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 part of the brainstem contains the main cardiovascular and respiratory centres?
AMedulla oblongata
BCerebellum
CMidbrain
DPons
2.Human anatomyHard
A footballer takes a blow to the outside of the planted knee. Which combination of injuries is classically called the "unhappy triad"?
AAnterior cruciate ligament, posterior cruciate ligament and patellar ligament
BPosterior cruciate ligament, lateral collateral ligament and lateral meniscus
CAnterior cruciate ligament, medial collateral ligament and medial meniscus
DMedial collateral ligament, lateral collateral ligament and patella
3.Human physiologyMedium
Which receptors respond mainly to a fall in arterial oxygen levels?
ABaroreceptors in the carotid sinus
BStretch receptors in the lungs
CCentral chemoreceptors in the medulla
DPeripheral chemoreceptors in the carotid and aortic bodies
4.Human physiologyHard
Why does breathing 100% oxygen correct hypoxaemia from a true right-to-left shunt only poorly?
AThe shunted blood never comes into contact with alveolar gas
BThe extra oxygen is all exhaled as CO₂
CPure oxygen damages haemoglobin
DShunted blood is already fully saturated
5.MicrobiologyMedium
In which phase of the bacterial growth curve do the cells divide at their fastest constant rate?
ALag (adaptation) phase
BStationary (plateau) phase
CDeath phase
DLog (exponential) phase
6.MicrobiologyHard
A healthcare worker's blood test is positive for anti-HBs (hepatitis B surface antibody) and negative for HBsAg and anti-HBc. What does this most likely mean?
APast natural infection that has cleared
BImmunity from hepatitis B vaccination
CCurrent acute hepatitis B infection
DChronic hepatitis B infection
7.PharmacologyMedium
A patient is starting olanzapine. Which monitoring is most important over the following months?
AINR and platelet count
BWeight, blood glucose and blood lipids
CHearing tests and vision tests every month
DPeak expiratory flow
8.PharmacologyHard
Morphine 4 mg IV is ordered. The ampoule contains 10 mg/mL. What volume should be drawn up?
A0.04 mL
B0.4 mL
C4 mL
D2.5 mL
9.Arterial blood gas interpretationMedium
A practice arterial blood gas shows pH 7.38, PaCO₂ 51 mmHg (6.8 kPa) and HCO₃⁻ 29 mmol/L. The primary disorder is respiratory acidosis. How far is it compensated? Click your answer.
pH 7.38PaCO₂ 51 mmHg (6.8 kPa)HCO₃⁻ 29 mmol/L
1uncompensated
2partially compensated
3fully compensated
10.Arterial blood gas interpretationHard
Practice values: Na⁺ 143, Cl⁻ 112 and HCO₃⁻ 11 mmol/L. Calculate the delta ratio (AG − 12)/(24 − HCO₃⁻), using AG = Na⁺ − (Cl⁻ + HCO₃⁻). Give two decimals.
Na⁺ 143 mmol/LCl⁻ 112 mmol/LHCO₃⁻ 11 mmol/L
delta ratio =
11.ECG rhythm strip interpretationMedium
Measure the QT interval and the R–R interval on this regular lead II strip, then calculate the rate-corrected QTc with Bazett’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
Interpret this lead II strip with the five steps in order: rate, rhythm, P waves, PR interval, QRS duration. At which step do you find the first abnormal finding? Click it.
lead II, 25 mm/s, 10 mm/mV1 small box = 0.04 s, 1 large box = 0.20 sstrip length 6 s
1rate
2rhythm
3P waves
4PR interval
5QRS duration
13.Evidence-based practice statisticsMedium
Practice exercise (made-up values, not clinical guidance). In a randomised trial, 110 patients received a new treatment and 250 received the control treatment. The table shows how many in each group had the outcome (an unwanted event such as a stroke). Find the absolute risk reduction ARR as a percentage. Round to one decimal place.
treatment: 32/110control: 98/250
ARR = %
14.Evidence-based practice statisticsHard
Practice exercise (made-up values, not clinical guidance). A patient has a pre-test probability of 5% for a disease. A test with sensitivity 84% and specificity 89% comes back positive. Find the post-test probability p′ of the disease as a percentage. Round to one decimal place.
pre-test 5%Se = 84%Sp = 89%positive
p′ = %
3Solutions
1.Human anatomy
Answer: A · Medulla oblongata
Why this is the answer
The medulla oblongata contains the vital centres that regulate heart rate, blood pressure and breathing rhythm.
Answer
A · Medulla oblongata
2.Human anatomy
Answer: C · Anterior cruciate ligament, medial collateral ligament and medial meniscus
Why this is the answer
A valgus force with rotation tears the MCL, the ACL and the medial meniscus, which is firmly attached to the MCL.
Answer
C · Anterior cruciate ligament, medial collateral ligament and medial meniscus
3.Human physiology
Answer: D · Peripheral chemoreceptors in the carotid and aortic bodies
Why this is the answer
The carotid and aortic bodies detect low PaO₂ (and also high CO₂ and H⁺). The central chemoreceptors respond to CO₂ but not directly to oxygen.
Answer
D · Peripheral chemoreceptors in the carotid and aortic bodies
4.Human physiology
Answer: A · The shunted blood never comes into contact with alveolar gas
Why this is the answer
Shunted blood bypasses ventilated alveoli and mixes deoxygenated blood into arterial blood. Extra inspired oxygen cannot reach it, unlike in V/Q mismatch or hypoventilation.
Answer
A · The shunted blood never comes into contact with alveolar gas
5.Microbiology
Answer: D · Log (exponential) phase
Why this is the answer
After the lag phase of adjustment, cells enter the log phase and grow exponentially. Growth stops in the stationary phase as nutrients run out and waste builds up.
Answer
D · Log (exponential) phase
6.Microbiology
Answer: B · Immunity from hepatitis B vaccination
Why this is the answer
The vaccine contains HBsAg, so a vaccinated person makes anti-HBs but no anti-HBc. Past natural infection gives both anti-HBs and anti-HBc, and ongoing infection shows HBsAg.
Answer
B · Immunity from hepatitis B vaccination
7.Pharmacology
Answer: B · Weight, blood glucose and blood lipids
Why this is the answer
Olanzapine and several other atypical antipsychotics cause weight gain, insulin resistance and dyslipidaemia. Weight, waist size, glucose and lipids are checked regularly.
Answer
B · Weight, blood glucose and blood lipids
8.Pharmacology
Answer: B · 0.4 mL
Why this is the answer
4 mg ÷ 10 mg/mL = 0.4 mL. The unused remainder of a controlled drug is discarded with a witness.
Answer
B · 0.4 mL
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 7.38: within 7.35–7.45
PaCO₂ 51 mmHg: high (35–45)
HCO₃⁻ 29 mmol/L: high (22–26)
Compensation
fully compensated
Answer
fully compensated
10.Arterial blood gas interpretation
Answer: delta ratio = 0.6154
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 = 143 − (112 + 11) = 20
Delta ratio
(20 − 12)/(24 − 11) = 8/13 = 0.6154
Answer
delta ratio = 0.6154
Below 1: an added normal anion gap acidosis.
11.ECG rhythm strip interpretation
Answer: QTc = 0.415 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 = 9.5 × 0.04 = 0.38 s
RR = 21 × 0.04 = 0.84 s
Bazett’s formula
QTc = 0.38/√0.84 = 0.38/0.9165 = 0.415 s
Answer
QTc = 0.415 s
Within the usual limits (about 0.45 s for men, 0.46 s for women). (415 ms)
12.ECG rhythm strip interpretation
Answer: PR interval
Five steps for every strip: 1. rate (300 or 1500 method, or the 6-second method if irregular); 2. rhythm: are the R–R intervals equal; 3. P waves: present, upright in lead II, one before every QRS and all alike; 4. PR interval: 0.12–0.20 s and constant; 5. QRS duration: below 0.12 s. Only after the five steps name the rhythm. Normal sinus rhythm passes all five: 60–100/min, regular, upright P before every QRS, PR 0.12–0.20 s, QRS below 0.12 s.
Five steps
1. Rate: R–R = 20 small boxes, 1500 ÷ 20 ≈ 75 per minute.
2. Rhythm: regular, every R–R is 0.80 s.
3. P waves: upright, one before every QRS, all alike.
4. PR interval: 0.26 s, constant, longer than 0.20 s.
5. QRS: 0.10 s, narrow (below 0.12 s).
First abnormal step
PR interval → first-degree AV block
Answer
PR interval
13.Evidence-based practice statistics
Answer: ARR = 10.1 %
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.
Event rates
EER = 32/110 = 0.2909
CER = 98/250 = 0.392
Absolute risk reduction
ARR = 0.392 − 0.2909 = 0.1011 ≈ 10.1%
Answer
ARR = 10.1%
14.Evidence-based practice statistics
Answer: p′ = 28.7 %
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.