Read the pH: below 7.35 is acidosis, above 7.45 alkalosis.
Find the value that explains it: PaCO₂ moving opposite to the pH is respiratory, HCO₃⁻ moving with the pH is metabolic.
Look at the other value: normal means uncompensated, abnormal in the compensating direction means partial compensation, and a normal pH with both abnormal means full compensation.
In a metabolic acidosis, compare PaCO₂ with Winter’s formula, 1.5·HCO₃⁻ + 8 ± 2, and calculate the anion gap Na⁺ − (Cl⁻ + HCO₃⁻).
Check a high anion gap with the delta ratio and, when albumin is low, with the corrected gap.
ECG rhythm strip interpretation
Paper (25 mm/s, 10 mm/mV)
1 small box = 0.04 s, 1 large box = 0.20 s, 10 mm = 1 mV
Heart rate, regular rhythm
300 ÷ large boxes = 1500 ÷ small boxes = 60 ÷ RR (s)
Heart rate, irregular rhythm
QRS complexes in 6 s × 10
Normal PR interval
0.12–0.20 s
Normal QRS duration
< 0.12 s
QTc (Bazett)
QT/√RR
QTc (Fridericia)
QT/∛RR
QTc (Framingham)
QT + 154·(1 − RR)
Sinus rates
bradycardia < 60, normal 60–100, tachycardia > 100 /min
Escape rates
junctional 40–60, ventricular 20–40 /min
Fast rhythms
SVT 150–250 /min (narrow), VT > 100 /min (wide)
Method
Rate: for a regular rhythm divide 300 by the large boxes or 1500 by the small boxes between two R waves; for an irregular rhythm count the QRS complexes in 6 seconds and multiply by 10.
Rhythm: compare the R–R intervals. Equal is regular; a repeating pattern is regularly irregular; no pattern at all is irregularly irregular.
P waves: is there an upright P wave in lead II before every QRS, all alike? Look for fibrillatory or sawtooth waves, inverted P waves and P waves without a QRS.
PR interval: from the start of P to the start of the QRS, normally 0.12–0.20 s and constant. Watch whether it lengthens, stays fixed before a dropped beat, or varies at random.
QRS duration: below 0.12 s is narrow; 0.12 s or more is wide. Then name the rhythm, and for the QT use QTc = QT/√RR (Bazett) with times in seconds.
Evidence-based practice statistics
Sensitivity and specificity
Se = a/(a + c), Sp = d/(b + d)
Predictive values
PPV = a/(a + b), NPV = d/(c + d)
Accuracy
(a + d)/n
Likelihood ratios
LR+ = Se/(1 − Sp), LR− = (1 − Se)/Sp
PPV at prevalence p
Se·p / (Se·p + (1 − Sp)(1 − p))
Odds
odds = p/(1 − p); p = odds/(1 + odds); post-test odds = pre-test odds × LR
Risk measures
RR = EER/CER, ARR = CER − EER, RRR = ARR/CER
Number needed to treat
NNT = 1/ARR, rounded up; NNH = 1/(EER − CER)
Odds ratio
OR = ad/(bc)
Incidence rate
new cases / person-time
Method
Draw the 2 × 2 table and label a, b, c and d before you calculate.
Read sensitivity and specificity down the columns, predictive values along the rows.
For another prevalence, imagine 10 000 people and fill the table with Se, Sp and the prevalence.
For a likelihood ratio, convert the probability to odds, multiply, and convert back.
For a trial, find EER and CER first; then RR = EER/CER, ARR = CER − EER and NNT = 1/ARR rounded up.
2Practice problems
1.Human anatomyMedium
Which cranial nerve controls movement of the tongue?
AFacial nerve (VII)
BGlossopharyngeal nerve (IX)
CVagus nerve (X)
DHypoglossal nerve (XII)
2.Human anatomyHard
An ECG shows an infarct in leads II, III and aVF (inferior wall). Which coronary artery is most commonly blocked?
ALeft anterior descending artery
BLeft main coronary artery
CFirst diagonal branch
DRight coronary artery
3.Human physiologyMedium
What does the QRS complex on an ECG represent?
AAtrial depolarisation
BAtrial repolarisation
CVentricular repolarisation
DVentricular depolarisation
4.Human physiologyHard
A healthy person stands up quickly after lying down. Which reflex response prevents fainting?
AThe chemoreceptor reflex raises breathing rate and arterial oxygen levels
BADH release immediately doubles blood volume
CThe Bainbridge reflex lowers heart rate
DThe baroreceptor reflex raises heart rate and vasoconstriction
5.MicrobiologyMedium
How does an mRNA vaccine produce immunity?
ABody cells use the mRNA to make a viral protein that the immune system learns to recognise
BThe mRNA kills the virus directly in the blood
CThe mRNA is inserted permanently into human DNA
DThe mRNA contains weakened live virus
6.MicrobiologyHard
A healthcare worker's blood test is positive for anti-HBs (hepatitis B surface antibody) and negative for HBsAg and anti-HBc. What does this most likely mean?
ACurrent acute hepatitis B infection
BImmunity from hepatitis B vaccination
CChronic hepatitis B infection
DPast natural infection that has cleared
7.PharmacologyMedium
Aciclovir is mainly used to treat infections caused by:
AHIV
BInfluenza A virus
CBacteria that produce beta-lactamase
DHerpes viruses such as herpes simplex and varicella-zoster
8.PharmacologyHard
A nurse gives 2 mg from a 4 mg ampoule of morphine. What should happen to the remaining 2 mg?
AIt is discarded in front of a witness, and both sign the record
BIt is kept labelled in the patient's drawer for the next dose
CIt is poured down the sink without telling anyone
DIt is given to the next patient who needs morphine
9.Arterial blood gas interpretationMedium
Practice values: Na⁺ 147 mmol/L, glucose 11.1 mmol/L and urea 17.9 mmol/L. Calculate the serum osmolality Osm = 2·Na⁺ + glucose + urea to the nearest whole number.
Na⁺ 147 mmol/Lglucose 11.1 mmol/Lurea 17.9 mmol/L
Osm = mOsm/kg
10.Arterial blood gas interpretationHard
Practice values: Na⁺ 140, Cl⁻ 96 and HCO₃⁻ 9 mmol/L. Calculate the delta ratio (AG − 12)/(24 − HCO₃⁻), using AG = Na⁺ − (Cl⁻ + HCO₃⁻). Give two decimals.
Na⁺ 140 mmol/LCl⁻ 96 mmol/LHCO₃⁻ 9 mmol/L
delta ratio =
11.ECG rhythm strip interpretationMedium
Interpret this lead II rhythm strip. Which rhythm is it? Click your answer.
lead II, 25 mm/s, 10 mm/mV1 small box = 0.04 s, 1 large box = 0.20 sstrip length 6 s
1sinus rhythm with PVCs
2atrial flutter, 2:1 conduction
3atrial fibrillation
4atrial flutter, 4:1 conduction
12.ECG rhythm strip interpretationHard
Interpret this lead II strip with the five steps in order: rate, rhythm, P waves, PR interval, QRS duration. At which step do you find the first abnormal finding? Click it.
lead II, 25 mm/s, 10 mm/mV1 small box = 0.04 s, 1 large box = 0.20 sstrip length 6 s
1rate
2rhythm
3P waves
4PR interval
5QRS duration
13.Evidence-based practice statisticsMedium
Practice exercise (made-up values, not clinical guidance). A test has a sensitivity of 71% and a specificity of 67%. Find the negative likelihood ratio LR−. Round to two decimal places.
Se = 71%Sp = 67%
LR− =
14.Evidence-based practice statisticsHard
Practice exercise (made-up values, not clinical guidance). A treatment reduces the risk of an event by 28% in relative terms (RRR = 28%). In a patient group the risk without treatment (CER) is 17% over five years. Find the number needed to treat NNT for five years to prevent one event. Round up to a whole number of patients.
RRR = 28%CER = 17%
NNT =
3Solutions
1.Human anatomy
Answer: D · Hypoglossal nerve (XII)
Why this is the answer
The hypoglossal nerve supplies the intrinsic and most extrinsic tongue muscles. A lesion makes the protruded tongue deviate towards the affected side.
Answer
D · Hypoglossal nerve (XII)
2.Human anatomy
Answer: D · Right coronary artery
Why this is the answer
The inferior wall is usually supplied by the right coronary artery through the posterior descending artery (right-dominant circulation), so inferior infarcts are mostly RCA occlusions.
Answer
D · Right coronary artery
3.Human physiology
Answer: D · Ventricular depolarisation
Why this is the answer
The P wave is atrial depolarisation, the QRS complex ventricular depolarisation and the T wave ventricular repolarisation. Atrial repolarisation is hidden in the QRS.
Answer
D · Ventricular depolarisation
4.Human physiology
Answer: D · The baroreceptor reflex raises heart rate and vasoconstriction
Why this is the answer
Blood pools in the legs and pressure falls; baroreceptors fire less, which increases sympathetic and decreases vagal activity within seconds, raising heart rate and vascular tone.
Answer
D · The baroreceptor reflex raises heart rate and vasoconstriction
5.Microbiology
Answer: A · Body cells use the mRNA to make a viral protein that the immune system learns to recognise
Why this is the answer
The mRNA is translated by cells into an antigen, such as a coronavirus spike protein. The immune system responds to it. The mRNA is broken down and does not enter the nucleus.
Answer
A · Body cells use the mRNA to make a viral protein that the immune system learns to recognise
6.Microbiology
Answer: B · Immunity from hepatitis B vaccination
Why this is the answer
The vaccine contains HBsAg, so a vaccinated person makes anti-HBs but no anti-HBc. Past natural infection gives both anti-HBs and anti-HBc, and ongoing infection shows HBsAg.
Answer
B · Immunity from hepatitis B vaccination
7.Pharmacology
Answer: D · Herpes viruses such as herpes simplex and varicella-zoster
Why this is the answer
Aciclovir is phosphorylated by viral thymidine kinase in infected cells and then blocks viral DNA polymerase. It is used for cold sores, genital herpes, chickenpox and shingles.
Answer
D · Herpes viruses such as herpes simplex and varicella-zoster
8.Pharmacology
Answer: A · It is discarded in front of a witness, and both sign the record
Why this is the answer
Unused controlled drugs are wasted in front of a witness according to policy, and both staff document the waste. This prevents diversion and keeps the stock count accurate.
Answer
A · It is discarded in front of a witness, and both sign the record
9.Arterial blood gas interpretation
Answer: Osm = 323 mOsm/kg
Derived values: calculated serum osmolality is 2·Na⁺ + glucose + urea with all values in mmol/L (2·Na⁺ + glucose/18 + BUN/2.8 with glucose and BUN in mg/dL); normal is about 275–295 mOsm/kg. The P/F ratio is PaO₂ divided by FiO₂ as a fraction. The alveolar oxygen pressure is PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8 mmHg at sea level, and the A–a gradient is PAO₂ − PaO₂.
Calculated osmolality
Osm = 2·147 + 11.1 + 17.9 = 323 ≈ 323 mOsm/kg
Answer
Osm = 323 mOsm/kg
10.Arterial blood gas interpretation
Answer: delta ratio = 1.533
Expected compensation: in metabolic acidosis Winter’s formula predicts PaCO₂ = 1.5·HCO₃⁻ + 8 ± 2 mmHg; a measured value above that range points to an additional respiratory acidosis, below it to an additional respiratory alkalosis. The anion gap AG = Na⁺ − (Cl⁻ + HCO₃⁻), normally about 8–12 mmol/L, separates high-gap from normal-gap metabolic acidosis; low albumin hides a high gap (add 2.5 per g/dL that albumin is below 4.0). The delta ratio (AG − 12)/(24 − HCO₃⁻) between about 0.8 and 2 fits a pure high-gap acidosis (some texts use 1 instead of 0.8), below that an added normal-gap acidosis, and above 2 an added metabolic alkalosis.
Anion gap
AG = 140 − (96 + 9) = 35
Delta ratio
(35 − 12)/(24 − 9) = 23/15 = 1.533
Answer
delta ratio = 1.533
Between 1 and 2: a pure high anion gap acidosis.
11.ECG rhythm strip interpretation
Answer: atrial fibrillation
Rhythm criteria (lead II): sinus rhythm has an upright P wave before every QRS, a constant PR of 0.12–0.20 s and a narrow QRS: normal sinus rhythm at 60–100/min, sinus bradycardia below 60, sinus tachycardia above 100. Atrial fibrillation: no P waves, a fibrillatory baseline and an irregularly irregular R–R. Atrial flutter: sawtooth flutter waves at about 250–350/min with a fixed conduction ratio such as 2:1 or 4:1. First-degree AV block: sinus rhythm with a constant PR above 0.20 s. Junctional rhythm: narrow QRS at 40–60/min with absent or inverted P waves (PR below 0.12 s, or P after the QRS). A PVC is an early, wide (0.12 s or more), bizarre QRS without a P wave, with the T wave in the opposite direction and usually a full compensatory pause; in bigeminy every second beat is a PVC.
Five steps
1. Rate: 10 QRS complexes in 6 s, × 10 = 100 per minute.
2. Rhythm: irregularly irregular, R–R from 0.44 to 0.76 s with no pattern.
3. P waves: none; the baseline shows small irregular fibrillatory waves.
4. PR interval: not measurable.
5. QRS: 0.08 s, narrow (below 0.12 s).
Interpretation
atrial fibrillation
Answer
atrial fibrillation
12.ECG rhythm strip interpretation
Answer: QRS duration
Five steps for every strip: 1. rate (300 or 1500 method, or the 6-second method if irregular); 2. rhythm: are the R–R intervals equal; 3. P waves: present, upright in lead II, one before every QRS and all alike; 4. PR interval: 0.12–0.20 s and constant; 5. QRS duration: below 0.12 s. Only after the five steps name the rhythm. Normal sinus rhythm passes all five: 60–100/min, regular, upright P before every QRS, PR 0.12–0.20 s, QRS below 0.12 s.
Five steps
1. Rate: R–R = 19 small boxes, 1500 ÷ 19 ≈ 79 per minute.
2. Rhythm: regular, every R–R is 0.76 s.
3. P waves: upright, one before every QRS, all alike.
4. PR interval: 0.18 s, constant, within 0.12–0.20 s.
5. QRS: 0.16 s, wide (0.12 s or more).
First abnormal step
QRS duration → sinus rhythm with wide QRS
Answer
QRS duration
13.Evidence-based practice statistics
Answer: LR− = 0.43
Likelihood ratios and risk measures: LR+ = sensitivity/(1 − specificity), LR− = (1 − sensitivity)/specificity. In a trial, EER and CER are the event rates with and without treatment: RR = EER/CER, ARR = CER − EER, RRR = ARR/CER and NNT = 1/ARR, rounded up. In a case-control study only the odds ratio OR = ad/(bc) can be used. An incidence rate = new cases/person-time.
Negative likelihood ratio
LR− = (1 − 0.71)/0.67 = 0.4328 ≈ 0.43
Answer
LR− = 0.43
14.Evidence-based practice statistics
Answer: NNT = 22
Predictive values depend on prevalence: PPV = Se·p/(Se·p + (1 − Sp)(1 − p)) and NPV = Sp(1 − p)/(Sp(1 − p) + (1 − Se)p). With likelihood ratios: pre-test odds = p/(1 − p), post-test odds = pre-test odds × LR, probability = odds/(1 + odds). For a population of N, true positives = N·p·Se and false positives = N·(1 − p)·(1 − Sp). NNT = 1/(CER × RRR), rounded up.
Absolute risk reduction
ARR = 0.17·0.28 = 0.0476
Number needed to treat
NNT = 1/0.0476 = 21.01 → 22
Answer
NNT = 22
The same relative effect gives a smaller NNT when the baseline risk is higher.