How to investigate disorders of acid-base

• The normal value of H+ in plasma is 35-45 nmol/l
• The normal value of pH in plasma is 7.35-7.45
• There are three ways to correct blood pH

  • buffers (rapid)
  • lungs (fast)
  • kidneys (slow)
• Buffers correct blood pH by binding H+ and thus reducing acidity
• Buffers include proteins (e.g., Hb), and bicarbonate
• lungs correct blood pH by removing CO2
• The lungs are limited on how much CO2 can be expelled as it is dependent on bicarbonate reserves
• The kidneys correct blood pH by excreting H+ and regenerating bicarbonate
• Acid is produced through:

  • cellular respiration - produces CO2, reacts with water to produce carbonic acid and bicarbonate
  • metabolic processes- give rise to non-volatile acids (because cant be removed by lungs): ketones, lactate, etc.
• acidaemia is a plasma H+>45 nmol/l , pH<7.35
• • alkalaemia is a plasma H+<35 nmol/l , pH>7.45
• Respiratory cause of acid-base disorders are:

  • hypoventilation increases CO2, causing respiratory acidosis
  • hyperventilation decreases CO2, causing respiratory alkalosis
• Metabolic cause of acid-base disorders are:

  • overproduction/impaired excretion of H+ or unusual loss of bicarbonate can cause metabolic acidosis
  • unusual loss of H+ or unusually high ingestion of bicarbonate can cause metabolic alkalosis
• disorder classifications

  • primary acid-base disturbance- caused by an underlying pathology
  • acid-base disturbances resulting from body’s attempt to compensate for primary acid-base disturbance
• Arterial blood gas collection is usually done through the radial artery, and can also be taken from femoral in peri/arrest situation
• Arterial blood gas collected from vein is more painful, and local anaesthetic should be considered
• Arterial blood gas needs to be collected in a special syringe with an anticoagulant (assuming no bubbles, stable for ∼10 mins, ∼60 mins if on ice), and should not be sent via pneumatic tube (pod system)
• for acid-base interpretations, only need 4 values are required pO2, pCO2, H+ (pH), HCO3. Depending on machine other values can also be given which are all calculated e.g., standard bicarb, bace excess and anion gap
• bace excess is the amount of H+ ions per litre of blood, required to return H+ back to reference range at a reference range pCO2 (∼ 5.3 kPa)
• bace excess is negative in metabolic acidosis and positive in metabolic alkalosis, and its reference range is -3 to 3 mmol/l
• standard bicarbonate is what the HCO3 would be if pCO2 were reference range (∼5.3 kPa)
• standard bicarbonate should only be degenerated in a metabolic disorder
• standard bicarbonate is in reference range in respiratory disorder, equivalent to actual bicarb, and in mixed respiratory and metabolic disorders its significantly different from actual bicarb
• the reference range of standard bicarbonate is 21-29 mmol/l
• • anion gap is the difference between the most abundant cation and anion, and can be useful in narrowing differential of metabolic acidosis
• reference range for anion gap is 6-18 mmol/l
• anion gap is elevated in some types of metabolic acidosis, normal in others
• anion gap value depends on what bicarb ions are replaced with:

  • no change: replaced with Cl ions (hyperchloremia acidosis)
  • elevated: replaced with anions corresponding to lactate, keto-acids
• Blood gas analysis approach should be:

  • is the patient adequately oxygenated
  • what is their pH (H+)
  • is there a pCO2 disturbance
  • is there a bicarbonate disturbance
• blood gas interpretation plots:

  • shaded areas define usual behaviour of acid-base disorders
  • can be helpful where one or more pathologies simultaneously driving acid-base disorders (mixed disorders)
  • mixed/multiple disorders will appear outside the shaded areas
• causes of metabolic acidosis include:

  • increased acid formation
  • reduced acid excretion
  • loss of bicarbonate
• increased acid formation can occur due to:

  • ketoacidosis - diabetes, starvation, alcoholic
  • lactic acidosis - tissue hypoxia (sepsis, anaemia, major haem…, cardio respiratory arrest, peripheral vascular disease, general seizure)
  • poisoning - salicylate, methanol
  • inherited metabolic disorders (usually present in early childhood
• reduced acid excretion can occur due to:

  • renal failure (CKD, AKI)
  • renal tubular acidosis (types 1 & 4)
• loss of bicarbonate can occur due to:

  • gastrointestinal- severe diarrhoea, high-output small bowel fistula
  • renal tubular acidosis type 2
• metabolic acidosis can affect:

  • cardiovascular
  • oxygen delivery
  • nervous system
  • potassium homeostasis
  • bone minerality
• cardiovascular effects of metabolic acidosis is negative inotropic effect (if severe)
• oxygen delivery effects of metabolic acidosis includes:

  • acutely: H+ causes right shift of oxygen-haemoglobin dissociation curve (facilitates O2 delivery)
  • after several hours: H+ reduces 2,3-DPG causing left shift of curve (impairs O2 delivery)
• nervous system effects of metabolic acidosis are impaired consciousness (little correlation with H+)
• potassium homeostasis effects of metabolic acidosis include:

  • potassium leakage from cells causing high plasma K+, may also be lost renally
  • if above sustained, total body K+ may be depleted
• bone minerality effects of metabolic acidosis are the buffering effect that chronic acidaemia has on bone, leading to decalcification