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 trapezius and sternocleidomastoid muscles?
AFacial nerve (VII)
BAccessory nerve (XI)
CVagus nerve (X)
DHypoglossal nerve (XII)
2.Human anatomyHard
After a stroke, a patient speaks fluently but uses meaningless words and cannot understand speech. Where is the lesion most likely to be?
AInferior frontal gyrus of the dominant hemisphere
BCerebellar hemisphere
CPosterior superior temporal gyrus of the dominant hemisphere
DPrecentral gyrus of the non-dominant hemisphere
3.Human physiologyMedium
Mean arterial pressure is approximately equal to:
ACardiac output × total peripheral resistance
BHeart rate × stroke volume
CCardiac output ÷ heart rate
DStroke volume × total peripheral vascular resistance
4.Human physiologyHard
In a patient with primary hypothyroidism, you would expect:
ALow TSH and low free T4
BHigh TSH and high free T4
CHigh TSH and low free T4
DLow TSH and high free T4
5.MicrobiologyMedium
In the chain of infection, what is the "reservoir"?
AThe place where the pathogen normally lives and multiplies
BThe way the pathogen enters a new host
CThe route by which the pathogen leaves the infected host's body
DThe person who is most at risk of infection
6.MicrobiologyHard
After eating home-canned vegetables, a patient develops double vision, difficulty swallowing and a descending flaccid paralysis. What is the most likely cause?
AEndotoxin from Gram-negative bacteria
BTetanus toxin, which blocks inhibitory neurotransmitters
CA viral infection of the motor neurons
DBotulinum toxin, which blocks acetylcholine release
7.PharmacologyMedium
Which approach is usually recommended to prevent constipation in a patient on regular opioids?
AAn antidiarrhoeal drug at night to slow the gut
BA stimulant laxative such as senna, often with a stool softener
CNo laxative until the patient has not opened the bowels for a week
DBulk-forming fibre alone, with fluid intake restricted
8.PharmacologyHard
Some receptors show activity even without an agonist. How does an inverse agonist differ from a neutral antagonist at such receptors?
AIt increases activity above the baseline
BIt reduces activity below the baseline level
CIt has no effect when given alone
DIt destroys the receptor permanently
9.Arterial blood gas interpretationMedium
A practice arterial blood gas shows pH 7.29, PaCO₂ 63 mmHg (8.4 kPa) and HCO₃⁻ 29 mmol/L. The primary disorder is respiratory acidosis. How far is it compensated? Click your answer.
pH 7.29PaCO₂ 63 mmHg (8.4 kPa)HCO₃⁻ 29 mmol/L
1uncompensated
2partially compensated
3fully compensated
10.Arterial blood gas interpretationHard
A practice gas shows a respiratory acidosis: pH 7.31, PaCO₂ 72 mmHg (9.6 kPa), HCO₃⁻ 35 mmol/L. Use the rules ‘HCO₃⁻ rises 1 mmol/L per 10 mmHg (acute) or 3.5 mmol/L per 10 mmHg (chronic)’ to classify it. Click your answer.
pH 7.31PaCO₂ 72 mmHg (9.6 kPa)HCO₃⁻ 35 mmol/L
1acute
2chronic
3acute on chronic
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 flutter, 3:1 conduction
2normal sinus rhythm
3atrial fibrillation
4sinus tachycardia
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 test has a sensitivity of 86% and a specificity of 82%. Find the positive likelihood ratio LR+. Round to two decimal places.
Se = 86%Sp = 82%
LR+ =
14.Evidence-based practice statisticsHard
Practice exercise (made-up values, not clinical guidance). 10 000 people are screened with a test of sensitivity 95% and specificity 92%. The prevalence of the disease is 3%. How many true-positive results TP are expected?
N = 10 000p = 3%Se = 95%Sp = 92%
TP =
3Solutions
1.Human anatomy
Answer: B · Accessory nerve (XI)
Why this is the answer
The accessory nerve (XI) supplies sternocleidomastoid and trapezius. Damage weakens head turning and shoulder shrugging.
Answer
B · Accessory nerve (XI)
2.Human anatomy
Answer: C · Posterior superior temporal gyrus of the dominant hemisphere
Why this is the answer
This is Wernicke's (receptive) aphasia, caused by damage to Wernicke's area in the posterior superior temporal gyrus. Broca's area in the inferior frontal gyrus causes non-fluent aphasia.
Answer
C · Posterior superior temporal gyrus of the dominant hemisphere
3.Human physiology
Answer: A · Cardiac output × total peripheral resistance
Why this is the answer
Pressure = flow × resistance, so MAP ≈ CO × TPR. Changes in either cardiac output or vascular resistance change blood pressure.
Answer
A · Cardiac output × total peripheral resistance
4.Human physiology
Answer: C · High TSH and low free T4
Why this is the answer
When the thyroid fails, low T4 removes negative feedback and the pituitary raises TSH. Low TSH with low T4 would point to secondary (pituitary) hypothyroidism.
Answer
C · High TSH and low free T4
5.Microbiology
Answer: A · The place where the pathogen normally lives and multiplies
Why this is the answer
The chain of infection links the agent, reservoir, portal of exit, mode of transmission, portal of entry and susceptible host. Breaking any link stops transmission.
Answer
A · The place where the pathogen normally lives and multiplies
6.Microbiology
Answer: D · Botulinum toxin, which blocks acetylcholine release
Why this is the answer
Clostridium botulinum can grow in poorly canned, low-acid foods and produce botulinum toxin. The toxin blocks acetylcholine release at nerve endings, causing descending flaccid paralysis.
Answer
D · Botulinum toxin, which blocks acetylcholine release
7.Pharmacology
Answer: B · A stimulant laxative such as senna, often with a stool softener
Why this is the answer
Opioid-induced constipation does not usually resolve with time, so a laxative is started with the opioid. Stimulant laxatives counter reduced motility; bulk-forming laxatives alone can worsen symptoms if fluid intake is low.
Answer
B · A stimulant laxative such as senna, often with a stool softener
8.Pharmacology
Answer: B · It reduces activity below the baseline level
Why this is the answer
A neutral antagonist simply blocks agonists and leaves baseline activity unchanged. An inverse agonist stabilises the inactive receptor state, so constitutive activity falls below baseline.
Answer
B · It reduces activity below the baseline level
9.Arterial blood gas interpretation
Answer: partially compensated
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 7.29: acidaemia (below 7.35)
PaCO₂ 63 mmHg: high (35–45)
HCO₃⁻ 29 mmol/L: high (22–26)
Compensation
partially compensated
Answer
partially compensated
10.Arterial blood gas interpretation
Answer: chronic
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 HCO₃⁻
acute: 24 + 1·3.2 = 27.2 mmol/L
chronic: 24 + 3.5·3.2 = 35.2 mmol/L
Compare
measured: 35 mmol/L → chronic
Answer
chronic
11.ECG rhythm strip interpretation
Answer: atrial flutter, 3:1 conduction
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: R–R = 18 small boxes, 1500 ÷ 18 ≈ 83 per minute.
2. Rhythm: regular, every R–R is 0.72 s.
3. P waves: none; regular sawtooth flutter waves every 0.24 s (250 per minute), 3 for each QRS.
4. PR interval: not measurable.
5. QRS: 0.06 s, narrow (below 0.12 s).
Interpretation
atrial flutter, 3:1 conduction
Answer
atrial flutter, 3:1 conduction
12.ECG rhythm strip interpretation
Answer: atrial rate = 79 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 = 19 small boxes = 0.76 s → 1500 ÷ 19 = 78.95 ≈ 79 per minute
Ventricular rate
R–R = 42 small boxes = 1.68 s → 1500 ÷ 42 = 35.71 ≈ 36 per minute
Answer
atrial rate = 79 bpm
Answer
atrial rate = 79 bpm
Third-degree block with a wide-complex ventricular escape (usually 20–40 per minute).
13.Evidence-based practice statistics
Answer: LR+ = 4.78
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.
Positive likelihood ratio
LR+ = 0.86/(1 − 0.82) = 4.778 ≈ 4.78
Answer
LR+ = 4.78
14.Evidence-based practice statistics
Answer: TP = 285
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.