My patient is post on pump cabg, post intraop CPR status,on IABP Passing borderline urine, improved by 20mg stat dose of lasix Was having increasing trends of lactate,then settled down Patient is intubated Now the patient start showing metabolic alkalosis What could be the causes and how to treat
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metabolic alkalosis post cardiac surgery CABG
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I now have all the information needed for a thorough, clinically focused answer. Let me synthesize this for the clinical scenario.
Metabolic Alkalosis in a Post-CABG / Post-CPR Patient on IABP
This patient has a near-perfect storm for metabolic alkalosis. Here is a systematic breakdown:
Why This Patient is at High Risk
Before listing causes, understand the two key questions:
- What generated the alkalosis? (generation phase)
- What is preventing the kidneys from excreting the excess HCO3-? (maintenance phase)
In this patient, multiple simultaneous generators AND maintainers are almost certainly operating at once.
Causes - Specific to This Patient
1. Post-Lactic Acidosis "Rebound Alkalosis" (most likely primary driver)
This patient had a rising lactate trend that "settled down" - which means lactate and organic anions were being metabolized. As Harrison's explains: "When an underlying stimulus for the generation of lactic acid is corrected, such as correction of shock by volume restoration, the lactate is metabolized to yield an equivalent amount of HCO3-. If there have been exogenous sources of HCO3-, this additional HCO3- will be additive... together may create a surfeit of HCO3- ('rebound alkalosis')."
This is extremely common post-resuscitation. The metabolic acidosis phase consumed buffers and drove compensatory renal HCO3 retention - once the lactate clears, the retained HCO3 remains.
2. Furosemide (Loop Diuretic) - Contraction Alkalosis
The 20 mg furosemide bolus given to the patient is a direct cause. Loop diuretics:
- Acutely reduce ECF volume without altering total body bicarbonate content
- The serum HCO3- rises as the reduced ECF "contracts" around the same HCO3- pool - this is contraction alkalosis
- Chronically, they also stimulate distal salt delivery, increasing H+ and K+ secretion
- The alkalosis is maintained by secondary hyperaldosteronism and K+ depletion (Harrison's, Current Surgical Therapy 14e, Miller's Anesthesia)
3. Post-CPR Sodium Bicarbonate Administration
If the patient received NaHCO3 during the intraoperative CPR (which is standard ACLS practice), any excess bicarbonate - especially as the acidosis resolves with ROSC and improving perfusion - becomes a direct HCO3 load. As noted in Murray & Nadel's: "As the underlying disorders improve, both ketone bodies and lactate may be metabolized to HCO3, resulting in the development of posttherapeutic alkalosis."
4. Hypokalemia
Post-cardiac surgery patients almost universally develop hypokalemia from:
- Cardiopulmonary bypass-induced dilution
- Stress response / catecholamine surge (K+ shift intracellularly)
- Diuretic use
Hypokalemia directly generates metabolic alkalosis by:
- Upregulating H+,K+-ATPases in the collecting duct, increasing H+ secretion
- Stimulating renal ammoniagenesis
- The alkalosis from severe K+ depletion is resistant to saline - only K+ replacement corrects it (Harrison's)
5. Post-Hypercapnia Alkalosis (Ventilated Patient)
The patient is intubated. If there was any period of hypoventilation or CO2 retention (common peri-operatively, under sedation), the kidneys compensated by retaining HCO3. If the ventilator settings are now correcting PaCO2 too aggressively or the CO2 retention phase resolved, the HCO3 that was renally generated remains - this is post-hypercapnic alkalosis. Miller's Anesthesia: "Prolonged respiratory failure, with associated hypercarbia, leads to additional metabolic alkalosis due to chloride loss in urine."
6. Hypoalbuminemia
Post-bypass patients have dilutional hypoalbuminemia. Miller's Anesthesia specifically flags this: "The most common single disturbance in acid-base chemistry in critically ill patients is hypoalbuminemia... associated with metabolic alkalosis." Albumin is a weak acid; lower levels raise the pH. This may be masking a co-existing acidosis and inflating the apparent alkalosis.
Practical note: Correct the anion gap for albumin. For every 1 g/dL fall in albumin below 4 g/dL, the anion gap falls ~2.5 mEq/L, potentially hiding an underlying high-AG acidosis.
7. Nasogastric Tube Suctioning (if in place)
If an NGT is on suction - a common ICU practice in this patient population - loss of HCl from gastric secretions is a direct H+ loss, generating HCO3- accumulation. This is the classic hypochloremic metabolic alkalosis.
8. Contraction from Diuresis / Poor Oral Intake
The patient is passing borderline urine, is intubated, and has received a diuretic. Insensible losses, fever if present, evaporation from ventilator circuits - all cause free water and Na/Cl loss, contracting the ECF around a fixed HCO3 pool.
Classification (Clinically Useful)
| Category | Cause in This Patient | Urine Cl- |
|---|---|---|
| Chloride-responsive | Contraction alkalosis (furosemide), NG suction, post-lactic rebound | < 20 mEq/L |
| Chloride-resistant | Hypokalemia (severe), hyperaldosteronism (stress/IABP) | > 40 mEq/L |
| Mixed | Post-hypercapnia + hypoalbuminemia | Variable |
Urine Cl- is the key bedside discriminator. In this patient it is likely low (< 20), pointing to chloride-responsive causes as dominant.
Consequences to Monitor For
- Arrhythmias - alkalosis increases myocardial irritability; ventricular arrhythmias especially with concurrent hypokalemia. This patient on IABP has zero tolerance for arrhythmia.
- Impaired O2 delivery - alkalosis left-shifts the oxyhemoglobin dissociation curve, reducing O2 offloading to tissues. Dangerous in a post-CPR patient with residual low-output state.
- Coronary vasospasm - alkalosis causes peripheral and coronary vasoconstriction, worsening ischemia in fresh grafts.
- Decreased respiratory drive - alkalosis suppresses the carotid body, potentially causing apnoeas post-extubation if weaning is attempted.
- Ionized hypocalcemia - alkalosis increases Ca2+ binding to albumin, reducing ionized Ca2+ - dangerous for a post-cardiac surgery myocardium.
- Hypokalemia worsening - alkalosis drives K+ intracellularly, exacerbating any existing deficit.
Treatment
Step 1: Treat the Underlying Causes
A. Volume and Chloride Replacement (for chloride-responsive alkalosis)
- If the patient is not volume-overloaded, give isotonic NaCl (0.9% saline) - provides Cl- to allow the kidneys to excrete excess HCO3-
- If fluid-restricted (cardiac failure, IABP-dependent), this must be done carefully and judiciously
- Current Surgical Therapy: "Fluid administration to edematous patients with alkalosis is usually inappropriate" - if there is pulmonary edema, saline is contraindicated
B. Potassium Replacement (mandatory)
- Give KCl - potassium AND chloride simultaneously address two drivers
- Target K+ > 4.0 mEq/L in post-cardiac surgery patients (higher than routine ICU targets)
- Severe hypokalemic alkalosis does NOT respond to saline alone - K+ is essential
- Consider also checking and correcting magnesium (hypomagnesemia sustains hypokalemia)
C. Stop/Reduce Furosemide
- If the patient's urine output is now adequate, hold further furosemide
- If diuresis is still needed for fluid overload, switch to acetazolamide (carbonic anhydrase inhibitor)
D. Acetazolamide - ideal in this scenario
- Blocks proximal tubular HCO3- reabsorption, causing bicarbonaturia
- Dose: 250-500 mg IV
- Particularly useful in post-cardiac surgery patients on ventilators who need diuresis but have alkalosis - it corrects alkalosis while maintaining diuresis
- Murray & Nadel's: "The carbonic anhydrase inhibitor, acetazolamide, may be helpful in patients with post-hypercapnic alkalosis"
- Watch for: K+ wasting (supplement), and CO2 retention if severe lung disease (less relevant here)
E. Ventilator Adjustment
- If alkalosis is partly from hypoventilation/CO2 retention (post-hypercapnic), do NOT abruptly normalize PaCO2 on the ventilator - this will worsen the alkalosis
- Allow a gradual decrease in PCO2 to allow renal compensation to dissipate
- Target pH 7.40-7.45, not rushing to normalize
F. Check and Stop Hidden HCO3- Sources
- Review IV infusions for sodium bicarbonate or citrate-containing products (FFP, packed RBCs - citrate metabolizes to HCO3-)
- Blood transfusions given during cardiac surgery are a significant hidden alkali load
Step 2: If Refractory and Severe (pH > 7.55)
- HCl infusion (0.1-0.2 N HCl via central line, confirmed tip position in SVC to avoid tissue necrosis) - 100-200 mEq/L concentration, infused slowly via central access. This is a last resort but highly effective.
- KCl IV concentrated replacement (via central line with ECG monitoring)
- Dialysis - if the patient progresses to renal replacement therapy anyway (borderline urine output), CRRT citrate/bicarbonate-free protocol can help correct alkalosis, though Miller's Anesthesia notes this is limited
Practical Priority Order for This Patient
- Check ABG + electrolytes (K+, Cl-, Mg2+, Ca2+, albumin, lactate) - confirm degree and mixed nature
- Check urine Cl- - distinguishes responsive vs. resistant
- Replace K+ and Mg2+ aggressively
- Assess fluid status - echo/CVP/IABP waveform: if no pulmonary edema, give NaCl; if congested, hold
- Give acetazolamide 250-500 mg IV if diuresis still needed or pH >7.50
- Ventilator check - ensure no iatrogenic hyperventilation; normalize PCO2 gradually
- Audit all infusions for hidden bicarbonate sources
- Reassess in 4-6 hours with repeat ABG
Sources: Harrison's Principles of Internal Medicine 22E - Table 38-6 | Murray & Nadel's Textbook of Respiratory Medicine | Current Surgical Therapy 14e | Miller's Anesthesia 10e
Will metolazone helps in this case.
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No - Metolazone Will Worsen the Metabolic Alkalosis
Short answer: Metolazone is contraindicated for treating the alkalosis in this patient and will actively make it worse.
Why Metolazone Worsens Metabolic Alkalosis
Metolazone is a thiazide-like diuretic - it inhibits the Na+/Cl- cotransporter (NCC) in the distal convoluted tubule. This gives it the same acid-base consequences as all thiazides and loop diuretics:
| Mechanism | Effect on Acid-Base |
|---|---|
| Reduces ECF volume without reducing total body HCO3- | Contraction alkalosis - HCO3- concentrates |
| Increases distal Na+ delivery → more K+ and H+ secreted | Generates new HCO3- |
| Causes chloride wasting | Hypochloremic alkalosis - removes the anion needed to excrete HCO3- |
| Causes hypokalemia | Sustains alkalosis - K+ depletion independently drives H+ secretion |
(Katzung's Basic & Clinical Pharmacology 16e: thiazide class toxicity explicitly listed as "hypokalemic metabolic alkalosis")
Metolazone is actually the most potent of the thiazide agents at generating these effects when combined with a loop diuretic - it is specifically "popular for use with loop agents for synergistic effects." That synergy applies to both diuresis AND to alkalosis generation.
When Metolazone IS Used in Post-CABG Patients
Metolazone is commonly used in cardiac ICUs to break diuretic resistance in fluid-overloaded patients who are not responding to furosemide alone. In that context it is appropriate - but the trade-off is:
- Massive electrolyte losses - K+, Mg2+, Cl-
- Worsening of any pre-existing metabolic alkalosis
- Textbooks explicitly note: "Close monitoring of electrolytes is essential with such dual diuretic therapy because hypokalemia and metabolic alkalosis are common side effects" (Campbell Walsh Wein Urology / NKF Primer)
What This Means for Your Patient
Your patient already HAS metabolic alkalosis from:
- Post-lactic rebound
- Furosemide already given
- Probable hypokalemia
Adding metolazone at this point would:
- Further contract ECF → worsen alkalosis
- Dump more Cl- → remove the means of renal HCO3- excretion
- Cause severe hypokalemia → independently sustain the alkalosis
- Risk dangerous arrhythmias in a post-CPR heart on IABP
What to Use Instead
| Goal | Agent | Rationale |
|---|---|---|
| Need more diuresis AND want to fix alkalosis | Acetazolamide 250-500 mg IV | Blocks HCO3- reabsorption → bicarbonaturia; maintains diuresis without worsening alkalosis |
| Correct K+ and alkalosis simultaneously | IV KCl replacement | KCl provides Cl- to enable HCO3- excretion AND corrects K+ deficit |
| Volume-tolerant and chloride-responsive | Normal saline (judiciously) | Replenishes Cl- pool |
| Hyperaldosteronism maintaining alkalosis | Spironolactone | Blocks aldosterone-driven H+ secretion; also K+-sparing |
Bottom line: In this specific patient, metolazone is the wrong drug. If you still need increased diuresis for fluid overload despite the alkalosis, the correct combination is furosemide + acetazolamide, not furosemide + metolazone.
(Katzung's Basic & Clinical Pharmacology 16e | Murray & Nadel's Respiratory Medicine | NKF Primer on Kidney Diseases)
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