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 muscle is the main extensor of the elbow?
ATriceps brachii
BBrachialis
CCoracobrachialis
DBiceps brachii
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
Which cells in the stomach secrete hydrochloric acid?
AG cells
BGoblet cells
CChief cells
DParietal cells
4.Human physiologyHard
Which transporter in the thick ascending limb of the loop of Henle is key to building the medullary concentration gradient?
AThe epithelial sodium channel (ENaC)
BThe sodium–glucose cotransporter (SGLT2)
CAquaporin-2 water channels
DThe Na⁺–K⁺–2Cl⁻ cotransporter (NKCC2)
5.MicrobiologyMedium
How does an autoclave sterilise instruments?
AWith ultraviolet light only
BWith alcohol vapour at room temperature
CWith dry heat at about 60 °C
DWith steam under pressure at about 121 °C
6.MicrobiologyHard
A traveller returns from sub-Saharan Africa with fever. Malaria is suspected. Which species is most likely to cause severe and cerebral malaria?
APlasmodium vivax
BPlasmodium ovale
CPlasmodium malariae
DPlasmodium falciparum
7.PharmacologyMedium
How are most medication doses for children calculated?
ABy the child's height alone
BBy giving half of the adult dose to all children under 12
CBy the child's age in years only
DBy body weight in kilograms, or sometimes body surface area
8.PharmacologyHard
A patient who used cocaine has chest pain, agitation, a high temperature, sweating, tachycardia and dilated pupils. Which drug class is commonly first-line for the agitation and sympathetic overactivity?
AAntihistamines
BBenzodiazepines
CAnticholinergic drugs
DOpioid antagonists
9.Arterial blood gas interpretationMedium
A practice gas shows PaCO₂ 25 mmHg (3.3 kPa). Use the rule that in acute respiratory alkalosis HCO₃⁻ falls by 2 mmol/L for every 10 mmHg that PaCO₂ is below 40, starting from HCO₃⁻ 24 mmol/L, to find the expected HCO₃⁻.
A practice gas shows a metabolic acidosis: pH 7.10, PaCO₂ 37 mmHg (4.9 kPa), HCO₃⁻ 11 mmol/L. Compare the measured PaCO₂ with Winter’s formula. Click your interpretation.
pH 7.10PaCO₂ 37 mmHg (4.9 kPa)HCO₃⁻ 11 mmol/L
1appropriate compensation
2added respiratory acidosis
3added respiratory alkalosis
11.ECG rhythm strip interpretationMedium
This lead II strip shows atrial flutter. How many flutter waves per QRS complex are there (the n of n:1 conduction)?
lead II, 25 mm/s, 10 mm/mV1 small box = 0.04 s, 1 large box = 0.20 sstrip length 6 s
flutter waves per QRS =
12.ECG rhythm strip interpretationHard
A monitored patient is found unresponsive and not breathing normally, and no carotid pulse can be felt. The monitor shows this organised rhythm. What is this situation called? 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
1ventricular fibrillation
2sinus bradycardia
3asystole
4pulseless electrical activity (PEA)
13.Evidence-based practice statisticsMedium
Practice exercise (made-up values, not clinical guidance). In a randomised trial, 910 patients received a new treatment and 640 received the control treatment. The table shows how many in each group had the outcome (an unwanted event such as a stroke). Find the relative risk RR of the event with treatment. Round to two decimal places.
treatment: 45/910control: 45/640
RR =
14.Evidence-based practice statisticsHard
Practice exercise (made-up values, not clinical guidance). A screening test has a sensitivity of 81% and a specificity of 83%. It is used in a population where the prevalence is 0.5%. Find the negative predictive value NPV as a percentage. Round to one decimal place.
Se = 81%Sp = 83%p = 0.5%
NPV = %
3Solutions
1.Human anatomy
Answer: A · Triceps brachii
Why this is the answer
Triceps brachii inserts on the olecranon of the ulna and extends the elbow. Biceps and brachialis are elbow flexors.
Answer
A · Triceps brachii
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 · Parietal cells
Why this is the answer
Parietal cells secrete HCl and intrinsic factor. Chief cells secrete pepsinogen and G cells secrete gastrin.
Answer
D · Parietal cells
4.Human physiology
Answer: D · The Na⁺–K⁺–2Cl⁻ cotransporter (NKCC2)
Why this is the answer
NKCC2 moves Na⁺, K⁺ and Cl⁻ out of the tubule into the interstitium without water, building the medullary gradient. Loop diuretics such as furosemide block it.
Answer
D · The Na⁺–K⁺–2Cl⁻ cotransporter (NKCC2)
5.Microbiology
Answer: D · With steam under pressure at about 121 °C
Why this is the answer
Pressurised steam reaches about 121 °C and kills all microorganisms, including spores, typically in about 15 minutes or more depending on the load.
Answer
D · With steam under pressure at about 121 °C
6.Microbiology
Answer: D · Plasmodium falciparum
Why this is the answer
P. falciparum causes very high parasite levels and makes infected red cells stick in small vessels, including in the brain. It is common in sub-Saharan Africa and is a medical emergency.
Answer
D · Plasmodium falciparum
7.Pharmacology
Answer: D · By body weight in kilograms, or sometimes body surface area
Why this is the answer
Children's doses are usually weight-based, sometimes using body surface area, and must not exceed the adult maximum. Accurate, recent weight in kilograms is essential.
Answer
D · By body weight in kilograms, or sometimes body surface area
8.Pharmacology
Answer: B · Benzodiazepines
Why this is the answer
Cocaine causes a sympathomimetic toxidrome. Benzodiazepines reduce agitation, tachycardia, hypertension and hyperthermia, while cooling and cardiac monitoring are also given.
Answer
B · Benzodiazepines
9.Arterial blood gas interpretation
Answer: HCO₃⁻ = 21 mmol/L
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.
Change in PaCO₂
|25 − 40| = 15 mmHg → 1.5 × 10 mmHg
Expected HCO₃⁻
HCO₃⁻ = 24 − 2·1.5 = 21 mmol/L
Answer
HCO₃⁻ = 21 mmol/L
10.Arterial blood gas interpretation
Answer: added respiratory acidosis
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: 37 mmHg → added respiratory acidosis
Answer
added respiratory acidosis
11.ECG rhythm strip interpretation
Answer: flutter waves per QRS = 4
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.
Flutter waves
F–F = 6 small boxes = 0.24 s → 1500 ÷ 6 = 250 per minute
QRS complexes
R–R = 24 small boxes = 0.96 s → 1500 ÷ 24 = 63 per minute
conduction ratio = 250 ÷ 63 = 4:1
Answer
flutter waves per QRS = 4
12.ECG rhythm strip interpretation
Answer: pulseless electrical activity (PEA)
Pulseless electrical activity (PEA): PEA cannot be seen on a strip. The monitor shows organised electrical activity (a rhythm other than VF, VT or asystole) but the patient has no palpable pulse, so it is recognised by checking the patient. In adult resuscitation guidelines PEA and asystole are the non-shockable cardiac arrest rhythms: high-quality CPR, epinephrine as soon as possible and a search for reversible causes (the Hs and Ts, such as hypovolaemia, hypoxia, tension pneumothorax and cardiac tamponade). VF and pulseless VT are the shockable rhythms.
What the strip shows
1. Rate: R–R = 29 small boxes, 1500 ÷ 29 ≈ 52 per minute.
2. Rhythm: regular, every R–R is 1.16 s.
3. P waves: upright, one before every QRS, all alike.
4. PR interval: 0.14 s, constant, within 0.12–0.20 s.
5. QRS: 0.08 s, narrow (below 0.12 s).
sinus bradycardia
What the patient shows
no pulse despite organised electrical activity: pulseless electrical activity (PEA), a non-shockable cardiac arrest rhythm
Answer
pulseless electrical activity (PEA)
Answer
pulseless electrical activity (PEA)
PEA is recognised at the bedside, never from the strip alone. This is exam practice, not clinical guidance.
13.Evidence-based practice statistics
Answer: RR = 0.7
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.
Event rates
EER = 45/910 = 0.04945
CER = 45/640 = 0.07031
Relative risk
RR = 0.04945/0.07031 = 0.7033 ≈ 0.7
Answer
RR = 0.7
14.Evidence-based practice statistics
Answer: NPV = 99.9 %
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.