ARTERIAL BLOOD GAS

ABG

 1. Acid-base Balance
 2. Blood oxygenation assessment
Arterial pH

 Maintained at 7.35-7.45 by
 Intracellular and extracellular buffering
 Respiratory mechanisms
 Renal mechanisms
 Arterial CO2 tension (PaCO2)
 Controlled by the CNS and respiratory system
 Plasma bicarbonate
 Controlled by the kidneys
 By retention or excretion of acid or alkali
CLINICAL MANIFESTATIONS

 LOW ARTERIAL PH

- generalized depressive effect

- drowsiness and lethargy
- very low pH (<7.1) – coma
- < 6.8 for any extended period –
incompatible with life
CLINICAL MANIFESTATIONS

 HIGH BLOOD pH

- excitatory to the CNS

- irritability or tetany
- serious arrhythmias
- very high pH = convulsions
- > 7.8 – incompatible with life
pH Symptoms
7.80 Death
Convulsions
Arrhythmias
Irritability
7.40 Normal
Drowsiness
Lethargy
Coma
6.80 Death
PaCO2

 Usual steady state PaCO2 at 40 mmHg

 Under-excretion causes hypercapnia
 Over-excretion produces hypocapnea
 Regulated primarily by neural respiratory factors
and not by rate of CO2 production
 May be due to compensatory changes in
response to a primary alteration in the plasma
HCO3
HCO3

 24 (22-26)
 Regulated by
 Reabsorption of filtered HCO3
 Formation of titratable acid
 Excretion of ammonia in the urine
 Kidneys filter 4000 mmol per day
 80-90% in the proximal tubule
Acid-Base Disorders

 Primary respiratory disturbances invoke

compensatory metabolic responses and vice
versa in an attempt to drive pH to normal
 The degree of compensation can be predicted
 Any abnormal compensation define a mixed
acid-base disturbance
Acid-Base Disorders
Acid-Base Disorders
 Causes

Acidosis
Alkalosis
NAGMA HAGMA

• GI Bicarbonate loss Lactic acidosis • Diuretics

• Renal acidosis Ketoacidosis • Antacids
• Drug induced Toxins • Volume
hyperkalemia Renal failure depletion
MUDPILES • Cushing
syndrome
• CAH
• Bartter syndrome
• Gitelman
syndrome
Acid-Base Disorders
Acid-Base Disorders
 Causes

Acidosis Alkalosis

COPD Anxiety
Neuromuscular diseases Pneumonia
Chest wall disorders High altitude
Obesity/OSA Pregnancy
CNS depression Sepsis
Drugs Drugs
Anion gap

 Measure of unmeasured anions in the blood

 Major anions: Cl- and HCO3
 Others: phosphates, sulfates and proteins
 Major Cation: Na
 Others: Ca, K, Mg
 Anion Gap: unmeasured anions – unmeasured
cations
 (Na)- (Cl+HCO3)
 8-10 mmol/L
Compensatory Responses in Simple Acid-Base Disturbances

 Metabolic Acidosis Limit

 PCO2= (1.5 x HCO3) + 8 + 2 10 mmhg
 Metabolic Alkalosis
 Each mEq inc. in HCO3=
 0.5-1.0 mmhg inc in PCO2 55 mmhg
 Acute Respiratory Acidosis
 HCO3 inc by 3-4 mEq/L 30 mmhg
 Acute Respiratory Alkalosis
 HCO3 dec by 2-4 mEq/L 18 mEq/L
 Chronic Respiratory Acidosis
 Each mEq inc in PCO2=
 0.4 meq/L inc in HCO3 45mEq/L
 Chroinc Respiratory Alkalosis
 Each mEq dec in Pco2 =
 0.5 mEq dec in Hco3 12-15mEq/L
PRIMARY PROBLEM

 RESPIRATORY

40 – ACTUAL PCO2 X 100

40

 METABOLIC

24 – ACTUAL HCO3 X 100

24
EXAMPLE 1
 pH - 7.28
 PaCO2 - 60 mmHg
 HCO3 - 22 mEq/L
 Solution:
 Expected HCO3= 24 + (Actual PaCO2 – Desired PaCO2) x 0.1
 24 + (60-40) x 0.1
 24 + 2
 26
 Acute Respiratory acidosis, uncompensated
if ph is between 7.35-7.45
 24 + (Actual PCO2 – Desired PCO2) x 0.4
 24 + (60-40) x 0.4
 24 + (20 x 0.4)
 24 + 8
 32
 Chronic Respiratory Acidosis, uncompensated
EXAMPLE 2
 pH - 7.49
 PaCO2 - 32 mmHg
 HCO3 - 26 mEq/L
 Solution:
 Expected HC03 = 24 –(Desired PaCo2 – Actual PaCo2) x 0.2
 24- (40-32) x 0.2
 24- (8 x 0.2)
 24- 1.6
 22.4
 Acute Respiratory Alkalosis, uncompensated
if ph is between 7.35-7.45
 24- (40-32) x 0.4
 24- (8 x 0.4)
 24- 3.2
 20.8
 Chronic Respiratory Alkalosis, uncompensated
Example 3
 pH: 7.50
 PaCo2: 36
 Hco3: 32
 Solution:
 Expected PaCo2: Actual HCo3 + 15
 32 + 15
 47 or
 [(Desired PaCo2 – Actual PaCo2) x 0.75] + 40
 [(40-36) x 0.75] + 40
 (4x0.75)+ 40
 3+40
 44
 Uncompensated Metabolic Alkalosis
Example 4
 pH: 7.10
 PaCo2: 45
 HCo3: 14
 Solution:
 Expected PaCo2: (1.5 x HCO3) + 8
 (1.5 x 14) + 8
 21+8
 29 0r
 Actual HC03 + 15
 14 + 15
 29
 Uncompensated Metabolic Acidosis
 CLASSIFICATION PaO2 (mmHg)
Hyperoxemia >100
Normoxemia 80 – 100
Mild Hypoxemia 60 – 79
Moderate Hypoxemia 45 – 59
Severe Hypoxemia < 45
QUANTIFYING
PULMONARY
DYSFUNCTION
Indices of Oxygenation

1. pO2
2. P/F ratio
3. O2 saturation
4. aADO2 – difference in O2 in the alveolus and
the arterial blood (alveolar-arterial O2
gradient or difference)
pAO2 = 713 (FiO2) – pCO2
0.8
aADO2 = pAO2 – paO2
Normal ≤ 20
Oxygenation

Expected pO2 at room air:

 For age ≤ 60 y.o. = 80-100
 For age > 60 y.o. = 80 - # of years above
60
eg. 70 y.o. = 80-10 = 70

Ideal PO2 for age: 103.5 - 0.42(age)

Oxygenation

At Room Air:
- Compare actual pO2 to expected pO2
Actual < Expected = HYPOXEMIC
Actual > Expected = NON-HYPOXEMIC

On supplemental O2:
- Expected P/F ratio (pO2/FIO2)
For age ≤ 60 y.o.: 400-500
For age > 60 y.o.: 400 – (age above 60 x 5)
eg. 70 y.o.: 400 – (10 x 5) = 350
FiO2 per nasal cannula:
= (O2 in L/min) x 4 + 20
eg. O2 at 2 lpm = 2 x 4 + 20 = 28%

Actual P/F ratio:

P/F = paO2 (actual from ABG)
FiO2 (expressed in decimal)
FiO2 Requirement

Room air = 0.21

Nasal cannula= # of liters x 4 + 20/100

Face mask = # LPM x 10 – 20

ET = # LPM x 10
OXYGENATION RATIO

PULMONARY OXYGENATION
STATUS RATIO
Normal 400 – 500
Moderate Pulmonary 200 – 399
dysfunction
Substantial Pulmonary < 200
dysfunction
4 MECHANISMS

1. Hypoventilation
2. Absolute shunting
3. Relative Shunting (V/Q mismatch)
4. Diffusion defects
Hypoventilation

 Quantitate the efficiency of oxygen loading

 Causes: neuromuscular disorders, chest wall
trauma, CNS depression, anesthesia effect, high
cervical spine trauma
Absolute Shunting

 Blood passing from the right side of the heart to

the left side of the heart without being exposed to
alveolar oxygen
 Anatomic or capillary in nature
 Does not respond to administration of
supplemental oxygen
Capillary Shunting

 Alveolar consolidation (filling with fluid)

 Collapse
 Causes: ARDS, left-sided heart failure, pneumonia,
atelectasis
 Pulmonary edema – single greatest cause of
severe, absolute capillary shunting
Anatomic Shunting

 Congenital cardiovascular abnormalities

 Persistent fetal circulation
Relative Shunting

 Most common
 V/Q perfusion mismatch
 Perfusion in excess of ventilation
 Good PaO2 response to small increments of
oxygen therapy
 Uneven distribution of ventilation secondary to
increased pulmonary secretions
 COPD
Diffusion defects

 Anatomic impedance to oxygen transfer in the

lungs, due to a thickened alveolar-capillary
membrane
 Responds to oxygen therapy
 Can be included to relative shunting
PHYSIOLOGIC SHUNTING

 Percentage of the venous blood that remains

unoxygenated after traveling from the right side
of the heart to the left side of the heart
I. (alveolar) PAO2 = 713 (FiO2) – pC02/0.8

II. (arterial) PaO2 = PAO2 (O2 for age/PO2)

III. FiO2 requirement (minimum)

PaO2 + pCO2/0.8
------------------------ x100
713
Exercise

 pH 7.36
 pCO2 46
 HCO3 26
 pO2 154
 FIO2 100%
 Age 42
Compensated Respiratory Acidosis with
Hypoxemia with Overcorrected FIO2 at 100%

 Respiratory
Acidosis (Acute)  Get O2 for Age
Exp HCO3= 24+ =103.5- (0.42 X
[(Actual PCO2- age)
Desired PCO2) X =103.5- (0.42 x 42)
0.1] =103.5- 17.64
24+ [(46-40) x 0.1] =85.86
24 + (6 x 0.1)
24 + 0.6
26.6
Compute for PF=
Actual/FIO2
= 154/100
= 154
New FIO2

 I = 713 (FIO2) – Pco2/0.8

= 713 (100) – 46/0.8.
= 713- 57.5
=655
 II = I (O2 for age/po2)
= 655 (85.86/154)
= 655 x 0.55
=360.25
 III = II +[( PCO2/0.8) x 100]/713
=360.25 + (57.5 x 100) / 713
=41750/713
58.55
Exercise:
 Age; 42
Ph=7.56
PC02= 34.4
P02= 156
HC03= 30.9
FI02= 40% MV
Combined Metabolic Alkalosis with underlying
Respiratory Alkalosis with hypoxemia with
overcorrected FIO2 at 40% FIO2

 Respiratory  Metabolic
= 40-PC02x100  =24-HC03x100
40
 24
= 40-34.4x100
40  24-30.9x100
=5.6x100  24
40
=560  -6.9x100
40  24
=14  -690
24
 28.75
 P/F ratio= 156/.40
 =390
 O2 for age= 103.5 – (0.42x42)
 =103.5-17.64
 =85.86
New FIO2

I = 713 (.40) – PCO2/.8

= 285- 43
= 242
II = I (O2 for age/PO2)
= 242 (85.86/156)
= 242 x .55
= 133
III = II [(PCO2/0.8) x 100]/ 713
= 133 ( 43 x 100) x 100/713
= 24
Exercise

 Age: 79
 pH: 7.32
 PCO2: 28
 PO2: 101
 HCO3: 14
 FIO2: 2 lpm via NC
Metabolic Acidosis, uncompensated,
non-hypoxemic, with overcorrected
FIO2 at 2lpm Nc
 Expected PCO2= (1.5 x  O2 for Age
HCO3) + 8
 103.5 –(0.42 x 79)
 (1.5 x 14) + 8
 103.5 – 33.18
 21+ 8
 70.32
 29
 P/F ratio= 101/.28
 360
 Expected P/F ratio
 400- (19x5)
 400 – 95
 305
New FIO2

 I = 713 (FIO2)- PCO2/0.8

=713 (.28) -35
=1.99.64-35
= 164
 II =I (O2 for age/PO2)
= 164 x 0.69
=113.16
 III = II x [(PCO2/0.8) x 100]/713
= 113.16 x (35 x 100)/713
= 20. 77
Exercise

 Age: 54
 pH: 7.53
 PCO2: 45.2
 HCO3: 37.8
 PO2: 78
 FIO2: 8l FM
Metabolic Alkalosis, uncompensated
with hypoxemia, uncorrected @ 8LPM
via N.C
 Expected PCO2= Actual  PF ratio= 78/0.60
HCO3 + 15
 130
 37.8 + 15
 O2 for age=103.5-(.42 x
 52.8 or age)
 [(Desired HCO3-Actual  80.82
HCO3)x0.75] + 40
 10.35 + 40
 50.35
New FIO2

 I = 713 (FIO2) – PCO2/0.8

= 713 (.60)- 56.6
= 427- 56.5
= 371
 II = I (O2 for age/po2)
= 371 x 1.03
= 382
 III = III + [(Pco2/0.8) x 100]/ 713
= 382 + (56.6 x 100)/ 713
= 61.5
Exercise

 Age: 68
 pH: 7.24
 pCO2: 94.1
 pO2: 60
 HCO3: 40.4
 FIO2: 2l NC
Acute Respiratory Acidosis,
uncompensated with hypoxemia,
uncorrected @ 2 lpm via N.C
 Expected HCO3=24+  O2 for age=103.5-
(Actual HCO3-Desired (0.42xage)
HCO3)x0.1
 103.5-28.56
 24+ (54.1x 0.1)
 74.94
 29.41
 PF ratio= 60/.28
 214
 PF for age=400-(8x5)
 360
New FIO2

 I = 713 (FIO2) – PCO2/.08

= 713 (0.28) – 117.625
= 82.015
 II = I (O2 for age/PO2)
= 82.015 (74.94/60)
= 102
 III = III + [(PCO2/0.8) x 100] /713
= 102 + (117.625) x 100/ 713
= 30
Henderson-Hasselbalch equation: to determine if
acid is chronic or acute

 A. H+ = 24x PCO2/ HCO3

= 24 x 94.1/40.4
= 55.68
 B. Delta H+= [H+] – Desired PCO2
= 55.68 – 40
= 15.68
 C. Delta PaCO2= Actual PCO2- Desired PCO2
= 94.1 - 40
= 54.1
 D. Delta [H+]/Delta PCO2
= 15.68/54.1
= 0.3 = Acute on top of chronic
Interpretation

 0.3 – 0.7
 < 0.3 chronic hypercapnia
 > 0.7 acute hypercapnia
 0.3 -0.7 acute on top of chronic
Exercise

 Age: 32
 pH: 7.41
 pCO2: 36.7
 pO2: 56
 HCO3: 23.5
 FIO2: 4l NC
Normal Acid base balance with
Hypoxemia uncorrected @ 4l via NC

 O2 for age=103.5-
(0.24x32)
 PF ratio= 56/0.36
 103.5-13.44
 155
 90.06
New FIO2

 I = 713 (FIO2) – PCO2/.8

= 713 (0.36) – 45.875
= 256.68.-45.875
= 210.805
 II = I (O2 for age/PO2)
= 210.805 x 1.6-8
= 338.97
 III = II [PCO2/0.8 )x 100]/713
= 338.97 (45.875 x 100)/ 713
= 53.97
HCO3 Deficit

 Metabolic acidosis
 18 to 20 (desired)-actual HCO3 x (0.4 to 0.6) x
weight
 Half of computed is given SIVP then half as 24
hour drip
 Repeat ABG post correction

 Example:
 pH 7.18
 PCO2 27
 HCO3 9
Thank you for your attention