Blood Gas Analysis--Rules To Live By

1.Is there ACIDEMIA or ALKALEMIA

a. Alkalemia and Acidemia refer only to the net pH of blood, and do not describe the process that led to the alteration of the pH.
Acidemic pH <>7.45

b. Whatever side of 7.4 the pH is on, the process that caused it to shift to that side is the primary abnormality. The body does not fully compensate for primary acid base disorders. After this step one should search for the underlying primary disorder. What processes, metabolic or respiratory brought the pH to either side of 7.4.

2. Identify abnormalities in PCO2.

a. Identify whether the PCO2 is normal, inappropriate, and whether there is a respiratory acidosis or alkalosis (i.e. the CO2 could be in the normal range but is the patient tachypneic and is CO2 inappropriately high).

• For acute increases in PCO2 of 10 mm Hg there should be a rise of 0.08 in the pH.
• For acute decreases in PCO2 of 10 mm Hg there should be a fall of 0.08 in the pH.

b. Winter’s rule states that the last 2 digits of pH greater than 7 predicts the PCO2 in a compensatory respiratory alkalosis in a patient with a primary acidosis. So if a patient has a metabolic acidosis, say pH 7.25, one should expect a PCO2 of 25. If the CO2 is higher, than the patient is not having an appropriate compensation, suggesting respiratory dysfunction.

c. At this point one should decide whether the change in pH could be explained by your PCO2. If not, consider ordering a set of electrolytes to review the causes for a metabolic acidosis. One would evaluate the bicarbonate level (on the ABG the Bicarbonate level is a calculated value not a true value).

• In addition, evaluating the anion gap can direct you toward the cause for an acidosis.

3.If pH change is appropriate for PCO2 change go to the evaluation of oxygenation.

4.Identify abnormalities in bicarbonate.

a.Normal bicarbonate is 22-24

• If the value is lower than 22, there is a metabolic acidosis.
• If the value is higher than 24, then there is a metabolic alkalosis.

b.Metabolic Acidosis is commonly divided into anion gap and nonanion gap acidosis. If the Bicarbonate value is less than 22 one must calculate the anion gap; if the anion gap is greater than 17, it suggests anionic gap acidosis.

Normal Anion Gap 12 +/- 2. If AG > 17, a metabolic acidosis is probably present.

• Anion Gap acidosis differential is remembered by stating this antonym: CAT MUDPILES.
  
• C for cyanide or carbon monoxide, A for alcohol, T for toluene, M for methanol, U for uremia, D for DKA, P for paraldehyde, I for iron and isoniazid, L for lactic acidosis, E for ethylene glycol, and S for strycnine.
  
• The body does not generate a large anion gap to compensate even for a chronic alkalosis. For each decrease in serum bicarbonate there should be an equal rise in the AG.

c. Calculate the excess anion gap. The excess anion gap = total anion gap minus the normal anion gap (12 mmol per liter). Add the excess anion gap to the measured bicarbonate concentration; if the sum is greater than a normal serum bicarbonate (> 30 mmol per liter) there is an underlying metabolic alkalosis; if the sum is less than a normal bicarbonate (< 23) there is an underlying nonanion gap metabolic acidosis (.i.e. 1 mmol of bicarbonate titrates 1 mmol of acid titrates which forms Na acid which is a unmeasured cation which increases the anion gap). If the value is not less than 23 or greater than 30 the no additional process is going on. d. If there is no anion gap but the bicarbonate is low, a urinary anion gap can help differentiate renal vs. gut as loss of bicarbonate. (Na + K)- Cl-correlates with ammonium excretion. As ammonium excretion increases the urinary gap narrows. So with diarrhea (gap is negative), ammonium excretion unimpaired and increases with acidosis. With RTA (gap is positive) the gap widens since they have lost the ability to absorb bicarb and form ammonium.

5.Assessment of Oxygenation

PaO2 of < 60 and Saturation < 90 is hypoxia. A-a gradient is important to evaluate. One can estimate the Alveolar O2 by multiplying 5 X the FiO2. Alveolar O2 = (760-47)FIO2 - PCO2/.8 or 5 X FIO2. If the predicted PaO2 is much different than the true PaO2 than further evaluation for causes should be done. The main lesson here is to understand that by looking at the ABG one can assess if there is an inappropriate FiO2 requirement. Normally the A-a is < 60 - 100. If it is larger the ability of the patient to oxygenate is compromised.

6.Venous Blood Gas

In patients with normal cardiac output, central venous pH is lower than arterial by an average of 0.03 units, with venous PCO2 being higher by about 6 mm Hg. With severe circulatory failure, the pH differs by 0.1 unit with pCO2 difference of 24. With a cardiopulmonary arrest, a venous blood gas could have a pH difference of 0.35 and a CO2 difference of 56 when compared to an arterial blood gas.

Category: Biochemistry Notes