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 part of the nephron does most reabsorption of water and sodium take place?
ALoop of Henle
BProximal convoluted tubule
CDistal convoluted tubule
DCollecting duct
2.Human anatomyHard
Why can an inflamed gallbladder that irritates the diaphragm cause pain at the tip of the right shoulder?
ABile travels up the lymphatics to the shoulder
BThe vagus nerve supplies the skin of the shoulder
CThe gallbladder is attached to the right clavicle
DThe phrenic nerve shares spinal segments C3–C5 with the skin of the shoulder
3.Human physiologyMedium
The Frank–Starling mechanism describes that:
AHeart rate rises when arterial blood pressure falls suddenly
BContractility falls when temperature rises
CThe atria contract after the ventricles
DStroke volume rises when ventricular filling increases
4.Human physiologyHard
During exercise, blood flow to active skeletal muscle rises greatly. What is the main cause?
ASympathetic stimulation constricts the muscle arterioles via alpha receptors
BLocal metabolites such as CO₂, H⁺, K⁺ and adenosine dilate the arterioles
CThe parasympathetic system dilates all arteries
DThe heart pumps blood only to the muscles
5.MicrobiologyMedium
Which is a function of the complement system?
AProducing antibodies against newly encountered antigens
BPresenting antigens to T cells
CForming pores in the membranes of some bacteria
DMaking new red blood cells
6.MicrobiologyHard
Why can a person get influenza many times but measles usually only once?
AInfluenza attacks and destroys the memory cells formed after infection
BMeasles is caused by a bacterium
CInfluenza viruses change their surface antigens often
DMeasles virus has no surface antigens
7.PharmacologyMedium
Patients taking metronidazole are usually advised to avoid:
ASunlight
BAlcohol
CDairy products
DGrapefruit juice
8.PharmacologyHard
A 19-year-old woman is starting isotretinoin for severe acne. Which requirement is most important?
AReliable contraception and pregnancy tests, as it is highly teratogenic
BA high-vitamin A diet to boost the effect
CWeekly hearing tests and monthly eye examinations during treatment
DTaking the drug only at weekends
9.Arterial blood gas interpretationMedium
A practice arterial blood gas shows pH 7.60, PaCO₂ 20 mmHg (2.7 kPa) and HCO₃⁻ 19 mmol/L. The primary disorder is respiratory alkalosis. How far is it compensated? Click your answer.
pH 7.60PaCO₂ 20 mmHg (2.7 kPa)HCO₃⁻ 19 mmol/L
1uncompensated
2partially compensated
3fully compensated
10.Arterial blood gas interpretationHard
A practice gas shows a respiratory acidosis: pH 7.22, PaCO₂ 85 mmHg (11.3 kPa), HCO₃⁻ 34 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 7.22PaCO₂ 85 mmHg (11.3 kPa)HCO₃⁻ 34 mmol/L
1acute
2chronic
3acute on chronic
11.ECG rhythm strip interpretationMedium
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
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 fibrillation
2third-degree (complete) AV block
3sinus bradycardia
4asystole
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, 99 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 87% and a specificity of 92%. It is used in a population where the prevalence is 30%. Find the positive predictive value PPV as a percentage. Round to one decimal place.
Se = 87%Sp = 92%p = 30%
PPV = %
3Solutions
1.Human anatomy
Answer: B · Proximal convoluted tubule
Why this is the answer
The proximal convoluted tubule reabsorbs about two thirds of the filtered sodium and water, plus nearly all glucose and amino acids.
Answer
B · Proximal convoluted tubule
2.Human anatomy
Answer: D · The phrenic nerve shares spinal segments C3–C5 with the skin of the shoulder
Why this is the answer
Diaphragmatic irritation is carried by the phrenic nerve (C3–C5). The brain refers it to skin supplied by the same segments, via the supraclavicular nerves over the shoulder.
Answer
D · The phrenic nerve shares spinal segments C3–C5 with the skin of the shoulder
3.Human physiology
Answer: D · Stroke volume rises when ventricular filling increases
Why this is the answer
Greater end-diastolic volume stretches the myocardium, which increases the force of contraction and the stroke volume, matching output to venous return.
Answer
D · Stroke volume rises when ventricular filling increases
4.Human physiology
Answer: B · Local metabolites such as CO₂, H⁺, K⁺ and adenosine dilate the arterioles
Why this is the answer
Active hyperaemia: metabolites produced by working muscle relax arteriolar smooth muscle locally, overriding sympathetic vasoconstriction in that tissue.
Answer
B · Local metabolites such as CO₂, H⁺, K⁺ and adenosine dilate the arterioles
5.Microbiology
Answer: C · Forming pores in the membranes of some bacteria
Why this is the answer
Complement proteins coat microbes for phagocytosis (opsonisation), attract immune cells and form the membrane attack complex that punches holes in some bacteria.
Answer
C · Forming pores in the membranes of some bacteria
6.Microbiology
Answer: C · Influenza viruses change their surface antigens often
Why this is the answer
Influenza haemagglutinin and neuraminidase change through drift (small mutations) and shift (reassortment), so old antibodies may not recognise new strains. Measles is antigenically stable.
Answer
C · Influenza viruses change their surface antigens often
7.Pharmacology
Answer: B · Alcohol
Why this is the answer
Metronidazole can cause a disulfiram-like reaction with alcohol, with flushing, nausea and palpitations. Alcohol is avoided during treatment and for a short time after.
Answer
B · Alcohol
8.Pharmacology
Answer: A · Reliable contraception and pregnancy tests, as it is highly teratogenic
Why this is the answer
Isotretinoin causes severe birth defects. Pregnancy prevention programmes require effective contraception and regular pregnancy tests before, during and after treatment.
Answer
A · Reliable contraception and pregnancy tests, as it is highly teratogenic
9.Arterial blood gas interpretation
Answer: partially 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 7.60: alkalaemia (above 7.45)
PaCO₂ 20 mmHg: low (35–45)
HCO₃⁻ 19 mmol/L: low (22–26)
Compensation
partially compensated
Answer
partially compensated
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 + 1·4.5 = 28.5 mmol/L
chronic: 24 + 3.5·4.5 = 39.75 mmol/L
Compare
measured: 34 mmol/L → acute on chronic
Answer
acute on chronic
11.ECG rhythm strip interpretation
Answer: QTc = 0.458 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 = 21 × 0.04 = 0.84 s
Bazett’s formula
QTc = 0.42/√0.84 = 0.42/0.9165 = 0.458 s
Answer
QTc = 0.458 s
Between the usual limits for men (about 0.45 s) and women (about 0.46 s): borderline, depending on the reference used. (458 ms)
12.ECG rhythm strip interpretation
Answer: asystole
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: no countable ventricular rate.
2. Rhythm: none; an almost flat line.
3. P waves: none identifiable.
4. PR interval: not measurable.
5. QRS: no QRS complexes.
Interpretation
asystole
Answer
asystole
A flat line is always checked: look at the patient, the leads and the gain, and view a second lead, because a loose electrode or fine VF can look like asystole.
13.Evidence-based practice statistics
Answer: IR = 21.9 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 = 390·5 + 190·2 + 550·4 = 4530
Incidence rate
IR = 99/4530·1000 = 21.9 per 1000 person-years
Answer
IR = 21.9 per 1000 person-years
14.Evidence-based practice statistics
Answer: PPV = 82.3 %
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.
True and false positives (per person)
Se·p = 0.87·0.3 = 0.261
(1 − Sp)·(1 − p) = 0.08·0.7 = 0.056
Positive predictive value
PPV = 0.261/(0.261 + 0.056) = 0.8233 ≈ 82.3%
Answer
PPV = 82.3%
The lower the prevalence, the lower the PPV of the same test.