Use this deck to practise a method, not just definitions

The ACEM Primary assesses foundational sciences and their application to emergency medicine. It typically includes written components and structured oral or viva-style assessment. Candidates may need to explain mechanisms, interpret data and apply core principles to acute presentations under time pressure. Check the official ACEM website for current examination information rather than relying on a study guide for format details.

Acid–base interpretation is a useful area to practise because it combines definitions, equations, data interpretation and clinical reasoning. For each card below, try to give a short answer aloud before revealing the back. Then grade your recall honestly: use the lowest grade that matches how readily and accurately you answered. A correct answer that took a long pause, missed a unit or needed a prompt is not the same as fluent recall.

A MySummaries deck on acid–base interpretation could look like this:

Cards — ACEM Primary — Acid–base interpretation16 due

What pH defines acidemia, and what pH defines alkalaemia?

Acidemia: arterial pH <7.35. Alkalaemia: arterial pH >7.45.

All 16 cards
What pH defines acidemia, and what pH defines alkalaemia?Acidemia: arterial pH <7.35. Alkalaemia: arterial pH >7.45.
What are usual adult reference ranges for arterial pH, PaCO₂ and HCO₃⁻?pH 7.35–7.45; PaCO₂ 35–45 mmHg; HCO₃⁻ 22–26 mmol/L. Use the local laboratory’s ranges when interpreting a real result.
State the Henderson–Hasselbalch relationship used to understand blood pH. Include the constants and units. pH = 6.1 + log₁₀[HCO₃⁻/(0.03 × PaCO₂)]. HCO₃⁻ is in mmol/L and PaCO₂ in mmHg; 0.03 is the CO₂ solubility coefficient in mmol·L⁻¹·mmHg⁻¹.
How do you calculate the anion gap without potassium?Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻), using concentrations in mmol/L. The reference range depends on the laboratory and its measurement method.
How do you correct the anion gap for a low albumin, with albumin in g/L?Corrected anion gap = measured anion gap + 0.25 × (40 − albumin in g/L), in mmol/L. This is an estimate; interpret it with the clinical picture and laboratory ranges.
In metabolic acidosis, what PaCO₂ does Winter’s formula predict?Expected PaCO₂ = (1.5 × HCO₃⁻) + 8 ± 2 mmHg. A measured PaCO₂ outside the predicted range suggests an additional respiratory disorder.
HCO₃⁻ is 12 mmol/L in metabolic acidosis. What PaCO₂ range does Winter’s formula predict?24–28 mmHg: (1.5 × 12) + 8 = 26 mmHg, with an expected range of ±2 mmHg.
What does a PaCO₂ above Winter’s predicted range suggest in metabolic acidosis?Concurrent respiratory acidosis or inadequate respiratory compensation. Consider causes of impaired ventilation, alongside the cause of the metabolic acidosis.
In acute respiratory acidosis, how does HCO₃⁻ usually change for each 10 mmHg rise in PaCO₂?It rises by about 1 mmol/L per 10 mmHg PaCO₂ increase above 40 mmHg, through short-term buffering.
In chronic respiratory acidosis, how does HCO₃⁻ usually change for each 10 mmHg rise in PaCO₂?It rises by about 3.5–4 mmol/L per 10 mmHg PaCO₂ increase above 40 mmHg, reflecting renal adaptation over time.
In acute respiratory alkalosis, how does HCO₃⁻ usually change for each 10 mmHg fall in PaCO₂?It falls by about 2 mmol/L per 10 mmHg PaCO₂ decrease below 40 mmHg.
In chronic respiratory alkalosis, how does HCO₃⁻ usually change for each 10 mmHg fall in PaCO₂?It falls by about 4–5 mmol/L per 10 mmHg PaCO₂ decrease below 40 mmHg, as renal bicarbonate excretion develops.
What PaCO₂ is expected as compensation in metabolic alkalosis?A commonly used estimate is PaCO₂ = 40 + 0.7 × (HCO₃⁻ − 24) mmHg, with a broad expected range of about ±5 mmHg. Check whether the result fits the clinical context.
What does compensation that exceeds the expected range usually indicate?A second primary acid–base disorder. Compensation alone does not usually drive the pH past the normal range into the opposite disturbance.
Why can salicylate toxicity produce both respiratory alkalosis and metabolic acidosis?Salicylates stimulate the respiratory centre, lowering PaCO₂, and can also cause organic-acid accumulation, lowering HCO₃⁻ and increasing the anion gap.
What does a low urine chloride, often <20 mmol/L, support in a patient with metabolic alkalosis?It supports chloride-responsive metabolic alkalosis, commonly associated with chloride depletion or volume contraction. Interpret it alongside history, volume status and treatment.

That is a MySummaries deck, filled with ACEM Primary material. Yours is written from your own notes. Start free

Work through the cards in useful passes

Pass one: identify the primary process. For each question, say whether it asks for a definition, a reference value, a calculation or a mechanism. On an ABG, establish whether the blood is acidemic or alkalaemic, then ask which change in PaCO₂ or HCO₃⁻ could explain the pH. Do not start naming causes before you have established the pattern.

Pass two: calculate expected compensation. Winter’s formula is especially useful because it gives a testable expected range in metabolic acidosis. Calculate it rather than relying on “the CO₂ should be low”. Then compare the measured PaCO₂ with that range. If it does not fit, state that a mixed disorder is possible and explain which additional process the result suggests.

Pass three: link the pattern to a mechanism and an acute-care question. For example, a low PaCO₂ could reflect appropriate compensation, a primary respiratory alkalosis, or both. A candidate should connect the numbers to ventilation and the likely clinical context, not stop at a label. In an acute presentation, also consider whether the patient’s ventilation is failing or changing, and whether immediate assessment is needed. These cards are for science revision, not a substitute for clinical assessment or local protocols.

After each answer, use the four grade buttons according to your actual recall. Grade a correct but hesitant calculation lower than an immediate, accurate response. If you missed a unit, confused acute and chronic compensation, or could not explain the mechanism, mark that specific weakness mentally before moving on. The card tells you what to revisit; it does not prove that you can yet apply the fact in a longer answer.

The core facts to keep available

The deck is deliberately wider than the minimum set of facts to retrieve quickly. These four points are a compact check before you attempt a longer ABG interpretation:

Must not miss coreACEM Primary — Acid–base interpretation
Usual adult ABG ranges: pH 7.35–7.45, PaCO₂ 35–45 mmHg, HCO₃⁻ 22–26 mmol/L; check the local laboratory range.
Anion gap without potassium = Na⁺ − (Cl⁻ + HCO₃⁻); the reference range varies by laboratory.
In metabolic acidosis, expected PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2 mmHg (Winter’s formula).
A measured PaCO₂ outside the expected compensation range raises concern for a second primary respiratory disorder.

When these are fluent, practise speaking a compact interpretation: name the primary process, give the key evidence, check compensation, then state the likely clinical significance. If you need to calculate, say the formula and units as you work. That makes errors easier to spot and keeps the explanation organised under questioning.

Turn repeated misses into a specific repair task

A missed card is more useful when it leads to one small task. If you repeatedly mix up acute and chronic respiratory compensation, compare the two figures and explain why renal adaptation changes the bicarbonate response over time. If you miss Winter’s formula, do a fresh calculation and check the arithmetic against the predicted range. If you remember the equation but cannot say what an out-of-range PaCO₂ means, practise the interpretation rather than repeating the equation alone.

A remediation prompt from a repeated error could look like this:

Remediation tray

You lost this mark twice: HCO₃⁻ is 10 mmol/L in metabolic acidosis. Calculate the expected PaCO₂ using Winter’s formula, include the range, and explain what a measured PaCO₂ of 35 mmHg suggests.

Add cardDismiss

A complete response would calculate 23 mmHg, with an expected range of 21–25 mmHg. A measured PaCO₂ of 35 mmHg is above that range and suggests an additional respiratory acidosis or inadequate ventilation. Practise saying both the calculation and the implication: knowing the number without interpreting it leaves the clinical reasoning unfinished.

How MySummaries helps

Build a revision board from your own PDFs, slides or photographed notes, then generate flashcards from that material. Use spaced repetition to revisit difficult facts, and connect the cards to written mock exams or viva practice when you need to apply them in longer answers. Start at portal.mysummaries.app.