A 68-year-old woman is brought into the Emergency Department because of severe difficulty breathing. She complains that for the past few days she has had a progressive fever and productive cough. She was diagnosed a few weeks ago with idiopathic focal segmental glomerulosclerosis (FSGS), and her other medical problems include hypertension and gastroesophageal reflux disease (GERD). Her vitals show a temperature of 38.3°C, blood pressure of 104/74 mmHg, heart rate of 98 beats per minute, respiratory rate of 22 breaths per minute, and oxygen saturation of 93% on room air. Her laboratory values are shown below.
An arterial blood gas shows a pH of 7.29 and a PaCO2 of 32 mmHg.
Which of the following is the likely cause of this patient’s acid/base status?a. Renal tubular acidosis
Lactic acidosis. This patient has sepsis (meets 3/4 systemic inflammatory response syndrome criteria with the likely infectious source being pneumonia) and has an anion gap metabolic acidosis (explained later). The recent diagnosis of nephrotic syndrome put her at an increased risk of infection because of immunoglobulin loss in the urine. In addition, patients with nephrotic syndrome are also at risk for thrombosis, protein malnutrition, and hypovolemia. Generally speaking, acid/base problems on the shelf examination will consist of a primary disturbance with a compensatory process (Table below); there will likely be no mixed acid/base problems. (Note: a major exception to this is salicylate overdose, in which there will initially be a primary anion gap metabolic acidosis with a primary respiratory alkalosis.) The first step in any acid–base problem is determining if there is an anion gap using the equation: Na+ − (Cl− + HCO3 − ). If so, then there is at least one type of metabolic acidosis (one can determine if there is another metabolic acidosis by checking the delta–delta, but this is probably beyond the scope of the examination). Next, check the pH, bicarbonate, and PaCO2 to determine the primary acid/base disorder. This will open up a list of potential differential diagnoses, only one of which will fit the findings in the vignette. An important part of this question is recognizing the anion gap metabolic acidosis. The initial calculated anion gap is 10 (normal range 6 to 12); however, this needs to be corrected for the low albumin. For every drop in albumin by 1, the expected anion gap drops by 2.5. The corrected anion gap is therefore 15, so there is an anion gap metabolic acidosis. Because this patient is septic, she likely has lactic acidosis. (She likely has a mixed acid/base picture with a primary respiratory alkalosis as a result of hypoxemia from pneumonia, but this is not important for the question.) (A, D) Both renal tubular acidosis and excessive IV saline administration will cause a nonanion gap metabolic acidosis. (B) Hyperaldosteronism will cause a metabolic alkalosis.