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Chapter: Medicine Study Notes : Endocrine and Electrolytes

Acid-Base balance - Electrolytes

Metabolism produces two acids: o Volatile: carbonic o Non-volatile: eg lactic

Acid-Base balance




·        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

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