A 40 year old 70 kg male has gram negative sepsis and has developed bilateral pulmonary infiltrates. The following are data from blood gas analysis.
|
Test |
Value |
Normal Range |
|
FiO2 |
0.5 |
|
|
pH* |
7.31 |
7.35 – 7.45 |
|
pCO2* |
31 mmHg (4 kPa) |
35 – 45 (4.6 – 5.9) |
|
pO2 |
110 mmHg (14.5 kPa) |
80 – 110 (10.5 – 14.5) |
|
Bicarbonate* |
15.1 mmol/L |
24 – 32 |
|
Standard Base Excess* |
-10.0 mmol/L |
-2.0 – +2.0 |
a) Could this blood gas be consistent with the definition of acute respiratory distress syndrome (ARDS)? Give your reasoning.
b) What dose of sodium bicarbonate (in mmol) would be required to reverse the metabolic acidosis? Show your calculation method.
a) Could this blood gas be consistent with the definition of acute respiratory distress syndrome (ARDS)? Give your reasoning.
No. The P/F ratio is 220. By definition, the problem would be acute lung injury rather that ARDS at this stage.
b) What dose of sodium bicarbonate (in mmol) would be required to reverse the metabolic acidosis? Show your calculation method.
Dose sodium bicarbonate = Wt (kg) x 0.3 x -SBE = 70 x 0.3 x 10 = 210 mmol
Unlike the majority of these questions about ABGs, this one does not require indepth interpretation of the acid-base disturbance.
The first question refers to the now-extinct definition of ARDS, which is distinct from the new definition of ARDS. The issues surrounding ARDS classification are discussed in full elsewhere, and I will not rant about this extensively, except to mention the following ranges of the PaO2/FiO2 ratio:
This patient has an P/F ratio which is 110/0.5 = 220; thus the patient falls into a "mild" ARDS category, or "Acute Lung Injury" according to the old definition. Since this paper was offered to the candidates in 2011, several definitions have come and gone, and at the time of writing (2am on a Saturday morning of December 2024) the most recent ARDS definition is Matthay et al, 2024.
Now, as for the bicarbonate:
There are actually several methods of estimating the amount of bicarbonate required to titrate somebody's body fluids back to normal pH. They are all equally inaccurate. Assumptions are made about the volume of distribution of bicarbonate, which is about 50% of body weight at normal pH and about 100% at a low pH. These things are discussed in greater detail in a distant chapter where I attempted to explain to myself the physiological response to an infusion of sodium bicarbonate.
For example, the following equations give an answer as bicarbonate dose in mmol.
Reversal of metabolic acidosis with bicarbonate, and the various equation used to calculate it, is discussed at great length in a 2008 article by Sabatini and Kurtzman.
The old definition of ARDS and ALI is described in this seminal paper:
Bernard G, Artigas A, Brigham K, Carlet J, Falke K, Hudson L, Lamy M, Legall J, Morris A, Spragg R (1994). "The American-European Consensus Conference on ARDS. Definitions, mechanisms, relevant outcomes, and clinical trial coordination". Am J Respir Crit Care Med 149 (3 Pt 1): 818–24
However, it is not available as free full text.
Matthay, Michael A., et al. "A new global definition of acute respiratory distress syndrome." American journal of respiratory and critical care medicine 209.1 (2024): 37-47.
Schwartz, William B., and Arnold S. Relman. "A Critique of the Parameters Used in the Evaluation of Acid-Base Disorders: Whole-Blood Buffer Base and Standard Bicarbonate Compared with Blood pH and Plasma Bicarbonate Concentration." New England Journal of Medicine 268.25 (1963): 1382-1388.
Sabatini, Sandra, and Neil A. Kurtzman. "Bicarbonate therapy in severe metabolic acidosis." Journal of the American Society of Nephrology 20.4 (2009): 692-695.
Garella, Serafino, Clare L. Dana, and Joseph A. Chazan. "Severity of metabolic acidosis as a determinant of bicarbonate requirements." New England Journal of Medicine 289.3 (1973): 121-126.
Fernandez, Pedro C., Raphael M. Cohen, and George M. Feldman. "The concept of bicarbonate distribution space: the crucial role of body buffers."Kidney international 36.5 (1989): 747.