Ward 6 Flashcards
How many values are find in ABGs?
Six:
pH
PaO2
PaCO2
HCO3 (bicarbonate)
SaO2
BE (base excess)
What do ABGs try to tell us?
ABGs tell us about activity in two systems; the respiratory system and the ‘metabolic’ system. If one system is disturbed, the other tries to restore balance. Both systems are primarily concerned with keeping blood pH in the normal range. Even for the respiratory system, pH (rather than oxygen) is the priority
What are the main steps to take when looking at ABGs?
1- Look at pH
2- Look at PaCO2
3- Look at the base picture
4- Look at oxygen
What should you do when looking for pH in ABGs?
Decide whether this is an ‘acidosis’ or ‘alkalosis’ (if it is within the normal range, note whether it is sitting towards the ‘acidotic’ or ‘alkalotic’ end of that range)
Normal pH ABG
7.35 and 7.45.
What should you do when looking at PaCO2 in ABG?
- Ask the question: is the PaCO2 contributing to, or attempting to compensate for, the problem. If, for example, the problem is an acidosis and the PaCO2 is low, then clearly the respiratory system is attempting to compensate. Thus, one can conclude that the problem is metabolic (similarly with other combinations). Therefore, after looking at only two numbers (pH and PaCO2), most of the interpretation is done.
- The other numbers (actual bicarbonate [aHCO3], base excess [BE], PaO2 and so on) might do nothing more than confirm this conclusion. However, they can sometimes add information about time course or provide information on additional derangements, but they will not contradict the conclusion that has already been reached
What should you do when looking at the base picture in ABG?
Actual bicarbonate (aHCO3) vs standard bicarbonate (sHCO3) – what’s the difference? What is perhaps surprising is that, after many years of looking at ABGs, those intelligent, enquiring minds have seemingly never once pondered that question.
aHCO3 is the actual measurement of bicarbonate in that actual blood sample (hence the name). The problem with this measurement is that it is markedly affected by PaCO2. If the PaCO2 is high, the aHCO3 is dragged higher and vice versa. What one would like to know is what the HCO3 would have been had the PaCO2 been normal. It is this value that would provide a direct handle on what the metabolic system is doing. One can calculate the value if aHCO3 and PaCO2 are known, although most blood gas machines do this automatically, known as the sHCO3
What is base excess?
Base excess (BE) measures all bases, not just bicarbonate. However, because bicarbonate is the greater part of the base buffer, for most practical interpretations, BE provides essentially the same information as bicarbonate. In simple terms, a high BE excess is the same as a high HCO3
Normal level for base excess?
One could probably have guessed that the expected value of BE was zero (the clue is in the word: ‘excess’). Therefore, a tight range around zero (−3 to +3) is normal
Normal PaCO2
35-45 mm Hg
Normal PaO2
75-100 mm Hg
Interpret this ABG
What is type 2 respiratory failure?
- Type 2 respiratory failure is extremely an issue of ventilation, that is, the business of pumping air in and out of the lungs. When underventilation occurs, for what ever reason (eg muscular weakness or opiate overdose), the PaCO2 will increase (the definition of underventilation) and PaO2 must decrease (even if the lungs are perfectly healthy).
- Type 2 respiratory failure results from underventilation, which can occur even in the context of healthy lungs
- defined as an increase in arterial carbon dioxide (CO2) (PaCO)> 45 mmHg with a pH < 7.35 due to respiratory pump failure and/or increased CO2 production
Respiratory failure types
Type 1, type 2, or type1+2
What is type 1 resp failure?
An acute type of resp failure with the distinguishing characteristic being a partial pressure of oxygen (PaO2) < 60 mmHg with a normal or decreased partial pressure of carbon dioxide (PaCO2)
