Tuesday July 1, 2008
Introduction: In patients with acute respiratory distress syndrome (ARDS), permissive hypercapnia is a strategy to decrease airway pressures to prevent ventilator-induced lung damage by lowering tidal volumes and tolerating higher arterial carbon dioxide tension. A pure respiratory acidosis generally does not require alkali therapy. Alkali therapy is indicated for either a metabolic acidosis or a mixed acidosis. The choice of buffer is based on the type of acidosis, cardiorespiratory status, and lung mechanics.
Problem with NaHCO3: Slow infusions of NaHCO3 can be used to treat non-anion gap metabolic acidosis and some forms of increased anion gap acidosis. But using NaHCO3 to treat type A (hypoxia-related) lactic acidosis can be hazardous, particularly under conditions of hypoxemia, inadequate circulation, and limited alveolar ventilation.
THAM: Under above circumstances, THAM is the preferable buffer because it does not increase PaCO2 and is excreted by the kidneys. Tromethamine (THAM) is a sodium-free alkalinizing agent that acts as a hydrogen ion (proton) acceptor. It is a weak base that combines with hydrogen ions from carbonic acid to form bicarbonate and cationic buffer. Administration of tromethamine decreases hydrogen ion concentration, which results in a decrease in carbon dioxide concentrations and an increase in bicarbonate concentrations. The administration of Tham also increases urine output through osmotic diuresis. Excretion of electrolytes and CO2 is also increased. Urine pH is raised along with the excretion of electrolytes.
Dose in ml's of 0.3M THAM = (1.1) (Wt. in Kg) (normal HCO3 – Pt’s HCO3)
Dose in ml’s of 0.3M THAM = body wt in kg X base deficit in MEq/L x 1.1
Total dose should be administered over a period not less than 1 hour via central line. .3M THAM solution is available as premix and is contra-indicated in renal failure, anuria and hyperkalemia. It may cause transient hypoglycemia and respiratory depression.