acid / base balance – interpretation of results comep oc 2010

CoMEP TEACHING

ACID / BASE BALANCE – INTERPRETATION OF RESULTS

Dr. Alan S. Hutchison

Clinical Biochemistry

South Glasgow

Getting over the fear factor: ….

• (A little) physiology

• Simple acid-base disorders

• Mixed acid-base disorders

• Volatile acidsH+ + HCO3

- H2CO3 H2O + CO2

15,000 – 25,000 mmol/day

• Non-volatile acids, e.g.Lactic acid, ketoacids, free fatty acids,

sulphuric acid, phosphoric acid50 – 100 mmol/day under non-pathological

conditions

Disposal of non-volatile acids

• Buffering

• Respiratory system

• Renal regeneration

HCO3- / H2CO3 buffering system

• 60% of blood buffering

• Central role in respiratory mechanisms

• Central role in renal handling of H+

Disposal of non-volatile acids

• 2H+ + SO42- + 2Na+ + 2 HCO3

-

2Na+ + SO42- + 2H2O + 2CO2 [lungs]

• Carbon dioxide is removed via the lungs

• Bicarbonate is regenerated in the renal tubules

Blood gas analysis

• Measured[H+] (36 – 43 nmol/L) pH (7.44 – 7.37)

P CO2 (a = 4.6 – 6.0 kPa, v = 4.8 – 6.7 kPa)

P O2 (a = 10.5 – 13.5 kPa, v = 4.0 – 6.7 kPa)Hb (sometimes)

• Calculatedactual bicarbonate (23 – 30 mmol/L),standard bicarbonate, base excess, oxygen content etc.

Those of a nervous disposition, look away now……

Henderson-Hasselbalch Equation

pH = pK + log [HCO3- ] / [H2CO3]

= pK + log [HCO3- ] / α P CO2

(α = 0.03 for mmHg, 0.23 for kPa)

[H+] = 24 x P CO2 / [HCO3- ] (for mmHg)

[H+] = 180 x P CO2 / [HCO3- ] (for kPa)

Simple acid / base disorders

Disorder [H+] Primary derangement

Compensation

Metabolic acidosis

[HCO3- ] P CO2

Metabolic alkalosis

[HCO3- ] P CO2

Respiratory acidosis

P CO2 [HCO3- ]

Respiratory alkalosis

P CO2 [HCO3- ]

METABOLIC ACIDOSISAnion – cation gap= [Na+] – ([Cl-] + [HCO3

- ])Normally 8 – 16 mmol/L, depending on

methodology

[H+] nmol/L (36-43)

pH Pa CO2 kPa (4.6-6.0)

[HCO3- ]

mmol/L (23-30)

60 7.22 2.5 8

Causes of metabolic acidosis

• Anion – cation gap increased:

renal failure, ketoacidosis, lactic acidosis, toxins e.g. salicylate, inborn errors of metabolism

• Anion – cation gap normal:

renal tubular acidosis, diarrhoea, carbonic anhydrase inhibitors, ureteric diversion, mild renal failure

[H+] nmol/L (36-43)

pH Pa CO2 kPa (4.6-6.0)

[HCO3- ]

mmol/L (23-30)

32 7.49 6.3 35

METABOLIC ALKALOSIS

Causes:

vomiting, n/g suction, diarrhoea, villous adenoma of colon, diuretics, penicillin, Conn’s, Cushing’s & Bartter’s syndromes, liquorice ingestion, re-feeding, bicarbonate infusion

RESPIRATORY ACIDOSISCauses: Any cause of hypoventilation, e.g. CNS depression, neuromuscular disorders, thoracic cage limitation, acute or chronic OPD, ventilator

malfunction, cardiac arrest

[H+] nmol/L (36-43)

pH Pa CO2 kPa (4.6-

6.0)

[HCO3- ]

mmol/L (23-30)

49 7.31 9.7 28 (acute)

45 7.35 10.0 39 (chronic)

[H+] nmol/L (36-43)

pH Pa CO2 kPa (4.6-

6.0)

[HCO3- ]

mmol/L (23-30)

34 7.47 3.3 22 (acute)

40 7.40 3.3 17 (chronic)

RESPIRATORY ALKALOSIS

Causes: Any cause of hyperventilation, e.g. anxiety, CNS lesions, stimulants, fever, toxaemia, hyperthyroidism, pregnancy, hepatic disease, LVF, PTE, pneumonia

Mixed acid / base disorders

• Respiratory acidosis / metabolic acidosis• Respiratory acidosis / metabolic alkalosis• Respiratory alkalosis / metabolic acidosis• Respiratory alkalosis / metabolic alkalosis• Metabolic acidosis / metabolic alkalosis

• Metabolic acidosis / metabolic alkalosis / respiratory acidosis

• Metabolic acidosis / metabolic alkalosis / respiratory alkalosis

Expected degrees of compensation

• Metabolic acidosisP CO2 (kPa) = 0.2[HCO3

- ] + 1.05 +/- 0.26

If bicarbonate = 10 mmol/l, expected P CO2 = 2.79 – 3.31 kPa

OR

P CO2 = 0.13 – 0.17 x [HCO3- ]

If bicarbonate = 10 mmol/l, expected P CO2 = 3.05 kPa

Expected degrees of compensation

• Metabolic alkalosisP CO2 (kPa) = 0.12[HCO3

- ] + 1.18

OR

P CO2 = 0.08 kPa for each [HCO3- ] in

mmol/L

Expected degrees of compensation

• Respiratory acidosis

Acute: [HCO3- ] increases by 1 mmol/L for

every 1.3 kPa increase in P CO2

Chronic: [HCO3- ] increases by 3.5

mmol/L for every 1.3 kPa increase in P CO2

Expected degrees of compensation

• Respiratory alkalosis

Acute: [HCO3- ] falls by 2 mmol/L for every

1.3 kPa fall in P CO2

Chronic: [HCO3- ] falls by 5 mmol/L for

every 1.3 kPa fall in P CO2

72-year-old man post cardiac arrest

[H+] nmol/L (36-43)

pH Pa CO2 kPa (4.6-

6.0)

[HCO3- ]

mmol/L (23-30)

118 6.93 8.9 14

72-year-old man post cardiac arrest

[H+] nmol/L (36-43)

pH Pa CO2 kPa (4.6-

6.0)

[HCO3- ]

mmol/L (23-30)

118 6.93 8.9 14

Mixed metabolic acidosis and respiratory acidosis

62-year-old woman with congestive cardiac failure

[H+] nmol/L (36-43)

pH Pa CO2 kPa (4.6-

6.0)

[HCO3- ]

mmol/L (23-30)

21 7.64 3.5 31

62-year-old woman with congestive cardiac failure

[H+] nmol/L (36-43)

pH Pa CO2 kPa (4.6-

6.0)

[HCO3- ]

mmol/L (23-30)

21 7.64 3.5 31

Mixed metabolic alkalosis and respiratory alkalosis

45-year-old woman with pulmonary thrombo-embolism

[H+] nmol/L (36-43)

pH Pa CO2 kPa (4.6-

6.0)

[HCO3- ]

mmol/L (23-30)

35 7.45 2.7 14

45-year-old woman with pulmonary thrombo-embolism

[H+] nmol/L (36-43)

pH Pa CO2 kPa (4.6-

6.0)

[HCO3- ]

mmol/L (23-30)

35 7.45 2.7 14

Mixed metabolic acidosis and respiratory alkalosis

80-year-old man with cor pulmonale

[H+] nmol/L (36-43)

pH Pa CO2 kPa (4.6-

6.0)

[HCO3- ]

mmol/L (23-30)

33 7.48 9.3 51

80-year-old man with cor pulmonale

[H+] nmol/L (36-43)

pH Pa CO2 kPa (4.6-

6.0)

[HCO3- ]

mmol/L (23-30)

33 7.48 9.3 51

Mixed metabolic alkalosis and respiratory acidosis

72-year-old man with l.v.f. treated with a diuretic

[H+] nmol/L (36-43)

pH Pa CO2 kPa (4.6-

6.0)

[HCO3- ]

mmol/L (23-30)

[Na+] mmol/L (135-145)

[Cl-] mmol/L (97-107)

Anion-cation

gap (8-16)

21 7.67 4.0 34 140 94 12

Mixed metabolic alkalosis and respiratory alkalosis

26 7.58 4.0 28 140 94 18

Add a lactic acidosis