Clinical Aspect of
Interpretation of Blood
Gas Analysis
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.