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 bone of the skull contains the foramen magnum?
ATemporal bone
BOccipital bone
CParietal bone
DSphenoid bone
2.Human anatomyHard
In the femoral triangle, what is the order of the main structures from lateral to medial?
AFemoral artery, femoral nerve, femoral vein
BFemoral nerve, femoral artery, femoral vein
CFemoral vein, femoral artery, femoral nerve
DFemoral nerve, femoral vein, femoral artery
3.Human physiologyMedium
Which ABO blood group has both anti-A and anti-B antibodies in the plasma?
AGroup A
BGroup AB
CGroup B
DGroup O
4.Human physiologyHard
Why does fetal haemoglobin have a higher affinity for oxygen than adult haemoglobin?
AIt contains more iron per haem
BIt binds 2,3-BPG less strongly
CFetal blood has a higher pH
DIt has five subunits instead of four
5.MicrobiologyMedium
Which laboratory test separates Staphylococcus aureus from most other staphylococci?
AThe acid-fast stain
BThe coagulase test
CThe catalase test
DThe Gram stain
6.MicrobiologyHard
Primary syphilis typically presents with:
ADamage to the heart and brain
BA widespread rash on the palms and soles
CA painless ulcer (chancre) at the site of infection
DA painful cluster of small blisters on the genitals or the mouth
7.PharmacologyMedium
Which type of diuretic acts on the thick ascending limb of the loop of Henle?
AThiazide diuretics
BOsmotic diuretics
CPotassium-sparing diuretics
DLoop diuretics such as furosemide
8.PharmacologyHard
On admission, a patient's home medicine list includes warfarin for atrial fibrillation, but warfarin is not on the admission orders. What should the nurse do?
AGive the patient's own warfarin brought from home without an order
BClarify the omission with the prescriber (medication reconciliation)
CTell the patient to stop warfarin permanently
DAssume the omission is intended and do nothing
9.Arterial blood gas interpretationMedium
A practice gas shows PaCO₂ 77 mmHg (10.3 kPa). Use the rule that in chronic respiratory acidosis HCO₃⁻ rises by 3.5 mmol/L for every 10 mmHg that PaCO₂ is above 40, starting from HCO₃⁻ 24 mmol/L, to find the expected HCO₃⁻.
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
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 I (Wenckebach)
2second-degree AV block type II
3first-degree AV block
4third-degree (complete) AV block
13.Evidence-based practice statisticsMedium
Practice exercise (made-up values, not clinical guidance). A case-control study compares 230 patients with a disease (cases) and 380 people without it (controls). The table shows how many in each group were exposed to a risk factor. Find the odds ratio OR for the exposure. Round to two decimal places.
a = 76b = 209c = 154d = 171
OR =
14.Evidence-based practice statisticsHard
Practice exercise (made-up values, not clinical guidance). A screening test has a sensitivity of 95% and a specificity of 97%. 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 = 95%Sp = 97%p = 0.5%
PPV = %
3Solutions
1.Human anatomy
Answer: B · Occipital bone
Why this is the answer
The foramen magnum is a large opening in the occipital bone through which the medulla oblongata continues as the spinal cord.
Answer
B · Occipital bone
2.Human anatomy
Answer: B · Femoral nerve, femoral artery, femoral vein
Why this is the answer
From lateral to medial: nerve, artery, vein, then empty space and lymphatics (NAVEL). This is why the femoral artery pulse lies between the nerve and vein.
Answer
B · Femoral nerve, femoral artery, femoral vein
3.Human physiology
Answer: D · Group O
Why this is the answer
Group O people have no A or B antigens and make both anti-A and anti-B. Group AB has both antigens and neither antibody.
Answer
D · Group O
4.Human physiology
Answer: B · It binds 2,3-BPG less strongly
Why this is the answer
The gamma chains of HbF bind 2,3-BPG poorly, so the curve is shifted left. This lets fetal blood take oxygen from maternal blood in the placenta.
Answer
B · It binds 2,3-BPG less strongly
5.Microbiology
Answer: B · The coagulase test
Why this is the answer
S. aureus produces coagulase, which converts fibrinogen to fibrin. All staphylococci are Gram-positive and catalase-positive, so those tests do not separate them.
Answer
B · The coagulase test
6.Microbiology
Answer: C · A painless ulcer (chancre) at the site of infection
Why this is the answer
Treponema pallidum causes a painless chancre in the primary stage. The secondary stage brings rash and systemic symptoms, and tertiary syphilis can damage the heart and nervous system. Painful blisters suggest herpes.
Answer
C · A painless ulcer (chancre) at the site of infection
7.Pharmacology
Answer: D · Loop diuretics such as furosemide
Why this is the answer
Loop diuretics block the Na⁺/K⁺/2Cl⁻ cotransporter in the thick ascending limb and are the most powerful diuretics.
Answer
D · Loop diuretics such as furosemide
8.Pharmacology
Answer: B · Clarify the omission with the prescriber (medication reconciliation)
Why this is the answer
Medication reconciliation compares the patient's home medicines with current orders to find omissions, duplications and errors. The nurse raises the difference with the prescriber, who decides whether to continue, hold or change the drug.
Answer
B · Clarify the omission with the prescriber (medication reconciliation)
9.Arterial blood gas interpretation
Answer: HCO₃⁻ = 36.95 mmol/L
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.
Change in PaCO₂
|77 − 40| = 37 mmHg → 3.7 × 10 mmHg
Expected HCO₃⁻
HCO₃⁻ = 24 + 3.5·3.7 = 36.95 mmol/L
Answer
HCO₃⁻ = 36.95 mmol/L
10.Arterial blood gas interpretation
Answer: AGc = 18.5 mmol/L
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 = 143 − (111 + 15) = 17
Correction for albumin
17 + 0.25·(40 − 34) = 18.5 mmol/L
Answer
AGc = 18.5 mmol/L
Above about 12 after correction: a high anion gap.
11.ECG rhythm strip interpretation
Answer: QTc = 0.5 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 × 0.04 = 0.40 s
RR = 16 × 0.04 = 0.64 s
Bazett’s formula
QTc = 0.40/√0.64 = 0.40/0.8 = 0.5 s
Answer
QTc = 0.5 s
Above the usual limits for both men (about 0.45 s) and women (about 0.46 s): prolonged. (500 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 = 51 small boxes, 1500 ÷ 51 ≈ 29 per minute.
2. Rhythm: R–R regular (2.04 s) and P–P regular (0.80 s), but the two are independent.
3. P waves: upright and regular at 75 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: 0.16 s, wide (0.12 s or more).
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: OR = 0.4
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.
Odds of exposure
cases: 76/154 = 0.4935
controls: 209/171 = 1.222
Odds ratio
OR = (76·171)/(209·154) = 0.4038 ≈ 0.4
Answer
OR = 0.4
OR below 1: exposure is less common among the cases, which suggests a protective effect.
14.Evidence-based practice statistics
Answer: PPV = 13.7 %
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.