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.
Clinical pharmacokinetics
Rate constant and half-life
k = ln 2/t½ = 0.693/t½
First-order decay
C = C₀·e−kt = C₀·(1/2)t/t½
Two levels
k = ln(C₁/C₂)/Δt
Volume and clearance
Vd = dose/C₀, CL = k·Vd
Loading dose
LD = Vd·C/(S·F)
Maintenance rate
R = CL·Css/(S·F)
Average steady state
Css,avg = S·F·D/(CL·τ)
Steady state
1 − (1/2)ⁿ after n half-lives; ≈ 97% after 5
Infusion
C = (R/CL)(1 − e−kt)
Interval and dose from peak and trough
τ = ln(Cpeak/Ctrough)/k, D = Vd·(Cpeak − Ctrough)
Cockcroft–Gault
CrCl = (140 − age) × kg/(72 × SCr mg/dL), × 0.85 for women
Pick the relation: C = C₀·e−kt, LD = Vd·C/(S·F), maintenance rate = CL·Css/(S·F), τ = ln(Cpeak/Ctrough)/k.
For kidney function, estimate CrCl with Cockcroft–Gault first, then scale the dose or the interval.
Check the result: levels fall by half every half-life, and steady state takes about five half-lives.
2Practice problems
1.Lab values & electrolytesMedium
A patient has a sodium of 124 mEq/L, a headache and new confusion. Which nursing measure is most appropriate?
AEncourage the patient to drink plenty of plain water
BPut seizure precautions in place and monitor neurological status
CGive the next dose of the prescribed thiazide diuretic early
DKeep the patient flat and limit neurological checks to once a shift
2.Lab values & electrolytesHard
An older adult has bone pain, fatigue and frequent infections. Results show anemia, a calcium of 12.4 mg/dL (3.1 mmol/L), a raised total protein and a rising creatinine. Which condition should be suspected?
AMultiple myeloma
BPrimary hypothyroidism
CIron-deficiency anemia
DAcute viral hepatitis
3.Arterial blood gas interpretationMedium
Practice values: Na⁺ 130 mmol/L, glucose 5.7 mmol/L and urea 9 mmol/L. Calculate the serum osmolality Osm = 2·Na⁺ + glucose + urea to the nearest whole number.
Na⁺ 130 mmol/Lglucose 5.7 mmol/Lurea 9 mmol/L
Osm = mOsm/kg
4.Arterial blood gas interpretationHard
At sea level, a practice gas taken on 35% oxygen shows PaO₂ 89 mmHg (11.9 kPa) and PaCO₂ 54 mmHg (7.2 kPa). Use PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8 to find the A–a gradient.
Practice exercise (made-up values, not clinical guidance). A 79 kg patient is to be loaded intravenously with aminophylline to reach a theophylline level of 10 mg/L. The volume of distribution is 0.5 L/kg and the salt factor is S = 0.8. Find the loading dose LD of aminophylline in mg.
79 kgVd = 0.5 L/kgC = 10 mg/LS = 0.8
LD = mg
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A 77 kg patient has a phenytoin level of 12 mg/L; the target is 20 mg/L. The drug is given intravenously as phenytoin sodium (S = 0.92), and the volume of distribution is 0.7 L/kg. What extra loading dose LD raises the level to the target?
77 kgVd = 0.7 L/kgC = 12 → 20 mg/LS = 0.92
LD = mg
7.Clinical nursingMedium
A nurse suspects that an older patient is being financially abused by a relative. What should the nurse do?
AConfront the relative directly at the bedside about it
BKeep it secret because it is a family matter
CWait until the patient has been discharged
DRaise the concern through the local safeguarding (adult protection) procedure
8.Clinical nursingHard
A patient starting a DOAC is taught about missed doses. Which statement shows a need for further teaching?
A"I will report any blood in my urine."
B"If I forget a dose, I'll take two the next day to catch up."
C"I will take it at the same time each day."
D"I will tell other doctors and my dentist I take an anticoagulant."
3Solutions
1.Lab values & electrolytes
Answer: B · Put seizure precautions in place and monitor neurological status
Why this is the answer
Hyponatremia can cause cerebral edema and seizures. The nurse monitors neurological status, takes seizure precautions and reports the result. Extra water and thiazides can lower the sodium further.Checked against:Low blood sodium · MedlinePlus (NIH) (2025)
Answer
B · Put seizure precautions in place and monitor neurological status
2.Lab values & electrolytes
Answer: A · Multiple myeloma
Why this is the answer
Multiple myeloma typically causes high calcium from bone destruction, kidney damage, anemia and bone lesions, plus a raised protein from a monoclonal antibody. Recurrent infections reflect poor production of normal antibodies.Checked against:Multiple Myeloma · StatPearls (NCBI Bookshelf) (2023)
Answer
A · Multiple myeloma
3.Arterial blood gas interpretation
Answer: Osm = 275 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 = 2·130 + 5.7 + 9 = 274.7 ≈ 275 mOsm/kg
Answer
Osm = 275 mOsm/kg
4.Arterial blood gas interpretation
Answer: A–a gradient = 93.05 mmHg
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.
Clearance and volume: at steady state the dosing rate equals the elimination rate, so the maintenance rate = CL × Css. A loading dose is LD = Vd × C/(S × F), with S the salt factor (fraction of active drug in the salt) and F the bioavailability (1 for IV). From two levels, k = ln(C₁/C₂)/Δt; for an IV bolus Vd = dose/C₀ and CL = k × Vd. The numbers are for practice only; at the bedside always follow local protocols and have high-risk doses double-checked.
Volume of distribution
Vd = 0.5·79 = 39.5 L
Amount of active drug needed
Vd·C = 39.5·10 = 395 mg
Loading dose
LD = 395/(0.8·1) = 493.8 mg
Answer
LD = 493.8 mg
In practice the dose is rounded to a convenient amount.
6.Clinical pharmacokinetics
Answer: LD = 468.7 mg
Designing a regimen: with rapid IV doses the level falls from peak to trough during the interval, so τ = ln(Cpeak/Ctrough)/k and the dose that restores the peak is Vd × (Cpeak − Ctrough). The average steady-state level is Css,avg = S·F·dose/(CL·τ). An infusion rises as C = (R/CL)(1 − e−kt). For a drug cleared by the kidneys, the dose is scaled with the creatinine clearance. The numbers are for practice only; at the bedside always follow local protocols and have high-risk doses double-checked.
Volume of distribution
Vd = 0.7·77 = 53.9 L
Rise needed
20 − 12 = 8 mg/L
Loading dose
LD = 53.9·8/(0.92·1) = 468.7 mg
Answer
LD = 468.7 mg
In practice the dose is rounded to a convenient amount.
7.Clinical nursing
Answer: D · Raise the concern through the local safeguarding (adult protection) procedure
Why this is the answer
Financial abuse is a form of adult abuse. Nurses report concerns through safeguarding procedures, document facts accurately and involve the patient as appropriate.
Answer
D · Raise the concern through the local safeguarding (adult protection) procedure
8.Clinical nursing
Answer: B · "If I forget a dose, I'll take two the next day to catch up."
Why this is the answer
Doubling up on an anticoagulant increases the bleeding risk. Patients follow the specific missed-dose advice for their drug and never take a double dose.
Answer
B · "If I forget a dose, I'll take two the next day to catch up."