Compare conditions

Put any two — or three — conditions side by side, adult or pediatric, to spot the look-alike differences fast, row by row.

Acid-base

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In one line

·For every blood gas, ask: which way is pH moving, is the main problem CO₂ or bicarbonate, is the body fixing it the right amount, does anion gap show hidden acids, then fix what broke—not the number itself.

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Normal physiology

·Three teams keep pH between 7.35 and 7.45: buffers (chemicals that grab or release H⁺ instantly), lungs (control CO₂ by breathing faster or slower), and kidneys (control bicarbonate over days by dumping or saving it). Buffers react in seconds, lungs adjust in minutes to hours, kidneys fine-tune over days. Keep that picture in your head, because every abnormal finding is a change from this normal teamwork.

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What goes wrong

·Usually one broken thing upstream explains all the strange numbers together.

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Hallmark signs

·Kussmaul breathing (deep, fast, sighing breaths)
·Confusion, drowsiness, or coma
·Nausea and vomiting
·Muscle weakness or paralysis
·Muscle spasms, tingling in fingers and lips, or seizures (tetany)
·Heart racing, fluttering, or dangerous rhythms
·Belly pain
·Fruity or sweet breath smell

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Red flags · escalate now

·pH below 7.20 or above 7.60 — risk of deadly heart rhythms, seizures, or coma
·Potassium above 6.5 mEq/L with peaked T waves or wide QRS on ECG — heart can stop beating any second
·Anion gap above 30 with confusion — think poisoning (methanol, antifreeze) or septic shock with severe lactic acidosis (acid from oxygen-starved tissues)
·Bicarbonate below 10 mEq/L in diabetic ketoacidosis (the acid crisis of missing insulin) — brain can swell if you correct pH too fast; do NOT give bicarbonate unless pH is below 6.9 (ADA 2024)
·Kussmaul breathing with fruity breath and high blood sugar — diabetic ketoacidosis (the acid crisis of missing insulin) until proven otherwise; check blood or urine ketones and start insulin right away

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Workup

·Arterial blood gas (ABG—blood drawn from an artery in the wrist) or venous blood gas (VBG—blood drawn from a vein)
·Basic metabolic panel (BMP—blood test for sodium, potassium, chloride, bicarbonate, blood urea nitrogen, creatinine, and glucose); calculate anion gap: sodium minus (chloride plus bicarbonate), normal range 8 to 12 mEq/L
·Serum lactate (lactic acid level in the blood)
·Serum or urine ketones (beta-hydroxybutyrate is most accurate; urine dipstick detects acetoacetate)
·Delta-delta calculation: (measured anion gap minus 12) divided by (24 minus measured bicarbonate)
·Serum osmolal gap: measured serum osmolality minus calculated osmolality [2×sodium + glucose/18 + BUN/2.8]; normal gap is less than 10 mOsm/kg
·Serum salicylate level (aspirin level); toxic level is above 30 mg/dL, severe toxicity above 100 mg/dL
·Arterial or venous blood gas with co-oximetry (measures carboxyhemoglobin and methemoglobin)

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Treatment

·Identify the primary disorder: Look at pH first. If pH is low (below 7.35), the primary problem is too much acid in the blood (acidosis). If pH is high (above 7.45), it is too little acid in the blood (alkalosis). Then look at PaCO₂ and bicarbonate—whichever one moved in the same direction as pH (both low or both high) is the primary problem. Calculate anion gap if metabolic acidosis (acid building up in the blood) is present.
·Obtain detailed history and perform targeted physical exam: Ask about vomiting, diarrhea, shortness of breath, chest pain, diabetes, medicines (diuretics, aspirin, metformin), kidney or lung disease, alcohol or drug use, possible toxic ingestion. Examine breathing pattern, mental status, skin blood flow (perfusion), heart rhythm, and abdomen.
·Treat the underlying cause: IV regular insulin 0.1 units/kg/hour plus IV 0.9% saline 500–1000 mL/hour initially for diabetic ketoacidosis (the acid crisis of missing insulin) per ADA; IV fluids (crystalloid 30 mL/kg in first 3 hours) plus IV antibiotics within 1 hour plus vasopressors (norepinephrine first-line) to restore blood flow (perfusion) and clear lactate in septic shock per Surviving Sepsis Campaign; oxygen plus non-invasive ventilation (BiPAP) or intubation and mechanical ventilation for COPD with respiratory acidosis (acid building up from trapped carbon dioxide) per ATS/GOLD; stop metformin and give IV fluids for metformin-associated lactic acidosis (acid from oxygen-starved tissues); fomepizole 15 mg/kg IV load plus hemodialysis for methanol or ethylene glycol; IV sodium bicarbonate to alkalinize urine plus hemodialysis for severe aspirin toxicity; hemodialysis for kidney failure (uremic too much acid in the blood (acidosis)) per KDIGO.
·Give IV sodium bicarbonate (50–100 mEq in 1 liter of D5W over 30–60 minutes) ONLY if pH is below 7.10 AND the cause cannot be corrected quickly, OR in specific situations: tricyclic antidepressant overdose with wide QRS (above 100 ms) or arrhythmia (to overcome sodium-channel blockade), severe high potassium (hyperkalemia) (potassium above 6.5 mEq/L) with ECG changes per AHA, aspirin or methotrexate poisoning (to alkalinize urine and trap drug for faster excretion), or rhabdomyolysis with myoglobinuria (to prevent kidney damage).
·Monitor and manage potassium closely: Measure potassium every 2–4 hours during treatment. If potassium is above 5.5 mEq/L with ECG changes (peaked T waves, wide QRS), give IV calcium gluconate 1–2 grams over 5 minutes to protect the heart, then insulin 10 units IV plus dextrose 25 grams IV to shift potassium into cells per AHA. If potassium is normal or low at presentation, start potassium replacement (20–40 mEq/L in IV fluids) before or with insulin in diabetic ketoacidosis (the acid crisis of missing insulin) per ADA.
·Provide ventilatory support as needed: Supplemental oxygen by nasal cannula or non-rebreather mask to maintain SpO₂ above 90%; non-invasive positive-pressure ventilation (BiPAP) for COPD flare-up (exacerbation) with respiratory acidosis (acid building up from trapped carbon dioxide) if alert and cooperative; endotracheal intubation and mechanical ventilation if mental status is declining (GCS below 8), respiratory muscles are exhausted (paradoxical breathing, accessory muscle use), or PaCO₂ is rising above 60 mmHg with worsening acidemia per ATS guidelines. Adjust minute ventilation (respiratory rate × tidal volume) to target PaCO₂.
·Consider renal replacement therapy (hemodialysis or continuous veno-venous hemofiltration): Indications include pH below 7.10 despite maximal therapy, severe kidney failure with too much acid in the blood (acidosis) (creatinine above 4 mg/dL, GFR below 15 mL/min), severe electrolyte disturbances (potassium above 6.5 mEq/L refractory to medical therapy), toxic alcohol ingestion (methanol, ethylene glycol), or salicylate toxicity with level above 100 mg/dL per KDIGO and toxicology guidelines.

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NCLEX trap

·The low CO2 is actually helping — it means the lungs are working hard to fix the problem. When blood becomes too acidic (pH drops), the lungs speed up and blow off CO2, like opening a window to let steam escape. This pushes the pH back up toward normal. Do not give bicarbonate just to chase a number on paper. Treat the root cause: give insulin and fluids for DKA, or restore blood flow for lactic acidosis (acid from oxygen-starved tissues) (when cells make too much lactic acid because they are starving for oxygen). Once you fix the upstream problem, pH will fix itself. Sodium bicarbonate is saved for life-threatening acidemia — pH below 6.9 to 7.0 in DKA (per ADA and Endocrine Society) — and for specific poisonings like aspirin, methanol, antifreeze (ethylene glycol), and tricyclic antidepressants, or very high potassium with severe too much acid in the blood (acidosis).
·Always do the math first — calculate the anion gap and the delta-delta ratio. High bicarbonate plus high pH can be too little acid in the blood (alkalosis), but it can also hide a second, dangerous too much acid in the blood (acidosis) underneath (a mixed disorder). Normal saline works only if the too little acid in the blood is from vomiting or diuretics that caused volume loss and chloride loss (called saline-responsive). If the too little acid in the blood is from too much aldosterone (a hormone that holds onto sodium and dumps acid), giving saline overloads the patient with fluid and does not fix the too little acid in the blood (called saline-unresponsive). Check the urine chloride: low urine chloride (below 20 mEq/L) means saline will help; high urine chloride (above 20) means you need to treat the hormone problem or stop the diuretic, not give more saline.
·Low pH plus low CO2 is metabolic acidosis (acid building up in the blood) with the lungs helping (respiratory compensation). The lungs are already working overtime, breathing fast to blow off CO2 and raise the pH. If you put the patient on a ventilator and set a normal CO2 target, you take away the body's own fix and make the pH crash worse. Find and treat the real problem: insulin for DKA, fluids and vasopressors (blood pressure medicines like norepinephrine) to restore blood flow in shock causing lactic acidosis (acid from oxygen-starved tissues), dialysis for kidney failure toxins (uremia), or antidote for poisons. Only intubate (put a breathing tube in) if the patient cannot protect their airway, is too tired to keep breathing, or is about to stop breathing.
·Lactic acidosis (acid from oxygen-starved tissues) in shock is a red flag that cells are not getting enough oxygen and are making energy the emergency backup way (without oxygen), which produces lactic acid. Bicarbonate does not fix shock or get more oxygen to the cells. Treat the shock: give IV fluids to restore volume, vasopressors like norepinephrine or epinephrine to raise blood pressure, antibiotics for sepsis (infection in the blood), stop bleeding, and support the heart. Lactate will clear on its own once blood flow and oxygen delivery are restored. Sodium bicarbonate in lactic acidosis can actually make things worse — it creates more CO2 inside the cells, overloads the patient with fluid and sodium, makes it harder for oxygen to leave the red blood cells and feed the tissues (shifts the oxygen-hemoglobin dissociation curve left), and can cause rebound too little acid in the blood (alkalosis). Current guidelines (Surviving Sepsis Campaign, SCCM) do not recommend routine bicarbonate for lactic acidosis unless pH is very low (below 7.15), and even then only as a temporary bridge while you treat the cause.
·Always calculate the anion gap first. A low bicarbonate with a high anion gap (above 12 by most lab standards) is high-anion-gap metabolic acidosis (acid building up in the blood) — a true emergency. Missing poisons like methanol, antifreeze (ethylene glycol), severe lactic acidosis (acid from oxygen-starved tissues), aspirin poisoning, or ketoacidosis can kill the patient. These need urgent antidotes, dialysis, or specific treatments. Normal-anion-gap too much acid in the blood (acidosis) (anion gap 8 to 12) comes from losing bicarbonate in diarrhea or the kidneys not holding onto bicarbonate properly (renal tubular too much acid in the blood, or RTA). The anion gap tells you which door to walk through: high anion gap means emergency acid load (toxin, ketones, lactate, severe waste buildup in the blood (uremia)) — find and treat the source immediately. Normal anion gap means bicarbonate loss — treat the diarrhea or RTA. Do not skip this step.
·Check the pH first. If pH is low with low CO2, this is metabolic acidosis (acid building up in the blood) and the hyperventilation is the body's repair mechanism — the lungs are blowing off CO2 to raise the pH back up. Slowing the breathing takes away the compensation and makes the acidemia worse. Find the upstream acid source: diabetic ketoacidosis (the acid crisis of missing insulin), lactic acidosis (acid from oxygen-starved tissues) from sepsis or shock, diarrhea, kidney failure (uremia building up), or toxin ingestion (MUDPILES mnemonic: Methanol, Severe waste buildup in the blood (Uremia), DKA, Propylene glycol or Paraldehyde, Iron or Isoniazid, Lactic acidosis, Ethylene glycol, Salicylates). Once you treat the underlying cause and the acid load is gone, the breathing rate will slow on its own as pH normalizes. If pH is high with low CO2, then it is primary respiratory alkalosis (blowing off too much carbon dioxide) (from hyperventilation due to pain, anxiety, low oxygen (hypoxia) (low blood oxygen), pregnancy, liver failure, or brain lesion), and you address the cause of the hyperventilation — but never assume this without checking the pH and anion gap.

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