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 patient has Hb 106 g/L, SaO₂ 89% and PaO₂ 56 mmHg (7.5 kPa). Find the arterial oxygen content CaO₂ = 1.34 × Hb (g/dL) × SaO₂ + 0.003 × PaO₂ (mmHg) in mL O₂/dL.
Hb 106 g/LSaO₂ 89%PaO₂ 56 mmHg (7.5 kPa)
CaO₂ = mL/dL
2.Oxygen therapy & respiratory calculationsHard
Practice exercise (made-up values, not clinical guidance). A transfer will take 90 minutes. The size D cylinder (factor 0.16 L/psi) reads 800 psi. Keeping a safe residual of 200 psi, what is the highest whole-number flow Q in L/min that lasts the whole transfer?
90 min800 psif = 0.16 L/psi200 psi
Q = L/min
3.Arterial blood gas interpretationMedium
Practice values: Na⁺ 132 mmol/L, glucose 7.1 mmol/L and urea 16.7 mmol/L. Calculate the serum osmolality Osm = 2·Na⁺ + glucose + urea to the nearest whole number.
Na⁺ 132 mmol/Lglucose 7.1 mmol/Lurea 16.7 mmol/L
Osm = mOsm/kg
4.Arterial blood gas interpretationHard
A practice gas shows pH 7.31, PaCO₂ 27 mmHg (3.6 kPa) and HCO₃⁻ 13 mmol/L, with Na⁺ 146 and Cl⁻ 95 mmol/L. Use the anion gap and the delta ratio to click the best interpretation.
Practice exercise (made-up values, not clinical guidance). Use the NEWS2 chart (SpO₂ scale 1) for these observations: respiration rate 14/min, SpO₂ 99% on room air, blood pressure 88/52 mmHg, pulse 76/min, responds only to pain, temperature 36.7 °C. What is the NEWS2 total score?
RR 14SpO₂ 99%airSBP 88HR 76PT 36.7 °C
NEWS2 =
6.Clinical assessment scoresHard
Practice exercise (made-up values, not clinical guidance). A practice patient is assessed with the Braden scale: sensory perception: very limited; moisture: constantly moist; activity: bedfast; mobility: completely immobile; nutrition: very poor; friction and shear: no apparent problem. Use the bands 19–23 not at risk, 15–18 mild, 13–14 moderate, 10–12 high and 9 or below very high. Click the risk level.
1not at risk
2mild risk
3moderate risk
4high risk
5very high risk
7.Med-surg: cardiovascular & respiratoryMedium
According to current guidance, when should oxygen be given to a patient with an acute coronary syndrome?
AOnly once the troponin result is available
BOnly when SpO₂ is below the target range
CNever, because oxygen harms heart muscle
DAlways at 15 L/min, whatever the saturation
8.Med-surg: cardiovascular & respiratoryHard
A patient with asthma needs their reliever every three hours and their peak flow is falling well below their personal best. What does their action plan most likely advise?
ADo extra exercise to open the airways
BWait two weeks to see whether it improves without any change
CStop the preventer inhaler for a few days to rest the lungs
DFollow the worsening-asthma steps and get a same-day review
3Solutions
1.Oxygen therapy & respiratory calculations
Answer: CaO₂ = 12.81 mL/dL
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.
Hb in g/dL
106 g/L = 10.6 g/dL
Bound oxygen
1.34·10.6·0.89 = 12.64 mL/dL
Dissolved oxygen
0.003·56 = 0.168 mL/dL
Oxygen content
CaO₂ = 12.64 + 0.168 = 12.81 mL/dL
Answer
CaO₂ = 12.81 mL/dL
2.Oxygen therapy & respiratory calculations
Answer: Q = 1 L/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
(800 − 200)·0.16 = 96 L
Highest flow
96/90 = 1.067 L/min → Q = 1 L/min
Answer
Q = 1 L/min
3.Arterial blood gas interpretation
Answer: Osm = 288 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 = 2·132 + 7.1 + 16.7 = 287.8 ≈ 288 mOsm/kg
Answer
Osm = 288 mOsm/kg
4.Arterial blood gas interpretation
Answer: added metabolic alkalosis
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 = 146 − (95 + 13) = 38
Delta ratio
(38 − 12)/(24 − 13) = 2.364
added metabolic alkalosis
Answer
added metabolic alkalosis
5.Clinical assessment scores
Answer: NEWS2 = 6
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 14/min → 0
SpO₂ 99% → 0
on air → 0
SBP 88 mmHg → 3
HR 76/min → 0
responds only to pain (P) → 3
T 36.7 °C → 0
Add
NEWS2 = 0 + 0 + 0 + 3 + 0 + 3 + 0 = 6
Answer
NEWS2 = 6
6.Clinical assessment scores
Answer: very high risk
Braden scale: six subscales for pressure-injury risk. Sensory perception, moisture, activity, mobility and nutrition score 1 to 4, from the worst level (1) to no problem (4); friction and shear scores 1 to 3. The total runs from 6 to 23, and a lower total means a higher risk. Common bands: 19–23 not at risk, 15–18 mild risk, 13–14 moderate risk, 10–12 high risk and 9 or below very high risk.
Score each subscale
sensory perception: very limited → 2
moisture: constantly moist → 1
activity: bedfast → 1
mobility: completely immobile → 1
nutrition: very poor → 1
friction and shear: no apparent problem → 3
Add
Braden total = 2 + 1 + 1 + 1 + 1 + 3 = 9
Risk level
9 → very high risk
Answer
very high risk
7.Med-surg: cardiovascular & respiratory
Answer: B · Only when SpO₂ is below the target range
Why this is the answer
Routine high-flow oxygen in patients who are not hypoxaemic offers no benefit and may increase infarct size. Oxygen is given when saturation falls below the target (for example below 90% in some guidelines, below 94% in others).
Answer
B · Only when SpO₂ is below the target range
8.Med-surg: cardiovascular & respiratory
Answer: D · Follow the worsening-asthma steps and get a same-day review
Why this is the answer
Personal asthma action plans set steps based on symptoms and peak flow compared with the personal best. Increasing reliever need and a falling peak flow mean worsening asthma: treatment is increased as the plan says (sometimes with oral steroids) and urgent review is sought. Severe signs need emergency care.
Answer
D · Follow the worsening-asthma steps and get a same-day review