University Hospital of Wales Paediatric Intensive Care Unit Guideline Printed on Wed 23-jul-08
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Noah's Ark Childrens Hospital for Wales
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Interpretation of acid base results - theory and background

 

Conventional theory

Normal ph is in the range of 7.42 to 7.32

Derangements that are due primarily to fluctuations in the pCO2 are termed "respiratory", all others are "metabolic".

There are 3 major buffer systems in the body:
1. Carbonic acid (pCO2, bicarbonate)
2. Electrolytes (Na, K, Cl, Lactate, Ca, Mg)
3. Weak acids (Albumin)

Other buffers include phosphate, haemoglobin, proteins and ammonia.

In a blood gas bicarbonate and base excess are NOT directly measured. They are calculated from the pH and pCO2 in the Henderson-Hasselbach equation.

Henderson-Hasselbach equation

pH = 6.1 + (HCO3/pCO2 mmHg x 0.003)

However, in-vivo both bicarbonate and base excess do not correlate well with acid base disturbance because they assume that all other blood components and electrolytes are normal which is seldom the case in PICU-patients.

Stewart's strong ion model

In Stewart's strong ion model, the bicarbonate space has to compete with other negatively charged anions in order to maintain electroneutrality.

Stewart's strong ion model:

Na+ + K+ + Ca2+ + Mg2+ = Cl- + HCO3- + Lactate + Albumin

This means:

Interpreting blood gases

Practically, how can you interpret a blood gas?

Chloride

Cl- must always be interpreted relative to Na+.

The chloride to sodium ratio (Cl- / Na+) is normally between 72-80%
(normal Cl- = 106, normal Na+ =140, 106/140 = 0.75 = 75% )

Chloride is acidifying if Cl/Na >80%, and is alkalizing if Cl/Na <72%

The effect of chloride and sodium on the base excess can be simplified to the following formula: Base excess due to chloride & sodium = Na - Cl - 32

Example:

BE = -10 mEq/L
Na = 140 Cl = 112,
Then chloride accounts for 140 - 112 - 32 = -6 mEq/L
i.e. Chloride is acidifying by 6 mEq/L or 60% of the base excess. The remaining 4mEq/L could be explained by other anionic acids (e.g. lactate)

 

Albumin

Albumin is a weak acid and has a charge in mEq/L of about 25% of the concentration in g/L (i.e. 40g/L has a charge of 10 mEq/L). If albumin is low the base excess will be increased by the alkalinizing effect of low albumin by:

Albumin effect base excess = (42-Albumin (g/L)) x 0.25

 

Example:

BE = - 4 mEq/L
Albumin = 32
Then albumin will reduce base excess by (42 - 32 ) X 0.25 = + 2.5 mEq/L
i.e. Albumin is alkalinizing by 2.5 mEq/L, so true base excess = -6.5 mEq/L

 

 

The anion gap (Na + K) - (CL + HCO3) is also falsely lowered by a low albumin.
To correct the anion gap:

anion gap + (42-albumin (g/L)) x 0.25

Example:

Blood gas / U + Es:

pH 7.03 Na 128
pCO2 3.8 kPa K 4
Bicarbonate 10mmol/l Cl 109
Base Excess -10 mEq/L Alb 24

Calculations:

Cl/Na = 109/128 = 0.85 = 85% (acidifying)

BE chloride = 128 - 109 - 32 = -13 mEq/L

BE albumin = (42-24) x 0.25 = + 4.5 (alkalizing)

BE unmeasured = -10 - (-13) - (+ 4.5) = - 1.5

Anion gap = 128 + 4 - 109 - 10 = 13

Anion gap corrected for albumin = 13 + ((42-24) x 0.25) = 17.5

Interpretation:

Metabolic acidosis (not respiratory since pCO2 is low)
Some respiratory compensation
Severe hyperchloraemia is main cause acidosis
Minimal degree of "tissue" acidosis as anion gap only slightly raised
Albumine is very low and alkalizing by a significant amount (4.5 mEq/L)