ASSESSMENT OF ACID BASE STATUS Flashcards
Acid-base status can be fully characterized by the three parameters
defined in the Henderson-Hasselbalch equation:
pH
PCO2
and plasma HCO3− concentration.
If two of these parameters are known, the third can be calculated.
Normal plasma HCO3−
22 – 26 mmol/L
Normal plasma PCO2
4.7-6.0kPa (35 - 45 mmol/L)
Specimen collection and transport
As with any patient’s sample for laboratory analysis, a correct specimen must be taken and stored in an appropriate container and be delivered to the laboratory in a timely way. This is essential for samples taken for acid-base analyses.
The sample must be:
Arterial blood or arterialized capillary blood; the latter is not easy to obtain as it needs an arterial cannula in situ.
Heparinized, to prevent clotting, but only using a small amount of heparin as heparin is acidic and an excess can dilute the sample and potentially cause haemolysis.
Well mixed, but free from bubbles. Any air bubbles will increase the PO2 and decrease the PCO2 levels.
Chilled or delivered to the laboratory without delay, as chilling reduces glycolysis and the production of lactate.
Normal plasma PO2
12 – 14.6 kPa (90 - 110 mmol/L)
Assessment of acid-base status
- H+/pH tells us whether there is an overt acidosis or alkalosis
- HCO3- tells us whether there is a metabolic disturbance
- pCO2 tells us whether there is a respiratory disturbance
- pO2 does not directly affect acid-base status but gives an indication of respiratory function and tissue oxygenation
Step 5: Identify underlying cause of acid–base disturbance
Once the type of acid–base disorder has been identified it is important to establish the underlying cause of the disorder.
This requires a careful history, examination, and a variety of biochemical tests.
Cause may be obvious if patient presents with an acute abdomen & has grossly raised serum amylase level.
However, it may be a careful drug history that provides the diagnosis in an elderly diabetic patient with renal impairment who has been commenced on metformin and has developed a lactic acidosis as a Cx of this Rx.
Steps to the clinical assessment of acid–base status
Step 1: What is the pH?
Step 2: Check the bicarbonate (HCO3)
Step 3: Check the PCO2
+/- Step 4: Assess the anion gap
Step 5: Identify underlying cause of acid–base disturbance
Step 1: What is the pH (normal 7.35–7.45)
pH < 7.35 implies acidaemia
pH >7.45 implies alkalaemia
pH within the reference interval implies either no acid–base disturbance, there is compensation, or a complex disorder where acidosis and alkalosis exactly cancel each other out.
Step 2: Check the bicarbonate (HCO3) (22–30mmol/L)
HCO3 <22 implies metabolic acidosis
HCO3 >30 implies metabolic alkalosis
Step 3: Check the PCO2
(35–45mmHg [4.7 – 6.0kPa])
PCO2 >40 mmHg implies respiratory acidosis
PCO2 <40 mmHg implies respiratory alkalosis
Step 4: Assess the anion gap
The anion gap is the calculated difference between cations and anions in the blood. The anion gap (AG) can be calculated from the equation: AG = (Na+ + K+) – (Cl- + HCO-3) which is roughly equal to 20.
Differences in how mixed vs simple disturbances proceed
A low pH indicates acidaemia and in simple disturbances the other two analytes tend to proceed in the same direction: In simple metabolic acidosis the expected results are low levels for both [HCO3−] (primary lesion) and Pco2 (compensation by respiratory alkalosis). In simple respiratory acidosis the reverse is expected high Pco2 (primary lesion) and high [HCO3−] (compensatory response).
If the [HCO3−] and the Pco2 proceed in opposite directions, contrary to what is expected; a high Pco2 and a low [HCO3−], then the patient has a mixed respiratory and metabolic acidosis, in which case the pH tend to be very low.
