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 breathes 26 times a minute with a tidal volume of 480 mL. The anatomical dead space is 140 mL. Find the alveolar ventilation V̇A in L/min.
VT 480 mL26/minVD 140 mL
V̇A = L/min
2.Oxygen therapy & respiratory calculationsHard
Practice exercise (made-up values, not clinical guidance). A patient has Hb 120 g/L, SaO₂ 93%, PaO₂ 66 mmHg (8.8 kPa) and a cardiac output of 4.8 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 gas shows PaCO₂ 72 mmHg (9.6 kPa). Use the rule that in acute respiratory acidosis HCO₃⁻ rises by 1 mmol/L for every 10 mmHg that PaCO₂ is above 40, starting from HCO₃⁻ 24 mmol/L, to find the expected HCO₃⁻.
At sea level, a practice gas taken on 35% oxygen shows PaO₂ 95 mmHg (12.7 kPa) and PaCO₂ 49 mmHg (6.5 kPa). Use PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8 to find the A–a gradient.
Practice exercise (made-up values, not clinical guidance). Use the NEWS2 chart (SpO₂ scale 1) for these observations: respiration rate 12/min, SpO₂ 86% on room air, blood pressure 150/90 mmHg, pulse 64/min, alert, temperature 37.0 °C. What is the NEWS2 total score?
RR 12SpO₂ 86%airSBP 150HR 64AT 37.0 °C
NEWS2 =
6.Clinical assessment scoresHard
Practice exercise (made-up values, not clinical guidance). A practice patient is assessed with the Morse fall scale: history of falling in the last 3 months: no; secondary diagnosis: no; ambulatory aid: none, bed rest or nurse assists; IV therapy or saline lock: no; gait: normal, bed rest or wheelchair; mental status: knows own limits. Using the cut-offs 0–24 low, 25–44 moderate and 45 or more high, click the fall-risk level.
1low risk
2moderate risk
3high risk
7.Med-surg: cardiovascular & respiratoryMedium
A 70 kg patient has passed 20 mL of urine per hour for the last three hours. How should this be interpreted?
ANormal, because any measurable urine output is reassuring
DHigh, suggesting fluid overload that needs a diuretic
8.Med-surg: cardiovascular & respiratoryHard
A patient with end-stage COPD has severe breathlessness at rest despite optimal treatment and wants comfort-focused care. Which measure is recognised to help?
AStopping all medicines, including inhalers
BEncouraging long walks to build fitness
CHigh-flow oxygen at 15 L/min whatever the oxygen saturation is
DA low-dose opioid with a hand-held fan and calm positioning
3Solutions
1.Oxygen therapy & respiratory calculations
Answer: V̇A = 8.84 L/min
Ventilation: minute ventilation V̇E = tidal volume × respiratory rate (normal adult about 5–8 L/min). Alveolar ventilation V̇A = (tidal volume − dead space) × rate, because the air in the airways (about 150 mL in an adult) takes no part in gas exchange. Lung-protective ventilation uses about 6 mL per kg of predicted body weight (PBW): men 50 + 0.91 × (height in cm − 152.4) kg, women 45.5 + 0.91 × (height in cm − 152.4) kg (with inches: 2.3 per inch above 60 in).
Alveolar part of a breath
480 − 140 = 340 mL
Alveolar ventilation
V̇A = 340·26 = 8840 mL/min = 8.84 L/min
Answer
V̇A = 8.84 L/min
2.Oxygen therapy & respiratory calculations
Answer: DO₂ = 727.3 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
120 g/L = 12 g/dL
CaO₂ = 1.34·12·0.93 + 0.003·66 = 15.15 mL/dL
Oxygen delivery
DO₂ = 15.15·4.8·10 = 727.3 mL/min
Answer
DO₂ = 727.3 mL/min
A normal resting adult value is about 1000 mL/min.
3.Arterial blood gas interpretation
Answer: HCO₃⁻ = 27.2 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
|72 − 40| = 32 mmHg → 3.2 × 10 mmHg
Expected HCO₃⁻
HCO₃⁻ = 24 + 1·3.2 = 27.2 mmol/L
Answer
HCO₃⁻ = 27.2 mmol/L
4.Arterial blood gas interpretation
Answer: A–a gradient = 93.3 mmHg
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.
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 12/min → 0
SpO₂ 86% → 3
on air → 0
SBP 150 mmHg → 0
HR 64/min → 0
alert (A) → 0
T 37.0 °C → 0
Add
NEWS2 = 0 + 3 + 0 + 0 + 0 + 0 + 0 = 3
Answer
NEWS2 = 3
6.Clinical assessment scores
Answer: low risk
Morse fall scale: six items. History of falling: no 0, yes 25. Secondary diagnosis: no 0, yes 15. Ambulatory aid: none, bed rest or nurse assistance 0; crutches, cane or walker 15; holding on to furniture 30. IV therapy or saline lock: no 0, yes 20. Gait: normal, bed rest or wheelchair 0; weak 10; impaired 20. Mental status: knows own limits 0; overestimates or forgets limits 15. The total runs from 0 to 125. Risk cut-offs differ between hospitals; a common set is 0–24 low, 25–44 moderate and 45 or more high.
Score each item
history of falling in the last 3 months: no → 0
secondary diagnosis: no → 0
ambulatory aid: none, bed rest or nurse assists → 0
IV therapy or saline lock: no → 0
gait: normal, bed rest or wheelchair → 0
mental status: knows own limits → 0
Add
Morse total = 0 + 0 + 0 + 0 + 0 + 0 = 0
Risk level
0 → low risk
Answer
low risk
7.Med-surg: cardiovascular & respiratory
Answer: B · Below 0.5 mL/kg/h, suggesting poor kidney perfusion
Why this is the answer
A urine output below about 0.5 mL/kg/h (35 mL/h for 70 kg) for several hours is oliguria and may indicate hypovolaemia, shock or acute kidney injury. It should be reported along with the other observations, and the catheter checked for blockage.
Answer
B · Below 0.5 mL/kg/h, suggesting poor kidney perfusion
8.Med-surg: cardiovascular & respiratory
Answer: D · A low-dose opioid with a hand-held fan and calm positioning
Why this is the answer
Low-dose oral opioids can ease refractory breathlessness in advanced disease. A fan blowing air across the face, upright positioning, breathing techniques and anxiety management also help. Oxygen is used only if it relieves symptoms or corrects hypoxaemia.
Answer
D · A low-dose opioid with a hand-held fan and calm positioning