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 94 g/L, SaO₂ 94% and PaO₂ 72 mmHg (9.6 kPa). Find the arterial oxygen content CaO₂ = 1.34 × Hb (g/dL) × SaO₂ + 0.003 × PaO₂ (mmHg) in mL O₂/dL.
Hb 94 g/LSaO₂ 94%PaO₂ 72 mmHg (9.6 kPa)
CaO₂ = mL/dL
2.Oxygen therapy & respiratory calculationsHard
Practice exercise (made-up values, not clinical guidance). A patient with acute hypoxaemia and bilateral opacities, not explained by heart failure, has PaO₂ 11.2 kPa on 70% oxygen with a PEEP of 15 cmH₂O. Grade the ARDS severity with the Berlin definition. Click your answer.
PaO₂ 11.2 kPaFiO₂ 70%PEEP 15 cmH₂O
1above 300: no ARDS grade
2mild
3moderate
4severe
5not gradable: PEEP below 5
3.Arterial blood gas interpretationMedium
Practice values: Na⁺ 138 mmol/L, glucose 9.5 mmol/L and urea 24 mmol/L. Calculate the serum osmolality Osm = 2·Na⁺ + glucose + urea to the nearest whole number.
Na⁺ 138 mmol/Lglucose 9.5 mmol/Lurea 24 mmol/L
Osm = mOsm/kg
4.Arterial blood gas interpretationHard
Practice values: Na⁺ 134, Cl⁻ 105, HCO₃⁻ 20 mmol/L and albumin 37 g/L. Find the albumin-corrected anion gap AGc = AG + 0.25·(40 − albumin).
Practice exercise (made-up values, not clinical guidance). A patient has a pulse of 120/min and a blood pressure of 156/94 mmHg. Find the shock index SI = HR/SBP. Round to two decimal places.
HR 120/minBP 156/94 mmHg
SI =
6.Clinical assessment scoresHard
Practice exercise (made-up values, not clinical guidance). Use the NEWS2 chart (SpO₂ scale 1) for these observations: respiration rate 14/min, SpO₂ 97% on room air, blood pressure 150/90 mmHg, pulse 76/min, alert, temperature 36.7 °C. Which clinical risk group does the patient fall into? Click it.
RR 14SpO₂ 97%airSBP 150HR 76AT 36.7 °C
1low
2low–medium
3medium
4high
7.Med-surg: cardiovascular & respiratoryMedium
After a bronchoscopy with throat anaesthesia, when can the patient eat and drink?
AImmediately after the scope is removed
BWhen the gag and cough reflexes have returned
CAfter a full 24 hours without food
DOnly after a chest X-ray has been reported
8.Med-surg: cardiovascular & respiratoryHard
One hour after cardiac surgery, a mediastinal chest drain has drained 350 mL of bright red blood and the heart rate is rising. What should the nurse do?
ARecord the volume and continue hourly checks
BReport it to the surgical team at once
CClamp the drain to stop further blood loss
DStrip the tubing hard every five minutes
3Solutions
1.Oxygen therapy & respiratory calculations
Answer: CaO₂ = 12.06 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
94 g/L = 9.4 g/dL
Bound oxygen
1.34·9.4·0.94 = 11.84 mL/dL
Dissolved oxygen
0.003·72 = 0.216 mL/dL
Oxygen content
CaO₂ = 11.84 + 0.216 = 12.06 mL/dL
Answer
CaO₂ = 12.06 mL/dL
2.Oxygen therapy & respiratory calculations
Answer: moderate
P/F ratio and ARDS (Berlin definition): P/F = PaO₂ (mmHg)/FiO₂ (as a fraction). With PEEP or CPAP of at least 5 cmH₂O, ARDS is graded mild for 200 < P/F ≤ 300, moderate for 100 < P/F ≤ 200 and severe for P/F ≤ 100 mmHg. The other Berlin criteria (onset within a week, bilateral opacities, not fully explained by heart failure) must also be met. PaO₂ in kPa × 7.5 gives mmHg. The ROX index (SpO₂/FiO₂)/respiratory rate is used to follow patients on high-flow nasal oxygen.
PEEP
PEEP 15 ≥ 5 cmH₂O
PaO₂ in mmHg
11.2·7.5 = 84 mmHg
P/F ratio
P/F = 84/0.7 = 120 mmHg
Berlin severity
moderate
Answer
moderate
3.Arterial blood gas interpretation
Answer: Osm = 310 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 = 2·138 + 9.5 + 24 = 309.5 ≈ 310 mOsm/kg
Answer
Osm = 310 mOsm/kg
4.Arterial blood gas interpretation
Answer: AGc = 9.75 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.
Anion gap
AG = 134 − (105 + 20) = 9
Correction for albumin
9 + 0.25·(40 − 37) = 9.75 mmol/L
Answer
AGc = 9.75 mmol/L
Still within about 8–12 after correction.
5.Clinical assessment scores
Answer: SI = 0.77
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 = 120/156 = 0.7692 ≈ 0.77
Answer
SI = 0.77
Above the usual 0.5–0.7.
6.Clinical assessment scores
Answer: low
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 97% → 0
on air → 0
SBP 150 mmHg → 0
HR 76/min → 0
alert (A) → 0
T 36.7 °C → 0
Total and red scores
NEWS2 = 0 + 0 + 0 + 0 + 0 + 0 + 0 = 0
No parameter scores 3.
Risk group
low
Answer
low
7.Med-surg: cardiovascular & respiratory
Answer: B · When the gag and cough reflexes have returned
Why this is the answer
Local anaesthetic sprayed on the throat abolishes the gag and cough reflexes for a time, so food or fluid could be aspirated. The nurse checks that the reflexes have returned (often after about one to two hours) before giving sips of water and then food.
Answer
B · When the gag and cough reflexes have returned
8.Med-surg: cardiovascular & respiratory
Answer: B · Report it to the surgical team at once
Why this is the answer
Drainage of more than about 100–200 mL/h (or a sudden large volume of fresh blood), especially with tachycardia or hypotension, may indicate surgical bleeding and needs urgent review. Clamping could cause cardiac tamponade. A sudden stop in drainage with falling blood pressure can also signal tamponade.