Arterial blood gas analysis

Arterial Blood gas analysis
Dr. Samaresh Das
@ samaresh

Alveolar Ventilation
Alveolar Ventilation (V'A) : Gas exchange of lung during normal
breathing.
o High air exchange in functioning alveoli, higher alveolar
ventilation, would bring in fresh oxygen-rich air and efflux
carbon dioxide air rapidly
o Healthy subjects the partial pressure of alveolar carbon
dioxide (PACO2) is equivalent to the arterial carbon dioxide
(PaCO2), we substitute the arterial variable for that of the
alveolar value
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Oxygenation
Oxygenation : Process of adding oxygen to the body system.
o clinical signs alone - Cyanosis, pallor and other physical findings
are not reliable.
o Saturation of peripheral oxygen (SpO2) levels measured with a
pulse oximeter correlate highly with arterial oxygenation
concentrations (PO2)
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Basic Terminology
PH: Free H+ conc. , PH Inversely related to H+
Acid : Substance that can donate H+, lowers PH
Base : Substance that can accept H+, raise PH
Acidemia : PH <7.35 , raise H+
Alkalemia : PH >7.45, lower H+
Acidosis : Process/disease , ↓ PH , ↑ acid or ↓ alkali
Alkalosis : Process /disease ,↑ PH , ↓ acid or ↑ alkali
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Why Order an ABG?
o Aids in establishing a diagnosis
o Helps guide treatment plan
o Aids in ventilator management
o Improvement in acid/base management allows for
optimal function of medications
o Acid/base status may alter electrolyte levels critical to
patient .
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Logistics
• Where to place -- the options
– Radial
– Femoral
– Brachial
– Dorsalis Pedis
• When to Place an arterial line --
– Need for continuous BP monitoring
– Need for multiple ABGs
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Technical Errors
Air bubbles
o PO2 150 mmHg & PCO2 0 mm Hg in air bubble.
o Discard sample if excessive air bubbles
o Seal with cork/cap after taking sample
Fever or Hypothermia
o Most ABG analyzers report data at N body temp
o If severe hyper/hypothermia, values of pH & PCO2 at 37 C
can be significantly diff from pt’s actual values
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Acid Base Status
o Assessment via bicarbonate-carbon dioxide buffer
system in blood.
o H+ = 24 × ( Pco2 / HCO3)
o PCO2 / HCo3 identifies the primary acid base disoders
and secondary response
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Sample collection
o Only the person who collect the sample can tell if he has
drawn a pulsating blood
o Partly mixed sample- Difficult to recognize
Arterial Venous
PH 7.35 - 7.45 7.36- 7.39
Pco2 35- 45 44 - 48
Po2 80 - 100 38 - 42
Hco3 24- 26 20 - 24
Sao2 95- 100 % 75%
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Primary Acid Base Disorders &
secondary response
Primary
Disorder
Primary change Secondary
Response
Resp. Acidosis ↑ PcO2 ↑ Hco3
Resp. Alkalosis ↓ Pco2 ↓ Hco3
Met. Acidosis ↓ Hco3 ↓ Pco2
Met. Alkalosis ↑ Hco3 ↑ Pco2
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During compensation HCO3¯ &
PaCO2 move in the same
direction
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Compensation
Respiratory compensation
is always FAST …12-24 hrs
Metabolic compensation
• is always SLOW...2 - 3
days
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Response to Metabolic Acidosis
o Secondary response decrease Pco2 by increase ventilation
o Appears in 30-120 minute & can take 12-24 hrs
o Expected Paco2 = 40 - ( 1.2 × ∆ Hco3)
EXAMPLE:
Metabolic acidosis with a plasma HCO3 of 10 mEq/L,
∆ HCO3 is 24 – 10 = 14 mEq/L,
so the expected PaCO2 is 40 – (1.2 × 14) = 23 mm Hg.
o If the PaCO2 is >23 mm Hg, there is a secondary respiratory
acidosis.
o If the PaCO2 is <23 mmhg , there is secondary respiratory alkalosis
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Metabolic alkalosis
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o Secondary response : Increase Pco2 by decrease
ventilation
o This response is not as vigorous as the response to
metabolic acidosis
o Expected Paco2 = 40 + ( 0.7 × ∆ Hco3 )
o EXAMPLE: Metabolic alkalosis with plasma HCO3 of
40 mEq/L, so ∆ HCO3 is 40 – 24 = 16 mEq/L,
o So expected PaCO2 is 40 + ( 0.7 × 16) = 51 mm Hg.
Response to Metabolic Alkalosis
@ samaresh

Respiratory acidosis
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o Secondary response of PaCO2 occurs in the kidneys,
o HCO3 absorption in the proximal tubes
o Response is relatively slow, take 2 or 3 days to reach
completion.
o Because of delay, respiratory acid-base disorders are
separated into acute and chronic disorders.
EXAMPLE: Acute increase in PaCO2 to 60 mm Hg,
Expected HCO3 = 24+ (0.1 × ∆ co2 )
For an chronic respiratory acidosis
o Expected HCO3 = 24+ (0.4 × ∆ co2 )
Response to Respiratory Acidosis
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Respiratory alkalosis
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EXAMPLE:
For an acute decrease in PaCO2 to 24 mm Hg,
o Expected HCO3 = 24 - (0.2 × ∆ co2 )
For an chronic decrease in PaCO2 to 20 mm Hg,
o Expected HCO3 = 24 - (0.4 × ∆ co2 )
Response to Respiratory Alkalosis
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Stepwise approach to
Acid Base analysis
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Stepwise approach to Acid Base
analysis
o Structured, rule-based approach to the diagnosis of
primary, secondary, and mixed acid-base disorders using
the relationships between the PH, PCO2, and HCO3
Normal Values :
o pH = 7.35–7.45
o PCO2 = 35–45 mm Hg
o HCO3 = 22–26 mEq/L
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Acidemic pH < 7.35
Alkalemic pH > 7.45
analysis
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o Stage I: PaCO2 and pH are used to identify the primary acid-
base disorder.
o Rule 1: If PaCO2 and/or the pH is outside the normal range,
there is an acid-base disorder.
o Rule 2: If PaCO2 and pH both abnormal, compare the
directional change.
o 2a. If PaCO2 and pH change in same direction, there is
primary metabolic acid-base disorder.
o 2b. If PaCO2 and pH change in opposite directions, there is
primary respiratory acid-base disorder.
analysis
@ samaresh

EXAMPLE:
o pH = 7.23 , PaCO2 = 23 mm Hg.
o The pH and PaCO2 , both reduced (indicating a primary
metabolic disorder) and the pH acidemic , so the
diagnosis is primary metabolic acidosis.
analysis
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Rule 3: If only pH or PaCO2 is abnormal, the condition is mixed
metabolic and respiratory disorder
o 3a. If PaCO2 is abnormal, the directional change in PaCO2
identifies the type of respiratory disorder (e.g., high PaCO2
indicates a respiratory acidosis), and the opposing metabolic
disorder.
o 3b. If the pH is abnormal, the directional change in pH
identifies the type of metabolic disorder (e.g., low pH
indicates a metabolic acidosis) and the opposing respiratory
disorder
analysis
@ samaresh

EXAMPLE:
o pH = 7.38 and PaCO2 = 55 mm Hg.
o Only PaCO2 is abnormal, so there is a mixed metabolic and
respiratory disorder.
o The PaCO2 is elevated, indicating a respiratory acidosis, so
the metabolic disorder must be a metabolic alkalosis
o So condition is a mixed respiratory acidosis and metabolic
alkalosis. Both disorders are equivalent in severity because
the pH is normal
analysis
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Stage II: The goal in Stage II is to determine if there is an
additional acid-base disorder.
o Rule 4: For primary metabolic disorder, if the measured
PaCO2 is higher than expected, there is a secondary
respiratory acidosis, and if the measured PaCO2 is less
than expected, there is a secondary respiratory alkalosis
analysis
@ samaresh

EXAMPLE:
o PaCO2 = 23 mm Hg, the pH = 7.32, and the HCO3 = 16 mEq/L. The
pH and PCO2 change in the same direction, indicating a primary
metabolic disorder, and the pH is acidemic, so the disorder is a
primary metabolic acidosis.
o Expected PaCO2 is 40 –1.2×(24 – 16) = 30 mm Hg.
o The measured PaCO2 (23 mm Hg) ,
o so there is an additional respiratory alkalosis. Therefore, this
condition is a primary metabolic acidosis with a secondary
respiratory alkalosis
analysis
@ samaresh

Rule 5: For a primary respiratory disorder, a normal or near-
normal HCO3 indicates that the disorder is acute.
 Respiratory
compensation
is always FAST …12-24
hrs
 Metabolic compensation
• is always SLOW...2 - 3
days
analysis
@ samaresh

o Rule 6: Primary respiratory disorder where HCO3 is abnormal,
determine the expected HCO3 for a chronic respiratory disorder.
o 6a. For chronic respiratory acidosis, if the HCO3 is lower than
expected, there is an incomplete renal response, and if the HCO3 is
higher than expected, there is a secondary metabolic alkalosis.
o 6b. For a chronic respiratory alkalosis, if the HCO3 is higher than
expected, there is an incomplete renal response, and if the HCO3 is
lower than expected, there is a secondary metabolic acidosis
analysis
@ samaresh

EXAMPLE:
PCO2 = 23 mm Hg, pH = 7.54, and the HCO3 = 32 mEq/L.
o The PaCO2 and pH change in opposite directions, indicating a
primary respiratory disorder, and the pH is alkaline, so the disorder
is a primary respiratory alkalosis.
o HCO3 is abnormal, indicating this is not an acute respiratory
alkalosis.
analysis
@ samaresh

PCO2 = 23 mm Hg, pH = 7.54, and the HCO3 = 37 mEq/L.
Chr. Resp. Alk expected HCO3 = 24 + 0.4 ×(40 – 23) = 31
mEq/L.
o If measured HCO3 < 31 mEq/L, condition would be a
chr. Resp. alkalosis with incomplete renal response
o If measured HCO3 is > 31 mEq/L, indicate secondary
metabolic alkalosis
analysis
@ samaresh

• ----- XXXX Diagnostics ------
• Blood Gas Report
• Measured 37.0
o
C
• pH 7.523
• pCO2 30.1 mm Hg
• pO2 105.3 mm Hg
• Calculated Data
• HCO3 act 22
mmol / L
• O2 Sat 98.3 %
• pO2 (A - a) 8 mm Hg D
• pO2 (a / A) 0.93
• Entered Data
• FiO2 21.0 %
Case 1
30 year old female with
sudden onset of
dyspnea.
No Cough or Chest
Pain
Vitals normal but RR
26,
anxious.
@ samaresh

• Measured 37.0
o
C
• pH 7.301
• pCO2 76.2 mm Hg
• pO2 45.5 mm Hg
• Calculated Data
• HCO3 act 35.1 mmol / L
• O2 Sat 78%
• pO2 (A - a) 9.5 mm Hg D
• pO2 (a / A) 0.83
• Entered Data
• FiO2 21 %
Case 2
60 year old male
smoker
with progressive
respiratory distress
and somnolence.
@ samaresh

• Measured 37.0
o
C
• pH 7.23
• pCO2 23 mm Hg
• pO2 110.5 mm Hg
• Calculated Data
• HCO3 act 14 mmol / L
• O2 Sat %
• pO2 (A - a) mm Hg D
• pO2 (a / A)
• Entered Data
• FiO2 21.0%
Case 3
28 year old
diabetic with
respiratory
distress
fatigue and
loss of appetite.
@ samaresh

 8) I shall practice gentle
mechanical ventilation and
not to try bring ABG to
perfect normal.
 9) I shall treat the patient, not
the ABG report.
 10) I shall always correlate
ABG report clinically.
@ samaresh

Arterial blood gas analysis

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