Metabolic Acidosis and Colonic Pathology of Renal Disease

Today, we discussed the case of a middle-aged man with history of CKD who presented with abdominal pain/diarrhea, altered mental status, AKI, and numerous metabolic derangements (AG metabolic acidosis + metabolic alkalosis + respiratory acidosis). His AKI and AMS improved with IV hydration.
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TOP PEARLS

  • See below for a step-wise approach to acid-base evaluation.
  • When a patient has an anion-gap metabolic acidosis, calculate the delta gap to assess for evidence of other concomitant metabolic abnormalities (such as metabolic alkalosis or non-gap metabolic acidosis).
  • ESRD patients are predisposed to unique colonic pathology, including uremic colitis (more below).

Acid-Base evaluation (Flashback to our pearls from earlier this year):

  1. What is the primary disorder? Primary acidosis if pH <7.4, alkalosis if >7.4
  2. Is it metabolic or respiratory?
    1. If acidosis, high CO2 implies respiratory, low bicarb implies metabolic à If elevated anion gap, a metabolic acidosis is present (but may not be primary)
    2. If alkalosis, low CO2 implies respiratory, high bicarb implies metabolic
  3. Is there a 2nd disorder too? Compensation formulas:
    a. Metabolic acidosis Expected pCO2 = 1.5 x bicarb + 8 +/- 2 (Winter’s formula)
    b. Metabolic alkalosis Expected pCO2 = 0.7 x bicarb + 20 +/- 5 c. There are also compensation formulas for acute and chronic respiratory acid-base disorders: http://www.anaesthesiamcq.com/AcidBaseBook/ab9_3.php If calculated value does not match expected value, there is a 2nd disorder.
  4. Is anion gap elevated (*corrected for albumin)? Calculate the “delta gap” or “delta ratio.”
    1. Normal anion gap = 12, Normal HCO3 = 24
    2. Delta gap = (AG – 12) – (24 – HCO3) If Delta gap >6, superimposed metabolic alkalosis If Delta gap <-6, superimposed non-gap metabolic acidosis
    3. Delta ratio = (AG – 12)/(24 – HCO3) If Delta ratio >1, superimposed metabolic alkalosis If Delta ratio <1, superimposed non-gap metabolic acidosis

Colonic Manifestations of ESRD or Dialysis Patients

  • Uremic colitis: Harry’s pearl from report. Long-standing, untreated uremia leading to colonic ulcerations and pseudomembranes.
  • Colonic ischemia: vascular disease is accelerated in ESRD, increasing risk for ischemic colitis.
  • Spontaneous colonic perforation: etiology is largely unknown in ESRD patients, but some identified causes include aluminum-containing antacids, barium studies, or dehydration.
  • Colonic necrosis: Sodium polystyrene sulfonate (used to treat hyperkalemia in patients with renal failure) can lead to colonic necrosis.
  • Fecal impaction: complication of usage of phos binders, analgesic, iron, etc.
  • Dialysis-related amyloidosis: causes dysmotility, intestinal necrosis, perforation.
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