Acid-Base balance - Electrolytes

Acid-Base balance

Physiology

·        Metabolism produces two acids:

o  Volatile: carbonic

o  Non-volatile: eg lactic

·        Buffer systems:

o  H+ + HCO₃- « H₂CO₃  (H₂O + CO₂)

o  Henderson Hasselbach Equation:

o  Normal range for pH is 7.35 – 7.45 (=45 – 35 nmol/L of H+ ion)

o  Range of pH compatible with life is about 6.8 – 7.8 = H+ concentration of 160 – 16 nmol/l

o  Lots of other buffering systems

·        Compensation:

o  Never complete

o  Respiratory: pH measured in the medulla.  Compensates rapidly

o  Renal:

§  Alter bicarbonate reabsorption

§  Titratable acid excretion: organic buffers in tubules acidifies urine. Excretes 30 – 50% of acid produced each day

§  NH4 excretion: formed in tubules, ­ takes days.  Excretes 50 – 70% of acid

Respiratory Alkalosis

·        Hyperventilation

·        Causes:

o  Hypoxia

o  Lung disease: PE, asthma 

o   Anxiety

o   Fever, sepsis

o   Salicylate overdose: stimulates respiration, will subsequently develop metabolic acidosis

·        ¯PaCO₂, ­pH, initial alterations in [HCO₃] are minimal, if it persists then kidneys compensate

·        Compensation:

o   Acute: HCO₃ ¯ by 2 for each 10 ¯ PCO₂

o   Chronic: HCO₃ ¯ by a further 3 (ie total of 5) for each 10 ¯ PCO₂ [renal loss of HCO₃]

Respiratory Acidosis

·        Hypoventilation

·        Causes:

o   PCO₂ excretion lags production – eg severe asthma (initially asthmatics hyperventilate)

o   Pulmonary disease, muscular diseases, etc

o   CNS depression: primary or drugs/toxins

o   Asphyxia, smoke inhalation

·        As PCO₂­ then CO₂ + H₂0 ® H+ + HCO₃-

·        ­PaCO₂ ® ¯pH, initial alterations in [HCO₃] are minimal, if it persists then kidneys compensate (­HCO₃ reabsorption, ­NH3 formation and excretion):

o   Acute: HCO₃­ by 1 for each 10 ­PCO₂ 

o   Chronic: HCO₃­ by a further 2.5 (ie 3.5 of total) for each 10 ­PCO₂

o   For example:

Metabolic acidosis

·        Net gain of acid

·        Causes:

o   Accumulation of acid (anion gap > 18 mmol/L): ­H+ (ketoacidosis, lactic acidosis, ingestion of salicylates, methanol), renal failure (failure to excrete H+)

o   ¯HCO₃ (anion gap < 18 mmol): GI tract loss (eg diarrhoea), renal loss (eg ¯carbonic anhydrase), hypoaldosteronism

·        Compensation:

o   Rapid: PCO₂¯ by 1.2 for each ¯1 in HCO₃ (baseline = 24) - rapid

o   Slow: ­HCO₃ reabsorption and ­NH4 excretion by the kidneys

Metabolic alkalosis

·        Net loss of acid

·        Causes:

o   Loss of H+:

§  Vomiting (suspect surreptitious if low Cl)

§  NG suction

§  Renal loss (hyperaldosteronism)

o   Increase in HCO₃ reabsorption:

§  K depletion (Conn‟s, Cushing‟s, drugs, diuretics).

§  Volume depletion, eg ­Aldosteronism ® ­Na/H exchange

o   Gain in alkali: eg NaHCO₃ administration

·        Compensation:

o   PCO₂ ­ by 0.6 for each 1 ¯ in HCO₃.  Limited by hypoxia

o   Final compensation is by renal excretion of HCO₃ – requires correction of Cl, K and volume

Summary of compensation rules

Mixed Acid/Base disorders

·        Suspect if:

o  Clinical grounds

o  Compensation rules not obeyed

o  Normal pH but abnormal PCO₂ and HCO₃

·        Examples:

o  Respiratory + Metabolic Acidosis:  Pulmonary oedema + cardiac arrest

o  Respiratory + Metabolic Alkalosis: Over-ventilation + Nasogastric suction

o  Respiratory Alkalosis + Metabolic Acidosis: Septic shock or Salicylate OD

o  Respiratory Acidosis + Metabolic Alkalosis: CORD + Diuretic

o  Metabolic Acidosis + Metabolic Alkalosis: Renal failure + vomiting

Interpreting Blood Gas Results

·        Arterial blood taken in 2 ml syringe containing heparin (to stop clotting) and transported on ice

·        Look at pH: 7.36 to 7.44 is normal

·        Look at PCO₂.  If < 36 then hyperventilation.  If > 44 then hypoventilation. 

·        Look at HCO₃. If < 22 then metabolic acidosis. If > 26 then metabolic alkalosis. But HCO₃ depends on PCO₂. So (to work out if its just compensation, or there is a metabolic problem as well as a respiratory one):

o  For acute changes (hours): a fall in PaCO₂ ® a normal HCO₃ 2 less for every 10 mmHg ¯ in PaCO₂. A rise in PaCO₂ ® normal HCO₃ 1 greater for every 10 mm Hg ­ in PaCO₂

o  For chronic changes (days): a rise in PaCO₂ results in a normal HCO₃ 4 greater for every 10 change in PaCO₂

Base Excess

·        Given on all arterial blood gas results

·        = Concentration of titratable base when titrating blood or plasma with a strong acid or base to a plasma pH of 7.40 at PCO₂ of 40 mmHg at 37C

·        Intent is to remove the impact of the respiratory component leaving just the metabolic component:

o  If +ive: metabolic alkalosis ® deficit of non-carbonic acid

o  If –ive: metabolic acidosis ® excess of non-carbonic acid

·        BUT recognises normal compensation as an extra disturbance. May be useful for an anaesthetist (eg simple and acute disturbances