ABG by a taecher

Arterial blood gas analysis
THANKS TO:
Dr. Chandramohan M,
Intensivist,
Bangalore

Decision to intubate
• The decision to intubate should be primarily made on
clinical grounds and the figures in ABG values like
PaO2<60 mmHg, PCO2>55mmHg etc should be used as
a guideline only
Consider two scenarios:
• A 45 yr old patient with chronic neurological weakness
with PCO2-60mmHg, PO2-58mmHg and with HR-80/min,
BP- 130/80mmHg, RR-14/min, conscious, comfortable
• A 24yr old asthmatic with PCO2-60mmHg, PO2-58mmHg
and with HR-120/min,BP-100/70 mmHg,RR-40/min, in
severe respiratory distress, drowsy

Which of the following cases would warrant
immediate ventilatory support?
a. A 50-year-old man is comatose from drug overdose.
PaCO2 is 51 mm Hg, PaO2 is 76 mm Hg, and pH is 7.31
while breathing room air.
b. A 29-year-old man is restless and in severe respiratory
distress; he is breathing 42 times/min. PaCO2 is 33 mm
Hg. pH is 7.42, and PaO2 is 47 mm Hg while breathing
60% oxygen through a face mask.
c. A 61-year-old woman who has severe emphysema is
alert but is in moderate respiratory distress; her
respiratory rate is 24/min. PaO2 is 75 mm Hg while
breathing nasal oxygen at 2 L/min, PaCO2 is 59 mm Hg,
and the pH is 7.34. Her chest x-ray is clear.

d. A 29-year-old woman is suffering from diabetic
ketoacidosis. Her pH is 7.15, PaCO2 is 26 mm Hg and
PaO2 is 110 mm Hg while breathing room air.
e. A 31-year-old drug addict responds briefly to the
administration of Narcan (a narcotic antagonist) by
opening her eyes and crying out and then lapses back
into a state of semi-stupor. PaCO2 is 31 mm Hg. pH is
7.38, and PaO2 is 90 mm Hg while breathing nasal
oxygen at 3 L/min.

Indications for Definitive
Airway/Ventilatory support
Need for Airway Protection Need for Ventilation
Unconscious Apnea
• Neuromuscular Paralysis
• Unconscious
Severe Maxillofacial Injuries Inadequate Respiratory Effort
• Tachypnea
• Hypoxia
• Hypercarbia
• Cyanosis
Risk for aspiration
• Bleeding
• Vomiting
Severe head injury with need for
controlling PaCO2 level
Risk for obstruction
• Neck hematoma
• Laryngeal, tracheal injury/burn
• Stridor
• Any patient in cardiac arrest
• Haemodynamic instability due
to septic or cardiogenic shock

What is an ABG?
• The Components
– pH / PaCO2 / PaO2 / HCO3 / O2sat / BE
• Desired Ranges
– pH : 7.35 - 7.45
– PaCO2 : 35-45 mmHg
– PaO2 : 80-100 mmHg
– HCO3 : 22-26 mEq/L
– O2sat : 93-100%
– Base Excess : +/-2 mEq/L

Why Order an ABG?
• Aids in establishing a diagnosis
• Helps guide treatment plan
• Aids in ventilator management
• Improvement in acid/base management
allows for optimal function of medications
• Acid/base status may alter electrolyte
levels critical to patient status/care

Approach to ABG Interpretation
Assessment
of Acid-Base
Status
Assessment of
Oxygenation &
ventilatory
Status
There is an interrelationship, but less
confusing if considered separately…..

The Key to Blood Gas Interpretation:
Four Equations, Three Physiologic Processes
Equation Physiologic Process
1. PaCO2 equation Alveolar ventilation
2. Alveolar gas equation Oxygenation
3. Oxygen content equation Oxygenation
4. Henderson-Hasselbalch Acid-base balance
equation

Assessment of Ventilatory Status….

Breathing pattern’s effect on PaCO2
Patient Vt f Ve Description
A (400)(20) = 8.0L/min (slow/deep)
B (200)(40) = 8.0L/min (fast/shallow)
Patient Vt-Vd f Va
A (400-150)(20) =5.0L/min (slow/deep)
B (200-150)(40) =2.0L/min (fast/shallow)
PaCO2 level is dependent on alveolar ventilation

Condition State of
PaCO2 in blood alveolar ventilation
> 45 mm Hg Hypercapnia Hypoventilation
35 - 45 mm Hg Eucapnia Normal ventilation
< 35 mm Hg Hypocapnia Hyperventilation
PaCO2 abnormalities…

• Assessment of Oxygenation….

ASSESSMENT OF OXYGENATION
• How much oxygen is in the blood?
PaO2 vs. SaO2 vs. CaO2
• Alveolar-arterial O2 tension difference
• PaO2/FIO2 ratio

Alveolar Gas Equation
• PAO2 = PIO2 - 1.2 (PaCO2)*
• Where PAO2 is the average alveolar PO2, and PIO2 is the partial
pressure of inspired oxygen in the trachea
PIO2 = FIO2 (PB – 47 mm Hg)
• FIO2 is fraction of inspired oxygen and PB is the barometric pressure.
47 mm Hg is the water vapor pressure at normal body
temperature.
* Note: This is the “abbreviated version” of the AG equation,
suitable for most clinical purposes. In the longer version, the
multiplication factor “1.2” declines with increasing FIO2, reaching
zero when 100% oxygen is inhaled. In these exercises “1.2” is
dropped when FIO2 is above 60%.

P(A-a)O2
• P(A-a)O2 is the alveolar-arterial difference in partial
pressure of oxygen.
• PAO2 is always calculated based on FIO2, PaCO2, and
barometric pressure.
• PaO2 is always measured on an arterial blood sample in
a “blood gas machine.”
• Normal P(A-a)O2 ranges from @ 5 to 25 mm Hg breathing
room air (it increases with age).
• 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.

• PaO2 ….is the pressure exerted by
dissolved oxygen, not a quantity of
oxygen
• To quantify oxygen calculate CaO2

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 Hb, PaO2 cannot tell us “how much”
oxygen is in the blood;
OXYGEN SATURATION: SaO2
• The percentage of all the available heme binding sites saturated
with oxygen is the Hb oxygen saturation (in arterial blood, the SaO2).
OXYGEN CONTENT: CaO2
• Only CaO2 (units ml O2/dl) tells us how much oxygen is in the blood;
this is because CaO2 is the only value that incorporates the Hb
content. Oxygen content can be measured directly or calculated
by the oxygen content equation:
CaO2 = (Hb x 1.34 x SaO2) + (.003 x PaO2)
Oxygen delivery = Cardiac output x CaO2

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 is 50%.
Point “X” is discussed on later slide.
CaO2
CaO2CaO2

CO Does Not Affect PaO2 – Be Aware!
• Review the O2 dissociation curve shown on a previous
slide. “X” represents the 2nd
set of blood gases for a
patient who presented to the ER with headache and
dyspnea & h/o exposure to smoke in a closed room
• His first blood gases showed PaO2 80 mm Hg, PaCO2 38
mm Hg, pH 7.43. SaO2 on this first set was calculated from
the O2-dissociation curve as 97%, and oxygenation was
judged normal.
• He was sent out from the ER and returned a few hours
later with mental confusion
• This time both SaO2 and COHb were measured (SaO2
shown by “X”): PaO2 79 mm Hg, PaCO2 31 mm Hg, pH
7.36, SaO2 53%, carboxyhemoglobin 46%.
• CO poisoning was missed on the first set of blood gases because SaO2 was
not measured!

Which patient is more hypoxemic, and why?
• Patient A: pH 7.48, PaCO2 34 mm Hg, PaO2 85 mm Hg, SaO2 95%,
Hemoglobin 7 gm%
• Patient B: pH 7.32, PaCO2 74 mm Hg, PaO2 59 mm Hg, SaO2 85%,
Hemoglobin 15 gm%
• Patient A:
Arterial oxygen content = .95 x 7 x 1.34 = 8.9 ml O2/dl
• Patient B:
Arterial oxygen content = .85 x 15 x 1.34 = 17.1 ml O2/dl
• Patient A, with the higher PaO2 but the lower hemoglobin content, is
more hypoxemic.
• In this problem the amount of oxygen molecules contributed by the
dissolved fraction is negligible and will not affect the answer.

Assessment of acid base balance…

Basics
•Nano equivalent =1×10-9
•[H+]= 40 nEq/L (16 to 160 nEq/L) at pH-7.4
•For every 0.3 pH change = [H+] double

Very fast 80% in ECF
Starts within minutes
good response by 2hrs,
complete by 12-24 hrs
Starts after few hrs
complete by 5-7 days

Acid-base Balance
Henderson-Hasselbalch Equation
[HCO3
-
]
pH = pK + log -------------
.03 [PaCO2]
For teaching purposes, the H-H equation can be
shortened to its basic relationships:
HCO3
-
pH ~ ---------
PaCO2

Abnormal Values
pH < 7.35
• Acidosis (metabolic
and/or respiratory)
pH > 7.45
• Alkalosis (metabolic
and/or respiratory)
paCO2 > 45 mm Hg
• Respiratory acidosis
(alveolar
hypoventilation)
paCO2 < 35 mm Hg
• Respiratory alkalosis
(alveolar
hyperventilation)
HCO3 < 22 meq/L
• Metabolic acidosis
HCO3 > 26 meq/L
• Metabolic alkalosis

SIMPLE ACID-BASE DISORDER
Simple acid-base disorder – a single
primary process of acidosis or alkalosis
with or with out compensation
1.pH=7.2 PCO2 = 60 mmHg, HCO3-24mEq/L-no
compensation
2.pH 7.36, PaCO2 53 ,HCO3 30-with compensation

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.
Clues to a mixed disorder:
• Normal pH with abnormal HCO3 or CO2
• PaCO2 and HCO3 move in opposite directions
• pH changes in an opposite direction for a
known primary disorder

Characteristics of 1° acid-base disorders
DISORDER PRIMARY
RESPONSES
COMPENSATORY
RESPONSE
Metabolic
acidosis
↑ [H+
] ↓ PH ↓ HCO3
-
↓ pCO2
Metabolic
alkalosis
↓ [H+
] ↑ PH ↑ HCO3
-
↑ pCO2
Respiratory
acidosis
↑ [H+
] ↓ PH ↑ pCO2 ↑ HCO3
-
Respiratory
alkalosis
↓ [H+
] ↑ PH ↓ pCO2 ↓ HCO3
-

Compensation…The Rules..
The body always tries to normalize the pH
so…
• CO2 and HCO3 should rise and fall
together in simple disorders
• Compensation never overcorrects the pH
• Lack of compensation in an appropriate
time interval defines a 2nd disorder
• Compensatory responses require normally
functioning lungs and kidneys

RENAL & RESPIRATORY COMPENSATIONS TO 1° ACID-
BASE DISTURBANCES
Disorder Compensatory response
Metabolic acidosis PCO2
↓ 1.2 mmHg per 1.0 meq/L ↓ HCO3
1. Compensation complete in 12-24hrs
2. Limit of CO2 is 10mmHg
Metabolic alkalosis PCO2
↑ 0.7 mmHg per 1.0 meq/L ↑HCO3
-
1.Compensation is complete (pCO2 levels out) in 12-24
hours.
2.The limit of compensation is a pCO2 of 60 mmHg

Respiratory acidosis [HCO3-] ↑
Acute 1.0 meq/L per 10 mmHg ↑ Pco2
Chronic 3.5 meq/L per 10 mmHg ↑ Pco2
Respiratory alkalosis [HCO3-] ↓
Acute 2.0 meq/L per 10 mmHg ↓ Pco2
Chronic 4.0 meq/L per 10 mmHg ↓ Pco2

Expected changes in pH for a 10-mm Hg change in
PaCO2 resulting from either primary respiratory
acidosis or respiratory alkalosis:
ACUTE CHRONIC
•Respiratory Acidosis
pH ↓ by 0.07 pH ↓ by 0.03
•Respiratory Alkalosis
pH ↑ by 0.08 pH ↑ by 0.03

Anion Gap
AG = [Na+
] - [Cl-
+HCO3-
]
• Elevated anion gap represents
metabolic acidosis
• Normal value: 10 ± 2 mmol/L
• Major unmeasured anions
– albumin
– phosphates
– sulfates
– organic anions

Na+
Unmeasured
cations
Unmeasured
anions
Cl-
HCO3-
‘Mind the gap’
cations = Anions
Anion gap =
metabolic
acidosis

Six steps for ABG ANALYSIS
• 1. The first step - Look at the pH - Label it.
pH of 7.30, PaCO2 of 80 mm Hg, and
HCO3- of 27 mEq/L.
• ACIDOSIS

• 2. The second step look at -pCO2. Label it.
• pH of 7.30, PaCO2 of 80 mm Hg, and
HCO3- of 27 mEq/L.
• Increased
Normal pCO2 levelsNormal pCO2 levels
are 35-45mmHg.are 35-45mmHg.
Below 35 is alkalotic,Below 35 is alkalotic,
above 45 is acidic.above 45 is acidic.

• 3. The third step is to look at the HCO3-
Label it.
pH of 7.30, PaCO2 of 80 mm Hg, and HCO3- of 27 mEq/L
• INCREASED
A normal HCO3 level is 22-26A normal HCO3 level is 22-26
mEq/L. If the HCO3 is below 22,mEq/L. If the HCO3 is below 22,
the patient is acidotic. If thethe patient is acidotic. If the
HCO3 is above 26, the patientHCO3 is above 26, the patient
is alkaloticis alkalotic

4.Next match either the pCO2 or the
HCO3 with the pH to determine the
acid-base disorder.
• pH of 7.30, PaCO2 of 80 mm Hg, and
HCO3- of 27 mEq/L
• pH is on acidotic side & PCO2 is increased.
So it is respiratory acidosis

• 5. Fifth, does either the CO2 or HCO3
go in the opposite direction of the pH?
• pH of 7.30, PaCO2 of 80 mm Hg, and HCO3-
of 27 mEq/L
• To find the primary and what is
compensatory
• HCO3 is going in opposite direction of
pH. So it is metabolic compensation

Is the compensation full or partial??
• Do the calculations….
pH of 7.30, PaCO2 of 80 mm Hg, and HCO3- of
27 mEq/L
• PCO2 is increased by =40
• HCO3-=should be increased by 4
i.e. 24+4=28( for full compensation)

• 6. Calculate the anion gap if it is more
there is Metabolic acidosis
AG = [Na+] - [Cl- +HCO3-AG = [Na+] - [Cl- +HCO3-]]

• Don’t forget to assess ventilation and
oxygenation status ….

A patient’s in acute respiratory distress,
ABG 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)?
Example…

• 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 by 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 by 3 mEq/L.
• Decreased perfusion leading to mild lactic acidosis
would explain the metabolic component.

Importance of History/Patients
•pH 7.08
•PCO2 80
•HCO3-24mEq/l
•80KG Man post
gastrectomy on CMV 6L
of mv
•Acidosis
•No compensation
•Expected HCO3-
24+4=28mEq/L
•pH 7.08
•PCO2 80
•HCO3-24mEq/l
•Diabetic + Chronic COPD
•Baseline PCO2 -80
•Stopped insulin few days
back
•DKA
•Expected HCO3-
=24+4×3.5=24+14=38
RESPIRATORY
ACIDOSIS METABOLIC
ACIDOSIS

Sample ABG
• pH 7.45
• PaCO2 38
• HCO3 23
• Analysis:
• Cause:??

Case 1
• Mr. A is a 60 year-old with pneumonia. He is admitted
with dyspnea, fever, and chills. His blood gas is below:
pH 7.28
CO2 56
PO2 70
HCO3 25
SaO2 89%
• What is your interpretation?
• What interventions would be appropriate for Mr. A?
• Mr. A has an uncompensated respiratory acidosis with
hypoxemia as a result of his pneumonia

Case 2
• Ms. B is a 24 year-old college student. She has acute GE
and is complaining a of a 3 day history of watery
diarrhea. A blood gas is obtained to assess her
acid/base balance:
pH 7.28
CO2 43
pO2 88
HCO3 20
SaO2 96%
• What interventions would be appropriate for Ms. B?
• Ms. B has an uncompensated metabolic acidosis. This is
due to excessive bicarbonate loss from her diarrhea.

Case 3
• Mr. C is a 80 year-old nursing home resident admitted
with urosepsis. Over the last two hours he has developed
shortness of breath and is becoming confused. His ABG
shows the following results:
pH 7.02
CO2 55
pO2 77
HCO3 14
SaO2 89%
• What interventions would be appropriate for Mr. C?
• Mr. C has a metabolic and respiratory acidosis with
hypoxemia. The metabolic acidosis is caused by his
sepsis. The respiratory acidosis is secondary to respiratory
failure.

Case 4
• 4. Mrs. D is a thin, elderly-looking 61 year-old
COPD patient. She has an ABG done as part of
her routine care in the pulmonary clinic. The
results are as follows:
pH 7.37
CO2 63
pO2 58
HCO3 35
SaO2 89%
• What interventions would be appropriate for
Mrs. D?
• Mrs. D has a fully-compensated respiratory
acidosis with hypoxemia

Case 5
• Ms. E is a 17 year-old with intractable vomiting. She has
some electrolyte abnormalities, so a blood gas is
obtained to assess her acid/base balance.
pH 7.50
CO2 36
pO2 92
HCO3 27
SaO2 97%
• What interventions would be appropriate for Ms. E?
• Ms. E has an uncompensated metabolic alkalosis.

Case 6
• Mr. F is a 18 year-old comatose, quadriplegic
patient who has the following ABG done as part
of a medical workup:
pH 7.44
CO2 22
pO2 96
HCO3 16
SaO2 98%
• What interventions would be appropriate for Mr.
F ?
• His blood gas shows a fully-compensated
respiratory alkalosis

Case 7• A 76 yrs, female admitted with right sided weakness, visual
disturbance, slurred speech. Started on NG feeding but has large
vomit 24 hrs later. She initially appears well later she became
agitated, distressed, pyrexial
• PEx: RS-basal coarse crepts present, CNS-confused, other findings
same as before
• PR-100/min, RR- 29/min, BP-110/70 mmHg, O2%-92% with
supplementary O2 ( reservoir mask + O2)
FIO2 .60 Na+
136 mEq/L
pH 7.4 K+
3.8 mEq/L
PaCO2 33 mm Hg Cl-
99 mEq/L
PaO2 65 mm Hg lactate 1.5 mmol/L
SaO2 92%
HCO3
-
21 mEq/L
%COHb 2.1%
Hb 13 gm%
How would you characterize her state of oxygenation, ventilation,
and acid-base balance? What is the likely diagnosis??

• Type I respiratory impairment, mild
respiratory alkalosis balanced by
metabolic acidosis
• Aspiration pneumonia

Case 8
• Mrs. H is found pulseless and not breathing this
morning. After a couple minutes of CPR she
responds with a pulse and starts breathing on
her own. A blood gas is obtained:
pH 6.89
CO2 70
pO2 42
HCO3 13
SaO2 50%
• What interventions would be appropriate for
Mrs. H?
• Mrs. H has a severe metabolic and respiratory
acidosis with hypoxemia

Case 9
• Mr. X is in respiratory distress. He has a history of
Type-I diabetes mellitus and is now febrile. His
ABG shows:
pH 7.00
CO2 59
pO2 86
HCO3 14
SaO2 91%
• What interventions would be appropriate for Mr.
X?
• Mr. X has a metabolic and respiratory acidosis
with hypoxemia.

Case 10
• Ms. Y was admitted for a drug overdose. She is being
mechanically ventilated and a blood gas is obtained to
assess her for weaning. The results are as follows:
pH 7.54
CO2 19
pO2 100
HCO3 16
SaO2 98%
• What interventions would be appropriate for Ms. Y?
• Mrs. Y is being overventilated which caused a partially-
compensated respiratory alkalosis

Case 11
A 46-year-old man has been in the hospital for 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 7.41
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/dl
How would you characterize his state of oxygenation,
ventilation, and acid-base balance?

• Normal pH with very low bicarbonate and
PaCO2 indicates combined respiratory
alkalosis and metabolic acidosis.

Case 12
• A 59 yrs, male, chronic alcoholic, h/o severe upper
abdominal pain x 3 days, breathlessness present,
excessive alcohol consumption for past few weeks
• PEx: looks sick, shortness of breath present, epigastric tenderness
present, CXR- few b/l scattered opacites
• PR-120/min, RR- 28/min, BP-75/60 mmHg, O2%-98%
• On 8 l/min O2 with mask & reservoir bag
Hb 11 gm% Na+
142 mEq/L
pH 7.31 K+
3.9 mEq/L
PaCO2 24 mm Hg Cl-
101 mEq/L
PaO2 81 mm Hg lactate 4 mmol/L
SaO2 98% iCa+ 0.8 mmol/L
HCO3
-
15 mEq/L
BE -12 mEq/L
How would you characterize his state of oxygenation, ventilation,
and acid-base balance? What is the probable diagnosis and furhter
action ??

• Type I respiratory impairment, severe
metabolic acidosis with partial
compensation most likely due to acute
pancreatitis

• A 55 yr, female c/o sudden onset of breathlessness & left sided chest
pain, underwent knee replacement operation 4 days back, no
relevant past medical history
• PEx: slight shortness of breath, CVS/RS-NAD, no clinical evidence of
DVT, CXR-NAD, ECG-only tachycardia
• PR-98/min, RR- 20/min, BP-150/90 mmHg, temp-36.6 degrees,O2%-
99%
Case 13
FIO2 .21 Na+
136 mEq/L
pH 7.43 K+
3.8 mEq/L
PaCO2 37 mm Hg Cl-
99 mEq/L
SaO2 99%
HCO3
-
25.8 mEq/L
%COHb 2.1%
Hb 10 gm%
and acid-base balance? Does she require any further work up??

• Normal gas exchange, normal acid base status
• PA02=(.21 x 713) + (37 x 1.2), PaO2 91 mm Hg
• P(A-a)02= 106-91=15 mmHg
• Patient is high risk for PE
• Normal ABG never excludes it and she requires
V/Q scan or CTPA

Case 14
• A 36 yr, male with alprazolam tab consumption x 3,
slightly drowsy but easily arousable, no relevant past
medical history
• PEx: RS/CVS-NAD
• PR-80/min, RR- 14/min, BP-110/70 mmHg, temp-36.6 degrees,
SpO2%-99%
FIO2 .21 Na+
138 mEq/L
pH 7.37 K+
3.8 mEq/L
PaCO2 41 mm Hg Cl-
104 mEq/L
SaO2 74%
HCO3
-
24 mEq/L
%COHb 2.1%
Hb 13 gm%
and acid-base balance? What is the likely explanation for low
PaO2??

• Appearance of severe type I respiratory failure,
normal acid base status
• There is marked discrepancy between SaO2 by
pulse oximeter and that calculated by ABG
• It is a venous sample & repeat ABG should be
done

Case 15
• A 70 yrs, male, comes to casualty with shortness of breath
and excessive tiredness, h/o chronic PR bleeding present
• Past medical history-k/c/o HTN, IHD
• PEx: RS/CVS-NAD
• PR-110/min, RR- 23/min, BP-140/90 mmHg, O2%-99% on air
FIO2 .21 Na+
138 mEq/L
pH 7.49 K+
3.8 mEq/L
PaCO2 25 mm Hg Cl-
104 mEq/L
SaO2 99%
HCO3
-
22 mEq/L
%COHb 2.1%
Hb 6.7 gm%
and acid-base balance? What is the likely explanation for
breathlessness??

• Hyperventilation- no impairment of oxygenation
but hypoxemia due to anaemia
• Uncompensated respiratory alkalosis
• Patient has severe anaemia due to iron
deficiency/chronic rectal bleeding

Case 16
• A 21yrs,female, known asthmatic, with 6hr h/o worsening
breathlessness & wheeze, no relief from salbutamol
inhaler
• PEx: tachypnoeic, using accessory muscles, just
managing to speak in full sentences
• PR-115/min, RR- 30/min, BP-110/80 mmHg, O2%-96% on air
FIO2 .21 Na+
138 mEq/L
pH 7.38 K+
3.8 mEq/L
PaCO2 43 mm Hg Cl-
104 mEq/L
SaO2 96%
HCO3
-
24 mEq/L
Hb 12 gm%
How would you characterize her state of oxygenation, ventilation,
and acid-base balance? Would you be concerned?? If so why??

• Mild type I respiratory impairment
• PaCO2-high end of the normal range
• Life threatening attack

Treat the patient not the ABG!!!
• “ABG’’ should supplement clinical judgment not
substitute it”
• Treat the underlying clinical condition(s); this will
usually suffice to correct most acid-base
disorders.
• If there is concern that acidemia or alkalemia is
life-threatening, aim toward correcting pH into
the range of 7.30 - 7.52 ([H+
] = 50-30 nM/L).

• THANK YOU FOR PATIENT HEARING…

A patient is admitted to the ICU with the
following lab values:
BLOOD GASES
pH: 7.40
PCO2: 38
HCO3: 24
PO2: 72
ELECTROLYTES, BUN & CREATININE
Na: 149
K: 3.8
Cl: 100
CO2: 24
BUN: 110
Creatinine: 8.7
What is(are) the acid-base disorder(s)?
(in this case venous CO2=arterial HCO3-)

Step 1: Anion gap
AG = Na+ - (Cl + CO2)= 149 - (100 + 24) = 25
This high an AG indicates an anion gap metabolic acidosis.
Step 2: Delta anion gap
calculated AG= 25 mEq/L
normal AG = 12 mEq/L
25 - 12 = 13 mEq/L; this is the excess or delta anion gap
Step 3: Delta serum CO2 = normal CO2 - measured CO2
=27 (average normal venous CO2) - 24 = 3 mEq/L
Step 4: Bicarbonate Gap = delta AG - delta CO2 = 13 - 3 =
10 mEq/L
This means the measured bicarbonate is 10 mEq/L higher
than expected from the excess AG, indicating (in this
case) a metabolic alkalosis. Thus this patient, with normal
pH and PaCO2, has BOTH metabolic acidosis and
metabolic alkalosis. The patient was both uremic
(causing metabolic acidosis) and had been vomiting
(metabolic alkalosis)

pH < 7.35
Acidosis
pH > 7.35
Alkalosis
pCO2 > 40
Respiratory
HCO3 < 24
Metabolic
pCO2 < 40
Respiratory
HCO3 > 24
Metabolic
PaCO2 ↑10
→HCO3 ↑4
PaCO2 ↑10
→HCO3 ↑1
PaCO2 ↓10
→HCO3 ↓4
PaCO2 ↓10
→HCO3 ↓2
PaCO2 ↑7
→HCO3 ↑10
Urine Cl < 10
Cl ResponsiveAnion Gap < 12
Non-Anion Gap
Anion Gap > 12
Anion Gap
Urine Cl > 10
Cl Unresponsive
Interpreting ABGs
Osm Gap > 10
Methanol
Ethylene Glycol
Osmolar Gap < 10
Ketoacidosis
Lactic acidosis
Uremia
Aspirin/salicylate tox
Diarrhea
Renal tubular acidosis
Acetazolamide
Total parenteral nutrition
Ureteral diversion
Pancreas transplant
CNS depressants
Neuromuscular disorder
Thoracic cage abnormalities
Obstructive lung disease
Obesity/hypoventilation syndrome
Myxedema coma
Anxiety/pain
Sepsis
CNS (stroke)
Aspirin OD
Chronic liver disease
Pulmonary embolism
Pregnancy
Hyperthyroidism
Loss of body fluids:
Vomiting
Nasogastric suctioning
Diuretic use
Excess body fluids:
Exogenous steroids
Cushing’s syndrome
Hyperaldosteronism
Bartter’s syndrome
=Na - (Cl+HCO3)
Acute
Chronic
PaCO2 ↓12
→HCO3 ↓10
Compensation:
If:
ΔPCO2/ΔHCO3
=
CO2/HCO3ratio
Then it is comp.
Acute
Chronic
(2xNa) + (Glu/18) +
(BUN/2.8) = calculated
serum osmoles
HCO3 loss Extra H+

P(A-a)O2: Test Your Understanding - Answers
• a) PAO2 = .40 (760 - 47) - 1.2 (50) = 225 mm Hg; P(A-a)O2 = 225 - 150
= 75 mm Hg
The P(A-a)O2 is elevated but actually within the expected range for
supplemental oxygen at 40%, so the patient may or may not have
a defect in gas exchange.
• b) PAO2 = .28 (713) - 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.
• c) PAO2 = .21 (713) - 1.2 (15) = 150 - 18 = 132 mm Hg; P(A-a)O2 =
132 - 120 = 12 mm Hg
Hyperventilation can easily raise PaO2 above 100 mm Hg when the
lungs are normal, as in this case.

d) PAO2 = .80 (713) - 40 = 530 mm Hg (Note that the factor 1.2
is dropped since FIO2 is above 60%)
P(A-a)O2 = 530 - 350 = 180 mm Hg
P(A-a)O2 is increased. Despite a very high PaO2, the lungs
are not transferring oxygen normally.
e) PAO2 = .21 (713) - 1.2 (72) = 150 - 86 = 64 mm Hg; P(A-a)O2 =
64 - 80 = -16 mm Hg
A negative P(A-a)O2 is incompatible with life (unless it is a
transient unsteady state, such as sudden fall in FIO2 -- not
the case here). In this example, negative P(A-a)O2 can be
explained by any of the following: incorrect FIO2, incorrect
blood gas measurement, or a reporting or transcription
error.
P(A-a)O2: Test Your Understanding - Answers

PaCO2 and Alveolar Ventilation: Q & A
1. What is the PaCO2 of a patient with respiratory
rate 24/min, tidal volume 300 ml, dead space
volume 150 ml, CO2 production 300 ml/min? The
patient shows some evidence of respiratory distress.
First, you must calculate the alveolar ventilation.
Since minute ventilation is 24 x 300 or 7.2 L/min, and
dead space ventilation is 24 x 150 or 3.6 L/min,
alveolar ventilation is 3.6 L/min. Then
300 ml/min x .863
PaCO2 = -----------------------
3.6 L/min
PaCO2 = 71.9 mm Hg

Alveolar Gas Equation
PAO2 = PIO2 - 1.2 (PaCO2)
where PIO2 = FIO2 (PB – 47 mm Hg)
Except in a temporary unsteady state, alveolar PO2
(PAO2) is always higher than arterial PO2 (PaO2). As a
result, whenever PAO2 decreases, PaO2 also decreases.
Thus, from the AG equation:
• If FIO2 and PB are constant, 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 leads to hypoxemia).

Alveolar Gas Equation:
Q & A
1. What is the PAO2 at sea level in the following
circumstances? (Barometric pressure = 760 mm Hg)
a) FIO2 = 1.00, PaCO2 = 30 mm Hg
b) FIO2 = .21, PaCO2 = 50 mm Hg
c) FIO2 = .40, PaCO2 = 30 mm Hg
To calculate PAO2 the PaCO2 must be subtracted from
the PIO2. Again, the barometric pressure is 760 mm Hg
since the values are obtained at sea level. In part a,
the PaCO2 of 30 mm Hg is not multiplied by 1.2 since
the FIO2 is 1.00. In parts b and c, PaCO2 is multiplied by
the factor 1.2.
a) PAO2 = 1.00 (713) - 30 = 683 mm Hg
b) PAO2 = .21 (713) - 1.2 (50) = 90 mm Hg
c) PAO2 = .40 (713) - 1.2 (30) = 249 mm Hg

Ventilation-perfusion Imbalance
• A normal amount of ventilation-perfusion (V-Q)
imbalance accounts for the normal P(A-a)O2.
• By far the most common cause of low PaO2 is an
abnormal degree of ventilation-perfusion imbalance
within the hundreds of millions of alveolar-capillary units.
Virtually all lung disease lowers PaO2 via V-Q imbalance,
e.g., asthma, pneumonia, atelectasis, pulmonary
edema, COPD.
• Diffusion barrier is seldom a major cause of low PaO2 (it
can lead to a low PaO2 during exercise).

Respiratory and Renal Compensatory
Mechanisms for ACID-BASE Disturbance

Quantification of Dead space
VD
VVT
=
25-40% is normal
In MV pts till 55% is normal
More than 60% is abnormal
dead space
Volume not taking
part in gas
exchange=dead
space
Effective alveolar
ventilation
=MV-VD

PCO2 vs. Alveolar Ventilation
The relationship is shown for
metabolic CO2 production rates
of 200 ml/min and 300 ml/min
(curved lines).
A fixed decrease in VA(x-axis) in
the hypercapnic patient will
result in a greater rise in PaCO2
(y-axis) than the same VA
change when PaCO2 is low or
normal.
This graph also shows that if VA is
fixed, an ↑ in CO2 production will
result in an ↓ in PaCO2.

Problems: Oxygenation
• Room Air, PaO2 = 45, PaCO2 =30
– PAO2 = 150 – 1.2(30) = 114 mm Hg
– P(A-a)O2 = 114 - 45 = 69 elevated
• Now on 100% O2, PaO2 = 65, PaCO2= 32
– minimal elevation in PaO2
– shunt major cause of hypoxemia

Problems: Oxygenation
• Room Air, PaO2 = 45, PaCO2 = 45
– PAO2 = 150 - 1.2(45) = 96
– P(A-a)O2 = 96 - 45 = 51
• Now on 100% O2, PaO2 =555, PaCO2 = 48
– PAO2 = 1.0(760 - 47) - 1.2(48) = 655
– P(A-a)O2 = 655 - 555 = 100
– Dramatic increase in PaO2
– V/Q mismatch major cause of hypoxemia

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