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.

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College Answer

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

Discussion

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:

  • 300-200 = "mild" ARDS, ~ 27% mortality
  • 200-100 = "moderate" ARDS ~32% mortality
  • under 100 = "Severe" ARDS, ~ 45% mortality

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. The new ARDS definition has replaced the old as of 2012, about 1 year after the candidates were exposed to this question paper.

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.

  • body weight in kg × 1
  • body weight × 0.3 × SBE
  • body weight × 0.3 × (desired HCO3- - measured HCO3-)
  • body weight × ( 0.4 + 2.6 / measured HCO3-)

References

References

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.

Though the JAMA site is still paywalled, The Sociedad Uruguaya de Neonatologia Y Pediatria Intensiva have mirrored the Berlin Definition statement article for your viewing pleasure:

CV. Marco Ranieri, MD (2012). "The ARDS Definition Task Force*. Acute Respiratory Distress Syndrome: The Berlin Definition". JAMA 307 (23): 2526-2533.

The new definition is discussed here; it was revised by the ARDS Definition Task Force: "Acute Respiratory Distress Syndrome." Jama307.23 (2012): 2526-2533.

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.