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
The rotator cuff consists of supraspinatus, infraspinatus, teres minor and which other muscle?
ALatissimus dorsi
BSubscapularis
CDeltoid
DTeres major
2.Human anatomyHard
Injury to the common fibular (peroneal) nerve at the neck of the fibula typically causes:
AWeak hip abduction
BLoss of sensation on the sole of the foot
CInability to flex the knee
DFoot drop
3.Human physiologyMedium
The hormone secretin is released when acid enters the duodenum. What is its main effect?
ABicarbonate secretion from the pancreas
BInsulin release from the endocrine pancreas
CAcid secretion from the stomach
DContraction of the gallbladder
4.Human physiologyHard
Hodgkin and Huxley's classic studies of the ionic basis of the action potential used which preparation?
AThe squid giant axon
BThe cat spinal cord
CThe frog sciatic nerve–muscle preparation
DCultured human cortical neurons
5.MicrobiologyMedium
Which cells does HIV mainly infect and destroy?
ARed blood cells
BPlatelets
CCD4 T helper cells
DNeutrophils
6.MicrobiologyHard
What is the key difference between bactericidal and bacteriostatic antibiotics?
ABactericidal drugs kill bacteria; bacteriostatic drugs stop growth and rely on the immune system to clear them
BBacteriostatic drugs always work faster than bactericidal drugs
CBactericidal drugs work only outside the body
DBactericidal drugs work only on viruses; bacteriostatic drugs work on bacteria
7.PharmacologyMedium
An IV site is cool, swollen and pale, and the infusion is running slowly. What should the nurse do?
AStop the infusion, remove the cannula and elevate the limb
BApply a tight bandage over the site
CFlush the cannula forcefully with saline
DIncrease the infusion rate to push through and clear the blockage
8.PharmacologyHard
How does levonorgestrel emergency contraception mainly work?
AIt kills sperm in the vagina
BIt ends an established pregnancy
CIt causes an immediate period
DIt delays or prevents ovulation
9.Arterial blood gas interpretationMedium
A practice arterial blood gas shows pH 7.38, PaCO₂ 26 mmHg (3.5 kPa) and HCO₃⁻ 15 mmol/L. The primary disorder is metabolic acidosis. How far is it compensated? Click your answer.
pH 7.38PaCO₂ 26 mmHg (3.5 kPa)HCO₃⁻ 15 mmol/L
1uncompensated
2partially compensated
3fully compensated
10.Arterial blood gas interpretationHard
At sea level, a practice gas taken on 21% oxygen shows PaO₂ 76 mmHg (10.1 kPa) and PaCO₂ 33 mmHg (4.4 kPa). Use PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8 to find the A–a gradient.
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 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, 173 new cases occurred. Find the incidence rate IR per 1000 person-years. Round to one decimal place.
Practice exercise (made-up values, not clinical guidance). A screening test has a sensitivity of 85% and a specificity of 99%. It is used in a population where the prevalence is 0.5%. Find the positive predictive value PPV as a percentage. Round to one decimal place.
Se = 85%Sp = 99%p = 0.5%
PPV = %
3Solutions
1.Human anatomy
Answer: B · Subscapularis
Why this is the answer
SITS: supraspinatus, infraspinatus, teres minor and subscapularis. Teres major is close by but is not part of the cuff.
Answer
B · Subscapularis
2.Human anatomy
Answer: D · Foot drop
Why this is the answer
The common fibular nerve supplies the dorsiflexors and evertors. Damage at the fibular neck causes foot drop and a high-stepping gait. The sole is supplied by the tibial nerve.
Answer
D · Foot drop
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: A · The squid giant axon
Why this is the answer
The squid giant axon is up to about 1 mm wide, so voltage-clamp electrodes could be inserted. This showed that sodium inflow and potassium outflow produce the action potential.
Answer
A · The squid giant axon
5.Microbiology
Answer: C · CD4 T helper cells
Why this is the answer
HIV binds CD4 and a co-receptor on T helper cells. Loss of these cells weakens both antibody and cell-mediated immunity, leading to AIDS.
Answer
C · CD4 T helper cells
6.Microbiology
Answer: A · Bactericidal drugs kill bacteria; bacteriostatic drugs stop growth and rely on the immune system to clear them
Why this is the answer
Bacteriostatic drugs halt multiplication so host defences can eliminate the bacteria. Bactericidal drugs are preferred when defences are weak or hard to reach, such as in endocarditis or neutropenia.
Answer
A · Bactericidal drugs kill bacteria; bacteriostatic drugs stop growth and rely on the immune system to clear them
7.Pharmacology
Answer: A · Stop the infusion, remove the cannula and elevate the limb
Why this is the answer
These signs suggest infiltration. The infusion is stopped and the cannula removed, the limb is elevated, and a new cannula is placed elsewhere; vesicant drugs need the extravasation protocol.
Answer
A · Stop the infusion, remove the cannula and elevate the limb
8.Pharmacology
Answer: D · It delays or prevents ovulation
Why this is the answer
Levonorgestrel emergency contraception mainly prevents or delays ovulation and does not disrupt an implanted pregnancy. Its effectiveness falls the longer it is delayed after unprotected sex.
Answer
D · It delays or prevents ovulation
9.Arterial blood gas interpretation
Answer: fully compensated
Reading a blood gas (ROME): first the pH: below 7.35 is acidaemia, above 7.45 alkalaemia. Then find the value that explains it. PaCO₂ (normal 35–45 mmHg, 4.7–6.0 kPa) is the respiratory part and moves Opposite to the pH in a respiratory disorder; HCO₃⁻ (normal 22–26 mmol/L) is the metabolic part and moves in the same direction as the pH (Respiratory Opposite, Metabolic Equal). The other value shows the compensation: normal means uncompensated; abnormal in the direction that pulls the pH back while the pH is still abnormal means partially compensated; a normal pH with both values abnormal means fully compensated. Ranges differ slightly between laboratories; these are practice values.
ROME, value by value
pH 7.38: within 7.35–7.45
PaCO₂ 26 mmHg: low (35–45)
HCO₃⁻ 15 mmol/L: low (22–26)
Compensation
Clinically, also check the expected compensation: Winter’s formula expects PaCO₂ 28.5–32.5 mmHg, so the measured 26 mmHg points to an additional respiratory alkalosis (a mixed disorder). The answer above is the ROME exam classification.
fully compensated
Answer
fully compensated
10.Arterial blood gas interpretation
Answer: A–a gradient = 32.48 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.
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 × 0.04 = 0.36 s
RR = 18 × 0.04 = 0.72 s
Bazett’s formula
QTc = 0.36/√0.72 = 0.36/0.8485 = 0.424 s
Answer
QTc = 0.424 s
Within the usual limits (about 0.45 s for men, 0.46 s for women). (424 ms)
12.ECG rhythm strip interpretation
Answer: rate
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 = 13 small boxes, 1500 ÷ 13 ≈ 115 per minute.
2. Rhythm: regular, every R–R is 0.52 s.
3. P waves: upright, one before every QRS, all alike.
4. PR interval: 0.14 s, constant, within 0.12–0.20 s.
5. QRS: 0.08 s, narrow (below 0.12 s).
First abnormal step
rate → sinus tachycardia
Answer
rate
13.Evidence-based practice statistics
Answer: IR = 41.1 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 = 360·4 + 470·5 + 210·2 = 4210
Incidence rate
IR = 173/4210·1000 = 41.1 per 1000 person-years
Answer
IR = 41.1 per 1000 person-years
14.Evidence-based practice statistics
Answer: PPV = 29.9 %
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.