Module 11: Arterial blood gas analysis

Question 1

ABG interpretation
- key pieces of information contained?

Answer 1

5 key pieces:
pH
PaCO2 (partial pressure of carbon dioxide in arterial blood)
Bicarbonate
Base excess (BE)
PaO2 (partial pressure of oxygen in arterial blood).

Also other values that may be very relevant in resuscitation: Blood glucose, K, lactate, Hb

Question 2

The 6-step approach

Answer 2

Step 1- How is the patient?

Step 2- Is the patient hypoxaemic?

Step 3- What is the pH?

Step 4- What happened to the PaCO2?

Step 5- What has happened to the base excess (BE) or bicarbonate?

Step 6- Are there any other important values to consider?

Question 3

Bicarbonate

Answer 3

The body’s buffer. Generated by kidneys. Mops up excess H+. This H+ mopped up by HCO3 go on to produced CO2 and H2O2.

Acids not excreted by the resp system can be excreted in urine. Depending on body’s acid load, urine will either be alkaline or acid.

Acute increase in acid =
↑respiratory system CO2 excretion and ↑
renal bicarbonate production (nb: kidneys are slow to respond!).

pH < 7.35, with a low bicarbonate = metabolic acidosis i.e. of the kidneys have not yet responded

Excess bicarbonate OR acid loss e.g. persistent vomiting —> reduces H+and increases pH >7.45 = metabolic alkalosis.

Question 4

Base excess

Answer 4

A measure of the amount ofexcess acidorbase in blood as a result of ametabolic derangement.

How much strong acid or base is needed to restore blood to normal (pH 7.4)?

Normal BE +2 to -2 mmol L-1. A base excess that is:

More negativeBE and acidotic pH = metabolic acidosis.

More positiveBE and alkalotic pH = metabolic alkalosis

Question 6

PaCO2

Answer 6

CO2 is a waste product of metabolism. Normally it is transported in the blood:
combined either with protein or haemoglobin
dissolved in plasma, where it reacts with water to form hydrogen ions (H+) and bicarbonate (HCO3-).
CO2 behaves as an acid when dissolved in water (creates H+ ions).

The normal PaCO2 is 5.3 kPa with a range of 4.7-6.0 kPa.

The key determinant of PaCO2 is alveolar ventilation
-Hypoventilation = respiratory acidosis
- Hyperventilation = respiratory alkalosis

Question 7

PaO2

Answer 7

Concentration of oxygen in inspired air is 21%. Since atmospheric pressure is about 100 kPa, this gives a partial pressure of oxygen of 21 kPa. This is reduced to 13 kPa by the time it reaches the alveoli due to the addition of water vapour and CO2.

The partial pressure of oxygen in arterial blood is always lower than alveolar; the extent of this gradient is determined by the presence of any lung disease. In a healthy individual breathing air, the PaO2 is normally at least 11 kPa (i.e. only about 10 kPa lower than the inspired partial pressure).

As a rule of thumb the PaO2 should be numerically 10 less than the inspired concentration. For example, 40% inspired oxygen should result in a PaO2 of approximately 30 kPa. Lung injury increases the gap between inspired concentration and PaO2. For someone breathing 50% oxygen a PaO2 of 13 kPa is not “normal”.