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.
falls 25, diagnosis 15, aid 15/30, IV 20, gait 10/20, mental status 15
NEWS2 risk
0–4 low; single 3 low–medium; 5–6 medium; ≥ 7 high
Mean arterial pressure
MAP = (SBP + 2·DBP)/3 = DBP + PP/3
Pulse pressure and shock index
PP = SBP − DBP, SI = HR/SBP
Body mass index
BMI = kg/m² = 703·lb/in²
Method
Find the score sheet that the question uses and its range (GCS 3–15, APGAR 0–10, Braden 6–23, Morse 0–125).
Score every item separately, matching the description to its level.
Add the item scores and check that the total lies inside the range.
Apply the cut-offs: for NEWS2 also look for any single parameter scoring 3.
For derived values use MAP = (SBP + 2·DBP)/3, PP = SBP − DBP, SI = HR/SBP and BMI = kg/m².
Med-surg: cardiovascular & respiratory
Method
Name the problem: pump failure, blocked artery, rhythm, clot, airway narrowing, infection or a device problem.
Check the danger signs first: hypoxia, hypotension, chest pain, new confusion, a cold pulseless limb.
Match the oxygen to the patient: 94–98% for most, 88–92% for those at risk of CO₂ retention.
Link each drug to its test and its risk: warfarin and INR, heparin and aPTT and platelets, diuretics and potassium.
Choose the safe first action, then escalate and reassess.
2Practice problems
1.Oxygen therapy & respiratory calculationsMedium
Practice exercise (made-up values, not clinical guidance). A size D oxygen cylinder (factor 0.16 L/psi) reads 1650 psi and runs at 5 L/min. Keeping a safe residual of 200 psi, how many minutes t will it last? Round down to a whole minute.
1650 psif = 0.16 L/psi5 L/min200 psi
t = min
2.Oxygen therapy & respiratory calculationsHard
Practice exercise (made-up values, not clinical guidance). A cylinder with a water capacity of 10 L is filled to 75 bar and runs at 4 L/min. The gas volume is about the water capacity × the pressure. In this exercise the cylinder is changed at 20 bar. How many minutes t until it must be changed? Round down to a whole minute.
10 L75 bar4 L/min20 bar
t = min
3.Arterial blood gas interpretationMedium
A practice gas shows a metabolic acidosis with HCO₃⁻ 10 mmol/L. Use Winter’s formula to find the expected PaCO₂ (the centre of the range) in mmHg.
HCO₃⁻ 10 mmol/L
PaCO₂ = mmHg
4.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
5.Clinical assessment scoresMedium
Practice exercise (made-up values, not clinical guidance). A patient has a pulse of 107/min and a blood pressure of 109/65 mmHg. Find the shock index SI = HR/SBP. Round to two decimal places.
HR 107/minBP 109/65 mmHg
SI =
6.Clinical assessment scoresHard
Practice exercise (made-up values, not clinical guidance). An adult weighs 59 kg and is 174 cm tall. Calculate the BMI and click the WHO weight category.
59 kg174 cm
1underweight
2healthy weight
3overweight
4obesity class I
5obesity class II
6obesity class III
7.Med-surg: cardiovascular & respiratoryMedium
A patient has central chest pain and a STEMI on the ECG. What does this usually require?
AEmergency reperfusion, preferably by primary PCI
BAdmission and a repeat ECG the next morning
COral beta-blocker and discharge if pain settles
DA troponin result before any treatment decision is made
8.Med-surg: cardiovascular & respiratoryHard
A patient with severe sepsis becomes increasingly breathless over 24 hours. SpO₂ stays low despite high-flow oxygen, and the chest X-ray shows bilateral infiltrates. The heart function is normal. Which condition is most likely?
ASpontaneous pneumothorax
BAcute respiratory distress syndrome
CAn acute asthma attack
DAcute left ventricular failure after an MI
3Solutions
1.Oxygen therapy & respiratory calculations
Answer: t = 46 min
Cylinder duration: usable litres = (gauge pressure − safe residual) × cylinder factor, with the safe residual usually 200 psi. Cylinder factors in L/psi: D 0.16, E 0.28, H (and K) 3.14. Duration (min) = usable litres/flow (L/min). Round a duration down, never up, so the cylinder is changed in time. In bar-based systems, the gas volume is the water capacity (L) × the pressure (bar).
Usable litres
(1650 − 200)·0.16 = 232 L
Duration
232/5 = 46.4 min → t = 46 min
Answer
t = 46 min
2.Oxygen therapy & respiratory calculations
Answer: t = 137 min
Cylinder duration: usable litres = (gauge pressure − safe residual) × cylinder factor, with the safe residual usually 200 psi. Cylinder factors in L/psi: D 0.16, E 0.28, H (and K) 3.14. Duration (min) = usable litres/flow (L/min). Round a duration down, never up, so the cylinder is changed in time. In bar-based systems, the gas volume is the water capacity (L) × the pressure (bar).
Usable gas
10·(75 − 20) = 550 L
Duration
550/4 = 137.5 min → t = 137 min
Answer
t = 137 min
That is about 2 h 17 min.
3.Arterial blood gas interpretation
Answer: PaCO₂ = 23 mmHg
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.
Winter’s formula
PaCO₂ = 1.5·10 + 8 = 23 mmHg
range: 21–25 mmHg
Answer
PaCO₂ = 23 mmHg
4.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
5.Clinical assessment scores
Answer: SI = 0.98
Blood pressure values: pulse pressure PP = SBP − DBP. Mean arterial pressure MAP ≈ (SBP + 2·DBP)/3 = DBP + PP/3, an estimate based on the heart spending about two thirds of each beat in diastole at a normal heart rate (less accurate when the heart rate is fast); a MAP of at least about 65 mmHg is a common target for organ perfusion. The shock index SI = heart rate/SBP is normally about 0.5–0.7; a value above about 1 is a warning sign of shock.
Shock index
SI = 107/109 = 0.9817 ≈ 0.98
Answer
SI = 0.98
Above the usual 0.5–0.7.
6.Clinical assessment scores
Answer: healthy weight
Body mass index: BMI = weight (kg)/height (m)², with the height converted to metres first. With pounds and inches, BMI = 703 × weight (lb)/height (in)². WHO adult categories: below 18.5 underweight, 18.5–24.9 healthy weight, 25.0–29.9 overweight, 30.0–34.9 obesity class I, 35.0–39.9 obesity class II and 40 or more obesity class III.
BMI
BMI = 59/1.74² = 19.49
Category
healthy weight
Answer
healthy weight
7.Med-surg: cardiovascular & respiratory
Answer: A · Emergency reperfusion, preferably by primary PCI
Why this is the answer
A STEMI usually reflects total occlusion of a coronary artery. Emergency reperfusion, preferably primary PCI (or thrombolysis if PCI cannot be done in time), is needed as soon as possible; "time is muscle". The decision is made on the ECG and symptoms, not on the troponin result.
Answer
A · Emergency reperfusion, preferably by primary PCI
8.Med-surg: cardiovascular & respiratory
Answer: B · Acute respiratory distress syndrome
Why this is the answer
ARDS is acute, non-cardiogenic pulmonary oedema caused by widespread inflammation, triggered by sepsis, pneumonia, trauma or pancreatitis. It causes severe hypoxaemia that responds poorly to oxygen, with bilateral infiltrates not explained by heart failure. Treatment includes lung-protective ventilation and often prone positioning.