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 sympathetic nervous system leaves the central nervous system from which spinal segments?
ASacral only (S2–S4)
BCervical only
CThoracic and upper lumbar (T1–L2)
DCranial and sacral
2.Human anatomyHard
Which coronary artery most often supplies the sinoatrial (SA) node?
ALeft anterior descending artery
BRight coronary artery
CCircumflex artery
DLeft main coronary artery
3.Human physiologyMedium
What is the main effect of vagus nerve stimulation on the heart?
AIt slows the heart rate
BIt increases ventricular contractility strongly
CIt constricts the coronary arteries
DIt increases the heart rate
4.Human physiologyHard
In pernicious anaemia, autoimmune destruction of gastric parietal cells leads to anaemia because:
AIntrinsic factor is lost, so vitamin B12 is not absorbed
BGastrin is lost, so the bone marrow is no longer stimulated
CPepsin is lost, so haemoglobin cannot be made
DAcid is lost, so iron is never absorbed in any form
5.MicrobiologyMedium
Why are Gram-negative bacteria often harder to treat with antibiotics?
ATheir outer membrane blocks many drugs
BThey have a thicker peptidoglycan layer
CThey divide much more slowly
DThey have no ribosomes
6.MicrobiologyHard
An older adult develops a painful blistering rash in a band on one side of the chest. What explains this pattern?
ABacteria spread along the lymphatic vessels of the chest
BA new chickenpox infection spreads through the blood to one side
CVaricella-zoster virus reactivates from one sensory ganglion and travels along its nerve
DAn allergic reaction to clothing affects one dermatome
7.PharmacologyMedium
Which receptor do beta blockers such as metoprolol mainly block?
ABeta-1 adrenergic receptors in the heart
BAlpha-1 receptors in blood vessels
CBeta-3 receptors in fat tissue
DMuscarinic receptors in the heart
8.PharmacologyHard
A patient who takes clonidine stopped it suddenly two days ago. He is now agitated, has a headache and his blood pressure is very high. What is the most likely explanation?
ARebound hypertension from abrupt clonidine withdrawal
BClonidine toxicity due to slow excretion by the kidneys
CNormal variation unrelated to the medication
DAn allergic reaction to clonidine
9.Arterial blood gas interpretationMedium
Practice values: Na⁺ 149 mmol/L, glucose 20.4 mmol/L and urea 3.9 mmol/L. Calculate the serum osmolality Osm = 2·Na⁺ + glucose + urea to the nearest whole number.
Na⁺ 149 mmol/Lglucose 20.4 mmol/Lurea 3.9 mmol/L
Osm = mOsm/kg
10.Arterial blood gas interpretationHard
At sea level, a practice gas taken on 28% oxygen shows PaO₂ 70 mmHg (9.3 kPa) and PaCO₂ 46 mmHg (6.1 kPa). Use PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8 to find the A–a gradient.
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
1atrial fibrillation
2sinus rhythm with PVCs
3atrial flutter, 4:1 conduction
4sinus arrhythmia
12.ECG rhythm strip interpretationHard
Identify the rhythm on this 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
1ventricular tachycardia
2sinus tachycardia
3atrial fibrillation
4atrial flutter, 2:1 conduction
13.Evidence-based practice statisticsMedium
Practice exercise (made-up values, not clinical guidance). In a randomised trial, 800 patients received a new treatment and 320 received the control treatment. The table shows how many in each group had a side effect. Find the number needed to harm NNH: how many patients must be treated for one extra patient to have the side effect? Round up to a whole number of patients.
treatment: 259/800control: 58/320
NNH =
14.Evidence-based practice statisticsHard
Practice exercise (made-up values, not clinical guidance). Without a drug, 34% of patients have a certain side effect during a year. The drug raises this risk with a relative risk of RR = 1.98. Find the number needed to harm NNH: how many patients must take the drug for a year for one extra patient to have the side effect? Round up to a whole number of patients.
control 34%RR = 1.98
NNH =
3Solutions
1.Human anatomy
Answer: C · Thoracic and upper lumbar (T1–L2)
Why this is the answer
Sympathetic preganglionic neurons lie in the lateral horn from T1 to L2. The parasympathetic system has a craniosacral outflow.
Answer
C · Thoracic and upper lumbar (T1–L2)
2.Human anatomy
Answer: B · Right coronary artery
Why this is the answer
The SA nodal artery arises from the right coronary artery in roughly 60% of people and from the circumflex artery in the rest.
Answer
B · Right coronary artery
3.Human physiology
Answer: A · It slows the heart rate
Why this is the answer
The vagus releases acetylcholine on muscarinic receptors in the SA and AV nodes, slowing pacemaker firing and AV conduction.
Answer
A · It slows the heart rate
4.Human physiology
Answer: A · Intrinsic factor is lost, so vitamin B12 is not absorbed
Why this is the answer
Parietal cells secrete intrinsic factor, needed for B12 absorption in the terminal ileum. B12 deficiency impairs DNA synthesis and causes megaloblastic anaemia.
Answer
A · Intrinsic factor is lost, so vitamin B12 is not absorbed
5.Microbiology
Answer: A · Their outer membrane blocks many drugs
Why this is the answer
The outer membrane of Gram-negative bacteria, with lipopolysaccharide and porins, keeps many antibiotics out and adds efflux and enzyme defences.
Answer
A · Their outer membrane blocks many drugs
6.Microbiology
Answer: C · Varicella-zoster virus reactivates from one sensory ganglion and travels along its nerve
Why this is the answer
After chickenpox, VZV stays latent in a dorsal root or cranial nerve ganglion. When immunity weakens it can reactivate, travelling down that nerve to cause shingles in its dermatome.
Answer
C · Varicella-zoster virus reactivates from one sensory ganglion and travels along its nerve
7.Pharmacology
Answer: A · Beta-1 adrenergic receptors in the heart
Answer: A · Rebound hypertension from abrupt clonidine withdrawal
Why this is the answer
Clonidine is a central alpha-2 agonist. Stopping it suddenly lets sympathetic activity surge, causing rebound hypertension, so it is tapered gradually.
Answer
A · Rebound hypertension from abrupt clonidine withdrawal
9.Arterial blood gas interpretation
Answer: Osm = 322 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 = 2·149 + 20.4 + 3.9 = 322.3 ≈ 322 mOsm/kg
Answer
Osm = 322 mOsm/kg
10.Arterial blood gas interpretation
Answer: A–a gradient = 72.14 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.
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: 8 QRS complexes in 6 s, × 10 = 80 per minute.
2. Rhythm: irregularly irregular, R–R from 0.54 to 1.00 s with no pattern.
3. P waves: none; the baseline shows small irregular fibrillatory waves.
4. PR interval: not measurable.
5. QRS: 0.08 s, narrow (below 0.12 s).
Interpretation
atrial fibrillation
Answer
atrial fibrillation
12.ECG rhythm strip interpretation
Answer: ventricular tachycardia
Ventricular rhythms: ventricular tachycardia is a run of three or more wide (0.12 s or more) complexes in a row at more than 100/min, usually 150–250/min, regular, without related P waves. Ventricular fibrillation is a chaotic, irregular waveform with no identifiable P waves, QRS complexes or T waves and no countable rate. Asystole is an almost flat line with no ventricular activity; a flat line is always confirmed by checking the patient, the leads and the gain and a second lead. In resuscitation guidelines VF and pulseless VT are the shockable rhythms and asystole is not.
Five steps
1. Rate: R–R = 10 small boxes, 1500 ÷ 10 ≈ 150 per minute.
2. Rhythm: regular, every R–R is 0.40 s.
3. P waves: none identifiable.
4. PR interval: not measurable.
5. QRS: 0.16 s, wide (0.12 s or more).
Interpretation
ventricular tachycardia
Answer
ventricular tachycardia
In resuscitation guidelines VF and pulseless VT are the shockable rhythms. This is identification practice only.
13.Evidence-based practice statistics
Answer: NNH = 8
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.
Event rates
EER = 259/800 = 0.3237
CER = 58/320 = 0.1812
Number needed to harm
ARI = 0.3237 − 0.1812 = 0.1425
NNH = 1/0.1425 = 7.018 → 8
Answer
NNH = 8
About one in every 8 treated patients has the side effect because of the treatment.
14.Evidence-based practice statistics
Answer: NNH = 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.