In all seriously ill pts – an ABG should be performed to rule out acid-base disturbances.
Metabolic acidosis: pO2 <8kPa (sats <90%), pH <7.35, pCO2 <5kPa
- Seen in diabetic ketoacidosis, lactic acidosis and renal failure.
- Severe acidosis results in cardiac depression and death.
- Body responds to acidosis with respiratory compensation. Increased RR (deep fast breaths = Kussmaul’s respiration) to drop excess CO2 and reduce acidosis.
- Four main aetiologies:
- o Ingestion of acid (=salicylate, methanol, ethylene glycol poisoning).
- o Accumulation of endogenous acids (=lactic acid)
- o Loss of alkali in severe diarrhoea (=biliary or enteric fistualae, renal tubular necrosis).
- o Failure of elimination of acid in renal failure and dital tubular acidosis.
- The anion gap can be useful in diagnosis of metabolic acidosis (difference between major cation [sodium] and major anions [chloride or bicarbonate]). It should be 12 +/- 2. >14 = addition of anions (lactate/ketones), <10 = hyperchloraemic acidosis = bicarbonate loss/renal tubular acidosis.
Metabolic alkalosis: pO2 <8kPa (sats <90%), pH >7.45, pCO2 >5kPa
- o Excess intake of alkali (ant-acid intake with milk = milk-alkali syndrome)
- o Loss of acid (=vomiting, seen in pyloric stenosis).
- o Increased renal loss of bicarbonate (seen in hyperaldosteronism and hypokalaemia).
- There may be respiratory compensation with reduced RR, rise in CO2 and fall in pH.
Respiratory acidosis: pO2 <8kPa (sats <90%), pH <7.35, pCO2 >5kPa
- If body unable to alter RR to achieve suitable pH = consider the need to upcoming ventilator support!
- Reduced RR = high CO2 = acidosis.
- o Sedation with opiates
- o Respiratory muscle weakness = GBS, poliomyelitis, MG.
- o Severe COPD
- p/c: (due to high CO2) = bounding pulse, papilloedema, metabolic (CO2) flap, elevated blood bicarbonate.
- Mx: correct underlying ventilatory defect. Artificial ventilation.
Respiratory alkalosis: pO2 <8kPa (sats <90%), pH >7.45, pCO2 <3.5kPa
- Result of increased ventilation, reduced CO2, rise in pH.
- o Response to hypoxaemia/tissue hypoxia
- o Undesired increased ventilation (=panic attack)
- o Excessive artificial ventilation
- o Stimulation of respiration by drugs
- o Fall in CO2 and rise of pH can be associated with fall in ionized Ca2 which produce symptoms of peripheral paraesthesia, light-headedness, carpopedal spasm. Chvostek’s sign may be present (tapping of facial nerve = brief facial muscle spasm [latent hypocalcaemic tetany]).
Mixed acid-base disorders: the body can compensate with respiration and metabolism (renal) – detailed history and calculation of anion gap can provide diagnosis.