Variants (1): - Respiratory acidosis (that is, ventilatory failure): the drop in pH is explained by the change in PaCO2.
- Respiratory alkalosis (that is, alveolar hyperventilation): the decreased PaCO2 explains the increased pH.
- Metabolic acidosis: reduced pH not explained by increased PaCO2. It is usually associated with an increased anion gap due to the accumulation of renal acids, lactic acids, and ketoacids (from diabetes or starvation). Metabolic acidosis without an increased anion gap is typically associated with an increase in plasma chloride concentrations; chloride ions replace plasma bicarbonate. Hyperchloraemic acidosis is usually caused by gastrointestinal loss or renal wasting of bicarbonate.
- Metabolic alkalosis: raised pH out of proportion to changes in PaCO2. It is associated with hypokalaemia, volume contraction, or exogenous alkali administration.
Measurements (1): - Arterial puncture may result in spasm, intraluminal clotting, or bleeding and haematoma formation, as well as a transient obstruction of blood flow. The radial artery at the wrist is the best site for obtaining an arterial sample because it is near the surface, relatively easy to palpate and stabilise, and usually has good collateral supply from the ulnar arteries. This can be confirmed by a modified Allen's test.
- It is kinder to patients to use local anaesthesia over the radial artery before puncture.
- Any sample with more than fine air bubbles should be discarded. Air bubbles result in gas equilibration between the air and the arterial blood, lowering the arterial carbon dioxide pressure (PaCO2) and increasing the arterial oxygen pressure (PaO2). The cellular constituents of blood remain metabolically active so arterial gas tensions in the sample will change over time. If the sample cannot be analysed quickly it should be cooled to 5° C immediately. The sample can then be stored for up to one hour with little clinically significant effect on result.
- Arterialised ear lobe samples provide an alternative to arterial puncture for occasional samples. A capillary tube is used to sample blood from a warmed, vasodilated earlobe. The PaCO2 values agree will with those obtained from arterial samples, but accuracy of the PaO2 depends on good technique in arterialisation of the ear lobe.
- Systemic assessement of arterial blood gas measurements:
- Step 1a: determine whether the PaCO2 is low (<4.7 kPa or <35 mmHg) indicating alveolar hyperventilation, normal (4.7-6 kPa or 35-45 mmHg), or high (>6 kPa or >45 mmHg) as in ventilatory failure. Calculate the respiratory pH to determine if there is any metabolic compensation or additional disorder.
- Step 1b: in the presence of a metabolic acidosis, calculate the anion gap to determine whether it has increased. No increase occurs with diarrhea or urinary loss of bicarbonate.
- Step 2a: assess arterial oxygenation. Arterial hypoxaemia in adults in defined as PaO2 <10.7 kPa or <80 mmHg breathing room air, although it is not usually treated as clinically important unless below 8 kPa or <60 mmHg, when oxygen saturation will be 90% or less.
- Step 2b: calculate the A-a gradient to determine whether carbon dioxide retention is related to an intrapulmonary cause.
References: - ABC of oxygen: assessing and interpreting arterial blood gases and acid-base balance. BMJ. 1998;317:1213: full text | pdf.
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