Acid-Base Disorders 2 (Metabolic Acidosis)

Question 1

Remarks on renal response to pulmonary acid-base disturbances

Answer 1

The renal response to pulmonary acid-base disturbances begins within 30 minutes of onset, but requires hours to days to achieve equilibrium

Question 2

delta gap

Answer 2

Refers to the relative change in the anion gap (AG)
May be more important than the actual AG value

Question 3

Virtually all AG values above _____ can be considered abnormal

Answer 3

15 mEq/L, even when there are no previous comparison values available
The common laboratory threshold is 12 mEq/L
Elevations of the AG are most commonly associated with METABOLIC ACIDOSIS

Question 4

Delta ratio

Answer 4

∆AG / ∆HCO3
<0.4 –> Pure NAGMA
0.4 -0.8 –> NAGMA + HAGMA
0.8 - 2.0 –> Pure HAGMA
>2.0 –> HAGMA + met alk (or preexisting compensated resp acid)

normal AG is 12. normal HCO3 is 24

Question 5

Albumin and metabolic acidosis

Answer 5

Hypoalbuminemia may mask the presence of a high AG metabolic acidosis (HAGMA).
For every drop in the albumin level by 1 g/dL, the normal AG range should be lowered by approximately 2.5

Question 6

What 2 parameters of blood gas results are the ones that are actually measured (and not just calculated)?

Answer 6

pH and PCO2
HCO3 is calculated via the Henderson-Hasselbalch equation

Question 7

Respiratory compensation in metabolic acidosis

Answer 7

1. With normal respiratory compensation, PCO2 decreases by 1 mm Hg for every net decrease in HCO3
2. In a maximally-compensated metabolic acidosis (which takes about 12-24 hours), Winter’s formula applies:
   Expected PaCO2 = [(1.5 x HCO3) +8] +/- 2

Question 8

Limits to the respiratory compensation for metabolic acidosis

Answer 8

1. Minute ventilation actually declines when pH decreases below 7.10
2. The development of metabolic acidosis that drives the pH below 7.10 is likely associated with a very high risk of inadequate ventilation response
3. Administration of HCO3 in the presence of hypoventilation may exacerbate respiratory acidosis, because the HCO3 is converted to CO2 and water

Question 9

The lowest PCO2 achievable is approximately

Answer 9

12 mm Hg
- This lower limit is obtainable PCO2 is due to resistance in airflow and increased CO2 generated by the exertion required for rapid ventilation, both offsetting the ventilator exhalation of CO2
- the superimposition of respiratory acidosis on a patient in such a condition will result in a RAPID decline of pH to levels at which organ function drops and pharmacotherapy will fail
- noninvasive or mechanical ventilation usually should be instituted in such situations to sure the ventilatory rate and volume are sufficient to prevent an increase in PCO2 at this critical time

Question 10

The differential diagnoses for HAGMA in emergency practice fall into 4 broad categories, namely

Answer 10

Renal failure
Ketoacidosis
Lactic acidosis
Ingestions

Question 11

The major ketone present in the serum of a patient with untreated diabetic or alcoholic ketoacidosis may be

Answer 11

B-hydroxybutyrate

Question 12

Remarks on lactic acidosis

Answer 12

Lactic acidosis is not a diagnosis, but a syndrome with its own differential diagnosis.

Question 13

Remarks on ethanol

Answer 13

Ethanol should never be considered the cause of an significant metabolic acidosis.
Look for other causes

Question 14

Triple acid-base disturbance

Answer 14

1. HAGMA
2. Metabolic alkalosis
3. Respiratory alkalosis
   Seen in sepsis (lactic acidosis) and salycylate poisoning

Question 15

Treatment of metabolic acidosis

Answer 15

1. The treatment of metabolic acidosis reflects the treatment of the underlying disorder and the restoration of normal tissue perfusion and oxygenation 🔸
2. The most important step 🔸 is to determine whether there is a respiratory component to the acidosis (i.e., a primary respiratory acidosis), because the treatment approach differs.
3. If there is inadequate respiratory compensation, the appropriate treatment will be to first correct the respiratory problem 🔸

Question 16

Bicarbonate therapy may be appropriate for limited indications

Answer 16

1. Severe hypobicarbonatemia (HCO3 <4 mEq/L)
   - insufficient buffer concentrations may lead to extreme increases in acidemia with small increases in acidosis
2. Severe acidemia (pH <7.00 to 7.15)
   - in cases of HAGMA with signs of shock or myocardial irritability that has not responsded to supportive measures includeing adequate ventilation and fluid resuscitation
3. Severe hyperchloremic acidemia
   - lost bicarbonate must be generated by kidneys and liver, which may require days

Question 17

When given, HCO3- can be dosed how?

Answer 17

1. 0.5 mEq/kg for each mEq/L rise in HCO3 desired
2. the goal is to restore adequate buffer capacity (HCO3 >8 mEq/L) or to achieve clinical improvement in shock or dysrhythmias
3. Bicarbonate should be given as slowly as the clinical situation permits
4. 75 mL of 8.4% sodium bicarbonate in 500 mL of D5W produces a nearly isotonic solution for infusion
5. Adequate time should be allowed for the desired effect to be achieved, with monitoring of acid-base balance