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 controls most of the muscles of facial expression?
ATrigeminal nerve (V)
BOculomotor nerve (III)
CFacial nerve (VII)
DHypoglossal nerve (XII)
2.Human anatomyHard
Where does the thoracic duct usually empty its lymph into the blood?
AInferior vena cava at the level of the kidneys
BJunction of the left subclavian and left internal jugular veins
CJunction of the right subclavian and right internal jugular veins
DRight atrium directly
3.Human physiologyMedium
What is the main function of pulmonary surfactant?
ATo trap dust in the bronchi
BTo reduce surface tension in the alveoli
CTo carry oxygen across the alveolar wall
DTo kill bacteria in the airways
4.Human physiologyHard
Why does a person with lactase deficiency get diarrhoea and bloating after drinking milk?
AMilk protein triggers an allergic reaction
BUndigested lactose draws water into the gut and is fermented by colonic bacteria
CLactose damages the villi directly
DLactose is absorbed too quickly and raises blood glucose
5.MicrobiologyMedium
Which organism is a common cause of infection in burns and in people with cystic fibrosis, and can produce a green pigment?
AStreptococcus pyogenes
BPseudomonas aeruginosa
CHelicobacter pylori
DClostridium tetani
6.MicrobiologyHard
Prions differ from all other infectious agents because they:
AAre killed by normal cooking and disinfection
BCan only infect bacteria
CContain only RNA, with no protein coat surrounding the genome
DContain no nucleic acid and consist of misfolded protein
7.PharmacologyMedium
A patient taking levodopa finds that the drug works poorly after large meals. Which advice is most appropriate?
ATake every dose with a large glass of milk and a protein snack
BDouble the dose after each meal
CTake the drug only at bedtime
DAvoid taking doses at the same time as high-protein meals
8.PharmacologyHard
A patient on disulfiram becomes flushed, nauseated and has palpitations after using a mouthwash. What is the most likely cause?
ALow blood glucose from skipping breakfast
BAn allergy to mint flavouring in the mouthwash
CDisulfiram toxicity from taking too many tablets by mistake
DAlcohol in the mouthwash caused a disulfiram–alcohol reaction
9.Arterial blood gas interpretationMedium
A practice arterial blood gas shows pH 7.52, PaCO₂ 39 mmHg (5.2 kPa) and HCO₃⁻ 31 mmol/L. The primary disorder is metabolic alkalosis. How far is it compensated? Click your answer.
pH 7.52PaCO₂ 39 mmHg (5.2 kPa)HCO₃⁻ 31 mmol/L
1uncompensated
2partially compensated
3fully compensated
10.Arterial blood gas interpretationHard
Practice values: Na⁺ 132, Cl⁻ 92 and HCO₃⁻ 17 mmol/L. Calculate the delta ratio (AG − 12)/(24 − HCO₃⁻), using AG = Na⁺ − (Cl⁻ + HCO₃⁻). Give two decimals.
Na⁺ 132 mmol/LCl⁻ 92 mmol/LHCO₃⁻ 17 mmol/L
delta ratio =
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
1sinus bradycardia
2normal sinus rhythm
3sinus arrhythmia
4sinus rhythm with PVCs
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
1torsades de pointes
2asystole
3ventricular fibrillation
4ventricular tachycardia
13.Evidence-based practice statisticsMedium
Practice exercise (made-up values, not clinical guidance). A test has a sensitivity of 63% and a specificity of 61%. Find the negative likelihood ratio LR−. Round to two decimal places.
Se = 63%Sp = 61%
LR− =
14.Evidence-based practice statisticsHard
Practice exercise (made-up values, not clinical guidance). A screening test has a sensitivity of 72% and a specificity of 84%. 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 = 72%Sp = 84%p = 0.5%
NPV = %
3Solutions
1.Human anatomy
Answer: C · Facial nerve (VII)
Why this is the answer
The facial nerve (cranial nerve VII) supplies the muscles of facial expression. Damage causes facial palsy, as in Bell's palsy.
Answer
C · Facial nerve (VII)
2.Human anatomy
Answer: B · Junction of the left subclavian and left internal jugular veins
Why this is the answer
The thoracic duct drains lymph from both legs, the abdomen and the left side of the upper body into the left venous angle. The right lymphatic duct drains the rest into the right venous angle.
Answer
B · Junction of the left subclavian and left internal jugular veins
3.Human physiology
Answer: B · To reduce surface tension in the alveoli
Why this is the answer
Surfactant, made by type II alveolar cells, lowers surface tension so alveoli do not collapse and the lungs are easier to inflate.
Answer
B · To reduce surface tension in the alveoli
4.Human physiology
Answer: B · Undigested lactose draws water into the gut and is fermented by colonic bacteria
Why this is the answer
Unabsorbed lactose acts osmotically, pulling water into the lumen. Colonic bacteria ferment it, producing gas and short-chain fatty acids.
Answer
B · Undigested lactose draws water into the gut and is fermented by colonic bacteria
5.Microbiology
Answer: B · Pseudomonas aeruginosa
Why this is the answer
Pseudomonas aeruginosa is an opportunistic pathogen of burns, ventilated patients and cystic fibrosis lungs. It is naturally resistant to many antibiotics.
Answer
B · Pseudomonas aeruginosa
6.Microbiology
Answer: D · Contain no nucleic acid and consist of misfolded protein
Why this is the answer
Prions are misfolded proteins that induce normal proteins to misfold. They contain no DNA or RNA and are very resistant to standard sterilisation.
Answer
D · Contain no nucleic acid and consist of misfolded protein
7.Pharmacology
Answer: D · Avoid taking doses at the same time as high-protein meals
Why this is the answer
Dietary amino acids compete with levodopa for absorption in the gut and transport into the brain. Spreading protein through the day or timing doses away from high-protein meals improves the effect.
Answer
D · Avoid taking doses at the same time as high-protein meals
8.Pharmacology
Answer: D · Alcohol in the mouthwash caused a disulfiram–alcohol reaction
Why this is the answer
Disulfiram reactions can occur with small amounts of alcohol in mouthwash, cough syrups, sauces or aftershave. Patients are taught to check labels and avoid all alcohol-containing products.
Answer
D · Alcohol in the mouthwash caused a disulfiram–alcohol reaction
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 7.52: alkalaemia (above 7.45)
PaCO₂ 39 mmHg: normal (35–45)
HCO₃⁻ 31 mmol/L: high (22–26)
Compensation
Clinically, also check the expected compensation: for a metabolic alkalosis PaCO₂ ≈ 0.7·HCO₃⁻ + 21 = 40.7–44.7 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: delta ratio = 1.571
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 − (92 + 17) = 23
Delta ratio
(23 − 12)/(24 − 17) = 11/7 = 1.571
Answer
delta ratio = 1.571
Between 1 and 2: a pure high anion gap acidosis.
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: 7 QRS complexes in 6 s, × 10 = 70 per minute.
2. Rhythm: irregular, R–R from 0.74 to 0.98 s, lengthening and shortening gradually.
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).
Interpretation
sinus arrhythmia
Answer
sinus arrhythmia
12.ECG rhythm strip interpretation
Answer: ventricular fibrillation
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: no countable ventricular rate.
2. Rhythm: chaotic, no repeating pattern.
3. P waves: none identifiable.
4. PR interval: not measurable.
5. QRS: no QRS complexes.
Interpretation
ventricular fibrillation
Answer
ventricular fibrillation
In resuscitation guidelines VF and pulseless VT are the shockable rhythms. This is identification practice only.
13.Evidence-based practice statistics
Answer: LR− = 0.61
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 − 0.63)/0.61 = 0.6066 ≈ 0.61
Answer
LR− = 0.61
14.Evidence-based practice statistics
Answer: NPV = 99.8 %
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.