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 cranial nerve controls movement of the tongue?
AVagus nerve (X)
BGlossopharyngeal nerve (IX)
CHypoglossal nerve (XII)
DFacial nerve (VII)
2.Human anatomyHard
A patient has a drooping eyelid, a dilated pupil and an eye that rests "down and out". Which nerve is most likely damaged?
AOptic nerve (II)
BOculomotor nerve (III)
CAbducens nerve (VI)
DTrochlear nerve (IV)
3.Human physiologyMedium
How is the ejection fraction calculated?
ACardiac output ÷ heart rate
BStroke volume ÷ end-diastolic volume
CEnd-diastolic volume ÷ end-systolic volume
DEnd-systolic volume ÷ stroke volume
4.Human physiologyHard
In Otto Loewi's classic experiment, fluid from a vagus-stimulated frog heart slowed a second heart. What did this show?
ANerve impulses spread through the fluid directly as electricity
BNerves pass signals to their targets by releasing a chemical
CThe heart beats without any nerve supply
DThe vagus nerve speeds up the heart
5.MicrobiologyMedium
How does an mRNA vaccine produce immunity?
AThe mRNA is inserted permanently into human DNA
BBody cells use the mRNA to make a viral protein that the immune system learns to recognise
CThe mRNA kills the virus directly in the blood
DThe mRNA contains weakened live virus
6.MicrobiologyHard
A deep, dirty wound becomes swollen and very painful, and gas can be felt under the skin. Which organism is most likely?
ANeisseria gonorrhoeae
BBordetella pertussis
CClostridium perfringens
DStreptococcus pneumoniae
7.PharmacologyMedium
During an oxytocin infusion, contractions become very frequent and the fetal heart rate shows late decelerations. What is the nurse's first action?
APosition the woman flat on her back
BIncrease the infusion to finish labour quickly
CGive a second oxytocin bolus
DStop the oxytocin infusion
8.PharmacologyHard
A 500 mg intravenous dose gives an initial plasma concentration of 10 mg/L. What is the volume of distribution?
A5 L
B5000 L
C500 L
D50 L
9.Arterial blood gas interpretationMedium
A practice gas shows a metabolic acidosis with HCO₃⁻ 16 mmol/L. Use Winter’s formula to find the expected PaCO₂ (the centre of the range) in mmHg.
HCO₃⁻ 16 mmol/L
PaCO₂ = mmHg
10.Arterial blood gas interpretationHard
A practice gas shows a metabolic acidosis: pH 7.30, PaCO₂ 25 mmHg (3.3 kPa), HCO₃⁻ 12 mmol/L. Compare the measured PaCO₂ with Winter’s formula. Click your interpretation.
pH 7.30PaCO₂ 25 mmHg (3.3 kPa)HCO₃⁻ 12 mmol/L
1appropriate compensation
2added respiratory acidosis
3added respiratory alkalosis
11.ECG rhythm strip interpretationMedium
Measure the QT interval on this lead II strip, in seconds.
lead II, 25 mm/s, 10 mm/mV1 small box = 0.04 s, 1 large box = 0.20 sstrip length 6 s
QT = s
12.ECG rhythm strip interpretationHard
This lead II strip shows second-degree AV block type I (Wenckebach). By how much does the PR interval increase from the first to the second beat of a group (ΔPR), in seconds?
lead II, 25 mm/s, 10 mm/mV1 small box = 0.04 s, 1 large box = 0.20 sstrip length 6 s
ΔPR = s
13.Evidence-based practice statisticsMedium
Practice exercise (made-up values, not clinical guidance). A new test was compared with the reference standard in a study (see the 2 × 2 table). Find the positive likelihood ratio LR+. Round to two decimal places.
a = 180b = 390c = 20d = 1110
LR+ =
14.Evidence-based practice statisticsHard
Practice exercise (made-up values, not clinical guidance). A treatment reduces the risk of an event by 31% in relative terms (RRR = 31%). In a patient group the risk without treatment (CER) is 13% over five years. Find the number needed to treat NNT for five years to prevent one event. Round up to a whole number of patients.
RRR = 31%CER = 13%
NNT =
3Solutions
1.Human anatomy
Answer: C · Hypoglossal nerve (XII)
Why this is the answer
The hypoglossal nerve supplies the intrinsic and most extrinsic tongue muscles. A lesion makes the protruded tongue deviate towards the affected side.
Answer
C · Hypoglossal nerve (XII)
2.Human anatomy
Answer: B · Oculomotor nerve (III)
Why this is the answer
The oculomotor nerve supplies most eye muscles, levator palpebrae and the pupil constrictor. Lesion leaves lateral rectus and superior oblique unopposed, so the eye goes down and out.
Answer
B · Oculomotor nerve (III)
3.Human physiology
Answer: B · 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
B · Stroke volume ÷ end-diastolic volume
4.Human physiology
Answer: B · Nerves pass signals to their targets by releasing a chemical
Why this is the answer
The fluid carried "Vagusstoff", later identified as acetylcholine. This was the first clear evidence for chemical neurotransmission.
Answer
B · Nerves pass signals to their targets by releasing a chemical
5.Microbiology
Answer: B · Body cells use the mRNA to make a viral protein that the immune system learns to recognise
Why this is the answer
The mRNA is translated by cells into an antigen, such as a coronavirus spike protein. The immune system responds to it. The mRNA is broken down and does not enter the nucleus.
Answer
B · Body cells use the mRNA to make a viral protein that the immune system learns to recognise
6.Microbiology
Answer: C · Clostridium perfringens
Why this is the answer
Clostridium perfringens causes gas gangrene in poorly oxygenated wounds. Its toxins destroy muscle and its metabolism produces gas, felt as crepitus. Urgent surgery and antibiotics are needed.
Answer
C · Clostridium perfringens
7.Pharmacology
Answer: D · Stop the oxytocin infusion
Why this is the answer
Excessive contractions reduce placental blood flow and cause fetal distress. The oxytocin is stopped first, then the woman is turned on her side, IV fluids and oxygen are given as per protocol and the provider is informed.
Answer
D · Stop the oxytocin infusion
8.Pharmacology
Answer: D · 50 L
Why this is the answer
Vd = 500 mg ÷ 10 mg/L = 50 L. This is close to total body water, so the drug spreads beyond the plasma but is not extensively stored in tissues.
Answer
D · 50 L
9.Arterial blood gas interpretation
Answer: PaCO₂ = 32 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 = 1.5·16 + 8 = 32 mmHg
range: 30–34 mmHg
Answer
PaCO₂ = 32 mmHg
10.Arterial blood gas interpretation
Answer: appropriate compensation
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
1.5·12 + 8 = 26 → 24–28 mmHg
Compare
measured: 25 mmHg → appropriate compensation
Answer
appropriate compensation
11.ECG rhythm strip interpretation
Answer: QT = 0.38 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
9.5 small boxes from the start of the QRS to the end of the T wave
Convert
QT = 9.5 × 0.04 s = 0.38 s
Answer
QT = 0.38 s
A rough rule: at normal rates the QT should be less than half the R–R interval (here R–R = 0.84 s).
12.ECG rhythm strip interpretation
Answer: ΔPR = 0.12 s
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.
March out the P waves
P–P = 1.00 s, regular; one P wave in every 3 is not conducted
PR in each group
PR₁ = 0.16 s, PR₂ = 0.28 s
Answer
ΔPR = 0.12 s
The biggest step in PR comes between the first and second beats of a group; later steps are smaller, which is why the R–R intervals shorten before the pause.
13.Evidence-based practice statistics
Answer: LR+ = 3.46
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.
Sensitivity and specificity
Se = 180/200 = 0.9
Sp = 1110/1500 = 0.74
Positive likelihood ratio
LR+ = 0.9/(1 − 0.74) = 3.462 ≈ 3.46
Answer
LR+ = 3.46
14.Evidence-based practice statistics
Answer: NNT = 25
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.
Absolute risk reduction
ARR = 0.13·0.31 = 0.0403
Number needed to treat
NNT = 1/0.0403 = 24.81 → 25
Answer
NNT = 25
The same relative effect gives a smaller NNT when the baseline risk is higher.