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?
ABrachial vein
BAxillary vein
CGreat saphenous vein
DMedian cubital vein
2.Human anatomyHard
Which ligament of the knee prevents the tibia from sliding forward relative to the femur?
AAnterior cruciate ligament
BPosterior cruciate ligament
CMedial collateral ligament
DPatellar ligament
3.Human physiologyMedium
How is the ejection fraction calculated?
AStroke volume ÷ end-diastolic volume
BEnd-systolic volume ÷ stroke volume
CCardiac output ÷ heart rate
DEnd-diastolic volume ÷ end-systolic volume
4.Human physiologyHard
Spirometry shows reduced FEV1 and reduced FVC, with a normal or high FEV1/FVC ratio. What pattern is this?
AA restrictive pattern, as in pulmonary fibrosis
BAn obstructive pattern, as in emphysema or chronic bronchitis
CAn obstructive pattern, as in asthma
DA normal result in an older adult
5.MicrobiologyMedium
In the chain of infection, what is the "reservoir"?
AThe person who is most at risk of infection
BThe way the pathogen enters a new host
CThe place where the pathogen normally lives and multiplies
DThe route by which the pathogen leaves the infected host's body
6.MicrobiologyHard
Which bacterial toxin causes the muscle spasms of tetanus?
ACholera toxin, which activates adenylate cyclase
BBotulinum toxin, which blocks acetylcholine release at the neuromuscular junction
CEndotoxin from the outer membrane
DTetanospasmin, which blocks inhibitory neurotransmitter release
7.PharmacologyMedium
A patient receiving chemotherapy has a very low neutrophil count and is going home. Which teaching is most important?
AEat plenty of raw shellfish to boost protein
BExercise in crowded gyms to build strength
CReport a temperature of 38 °C (100.4 °F) or higher straight away
DTake paracetamol regularly so that fevers do not show
8.PharmacologyHard
Why can an ACE inhibitor cause acute kidney injury in a patient with bilateral renal artery stenosis?
AThe drug crystallises in the renal tubules
BGFR depends on angiotensin II constricting the efferent arteriole
CACE inhibitors are directly toxic to the proximal tubule cells in the kidney
DACE inhibitors constrict the afferent arteriole
9.Arterial blood gas interpretationMedium
A practice arterial blood gas shows pH 7.56, PaCO₂ 49 mmHg (6.5 kPa) and HCO₃⁻ 42 mmol/L. The primary disorder is metabolic alkalosis. How far is it compensated? Click your answer.
pH 7.56PaCO₂ 49 mmHg (6.5 kPa)HCO₃⁻ 42 mmol/L
1uncompensated
2partially compensated
3fully compensated
10.Arterial blood gas interpretationHard
Practice values: Na⁺ 141, Cl⁻ 103, HCO₃⁻ 18 mmol/L and albumin 25 g/L. Find the albumin-corrected anion gap AGc = AG + 0.25·(40 − albumin).
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 arrhythmia
2second-degree AV block type II
3sinus rhythm with sinus arrest
4normal sinus rhythm
12.ECG rhythm strip interpretationHard
This lead II strip shows a conduction problem between the atria and the ventricles. 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
1first-degree AV block
2junctional rhythm
3third-degree (complete) AV block
4second-degree AV block type I (Wenckebach)
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, 7 new cases occurred. Find the incidence rate IR per 1000 person-years. Round to one decimal place.
310 × 2.5 years340 × 1 year7 new cases
IR = per 1000 person-years
14.Evidence-based practice statisticsHard
Practice exercise (made-up values, not clinical guidance). 10 000 people are screened with a test of sensitivity 84% and specificity 85%. The prevalence of the disease is 10%. How many true-positive results TP are expected?
N = 10 000p = 10%Se = 84%Sp = 85%
TP =
3Solutions
1.Human anatomy
Answer: D · 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
D · Median cubital vein
2.Human anatomy
Answer: A · Anterior cruciate ligament
Why this is the answer
The anterior cruciate ligament limits anterior translation of the tibia; a positive anterior drawer or Lachman test suggests an ACL tear.
Answer
A · Anterior cruciate ligament
3.Human physiology
Answer: A · Stroke volume ÷ end-diastolic volume
Why this is the answer
Ejection fraction is the proportion of blood in the full ventricle that is ejected per beat. A normal value is about 55–70%.
Answer
A · Stroke volume ÷ end-diastolic volume
4.Human physiology
Answer: A · A restrictive pattern, as in pulmonary fibrosis
Why this is the answer
In restrictive disease the lungs are small or stiff, so both FEV1 and FVC fall but the ratio is preserved. Obstruction lowers FEV1 more than FVC.
Answer
A · A restrictive pattern, as in pulmonary fibrosis
5.Microbiology
Answer: C · 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
C · The place where the pathogen normally lives and multiplies
6.Microbiology
Answer: D · Tetanospasmin, which blocks inhibitory neurotransmitter release
Why this is the answer
Tetanospasmin from Clostridium tetani blocks the release of GABA and glycine from inhibitory interneurons, causing spastic paralysis. Botulinum toxin, by contrast, causes flaccid paralysis.
Answer
D · Tetanospasmin, which blocks inhibitory neurotransmitter release
7.Pharmacology
Answer: C · Report a temperature of 38 °C (100.4 °F) or higher straight away
Why this is the answer
Neutropenic patients may have few signs of infection other than fever, and sepsis can develop quickly. A fever needs urgent assessment; good hand hygiene and avoiding sick contacts are also taught.
Answer
C · Report a temperature of 38 °C (100.4 °F) or higher straight away
8.Pharmacology
Answer: B · GFR depends on angiotensin II constricting the efferent arteriole
Why this is the answer
When renal perfusion is low, angiotensin II constricts the efferent arteriole to maintain glomerular pressure. Blocking it lets the pressure and GFR fall, so kidney function can drop sharply.
Answer
B · GFR depends on angiotensin II constricting the efferent arteriole
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 7.56: alkalaemia (above 7.45)
PaCO₂ 49 mmHg: high (35–45)
HCO₃⁻ 42 mmol/L: high (22–26)
Compensation
partially compensated
Answer
partially compensated
10.Arterial blood gas interpretation
Answer: AGc = 23.75 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.
Anion gap
AG = 141 − (103 + 18) = 20
Correction for albumin
20 + 0.25·(40 − 25) = 23.75 mmol/L
Answer
AGc = 23.75 mmol/L
Above about 12 after correction: a high anion gap.
11.ECG rhythm strip interpretation
Answer: sinus rhythm with sinus arrest
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. 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).
Five steps
1. Rate: 6 QRS complexes in 6 s, × 10 = 60 per minute.
2. Rhythm: regular (R–R 0.84 s) except for one pause of 1.92 s, which is not a whole number of P–P cycles.
3. P waves: upright, one before every QRS; no P wave at all during the pause.
4. PR interval: 0.18 s, constant, within 0.12–0.20 s.
5. QRS: 0.08 s, narrow (below 0.12 s).
Interpretation
sinus rhythm with sinus arrest
Answer
sinus rhythm with sinus arrest
12.ECG rhythm strip interpretation
Answer: third-degree (complete) AV block
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.
Five steps
1. Rate: R–R = 30 small boxes, 1500 ÷ 30 ≈ 50 per minute.
2. Rhythm: R–R regular (1.20 s) and P–P regular (0.84 s), but the two are independent.
3. P waves: upright and regular at 71 per minute, but not linked to the QRS complexes; some hide in a QRS or a T wave.
4. PR interval: varies from beat to beat; there is no fixed relation.
5. QRS: 0.08 s, narrow (below 0.12 s).
Interpretation
third-degree (complete) AV block
Answer
third-degree (complete) AV block
Type II and third-degree blocks are the ones that can suddenly progress, so textbooks rank them as more serious than type I.
13.Evidence-based practice statistics
Answer: IR = 6.3 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 = 310·2.5 + 340·1 = 1115
Incidence rate
IR = 7/1115·1000 = 6.3 per 1000 person-years
Answer
IR = 6.3 per 1000 person-years
14.Evidence-based practice statistics
Answer: TP = 840
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.