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 is the outermost layer of the meninges?
APia mater
BArachnoid mater
CDura mater
DChoroid plexus
2.Human anatomyHard
After a stroke, a patient speaks fluently but uses meaningless words and cannot understand speech. Where is the lesion most likely to be?
ACerebellar hemisphere
BPrecentral gyrus of the non-dominant hemisphere
CInferior frontal gyrus of the dominant hemisphere
DPosterior superior temporal gyrus of the dominant hemisphere
3.Human physiologyMedium
Which ABO blood group has both anti-A and anti-B antibodies in the plasma?
AGroup AB
BGroup O
CGroup A
DGroup B
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
Many antifungal drugs target ergosterol. Why does this work?
AErgosterol is the main structural component of bacterial cell walls
BErgosterol is only found in viruses
CHuman cells use ergosterol to make energy
DErgosterol is in fungal cell membranes but not in human cells
6.MicrobiologyHard
Staphylococcus aureus food poisoning starts only 1–6 hours after eating. Why so fast?
AThe bacteria invade the bloodstream immediately
BThe bacteria made a heat-stable toxin in the food before it was eaten
CThe bacteria multiply very fast inside the gut and invade the intestinal wall
DThe immune system reacts to the bacteria within minutes
7.PharmacologyMedium
Which advice helps reduce the most common side effect of varenicline?
ATake it with alcohol to reduce nausea
BTake it on an empty stomach at bedtime with a little milk
CTake double doses in the first week
DTake it after food with a full glass of water
8.PharmacologyHard
An older patient on amlodipine develops ankle swelling and is prescribed furosemide. She then becomes dizzy and falls. What does this sequence illustrate?
ADrug tolerance
BA type B allergic reaction
CA prescribing cascade
DEnzyme induction
9.Arterial blood gas interpretationMedium
Practice values: Na⁺ 140 mmol/L, glucose 26.2 mmol/L and urea 12.7 mmol/L. Calculate the serum osmolality Osm = 2·Na⁺ + glucose + urea to the nearest whole number.
Na⁺ 140 mmol/Lglucose 26.2 mmol/Lurea 12.7 mmol/L
Osm = mOsm/kg
10.Arterial blood gas interpretationHard
At sea level, a practice gas taken on 30% oxygen shows PaO₂ 94 mmHg (12.5 kPa) and PaCO₂ 28 mmHg (3.7 kPa). Use PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8 to find the A–a gradient.
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 tachycardia
2supraventricular tachycardia (SVT)
3accelerated junctional rhythm
4atrial fibrillation
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
1second-degree AV block type II
2second-degree AV block type I (Wenckebach)
3first-degree AV block
4sinus bradycardia
13.Evidence-based practice statisticsMedium
Practice exercise (made-up values, not clinical guidance). In a cohort study, exposed people contributed 7400 person-years with 89 new cases, and unexposed people 1900 person-years with 8 new cases. Find the incidence rate ratio IRR (exposed versus unexposed). Round to two decimal places.
exposed: 89/7400 PYnot exposed: 8/1900 PY
IRR =
14.Evidence-based practice statisticsHard
Practice exercise (made-up values, not clinical guidance). 10 000 people are screened with a test of sensitivity 70% and specificity 79%. The prevalence of the disease is 7%. How many true-positive results TP are expected?
N = 10 000p = 7%Se = 70%Sp = 79%
TP =
3Solutions
1.Human anatomy
Answer: C · Dura mater
Why this is the answer
From outside in the meninges are dura, arachnoid and pia mater. Cerebrospinal fluid flows in the subarachnoid space between arachnoid and pia.
Answer
C · Dura mater
2.Human anatomy
Answer: D · Posterior superior temporal gyrus of the dominant hemisphere
Why this is the answer
This is Wernicke's (receptive) aphasia, caused by damage to Wernicke's area in the posterior superior temporal gyrus. Broca's area in the inferior frontal gyrus causes non-fluent aphasia.
Answer
D · Posterior superior temporal gyrus of the dominant hemisphere
3.Human physiology
Answer: B · Group O
Why this is the answer
Group O people have no A or B antigens and make both anti-A and anti-B. Group AB has both antigens and neither antibody.
Answer
B · Group O
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: D · Ergosterol is in fungal cell membranes but not in human cells
Why this is the answer
Fungal membranes contain ergosterol, while human membranes contain cholesterol. Azoles block ergosterol synthesis and amphotericin binds ergosterol, so they target fungi selectively.
Answer
D · Ergosterol is in fungal cell membranes but not in human cells
6.Microbiology
Answer: B · The bacteria made a heat-stable toxin in the food before it was eaten
Why this is the answer
S. aureus produces enterotoxins in food left at warm temperatures. The toxin is already there when the food is eaten and survives reheating, so symptoms start quickly.
Answer
B · The bacteria made a heat-stable toxin in the food before it was eaten
7.Pharmacology
Answer: D · Take it after food with a full glass of water
Why this is the answer
Nausea is the most common adverse effect of varenicline and is reduced by taking it after food with water. Patients should also report changes in mood or behaviour.
Answer
D · Take it after food with a full glass of water
8.Pharmacology
Answer: C · A prescribing cascade
Why this is the answer
In a prescribing cascade, an adverse effect of one drug is mistaken for a new condition and treated with another drug, which brings new risks. Amlodipine oedema does not respond well to diuretics; reviewing the original drug is better.
Answer
C · A prescribing cascade
9.Arterial blood gas interpretation
Answer: Osm = 319 mOsm/kg
Derived values: calculated serum osmolality is 2·Na⁺ + glucose + urea with all values in mmol/L (2·Na⁺ + glucose/18 + BUN/2.8 with glucose and BUN in mg/dL); normal is about 275–295 mOsm/kg. The P/F ratio is PaO₂ divided by FiO₂ as a fraction. The alveolar oxygen pressure is PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8 mmHg at sea level, and the A–a gradient is PAO₂ − PaO₂.
Calculated osmolality
Osm = 2·140 + 26.2 + 12.7 = 318.9 ≈ 319 mOsm/kg
Answer
Osm = 319 mOsm/kg
10.Arterial blood gas interpretation
Answer: A–a gradient = 84.9 mmHg
Derived values: calculated serum osmolality is 2·Na⁺ + glucose + urea with all values in mmol/L (2·Na⁺ + glucose/18 + BUN/2.8 with glucose and BUN in mg/dL); normal is about 275–295 mOsm/kg. The P/F ratio is PaO₂ divided by FiO₂ as a fraction. The alveolar oxygen pressure is PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8 mmHg at sea level, and the A–a gradient is PAO₂ − PaO₂.
Alveolar oxygen
PAO₂ = 0.3·713 − 28/0.8 = 213.9 − 35 = 178.9 mmHg
Gradient
A–a = 178.9 − 94 = 84.9 mmHg
Answer
A–a gradient = 84.9 mmHg
11.ECG rhythm strip interpretation
Answer: supraventricular tachycardia (SVT)
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: R–R = 9 small boxes, 1500 ÷ 9 ≈ 167 per minute.
2. Rhythm: regular, every R–R is 0.36 s.
3. P waves: none visible; at this rate they are hidden in the QRS complexes or the T waves.
4. PR interval: not measurable.
5. QRS: 0.06 s, narrow (below 0.12 s), so the rhythm starts above the ventricles.
Interpretation
supraventricular tachycardia (SVT)
Answer
supraventricular tachycardia (SVT)
12.ECG rhythm strip interpretation
Answer: second-degree AV block type II
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: 6 QRS complexes in 6 s, × 10 = 60 per minute.
2. Rhythm: the P waves are regular; R–R is regular except for a long pause where a QRS is missing.
3. P waves: upright and regular (P–P 0.72 s, 83 per minute); one P in every 4 is not followed by a QRS.
4. PR interval: constant at 0.16 s in the conducted beats.
5. QRS: 0.08 s, narrow (below 0.12 s).
Interpretation
second-degree AV block type II
Answer
second-degree AV block type II
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: IRR = 2.86
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.
Incidence rates
exposed: 89/7400 = 12.03 per 1000 person-years
not exposed: 8/1900 = 4.211 per 1000 person-years
Rate ratio
IRR = 12.03/4.211 = 2.856 ≈ 2.86
Answer
IRR = 2.86
14.Evidence-based practice statistics
Answer: TP = 490
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.