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
Where does fertilisation most commonly take place?
AAmpulla of the uterine tube
BCervix
CBody of the uterus
DOvary
2.Human anatomyHard
Which muscle "unlocks" the fully extended knee by rotating the femur laterally on the tibia at the start of flexion?
AGastrocnemius
BBiceps femoris
CPopliteus
DSartorius
3.Human physiologyMedium
Since action potentials are all-or-none, how does a neuron signal a stronger stimulus?
ABy making each action potential larger
BBy making each action potential longer
CBy firing action potentials more often
DBy changing the neurotransmitter it releases
4.Human physiologyHard
A patient hyperventilating during a panic attack has tingling fingers. What would the arterial blood gas most likely show?
AHigh pH and high bicarbonate
BLow pH and high PaCO₂
CLow pH and low bicarbonate
DHigh pH and low PaCO₂
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 patient with suspected infectious pulmonary tuberculosis is admitted. Which precautions are needed?
ADroplet precautions with a surgical mask, and only within one metre
BContact precautions only, with gloves and an apron
CNo special precautions, because TB spreads only by blood
DAirborne precautions, ideally a negative-pressure room and respirator masks for staff
7.PharmacologyMedium
Which patient should usually not receive a live vaccine?
AA patient with mild seasonal allergies
BA patient receiving chemotherapy for leukaemia
CA healthy 1-year-old child
DA healthy adult nurse who works on a children's ward
8.PharmacologyHard
A patient on long-term lithium passes large amounts of dilute urine and is always thirsty. What is the most likely explanation?
ALithium causes type 1 diabetes by destroying the pancreatic beta cells
BThe patient is drinking too much and lithium is unrelated
CLithium blocks the kidney's response to ADH (nephrogenic diabetes insipidus)
DLithium increases ADH release from the pituitary
9.Arterial blood gas interpretationMedium
A practice arterial blood gas shows pH 7.37, PaCO₂ 61 mmHg (8.1 kPa) and HCO₃⁻ 34 mmol/L. The primary disorder is respiratory acidosis. How far is it compensated? Click your answer.
pH 7.37PaCO₂ 61 mmHg (8.1 kPa)HCO₃⁻ 34 mmol/L
1uncompensated
2partially compensated
3fully compensated
10.Arterial blood gas interpretationHard
A practice gas shows a respiratory acidosis: pH 7.30, PaCO₂ 82 mmHg (10.9 kPa), HCO₃⁻ 39 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 7.30PaCO₂ 82 mmHg (10.9 kPa)HCO₃⁻ 39 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
1junctional rhythm
2sinus tachycardia
3normal sinus rhythm
4sinus bradycardia
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 fibrillation
2torsades de pointes
3supraventricular tachycardia (SVT)
4idioventricular rhythm
13.Evidence-based practice statisticsMedium
Practice exercise (made-up values, not clinical guidance). In a cohort, groups of people were followed for different lengths of time (see the table: number of people and years of follow-up for each). During follow-up, 133 new cases occurred. Find the incidence rate IR per 1000 person-years. Round to one decimal place.
400 × 5 years560 × 2 years133 new cases
IR = per 1000 person-years
14.Evidence-based practice statisticsHard
Practice exercise (made-up values, not clinical guidance). A screening test has a sensitivity of 95% and a specificity of 90%. 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 = 95%Sp = 90%p = 3%
PPV = %
3Solutions
1.Human anatomy
Answer: A · Ampulla of the uterine tube
Why this is the answer
Fertilisation usually occurs in the ampulla of the uterine (fallopian) tube. The embryo then travels to the uterus to implant.
Answer
A · Ampulla of the uterine tube
2.Human anatomy
Answer: C · Popliteus
Why this is the answer
At full extension the knee locks by medial rotation of the femur. Popliteus laterally rotates the femur on the fixed tibia to unlock it so flexion can begin.
Answer
C · Popliteus
3.Human physiology
Answer: C · By firing action potentials more often
Why this is the answer
Stimulus strength is coded by the frequency of action potentials and by how many neurons are recruited, not by their amplitude.
Answer
C · By firing action potentials more often
4.Human physiology
Answer: D · High pH and low PaCO₂
Why this is the answer
Hyperventilation blows off CO₂ and causes acute respiratory alkalosis. Alkalosis increases calcium binding to albumin, lowering ionised calcium and causing tingling.
Answer
D · High pH and low PaCO₂
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 · Airborne precautions, ideally a negative-pressure room and respirator masks for staff
Why this is the answer
M. tuberculosis spreads in airborne droplet nuclei that can stay suspended for long periods. Patients need a single room with negative pressure where possible, and staff wear FFP2/FFP3 or N95 respirators.
Answer
D · Airborne precautions, ideally a negative-pressure room and respirator masks for staff
7.Pharmacology
Answer: B · A patient receiving chemotherapy for leukaemia
Why this is the answer
Live attenuated vaccines can cause serious infection in immunosuppressed people and are generally avoided in pregnancy. Inactivated vaccines are safe but may give a weaker response.
Answer
B · A patient receiving chemotherapy for leukaemia
8.Pharmacology
Answer: C · Lithium blocks the kidney's response to ADH (nephrogenic diabetes insipidus)
Why this is the answer
Lithium interferes with ADH action in the collecting duct, causing polyuria and polydipsia. Dehydration from this can raise lithium levels further, so fluid balance matters.
Answer
C · Lithium blocks the kidney's response to ADH (nephrogenic diabetes insipidus)
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 61 mmHg: high (35–45)
HCO₃⁻ 34 mmol/L: high (22–26)
Compensation
fully compensated
Answer
fully 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 + 1·4.2 = 28.2 mmol/L
chronic: 24 + 3.5·4.2 = 38.7 mmol/L
Compare
measured: 39 mmol/L → chronic
Answer
chronic
11.ECG rhythm strip interpretation
Answer: sinus bradycardia
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 = 34 small boxes, 1500 ÷ 34 ≈ 44 per minute.
2. Rhythm: regular, every R–R is 1.36 s.
3. P waves: upright, one before every QRS, all alike.
4. PR interval: 0.20 s, constant, within 0.12–0.20 s.
5. QRS: 0.08 s, narrow (below 0.12 s).
Interpretation
sinus bradycardia
Answer
sinus bradycardia
12.ECG rhythm strip interpretation
Answer: torsades de pointes
More ventricular rhythms: torsades de pointes is a polymorphic ventricular tachycardia, usually 150–250/min, in which the wide complexes change in height and twist around the baseline, pointing up for a few beats and then down. Textbooks link it to a prolonged QT interval, and magnesium sulfate is the drug resuscitation guidelines name for it. An idioventricular rhythm is a ventricular escape rhythm: regular, wide QRS (0.12 s or more), no P waves, at 20–40/min (an accelerated idioventricular rhythm runs at 40–100/min). Monomorphic VT has identical wide complexes at more than 100/min. VF is chaotic, without any repeating complex.
Five steps
1. Rate: very fast, the complexes are about 7 small boxes apart (about 214 per minute).
2. Rhythm: fast and roughly regular, but the height and direction of the complexes change in a repeating spindle pattern.
3. P waves: none identifiable.
4. PR interval: not measurable.
5. QRS: wide and polymorphic; the complexes twist around the baseline, pointing up for a few beats and then down.
Interpretation
torsades de pointes
Answer
torsades de pointes
Torsades de pointes is a polymorphic VT linked to a prolonged QT interval. This is identification practice only.
13.Evidence-based practice statistics
Answer: IR = 42.6 per 1000 person-years
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.
Person-years
PY = 400·5 + 560·2 = 3120
Incidence rate
IR = 133/3120·1000 = 42.6 per 1000 person-years
Answer
IR = 42.6 per 1000 person-years
14.Evidence-based practice statistics
Answer: PPV = 22.7 %
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 = 0.95·0.03 = 0.0285
(1 − Sp)·(1 − p) = 0.1·0.97 = 0.097
Positive predictive value
PPV = 0.0285/(0.0285 + 0.097) = 0.2271 ≈ 22.7%
Answer
PPV = 22.7%
The lower the prevalence, the lower the PPV of the same test.