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 superficial vein in the front of the elbow is most often used for venepuncture?
AMedian cubital vein
BBrachial vein
CAxillary vein
DGreat saphenous vein
2.Human anatomyHard
A typical posterolateral disc herniation between L4 and L5 most often compresses which nerve root?
AL4
BS1
CL5
DL3
3.Human physiologyMedium
Which cells in the stomach secrete hydrochloric acid?
AChief cells
BG cells
CParietal cells
DGoblet cells
4.Human physiologyHard
A young woman has muscle weakness that gets worse with repeated use and improves with an acetylcholinesterase inhibitor. What is the most likely mechanism?
AA defect in the dystrophin protein of muscle
BDemyelination of central nervous system tracts
CLoss of motor neurons in the spinal cord
DAntibodies against acetylcholine receptors at the neuromuscular junction
5.MicrobiologyMedium
Which antibody class is the first to rise in a new infection?
AIgG
BIgA
CIgE
DIgM
6.MicrobiologyHard
Primary syphilis typically presents with:
ADamage to the heart and brain
BA painless ulcer (chancre) at the site of infection
CA widespread rash on the palms and soles
DA painful cluster of small blisters on the genitals or the mouth
7.PharmacologyMedium
Which technique is correct when giving subcutaneous enoxaparin from a prefilled syringe?
AInject into the abdomen away from the navel and do not rub the site afterwards
BMassage the site well for one minute after the injection
CInject into a vein on the back of the hand
DExpel the air bubble from the syringe first, then inject into the deltoid muscle
8.PharmacologyHard
A patient with severe chronic hyponatraemia receives hypertonic saline. Why is the rate of correction strictly limited?
ARaising sodium too fast causes severe hypokalaemia and cardiac arrest
BFast correction causes immediate kidney failure
CRaising sodium too fast can cause osmotic demyelination in the brain
DHypertonic saline is toxic to the liver at high rates
9.Arterial blood gas interpretationMedium
A practice gas shows PaCO₂ 64 mmHg (8.5 kPa). Use the rule that in chronic respiratory acidosis HCO₃⁻ rises by 3.5 mmol/L for every 10 mmHg that PaCO₂ is above 40, starting from HCO₃⁻ 24 mmol/L, to find the expected HCO₃⁻.
A practice gas shows pH 7.28, PaCO₂ 24 mmHg (3.2 kPa) and HCO₃⁻ 11 mmol/L, with Na⁺ 132 and Cl⁻ 86 mmol/L. Use the anion gap and the delta ratio to click the best interpretation.
Measure the QT interval and the R–R interval on this regular lead II strip, then calculate the rate-corrected QTc with Fridericia’s formula QTc = QT/∛RR (both in seconds). Give the answer in seconds to three decimals.
lead II, 25 mm/s, 10 mm/mV1 small box = 0.04 s, 1 large box = 0.20 sstrip length 6 s
QTc = s
12.ECG rhythm strip interpretationHard
This patient has a pacemaker. Interpret the 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
1sinus rhythm with wide QRS
2paced rhythm, failure to capture
3idioventricular rhythm
4ventricular paced rhythm
13.Evidence-based practice statisticsMedium
Practice exercise (made-up values, not clinical guidance). A test has a sensitivity of 88% and a specificity of 68%. Find the negative likelihood ratio LR−. Round to two decimal places.
Se = 88%Sp = 68%
LR− =
14.Evidence-based practice statisticsHard
Practice exercise (made-up values, not clinical guidance). A screening test has a sensitivity of 90% and a specificity of 83%. It is used in a population where the prevalence is 3%. Find the positive predictive value PPV as a percentage. Round to one decimal place.
Se = 90%Sp = 83%p = 3%
PPV = %
3Solutions
1.Human anatomy
Answer: A · Median cubital vein
Why this is the answer
The median cubital vein crosses the cubital fossa between the cephalic and basilic veins. It is superficial, large and fairly fixed, so it is easy to puncture.
Answer
A · Median cubital vein
2.Human anatomy
Answer: C · L5
Why this is the answer
A posterolateral L4–L5 herniation usually catches the traversing L5 root, causing weakness of big toe extension and numbness on the top of the foot.
Answer
C · L5
3.Human physiology
Answer: C · Parietal cells
Why this is the answer
Parietal cells secrete HCl and intrinsic factor. Chief cells secrete pepsinogen and G cells secrete gastrin.
Answer
C · Parietal cells
4.Human physiology
Answer: D · Antibodies against acetylcholine receptors at the neuromuscular junction
Why this is the answer
In myasthenia gravis, autoantibodies reduce the number of functional acetylcholine receptors. Blocking acetylcholinesterase leaves more ACh to bind the remaining receptors.
Answer
D · Antibodies against acetylcholine receptors at the neuromuscular junction
5.Microbiology
Answer: D · IgM
Why this is the answer
IgM appears first in a primary response; IgG follows and provides long-term protection. A positive IgM therefore suggests recent infection.
Answer
D · IgM
6.Microbiology
Answer: B · A painless ulcer (chancre) at the site of infection
Why this is the answer
Treponema pallidum causes a painless chancre in the primary stage. The secondary stage brings rash and systemic symptoms, and tertiary syphilis can damage the heart and nervous system. Painful blisters suggest herpes.
Answer
B · A painless ulcer (chancre) at the site of infection
7.Pharmacology
Answer: A · Inject into the abdomen away from the navel and do not rub the site afterwards
Why this is the answer
Enoxaparin is given deep subcutaneously, usually in the abdomen at least 5 cm from the umbilicus. The air bubble in the prefilled syringe is not expelled, and rubbing the site increases bruising.
Answer
A · Inject into the abdomen away from the navel and do not rub the site afterwards
8.Pharmacology
Answer: C · Raising sodium too fast can cause osmotic demyelination in the brain
Why this is the answer
In chronic hyponatraemia the brain adapts to low osmolality. Correcting sodium too rapidly shrinks brain cells and can cause osmotic demyelination (central pontine myelinolysis), so sodium is checked often.
Answer
C · Raising sodium too fast can cause osmotic demyelination in the brain
9.Arterial blood gas interpretation
Answer: HCO₃⁻ = 32.4 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
|64 − 40| = 24 mmHg → 2.4 × 10 mmHg
Expected HCO₃⁻
HCO₃⁻ = 24 + 3.5·2.4 = 32.4 mmol/L
Answer
HCO₃⁻ = 32.4 mmol/L
10.Arterial blood gas interpretation
Answer: pure high-gap 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.
Anion gap
AG = 132 − (86 + 11) = 35
Delta ratio
(35 − 12)/(24 − 11) = 1.769
pure high-gap acidosis
Answer
pure high-gap acidosis
11.ECG rhythm strip interpretation
Answer: QTc = 0.488 s
QT and QTc: the QT interval runs from the start of the QRS complex to the end of the T wave. It shortens as the heart rate rises, so it is corrected for rate with the R–R interval in seconds: Bazett QTc = QT/√RR and Fridericia QTc = QT/∛RR. Bazett is the formula most textbooks use; it over-corrects at fast rates, where Fridericia is preferred. Limits differ between references: a QTc above about 0.45 s in men or 0.46 s in women is usually called prolonged, and above 0.50 s markedly prolonged.
Measure
QT = 11.5 × 0.04 = 0.46 s
RR = 21 × 0.04 = 0.84 s
Fridericia’s formula
QTc = 0.46/∛0.84 = 0.46/0.9435 = 0.488 s
Answer
QTc = 0.488 s
Above the usual limits for both men (about 0.45 s) and women (about 0.46 s): prolonged. (488 ms)
12.ECG rhythm strip interpretation
Answer: ventricular paced rhythm
Paced rhythms: a pacemaker impulse shows as a narrow vertical pacing spike. After a ventricular spike the QRS is wide (0.12 s or more), because the impulse spreads from the lead through the muscle instead of the conduction system; with right-ventricular pacing it is often mostly negative in lead II. A spike before each P wave means atrial pacing, and a dual-chamber (AV) pacemaker shows an atrial spike and a P wave, then, after the programmed AV delay, a ventricular spike and a wide QRS. Capture means every spike is followed by a depolarisation. Failure to capture is a spike without a P wave or QRS after it; failure to sense (spikes that ignore the patient’s own beats) and failure to pace (no spike when one is due) are the other classic malfunctions.
Five steps
1. Rate: R–R = 21 small boxes, 1500 ÷ 21 ≈ 71 per minute.
2. Rhythm: regular, every R–R is 0.84 s.
3. P waves: none visible; a sharp pacing spike comes just before every QRS.
4. PR interval: not measurable.
5. QRS: 0.16 s, wide, and every spike is followed by a QRS (capture).
Interpretation
ventricular paced rhythm
Answer
ventricular paced rhythm
The paced QRS is wide by nature; do not call it a bundle branch block or a ventricular rhythm.
13.Evidence-based practice statistics
Answer: LR− = 0.18
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 − 0.88)/0.68 = 0.1765 ≈ 0.18
Answer
LR− = 0.18
14.Evidence-based practice statistics
Answer: PPV = 14.1 %
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.
True and false positives (per person)
Se·p = 0.9·0.03 = 0.027
(1 − Sp)·(1 − p) = 0.17·0.97 = 0.1649
Positive predictive value
PPV = 0.027/(0.027 + 0.1649) = 0.1407 ≈ 14.1%
Answer
PPV = 14.1%
The lower the prevalence, the lower the PPV of the same test.