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 101 g/L, SaO₂ 86% and PaO₂ 51 mmHg (6.8 kPa). Find the arterial oxygen content CaO₂ = 1.34 × Hb (g/dL) × SaO₂ + 0.003 × PaO₂ (mmHg) in mL O₂/dL.
Hb 101 g/LSaO₂ 86%PaO₂ 51 mmHg (6.8 kPa)
CaO₂ = mL/dL
2.Oxygen therapy & respiratory calculationsHard
Practice exercise (made-up values, not clinical guidance). A transfer will take 75 minutes. The size D cylinder (factor 0.16 L/psi) reads 1000 psi. Keeping a safe residual of 200 psi, what is the highest whole-number flow Q in L/min that lasts the whole transfer?
75 min1000 psif = 0.16 L/psi200 psi
Q = L/min
3.Arterial blood gas interpretationMedium
Practice values: Na⁺ 144 mmol/L, glucose 18 mmol/L and urea 21.7 mmol/L. Calculate the serum osmolality Osm = 2·Na⁺ + glucose + urea to the nearest whole number.
Na⁺ 144 mmol/Lglucose 18 mmol/Lurea 21.7 mmol/L
Osm = mOsm/kg
4.Arterial blood gas interpretationHard
Practice values: Na⁺ 136, Cl⁻ 105 and HCO₃⁻ 11 mmol/L. Calculate the delta ratio (AG − 12)/(24 − HCO₃⁻), using AG = Na⁺ − (Cl⁻ + HCO₃⁻). Give two decimals.
Na⁺ 136 mmol/LCl⁻ 105 mmol/LHCO₃⁻ 11 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 18/min, SpO₂ 95% on room air, blood pressure 118/70 mmHg, pulse 82/min, alert, temperature 35.5 °C. What is the NEWS2 total score?
RR 18SpO₂ 95%airSBP 118HR 82AT 35.5 °C
NEWS2 =
6.Clinical assessment scoresHard
Practice exercise (made-up values, not clinical guidance). Use the NEWS2 chart (SpO₂ scale 1) for these observations: respiration rate 20/min, SpO₂ 98% on room air, blood pressure 84/50 mmHg, pulse 88/min, alert, temperature 36.4 °C. Which clinical risk group does the patient fall into? Click it.
RR 20SpO₂ 98%airSBP 84HR 88AT 36.4 °C
1low
2low–medium
3medium
4high
7.Med-surg: cardiovascular & respiratoryMedium
Why should chest drain tubing generally not be clamped?
AClamping permanently weakens and cracks the tubing
BClamping makes the fluid drain out of the chest much too quickly
CClamping causes the water in the seal to evaporate
DTrapped air can build up and cause a tension pneumothorax
8.Med-surg: cardiovascular & respiratoryHard
A patient with a large pericardial effusion has a systolic pressure that falls by more than 10 mmHg during inspiration. What is this finding called?
AA pulse deficit
BPulsus alternans
CA collapsing pulse
DPulsus paradoxus
3Solutions
1.Oxygen therapy & respiratory calculations
Answer: CaO₂ = 11.79 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
101 g/L = 10.1 g/dL
Bound oxygen
1.34·10.1·0.86 = 11.64 mL/dL
Dissolved oxygen
0.003·51 = 0.153 mL/dL
Oxygen content
CaO₂ = 11.64 + 0.153 = 11.79 mL/dL
Answer
CaO₂ = 11.79 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
(1000 − 200)·0.16 = 128 L
Highest flow
128/75 = 1.707 L/min → Q = 1 L/min
Answer
Q = 1 L/min
3.Arterial blood gas interpretation
Answer: Osm = 328 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 = 2·144 + 18 + 21.7 = 327.7 ≈ 328 mOsm/kg
Answer
Osm = 328 mOsm/kg
4.Arterial blood gas interpretation
Answer: delta ratio = 0.6154
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 = 136 − (105 + 11) = 20
Delta ratio
(20 − 12)/(24 − 11) = 8/13 = 0.6154
Answer
delta ratio = 0.6154
Below 1: an added normal anion gap acidosis.
5.Clinical assessment scores
Answer: NEWS2 = 2
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 18/min → 0
SpO₂ 95% → 1
on air → 0
SBP 118 mmHg → 0
HR 82/min → 0
alert (A) → 0
T 35.5 °C → 1
Add
NEWS2 = 0 + 1 + 0 + 0 + 0 + 0 + 1 = 2
Answer
NEWS2 = 2
6.Clinical assessment scores
Answer: low–medium
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 98% → 0
on air → 0
SBP 84 mmHg → 3
HR 88/min → 0
alert (A) → 0
T 36.4 °C → 0
Total and red scores
NEWS2 = 0 + 0 + 0 + 3 + 0 + 0 + 0 = 3
At least one parameter scores 3.
Risk group
low–medium
Answer
low–medium
A single score of 3 needs an urgent review even when the total is low.
7.Med-surg: cardiovascular & respiratory
Answer: D · Trapped air can build up and cause a tension pneumothorax
Why this is the answer
Clamping a drain while air is still leaking from the lung allows pressure to build in the pleural space, which can cause a tension pneumothorax. Clamping is only done briefly for specific reasons, such as changing the unit, and only as directed.
Answer
D · Trapped air can build up and cause a tension pneumothorax
8.Med-surg: cardiovascular & respiratory
Answer: D · Pulsus paradoxus
Why this is the answer
Pulsus paradoxus is an exaggerated inspiratory fall in systolic pressure (more than 10 mmHg). In tamponade, the right ventricle fills on inspiration and pushes the septum left, reducing left ventricular output. It also occurs in severe asthma. A collapsing pulse suggests aortic regurgitation.