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 H oxygen cylinder (factor 3.14 L/psi) reads 850 psi and runs at 6 L/min. Keeping a safe residual of 200 psi, how many minutes t will it last? Round down to a whole minute.
850 psif = 3.14 L/psi6 L/min200 psi
t = min
2.Oxygen therapy & respiratory calculationsHard
Practice exercise (made-up values, not clinical guidance). A patient has Hb 100 g/L, SaO₂ 98%, PaO₂ 103 mmHg (13.7 kPa) and a cardiac output of 4.5 L/min. With CaO₂ = 1.34 × Hb (g/dL) × SaO₂ + 0.003 × PaO₂ (mmHg), find the oxygen delivery DO₂ = CaO₂ × CO × 10 in mL/min.
A practice arterial blood gas shows pH 7.36, PaCO₂ 51 mmHg (6.8 kPa) and HCO₃⁻ 28 mmol/L. The primary disorder is respiratory acidosis. How far is it compensated? Click your answer.
pH 7.36PaCO₂ 51 mmHg (6.8 kPa)HCO₃⁻ 28 mmol/L
1uncompensated
2partially compensated
3fully compensated
4.Arterial blood gas interpretationHard
Practice values: Na⁺ 140, Cl⁻ 107 and HCO₃⁻ 17 mmol/L. Calculate the delta ratio (AG − 12)/(24 − HCO₃⁻), using AG = Na⁺ − (Cl⁻ + HCO₃⁻). Give two decimals.
Na⁺ 140 mmol/LCl⁻ 107 mmol/LHCO₃⁻ 17 mmol/L
delta ratio =
5.Clinical assessment scoresMedium
Practice exercise (made-up values, not clinical guidance). Use the NEWS2 chart (SpO₂ scale 1) for these observations: respiration rate 20/min, SpO₂ 93% on room air, blood pressure 92/56 mmHg, pulse 70/min, alert, temperature 39.0 °C. What is the NEWS2 total score?
RR 20SpO₂ 93%airSBP 92HR 70AT 39.0 °C
NEWS2 =
6.Clinical assessment scoresHard
Practice exercise (made-up values, not clinical guidance). An adult weighs 149 kg and is 169 cm tall. Calculate the BMI and click the WHO weight category.
149 kg169 cm
1underweight
2healthy weight
3overweight
4obesity class I
5obesity class II
6obesity class III
7.Med-surg: cardiovascular & respiratoryMedium
After repair of an abdominal aortic aneurysm, which regular assessment helps detect a blocked graft or clots in the legs?
AMeasuring the blood glucose every hour through the night
BTesting the gag reflex every four hours after surgery
CChecking leg pulses, colour, warmth and sensation
DChecking the pupils' response to light
8.Med-surg: cardiovascular & respiratoryHard
A patient with crushing chest pain has ST elevation in leads V1–V4. Which coronary artery is most likely blocked?
AThe left anterior descending artery
BThe circumflex artery
CThe right coronary artery and its branches
DThe left subclavian artery
3Solutions
1.Oxygen therapy & respiratory calculations
Answer: t = 340 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
(850 − 200)·3.14 = 2041 L
Duration
2041/6 = 340.2 min → t = 340 min
Answer
t = 340 min
That is about 5 h 40 min.
2.Oxygen therapy & respiratory calculations
Answer: DO₂ = 604.8 mL/min
Oxygen content and delivery: CaO₂ = 1.34 × Hb (g/dL) × SaO₂ + 0.003 × PaO₂ (mmHg), in mL O₂ per dL of blood, with SaO₂ as a fraction. Almost all oxygen is bound to haemoglobin; the dissolved part is small. Delivery DO₂ = CaO₂ × cardiac output (L/min) × 10 (mL/min), because 1 L = 10 dL; a normal resting adult value is about 1000 mL/min.
Oxygen content
100 g/L = 10 g/dL
CaO₂ = 1.34·10·0.98 + 0.003·103 = 13.44 mL/dL
Oxygen delivery
DO₂ = 13.44·4.5·10 = 604.8 mL/min
Answer
DO₂ = 604.8 mL/min
A normal resting adult value is about 1000 mL/min.
3.Arterial blood gas interpretation
Answer: fully 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 7.36: within 7.35–7.45
PaCO₂ 51 mmHg: high (35–45)
HCO₃⁻ 28 mmol/L: high (22–26)
Compensation
fully compensated
Answer
fully compensated
4.Arterial blood gas interpretation
Answer: delta ratio = 0.5714
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 = 140 − (107 + 17) = 16
Delta ratio
(16 − 12)/(24 − 17) = 4/7 = 0.5714
Answer
delta ratio = 0.5714
Below 1: an added normal anion gap acidosis.
5.Clinical assessment scores
Answer: NEWS2 = 5
NEWS2 (National Early Warning Score 2, Royal College of Physicians 2017): score each of the seven parameters with the chart (respiration rate, SpO₂ on scale 1, air or supplemental oxygen, systolic blood pressure, pulse, consciousness on the ACVPU scale and temperature) and add the scores. Clinical risk: 0–4 low; a score of 3 in any single parameter low–medium; 5–6 medium; 7 or more high. A total of 5 or more needs an urgent response.
Score each parameter
RR 20/min → 0
SpO₂ 93% → 2
on air → 0
SBP 92 mmHg → 2
HR 70/min → 0
alert (A) → 0
T 39.0 °C → 1
Add
NEWS2 = 0 + 2 + 0 + 2 + 0 + 0 + 1 = 5
Answer
NEWS2 = 5
6.Clinical assessment scores
Answer: obesity class III
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 = 149/1.69² = 52.17
Category
obesity class III
Answer
obesity class III
7.Med-surg: cardiovascular & respiratory
Answer: C · Checking leg pulses, colour, warmth and sensation
Why this is the answer
After aortic surgery, distal pulses, skin colour, temperature, capillary refill, movement and sensation are checked often and compared with baseline. Loss of pulses, pallor or pain suggests graft occlusion or emboli and must be reported at once. Urine output is also monitored for renal perfusion.
Answer
C · Checking leg pulses, colour, warmth and sensation
8.Med-surg: cardiovascular & respiratory
Answer: A · The left anterior descending artery
Why this is the answer
ST elevation in V1–V4 indicates an anteroseptal STEMI, usually from LAD occlusion. Anterior MIs damage a large area of muscle and carry a high risk of heart failure and arrhythmias. Leads II, III and aVF suggest an inferior MI, usually from the right coronary artery.