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 group on the front of the thigh extends the knee?
AGluteal muscles
BHamstrings
CQuadriceps femoris
DAdductors
2.Human anatomyHard
After axillary lymph node surgery, a patient's scapula sticks out from the chest wall when pushing against a wall. Which nerve has been damaged?
ALong thoracic nerve
BAxillary nerve
CThoracodorsal nerve
DSuprascapular nerve
3.Human physiologyMedium
In skeletal muscle, where does most of the calcium for contraction come from?
AThe nucleus
BThe extracellular fluid
CThe sarcoplasmic reticulum
DThe mitochondria
4.Human physiologyHard
In pernicious anaemia, autoimmune destruction of gastric parietal cells leads to anaemia because:
AAcid is lost, so iron is never absorbed in any form
BGastrin is lost, so the bone marrow is no longer stimulated
CIntrinsic factor is lost, so vitamin B12 is not absorbed
DPepsin is lost, so haemoglobin cannot be made
5.MicrobiologyMedium
Which cells does HIV mainly infect and destroy?
ANeutrophils
BCD4 T helper cells
CRed blood cells
DPlatelets
6.MicrobiologyHard
An older adult develops a painful blistering rash in a band on one side of the chest. What explains this pattern?
ABacteria spread along the lymphatic vessels of the chest
BA new chickenpox infection spreads through the blood to one side
CAn allergic reaction to clothing affects one dermatome
DVaricella-zoster virus reactivates from one sensory ganglion and travels along its nerve
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
BNo laxative until the patient has not opened the bowels for a week
CA stimulant laxative such as senna, often with a stool softener
DBulk-forming fibre alone, with fluid intake restricted
8.PharmacologyHard
A patient with angina is taught to use sublingual glyceryl trinitrate (nitroglycerin). Which statement shows a need for further teaching?
A"I will sit down before I take the tablet, because it can make me feel faint or dizzy."
B"I may get a headache after taking it."
C"I will replace my tablets before the expiry date."
D"If the pain is no better after one tablet, I will wait an hour before calling for help."
9.Arterial blood gas interpretationMedium
A practice arterial blood gas shows pH 7.37, PaCO₂ 18 mmHg (2.4 kPa) and HCO₃⁻ 10 mmol/L. The primary disorder is metabolic acidosis. How far is it compensated? Click your answer.
pH 7.37PaCO₂ 18 mmHg (2.4 kPa)HCO₃⁻ 10 mmol/L
1uncompensated
2partially compensated
3fully compensated
10.Arterial blood gas interpretationHard
A practice gas shows a respiratory acidosis: pH 7.22, PaCO₂ 85 mmHg (11.3 kPa), HCO₃⁻ 34 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 7.22PaCO₂ 85 mmHg (11.3 kPa)HCO₃⁻ 34 mmol/L
1acute
2chronic
3acute on chronic
11.ECG rhythm strip interpretationMedium
This lead II strip shows sinus rhythm with one pause. Measure the pause, the R–R interval across it, in seconds.
lead II, 25 mm/s, 10 mm/mV1 small box = 0.04 s, 1 large box = 0.20 sstrip length 6 s
pause = s
12.ECG rhythm strip interpretationHard
Identify the rhythm on this lead II strip. 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 tachycardia
2ventricular fibrillation
3torsades de pointes
4atrial fibrillation
13.Evidence-based practice statisticsMedium
Practice exercise (made-up values, not clinical guidance). A case-control study compares 100 patients with a disease (cases) and 680 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 = 36b = 360c = 64d = 320
OR =
14.Evidence-based practice statisticsHard
Practice exercise (made-up values, not clinical guidance). A patient has a pre-test probability of 15% for a disease. A test with sensitivity 70% and specificity 81% comes back positive. Find the post-test probability p′ of the disease as a percentage. Round to one decimal place.
pre-test 15%Se = 70%Sp = 81%positive
p′ = %
3Solutions
1.Human anatomy
Answer: C · Quadriceps femoris
Why this is the answer
Quadriceps femoris (rectus femoris and three vasti) extends the knee through the patellar ligament. The hamstrings flex it.
Answer
C · Quadriceps femoris
2.Human anatomy
Answer: A · Long thoracic nerve
Why this is the answer
The long thoracic nerve supplies serratus anterior, which holds the scapula against the chest wall. Damage causes a winged scapula.
Answer
A · Long thoracic nerve
3.Human physiology
Answer: C · The sarcoplasmic reticulum
Why this is the answer
Depolarisation of the T-tubules opens ryanodine receptors in the sarcoplasmic reticulum, releasing stored calcium into the cytoplasm.
Answer
C · The sarcoplasmic reticulum
4.Human physiology
Answer: C · Intrinsic factor is lost, so vitamin B12 is not absorbed
Why this is the answer
Parietal cells secrete intrinsic factor, needed for B12 absorption in the terminal ileum. B12 deficiency impairs DNA synthesis and causes megaloblastic anaemia.
Answer
C · Intrinsic factor is lost, so vitamin B12 is not absorbed
5.Microbiology
Answer: B · CD4 T helper cells
Why this is the answer
HIV binds CD4 and a co-receptor on T helper cells. Loss of these cells weakens both antibody and cell-mediated immunity, leading to AIDS.
Answer
B · CD4 T helper cells
6.Microbiology
Answer: D · Varicella-zoster virus reactivates from one sensory ganglion and travels along its nerve
Why this is the answer
After chickenpox, VZV stays latent in a dorsal root or cranial nerve ganglion. When immunity weakens it can reactivate, travelling down that nerve to cause shingles in its dermatome.
Answer
D · Varicella-zoster virus reactivates from one sensory ganglion and travels along its nerve
7.Pharmacology
Answer: C · 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
C · A stimulant laxative such as senna, often with a stool softener
8.Pharmacology
Answer: D · "If the pain is no better after one tablet, I will wait an hour before calling for help."
Why this is the answer
Unrelieved chest pain may be a heart attack, so help must not be delayed. In US teaching, emergency services are called if pain is not better 5 minutes after the first dose; UK advice is to repeat the dose after 5 minutes and call 999 if pain persists 5 minutes after the second. Sitting down prevents falls from hypotension, and headache is a common effect.
Answer
D · "If the pain is no better after one tablet, I will wait an hour before calling for help."
9.Arterial blood gas interpretation
Answer: fully 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 7.37: within 7.35–7.45
PaCO₂ 18 mmHg: low (35–45)
HCO₃⁻ 10 mmol/L: low (22–26)
Compensation
Clinically, also check the expected compensation: Winter’s formula expects PaCO₂ 21–25 mmHg, so the measured 18 mmHg points to an additional respiratory alkalosis (a mixed disorder). The answer above is the ROME exam classification.
fully compensated
Answer
fully compensated
10.Arterial blood gas interpretation
Answer: acute on 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 + 1·4.5 = 28.5 mmol/L
chronic: 24 + 3.5·4.5 = 39.75 mmol/L
Compare
measured: 34 mmol/L → acute on chronic
Answer
acute on chronic
11.ECG rhythm strip interpretation
Answer: pause = 1.68 s
Narrow-complex tachycardia and pauses: supraventricular tachycardia (SVT) is a regular tachycardia at about 150–250/min with narrow QRS complexes; the P waves are usually not visible because they are hidden in the QRS complex or the T wave. Sinus tachycardia in an adult at rest is usually below about 150/min and has a visible P wave before every QRS; atrial flutter with 2:1 conduction often runs at about 150/min and shows sawtooth waves. In sinus arrest the sinus node stops firing for a moment: a pause with no P wave and no QRS whose length is not a whole number of P–P cycles (a pause of exactly two or three cycles points to sinus exit block instead).
Measure
42 small boxes from R to R across the pause
Convert
pause = 42 × 0.04 s = 1.68 s
Compare with P–P
1.68 ÷ 0.72 ≈ 2.3 P–P cycles, not a whole number
Answer
pause = 1.68 s
No P wave appears in the pause, and the pause is not a whole number of P–P cycles: sinus arrest. A pause of exactly two or three cycles would point to sinus exit block.
12.ECG rhythm strip interpretation
Answer: ventricular tachycardia
Ventricular rhythms: ventricular tachycardia is a run of three or more wide (0.12 s or more) complexes in a row at more than 100/min, usually 150–250/min, regular, without related P waves. Ventricular fibrillation is a chaotic, irregular waveform with no identifiable P waves, QRS complexes or T waves and no countable rate. Asystole is an almost flat line with no ventricular activity; a flat line is always confirmed by checking the patient, the leads and the gain and a second lead. In resuscitation guidelines VF and pulseless VT are the shockable rhythms and asystole is not.
Five steps
1. Rate: R–R = 10 small boxes, 1500 ÷ 10 ≈ 150 per minute.
2. Rhythm: regular, every R–R is 0.40 s.
3. P waves: none identifiable.
4. PR interval: not measurable.
5. QRS: 0.14 s, wide (0.12 s or more).
Interpretation
ventricular tachycardia
Answer
ventricular tachycardia
In resuscitation guidelines VF and pulseless VT are the shockable rhythms. This is identification practice only.
13.Evidence-based practice statistics
Answer: OR = 0.5
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: 36/64 = 0.5625
controls: 360/320 = 1.125
Odds ratio
OR = (36·320)/(360·64) = 0.5
Answer
OR = 0.5
OR below 1: exposure is less common among the cases, which suggests a protective effect.
14.Evidence-based practice statistics
Answer: p′ = 39.4 %
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.