Lecturer: James Zangrilli
Lecture: 31
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Arterial Blood Gases
* This lecture is a review of how to clinically approach alterations in blood gases, and pH disturbances.
For a more in depth review see (pgs 252-258 of syllabus, Page 309 of physiology hand written notes,
page 772 of physiology syllabus notes.)
A. GENERAL INFORMATION ABOUT BLOOD GASES
I. Hypoxemia and gas exchange
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The transfer of gas from the alveoli to the blood is not perfect.
PAO2 ≠ PaO2
Normal PaO2 = 80 mmHg
Normal PaCO2 = 40 mmHg
1. A-a difference
o Difference between alveolar partial pressure of oxygen (PAO2) and arterial partial pressure of O2 (PaO2).
o Normal difference  20mmHg
o An A-a > 20 mmHg implies abnormal gas exchange.
 Normal causes of A-a difference:
o VQ mismatch d/t regions of lung that is perfused but poorly ventilated.
o Shunt: some blood must bypass the oxygen exchange portion of the lung entirely.
o Diffusion block: membranes, etc… resist diffusion of gases according to fick’s law.
* NOTE: hypoventilation and low inspired PO2 will not change the A-a.
2. Calculating PAO2
o The PAO2 depends on altitude (pressure of ambient air), Partial pressure (fraction) of oxygen in the air (usually
0.21) and the PaCO2 exiting the lung into the alveoli.
* PAO2 = 0.21(760 – 47) – (PaCO2/0.8)
 When at sea level this can be shortened to: PAO2 = 149 – (PaCO2/0.8)
* You must use this to calculate if a change in normal blood gases is potential d/t a problem with gas
exchange!!
II. Normal Acid Base Balance
* Normal pH = 7.4
* Normal HCO3 = 24 mmol/L
1. Buffers
o HCO3 (bicarbonate) is the body’s natural buffer to changes in pH.
o It can “absorb” acid but converting to carbonic acid which is then made into CO2 and water by carbonic
anhydrase.
o It is capable of maintaining a near normal pH as long as there is enough Bicarbonate in the system.
o Remember that measured HCO3 is going to be a little bit higher than expected because it is an indirect
measure which will also include any non-bound CO2 released as having come from bicarb.
2. Renal excretion
o The kidney is capable of excreting hydrogen while saving Bicarbonate to decrease acid in the system, or it is
cable of excreting bicarbonate when the pH is alkaline.
* This compensatory mechanism takes time!! And when it is in effect it suggests the condition is chronic.
B. ACID-BASE DISORDERS
I. Algorithm:
1. Data/normals
o You must have the pH, PaCO2 and HCO3
o Usually presented as pH/PaCO2/PaO2, HCO3… Normal = 7.4/40/80, 24
o Normal pH = 7.4
o Normal PaCO2 = 40 mmHg
o Normal HCO3 = 24 mmol
o Normal PaO2 = 80 mmHg
2. Determine the problem: Acidosis or alkalosis
o Acidosis = < 7.35
o Alkalosis = > 7.45
David Reilly
Lecturer: James Zangrilli
Lecture: 31
3. Determine the primary cause: Respiratory or Metabolic
o Acidosis:
 Respiratory: PaCO2 > 40 mmHg
 Metabolic: HCO3 < 24
o Alkalosis:
 Respiratory: PaCO2 < 40mmHg
 Metabolic: HCO3 > 24
4. Determine if the insult is acute or chronic based on secondary compensation
o Acute:
 Very little time for any kind of compensation – no alteration in secondary parameter will usually be
seen.
 Bicarbonate already in the blood can act immediately, thus a metabolic acidosis may present
rapidly by compensating with increased respiratory rate and blowing of CO2
 Compensation requiring renal function will take time.
o Chronic:
 A compensatory change is made to try and balance the pH.
 Example: Chronic respiratory acidosis from COPD  compensated with increased HCO3 by
preserving it in the kidneys and producing more of it.
5. Decide if the compensatory mechanism is appropriate for the initial insult.
* Especially important when assessing for mixed metabolic disorders.
o Acute disturbance in PaCO2 = 0.08 pH increase or decrease for every 10 mmHg change in PaCO2
o Metabolic acidosis = 1 mmHg decrease in PaCO2/ 1 decrease in HCO3
o Respiratory acidosis:
 Acute response: 1 increase HCO3 / 10 mmHg increase in PaCO2
 Chronic response: 3.5 increase in HCO3 / 10 mmHg incrase in PaCO2
o Metabolic alkalosis = 1 mmHg increase in PaCO2 / 1 increase in HCO3
o Respiratory Alkalosis:
 Acute response: 2 decrease in HCO3 / 10 mmHg decrease in PaCO2
 Chronic response: 5 decrease in HCO3 / 10 mmHg decrease in PaCO2
* Note: the only reason we cannot know the acute response to a metabolic alkalosis or acidosis is
because the HCO3 is directly effected by that process, it is not a response to a change in something
else (like respiration, etc…)
6. Check the anion gap
o Anion gap is mostly useful for determining if he possible cause of a metabolic acidosis.
o Normal = 12
o Anion Gap = Na – (Cl + HCO3)
David Reilly
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Lecturer: James Zangrilli
David Reilly
Lecture: 31
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