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 19 times a minute with a tidal volume of 500 mL. The anatomical dead space is 150 mL. Find the alveolar ventilation V̇A in L/min.
VT 500 mL19/minVD 150 mL
V̇A = L/min
2.Oxygen therapy & respiratory calculationsHard
Practice exercise (made-up values, not clinical guidance). A transfer will take 90 minutes. The size E cylinder (factor 0.28 L/psi) reads 1050 psi. Keeping a safe residual of 200 psi, what is the highest whole-number flow Q in L/min that lasts the whole transfer?
90 min1050 psif = 0.28 L/psi200 psi
Q = L/min
3.Arterial blood gas interpretationMedium
A practice gas shows PaCO₂ 80 mmHg (10.7 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⁻.
A practice gas shows a metabolic acidosis: pH 7.33, PaCO₂ 35 mmHg (4.7 kPa), HCO₃⁻ 18 mmol/L. Compare the measured PaCO₂ with Winter’s formula. Click your interpretation.
pH 7.33PaCO₂ 35 mmHg (4.7 kPa)HCO₃⁻ 18 mmol/L
1appropriate compensation
2added respiratory acidosis
3added respiratory alkalosis
5.Clinical assessment scoresMedium
Practice exercise (made-up values, not clinical guidance). A patient has a pulse of 114/min and a blood pressure of 73/40 mmHg. Find the shock index SI = HR/SBP. Round to two decimal places.
HR 114/minBP 73/40 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 30/min, SpO₂ 91% on room air, blood pressure 118/70 mmHg, pulse 104/min, alert, temperature 36.9 °C. Which clinical risk group does the patient fall into? Click it.
RR 30SpO₂ 91%airSBP 118HR 104AT 36.9 °C
1low
2low–medium
3medium
4high
7.Med-surg: cardiovascular & respiratoryMedium
Which emergency equipment should be kept at the bedside of a patient with a tracheostomy?
AA urinary catheter set, a blood glucose meter and a thermometer
BOnly a spare oxygen mask, since the tube rarely causes problems
CSpare tubes (same size and one smaller), suction and oxygen
DA nasogastric tube and a feeding pump
8.Med-surg: cardiovascular & respiratoryHard
A 25-year-old non-smoker has wheeze and cough that come and go, worse at night and after exercise. Spirometry improves markedly after a bronchodilator. Which diagnosis is most likely?
AIdiopathic pulmonary fibrosis
BCOPD
CHeart failure
DAsthma
3Solutions
1.Oxygen therapy & respiratory calculations
Answer: V̇A = 6.65 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
500 − 150 = 350 mL
Alveolar ventilation
V̇A = 350·19 = 6650 mL/min = 6.65 L/min
Answer
V̇A = 6.65 L/min
2.Oxygen therapy & respiratory calculations
Answer: Q = 2 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
(1050 − 200)·0.28 = 238 L
Highest flow
238/90 = 2.644 L/min → Q = 2 L/min
Answer
Q = 2 L/min
3.Arterial blood gas interpretation
Answer: HCO₃⁻ = 28 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
|80 − 40| = 40 mmHg → 4 × 10 mmHg
Expected HCO₃⁻
HCO₃⁻ = 24 + 1·4 = 28 mmol/L
Answer
HCO₃⁻ = 28 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
1.5·18 + 8 = 35 → 33–37 mmHg
Compare
measured: 35 mmHg → appropriate compensation
Answer
appropriate compensation
5.Clinical assessment scores
Answer: SI = 1.56
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 = 114/73 = 1.562 ≈ 1.56
Answer
SI = 1.56
1 or more: a warning sign of shock in this practice case.
6.Clinical assessment scores
Answer: high
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 91% → 3
on air → 0
SBP 118 mmHg → 0
HR 104/min → 1
alert (A) → 0
T 36.9 °C → 0
Total and red scores
NEWS2 = 3 + 3 + 0 + 0 + 1 + 0 + 0 = 7
At least one parameter scores 3.
Risk group
high
Answer
high
A total of 5 or more calls for an urgent clinical response.
7.Med-surg: cardiovascular & respiratory
Answer: C · Spare tubes (same size and one smaller), suction and oxygen
Why this is the answer
Bedside equipment usually includes spare tracheostomy tubes (same size and one size smaller), an obturator if used, suction with catheters, oxygen, a bag-valve-mask and tracheal dilators where policy requires. It is checked at every shift.
Answer
C · Spare tubes (same size and one smaller), suction and oxygen
8.Med-surg: cardiovascular & respiratory
Answer: D · Asthma
Why this is the answer
Asthma typically causes variable symptoms with reversible airflow obstruction. COPD usually affects older people with a smoking history and causes persistent, poorly reversible obstruction. Fibrosis gives a restrictive pattern.