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 29 times a minute with a tidal volume of 430 mL. The anatomical dead space is 180 mL. Find the alveolar ventilation V̇A in L/min.
VT 430 mL29/minVD 180 mL
V̇A = L/min
2.Oxygen therapy & respiratory calculationsHard
Practice exercise (made-up values, not clinical guidance). A ventilated woman is 169 cm tall and weighs 91 kg. Lung-protective ventilation is set at 6 mL per kg of predicted body weight. Using PBW = 45.5 + 0.91 × (height cm − 152.4) kg, find the tidal volume VT to the nearest mL.
169 cm91 kg6 mL/kg
VT = mL
3.Arterial blood gas interpretationMedium
A practice gas shows PaCO₂ 32 mmHg (4.3 kPa). Use the rule that in chronic respiratory alkalosis HCO₃⁻ falls by 5 mmol/L for every 10 mmHg that PaCO₂ is below 40, starting from HCO₃⁻ 24 mmol/L, to find the expected HCO₃⁻.
A practice gas shows a metabolic acidosis: pH 7.29, PaCO₂ 30 mmHg (4.0 kPa), HCO₃⁻ 14 mmol/L. Compare the measured PaCO₂ with Winter’s formula. Click your interpretation.
pH 7.29PaCO₂ 30 mmHg (4.0 kPa)HCO₃⁻ 14 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 20/min, SpO₂ 98% on room air, blood pressure 90/54 mmHg, pulse 64/min, alert, temperature 38.8 °C. What is the NEWS2 total score?
RR 20SpO₂ 98%airSBP 90HR 64AT 38.8 °C
NEWS2 =
6.Clinical assessment scoresHard
Practice exercise (made-up values, not clinical guidance). At 1 minute after birth, a practice newborn has pink skin all over, a heart rate of 155/min, a cry, cough or sneeze when stimulated, active movement and good breathing with a strong cry. Score the APGAR and click how the score is interpreted.
1reassuring
2moderately abnormal
3low
7.Med-surg: cardiovascular & respiratoryMedium
After an above-knee amputation, which measure helps prevent a hip flexion contracture?
ALying prone for periods and avoiding pillows under the stump
BKeeping the hip bent to reduce stump swelling
CKeeping a pillow under the stump at all times, day and night
DSitting in a chair for most of the day with the hips bent
8.Med-surg: cardiovascular & respiratoryHard
A patient who injects drugs has a fever, a new heart murmur and splinter haemorrhages. Before antibiotics are started, what is most important?
ATake a single blood culture from the cannula
BTake several sets of blood cultures from separate sites
CWait for an echocardiogram before taking any samples
DArrange an exercise ECG test
3Solutions
1.Oxygen therapy & respiratory calculations
Answer: V̇A = 7.25 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
430 − 180 = 250 mL
Alveolar ventilation
V̇A = 250·29 = 7250 mL/min = 7.25 L/min
Answer
V̇A = 7.25 L/min
2.Oxygen therapy & respiratory calculations
Answer: VT = 364 mL
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).
Predicted body weight
PBW = 45.5 + 0.91·(169 − 152.4) = 60.61 kg
Tidal volume
VT = 6·60.61 = 363.6 ≈ 364 mL
Answer
VT = 364 mL
3.Arterial blood gas interpretation
Answer: HCO₃⁻ = 20 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
|32 − 40| = 8 mmHg → 0.8 × 10 mmHg
Expected HCO₃⁻
HCO₃⁻ = 24 − 5·0.8 = 20 mmol/L
Answer
HCO₃⁻ = 20 mmol/L
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
1.5·14 + 8 = 29 → 27–31 mmHg
Compare
measured: 30 mmHg → appropriate compensation
Answer
appropriate compensation
5.Clinical assessment scores
Answer: NEWS2 = 4
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 90 mmHg → 3
HR 64/min → 0
alert (A) → 0
T 38.8 °C → 1
Add
NEWS2 = 0 + 0 + 0 + 3 + 0 + 0 + 1 = 4
Answer
NEWS2 = 4
6.Clinical assessment scores
Answer: reassuring
APGAR score: five signs, each scored 0, 1 or 2, checked at 1 and 5 minutes after birth. Appearance (colour): blue or pale 0, pink body with blue extremities 1, pink all over 2. Pulse: absent 0, below 100/min 1, 100/min or more 2. Grimace (reflex irritability): none 0, grimace 1, cry, cough or sneeze 2. Activity (tone): limp 0, some flexion 1, active movement 2. Respiration: absent 0, slow or irregular with a weak cry 1, good with a strong cry 2. A total of 7–10 is reassuring, 4–6 moderately abnormal and 0–3 low.
Score each sign
Appearance: pink skin all over → 2
Pulse: 155/min → 2
Grimace: a cry, cough or sneeze when stimulated → 2
Activity: active movement → 2
Respiration: good breathing with a strong cry → 2
Add
APGAR = 2 + 2 + 2 + 2 + 2 = 10
Interpretation
10 → reassuring
Answer
reassuring
7.Med-surg: cardiovascular & respiratory
Answer: A · Lying prone for periods and avoiding pillows under the stump
Why this is the answer
Prolonged hip flexion, such as long periods sitting or a pillow under the stump, can lead to a fixed contracture that makes fitting a prosthesis difficult. Lying prone for periods, keeping the hip extended and early physiotherapy help prevent it.
Answer
A · Lying prone for periods and avoiding pillows under the stump
8.Med-surg: cardiovascular & respiratory
Answer: B · Take several sets of blood cultures from separate sites
Why this is the answer
Suspected infective endocarditis requires multiple sets of blood cultures taken with careful aseptic technique, ideally before antibiotics, to identify the organism and guide weeks of IV therapy. Echocardiography looks for vegetations but should not delay cultures or, in a very sick patient, antibiotics.
Answer
B · Take several sets of blood cultures from separate sites