Acid-Base Balance Disorders

28/04/2013 by admin | Renal

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

  1. Seen in diabetic ketoacidosis, lactic acidosis and renal failure.
  2. Severe acidosis results in cardiac depression and death.
  3. Body responds to acidosis with respiratory compensation. Increased RR (deep fast breaths = Kussmaul’s respiration) to drop excess CO2 and reduce acidosis.
  4. Four main aetiologies:
    1. o Ingestion of acid (=salicylate, methanol, ethylene glycol poisoning).
    2. o Accumulation of endogenous acids (=lactic acid)
    3. o Loss of alkali in severe diarrhoea (=biliary or enteric fistualae, renal tubular necrosis).
    4. o Failure of elimination of acid in renal failure and dital tubular acidosis.
  5. 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

  1. Aetiologies:
    1. o Excess intake of alkali (ant-acid intake with milk = milk-alkali syndrome)
    2. o Loss of acid (=vomiting, seen in pyloric stenosis).
    3. o Increased renal loss of bicarbonate (seen in hyperaldosteronism and hypokalaemia).
  2. 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

 

  1. If body unable to alter RR to achieve suitable pH = consider the need to upcoming ventilator support!
  2. Reduced RR = high CO2 = acidosis.
  3. Aetiologies:
    1. o Sedation with opiates
    2. o Respiratory muscle weakness = GBS, poliomyelitis, MG.
    3. o Severe COPD
  4. p/c: (due to high CO2) = bounding pulse, papilloedema, metabolic (CO2) flap, elevated blood bicarbonate.
  5. Mx: correct underlying ventilatory defect. Artificial ventilation.

 

Respiratory alkalosis: pO2 <8kPa (sats <90%), pH >7.45, pCO2 <3.5kPa

  1. Result of increased ventilation, reduced CO2, rise in pH.
  2. Aetiologies:
    1. o Response to hypoxaemia/tissue hypoxia
    2. o Undesired increased ventilation (=panic attack)
    3. o Excessive artificial ventilation
    4. o Stimulation of respiration by drugs
    5. 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.

 

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