ABG & VBG - Arterial and Venous Blood Gas - Interpreting Results

 Normal Values

pH
7.35-7.45
PaCO2
4.7-6.0 kPa   /   35-45 mmHg
PaO2
9.3-13.3 kPa   /   80-100 mmHg
HCO3-
22-28 mmol/L
SaO2(Oxygen saturation)
92-98%
PA – pressure in the alveoli
Pa – pressure in the artery
To convert kPa to mmHg, multiply by 7.5

PaCO2 gives an indication of ventilation - how was is the patient breathing, how is their gas exchange
PaO2 is a measure of oxygenation

 

The oxygen dissociation curve

 
 
Note how:
  • The saturation declines rapidly at some points, and barely moves at other points. The curve starts its fast decline at about 90%, thus in the emergency situation, keeping oxygen saturations above 90% is important to avoid hypoxic injury (particularly hypoxic brain injury)
  • Various factors, including pH and temperature can shift the curve to the right or the left
 

Bicarbonate Buffering System

The bicarbonate buffering system is the method by which the body controls pH and is crucial to understand arterial and venous blood gas results. 

CO2  +  H2O   ↔   H2CO3   ↔    H+ + HCO3-

The equation demonstrates an equilibrium, between carbon dioxide, and hydrogen ions + bicarbonate. In normal physiology at a normal metabolic rate, this equilibrium exists to keep the pH between 7.35 and 7.45

  • Remember the pH is a logarithmic scale and as such, increases exponentially. The concentration of hydrogen ions at pH 7.1 is double that at pH 7.4. 

When a pathological abnormality occurs, this can cause various shifts in the equilibrium. We interpret these shifts to try to assess what the pathological abnormality is.

  • If concentration of Hydrogen ions increases, the pH will decrease, causing an acidosis. This causes the equation to shift to the left, and more CO2 is produced, of which some (or all) can be blown off by the lungs. This the mechanism of respiratory compensation.
  • If CO2 is not able to blown off effectively, then the concentration of CO2 increases, as thus then so will the concentration of hydrogen ions, and the pH will not be able to be resolved to normal. This is partial respiratory compensation. 
  • Most causes of acid-base disturbance are due to an acidosis. The causes of these are discussed in more detail below.
  • There can also be metabolic comepensation whereby the concentration of HCO3 is altered to try to keep the equilibrium in cases of respiratory dysfunction.
  • In a respiratory alkalosis, CO2 is blown off too quickly, thus the curve shifts to the left, to replace the CO2, and the concentration of hydrogen ions is lowered
  • In a metabolic alkalosis there is a disturbance due a loss of H+ or an excess of HCO3, causing the curve to shift.

Basic interpretation Rules

Interpretation is best done as part of an overall case review of a patient. Chronic as well as acute factors in the history can influence the result, particularly:
  • Renal disease
  • Diabetes
  • Drugs; diuretics, aspirin(+are they on oxygen?)
    • The Rule of 19 is a way of assessing whether or not the patient was on oxygen at the time of the sample.Add the PO2 and PCO2 – if the sum of these is >19 then likely to be on inspired oxygen. If the level is lower than this, they are likely to be breathing room air.
  • Symptoms and onset (lung disease?)

Basic Interpretation Rules

  1. Look at the pH - is it acidosis, alkalosis, or normal?
    1. If its acidotic, then the patient is acidotic
      1. If acidotic, calculate the anion gap to help differentiate the cause
    2. If its alkalotic, the patient is alkalotic
    3. ?If its it normal, there may be no acid-base dysfunction, or the patient could have a compensated acidosis or alkalosis. The CO2 and HCO3 values are required for further interpretation.
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