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 size E oxygen cylinder (factor 0.28 L/psi) reads 1150 psi and runs at 8 L/min. Keeping a safe residual of 200 psi, how many minutes t will it last? Round down to a whole minute.
1150 psif = 0.28 L/psi8 L/min200 psi
t = min
2.Oxygen therapy & respiratory calculationsHard
Practice exercise (made-up values, not clinical guidance). A patient has Hb 112 g/L, SaO₂ 91%, PaO₂ 62 mmHg (8.3 kPa) and a cardiac output of 7.9 L/min. With CaO₂ = 1.34 × Hb (g/dL) × SaO₂ + 0.003 × PaO₂ (mmHg), find the oxygen delivery DO₂ = CaO₂ × CO × 10 in mL/min.
Practice values: Na⁺ 133 mmol/L, glucose 14 mmol/L and urea 9.9 mmol/L. Calculate the serum osmolality Osm = 2·Na⁺ + glucose + urea to the nearest whole number.
Na⁺ 133 mmol/Lglucose 14 mmol/Lurea 9.9 mmol/L
Osm = mOsm/kg
4.Arterial blood gas interpretationHard
Practice values: Na⁺ 137, Cl⁻ 102, HCO₃⁻ 15 mmol/L and albumin 26 g/L. Find the albumin-corrected anion gap AGc = AG + 0.25·(40 − albumin).
Practice exercise (made-up values, not clinical guidance). A practice patient is assessed with the Morse fall scale: history of falling in the last 3 months: no; secondary diagnosis: yes; ambulatory aid: none, bed rest or nurse assists; IV therapy or saline lock: no; gait: impaired; mental status: overestimates or forgets limits. Find the Morse total MFS.
0–125
MFS =
6.Clinical assessment scoresHard
Practice exercise (made-up values, not clinical guidance). The team aims for a mean arterial pressure of at least 65 mmHg. A patient’s systolic pressure is 83 mmHg. Using MAP = (SBP + 2·DBP)/3, what is the lowest diastolic pressure DBP that still gives a MAP of 65 mmHg?
MAP 65 mmHgSBP 83 mmHg
DBP = mmHg
7.Med-surg: cardiovascular & respiratoryMedium
A patient on nasal cannula oxygen breathes mainly through the mouth. What is the most accurate statement?
AThe flow must be increased to 15 L/min to make it work at all
BIt can still work, because oxygen collects in the nasopharynx
CIt has no effect at all in mouth breathers and must be stopped
DThe cannula should be placed in the mouth
8.Med-surg: cardiovascular & respiratoryHard
A patient with heart failure has gained 2.5 kg in three days, has new crackles at both bases and now sleeps on four pillows. BP 138/84 mmHg, SpO₂ 93% on air. What is the priority?
ALay the patient flat to rest the heart and lower demand
BRecord the weight and check it again at the end of the next week
CSit the patient up, assess fully and report the fluid overload
DEncourage extra oral fluids to protect the kidneys
3Solutions
1.Oxygen therapy & respiratory calculations
Answer: t = 33 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
(1150 − 200)·0.28 = 266 L
Duration
266/8 = 33.25 min → t = 33 min
Answer
t = 33 min
2.Oxygen therapy & respiratory calculations
Answer: DO₂ = 1094 mL/min
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.
Oxygen content
112 g/L = 11.2 g/dL
CaO₂ = 1.34·11.2·0.91 + 0.003·62 = 13.84 mL/dL
Oxygen delivery
DO₂ = 13.84·7.9·10 = 1094 mL/min
Answer
DO₂ = 1094 mL/min
A normal resting adult value is about 1000 mL/min.
3.Arterial blood gas interpretation
Answer: Osm = 290 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 = 2·133 + 14 + 9.9 = 289.9 ≈ 290 mOsm/kg
Answer
Osm = 290 mOsm/kg
4.Arterial blood gas interpretation
Answer: AGc = 23.5 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 = 137 − (102 + 15) = 20
Correction for albumin
20 + 0.25·(40 − 26) = 23.5 mmol/L
Answer
AGc = 23.5 mmol/L
Above about 12 after correction: a high anion gap.
5.Clinical assessment scores
Answer: MFS = 50
Morse fall scale: six items. History of falling: no 0, yes 25. Secondary diagnosis: no 0, yes 15. Ambulatory aid: none, bed rest or nurse assistance 0; crutches, cane or walker 15; holding on to furniture 30. IV therapy or saline lock: no 0, yes 20. Gait: normal, bed rest or wheelchair 0; weak 10; impaired 20. Mental status: knows own limits 0; overestimates or forgets limits 15. The total runs from 0 to 125. Risk cut-offs differ between hospitals; a common set is 0–24 low, 25–44 moderate and 45 or more high.
Score each item
history of falling in the last 3 months: no → 0
secondary diagnosis: yes → 15
ambulatory aid: none, bed rest or nurse assists → 0
IV therapy or saline lock: no → 0
gait: impaired → 20
mental status: overestimates or forgets limits → 15
Add
Morse total = 0 + 15 + 0 + 0 + 20 + 15 = 50
Answer
MFS = 50
6.Clinical assessment scores
Answer: DBP = 56 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 = (3·65 − 83)/2 = 112/2 = 56 mmHg
Answer
DBP = 56 mmHg
7.Med-surg: cardiovascular & respiratory
Answer: B · It can still work, because oxygen collects in the nasopharynx
Why this is the answer
Studies show that mouth breathing still draws in oxygen from the nasopharynx, so nasal cannulae can work. The effect is judged by the SpO₂, and the device is changed if targets are not reached. Cannulae are never placed in the mouth.
Answer
B · It can still work, because oxygen collects in the nasopharynx
8.Med-surg: cardiovascular & respiratory
Answer: C · Sit the patient up, assess fully and report the fluid overload
Why this is the answer
Rapid weight gain with crackles and orthopnoea indicates fluid overload and worsening heart failure. Sitting upright reduces venous return and eases breathing. The nurse completes an assessment (oxygen trend, fluid balance, oedema) and escalates so the prescriber can adjust diuretics. More fluid or lying flat would worsen congestion.
Answer
C · Sit the patient up, assess fully and report the fluid overload