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 artery is most commonly used to feel the pulse at the wrist?
ARadial artery
BAxillary artery
CUlnar artery
DBrachial artery
2.Human anatomyHard
During a lumbar puncture, which ligament is the last one pierced before the needle enters the epidural space?
APosterior longitudinal ligament
BInterspinous ligament
CSupraspinous ligament
DLigamentum flavum
3.Human physiologyMedium
Which is a classic example of positive feedback in the body?
AOxytocin release during labour
BSweating when body temperature rises
CInsulin release after a meal
DADH release when plasma osmolality rises
4.Human physiologyHard
After a head injury, a patient passes large volumes of very dilute urine and plasma sodium rises. What is the most likely cause?
AUncontrolled diabetes mellitus
BExcess aldosterone
CLack of ADH release (cranial diabetes insipidus)
DExcess ADH release (SIADH)
5.MicrobiologyMedium
Toxoplasma gondii is a risk in pregnancy. Which advice reduces exposure?
AAvoid contact with dogs
BAvoid cleaning cat litter and avoid undercooked meat
CAvoid all fruit and vegetables
DAvoid swimming pools and all other public bathing areas
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?
ABacillus cereus
BSalmonella Typhi
CClostridium botulinum
DListeria monocytogenes
7.PharmacologyMedium
Bethanechol may be used for non-obstructive urinary retention. How does it work?
AIt increases urine production in the kidney
BIt blocks muscarinic receptors and relaxes the bladder wall muscle
CIt shrinks the prostate gland
DIt stimulates muscarinic receptors and contracts the bladder muscle
8.PharmacologyHard
A teenager took a large overdose of diphenhydramine. He is agitated, hot, flushed and confused, with dry skin, dilated pupils and a full bladder. What is this pattern?
AAnticholinergic toxidrome
BSerotonin syndrome
CCholinergic toxidrome
DOpioid toxidrome
9.Arterial blood gas interpretationMedium
Practice values: Na⁺ 143 mmol/L, glucose 30.1 mmol/L and urea 21.1 mmol/L. Calculate the serum osmolality Osm = 2·Na⁺ + glucose + urea to the nearest whole number.
Na⁺ 143 mmol/Lglucose 30.1 mmol/Lurea 21.1 mmol/L
Osm = mOsm/kg
10.Arterial blood gas interpretationHard
A practice gas shows pH 7.34, PaCO₂ 31 mmHg (4.1 kPa) and HCO₃⁻ 16 mmol/L, with Na⁺ 135 and Cl⁻ 86 mmol/L. Use the anion gap and the delta ratio to click the best interpretation.
Measure the QT interval and the R–R interval on this regular lead II strip, then calculate the rate-corrected QTc with Fridericia’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
This patient has a pacemaker. Interpret the lead II strip. 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 rhythm with wide QRS
2dual-chamber (AV) paced rhythm
3idioventricular rhythm
4junctional rhythm
13.Evidence-based practice statisticsMedium
Practice exercise (made-up values, not clinical guidance). A test has a sensitivity of 77% and a specificity of 73%. Find the negative likelihood ratio LR−. Round to two decimal places.
Se = 77%Sp = 73%
LR− =
14.Evidence-based practice statisticsHard
Practice exercise (made-up values, not clinical guidance). A treatment reduces the risk of an event by 46% in relative terms (RRR = 46%). In a patient group the risk without treatment (CER) is 11% 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 = 46%CER = 11%
NNT =
3Solutions
1.Human anatomy
Answer: A · Radial artery
Why this is the answer
The radial artery lies superficially on the thumb side of the wrist, against the radius, which makes the pulse easy to feel.
Answer
A · Radial artery
2.Human anatomy
Answer: D · Ligamentum flavum
Why this is the answer
The needle passes skin, supraspinous and interspinous ligaments, then ligamentum flavum into the epidural space, then dura and arachnoid into the subarachnoid space.
Answer
D · Ligamentum flavum
3.Human physiology
Answer: A · Oxytocin release during labour
Why this is the answer
Stretch of the cervix releases oxytocin, which strengthens contractions and causes more stretch. The loop ends with delivery. The other examples are negative feedback.
Answer
A · Oxytocin release during labour
4.Human physiology
Answer: C · Lack of ADH release (cranial diabetes insipidus)
Why this is the answer
Without ADH the collecting ducts stay impermeable to water, so dilute urine is produced and the patient loses free water, raising plasma sodium.
Answer
C · Lack of ADH release (cranial diabetes insipidus)
5.Microbiology
Answer: B · Avoid cleaning cat litter and avoid undercooked meat
Why this is the answer
Toxoplasma oocysts are shed in cat faeces and cysts are found in undercooked meat. Infection during pregnancy can damage the fetal brain and eyes.
Answer
B · Avoid cleaning cat litter and avoid undercooked meat
6.Microbiology
Answer: A · 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
A · Bacillus cereus
7.Pharmacology
Answer: D · It stimulates muscarinic receptors and contracts the bladder muscle
Why this is the answer
Bethanechol mimics acetylcholine at muscarinic receptors, contracting the detrusor muscle. It must not be used if there is mechanical obstruction to outflow.
Answer
D · It stimulates muscarinic receptors and contracts the bladder muscle
8.Pharmacology
Answer: A · Anticholinergic toxidrome
Why this is the answer
Anticholinergic toxicity causes hyperthermia, dry flushed skin, mydriasis, urinary retention, tachycardia and delirium. Dry skin helps distinguish it from serotonin syndrome and sympathomimetic poisoning, which cause sweating.
Answer
A · Anticholinergic toxidrome
9.Arterial blood gas interpretation
Answer: Osm = 337 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 = 2·143 + 30.1 + 21.1 = 337.2 ≈ 337 mOsm/kg
Answer
Osm = 337 mOsm/kg
10.Arterial blood gas interpretation
Answer: added metabolic alkalosis
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 = 135 − (86 + 16) = 33
Delta ratio
(33 − 12)/(24 − 16) = 2.625
added metabolic alkalosis
Answer
added metabolic alkalosis
11.ECG rhythm strip interpretation
Answer: QTc = 0.42 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 = 10.5 × 0.04 = 0.42 s
RR = 25 × 0.04 = 1.00 s
Fridericia’s formula
QTc = 0.42/∛1.00 = 0.42/1 = 0.42 s
Answer
QTc = 0.42 s
Within the usual limits (about 0.45 s for men, 0.46 s for women). (420 ms)
12.ECG rhythm strip interpretation
Answer: dual-chamber (AV) paced rhythm
Paced rhythms: a pacemaker impulse shows as a narrow vertical pacing spike. After a ventricular spike the QRS is wide (0.12 s or more), because the impulse spreads from the lead through the muscle instead of the conduction system; with right-ventricular pacing it is often mostly negative in lead II. A spike before each P wave means atrial pacing, and a dual-chamber (AV) pacemaker shows an atrial spike and a P wave, then, after the programmed AV delay, a ventricular spike and a wide QRS. Capture means every spike is followed by a depolarisation. Failure to capture is a spike without a P wave or QRS after it; failure to sense (spikes that ignore the patient’s own beats) and failure to pace (no spike when one is due) are the other classic malfunctions.
Five steps
1. Rate: R–R = 23 small boxes, 1500 ÷ 23 ≈ 65 per minute.
2. Rhythm: regular, every R–R is 0.92 s.
3. P waves: each P wave follows its own pacing spike (atrial pacing).
4. AV interval: the second spike comes 0.18 s after the first one (the programmed AV delay).
5. QRS: 0.14 s, wide, right after the second spike (ventricular pacing).
Interpretation
dual-chamber (AV) paced rhythm
Answer
dual-chamber (AV) paced rhythm
The paced QRS is wide by nature; do not call it a bundle branch block or a ventricular rhythm.
13.Evidence-based practice statistics
Answer: LR− = 0.32
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.
Negative likelihood ratio
LR− = (1 − 0.77)/0.73 = 0.3151 ≈ 0.32
Answer
LR− = 0.32
14.Evidence-based practice statistics
Answer: NNT = 20
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 = 0.11·0.46 = 0.0506
Number needed to treat
NNT = 1/0.0506 = 19.76 → 20
Answer
NNT = 20
The same relative effect gives a smaller NNT when the baseline risk is higher.