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 nerve supplies the diaphragm?
AVagus nerve
BAccessory nerve
CPhrenic nerve
DIntercostal nerves
2.Human anatomyHard
Why is the proximal pole of the scaphoid at risk of avascular necrosis after a waist fracture?
AIts only artery runs through the lunate
BIts blood supply enters mainly from the distal end
CIt is drained by a single vein that clots easily
DIt has no blood supply at all
3.Human physiologyMedium
What is anatomical dead space?
AThe air that stays in the lungs after full exhalation (residual volume)
BAlveoli that have collapsed completely
CThe volume of the conducting airways where no gas exchange happens
DThe space between the two pleural layers
4.Human physiologyHard
A patient hyperventilating during a panic attack has tingling fingers. What would the arterial blood gas most likely show?
AHigh pH and high bicarbonate
BLow pH and low bicarbonate
CLow pH and high PaCO₂
DHigh pH and low PaCO₂
5.MicrobiologyMedium
MRSA is resistant to methicillin because it has:
ANo cell wall at all
BAn altered penicillin-binding protein (PBP2a)
CA polysaccharide capsule that blocks the entry of all beta-lactams
DAn efflux pump for every antibiotic
6.MicrobiologyHard
A healthcare worker's blood test is positive for anti-HBs (hepatitis B surface antibody) and negative for HBsAg and anti-HBc. What does this most likely mean?
APast natural infection that has cleared
BCurrent acute hepatitis B infection
CImmunity from hepatitis B vaccination
DChronic hepatitis B infection
7.PharmacologyMedium
During a rapid infusion of vancomycin, a patient develops flushing and a red rash on the face, neck and upper body. What is the most likely cause and action?
AA sign that the infection is spreading; give a second dose
BA normal effect; increase the infusion rate to finish sooner
CAcute kidney failure; stop all IV fluids and antibiotics at once
DAn infusion reaction; stop or slow the infusion and inform the prescriber
8.PharmacologyHard
Which strategy is most effective at preventing errors with IV insulin infusions?
AWriting "U" for units on the order
BPreparing insulin infusions in advance and leaving them unlabelled
CUsing insulin syringes to measure large IV volumes
DAn independent double check and a smart pump with a drug library
9.Arterial blood gas interpretationMedium
A practice arterial blood gas shows pH 7.11, PaCO₂ 78 mmHg (10.4 kPa) and HCO₃⁻ 24 mmol/L. The primary disorder is respiratory acidosis. How far is it compensated? Click your answer.
pH 7.11PaCO₂ 78 mmHg (10.4 kPa)HCO₃⁻ 24 mmol/L
1uncompensated
2partially compensated
3fully compensated
10.Arterial blood gas interpretationHard
At sea level, a practice gas taken on 28% oxygen shows PaO₂ 91 mmHg (12.1 kPa) and PaCO₂ 58 mmHg (7.7 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 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 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
2third-degree (complete) AV block
3first-degree AV block
4second-degree AV block type I (Wenckebach)
13.Evidence-based practice statisticsMedium
Practice exercise (made-up values, not clinical guidance). In a cohort study, exposed people contributed 5200 person-years with 26 new cases, and unexposed people 1900 person-years with 19 new cases. Find the incidence rate ratio IRR (exposed versus unexposed). Round to two decimal places.
exposed: 26/5200 PYnot exposed: 19/1900 PY
IRR =
14.Evidence-based practice statisticsHard
Practice exercise (made-up values, not clinical guidance). A patient has a pre-test probability of 60% for a disease. A test with sensitivity 95% and specificity 72% comes back negative. Find the post-test probability p′ of the disease as a percentage. Round to one decimal place.
pre-test 60%Se = 95%Sp = 72%negative
p′ = %
3Solutions
1.Human anatomy
Answer: C · Phrenic nerve
Why this is the answer
The phrenic nerve (roots C3–C5) is the motor supply to the diaphragm. High cervical cord injury above C3 therefore stops spontaneous breathing.
Answer
C · Phrenic nerve
2.Human anatomy
Answer: B · Its blood supply enters mainly from the distal end
Why this is the answer
Branches of the radial artery enter the scaphoid mainly distally and flow backwards. A fracture through the waist can cut off the proximal pole, causing avascular necrosis.
Answer
B · Its blood supply enters mainly from the distal end
3.Human physiology
Answer: C · The volume of the conducting airways where no gas exchange happens
Why this is the answer
Anatomical dead space (about 150 ml) is air in the nose, trachea and bronchi. It is moved with every breath but never reaches the alveoli.
Answer
C · The volume of the conducting airways where no gas exchange happens
4.Human physiology
Answer: D · High pH and low PaCO₂
Why this is the answer
Hyperventilation blows off CO₂ and causes acute respiratory alkalosis. Alkalosis increases calcium binding to albumin, lowering ionised calcium and causing tingling.
Answer
D · High pH and low PaCO₂
5.Microbiology
Answer: B · An altered penicillin-binding protein (PBP2a)
Why this is the answer
The mecA gene codes for PBP2a, which has low affinity for beta-lactams, so the bacterium can keep building its cell wall despite methicillin.
Answer
B · An altered penicillin-binding protein (PBP2a)
6.Microbiology
Answer: C · Immunity from hepatitis B vaccination
Why this is the answer
The vaccine contains HBsAg, so a vaccinated person makes anti-HBs but no anti-HBc. Past natural infection gives both anti-HBs and anti-HBc, and ongoing infection shows HBsAg.
Answer
C · Immunity from hepatitis B vaccination
7.Pharmacology
Answer: D · An infusion reaction; stop or slow the infusion and inform the prescriber
Why this is the answer
Rapid vancomycin infusion releases histamine, causing flushing and itching of the upper body (formerly called "red man syndrome"). It is managed by stopping or slowing the infusion, and sometimes with an antihistamine.
Answer
D · An infusion reaction; stop or slow the infusion and inform the prescriber
8.Pharmacology
Answer: D · An independent double check and a smart pump with a drug library
Why this is the answer
Independent double checks, standard concentrations and smart pumps with dose limits reduce insulin errors. "U" for units is avoided because it can be misread as a zero.
Answer
D · An independent double check and a smart pump with a drug library
9.Arterial blood gas interpretation
Answer: uncompensated
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 7.11: acidaemia (below 7.35)
PaCO₂ 78 mmHg: high (35–45)
HCO₃⁻ 24 mmol/L: normal (22–26)
Compensation
uncompensated
Answer
uncompensated
10.Arterial blood gas interpretation
Answer: A–a gradient = 36.14 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 = 11.5 × 0.04 = 0.46 s
RR = 25 × 0.04 = 1.00 s
Fridericia’s formula
QTc = 0.46/∛1.00 = 0.46/1 = 0.46 s
Answer
QTc = 0.46 s
Between the usual limits for men (about 0.45 s) and women (about 0.46 s): borderline, depending on the reference used. (460 ms)
12.ECG rhythm strip interpretation
Answer: third-degree (complete) AV block
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: R–R = 29 small boxes, 1500 ÷ 29 ≈ 52 per minute.
2. Rhythm: R–R regular (1.16 s) and P–P regular (0.64 s), but the two are independent.
3. P waves: upright and regular at 94 per minute, but not linked to the QRS complexes; some hide in a QRS or a T wave.
4. PR interval: varies from beat to beat; there is no fixed relation.
5. QRS: 0.08 s, narrow (below 0.12 s).
Interpretation
third-degree (complete) AV block
Answer
third-degree (complete) AV block
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 = 0.5
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: 26/5200 = 5 per 1000 person-years
not exposed: 19/1900 = 10 per 1000 person-years
Rate ratio
IRR = 5/10 = 0.5
Answer
IRR = 0.5
14.Evidence-based practice statistics
Answer: p′ = 9.4 %
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.