acid / base balance – interpretation of results comep oc 2010
TRANSCRIPT
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