A normal gap acidosis dissected

The patient is a 38 year old man admitted with a bleeding ulcer. It is 3 days later, he is otherwise stable and has these labs. He has known polycystic kidney disease with an estimated GFR of 30. He weights approximately 70 kg.

So please address these questions:

1. What is the acid-base disorder?

This is a normal gap acidosis.  Note the decreased bicarbonate and anion gap of 9.  Is the respiratory compensation correct?  We use the Winter’s equation.  With a calculated bicarbonate (from the ABG) of 15, we would expect a pCO2 of approximately 30.5.  The actually pCO2 is close enough.

2. Can you likely confirm the etiology?

We checked the urine anion gap. Urine Na+ 85; Urine K+ 21; Urine Cl- 75.  This gives us a positive urine anion gap and that implies that little ammonium (NH4+) has gotten into the urine.

Thus, we know we have a renal cause.  The estimated GFR is ~30, thus I believe this patient had the acidosis of CKD.

3. Would you treat, and how?

We did decide to treat acutely to return the bicarbonate to approximately 22.  Since he weighed 70 kg, and is a relatively young man, we estimated that his total body water was around 40 liters.  Remember that the “bicarbonate space” is the total body water.  Given a deficit of 6 mEq per liter, we assumed that we needed to provide approximately 240 mEq of bicarbonate.

As a rule we try to correct halfway over the first day.  We added 2 amps of bicarbonate to a liter of D5/0.5 NS.  The next day his bicarbonate was 19.  We repeated on day 2.  His electrolyte panel after repletion:

4. Will he need long term treatment?

I believe that we should treat long term. I know of 3 reasons to treat persistent normal gap acidosis in CKD patients:

1. Decrease bone destruction
2. Improve overall nutrition – Some research data show a correlation between chronic acidosis and malnutrition.  This reason may be soft, but the treatment is very benign.
3. Delay dialysis – a recent study suggests that treating metabolic acidosis delays dialysis.  The article and the abstract:

Bicarbonate Supplementation Slows Progression of CKD and Improves Nutritional Status

Bicarbonate supplementation preserves renal function in experimental chronic kidney disease (CKD), but whether the same benefit occurs in humans is unknown. Here, we randomly assigned 134 adult patients with CKD (creatinine clearance [CrCl] 15 to 30 ml/min per 1.73 m²) and serum bicarbonate 16 to 20 mmol/L to either supplementation with oral sodium bicarbonate or standard care for 2 yr. The primary end points were rate of CrCl decline, the proportion of patients with rapid decline of CrCl (>3 ml/min per 1.73 m²/yr), and ESRD (CrCl <10 ml/min). Secondary end points were dietary protein intake, normalized protein nitrogen appearance, serum albumin, and mid-arm muscle circumference. Compared with the control group, decline in CrCl was slower with bicarbonate supplementation (5.93 versus 1.88 ml/min 1.73 m²; P < 0.0001). Patients supplemented with bicarbonate were significantly less likely to experience rapid progression (9 versus 45%; relative risk 0.15; 95% confidence interval 0.06 to 0.40; P < 0.0001). Similarly, fewer patients supplemented with bicarbonate developed ESRD (6.5 versus 33%; relative risk 0.13; 95% confidence interval 0.04 to 0.40; P < 0.001). Nutritional parameters improved significantly with bicarbonate supplementation, which was well tolerated. This study demonstrates that bicarbonate supplementation slows the rate of progression of renal failure to ESRD and improves nutritional status among patients with CKD.

I would treat this patient to maintain a bicarbonate of 22.  I would start either with 5 tablets of sodium bicarbonate each day.  Remember that a 650 mg bicarbonate tablet has 7.7 mEq of bicarbonate.  I usually start with approximately 0.5 mEq per kg.  This assumes a normal diet of 1 mEq per kg of acid that needs buffering and some remaining buffering from phosphate.

If the patient cannot tolerate sodium bicarbonate I use sodium citrate (Shohl’s solution or Bicitra) and would start with 15 cc twice a day.  Each cc converts to 1 mEq of bicarbonate.

Regardless of our starting point, we need to follow the patient closely and titrate our therapy to maintain the bicarbonate around 22.

I would appreciate comments, especially from those nephrologists who frequent this blog.  If any of my discussion remains obtuse, please call me out and I will try to explain better.