Clinical Aspect of

Interpretation of Blood

Gas Analysis

What Does Arterial

Blood Gas (ABG)

Measure

?

Pulmonary function tests are concern with
ventilation: the movement of air into and
out of the lung

MIXED VENOUS BLOOD

pH 7.36
PCO

2

46 mmHg

PO

2

40 mmHg

SO

2

75%

pH 7.40
PCO

2

40 mmHg

PO

2

95 mmHg

SO

2

95%

ARTERIAL BLOOD

External Respiration

Internal Respiration

What Information Does

Arterial

Blood Gas provide?

•

Arterial oxygenation

•

Alveolar ventilation

•

Respiratory/metabolic acid-base balance

•

Carboxyhemoglobin levels

Alveolar Ventilation

Equation

Inverse relationship between V

A

and

P

a

CO

2

K

V

V

PCO

A

CO





.

.

2

2

Arterial Blood Gas

Analysis

Indications

•

Evaluate adequacy of lung function

–

Ventilation, acid-base status

–

Oxygenation

•

Determine need for supplemental O

2

•

Monitor ventilatory support

•

Document severity or progression of
known pulmonary disease

•

Diagnose the toxicity of CO

Henderson-Hasselbalch

Equation

The relationship between pH, P

a

CO

2

,

HCO

3

-

1

20

,

,

]

[

log

3

2

3

or

Lungs

Kidneys

or

CO

H

HCO

pK

pH







Why the assessment of a

single buffer system is

adequate despite multiple

buffer systems?

•

Bicarbonate buffer system: of primary
importance, open system in communication with
external environment via kidneys and lungs

•

Hemoglobin buffer: of second importance

•

Phosphate buffer system

•

Plasma protein buffer system

•

All buffer systems are linked together through
H

+

Why the assessment of

carbonic acid is adequate?

•

Each day our
body produces
large amount of
acid from
metabolism. 99%
of the total acid
is in the form of
CO

2

. Only 1% is

fixed acid

Henderson-Hasselbalch

Equation

The relationship between pH, P

a

CO

2

,

HCO

3

-

1

20

,

,

]

[

log

3

2

3

or

Lungs

Kidneys

or

CO

H

HCO

pK

pH







Case 1: normal

0301

.

0

*

]

[

log

2

3

CO

P

HCO

pK

pH

a







Case 1: normal

0301

.

0

*

40

/

24

log

1

.

6

L

mEq

pH





Case 1: normal

L

mEq

L

mEq

pH

/

2

.

1

/

24

log

1

.

6 



L

mEq

L

mEq

pH

/

2

.

1

/

24

log

1

.

6 



Case 1: normal

1

20

log

1

.

6 



pH

Case 1: normal

4

.

7

3

.

1

1

.

6







pH

Case 2

Uncompensated Respiratory

Acidosis

0301

.

0

*

3

.

68

/

3

.

25

log

1

.

6

L

mEq

pH





Case 2

Uncompensated Respiratory

Acidosis

L

mEq

L

mEq

pH

/

2

.

1

/

24

log

1

.

6 



L

mEq

L

mEq

pH

/

06

.

2

/

3

.

25

log

1

.

6 



Case2

Uncompensated Respiratory

Acidosis

1

12

log

1

.

6 



pH

Case 2

Uncompensated Respiratory

Acidosis

18

.

7

08

.

1

1

.

6







pH

Case 3

Compensated Respiratory

Acidosis

0301

.

0

*

3

.

68

/

4

.

36

log

1

.

6

L

mEq

pH





Case3

Compensated Respiratory

Acidosis

1

7

.

17

log

1

.

6 



pH

Case 3

Compensated Respiratory

Acidosis

35

.

7

25

.

1

1

.

6







pH

PEARL: The compensations of either
the renal system or the respiratory
system can never be complete.

Clinically Relevant

Parameters (1)

Through the years,

opinions have changed
regarding what are the most
clinically relevant parameters.
Today, for a nearly complete
description of the oxygenation,
ventilation, and acid-base
status, pH, PaCO

2

, PaO

2

and

actual HCO

3

-

are generally

sufficient.

Clinically Relevant

Parameters (2)

Indeed, the literature or text
book contains literally several
parameters, i.e. standard HCO

3

-

,

buffer base (BB), base excess
(BE) from in vitro measures.
Because intro and in vivo
changes in response to
hypercapnia are different,
their actual clinical benefit is
limited.

Burton GG, Hodgkin JE, Ward JJ.

Respiratory care: A

guide to clinical practice. 1997, 260-265.

Primary Respiratory

Acidosis

•

Initiating event:
hypoventilation

•

Resultant effects: CO

2

retention

•

Compensation: HCO

3-

retention

via renal system

Primary Respiratory

Alkalosis

•

Initiating event: hyperventilation

•

Resultant effects: CO

2

elimination

•

Compensation: HCO

3-

elimination

via renal system

Primary Metabolic

Acidosis

•

Initiating event: renal, extrarenal

•

Resultant effects: HCO

3-

deficit

•

Compensation: CO

2

elimination

via respiratory system

Primary Metabolic

Alkalosis

•

Initiating event: renal, extrarenal

•

Resultant effects: HCO

3-

increase

•

Compensation: CO

2

retention via

respiratory system

Normal Range of Arterial

Blood Gases

Normal Range

Clinical Indication

pH

PCO

2

HCO

3

-

7.35-7.45

35-45
22-27

Acid-base parameter

Respiratory parameter

Metabolic parameter

Interpretation of Arterial

Blood Gases

Interpretation Strategies

Step 1

Was the blood gas specimen
obtained acceptably? Free of air
bubbles and clots? Analyzed
promptly and/or iced properly?

Air Contamination of

Sample

In vivo values Air contamination

PH

PCO

2

PO

2

7.40

40

95

7.45

30

110

Step 2

Did the blood gas analyzer function
properly? Was there a recent
acceptable calibration of all
electrodes? Was analyzer function
validated by appropriate quality
control?

Data Quality in Blood

Gases

•

Blood collected anaerobically

•

The specimen adequately anticoagulated

•

A 2-4 ml sample recommended

•

The specimen analyzed in a few minutes,
otherwise stored in ice within 1 hour

•

Equipment calibration and quality control

•

The specimen adequately identified

Step3

Determine acid-base

imbalance

The normal limits of pH is 7.35 - 7.45.

If below 7.35, acidosis is present; If

above 7.45, alkalosis is present.
Otherwise look for compensation.

Is pH within normal
limits?

Step 4

the cause of acid-base

imbalance?

Respiratory?

•

If PCO

2

>45 and pH <7.35, respiratory acidosis.

•

If PCO

2

>45 and pH 7.35-7.45, then

compensated respiratory acidosis

•

If PCO

2

<35 and pH >7.45, respiratory alkalosis

•

If PCO

2

<35 and pH 7.35-7.45, then

compensated respiratory alkalosis

Step 4

the cause of acid-base

imbalance?

Metabolic?

•

If HCO

3-

<22 and pH <7.35, metabolic acidosis.

•

If HCO

3-

<22 and pH 7.35-7.45, then

compensated metabolic acidosis

•

If HCO

3-

>27 and pH >7.45, metabolic alkalosis

•

If HCO

3-

>27 and pH 7.35-7.45, then

compensated metabolic alkalosis

Step 5

Oxygenation?

Is P

a

O

2

within normal limits of 80

to 100 mm Hg? If P

a

O

2

< 50 mm

Hg, severe hypoxemia is present.

The Hypoxemic State

Hypoxemia is defined as PaO2 < 80 m
m Hg while breathing room air. Whe
n patients are already on oxygen it i
s not necessary and may be dangerou
s to interrupt the oxygen therapy to
assess hypoxemia.

Step 6

Correlated with clinical

picture?

Are blood gas results consiste

nt with patient's clinical status
?

Case 1

•

pH 7.35

•

PCO

2

30 mm Hg

•

HCO

3-

16 mEq/L

What is your interpretation?

Case 2

•

pH 7.45

•

PCO

2

30 mm Hg

•

HCO

3-

20 mEq/L

What is your interpretation?

Case 3

•

pH 7.55

•

PCO

2

27 mm Hg

•

HCO

3-

23 mEq/L

•

PO

2

104 mm Hg

•

Get the plastic bag out!!!

What is your interpretation?

Case 4

•

pH 7.30

•

PCO

2

34 mm Hg

•

HCO

3-

24 mEq/L

•

Get the technician out!!!

What is your interpretation?

Case 5

A patient referred to PFT Lab. for
shortness of breath

Case 5

pH 7.28

HCO

3

-

25.8

mEq/L

PCO

2

51 mm Hg

PO

2

55 mm Hg

What is your interpretation?

Case 6

A 17 y/o diabetic, entered Emergency
with Kussmaul breathing

Case 6

Interpretation?

pH 7.05

HCO

3

-

5

mEq/L

PCO

2

12 mm Hg

PO

2

108 mm Hg

Case 7

34 y/o female, entered Emergency
in coma, drug overdose suspected

Case 7

pH 7.15

HCO

3

-

28

mEq/L

PCO

2

80 mm Hg

PO

2

42 mm Hg

What is your interpretation?

Case 8

A 63 y/o male, admitted for
elective knee surgery

Case 8

pH 7.36

BP

122/84

PCO

2

46 mm Hg

P 80,

regular

PO

2

41 mm Hg RR 15/min

Preoperative blood gas

Suggested panic values of

ABG

•

pH < 7.20

•

pH > 7.60

•

PaCO

2

> 65mmHg (check pH and

HCO

3-

to see compensation)

•

PaO

2

< 50mmHg (exception:

congenital cardiac malformations)

•

COHb > 20%

•

MetHb > 10%

Summary

Since arterial blood gas
analysis is the reflection
of efficiency or inefficiency
of several organ
systems, proper
interpretation is essential in
the care of critically ill
patients.