When to intubate in GCS?

GCS ≤ 8 requires intubation for airway protection. The patient cannot protect their own airway at this level of consciousness. Motor score is the most prognostically important component.

What is the best initial fluid for sepsis resuscitation?

Lactated Ringer's (LR) is the preferred resuscitation fluid. The SMART trial (2018) showed balanced crystalloids reduced death, new renal failure, and persistent renal dysfunction compared to normal saline.

When to start vasopressors in septic shock?

Start norepinephrine (first-line vasopressor) if MAP remains < 65 mmHg after initial 30 mL/kg crystalloid resuscitation, or earlier if patient is profoundly hypotensive. Do not delay pressors to complete full fluid resuscitation.

Why can serum uric acid be normal during acute gout?

During acute inflammation, IL-6 increases renal urate excretion and redistributes urate from plasma to tissues. Up to 40% of acute gout flares have normal serum uric acid. Joint aspiration with polarized microscopy is needed for definitive diagnosis.

How do you distinguish lupus flare from infection in SLE?

Key clues: CRP is low/normal in flare but elevated in infection. Complement (C3/C4) drops in flare but is normal/elevated in infection. Procalcitonin is elevated in bacterial infection but normal in flare. Anti-dsDNA rises in flare, stable in infection.

What is the difference between DKA and HHS?

DKA features acidosis (pH 600), hyperosmolality (> 320), and minimal ketosis -typically in Type 2 DM. DKA requires insulin drip; HHS requires aggressive fluid resuscitation first.

When to give tPA in acute ischemic stroke?

IV alteplase (tPA) can be given within 4.5 hours of last known well time in eligible patients. CT head must rule out hemorrhage first. For large vessel occlusion, mechanical thrombectomy can be performed up to 24 hours with favorable imaging.

What antibiotics for community-acquired pneumonia?

Inpatient non-ICU: ceftriaxone + azithromycin. ICU: ceftriaxone + azithromycin (add vancomycin if MRSA risk). Outpatient healthy: amoxicillin. Outpatient with comorbidities: augmentin + azithromycin or respiratory fluoroquinolone. Duration: 5 days if clinically stable for 48 hours.

How to interpret iron studies?

Iron deficiency: low ferritin (< 30), high TIBC, low iron, low transferrin saturation (< 20%). Anemia of chronic disease: normal/high ferritin (acute phase reactant), low TIBC, low iron. A ferritin of 50-100 in an inflamed patient may still represent iron deficiency.

What is the correction rate for hyponatremia?

Correct sodium no more than 8 mEq/L in 24 hours to avoid osmotic demyelination syndrome (ODS). For symptomatic severe hyponatremia with seizures, give 3% hypertonic saline 100-150 mL bolus over 10-20 minutes. If overcorrected, use D5W + DDAVP rescue protocol.

When to start antibiotics in meningitis?

Within 30-60 minutes. Do NOT delay antibiotics for LP or CT. Draw blood cultures, start empiric antibiotics (vancomycin + ceftriaxone ± ampicillin if > 50 or immunocompromised), then perform LP when possible. Dexamethasone should be given before or with the first antibiotic dose.

DOACs vs warfarin in antiphospholipid syndrome?

DOACs are contraindicated in APS. The TRAPS trial (2018) showed rivaroxaban was inferior to warfarin with more thrombotic events in triple-positive APS patients. Warfarin with INR target 2-3 is the standard for APS, indefinitely.

How to manage scleroderma renal crisis?

ACE inhibitors (captopril) are life-saving -start immediately and titrate aggressively. Do NOT hold for rising creatinine. Steroids > 15 mg prednisone per day precipitate scleroderma renal crisis. ARBs are NOT a substitute -evidence is only for ACEi.

What are the 4 pillars of heart failure treatment?

Guideline-directed medical therapy (GDMT) for HFrEF: (1) ARNI or ACEi/ARB, (2) beta-blocker (carvedilol, metoprolol succinate, or bisoprolol), (3) MRA (spironolactone or eplerenone), (4) SGLT2 inhibitor (dapagliflozin or empagliflozin). All four should be initiated and titrated to target doses.

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NephrologyCore Skill

Acid-Base Disorders

A systematic 5-step approach to every ABG. pH → primary disorder → compensation → anion gap → delta-delta. Master this and you'll never be stumped by an ABG again.

🧪 5-Step ABG Interpretation

Step 1 -Look at the pH

pH	Primary Process
< 7.35	Acidemia (acidosis is the dominant process)
7.35–7.45	Normal (or mixed disorder with complete compensation)
> 7.45	Alkalemia (alkalosis is the dominant process)

Step 2 -Identify the Primary Disorder

If pH < 7.35 (acidemia)	If pH > 7.45 (alkalemia)
PCO₂ > 40 → respiratory acidosis (hypoventilation)	PCO₂ < 40 → respiratory alkalosis (hyperventilation)
HCO₃ < 22 → metabolic acidosis	HCO₃ > 26 → metabolic alkalosis

Step 3 -Is Compensation Appropriate?

Primary Disorder	Expected Compensation
Metabolic acidosis	Winter's formula: expected PCO₂ = 1.5 × [HCO₃] + 8 (± 2). If actual PCO₂ ≠ expected → additional respiratory disorder.
Metabolic alkalosis	Expected PCO₂ = 0.7 × [HCO₃] + 21 (± 2). Or: PCO₂ rises ~0.7 for each 1 mEq/L rise in HCO₃.
Acute respiratory acidosis	HCO₃ rises 1 per 10 mmHg ↑ PCO₂
Chronic respiratory acidosis	HCO₃ rises 3.5 per 10 mmHg ↑ PCO₂
Acute respiratory alkalosis	HCO₃ falls 2 per 10 mmHg ↓ PCO₂
Chronic respiratory alkalosis	HCO₃ falls 5 per 10 mmHg ↓ PCO₂

If compensation is more or less than expected → there's a second (mixed) disorder. Example: metabolic acidosis with PCO₂ lower than Winter's predicted → concurrent respiratory alkalosis (sepsis?).

Step 4 -Calculate the Anion Gap

AG = Na − (Cl + HCO₃). Normal: 12 ± 4 (or 8 ± 4 if albumin-corrected). Correct for albumin: for every 1 g/dL albumin below 4 → add 2.5 to the AG.

MUDPILES (AG Elevated > 12)

Cause	Mechanism
M ethanol	Alcohol dehydrogenase converts methanol to formic acid, which inhibits cytochrome oxidase and damages the retina (optic disc edema, blindness). Treat with fomepizole + dialysis.
U remia	Failing kidneys cannot excrete sulfates, phosphates, urate, and other organic anions. Develops at GFR < 20.
D KA (also alcoholic KA, starvation KA)	Insulin deficiency (or alcohol-induced NADH excess) drives lipolysis and hepatic ketogenesis. Accumulating β-hydroxybutyrate and acetoacetate are the unmeasured anions.
P ropylene glycol	Solvent in IV lorazepam, phenobarbital, and diazepam infusions. Metabolized by alcohol dehydrogenase to lactic acid. Classic iatrogenic ICU cause after high-dose benzo drips.
I soniazid / Iron	INH depletes pyridoxine (B6) and inhibits GABA synthesis → refractory seizures + lactic acidosis. Treat with high-dose pyridoxine 1:1 (g per g of INH ingested). Iron OD disrupts mitochondrial oxidative phosphorylation → lactate.
L actic acidosis #1 cause in hospital	Type A: tissue hypoperfusion or hypoxia (sepsis, shock, ischemia, severe anemia, CO poisoning). Type B: drugs (metformin, linezolid, propofol, NRTIs, β₂ agonists), liver failure, malignancy (Warburg effect), thiamine deficiency, D-lactic from short bowel. See full breakdown in Differentials tab.
E thylene glycol	Antifreeze. Metabolized to glycolic acid (AG acidosis) and oxalic acid (calcium oxalate crystalluria → AKI). Treat with fomepizole + dialysis. Bedside clues: oxalate crystals in urine, fluorescence under Wood's lamp (some products).
S alicylates	Uncouples mitochondrial oxidative phosphorylation → AG acidosis. Also directly stimulates the medullary respiratory center → early respiratory alkalosis. The classic mixed disorder (AGMA + resp alkalosis) is the giveaway. Tinnitus is the bedside tell.

HARDUPS (AG Normal / Hyperchloremic)

Cause	Mechanism
H yperalimentation (TPN)	Cationic amino acids (lysine, arginine, histidine) in TPN generate HCl as they are metabolized. Modern TPN includes acetate buffer to offset this; classic boards still tests it.
A ddison's / Acetazolamide	Addison's (adrenal insufficiency): low aldosterone → impaired distal H⁺ and K⁺ excretion (mimics Type 4 RTA, hyperK). Acetazolamide: blocks proximal carbonic anhydrase → bicarbonate wasted in urine.
R TA	Type 1 (distal): α-intercalated cells cannot secrete H⁺ → urine pH > 5.5, hypoK, kidney stones (Sjögren, SLE). Type 2 (proximal): impaired HCO₃^- reabsorption → Fanconi syndrome, hypoK. Type 4: hypoaldosteronism (DM, ACEi/ARB, TMP-SMX, heparin, spironolactone) → hyperK. Most common RTA.
D iarrhea #1 cause of NAGMA	Direct loss of bicarbonate-rich small bowel and colonic secretions. Urine AG is negative (kidneys appropriately excrete NH₄⁺), distinguishing it from RTA.
U reteral diversion (ileal conduit, ureterosigmoidostomy)	Bowel mucosa exchanges urinary Cl^- for HCO₃^-. The longer the urine sits in contact with bowel, the more HCO₃^- is lost.
P ost-hypocapnia / Pancreatic fistula	Post-hypocapnia: chronic respiratory alkalosis caused renal HCO₃^- wasting as compensation; when PaCO₂ abruptly normalizes (e.g., post-intubation), bicarb is now inappropriately low. Pancreatic fistula or biliary drain: direct loss of HCO₃-rich secretions.
S aline (NS resuscitation)	Large-volume 0.9% NS delivers a Cl^- load that displaces HCO₃^- extracellularly = hyperchloremic dilutional acidosis. Switch to LR or PlasmaLyte for ongoing resuscitation. SMART 2018

Step 5 -Delta-Delta (if AG elevated)

Delta-delta = ΔAG / ΔHCO₃ = (AG − 12) / (24 − HCO₃)

Ratio	Interpretation
< 1	AG metabolic acidosis + concurrent non-AG metabolic acidosis (the bicarb dropped more than expected from the AG alone → additional acid or bicarb loss)
1–2	Pure AG metabolic acidosis (the drop in bicarb matches the rise in AG)
> 2	AG metabolic acidosis + concurrent metabolic alkalosis (bicarb is higher than expected → something is raising it -vomiting, diuretics, bicarb administration)

📋 Differentials

Metabolic Alkalosis

CLEVER PD (Causes of Metabolic Alkalosis)

Cause	Mechanism
C ontraction (volume depletion) SALINE-RESP	Loss of Cl⁻-rich extracellular fluid (vomiting, diuretics, sweating) makes the proximal tubule reabsorb Na⁺ paired with HCO₃^- instead of Cl⁻. Volume contraction also activates RAAS → aldosterone drives distal H⁺ secretion. Together: rising and maintained HCO₃^-.
L icorice (glycyrrhizic acid) SALINE-RESIST	Inhibits renal 11β-HSD2, which normally inactivates cortisol to cortisone in the collecting duct. Unopposed cortisol activates the mineralocorticoid receptor → apparent mineralocorticoid excess (Na retention, K⁺ and H⁺ wasting). Mimics primary hyperaldosteronism but with low renin AND low aldosterone.
E ndocrine (Conn's, Cushing's, Bartter, Gitelman) SALINE-RESIST	Conn's: autonomous aldosterone → distal Na/K and Na/H exchange. Cushing's: excess cortisol overwhelms 11β-HSD2. Bartter: defective NKCC2 in TAL = "endogenous loop diuretic" (hypoK, hypoCl, alkalosis, normal BP). Gitelman: defective NCC in DCT = "endogenous thiazide" (adds hypoMg).
V omiting / NG suction SALINE-RESP Common	Direct loss of HCl from gastric secretions raises serum HCO₃^- (every H⁺ lost = one HCO₃^- generated). Concurrent volume and Cl⁻ depletion add a contraction component, perpetuating the alkalosis until both are repleted.
E xcess alkali SALINE-RESP	Direct HCO₃^- load: IV sodium bicarbonate, milk-alkali syndrome (CaCO₃ antacids + milk → hyperCa, AKI, alkalosis; full breakdown in Hypercalcemia topic), massive transfusion (citrate metabolized to bicarb by the liver), TPN with acetate buffer overload.
R efeeding SALINE-RESP	Carb load triggers insulin surge → cellular uptake of K⁺, phos, Mg, and intracellular H⁺ shift. The resulting hypokalemia and intracellular acidosis maintain extracellular alkalosis. Watch closely in malnourished patients restarting nutrition. Full breakdown → Refeeding Syndrome topic.
P ost-hypercapnia SALINE-RESP	Chronic respiratory acidosis (COPD) caused renal HCO₃^- retention as compensation. When PaCO₂ abruptly normalizes (post-intubation overventilation, NIV, mechanical hyperventilation), HCO₃^- is left inappropriately high → unmasked metabolic alkalosis. Avoid by lowering ventilator support gradually.
D iuretics (loop, thiazide) RESIST (active) RESP (off)	Block Na/Cl reabsorption proximal to the collecting duct → increased distal Na⁺ delivery + secondary hyperaldosteronism → distal Na/K and Na/H exchange. Plus volume contraction, hypoK, and hypoCl. While dosed: urine Cl > 20 (resistant). After stopping: urine Cl falls, becomes responsive. K⁺-sparing diuretics (spironolactone, eplerenone, amiloride) do NOT cause alkalosis (and can correct it).

Urine Cl⁻	Category	Causes	Treatment
< 20 mEq/L	Chloride-responsive (saline-responsive)	Vomiting/NG suction (#1), diuretics (after stopping), post-hypercapnia	NS (volume + chloride repletion). Correct the deficit.
> 20 mEq/L	Chloride-resistant	Hyperaldosteronism (Conn syndrome), Cushing's, Bartter/Gitelman, active diuretic use, severe hypokalemia	Treat underlying cause. K⁺ repletion. Spironolactone if hyperaldosteronism.

Why NS and K⁺ correct chloride-responsive metabolic alkalosis:

NS (volume + Cl⁻): Volume depletion (from vomiting, NG suction, diuretics) makes the proximal tubule avidly reabsorb Na⁺. With Cl⁻ also depleted, Na⁺ has to be reabsorbed paired with HCO₃⁻ instead of Cl⁻, which keeps serum HCO₃⁻ high. Volume contraction also activates aldosterone, which drives distal Na⁺/H⁺ exchange and generates more HCO₃⁻. Giving NS restores volume (turns off RAAS) and supplies Cl⁻ so the proximal tubule pairs Na⁺ with Cl⁻ again, letting the kidney finally dump excess HCO₃⁻ in the urine.
K⁺ repletion: Hypokalemia maintains the alkalosis through three mechanisms. (1) Transcellular shift: K⁺ leaves cells in exchange for H⁺ moving in, producing intracellular acidosis but extracellular alkalosis. (2) Distal tubule: when K⁺ is low, the principal cell swaps Na⁺ for H⁺ instead of K⁺, dumping more H⁺ in the urine. (3) Proximal tubule: hypokalemia stimulates ammoniagenesis (NH₄⁺ excretion), and each NH₄⁺ excreted generates a new HCO₃⁻. Replacing K⁺ shuts all three down. You cannot fully correct metabolic alkalosis without correcting K⁺.

Renal Tubular Acidosis (RTA)

Type	Defect	pH	K⁺	Classic Association
Type 1 (Distal)	Can't secrete H⁺ in distal tubule	Urine pH > 5.5 (can't acidify)	↓ (hypoK)	Sjögren, SLE, nephrocalcinosis, amphotericin B
Type 2 (Proximal)	Can't reabsorb HCO₃ in proximal tubule	Urine pH < 5.5 (once threshold exceeded)	↓ (hypoK)	Fanconi syndrome, multiple myeloma, carbonic anhydrase inhibitors (acetazolamide)
Type 4 (Hypoaldo)	↓ Aldosterone or tubular resistance	Urine pH < 5.5	↑ (hyperK)	Most common RTA. Diabetic nephropathy, ACEi/ARBs, spironolactone, TMP-SMX, heparin

Pearl: Always calculate the corrected AG (for albumin) and delta-delta ratio to identify mixed disorders.

Lactic Acidosis: Types A and B

Cohen-Woods classification. Two types only: A (hypoperfusion) and B (everything else). B is subdivided into B1, B2, B3. Every named cause (MALA, propofol infusion, D-lactic acidosis) sits inside one of these buckets.

Type	Mechanism	Causes
Type A Most common	Tissue hypoperfusion or hypoxia. Anaerobic glycolysis surges.	Shock (septic, cardiogenic, hypovolemic, obstructive), regional ischemia (mesenteric, limb, compartment syndrome), severe hypoxemia, severe anemia, CO poisoning, post-arrest, seizures, heavy exertion.
Type B1 Underlying disease	Disease impairs lactate clearance or shifts metabolism without overt hypoperfusion.	Liver failure (impaired clearance), malignancy (Warburg effect; classic in leukemia/lymphoma), sepsis (overlaps with A), thiamine deficiency, diabetes, pheochromocytoma, short bowel / SIBO → D-lactic acidosis.
Type B2 Drugs / toxins	Mitochondrial toxicity, β₂ stimulation, or impaired clearance.	Metformin (MALA, especially with AKI), linezolid (> 2 wks), propofol infusion syndrome, NRTIs (didanosine, stavudine, zidovudine), β₂ agonists (albuterol, epinephrine, terbutaline), salicylates, cyanide, methanol, ethylene glycol, acetaminophen (late), cocaine, alcohol.
Type B3 Inborn errors	Genetic mitochondrial or enzyme defects.	Mitochondrial disorders (MELAS), pyruvate dehydrogenase deficiency, G6PD-related defects.

D-lactic acidosis (bacterial overgrowth): Short bowel syndrome and SIBO let gut bacteria (Lactobacillus, Streptococcus bovis, Bifidobacterium) ferment unabsorbed carbohydrate into D-lactate, the stereoisomer humans cannot metabolize efficiently (we lack effective D-lactate dehydrogenase). Triggered by carb-rich meals. Presents as episodic encephalopathy (slurred speech, ataxia, confusion; looks intoxicated) with a wide anion gap. The trap: the standard lactate assay measures L-lactate only, so it returns normal despite the gap. You must specifically order a D-lactate level. Treatment: low-carb diet, oral non-absorbable antibiotics (metronidazole, neomycin, vancomycin, rifaximin) to suppress overgrowth, IV fluids, bicarb if severe. Sits inside Type B1 in Cohen-Woods (the "D" refers to the molecular isomer, not a separate Cohen-Woods type).

Bedside framing: Lactate > 4 with hypotension → Type A, resuscitate first. Lactate elevated but patient looks well-perfused → Type B (drugs, liver, malignancy, thiamine). Persistent lactate despite resuscitation → recheck for missed ischemia (mesenteric, limb), failed source control, then consider metformin, linezolid, thiamine deficiency, malignancy.

Toxic Alcohols (suspect when AG ↑ + osmolar gap > 10)

Osmolar gap = measured osm − (2×Na + glu/18 + BUN/2.8). Normal < 10. The trap: the gap shrinks as the parent alcohol is metabolized to its toxic acid, so a late presentation can show a wide AG with a deceptively normal osmolar gap. Order both early and trend.

Toxin	Source	Toxic metabolite	Bedside clue	Treatment
Ethylene glycol	Antifreeze, brake fluid	Glycolic acid (AG ↑) then oxalic acid (calcium oxalate → AKI)	Calcium oxalate crystalluria, hypocalcemia (Ca binds oxalate), AKI, Wood's lamp fluorescence (some products contain fluorescein)	Fomepizole 15 mg/kg IV load → 10 mg/kg q12h. Hemodialysis if pH < 7.25, AKI, end-organ damage, or level > 50 mg/dL.
Methanol	Windshield washer, moonshine, industrial solvents	Formic acid (AG ↑, inhibits cytochrome oxidase)	Visual changes (snowstorm vision, optic disc edema, blindness), abdominal pain, putaminal hemorrhage on imaging	Fomepizole + emergent dialysis (lower threshold than EG: pH < 7.30 or any visual changes). Folinic acid 50 mg IV q4h enhances formate clearance.
Isopropanol	Rubbing alcohol, hand sanitizer	Acetone (NOT an acid)	Osmolar gap WITHOUT AGMA, ketones positive without acidosis, fruity breath, CNS depression. Mimics DKA but glucose normal.	Supportive only. Fomepizole NOT indicated (no toxic acid metabolite). Dialysis only if hemodynamic instability or coma.
Propylene glycol	IV solvent in lorazepam, phenobarbital, diazepam infusions	Lactic acid (Type B lactic acidosis)	ICU patient on prolonged benzo drip with rising lactate without hypoperfusion. AKI possible.	Stop the offending infusion. Fomepizole if severe. Dialysis for refractory cases.

When to give fomepizole empirically: Suspected toxic alcohol ingestion + any of (osmolar gap > 10, pH < 7.30, AG > 16, visual changes, AKI). Don't wait for the level. Fomepizole blocks alcohol dehydrogenase, halting metabolism to the toxic acid. Loading dose 15 mg/kg IV over 30 min. Ethanol drip is the fallback when fomepizole is unavailable (target ethanol level 100–150 mg/dL).

Respiratory Acidosis (PaCO₂ > 40)

Hypoventilation fails to clear CO₂. Acute (HCO₃ rises 1 per 10 mmHg ↑ PaCO₂) vs chronic (HCO₃ rises 3.5 per 10) distinction matters: chronic is renally compensated, acute is dangerous and may need ventilation.

Mechanism	Causes	Bedside flag
CNS depression	Opioids, benzodiazepines, alcohol, barbiturates, brainstem stroke, encephalitis, ICH, post-ictal	Pinpoint pupils + slow RR → naloxone trial. AMS + bradypnea.
Lung / airway	Severe COPD/asthma exacerbation (eventual fatigue), end-stage ILD, severe pneumonia, ARDS, pulmonary edema, upper airway obstruction	"Normalizing" PaCO₂ in a previously tachypneic asthmatic = pre-arrest. Intubate before the next ABG.
Neuromuscular	GBS, myasthenic crisis, ALS, polymyositis, severe hypoK/hypoPhos/hypoMg, organophosphates, botulism, high cervical cord injury	NIF less negative than −20 cmH₂O, FVC < 15 mL/kg, paradoxical breathing → intubate before ABG declares failure.
Chest wall / restriction	Severe kyphoscoliosis, morbid obesity (OHS), large pleural effusion, flail chest, abdominal compartment syndrome, tense ascites	Restrictive PFT pattern, low ERV. OHS often missed on the ward.
Iatrogenic	Permissive hypercapnia (asthma, ARDS protective ventilation), under-set vent rate or tidal volume, sedation oversedation, equipment failure	Check vent settings before treating the patient. Always.

Acute vs chronic at the bedside: Pure acute respiratory acidosis means HCO₃ ≈ 24 + (PaCO₂ − 40)/10. If HCO₃ is much higher than predicted, the kidneys have had days to compensate (chronic). A COPD patient with pH 7.36, PaCO₂ 65, HCO₃ 36 is at chronic baseline; the same numbers in a previously healthy patient mean impending failure.

Respiratory Alkalosis (PaCO₂ < 40)

Hyperventilation. Almost always centrally driven. Most common acid-base disorder in hospitalized patients, and frequently part of a mixed picture (sepsis, salicylates, hepatic encephalopathy).

Trigger	Mechanism / Notes
Hypoxemia	Carotid body drives ventilation. PE, pneumonia, pulmonary edema, high altitude, severe anemia. Always check SpO₂ and consider CTPA in unexplained tachypnea.
Sepsis	Cytokine-mediated central drive. Often the earliest acid-base abnormality, before lactate or hypotension appear.
Pain / anxiety	Voluntary or involuntary hyperventilation. Pure psychogenic hyperventilation is a diagnosis of exclusion (rule out PE, sepsis, ASA first).
Salicylate toxicity	Direct medullary stimulation + uncoupling of oxidative phosphorylation. Classic mixed AGMA + respiratory alkalosis. Tinnitus is the giveaway.
Hepatic encephalopathy / cirrhosis	Ammonia and progesterone-like metabolites stimulate the medullary center. Almost universal in advanced cirrhosis.
Pregnancy	Progesterone is a respiratory stimulant. Normal pregnant PaCO₂ is 28–32 with compensatory low HCO₃ (~18–20).
CNS lesions	Brainstem stroke, tumor, infection, head injury → central neurogenic hyperventilation.
Iatrogenic	Over-ventilation on the vent (rate or tidal volume too high). Re-check settings before treating.

Compensation rule: Acute → HCO₃ drops 2 per 10 mmHg ↓ PaCO₂. Chronic → HCO₃ drops 5 per 10. So a chronic respiratory alkalosis can present with a markedly low HCO₃ (15–18) that mimics metabolic acidosis. Always check the pH first; alkalemia rules out a primary metabolic acidosis.

🧪 Workup

Workup

ABG/VBG -VBG acceptable for pH/HCO₃
BMP -AG calculation
Albumin -corrected AG = AG + 2.5×(4−albumin)
Lactate -#1 AG acidosis in hospital
Ketones/BHB
Osmolality (serum + calculated) -osmol gap >10 = toxic alcohol
Urine AG -positive=RTA, negative=GI loss
APAP + salicylate -always in unexplained AG acidosis

🚨 Management

Management

AG acidosis: DKA→insulin. Lactic→treat shock. Toxic alcohols→fomepizole+dialysis. Uremia→dialysis. Stewart Approach, Kellum 2009
Non-AG acidosis: Diarrhea→IVF. RTA→bicarb. NS-induced→switch to LR.
Resp acidosis: COPD→BiPAP. Opioid→naloxone. NMD→intubation.
Met alkalosis: Cl-responsive→NS+KCl. Cl-resistant→treat cause.

💊 Medications

Medications

Drug	Dose	Route	Notes
NaHCO₃	50-150 mEq	IV	Only pH < 6.9. Monitor K⁺. BICAR-ICU, Jaber 2018
Fomepizole	15 mg/kg load	IV	Toxic alcohols
Acetazolamide	250 mg q6-12h	IV/PO	Refractory met alkalosis
KCl	20-40 mEq q2-4h	IV	Must correct K⁺ to fix met alkalosis

📋 On Rounds

pH 7.28, PCO₂ 24, HCO₃ 11, Na 140, Cl 100, AG 29. Interpret this ABG.

Step 1: pH 7.28 = acidemia. Step 2: HCO₃ 11 (low) = metabolic acidosis (primary). Step 3: Winter's formula: expected PCO₂ = 1.5(11) + 8 = 24.5 ± 2. Actual PCO₂ = 24 → appropriate respiratory compensation. Step 4: AG = 140 − (100 + 11) = 29 (elevated) → AG metabolic acidosis. Think MUDPILES: DKA? Lactic acidosis? Toxic ingestion? Step 5: Delta-delta = (29−12)/(24−11) = 17/13 = ~1.3 → pure AG metabolic acidosis.

Why do you correct the anion gap for albumin?

Albumin is a negatively charged protein that contributes ~12 mEq/L to the unmeasured anions (the normal AG). In hypoalbuminemic patients (cirrhosis, nephrotic syndrome, malnutrition, critical illness), the baseline AG is lower because there are fewer albumin anions. A "normal" AG of 12 in a patient with albumin of 2 actually represents a hidden AG elevation of ~5. Correction: for every 1 g/dL albumin below 4, add 2.5 to the calculated AG.

Walk me through the 5-step approach to ABG interpretation.

Step 1: Look at pH -acidemia (< 7.35) or alkalemia (> 7.45)? Step 2: Identify the primary disorder -does the pCO₂ explain it (respiratory) or the HCO₃ (metabolic)? Step 3: Is compensation appropriate? For metabolic acidosis: Winter's formula (expected pCO₂ = 1.5 × HCO₃ + 8 ± 2). If actual pCO₂ is HIGHER than expected → concurrent respiratory acidosis. If LOWER → concurrent respiratory alkalosis.

How do you calculate the corrected anion gap for albumin?

The anion gap is falsely low in hypoalbuminemia (albumin is the main unmeasured anion). Corrected AG = Calculated AG + 2.5 × (4.0 − measured albumin). For each 1 g/dL drop in albumin below 4, add 2.5 to the AG. Example: Na 140, Cl 108, HCO₃ 20, albumin 2.0 → raw AG = 140-108-20 = 12 (looks normal). Corrected AG = 12 + 2.5 × (4.0-2.0) = 17 (elevated!). Without correction, you'd miss a significant anion gap acidosis.

How many types of lactic acidosis are there?

Two (Cohen-Woods): Type A = tissue hypoperfusion or hypoxia (shock, ischemia, severe hypoxemia, anemia, CO poisoning) and Type B = everything else. Type B has 3 subtypes: B1 underlying disease (liver failure, malignancy, sepsis, thiamine deficiency, short bowel/SIBO), B2 drugs/toxins (metformin, linezolid, propofol, NRTIs, β₂ agonists), B3 inborn errors (mitochondrial, PDH deficiency).

Short bowel patient with episodic confusion and slurred speech after meals, wide anion gap, but lactate is normal. Diagnosis?

D-lactic acidosis from bacterial overgrowth. Gut bacteria (Lactobacillus, Strep bovis, Bifidobacterium) ferment unabsorbed carbs into D-lactate, which humans cannot clear (we only metabolize L-lactate efficiently). The standard lactate assay measures L-lactate only, so it reads normal despite the gap. Order D-lactate level to confirm. Treat: low-carb diet, oral non-absorbable antibiotics (metronidazole, neomycin, rifaximin). Cohen-Woods: Type B1.

Lactate is 8 but the patient is well-perfused, BP normal, no shock. What's on your differential?

Type B lactic acidosis. Think drugs (metformin/MALA, linezolid > 2 wks, propofol infusion syndrome, NRTIs, β₂ agonists like albuterol or epinephrine), liver failure (impaired clearance), malignancy (Warburg effect, classic in leukemia/lymphoma), thiamine deficiency (especially alcoholics, refeeding), D-lactic acidosis in short bowel. Sepsis can also cause Type B physiology even before overt hypotension.

Clinical Examples

📋 Case 1, Triple Acid-Base Disorder in Sepsis

Patient: 58M with septic shock from pneumonia. ABG: pH 7.22, pCO₂ 30, HCO₃ 12. Na 142, Cl 98, albumin 2.0. Lactate 8.4. Also receiving NS resuscitation for 24h.

Key findings: Step 1: acidemia. Step 2: low HCO₃ = metabolic acidosis. Step 3: Winter's: expected pCO₂ = 1.5(12)+8 = 26 ± 2. Actual 30 → pCO₂ HIGHER than expected → concurrent respiratory acidosis (tiring out?). Step 4: corrected AG = (142-98-12) + 2.5(4-2) = 32+5 = 37 → AG metabolic acidosis. Step 5: delta-delta = (37-12)/(24-12) = 25/12 = 2.1 → hidden metabolic alkalosis (from vomiting? contraction?).

Management:

Triple disorder: AG metabolic acidosis (lactic from sepsis) + respiratory acidosis (respiratory failure) + metabolic alkalosis (contraction from volume depletion)
Address the lactate: aggressive IVF (switch to LR, NS worsens non-AG acidosis), source control, antibiotics
The rising pCO₂ suggests respiratory fatigue → may need intubation soon (normally sepsis causes hyperventilation/low pCO₂)
Correct the albumin, always use corrected AG in ICU patients (hypoalbuminemia is nearly universal)
Recheck ABG in 2h, if pCO₂ continues to rise, intubate before arrest

Teaching point: A "normal" pCO₂ in a septic patient is alarming, they should be hyperventilating. A pCO₂ that is higher than Winter's predicted suggests respiratory muscle fatigue and impending respiratory arrest. This is a pre-intubation sign.

📋 Case 2, Non-Anion Gap Metabolic Acidosis (NAGMA)

Patient: 45F with chronic diarrhea from Crohn's disease. pH 7.30, pCO₂ 28, HCO₃ 14. Na 138, Cl 112, AG 12 (normal). Urine: Na 30, K 20, Cl 60.

Key findings: Normal AG + metabolic acidosis = NAGMA (non-anion gap metabolic acidosis). Hyperchloremic (Cl elevated). Use urine anion gap (UAG) to distinguish GI vs renal cause: UAG = Na+K-Cl = 30+20-60 = -10 (NEGATIVE).

Management:

Negative UAG = kidneys ARE appropriately excreting acid (NH4⁺) → GI bicarbonate loss (diarrhea is the cause)
If UAG were positive: kidneys CANNOT excrete acid → renal tubular acidosis (RTA)
Treat the diarrhea: optimize Crohn's therapy (5-ASA, immunomodulators, biologics per GI)
Oral sodium bicarbonate 650 mg TID for symptomatic acidosis (bridge while treating diarrhea)
Replace potassium (diarrheal losses), hypokalemia is common with GI bicarbonate loss

Teaching point: The urine anion gap is the key to NAGMA workup. Negative UAG = GI loss (diarrhea, kidneys working fine). Positive UAG = renal problem (RTA, kidneys can't excrete acid). Remember: "Negative = Normal kidneys, Positive = Problem in kidney."

📋 Case 3, Mixed Metabolic Alkalosis + Respiratory Alkalosis

Patient: 72F with COPD on chronic steroids, admitted for COPD exacerbation with NG tube on suction × 3 days. ABG: pH 7.58, pCO₂ 32, HCO₃ 38. K⁺ 2.8, Cl 88.

Key findings: Severely alkalemic (pH 7.58, dangerous). Two alkalosis disorders: metabolic alkalosis (HCO₃ 38, from NG suction losing HCl + volume contraction) AND respiratory alkalosis (pCO₂ 32, should be 48-52 for this HCO₃ level as compensation, but is much lower → concurrent respiratory alkalosis from anxiety/pain).

Management:

Replace volume: NS is chloride-rich (restores renal ability to excrete bicarb, "chloride-responsive" metabolic alkalosis)
Replace potassium aggressively: K⁺ 2.8 is dangerous + hypokalemia maintains metabolic alkalosis (kidneys retain H⁺ to excrete K⁺)
Review NG suction necessity, can it be discontinued or placed to gravity?
Address respiratory alkalosis: treat pain/anxiety (may be driving hyperventilation), optimize bronchodilators
Danger: pH > 7.55 causes seizures, arrhythmias, and shifts the oxygen dissociation curve left (impaired O₂ delivery)

Teaching point: Metabolic alkalosis is maintained by three things: volume depletion (kidneys reabsorb Na with HCO₃), hypokalemia (kidneys excrete H⁺ instead of K⁺), and chloride depletion. Fixing all three (NS + KCl) corrects "chloride-responsive" alkalosis. Urine Cl < 20 = chloride-responsive.

📣 Sample Presentation

One-Liner

"Mr. Young is a 32-year-old with type 1 DM presenting with ABG: pH 7.18, pCO₂ 22, HCO₃ 8. Na 140, Cl 102, AG 30. Consistent with anion gap metabolic acidosis with appropriate respiratory compensation."

Key Points to Cover on Rounds

5-step ABG: (1) Acidemia (pH 7.18), (2) Primary metabolic acidosis (low HCO₃ 8), (3) Respiratory compensation: expected pCO₂ by Winter's = 1.5(8)+8±2 = 18-22 → actual 22 (appropriate), (4) AG 30 (elevated, corrected for albumin). (5) Delta-delta: ΔAG=18, ΔHCO₃=16 → ratio 1.1 (pure AG acidosis). Etiology: DKA (glucose 420, ketones positive). No concurrent non-AG acidosis or metabolic alkalosis. Plan: treat DKA -insulin drip, fluids, electrolyte monitoring.

Monitoring

ABG q2-4h during correction
BMP q4-6h -K⁺ shifts with pH
AG trend
Lactate q2-4h if lactic acidosis
UOP

⚡ Summary

Summary

5-Step ABG

1) pH (acid vs alkalemia) → 2) Primary disorder (respiratory vs metabolic) → 3) Compensation adequate? → 4) AG (corrected for albumin) → 5) Delta-delta.

Winter's Formula

Expected pCO₂ = 1.5 × HCO₃ + 8 ± 2. If actual pCO₂ higher → concurrent respiratory acidosis. Lower → respiratory alkalosis.

AG Correction

Corrected AG = AG + 2.5 × (4.0 − albumin). For each 1 g/dL albumin below 4, add 2.5. Hypoalbuminemia hides AG acidosis.

Delta-Delta

ΔAG / ΔHCO₃: Ratio > 2 = hidden metabolic alkalosis. Ratio < 1 = hidden non-AG metabolic acidosis.

MUDPILES

M ethanol · U remia · D KA · P ropylene glycol · I NH/Iron · L actic acidosis · E thylene glycol · S alicylates

Non-AG Causes

HARDUP: Hyperalimentation, Acetazolamide, RTA, Diarrhea, Ureteral diversion, Pancreatic fistula. Also: NS resuscitation.

📄 One Pager

Nephrology / Critical Care · One Pager

ABG Interpretation

5 steps: pH → primary disorder → compensation → AG (corrected for albumin) → delta-delta. MUDPILES for AG acidosis. Winter's formula.

🧪 5-Step Approach

(1) pH (acid vs alkalemia). (2) Primary disorder (pCO₂ vs HCO₃). (3) Compensation appropriate? (Winter's formula for met acidosis). (4) AG corrected for albumin. (5) Delta-delta (ΔAG/ΔHCO₃).

🚨 AG Acidosis -MUDPILES

M ethanol · U remia · D KA · P ropylene glycol · I NH/Iron · L actic acidosis · E thylene glycol · S alicylates Check lactate, ketones, osmol gap, tox screen.

💊 Key Formulas

Winter's: expected pCO₂ = 1.5 × HCO₃ + 8 ± 2. AG = Na − Cl − HCO₃. Corrected AG = AG + 2.5 × (4 − albumin). Delta-delta: > 2 = hidden met alk, < 1 = hidden non-AG met acidosis.

💊 Key Drugs

Bicarb drip150 mEq in D5W (if pH < 6.9 in DKA)

Fomepizole15 mg/kg load (toxic alcohols)

Insulin drip0.1 u/kg/hr (DKA)

DialysisRefractory acidosis or toxic ingestion

⚠️ Pitfalls

Not correcting AG for albumin (misses hidden AG acidosis in ICU patients)
Treating number without finding cause
Bicarb for DKA (except pH < 6.9)
Missing mixed acid-base disorder