What do you do if your septic patient now has a lactate of 12 and a pH of 7.00? You’ve already started your fluids, antibiotics, pressors, and have ruled out any unaddressed source. Intuitively, your patient’s acidosis should be corrected by providing a base – specifically sodium bicarbonate. This is a common approach, although as we will see it is not always the best approach.
To start, let’s review the Bicarb/CO2 buffer system in the blood:
CO2 + H2O ⇆ H2CO3 ⇆ HCO3– + H+
As bicarb (HCO3–) increases, the equation favors the reverse direction thereby forming more carbon dioxide (CO2). This means that in order to expect an increase in serum pH, patients must be able to adequately expire the produced CO2. If they aren’t, the fear is that CO2 will move intracellularly and the equation will then shift in the forward direction thereby making the intracellular environment more acidotic. Although serum pH may improve, the intracellular pH which is unable to be measured may be worsened as has been shown in prior in vitro studies.
Additionally, if one is so acidotic that you need to give bicarb, they often have already lost the ability to change their minute ventilation adequately to compensate for their metabolic acidosis. As such, bicarbonate is best served on ventilated patients for whom you can control their minute ventilation without worrying if they are tiring out. If one gives bicarb to a patient who is already in respiratory failure, there is added potential for badness due to the mechanism illustrated above. To go with this concept, giving a rapid bolus of bicarbonate has the added risk of rapidly producing a large amount of carbon dioxide which would be much more difficult to expire than if the bicarbonate were given over a slower infusion.
So what does (or may) work?
There are several “proton scavengers” that have been formulated for the purpose of improving severe metabolic acidosis including Carbicarb (Na2CO3/Na3CO3) and THAM. The idea behind these agents is to allow for alkalinization of the blood while avoiding the production of substances which can diffuse intracellularly and again dissociate.
As such, these medications remain ionized after they have obtained a hydrogen ion thereby allowing themselves to be excreted in the urine. Unfortunately, large randomized controlled trials have not been done examining the efficacy of these medications despite showing promise in smaller studies.
Furthermore, new innovative techniques targeting specific hydrogen ion channels (such as the sodium/hydrogen exchange) are being developed to combat this issue.
Additionally, an underutilized modality in the management of severe acidosis is renal replacement therapy (remember the “A” in AEIOU). The logistics of performing this method of improving acidosis can be prohibitively difficult, however, as patients who are this ill often cannot wait for access to be placed and for therapy to start. If there is a patient who already has access, however, this may be an option in your otherwise hemodynamically stable septic patient, as these modalities can often worsen hemodynamic status. Continuous veno-venous hemofiltration is particularly effective as one can rapidly change the ion composition of the plasma based on the characteristics of the replacement fluid used.
In the end, good sepsis management including early identification and management of infectious sources remain the best treatments for severe lactic acidosis in the setting of septic shock.
Kimmoun A., Novy E., Auchet T., Ducrocq N., Levy B. Hemodynamic consequences of severe lactic acidosis in shock states: from bench to bedside. Critical Care. 2015;19, article 175 doi: 10.1186/s13054-015-0896-7.
Weingardt S. Podcast 96 – Acid Base in the Critically Ill – Part V – Enough with the Bicarb Already. EMCrit. April 14, 2013. Available at: http://emcrit.org/podcasts/enough-with-the-bicarb-already. Accessed October 16, 2015.
Sooriyakumaran P. Your Patient in Extremis: THAM to the Rescue? Academic Life in Emergency Medicine. http://www.aliem.com/patient-extremis-tham-rescue/. October 5, 2015. Accessed October 16, 2015.