Blood Gas Analysis
Carrie George, MD
Pediatric Critical Care Medicine
Adapted from Dr. Lara Nelson
Blood Gas Analysis
• Acid-base status
• Oxygenation
Anatomy of a Blood Gas
• pH/pCO2/pO2/HCO3
Base: metabolic
Oxygenation: lungs/ECMO
Acid: lungs/ECMO
The sum total of the acid/base balance,
on a log scale (pH=-log[H+])
Blood Gas Norms
pH
pCO2
pO2
HCO3
BE
Arterial
7.35-7.45
35-45
80-100
22-26
-2 to +2
Venous
7.30-7.40
43-50
~45
22-26
-2 to +2
Blood Gas Analysis
1. Determine if pH is acidotic or alkalotic
2. Determine cause:
1. Respiratory
2. Metabolic
3. Mixed
3. Check oxygenation
Acid-Base Regulation
• Three mechanisms to maintain pH
– Respiratory (CO2)
– Buffer (in the blood: carbonic acid/bicarbonate,
phosphate buffers, Hgb)
– Renal (HCO3-)
Acid-Base Equation: the
carbonic acid/bicarbonate
CO2 + H2O
Respiratory component
Acid
H2CO3
HCO3- + H+
Blood/renal component
Base
Acid vs. Alkaline Blood pH
• Arterial pH = 7.40
• Venous pH = 7.35
6.9
Acidosis
7.0
7.4
Neutral pH
7.5
Alkalosis
Etiology
• Respiratory
• Metabolic
• Mixed
Rule #1
• Every change in CO2 of 10 mEq/L causes
pH to change by 0.08 (or Δ1 = 0.007)
• Increased CO2 causes a decreases in pH
• Decreased CO2 causes an increase in pH
Respiratory Acidosis
• Hypercarbia from hypoventilation
• Findings:
– pCO2 increased therefore… pH decreases
• Example:
ABG : 7.32/50/ /25
Respiratory Alkalosis
• Hypercarbia from hypoventilation
• Findings:
– pCO2 decreased… therefore pH increases
• Example:
ABG – 7.45/32/ /25
Metabolic Changes
• Remember normal HCO3- is 22-26
Rule #2
• Every change in HCO3- of 10 mEq/L causes
pH to change by 0.15
• Increased HCO3- causes an increase in pH
• Decreased HCO3- causes a decrease in pH
Metabolic Acidosis
• Gain of acid – e.g. lactic acidosis
• Inability to excrete acid – e.g. renal tubular
acidosis
• Loss of base – e.g. diarrhea
• Example:
– ABG – 7.25/40/
/15
Metabolic Alkalosis
• Loss of acid – e.g. vomiting (low Cl and
kidney retains HCO3-)
• Gain of base – e.g. contraction alkalosis
(lasix)
• Example:
– ABG – 7.55/40/
/35
Mixed
• pH depends on the type, severity, and acuity
of each disorder
• Over-correction of the pH does not occur
Practical Application
1. Check pH
2. Check pCO2
3. Remember Rule #1
Every change in CO2 of 10 mEq/L causes pH
to change by 0.08
Practical Application cont.
4. Does this fully explain the results?
5. If not, remember Rule #2
Every change in HCO3- of 10 mEq/L
causes pH to change by 0.15
Example #1
•
•
•
•
ABG- 7.30/48/ /22
Acidotic or Alkalotic?
pCO2 High or Low?
pH change = pCO2 change?
Combined respiratory and metabolic acidosis
Example #2
•
•
•
•
ABG- 7.42/50/ /32
Acidotic or Alkalotic?
pCO2 High or Low?
pH change = pCO2 change?
Metabolic alkalosis with respiratory compensation
Oxygen Supply and Demand
Arterial oxygen depends on:
-Lungs ability to get O2 into the blood
-Ability of hemoglobin to hold enough O2
Bedside Questions of Oxygenation
• Does supply of O2 equal demand?
• Is O2 content optimal?
• Is delivery of O2 optimal?
Mixed Venous Saturation
SvO2: What is it?
-In simple terms, it is the O2 saturation of the blood
returning to the right side of the heart
- This reflects the amount of O2 left after the tissues
remove what they need
SvO2 = O2 delivered to tissues – O2 consumption
Oxygen Delivery
O2 transport to the tissues equals arterial O2
content x cardiac output
-DO2 = CaO2 x CO
- Normal DO2 = 1000 ml/min
Arterial Oxygen Content
• CaO2 = (1.34 x Hgb x SaO2) + (PaO2 x 0.0031)
• Normal CaO2 = 14 +/- 1 ml/ dl
• Example:
CaO2 = (1.34 x 10 x 95)+(78 x 0.0031)
= 12.97
If Hgb is 12, CaO2 = 15.52
If PaO22 is 150, CaO2 = 13.20
Mixed Venous Oxygen Content
• CvO2 = (1.34 x Hgb x SvO2) + (PvO2 x 0.0031)
• Normal CvO2 = 14 +/- 1 ml/dl
Oxygen Consumption
• VO2 = (CaO2 – CvO2) x CO
 Fick equation
• Normal VO2 = 131 +/- 2 ml/min
Mixed Venous Saturation
SvO2 = O2 delivered to tissues – O2 consumption
How do we know what it is?
- Calculate it
- Direct blood gas analysis, e.g. from a pulmonary
catheter
- Oximetry
Normal Mixed Venous Saturation
• Normal value
-68%-77%
-Change from arterial saturation of 20% to 30%
• Values less than 50% are worrisome, or a change of
40%- 50%
• Values less than 30% suggest anaerobic metabolism
• The most useful application is to follow trends
Oxygen Saturation and pO2
• An O2 saturation of 75% correlates with a
PaO2 of about 45 mmHg
• This is on the step portion of the oxygen
dissociation curve
Oxygen Dissociation Curve
Utility of MVO2
• Gives information about the adequacy of
oxygen delivery
• Suggests information about oxygen
consumption
• Can help determine the usefulness of clinical
interventions
Decreased MVO2
Oxygen delivery is not high enough to meet
tissue needs.
• Poor saturation
• Anemia
• Poor CO
• Increased tissue extraction
Increased MVO2
• Wedged PA catheter
• Improvement in previous poor situation
• Shunting
-Tissues no longer extracting oxygen
-How can you tell?
End-Organ Perfusion
• Brain
- Neurologic exam
• Kidneys
-Urine output
- Creatinine
• Lacitic acidosis
NIRS
• Near Infrared Regional Spectroscopy
• An alternative strategy for measuring
localized perfusion
How the INVOS System Works
• rSo2 index represents the balance of site-specific O2 delivery
and consumption
• It measures both venous (~75%) and arterial (~25%) blood
• Indicates adequacy of site-specific tissue perfusion in realtime
• Correlates positively with SvO2, but is site-specific and
noninvasive
• rSO2 is not a simple blood gas, it measures the amount of
oxyhemoglobin in the tissue
Cerebral/Peri-Renal NIRS
Monitoring
Cerebral rSO2
• Normal values:
- 30% less than the arterial saturation
- Even in cyanotic heart disease this is true
• Concentrating values :
- A change of 20% from baseline
- rSO2 < 60%
• As with MVO2 trends are the most helpful application
Peri-Renal rSO2
• Normal Values:
- 10%-15% less than the arterial saturation
- Even in cyanotic heart disease this is true
• Concerning values:
- A change of 20% from baseline
-rSO2 < 60%
• As with MVO2 trends are the most helpful application
Why Monitor Both?
• More information is always better
• Perfusion is differentially distributed, i.e.
generally cerebral blood flow is maintained at
the expense of other organs