Blood gases and
acid-base balance
ACoRN © 2005-07
What is pH?
• a measure of the acidity or alkalinity of a
solution
• the negative logarithm of [H+]
• the more acidic a solution, the smaller the
pH value
• neutral pH = 7
• change in pH of 1 = 10 fold change in [H+]
ACoRN © 2005-07
pH is determined by:
• dissolved CO2 = PCO2
• metabolic acids
• buffers
ACoRN © 2005-07
PCO2
• PCO2 indicates how well the lung is removing
CO2 from the blood (ventilation)
• CO2 (respiratory acid) accumulates when
breathing is insufficient: respiratory acidosis
+
CO2 + H2O ↔ H2CO3 ↔ H + HCO3
LUNG
-
KIDNEY
• Excessive elimination of CO2 leads to
respiratory alkalosis
ACoRN © 2005-07
Metabolic component
• Lactic acid accumulates when respiration at a
tissue level is insufficient, due to inadequate
oxygen delivery: metabolic acidosis
• The bicarbonate (base) buffer is the main
system that neutralizes metabolic acids to
maintain a physiologic pH
base
acid
pH
ACoRN © 2005-07
Metabolic component
• The base deficit (BD) is calculated to estimate
the amount of excess metabolic acid.
BD
“How much alkali needs
to be added to return
the pH to 7.40 if the
PCO2 is 40.”
ACoRN © 2005-07
Application
Arterial, capillary or venous samples are
nearly equally useful for the
determination of ventilation (PCO2), pH
and base deficit but quite different for
oxygenation (PO2, SaO2)
ACoRN © 2005-07
Oxygen
Dissolved oxygen
Ratio of 1 to 40
Dissolved
Bound
ACoRN © 2005-07
Normal blood gas values
pH
7.35 to 7.45
PCO2
35 to 45 mmHg
BD
-4 to +4 mmol/L
HCO3
22 to 26 mmol/L
PaO2
50 to 80 mmHg
ACoRN © 2005-07
Acceptable blood gas values
for acute respiratory illness
pH
PCO2
7.25 to 7.40
45 to 55 mmHg
BD
NA
HCO3
NA
PaO2
50 to 80 mmHg
ACoRN © 2005-07
Blood gas interpretation
1.
Is the pH
normal
decreased (acidosis)
increased (alkalosis)
2.
Is the primary problem
respiratory
metabolic
3.
Is there compensation
respiratory
renal
4.
Is the arterial oxygenation
normal
decreased (hypoxemia)
increased (hyperoxemia)
ACoRN © 2005-07
Blood gas interpretation
pH
PCO2
BD
Interpretation
↓
↑
N
respiratory acidosis
↓
N
↑
metabolic acidosis
↑
↓
N
respiratory alkalosis
↑
N
↓
metabolic alkalosis
1) Normal, acidosis or alkalosis?
2) Respiratory, metabolic, mixed, or compensated?
ACoRN © 2005-07
Blood gas interpretation
• most acid-base problems in babies result
from inadequate ventilation (respiratory
acidosis)
• acidosis with pH ≤ 7.25 and PCO2 ≥ 55
indicates poor ventilation requiring
correction (ventilation)
• in metabolic acidosis correct the cause generally poor tissue perfusion
ACoRN © 2005-07
• Hypoxia: inadequate delivery of oxygen to
tissues
• Generally due to arterial hypoxemia
– PaO2 less than 50 mmHg
– SaO2 (~ SpO2) less than 85%
ACoRN © 2005-07
Case 1: Billy
• Billy was delivered by emergency
Caesarean delivery for fetal distress
• RR 26, HR 124, his respiratory effort is
weak
• SpO2 90% in 50% oxygen
• He is hypotonic
• Blood gas:
– pH 7.16, PCO2 70, BD 7, HCO3 21
ACoRN © 2005-07
pH 7.16, PCO2 70, BD 7
pH
PCO2
BD
Interpretation
↓
↑
N
Respiratory acidosis
↓
N
↑
Metabolic acidosis
↑
↓
N
Respiratory alkalosis
↑
N
↓
Metabolic alkalosis
1) Normal, acidosis or alkalosis?
2) Respiratory, metabolic, mixed or compensated?
ACoRN © 2005-07
Case 1: Billy
• Billy had a asphyxial insult.
• Inadequate respirations  high PCO2
(respiratory acidosis).
• Tissue hypoxia  metabolic acidosis.
• Is this blood gas acceptable?
ACoRN © 2005-07
Case 2: Belinda
•
•
•
•
28 weeks’ gestation
Developed RDS
Is ventilated
Blood gas:
– pH 7.47, PCO2 28, HCO3 22, BD 4
ACoRN © 2005-07
pH 7.47, PCO2 28, BD 4
pH
PCO2
BD
Interpretation
↓
↑
N
Respiratory acidosis
↓
N
↑
Metabolic acidosis
↑
↓
N
Respiratory alkalosis
↑
N
↓
Metabolic alkalosis
1) Normal, acidosis or alkalosis?
2) Respiratory, metabolic, mixed or compensated?
ACoRN © 2005-07
Case 2: Belinda
• Belinda has respiratory alkalosis
• Excessive ventilation
• Correct by
– decreasing ventilator rate
– decreasing tidal volume or
– decreasing inspiratory pressure
ACoRN © 2005-07
Case 3: Jacinta
•
•
•
•
•
•
•
14 days, 38 weeks’ gestation, SGA
Mother is a substance user
Poor feeding and watery stools x 3 days
One wet diaper in the last 24 hours
Cool to touch and delayed capillary refill
Respirations easy but rapid
Blood gas: pH 7.36, PCO2 25, HCO3 12,
BD 10.
ACoRN © 2005-07
pH 7.36, PCO2 25, BD 8
pH
PCO2
BD
Interpretation
↓
↑
N
Respiratory acidosis
↓
N
↑
Metabolic acidosis
↑
↓
N
Respiratory alkalosis
↑
N
↓
Metabolic alkalosis
1) Normal, acidosis or alkalosis?
2) Respiratory, metabolic, mixed or compensated?
ACoRN © 2005-07
Case 3: Jacinta
• Compensated metabolic acidosis
• Fluid loss (diarrhea) plus poor intake 
dehydration + metabolic acidosis
• Mechanisms:
– tissue hypoperfusion  lactic acidosis
– bicarbonate losses in stool
– respiratory compensation by “blowing
off” CO2
ACoRN © 2005-07
?



ACoRN © 2005-07