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Acidosis

1.  Acidosis versus Acidemia
2.  Respiratory Acidosis
3.  Metabolic Acidosis
4.  Treatment of Acidosis

1.  Acidosis versus Acidemia

Acidosis refers to a physiological process which, if unopposed, would lead to a drop in blood pH.  Acidemia is defined as an arterial blood pH less than 7.35 (or venous pH less than 7.3).  If a person develops a respiratory acidosis, it will take some time for the kidney to compensate by making more bicarbonate.  Until this happens, the blood pH will be lower than normal.  However, once the kidney swings into action, it can compensate almost fully.  On the other hand, if a person has a metabolic acidosis, the normal response is tachypnea/hyperpnea.  The body tries to maximize ventilation in an effort to remove CO2 from the blood.  The compensation occurs rapidly, but not fully; the pH corrects in the right direction, but not back to normal.

Possible clinical manifestations of acidosis include hyperpnea (deep, pauseless breathing), decreased myocardial contractility, arrythmia, arteriolar dilatation, hypotension, and pulmonary edema.

2.  Respiratory Acidosis

Respiratory acidosis occurs when the alveolar exchange of CO2 is impeded.  Possible causes include obstructive or restrictive respiratory conditions, an acute airway obstruction, bracing of the thoracic cage due to injury or pain, limitation of respiratory excursion due to extreme obesity, or conditions affecting respiratory muscles or the nerves and pathways that drive them.

Clinical manifestations of hypercapnea include somnolence, hypertension, and retinal edema.

3.  Metabolic Acidosis

Metabolic Acidosis results from three possible causes:  acid equivalents are being added to blood either as metabolic by-products or from some exogenous source, or base equivalents are being depleted.  The metabolic acidoses are divided into two camps based on the serum anion gap.  The anion gap is the defined as:
 

Anion Gap = Na+ - (Cl- + HCO3-)
Normally, the anion gap is 10-12 mEq/L.  The anion gap relies on two assumptions.  First, that the blood is electroneutral (i.e., there must be as many negatively charged ions as positively charged), and  that the major cation in the blood is sodium.  Any change in the anion gap must reflect changes in chloride, bicarbonate, or other unspecified anions.  In acidoses with a normal anion gap, the body has lost bicarbonate either via the kidneys or the GI tract.  To maintain electroneutrality, there is a compensatory increase in chloride concentration, so "normal anion gap" metabolic acidosis is also referred to as "hyperchloremic" acidosis.  The other metabolic acidosis is "increased anion gap" metabolic acidosis.  For the gap to increase, there must be an increase in unmeasured anions such as lactic acid produced by anaerobic metabolism or salicylic acid from an aspirin overdose.

4.  Treatment of Acidosis

While the underlying cause of acidosis should be corrected, if plasma bicarbonate is less than 5 mmol/L, immediate correction with bicarbonate is indicated.  The bicarbonate should be administered in hypotonic solution and given as a continuous infusion over an hour.  In neonates, a 4.2% solution is used, while in pediatric patients and adults, an 8.4% solution is standard.  The amount of infuse can be calculated according to the following formula:
 
 

HCO3- mEq = kg * (15 - observed HCO3-) * 0.5

Before administration of bicarbonate, check the serum potassium as addition of HCO3- will result in an intracellular potassium shift, further lowering serum potassium with potential neuromuscular and cardiac consequences.  Remember that administration of sodium bicarbonate represents a signficant sodium load, so account for that in terms of other fluids which are administered.  Also consider that administration of bicarbonate will potentionally worsen the situation for a patient who is retaining CO2.  Since the bicarbonate will be converted to CO2, in the long run it can worsen a respiratory acidosis.  A better agent in this case would be THAM.
 
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Last modification: April 30, 1998