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 87 g/L, SaO₂ 98% and PaO₂ 97 mmHg (12.9 kPa). Find the arterial oxygen content CaO₂ = 1.34 × Hb (g/dL) × SaO₂ + 0.003 × PaO₂ (mmHg) in mL O₂/dL.
Hb 87 g/LSaO₂ 98%PaO₂ 97 mmHg (12.9 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 13.6 kPa on 35% oxygen with a PEEP of 6 cmH₂O. Grade the ARDS severity with the Berlin definition. Click your answer.
PaO₂ 13.6 kPaFiO₂ 35%PEEP 6 cmH₂O
1above 300: no ARDS grade
2mild
3moderate
4severe
5not gradable: PEEP below 5
3.Arterial blood gas interpretationMedium
A practice arterial blood gas shows pH 7.31, PaCO₂ 68 mmHg (9.1 kPa) and HCO₃⁻ 33 mmol/L. The primary disorder is respiratory acidosis. How far is it compensated? Click your answer.
pH 7.31PaCO₂ 68 mmHg (9.1 kPa)HCO₃⁻ 33 mmol/L
1uncompensated
2partially compensated
3fully compensated
4.Arterial blood gas interpretationHard
A practice gas shows a metabolic acidosis: pH 7.31, PaCO₂ 33 mmHg (4.4 kPa), HCO₃⁻ 16 mmol/L. Compare the measured PaCO₂ with Winter’s formula. Click your interpretation.
pH 7.31PaCO₂ 33 mmHg (4.4 kPa)HCO₃⁻ 16 mmol/L
1appropriate compensation
2added respiratory acidosis
3added respiratory alkalosis
5.Clinical assessment scoresMedium
Practice exercise (made-up values, not clinical guidance). Use the NEWS2 chart (SpO₂ scale 1) for these observations: respiration rate 30/min, SpO₂ 89% on supplemental oxygen, blood pressure 78/46 mmHg, pulse 140/min, alert, temperature 37.5 °C. What is the NEWS2 total score?
RR 30SpO₂ 89%O₂SBP 78HR 140AT 37.5 °C
NEWS2 =
6.Clinical assessment scoresHard
Practice exercise (made-up values, not clinical guidance). The team aims for a mean arterial pressure of at least 70 mmHg. A patient’s systolic pressure is 108 mmHg. Using MAP = (SBP + 2·DBP)/3, what is the lowest diastolic pressure DBP that still gives a MAP of 70 mmHg?
MAP 70 mmHgSBP 108 mmHg
DBP = mmHg
7.Med-surg: cardiovascular & respiratoryMedium
Why should a long-acting beta-2 agonist (LABA) never be used alone in asthma?
AWithout an inhaled steroid, it is linked with severe and fatal attacks
BIt cannot be inhaled without a nebuliser
CIt always causes immediate severe bronchospasm when used on its own
DIt works only when it is taken together with a course of antibiotics
8.Med-surg: cardiovascular & respiratoryHard
Twelve hours after a coronary stent, a patient has sudden severe chest pain again with new ST elevation in the same leads. What is the most likely cause?
APericarditis from the catheter
BAcute stent thrombosis
CIndigestion from the hospital meal
DNormal pain from the procedure site
3Solutions
1.Oxygen therapy & respiratory calculations
Answer: CaO₂ = 11.72 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
87 g/L = 8.7 g/dL
Bound oxygen
1.34·8.7·0.98 = 11.42 mL/dL
Dissolved oxygen
0.003·97 = 0.291 mL/dL
Oxygen content
CaO₂ = 11.42 + 0.291 = 11.72 mL/dL
Answer
CaO₂ = 11.72 mL/dL
2.Oxygen therapy & respiratory calculations
Answer: mild
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 6 ≥ 5 cmH₂O
PaO₂ in mmHg
13.6·7.5 = 102 mmHg
P/F ratio
P/F = 102/0.35 = 291.4 mmHg
Berlin severity
mild
Answer
mild
3.Arterial blood gas interpretation
Answer: partially 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 7.31: acidaemia (below 7.35)
PaCO₂ 68 mmHg: high (35–45)
HCO₃⁻ 33 mmol/L: high (22–26)
Compensation
partially compensated
Answer
partially compensated
4.Arterial blood gas interpretation
Answer: appropriate compensation
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.
Expected PaCO₂
1.5·16 + 8 = 32 → 30–34 mmHg
Compare
measured: 33 mmHg → appropriate compensation
Answer
appropriate compensation
5.Clinical assessment scores
Answer: NEWS2 = 14
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 30/min → 3
SpO₂ 89% → 3
on oxygen → 2
SBP 78 mmHg → 3
HR 140/min → 3
alert (A) → 0
T 37.5 °C → 0
Add
NEWS2 = 3 + 3 + 2 + 3 + 3 + 0 + 0 = 14
Answer
NEWS2 = 14
6.Clinical assessment scores
Answer: DBP = 51 mmHg
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.
Rearrange
DBP = (3·MAP − SBP)/2
Substitute
DBP = (3·70 − 108)/2 = 102/2 = 51 mmHg
Answer
DBP = 51 mmHg
7.Med-surg: cardiovascular & respiratory
Answer: A · Without an inhaled steroid, it is linked with severe and fatal attacks
Why this is the answer
LABA monotherapy in asthma is linked with severe attacks and asthma deaths because it relieves symptoms while airway inflammation continues. In asthma, LABAs are always given with an inhaled corticosteroid, often in one combination inhaler.
Answer
A · Without an inhaled steroid, it is linked with severe and fatal attacks
8.Med-surg: cardiovascular & respiratory
Answer: B · Acute stent thrombosis
Why this is the answer
Acute stent thrombosis presents with recurrent ischaemic pain and ST elevation, and needs emergency return to the catheter laboratory. Missed antiplatelet doses are a major risk factor, which is why dual antiplatelet adherence is emphasised.