to blood, but
  in terms of what the body really needs OXYGEN
  CONTENT patient (b) is definitely more
  hypoxemic.

34
The Key to Blood Gas Interpretation4 Equations,
3 Physiologic Processes These 4 equations,
crucial to understanding and interpreting
arterial blood gas data, provide the basic
foundation for understanding blood gas
interpretation.

• Equation Physiologic Process
• PaCO2 equation Alveolar ventilation
• VCO2 x 0.863
• PaCO2 ---------------
• VA
• Alveolar gas equation Oxygenation
• PAO2 PIO2 - 1.2 (PaCO2)
• where PIO2 FIO2 (PB 47 mm Hg)
• Oxygen content equation Oxygenation
• CaO2 quantity O2 bound to Hb quantity
  O2 dissolved in plasmaCaO2 (Hb x 1.34 x
  SaO2) (.003 x PaO2)
• Henderson-Hasselbalch equation Acid-base
  balance HCO3-
  
• pH --------
• PaCO2

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ltgt

• ACID-BASE
• traditionally the most difficult of the 3
  physiologic processes

• Acidemia blood pH lt 7.35
• Acidosis a primary physiologic process that,
  occurring alone, tends to cause acidemia,
• Alkalemia blood pH gt 7.45
• Alkalosis a primary physiologic process that,
  occurring alone, tends to cause alkalemia.
• Primary acid-base disorder One of the four
  acid-base disturbances manifested by an initial
  change in HCO3- or PaCO2. They are
• metabolic acidosis (MAc)
• metabolic alkalosis (MAlk)
• respiratory acidosis (RAc)
• respiratory alkalosis (Ralk)
• Compensation The change in HCO3- or PaCO2 that
  results from the primary event. Compensatory
  changes are not classified by the terms used for
  the four primary acid-base disturbances.
• For example, a patient who hyperventilates
  (lowers PaCO2) solely as compensation for MAc
  does not have a RAlk, the latter being a primary
  disorder that, alone, would lead to alkalemia.
  In simple, uncomplicated MAc the patient will
  never develop alkalemia.

36
Primary acid-base disorders Respiratory
alkalosis

• Respiratory alkalosis - A primary disorder where
  the first change is a lowering of PaCO2,
  resulting in an elevated pH. Compensation
  (bringing the pH back down toward normal) is a
  secondary lowering of bicarbonate (HCO3) by the
  kidneys this reduction in HCO3- is not metabolic
  acidosis, since it is not a primary process.
• Primary Event Compensatory
  Event
• HCO3- 9HCO3-
• 8pH ))))) 8 pH )))))
• 9PaCO2 9PaCO2

RESPIRATORY ALKALOSIS 9PaCO2 8 pH
Hypoxemia (includes altitude) Anxiety Sepsis
Any acute pulmonary insult, e.g., pneumonia, mild
asthma attack, early pulmonary edema, pulmonary
embolism
37
Primary acid-base disorders Respiratory acidosis

• Respiratory acidosis - A primary disorder where
  the first change is an elevation of PaCO2,
  resulting in decreased pH. Compensation
  (bringing pH back up toward normal) is a
  secondary retention of bicarbonate by the
  kidneys this elevation of HCO3- is not metabolic
  alkalosis, since it is not a primary process.
• Primary Event Compensatory
  Event
• HCO3-
  8 HCO3-
• 9pH ))))) 9pH )))))
• 8PaCO2 8PaCO2

RESPIRATORY ACIDOSIS 8PaCO2 9pH Central
nervous system depression (e.g., drug
overdose) Chest bellows dysfunction (e.g.,
Guillain-Barré syndrome, myasthenia gravis)
Disease of lungs and/or upper airway (e.g.,
chronic obstructive lung disease, severe asthma
attack, severe pulmonary edema)
38
Primary acid-base disorders Metabolic acidosis

• Metabolic Acidosis - A primary acid-base disorder
  where the first change is a lowering of HCO3-,
  resulting in decreased pH. Compensation
  (bringing pH back up toward normal) is a
  secondary hyperventilation this lowering of
  PaCO2 is not respiratory alkalosis, since it is
  not a primary process.
• Primary Event Compensatory Event
• 9HCO3- 9HCO3-
• 9pH ))))) 9pH )))))
• PaCO2 9PaCO2

AG Na - (Cl- HCO3) nl 12 /- 4
METABOLIC ACIDOSIS 9HCO3- 9pH Increased
anion gap (gt16 mEq/L) lactic acidosis
ketoacidosis drug poisonings (e.g., aspirin,
ethyelene glycol, methanol) Normal anion gap (lt
16 mEq/L) diarrhea some kidney problems, e.g.,
renal tubular acidosis, intersititial nephritis
39
Primary acid-base disorders Metabolic alkalosis

• Metabolic alkalosis - A primary acid-base
  disorder where the first change is an elevation
  of HCO3-, resulting in increased pH.
  Compensation is a secondary hypoventilation
  (increased PaCO2) which is not respiratory
  acidosis, since it is not a primary process.
  Compensation for metabolic alkalosis (attempting
  to bring pH back down toward normal) is less
  predictable than for the other three acid-base
  disorders.
• Primary Event
  Compensatory Event
• 8HCO3- 8HCO3-
• 8pH ))))) 8pH )))))
• PaCO2 8PaCO2

• METABOLIC ALKALOSIS 8HCO3- 8pH
• Chloride responsive (responds to NaCl or KCl
  therapy) contraction alkalosis, diuretics
  corticosteroids gastric suctioning vomiting
• Chloride resistant any hyperaldosterone state,
  e.g., Cushingss syndrome Bartters syndrome
  severe K depletion

40
Mixed Acid-base disorders are common

• In chronically ill respiratory patients, mixed
  disorders are probably more common than single
  disorders, e.g., RAc MAlk, RAc Mac, RAlk
  MAlk.
• In renal failure (and other conditions) combined
  MAlk MAc is also encountered. pH 7.40,
  PaCO240 mm Hg, HCO3- 24 mEq/L, AG24 mEq/L
• In sepsis patients, MAc Ralk is common. pH
  7.40, PCO220 mm Hg, HCO3- 12 mEq/L
• Always be on lookout for mixed acid-base
  disorders. They can be missed!

41
Tips to diagnosing mixed acid-base disorders

• TIP 1. Dont interpret any blood gas data for
  acid-base diagnosis without closely examining the
  venous electrolytes Na, K, Cl- and HCO3-.
• A venous HCO3- out of the normal range always
  represents some type of acid-base disorder
  (barring lab or transcription error).
• High venous HCO3- indicates metabolic alkalosis
  /or bicarbonate retention as compensation for
  respiratory acidosis
• Low venous HCO3- indicates metabolic acidosis
  /or bicarbonate excretion as compensation for
  respiratory alkalosis
• Note that venous HCO3- may be normal in the
  presence of two or more acid-base disorders.

42
Tips to diagnosing mixed acid-base disorders
(cont.)

• TIP 2 . Single acid-base disorders do not lead
  to normal blood pH. Although pH can end up in
  the normal range (7.35 - 7.45) with a mild single
  disorder, a truly normal pH with distinctly
  abnormal HCO3- and PaCO2 invariably suggests two
  or more primary disorders.
• Example pH 7.40, PaCO2 20 mm Hg, HCO3- 12
  mEq/L, in a patient with sepsis. Normal pH
  results from two co-existing and unstable
  acid-base disorders acute respiratory alkalosis
  and metabolic acidosis.

43
Tips to diagnosing mixed acid-base disorders
(cont.)

• TIP 3. Simplified rules predict the pH and HCO3-
  for a given change in PaCO2. If the pH or HCO3-
  is higher or lower than expected for the change
  in PaCO2, the patient probably has a metabolic
  acid-base disorder as well.
• Below are expected changes in pH and HCO3- (in
  mEq/L) for a 10 mm Hg change in PaCO2 resulting
  from either primary hypoventilation (respiratory
  acidosis) or primary hyperventilation
  (respiratory alkalosis).

• ACUTE CHRONIC
• Resp Acidosis
• pH 9 by 0.07 pH 9 by 0.03
• HCO3- 8 by 1 HCO3- 8 by 3-4
• Resp Alkalosis
• pH 8 by 0.08 pH 8 by 0.03
• HCO3- 9 by 2 HCO3- 9 by 5
• Units for HCO3- are mEq/L

44
Predicted changes in HCO3- for a directional
change in PaCO2 can help uncover mixed acid-base
disorders.

• A normal or slightly low HCO3- in the presence of
  hypercapnia suggests a concomitant metabolic
  acidosis, e.g.,
• pH 7.27, PaCO2 50 mm Hg, HCO3- 22 mEq/L.
• Based on the rule for increase in HCO3- with
  hypercapnia, it should be at least 25 mEq/L in
  this example that it is only 22 mEq/L suggests a
  concomitant metabolic acidosis.
• A normal or slightly elevated HCO3- in the
  presence of hypocapnia suggests a concomitant
  metabolic alkalosis, e.g.,
• pH 7.56, PaCO2 30 mm Hg, HCO3- 26 mEq/L.
• Based on the rule for decrease in HCO3 with
  hypocapnia, it should be at least 23 mEq/L in
  this example that it is 26 mEq/L suggests a
  concomitant metabolic alkalosis.

45
Tips to diagnosing mixed acid-base disorders
(cont.)

• TIP 4. In maximally-compensated metabolic
  acidosis, the numerical value of PaCO2 should be
  the same (or close to) the last two digits of
  arterial pH. This observation reflects the
  formula for expected respiratory compensation in
  metabolic acidosis
• Expected PaCO2 1.5 x venous
  HCO3 (8 2)
• In contrast, compensation for metabolic alkalosis
  (by increase in PaCO2) is highly variable, and in
  some cases there may be no or minimal
  compensation.

46
Acid-base disorders test your understanding
A patients arterial blood gas shows pH of 7.14,
PaCO2 of 70 mm Hg, and HCO3- of 23 mEq/L. How
would you describe the likely acid-base
disorder(s)?
Acute elevation of PaCO2 leads to reduced pH,
i.e., an acute respiratory acidosis. However, is
the problem only acute respiratory acidosis or is
there some additional process? For every 10 mm
Hg rise in PaCO2 (before any renal compensation),
pH falls about 0.07 units. Because this
patient's pH is down 0.26, or 0.05 more than
expected for a 30 mm Hg increase in PaCO2, there
must be an additional, metabolic problem. Also,
note that with acute CO2 retention of this
degree, the HCO3- should be elevated 3 mEq/L.
Thus a low-normal HCO3- with increased PaCO2 is
another way to uncover an additional, metabolic
disorder. Decreased perfusion leading to mild
lactic acidosis would explain the metabolic
component.
47
Acid-base disorders test your understanding
A 45-year-old man comes to hospital complaining
of dyspnea for three days. Arterial blood gas
reveals pH 7.35, PaCO2 60 mm Hg, PaO2 57 mm Hg,
HCO3- 31 mEq/L. How would you characterize his
acid-base status?
PaCO2 and HCO3- are elevated, but HCO3- is
elevated more than would be expected from acute
respiratory acidosis. Since the patient has been
dyspneic for several days it is fair to assume a
chronic acid-base disorder. Most likely this
patient has a chronic or compensated respiratory
acidosis. Without electrolyte data and more
history, you cannot diagnose an accompanying
metabolic disorder.
48
Acid-base disorders test your understanding
State whether each of the following statements is
true or false. a) Metabolic acidosis is always
present when the measured serum CO2 changes
acutely from 24 to 21 mEq/L. b) In acute
respiratory acidosis, bicarbonate initially rises
because of the reaction of CO2 with water and the
resultant formation of H2CO3. c) If pH and PaCO2
are both above normal, the calculated bicarbonate
must also be above normal. d) An abnormal serum
CO2 value always indicates an acid-base disorder
of some type. e) The compensation for chronic
elevation of PaCO2 is renal excretion of
bicarbonate. f) A normal pH with abnormal HCO3-
or PaCO2 suggests the presence of two or more
acid-base disorders. g) A normal venous HCO3-
value indicates there is no acid-base
disorder. h) Normal arterial blood gas values
rule out the presence of an acid-base disorder.
49
Acid-base disorders test your understanding

• State whether each of the following statements is
  true or false.
• a) Metabolic acidosis is always present when the
  measured serum CO2 changes acutely from 24 to 21
  mEq/L.
• In acute respiratory acidosis, bicarbonate
  initially rises because of the reaction of CO2
  with water and the resultant formation of H2CO3.
• c) If pH and PaCO2 are both above normal, the
  calculated bicarbonate must also be above normal.
• d) An abnormal serum CO2 value always indicates
  an acid-base disorder of some type.
• e) The compensation for chronic elevation of
  PaCO2 is renal excretion of bicarbonate.
• f) A normal pH with abnormal HCO3- or PaCO2
  suggests the presence of two or more acid-base
  disorders.
• g) A normal venous HCO3- value indicates there
  is no acid-base disorder.
• h) Normal arterial blood gas values rule out the
  presence of an acid-base disorder.

• a) false
• b) true
• c) true
• d) true
• e) false
• f) true
• g) false
• h) false

50
Summary ) Clinical and laboratory approach to
acid-base diagnosis

• Determine existence of acid-base disorder from
  ABG and/or venous electrolytes.
• Examine pH, PaCO2 and HCO3- for obvious primary
  acid-base disorder, and for deviations that
  indicate mixed acid-base disorders (TIPS 2
  through 4).
• Use a full clinical assessment (hx, phys exam,
  other lab data, previous ABGs) to explain each
  acid-base disorder. Co-existing clinical
  conditions may lead to opposing acid-disorders,
  so that pH can be high when there is an obvious
  acidosis, or low when there is an obvious
  alkalosis.
• Treat the underlying clinical condition(s) this
  will usually suffice to correct most acid-base
  disorders.
• Clinical judgment should always apply.

51
Arterial Blood Gases test your overall
understanding
Case 1. A 55-year-old man is evaluated in the
pulmonary lab for shortness of breath. His
regular medications include a diuretic for
hypertension and one aspirin a day. He smokes a
pack of cigarettes a day. FIO2 .21 HCO3- 30
mEq/L pH 7.53 COHb 7.8 PaCO2 37 mm
Hg Hb 14 gm PaO2 62 mm Hg CaO2 16.5 ml
O2 SaO2 87 How would you characterize his
state of ventilation, oxygenation and acid-base
balance?
52
Arterial Blood Gases test your overall
understanding
Case 1 - Discussion VENTILATION Adequate for
the patient's level of CO2 production the
patient is neither hyper- nor hypo-
ventilating. OXYGENATION The PaO2 and SaO2 are
both reduced on room air. Since P(A-a)O2 is
elevated (approximately 43 mm Hg), the low PaO2
can be attributed to V-Q imbalance, i.e., a
pulmonary problem. SaO2 is reduced, in part from
the low PaO2 but mainly from elevated
carboxyhemoglobin, which in turn can be
attributed to cigarettes. The arterial oxygen
content is adequate. ACID-BASE Elevated pH and
HCO3- suggest a state of metabolic alkalosis,
most likely related to the patient's diuretic
his serum K should be checked for hypokalemia.
53
Arterial Blood Gases test your overall
understanding
Case 2. A 46-year-old man has been in the
hospital two days, with pneumonia. He was
recovering but has just become diaphoretic,
dyspneic and hypotensive. He is breathing oxygen
through a nasal cannula at 3 l/min.
pH 7.40 PaCO2 20 mm Hg
COHb 1.0 PaO2 80 mm Hg
SaO2 95 Hb 13.3 gm
HCO3- 12 mEq/L CaO2 17.2 ml O2 How
would you characterize his state of ventilation,
oxygenation, and acid-base balance?
54
Arterial Blood Gases test your overall
understanding
Case 2 - Discussion VENTILATION PaCO2 is half
normal and indicates marked hyperventilation. OXYG
ENATION The PaO2 is lower than expected for
someone hyperventilating to this degree and
receiving supplemental oxygen, and points to
significant V-Q imbalance. The oxygen content is
adequate. ACID-BASE Normal pH with very low
bicarbonate and PaCO2 indicates combined
respiratory alkalosis and metabolic acidosis. If
these changes are of sudden onset the diagnosis
of sepsis should be strongly considered,
especially in someone with a documented infection.
55
Arterial Blood Gases test your overall
understanding
Case 3. A 58-year-old woman is being evaluated
in the emergency department for acute
dyspnea. FIO2 .21 pH 7.19 PaCO2 65
mm Hg COHb 1.1 PaO2 45 mm
Hg SaO2 90 Hb 15.1
gm HCO3- 24 mEq/L CaO2 18.3 ml O2 How
would you characterize her state of ventilation,
oxygenation and acid-base balance?
56
Arterial Blood Gases test your overall
understanding
Case 3 - Discussion VENTILATION The patient is
hypoventilating. OXYGENATION The patient's PaO2
is reduced for two reasons hypercapnia and V-Q
imbalance, the latter apparent from an elevated
P(A-a)O2 (approximately 27 mm Hg). ACID-BASE
pH and PaCO2 are suggestive of acute respiratory
acidosis plus metabolic acidosis the calculated
HCO3- is lower than expected from acute
respiratory acidosis alone. .
57
Arterial Blood Gases test your overall
understanding
Case 4. A 23-year-old man is being evaluated in
the emergency room for severe pneumonia. His
respiratory rate is 38/min and he is using
accessory breathing muscles. FIO2 .90 Na 154
mEq/L pH 7.29 K 4.1 mEq/L PaCO2 55 mm
Hg Cl- 100 mEq/L PaO2 47 mm Hg HCO3- 24
mEq/L SaO2 86 HCO3- 23 mEq/L COHb 2.1 Hb 13
gm CaO2 15.8 ml O2 How would you characterize
his state of ventilation, oxygenation and
acid-base balance?
58
Arterial Blood Gases test your overall
understanding
Case 4 - Discussion VENTILATION The patient is
hypoventilating despite the presence of
tachypnea, indicating significant dead space
ventilation. This is a dangerous situation that
suggests the need for mechanical ventilation.
OXYGENATION The PaO2 and SaO2 are both
markedly reduced on 90 inspired oxygen,
indicating severe ventilation-perfusion
imbalance. ACID-BASE The low pH (7.29), high
PaCO2 (55) and normal HCO3- all point to combined
acute respiratory acidosis and metabolic
acidosis. Anion gap is elevated to 30 mEq/L
indicating a clinically significant anion gap
(AG) acidosis, possibly from lactic acidosis.
AG Na - (Cl HCO3-) 154 - (100 24)
30 mEq/L
With an of AG of 30 mEq/L his venous HCO3- should
be much lower, to reflect buffering of the
increased acid. However, his venous HCO3- is
normal, indicating a primary process that is
increasing it, i.e., a metabolic alkalosis in
addition to a metabolic acidosis. The cause of
the alkalosis is as yet undetermined. In summary
this patient has respiratory acidosis, metabolic
acidosis and metabolic alkalosis.
59
Blood Gas Interpretation means analyzing the
data to determine patients state of
Ventilation
Oxygenation
Acid-Base
Oh, did I mention you cant learn this from a
lecture?
60
The best way to learn ABGs is to work on blood
gas problems with some knowledge of basic
physiology, then check your work for instant
feedback. This iterative process will teach
you blood gas interpretation.
Books
Web sites
You Cant Learn Arterial Blood Gases from a
Lecture You CAN learn ABGs from selected web
sites. See list at www.lakesidepress.com/ABGi
ndex.htm