Normal Physiology Gans.pdf · 5. Evaluate mixed acid-base disorder • Relate delta anion gap to...
Transcript of Normal Physiology Gans.pdf · 5. Evaluate mixed acid-base disorder • Relate delta anion gap to...
Normal Physiology
Pathophysiological Response to acidemia/alkalemia
Structured Approach
Simple Acid-Base disorders and compensatory responses
Anion Gap
Urinary Anion Gap/Urine Osmolar Gap
Delta-Delta or Delta Ratio
Osmolal Gap
Metabolic AcidosisMetabolic AlkalosisRespiratory AcidosisRespiratory Alkalosis
Arterial-Alveolar Oxygen Tension Difference References
References and Sources
versus
- The major titratable acid buffer is HPO42- (dependent on diet, PTH)
- less important buffers are creatinine and uric acid.
- Ammonium production and excretion
minimum attainable urine pH is 4.0 to 4.5: cannot excrete HCL (requires urine pH of 1.0)
Physiological Effectsof Metabolic Acidosis
Structured approach
1. History and Physical Exam: clues to what acid-base disorder might be present2. Look at the pH (If normal consider mixed acid-base disorder)3. Determine primary acid-base disorder and secondary (compensatory) response4. Consider the metabolic component
• Metabolic acidosis: anion gap• High • Normal (urinary anion gap, Na<20mmpl/l: urinary osmol gap, both indirect
measures of ammonium)• Metabolic alkalosis
• Chloride-responsive (Cl <25 mmol/l)• Chloride-resistant (Cl> 40 mmol/l)
• Urinary K <20 or >30 mmol/l)5. Evaluate mixed acid-base disorder
• Relate delta anion gap to the delta bicarbonate6. Evaluate osmolal gap in unexplained high anion gap metabolic acidosis, coma, toxic
ingestions7. Evaluate Respiratory component of acid base disorder
• Relate partial pressure of oxygen to ventilation (alveolar-arterial oxygen tension difference)
8. Verification of diagnosis!
Anion Gap
[Na+] + [K+] + [Ca2+] + [Mg2+] + [H+] + unmeasured cations=
[Cl−] + [HCO3−] + [CO32−] + [OH−] + albumin + phosphate + sulfate + lactate + unmeasured anions (e.g., inorganic anions)
There is NO GAP: sum of the positive and negative ion chargesin plasma are equal in vivo
reference ranges of 3.0 to 12.0 mmol per liter up to 8.5 to 15.0 mmol/l
A= chloride or unmeasured anion
G Glycols (ethylene, propylene)O 5-oxoproline (pyroglutamic acid)L L-Lactate, D D-Lactate
M MethanolA AspirinR Renal failure (GFR <20 ml/min)R RhabdomyolysisK Ketoacidosis
High Anion Gap
bicarbonate decreases relative to levels of sodium and chloride
High Anion Gap (Pitfalls)
• 50% patients with serum lactate between 3.0 and 5.0 mmolper liter have an anion gap within the reference range
• Adjust for albumin (weak acid, up to 75% of the gap):Decrease of 1 g/dl, add 2.3-2.5 mmol/l to the gap
Low or Negative Anion Gap
• high levels of cations: - lithium toxicity- monoclonal IgG gammopathy- high levels of calcium or magnesium.
• pseudohyperchloremia in bromide or iodide intoxication.
Urinary Anion Gap
Represents excretion of Ammonium NH4+ (as NH4Cl)
Negative in normal anion gap acidosis (Hyperchloremic acidosis should lead to increasedrenal excretion of ammonium)
If positive: consider renal failure, distal renal tubular acidosis, hypoaldosternonism
Unreliable: polyuria, urinary pH >6.5, ammonium excreted with other anion (ketoacid, salicylates, d-lactate, penicilline) or when UNa+< 20 mmol/l
Urinary Osmolal Gap
mmol/l
<40 mmol/l indicates impaired ammonium excretion, except in ketoacidosis
Normally, near zero or positive
Delta-ratio
-If ratio between 1 and 2, then pure elevated anion gap acidosis
- If < 1, then there is a simultaneous normal anion gap acidosispresent.
- if > 2, then there is a simultaneous metabolic alkalosis present ora compensated chronic respiratory acidosis.
Delta-ratio
-if <0.4 pure normal anion gap acidosis
- If < 1, then there is a simultaneous normal anion gap acidosis present
-If ratio between 1 and 2, then pure elevated anion gap acidosis
- if > 2, then there is a simultaneous metabolic alkalosis present or a pre-existing chronic respiratory acidosis.
50% of excess acid is buffered intracellularlyand by bone, not by HCO3-Excess anions remain in ECF
Increase in Anions > decrease in HCO3-
Lactic Acidosis: 1.6:1Ketoacidosis 1:1 (loss of ketones with urine)
Mixed acid base disorders usually produce arterial blood gas results that couldpotentially be explained by other mixed disorders. Oftentimes, the clinical picture will
help to distinguish.
Osmolal gap
Difference between measured serum osmolality and calculated serum osmolality
Calculated Osmolality: 2 x [Na+] + (Glucose (mg/dl)/18) + (BUN (mg/dl)/2.8)
ethanol (mg/dl)/3.7
Normal: -10-10 mosmol/kg
Increased: toxic alcohols (lactate, ketones)
Note:Acidic urine!
Alveolar-arterial Oxygen tension difference
Ventilation-Perfusion mismatch:5-10 mmHg in young15-20 mmHg in elderly
For every decade a person has lived, A-a difference increase by 2 mm Hg;A-a O2 difference = [Age/4] + 4
Stroomdiagram onderzoek Hypokaliemie