the indications for sodium bicarbonate (NaHCO3) administration?
Before 1986, NaHCO3 was routinely used
during CPR, even without knowledge of the patient’s acid–base status. Acidosis
inhibits myocardial contractility and also inhibits the effects of
catecholamines. However, this inhibitory effect on catecholamines does not
appear clinically signifi-cant at the range of pH commonly encountered and the
catecholamine doses administered during resuscitation. The myocardial
depressant effect of metabolic acidosis is delayed compared with that produced
by the intracellular acidosis that follows the administration of NaHCO3.
As is apparent from the equilibrium equation,
[HCO3−] + [H+] ⇔ [H2CO3] ⇔ [CO2] + [H2O]
every 50 mEq of bicarbonate administered
produces large amounts of CO2 gas. CO2 gas freely
diffuses across cellular membranes, and causes a paradoxical worsening of the
intra-cellular acidosis. Intracellular CO2 tensions of greater than
300 mmHg and pH values less than 6.1 have been recorded.
Carbicarb, a buffering agent that does not
produce as much CO2, has also been tried without significant improvements
in outcome following CPR. Another probable explanation for the ineffectiveness
of these buffering agents is that they also cause hypernatremia and
hyperosmolality. Hyperosmolar solutions may decrease aortic pressures, and
compromise survival. Initially, the leftward shift in the oxy-hemoglobin
saturation curve following the administration of NaHCO3 may
theoretically decrease oxygen availability.
Thus, NaHCO3 should only be given
when the results of arterial blood gas analysis indicate a significant
metabolic acidosis in the presence of severe acidemia (e.g., with an arterial
pH <7.20). It currently holds a Class III indication in hypercarbic acidosis
and thus may be harmful during CPR. NaHCO3 is indicated in known
hyperkalemia (Class I), bicarbonate-responsive acidosis (Class IIa), tricyclic
anti-depressant overdose (Class IIa), to alkalinize urine in aspirin or other
drug overdose (Class IIa), and for intubated and ventilated patients with a
long arrest time or return of circulation after prolonged CPR (Class IIb). When
NaHCO3 administration is planned, the correct full dose is
calculated as follows:
Patient’s weight (kg) × base deficit × 0.3
Many clinicians use half of the calculated dose
initially. If blood gas results are unobtainable, an empiric dose of 1 mEq/kg
can be administered in prolonged arrest situations.