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
In which structure do sperm mature and get stored after leaving the testis?
AProstate gland
BSeminal vesicle
CSeminiferous tubules
DEpididymis
2.Human anatomyHard
A stab wound cuts the left half of the spinal cord at T10. Which pattern of loss is expected below the lesion?
ARight-sided weakness and loss of proprioception, left-sided loss of pain and temperature
BLoss of all sensation and movement on the left only
CLeft-sided weakness and loss of proprioception, right-sided loss of pain and temperature
DLoss of pain and temperature on both sides only
3.Human physiologyMedium
The hormone secretin is released when acid enters the duodenum. What is its main effect?
ABicarbonate secretion from the pancreas
BAcid secretion from the stomach
CInsulin release from the endocrine pancreas
DContraction of the gallbladder
4.Human physiologyHard
In a patient with primary hypothyroidism, you would expect:
ALow TSH and high free T4
BLow TSH and low free T4
CHigh TSH and high free T4
DHigh TSH and low free T4
5.MicrobiologyMedium
Which organism causes typhoid fever?
ACampylobacter jejuni
BVibrio cholerae
CSalmonella Typhi
DShigella dysenteriae
6.MicrobiologyHard
Several people vomit within a few hours of eating fried rice that was cooked the day before and kept at room temperature. Which organism is the classic cause?
ASalmonella Typhi
BBacillus cereus
CListeria monocytogenes
DClostridium botulinum
7.PharmacologyMedium
During an infusion of a vesicant chemotherapy drug, the patient reports burning, and the site is swollen. What should the nurse do first?
ARemove the cannula at once and massage the area firmly
BStop the infusion, leave the cannula in and follow the protocol
CIncrease the infusion rate to finish quickly
DFlush the line with a large volume of saline
8.PharmacologyHard
Drug X gives a maximum response of 100% with an EC50 of 1 mg. Drug Y gives a maximum response of 100% with an EC50 of 10 mg. Which statement is correct?
AX has higher efficacy than Y
BY has higher efficacy than X
CY is more potent than X, and both have the same efficacy
DX is more potent than Y, and both have the same efficacy
9.Arterial blood gas interpretationMedium
Practice values: Na⁺ 128 mmol/L, glucose 15.6 mmol/L and urea 12.5 mmol/L. Calculate the serum osmolality Osm = 2·Na⁺ + glucose + urea to the nearest whole number.
Na⁺ 128 mmol/Lglucose 15.6 mmol/Lurea 12.5 mmol/L
Osm = mOsm/kg
10.Arterial blood gas interpretationHard
A practice gas shows a respiratory acidosis: pH 7.25, PaCO₂ 78 mmHg (10.4 kPa), HCO₃⁻ 33 mmol/L. Use the rules ‘HCO₃⁻ rises 1 mmol/L per 10 mmHg (acute) or 3.5 mmol/L per 10 mmHg (chronic)’ to classify it. Click your answer.
pH 7.25PaCO₂ 78 mmHg (10.4 kPa)HCO₃⁻ 33 mmol/L
1acute
2chronic
3acute on chronic
11.ECG rhythm strip interpretationMedium
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 bradycardia
2junctional rhythm
3normal sinus rhythm
4first-degree AV block
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). 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, 23 new cases occurred. Find the incidence rate IR per 1000 person-years. Round to one decimal place.
600 × 0.5 years290 × 5 years23 new cases
IR = per 1000 person-years
14.Evidence-based practice statisticsHard
Practice exercise (made-up values, not clinical guidance). A patient has a pre-test probability of 50% for a disease. A test with sensitivity 87% and specificity 80% comes back negative. Find the post-test probability p′ of the disease as a percentage. Round to one decimal place.
pre-test 50%Se = 87%Sp = 80%negative
p′ = %
3Solutions
1.Human anatomy
Answer: D · Epididymis
Why this is the answer
Sperm are made in the seminiferous tubules and then mature and are stored in the epididymis before passing into the ductus deferens.
Answer
D · Epididymis
2.Human anatomy
Answer: C · Left-sided weakness and loss of proprioception, right-sided loss of pain and temperature
Why this is the answer
In Brown-Séquard syndrome the corticospinal tract and dorsal columns are lost on the same side, while the spinothalamic tract, which crosses near entry, causes contralateral pain and temperature loss.
Answer
C · Left-sided weakness and loss of proprioception, right-sided loss of pain and temperature
3.Human physiology
Answer: A · Bicarbonate secretion from the pancreas
Why this is the answer
Secretin stimulates pancreatic duct cells to secrete bicarbonate-rich fluid, which neutralises acidic chyme so intestinal enzymes can work.
Answer
A · Bicarbonate secretion from the pancreas
4.Human physiology
Answer: D · High TSH and low free T4
Why this is the answer
When the thyroid fails, low T4 removes negative feedback and the pituitary raises TSH. Low TSH with low T4 would point to secondary (pituitary) hypothyroidism.
Answer
D · High TSH and low free T4
5.Microbiology
Answer: C · Salmonella Typhi
Why this is the answer
Salmonella enterica serotype Typhi causes typhoid fever, a systemic illness with prolonged fever. Some people become chronic carriers, often in the gallbladder.
Answer
C · Salmonella Typhi
6.Microbiology
Answer: B · Bacillus cereus
Why this is the answer
Bacillus cereus spores survive boiling. If cooked rice is left warm, the bacteria grow and make a heat-stable emetic toxin, so reheating does not make the rice safe.
Answer
B · Bacillus cereus
7.Pharmacology
Answer: B · Stop the infusion, leave the cannula in and follow the protocol
Why this is the answer
Suspected extravasation of a vesicant is an emergency. The infusion is stopped, the cannula is left in so that drug can be aspirated, and the protocol, which may include an antidote and cold or warm packs, is followed.
Answer
B · Stop the infusion, leave the cannula in and follow the protocol
8.Pharmacology
Answer: D · X is more potent than Y, and both have the same efficacy
Why this is the answer
A lower EC50 means less drug is needed for half the effect, so X is more potent. Both reach the same maximum, so their efficacy is equal.
Answer
D · X is more potent than Y, and both have the same efficacy
9.Arterial blood gas interpretation
Answer: Osm = 284 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 = 2·128 + 15.6 + 12.5 = 284.1 ≈ 284 mOsm/kg
Answer
Osm = 284 mOsm/kg
10.Arterial blood gas interpretation
Answer: acute on chronic
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 HCO₃⁻
acute: 24 + 1·3.8 = 27.8 mmol/L
chronic: 24 + 3.5·3.8 = 37.3 mmol/L
Compare
measured: 33 mmol/L → acute on chronic
Answer
acute on chronic
11.ECG rhythm strip interpretation
Answer: junctional rhythm
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.
Five steps
1. Rate: R–R = 29 small boxes, 1500 ÷ 29 ≈ 52 per minute.
2. Rhythm: regular, every R–R is 1.16 s.
3. P waves: none visible.
4. PR interval: not measurable.
5. QRS: 0.06 s, narrow (below 0.12 s).
Interpretation
junctional rhythm
Answer
junctional rhythm
12.ECG rhythm strip interpretation
Answer: ΔPR = 0.14 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 = 0.92 s, regular; one P wave in every 3 is not conducted
PR in each group
PR₁ = 0.16 s, PR₂ = 0.30 s
Answer
ΔPR = 0.14 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: IR = 13.1 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 = 600·0.5 + 290·5 = 1750
Incidence rate
IR = 23/1750·1000 = 13.1 per 1000 person-years
Answer
IR = 13.1 per 1000 person-years
14.Evidence-based practice statistics
Answer: p′ = 14 %
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.