Management of Elevated CO2 and BUN
This patient has respiratory acidosis with compensatory metabolic alkalosis (elevated CO2 of 43 mmHg) and prerenal azotemia (BUN 26 with creatinine 0.67, BUN/Cr ratio ~39), requiring assessment of volume status, respiratory function, and underlying causes rather than treatment for metabolic acidosis. 1
Understanding the Laboratory Values
Your patient's CO2 of 43 mmol/L on a basic metabolic panel reflects total serum CO2, which is predominantly bicarbonate (70-85%), not arterial PCO2. 1 This elevated level indicates either:
- Chronic respiratory acidosis with renal compensation - The kidneys retain bicarbonate to buffer chronic CO2 retention from conditions like COPD, chest wall deformities, or muscle weakness 1
- Metabolic alkalosis - Often from diuretic use causing chloride depletion and volume contraction 1
Critical distinction: A CO2 of 43 is NOT metabolic acidosis. Low bicarbonate (<22 mmol/L) indicates metabolic acidosis, while your patient has elevated bicarbonate. 1
Addressing the Elevated BUN
The BUN of 26 with creatinine 0.67 yields a BUN/creatinine ratio of approximately 39:1 (normal is 10-20:1), indicating prerenal azotemia rather than intrinsic kidney disease. 2
Causes to Evaluate:
- Volume depletion/dehydration - Check for dry mucous membranes, reduced skin turgor, orthostatic hypotension 2
- Heart failure - Assess for jugular venous distension, peripheral edema, pulmonary rales 2
- GI bleeding - Increased protein load from blood 2
- Increased protein catabolism - Catabolic states, high protein intake 2
Management of Prerenal Azotemia:
For volume depletion: Administer IV isotonic saline at 15-20 mL/kg/h for adults, monitoring BUN serially until normalization 2
For heart failure: Optimize heart failure management per guidelines, using diuretics cautiously if fluid overloaded while monitoring for worsening dehydration 2
Diagnostic Algorithm
Step 1: Obtain Arterial Blood Gas
Order an ABG to determine if this represents: 1
- Respiratory acidosis (elevated PaCO2 with compensatory high bicarbonate)
- Pure metabolic alkalosis (normal PaCO2 with elevated bicarbonate)
ABG is mandatory if: 1
- Patient has respiratory symptoms
- Known COPD, obesity hypoventilation syndrome, or neuromuscular disease
- Bicarbonate >35 mmol/L
Step 2: Assess Volume Status
- Check orthostatic vital signs 2
- Evaluate for signs of dehydration or fluid overload 2
- Review diuretic use (common cause of contraction alkalosis) 1
Step 3: Evaluate Respiratory Function
For patients with chronic respiratory conditions: Target oxygen saturation of 88-92% to avoid worsening hypercapnia 1
Management Based on Underlying Cause
If Diuretic-Induced Metabolic Alkalosis:
- Reduce or temporarily hold diuretics if bicarbonate rises significantly above 30 mmol/L and patient is volume depleted 1
- Replete chloride and volume with normal saline to restore volume and provide chloride 1
If Chronic Respiratory Acidosis:
- This elevated bicarbonate represents appropriate renal compensation, not a disorder requiring treatment 1
- Focus on optimizing underlying respiratory condition 1
- Monitor carefully during any interventions that affect ventilation 1
If Prerenal Azotemia from Volume Depletion:
- IV fluid resuscitation with isotonic saline 2
- Monitor BUN levels serially until normalization 2
- Follow fluid status continuously during rehydration 2
Common Pitfalls to Avoid
- Don't assume elevated BUN always indicates kidney dysfunction when creatinine is normal - this likely represents prerenal azotemia 2
- Don't treat elevated CO2 as metabolic acidosis - this is either respiratory acidosis with compensation or metabolic alkalosis, both requiring different management 1
- Don't aggressively correct compensated respiratory acidosis - the elevated bicarbonate is protective 1
- Avoid reducing protein intake in catabolic patients, as this worsens nitrogen balance without benefit 1