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-)|
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.