Anesthesiology APPLIED Exam › Critical care, resuscitation and trauma

Arterial blood gas and acid-base interpretation

Critical care, resuscitation and trauma·~5 min read· Physician-reviewed✓ Sourced · 4

Stem

A 67-year-old woman, 58 kg, is having an emergency laparotomy for small-bowel obstruction after three days of vomiting with large nasogastric losses. After induction, on FiO2 0.5, her arterial gas shows pH 7.30, PaCO2 32 mmHg, HCO3 16 mEq/L and PaO2 210 mmHg. Sodium is 138, chloride 98, albumin 2.0 g/dL and lactate 4 mmol/L. Blood pressure is 92/50.

Question

Walk me through how you interpret this gas, and what will you do about it?

Ideal answer

HeadlineA gap metabolic acidosis, most likely lactic from hypoperfusion or ischemic bowel, with a contraction alkalosis hiding how sick she is. I treat the cause, not the number.

Oxygenation, then pH. A PaO2 of 210 on 50% is acceptable; pH 7.30 is acidemia, and low bicarbonate with low CO2 makes it primarily metabolic.
Compensation: Winter's predicts 30 to 34. Her 32 fits: no hidden respiratory problem.¹
Gap: 138 minus 114 is 24. With albumin 2, I add about 5: near 29.¹ It rose far more than bicarbonate fell: a metabolic alkalosis from vomiting coexists.
Restore perfusion: balanced crystalloid rather than saline, norepinephrine if fluid-unresponsive, blood if the hemoglobin supports it, to bring the lactate down.² No bicarbonate here.
Bowel viability from the surgeon, the likely lactate source; ventilation matched to her compensation, repeat gas in 30 minutes for the trend.

Breaking down the answer

Winter's formula — expected PaCO2 = 1.5 × HCO3 + 8 ± 2.¹ Changes if: measured PaCO2 is higher, a concurrent respiratory acidosis: under-ventilation when intubated, fatigue when extubated, and that changes my extubation decision.
Albumin-corrected gap — it falls about 2.5 mEq/L for every 1 g/dL of albumin below 4.¹ A cirrhotic or septic patient with albumin 2 and a "normal" gap of 12 really has a gap near 17, which hides a lactic acidosis.
Delta-delta — gap rising more than bicarbonate falls: a coexisting metabolic alkalosis (vomiting, nasogastric suction, diuretics). Bicarbonate falling more: a coexisting non-gap acidosis, most often hyperchloremia from normal saline.¹ Finding the second disorder is where the examiner sees organization.
Lactate is a perfusion signal until proven otherwise — I hunt the hidden cause: bowel ischemia, compartment syndrome, unrecognized bleeding. Epinephrine and metformin raise lactate without hypoperfusion; I name them but do not assume them.
Bicarbonate's narrow indications — Surviving Sepsis suggests against it to improve hemodynamics or cut vasopressor dose, and suggests it in septic shock with pH 7.2 or less and acute kidney injury (AKIN stage 2–3).²
Respiratory acidosis in the OR — circuit and patient: rising CO2 production (MH, thyroid storm, sepsis, CO2 insufflation) versus falling elimination (hypoventilation, exhausted absorbent, incompetent valves, dead space).

Follow-up question

You hand over for a break; three hours into the resection you return to find she has had several liters of normal saline, and the gas reads pH 7.18, PaCO2 32, HCO3 12, sodium 140, chloride 115, albumin 2.5, lactate 6. How do you interpret it, and what do you do now?

Follow-up answer

HeadlineA mixed gap and non-gap metabolic acidosis, saline on top of lactate, and I am behind on ventilation; I tell the surgeon.

The gap is 13, corrected to about 17; Winter's predicts 24–28, so her 32 is only partial compensation. Lactate is 6 and rising: I ask about bowel viability and bleeding. I raise minute ventilation, switch from normal saline to a balanced crystalloid, give blood if the hemoglobin supports it, start norepinephrine if she is fluid-unresponsive, and recheck in 30 minutes. No bicarbonate to fix the blood pressure; if the pH stays at or below 7.2 and the kidneys are failing, I give 50–100 mEq while ventilating to clear the CO2.²

Pitfalls that cost points

Naming the disorder and stopping; the examiner wants the cause and the plan.
Calculating an uncorrected anion gap in a hypoalbuminemic patient.

Rapid-fire

Q: Why does normal saline cause acidosis? A: Its chloride of 154 mEq/L narrows the strong ion difference, lowering bicarbonate; balanced crystalloid avoids it, and in SMART also lowered major adverse kidney events (14.3% vs 15.4%).³

Q: When else do you give bicarbonate, and what is the catch? A: Hyperkalemia, or a non-gap acidosis from bicarbonate loss. It generates CO2, which crosses cells and worsens intracellular acidosis unless I raise minute ventilation to clear it.

Q: What is the expected compensation in acute respiratory acidosis? A: Bicarbonate rises about 1 mEq/L per 10 mmHg rise in PaCO2 acutely and about 4 mEq/L chronically.¹

Q: Why can a COPD patient be hard to wean after you normalize the CO2? A: Hyperventilating a chronic retainer to 40 leaves a high bicarbonate, a metabolic alkalosis that suppresses respiratory drive; I ventilate to his baseline.

Q: Why does hypothermia matter to the gas? A: Gases are analyzed at 37 °C; I use alpha-stat management in adults, interpreting the uncorrected values, because it preserves enzyme function and cerebral autoregulation.

Numbers to know

Item	Value
Winter's formula	PaCO2 = 1.5 × HCO3 + 8 ± 2¹
Normal anion gap	8–12 mEq/L (analyzer-dependent)¹
Albumin correction	+2.5 mEq/L per 1 g/dL below 4
Metabolic alkalosis compensation	PaCO2 = 0.7 × HCO3 + 20 ± 5¹
Bicarbonate in septic shock	pH ≤ 7.2 with AKI; not for hemodynamics²
Lactate	> 2 mmol/L with vasopressors despite fluid defines septic shock⁴

Sources

1.Mastering blood gas interpretation: A practical guide for primary care providers · South African Family Practice (PubMed Central), 2025 · pmc.ncbi.nlm.nih.govTable 2 compensation rules; anion gap 8-12 with +0.25 per g/L albumin below 40 g/L; delta gap.
2.Surviving sepsis campaign: international guidelines for management of sepsis and septic shock 2021 · SCCM / ESICM, 2021 · pmc.ncbi.nlm.nih.gov
3.Balanced Crystalloids versus Saline in Critically Ill Adults (SMART) · NEJM, 2018 · pubmed.ncbi.nlm.nih.gov
4.The Third International Consensus Definitions for Sepsis and Septic Shock (Sepsis-3) · SCCM / ESICM (JAMA), 2016 · ncbi.nlm.nih.gov

Facts on this page were checked against these sources on Oct 14, 2026. Guidelines change; confirm doses against current institutional protocols.