Acute physiology

Question 1

What are the reference ranges for ABG?

What is your approach to reading an ABG?

Answer 1

pH (7.35-7.45)
PaO2 (80-100mmHg)
PaCO2 (35-45mmHg)
HCO3- (22-26 mmol/L)
base excess (-2 to +2 mmol/L)
lactate (0.5-2 mmol/L)

Approach:
1. What is the pH?
- Acidotic (<7.35)
- Alkalotic (>7.45)
2. Is it a respiratory or a metabolic cause?
- Respiratory the pH will head in the “O”pposite direction as the C“O”2
- “M”etabolic the pH will head in the sa“M”e direction as the HCO3-
3. Is there any compensation?
- No compensation
o pH remains abnormal and the other value (where the problem isn’t occurring i.e. CO2 or HCO3- will remain normal as it has made no attempt to help normalise the pH
- Partial compensation
o pH is still abnormal, and the other value is abnormal in an attempt to help normalise the pH.
- Total compensation
o pH is normal as the other value is abnormal and has successfully normalised the pH

The base excess increases in metabolic alkalosis and decreases (or becomes more negative) in metabolic acidosis, but its utility in interpreting blood gas results is controversial.

While the base excess may give some idea of the metabolic nature of a disorder, it may also confuse the interpretation. The alkalaemia or acidaemia may be primary or secondary to respiratory acidosis or alkalosis. The base excess does not take into account the appropriateness of the metabolic response for any given disorder, thus limiting its utility when interpreting results

Question 2

14. Please look at these arterial blood gas results:
    pH: 7.30, PaO2: 66mmHg, PaCO2: 26mmHg,
    Base Excess: -6.5, Lactate: 9.4.
    What is a summary description of these results?

Answer 2

• Patients pH indicates that they are acidotic.
• Seems to be a metabolic cause as the pH is low and the base excess is low and the the lactate is high meaning there is lactic acidosis.
• there is partial respiratory compensation as the pH is abnormal but paCO2 is low in an attempt to secrete CO2 to normalise the pH.
• Pt is hypoxic
• lactate> 5 so severe lactic acidosis and since it is >8 there is a high mortality risk

Question 3

15. Please look at these arterial blood gas results: pH: 7.30, PaO2: 66mmHg, PaCO2: 26mmHg, Base Excess: -6.5, Lactate: 9.4.
    What causes a blood gas profile of this sort?

Answer 3

Causes of lactic acidosis can be broadly classified into type A and type B based on the presence of tissue hypoxia.

Type A lactic acidosis is when there is inadequate oxygenation of tissues and this triggers anaerobic metabolism within cells and lactate is produced as a by-product.

Type B lactic acidosis occurs in settings of adequate tissue oxygenation and normal haemodynamics and is usually a result of an underlying disease (hep/renal or malignancy), congenital (enzyme deficiency) or medications and toxins.

In this ABG there is evidence of hypoxia with low O2 of 66mmHg, which suggests Type A lactic acidosis. This can be a result of tissue hypoperfusion, impaired O2 carrying capacity or an increase in oxygen consumption.
Tissue hypoperfusion could be due to volume depletion, hypotension or ischemia and so conditions like haemorrhagic, cardiogenic or septic shock as well as MI and mesenteric ischemia can show a blood gas profile of this sort.

Furthermore, severe anemia, severe hypoxaemia due to respiratory failure and CO poisoning can impair O2 carrying capacity and present with this profile.

Finally, exercises and seizures can lead to an increased oxygen consumption and also present with a profile like this.