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 brain is chiefly responsible for coordination and balance?
ACerebellum
BThalamus
CHypothalamus
DMedulla oblongata
2.Human anatomyHard
After a difficult delivery with traction on the neck, a baby's arm hangs by the side, internally rotated with the forearm pronated ("waiter's tip"). Which part of the brachial plexus is injured?
ALower trunk (C8–T1)
BPosterior cord
CMedial cord
DUpper trunk (C5–C6)
3.Human physiologyMedium
Vital capacity is the sum of which lung volumes?
AInspiratory reserve volume and residual volume
BTidal volume and residual volume
CTidal volume, inspiratory reserve volume and expiratory reserve volume
DResidual volume and expiratory reserve volume
4.Human physiologyHard
In the renin–angiotensin–aldosterone system, where is angiotensin I mainly converted to angiotensin II?
AIn the kidneys' juxtaglomerular cells
BIn the lungs
CIn the liver
DIn the adrenal cortex
5.MicrobiologyMedium
How does an autoclave sterilise instruments?
AWith alcohol vapour at room temperature
BWith dry heat at about 60 °C
CWith ultraviolet light only
DWith steam under pressure at about 121 °C
6.MicrobiologyHard
Which bacterial toxin causes the muscle spasms of tetanus?
AEndotoxin from the outer membrane
BTetanospasmin, which blocks inhibitory neurotransmitter release
CBotulinum toxin, which blocks acetylcholine release at the neuromuscular junction
DCholera toxin, which activates adenylate cyclase
7.PharmacologyMedium
When does oseltamivir give the most benefit in influenza?
AAt any time, as the timing makes no difference
BWhen started within 48 hours of the first symptoms
COnly after symptoms have lasted at least one week
DOnly after a bacterial pneumonia has developed
8.PharmacologyHard
Probenecid has been used to raise plasma penicillin levels. How does it do this?
AIt blocks the tubular secretion of penicillin in the kidney
BIt displaces penicillin from plasma proteins
CIt inhibits the liver metabolism of penicillin
DIt increases the absorption of penicillin from the small intestine
9.Arterial blood gas interpretationMedium
A practice gas shows a metabolic acidosis with HCO₃⁻ 12 mmol/L. Use Winter’s formula to find the expected PaCO₂ (the centre of the range) in mmHg.
HCO₃⁻ 12 mmol/L
PaCO₂ = mmHg
10.Arterial blood gas interpretationHard
A practice gas shows a metabolic acidosis: pH 7.13, PaCO₂ 44 mmHg (5.9 kPa), HCO₃⁻ 14 mmol/L. Compare the measured PaCO₂ with Winter’s formula. Click your interpretation.
pH 7.13PaCO₂ 44 mmHg (5.9 kPa)HCO₃⁻ 14 mmol/L
1appropriate compensation
2added respiratory acidosis
3added respiratory alkalosis
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, 520 patients received a new treatment and 360 received the control treatment. The table shows how many in each group had the outcome (an unwanted event such as a stroke). Find the number needed to treat NNT to prevent one event. Round up to a whole number of patients.
treatment: 98/520control: 97/360
NNT =
14.Evidence-based practice statisticsHard
Practice exercise (made-up values, not clinical guidance). A screening test has a sensitivity of 88% and a specificity of 83%. It is used in a population where the prevalence is 10%. Find the positive predictive value PPV as a percentage. Round to one decimal place.
Se = 88%Sp = 83%p = 10%
PPV = %
3Solutions
1.Human anatomy
Answer: A · Cerebellum
Why this is the answer
The cerebellum fine-tunes movement, posture and balance. Damage causes ataxia and an unsteady gait.
Answer
A · Cerebellum
2.Human anatomy
Answer: D · Upper trunk (C5–C6)
Why this is the answer
Erb's palsy follows stretching of the upper trunk (C5–C6), paralysing deltoid, the external rotators and biceps. Lower trunk injury (Klumpke's) affects the small hand muscles instead.
Answer
D · Upper trunk (C5–C6)
3.Human physiology
Answer: C · Tidal volume, inspiratory reserve volume and expiratory reserve volume
Why this is the answer
Vital capacity = IRV + TV + ERV. It excludes the residual volume, which always stays in the lungs.
Answer
C · Tidal volume, inspiratory reserve volume and expiratory reserve volume
4.Human physiology
Answer: B · In the lungs
Why this is the answer
Renin (kidney) cleaves angiotensinogen (liver) to angiotensin I, which ACE, mainly on pulmonary capillary endothelium, converts to angiotensin II.
Answer
B · In the lungs
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: B · 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
B · Tetanospasmin, which blocks inhibitory neurotransmitter release
7.Pharmacology
Answer: B · When started within 48 hours of the first symptoms
Why this is the answer
Neuraminidase inhibitors work best early, while the virus is multiplying. Started within 48 hours, they shorten illness; in high-risk or hospitalised patients they may still be given later.
Answer
B · When started within 48 hours of the first symptoms
8.Pharmacology
Answer: A · It blocks the tubular secretion of penicillin in the kidney
Why this is the answer
Penicillins are excreted largely by organic anion transporters in the proximal tubule. Probenecid competes for these transporters, so penicillin is excreted more slowly and its level stays higher.
Answer
A · It blocks the tubular secretion of penicillin in the kidney
9.Arterial blood gas interpretation
Answer: PaCO₂ = 26 mmHg
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.
Winter’s formula
PaCO₂ = 1.5·12 + 8 = 26 mmHg
range: 24–28 mmHg
Answer
PaCO₂ = 26 mmHg
10.Arterial blood gas interpretation
Answer: added respiratory 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.
Expected PaCO₂
1.5·14 + 8 = 29 → 27–31 mmHg
Compare
measured: 44 mmHg → added respiratory acidosis
Answer
added respiratory acidosis
11.ECG rhythm strip interpretation
Answer: QTc = 0.491 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 = 15 × 0.04 = 0.60 s
Bazett’s formula
QTc = 0.38/√0.60 = 0.38/0.7746 = 0.491 s
Answer
QTc = 0.491 s
Above the usual limits for both men (about 0.45 s) and women (about 0.46 s): prolonged. (491 ms)
12.ECG rhythm strip interpretation
Answer: P waves
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: inverted, just before each QRS.
4. PR interval: 0.08 s, shorter than 0.12 s.
5. QRS: 0.06 s, narrow (below 0.12 s).
First abnormal step
P waves → accelerated junctional rhythm
Answer
P waves
13.Evidence-based practice statistics
Answer: NNT = 13
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 = 98/520 = 0.1885
CER = 97/360 = 0.2694
Number needed to treat
ARR = 0.2694 − 0.1885 = 0.08098
NNT = 1/0.08098 = 12.35 → 13
Answer
NNT = 13
Treating 13 patients prevents about one event.
14.Evidence-based practice statistics
Answer: PPV = 36.5 %
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 = 0.88·0.1 = 0.088
(1 − Sp)·(1 − p) = 0.17·0.9 = 0.153
Positive predictive value
PPV = 0.088/(0.088 + 0.153) = 0.3651 ≈ 36.5%
Answer
PPV = 36.5%
The lower the prevalence, the lower the PPV of the same test.