Saturday, 27 September 2014

Blood Gases

1.    Anion Gap
    (Na + K) - (Cl + HCO)     Normal = 10 - 16mmol
    >20    Primary metabolic acidosis regardless of pH or serum bicarbonate concentration

Low anion gap: dilution, low albumin, increased unmeasured cation (calcium, magnesium, lithium)
Non anion gap acidosis: HARD UPS
  Hyperalimentation, acetazolamide, renal tubular acidosis, diarrhoea, uretero-pelvic shunt, post-hypocapnia,  spironolactone
Raised Anion Gap: MUD PILES

Methanol, uraemia, DKA, paraldehyde, iron/ isoniazid, lactic acidosis, ethanol, ethylene glycol, salicylates

2.  Excess Anion Gap = total anion gap - 12 + measured bicarb        If >30    Metabolic alkalosis
        If <23 Non gap metabolic acidosis
3. Delta Gap = Anion Gap - 12 (normal anion gap) < 0.4     Hyperchloraemic normal anion gap acidosis
 < 1        High AG & normal AG acidosis
 1 to 2    Pure Anion Gap Acidosis
                 Lactic acidosis: average value 1.6
                 DKA more likely to have a ratio closer to 1 due to urine ketone loss
 > 2        High AG acidosis and a concurrent metabolic alkalosis or a pre-existing compensated respiratory acidosis

4. Osmolality -  2 (Na + K) + Glu + Urea
 - Calculated vs measured should be 15- 20mmol / kg H20
 - Elevation suggests the presence of exogenous osmotically active particles and includes 4 main groups:
    Alcohols - Ethylene Glycol, methanol, ethanol, acetone, isopropyl alcohol
    Sugars - Mannitol, Sorbitol
    Lipids - e.g. hypertryglyceridaemia
    Proteins - Hypergammaglobulinaemia

5. Expected CO2
The CO2 is a very swift compensatory change. The bicarb takes a while to change. This means you can use the bicarb to calculate what you would expect the CO2 to be. You can predict what you expect the CO2 to be. If it's not what you expect it to be, then there's a hidden acidosis or alkalosis.

Acute Respiratory Acidosis 1:10- The [HCO3] will increase by 1 mmol/l for every 10 mmHg elevation in pCO2 above 40 mmHg.
Expected [HCO3] = 24 + { (Actual pCO2 - 40) / 10 }

Chronic Respiratory Acidosis 4:10- The [HCO3] will increase by 4 mmol/l for every 10 mmHg elevation in pCO2 above 40mmHg.
Expected [HCO3] = 24 + 4 {(Actual pCO2 - 40) / 10}

Acute Respiratory Alkalosis  2:10

- The [HCO3] will decrease by 2 mmol/l for every 10 mmHg decrease in pCO2 below 40 mmHg.
Expected [HCO3] = 24 - 2 { ( 40 - Actual pCO2) / 10 }

Chronic Respiratory Alkalosis 5:10

Metabolic Acidosis 1 1/2 + 8

Expected pCO2 = 1.5 x [HCO3] + 8 (range: +/- 2)

Metabolic Alkalosis 0.7 + 20 Expected pCO2 = 0.7 [HCO3] + 20 (range: +/- 5)

6. Alveolar Gas Equation
Alveolar pO2 = inspired pO2 - arterial pCO2 x 1.2.

7. CausesRespiratory Alkalosis
Anxiety, pregnancy,  hypoxia, sepsis, lung disease, hepatic encephalopathy, CNS disease, drugs

Respiratory Acidosis Acute airway obstruction, pulmonary embolism etc.

Metabolic AlkalosisVomiting, NG suction, diuretic use, post hyper-capnia
Excess mineralocorticoid activity, diuretics, excess alkali administration

Lactic Acidosis
- Ethylene glycol has a metabolite called glycolate, which is similar in structure to lactate. This can be wrongly interpreted by blood gas machines as an abnormally elevated lactate
- Sodium Bicarbonate is not routinely recommended. It can cause fluid overload, post recovery metabolic alkalosis, hypernatraemia. Often get rebound raise in CO2. 

Venoarterial ParadoxDuring resuscitation and manual resuscitation, VBG and ABGs may be completely different. On the venous side of the circulation, the lungs are poorly perfused and there is a build up of CO2.

CO2 Units
from kPa to mmHg you multiply by 7.5
from mmHg to kPa you divide by 7.5

Pathophysiology Reminder

Chemoreceptors in the medulla and the carotid and aortic bodies sense changes in pCO2 and H+. This is then used to regulate alveolar ventilation. When ventilation rises, pCO2 falls. This occurs because ventilation lowers the alveolar pCO2 below that of mixed venous blood. CO2 molecules then diffuse into the alveolar gas, thus lowering the CO2 concentration in pulmonary capillary blood.

Shift to the right:
Alkalosis, raised pCO2, raised temperature, raised concentration of 2, 3-DPG

A shift to the left may be seen in:
Alkalosis, reduced pCO2, reduced temperature, reduced concentration of 2, 3-DPG

Henderson Hasselbach Equation : CO2 + H2O ? H2CO3 ? H+ + HCO3-


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