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 supplies the lateral rectus muscle of the eye?
AOptic nerve (II)
BAbducens nerve (VI)
COculomotor nerve (III)
DTrochlear nerve (IV)
2.Human anatomyHard
When a patient stands on the right leg only, the left side of the pelvis drops (positive Trendelenburg sign). Which nerve is most likely damaged?
ARight superior gluteal nerve
BRight femoral nerve
CRight inferior gluteal nerve
DLeft superior gluteal nerve
3.Human physiologyMedium
Low plasma albumin, as in severe liver disease, causes oedema mainly because:
ACapillary hydrostatic pressure falls
BThe capillaries become impermeable
CLymphatic drainage increases
DPlasma oncotic pressure falls
4.Human physiologyHard
In the renin–angiotensin–aldosterone system, where is angiotensin I mainly converted to angiotensin II?
AIn the adrenal cortex
BIn the kidneys' juxtaglomerular cells
CIn the lungs
DIn the liver
5.MicrobiologyMedium
How does an mRNA vaccine produce immunity?
AThe mRNA kills the virus directly in the blood
BThe mRNA is inserted permanently into human DNA
CThe mRNA contains weakened live virus
DBody cells use the mRNA to make a viral protein that the immune system learns to recognise
6.MicrobiologyHard
Joseph Lister reduced infection after surgery in the 1860s. What was his main innovation?
AGiving sulfonamide antibiotics after every operation
BUsing steam autoclaves to sterilise all surgical instruments before each operation
CVaccinating surgeons against staphylococci
DUsing carbolic acid (phenol) as an antiseptic on wounds and instruments
7.PharmacologyMedium
1000 mL of IV fluid is to be given over 8 hours with a pump. What rate should be set?
A80 mL/h
B125 mL/h
C100 mL/h
D250 mL/h
8.PharmacologyHard
Gentamicin is combined with high-dose intravenous furosemide. Which risk is especially increased?
ALiver failure
BBleeding
CHypoglycaemia
DDamage to hearing
9.Arterial blood gas interpretationMedium
A practice arterial blood gas shows pH 7.60, PaCO₂ 39 mmHg (5.2 kPa) and HCO₃⁻ 37 mmol/L. The primary disorder is metabolic alkalosis. How far is it compensated? Click your answer.
pH 7.60PaCO₂ 39 mmHg (5.2 kPa)HCO₃⁻ 37 mmol/L
1uncompensated
2partially compensated
3fully compensated
10.Arterial blood gas interpretationHard
A practice gas shows a metabolic acidosis: pH 7.27, PaCO₂ 25 mmHg (3.3 kPa), HCO₃⁻ 11 mmol/L. Compare the measured PaCO₂ with Winter’s formula. Click your interpretation.
pH 7.27PaCO₂ 25 mmHg (3.3 kPa)HCO₃⁻ 11 mmol/L
1appropriate compensation
2added respiratory acidosis
3added respiratory alkalosis
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
1atrial fibrillation
2normal sinus rhythm
3sinus arrhythmia
4sinus rhythm with PVCs
12.ECG rhythm strip interpretationHard
This lead II strip shows an AV block, so the atria and the ventricles beat at different rates. Measure the P–P interval and find the atrial rate in beats per minute (1500 method, whole number).
lead II, 25 mm/s, 10 mm/mV1 small box = 0.04 s, 1 large box = 0.20 sstrip length 6 s
atrial rate = bpm
13.Evidence-based practice statisticsMedium
Practice exercise (made-up values, not clinical guidance). A case-control study compares 400 patients with a disease (cases) and 130 people without it (controls). The table shows how many in each group were exposed to a risk factor. Find the odds ratio OR for the exposure. Round to two decimal places.
a = 277b = 47c = 123d = 83
OR =
14.Evidence-based practice statisticsHard
Practice exercise (made-up values, not clinical guidance). Without a drug, 18% of patients have a certain side effect during a year. The drug raises this risk with a relative risk of RR = 2.07. Find the number needed to harm NNH: how many patients must take the drug for a year for one extra patient to have the side effect? Round up to a whole number of patients.
control 18%RR = 2.07
NNH =
3Solutions
1.Human anatomy
Answer: B · Abducens nerve (VI)
Why this is the answer
The abducens nerve (VI) supplies lateral rectus. A lesion stops the eye moving outwards and causes horizontal double vision.
Answer
B · Abducens nerve (VI)
2.Human anatomy
Answer: A · Right superior gluteal nerve
Why this is the answer
The superior gluteal nerve supplies gluteus medius and minimus. If those of the standing (right) leg are weak, the pelvis sags on the opposite, unsupported side.
Answer
A · Right superior gluteal nerve
3.Human physiology
Answer: D · Plasma oncotic pressure falls
Why this is the answer
Albumin generates most of the plasma oncotic pressure that pulls fluid back into capillaries. When it falls, more fluid stays in the tissues.
Answer
D · Plasma oncotic pressure falls
4.Human physiology
Answer: C · In the lungs
Why this is the answer
Renin (kidney) cleaves angiotensinogen (liver) to angiotensin I, which ACE, mainly on pulmonary capillary endothelium, converts to angiotensin II.
Answer
C · In the lungs
5.Microbiology
Answer: D · 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
D · Body cells use the mRNA to make a viral protein that the immune system learns to recognise
6.Microbiology
Answer: D · Using carbolic acid (phenol) as an antiseptic on wounds and instruments
Why this is the answer
Lister applied carbolic acid to wounds, dressings and instruments, which greatly reduced post-operative infection. This started antiseptic surgery.
Answer
D · Using carbolic acid (phenol) as an antiseptic on wounds and instruments
7.Pharmacology
Answer: B · 125 mL/h
Why this is the answer
1000 mL ÷ 8 h = 125 mL/h. Infusion pumps are set in mL per hour.
Answer
B · 125 mL/h
8.Pharmacology
Answer: D · Damage to hearing
Why this is the answer
Both aminoglycosides and loop diuretics can damage the inner ear. The combination increases the risk of ototoxicity, so it needs caution and monitoring.
Answer
D · Damage to hearing
9.Arterial blood gas interpretation
Answer: uncompensated
Reading a blood gas (ROME): first the pH: below 7.35 is acidaemia, above 7.45 alkalaemia. Then find the value that explains it. PaCO₂ (normal 35–45 mmHg, 4.7–6.0 kPa) is the respiratory part and moves Opposite to the pH in a respiratory disorder; HCO₃⁻ (normal 22–26 mmol/L) is the metabolic part and moves in the same direction as the pH (Respiratory Opposite, Metabolic Equal). The other value shows the compensation: normal means uncompensated; abnormal in the direction that pulls the pH back while the pH is still abnormal means partially compensated; a normal pH with both values abnormal means fully compensated. Ranges differ slightly between laboratories; these are practice values.
ROME, value by value
pH 7.60: alkalaemia (above 7.45)
PaCO₂ 39 mmHg: normal (35–45)
HCO₃⁻ 37 mmol/L: high (22–26)
Compensation
Clinically, also check the expected compensation: for a metabolic alkalosis PaCO₂ ≈ 0.7·HCO₃⁻ + 21 = 44.9–48.9 mmHg, so the measured 39 mmHg points to an additional respiratory alkalosis (a mixed disorder). The answer above is the ROME exam classification.
uncompensated
Answer
uncompensated
10.Arterial blood gas interpretation
Answer: appropriate compensation
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.
Expected PaCO₂
1.5·11 + 8 = 24.5 → 22.5–26.5 mmHg
Compare
measured: 25 mmHg → appropriate compensation
Answer
appropriate compensation
11.ECG rhythm strip interpretation
Answer: sinus arrhythmia
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: 6 QRS complexes in 6 s, × 10 = 60 per minute.
2. Rhythm: irregular, R–R from 0.78 to 1.14 s, lengthening and shortening gradually.
3. P waves: upright, one before every QRS, all alike.
4. PR interval: 0.16 s, constant, within 0.12–0.20 s.
5. QRS: 0.06 s, narrow (below 0.12 s).
Interpretation
sinus arrhythmia
Answer
sinus arrhythmia
12.ECG rhythm strip interpretation
Answer: atrial rate = 71 bpm
AV blocks: march out the P waves and check which ones are followed by a QRS. First-degree: every P conducted, PR constant and above 0.20 s. Second-degree type I (Mobitz I, Wenckebach): the PR lengthens from beat to beat until one P wave is not conducted, then the cycle starts again (group beating). Second-degree type II (Mobitz II): the PR of the conducted beats stays constant, and P waves are suddenly not conducted; the QRS is often wide. Third-degree (complete) block: P waves and QRS complexes are each regular but independent, the PR varies at random and the atrial rate is faster than the escape rate (junctional 40–60/min with a narrow QRS, ventricular 20–40/min with a wide QRS). A 2:1 block cannot be called type I or type II from one strip.
Atrial rate
P–P = 21 small boxes = 0.84 s → 1500 ÷ 21 = 71.43 ≈ 71 per minute
Ventricular rate
R–R = 42 small boxes = 1.68 s → 1500 ÷ 42 = 35.71 ≈ 36 per minute
Answer
atrial rate = 71 bpm
Answer
atrial rate = 71 bpm
2:1 AV block: every second P wave is not conducted, so the atrial rate is twice the ventricular rate.
13.Evidence-based practice statistics
Answer: OR = 3.98
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.
Odds of exposure
cases: 277/123 = 2.252
controls: 47/83 = 0.5663
Odds ratio
OR = (277·83)/(47·123) = 3.977 ≈ 3.98
Answer
OR = 3.98
OR above 1: exposure is more common among the cases.
14.Evidence-based practice statistics
Answer: NNH = 6
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.