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 had a total thyroidectomy yesterday. Which laboratory value is most important to monitor in the first days after surgery?
ASerum potassium
BSerum glucose
CSerum sodium
DSerum calcium
2.Lab values & electrolytesHard
A patient with thalassemia major has received regular red cell transfusions for years. The ferritin is now very high. What does this most likely show, and what treatment is used?
AIron overload, treated with iron chelation therapy
BIron deficiency, treated with IV iron supplements
CAcute infection, treated with broad-spectrum antibiotics
DLiver failure, treated with a liver transplant at once
3.Arterial blood gas interpretationMedium
A practice gas shows a metabolic acidosis with HCO₃⁻ 16 mmol/L. Use Winter’s formula to find the expected PaCO₂ (the centre of the range) in mmHg.
HCO₃⁻ 16 mmol/L
PaCO₂ = mmHg
4.Arterial blood gas interpretationHard
Practice values: Na⁺ 137, Cl⁻ 86 and HCO₃⁻ 17 mmol/L. Calculate the delta ratio (AG − 12)/(24 − HCO₃⁻), using AG = Na⁺ − (Cl⁻ + HCO₃⁻). Give two decimals.
Na⁺ 137 mmol/LCl⁻ 86 mmol/LHCO₃⁻ 17 mmol/L
delta ratio =
5.Clinical pharmacokineticsMedium
Practice exercise (made-up values, not clinical guidance). An 81 kg patient receives a theophylline infusion. The clearance is 0.035 L/kg/h. What infusion rate R in mg/h gives a steady-state level of 10 mg/L?
81 kgCL = 0.035 L/kg/hCss = 10 mg/L
R = mg/h
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A 69 kg patient on regular gentamicin doses is at steady state. The target is a peak of 6 mg/L and a trough of 0.5 mg/L, and the volume of distribution is 0.3 L/kg. Assuming rapid IV doses, what dose D raises the level from the trough to the peak?
After thyroid surgery, a patient has tingling around the mouth and muscle cramps. Which electrolyte problem should be suspected?
AHypocalcaemia
BHyperkalaemia
CHypernatraemia
DHypermagnesaemia
8.Clinical nursingHard
A patient admitted after a fall takes many regular medicines. Which safety step on admission helps prevent medication errors?
AMedicines reconciliation against reliable sources, such as primary care records
BPrescribing only the medicines that the patient can remember taking
CDoubling the doses to make up for any missed while at home
DStopping all regular medicines until the day the patient is discharged
3Solutions
1.Lab values & electrolytes
Answer: D · Serum calcium
Why this is the answer
The parathyroid glands can be injured or removed during thyroidectomy, so parathyroid hormone falls and hypocalcemia can follow. The nurse monitors calcium and checks for tingling around the mouth, muscle cramps and Chvostek or Trousseau signs.Checked against:Hypoparathyroidism · MedlinePlus (NIH) (2026)
Answer
D · Serum calcium
2.Lab values & electrolytes
Answer: A · Iron overload, treated with iron chelation therapy
Why this is the answer
The body has no active way to excrete iron, so repeated transfusions cause iron overload, which can damage the heart, liver and endocrine glands. A very high ferritin reflects this, and iron chelators are used to remove the excess.Checked against:Thalassemia · MedlinePlus (NIH) (2026); Ferritin Blood Test · MedlinePlus (NIH) (2025)
Answer
A · Iron overload, treated with iron chelation therapy
3.Arterial blood gas interpretation
Answer: PaCO₂ = 32 mmHg
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.
Winter’s formula
PaCO₂ = 1.5·16 + 8 = 32 mmHg
range: 30–34 mmHg
Answer
PaCO₂ = 32 mmHg
4.Arterial blood gas interpretation
Answer: delta ratio = 3.143
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 − (86 + 17) = 34
Delta ratio
(34 − 12)/(24 − 17) = 22/7 = 3.143
Answer
delta ratio = 3.143
Above 2: an added metabolic alkalosis.
5.Clinical pharmacokinetics
Answer: R = 28.35 mg/h
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.
Clearance
CL = 0.035·81 = 2.835 L/h
Infusion rate
R = 2.835·10 = 28.35 mg/h
Answer
R = 28.35 mg/h
6.Clinical pharmacokinetics
Answer: D = 113.8 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 = 0.3·69 = 20.7 L
Dose
D = 20.7·(6 − 0.5) = 113.8 mg
Answer
D = 113.8 mg
In practice the dose is rounded to a convenient amount.
7.Clinical nursing
Answer: A · Hypocalcaemia
Why this is the answer
Damage to the parathyroid glands lowers calcium, causing perioral tingling, tetany and positive Chvostek or Trousseau signs.
Answer
A · Hypocalcaemia
8.Clinical nursing
Answer: A · Medicines reconciliation against reliable sources, such as primary care records
Why this is the answer
Medicines reconciliation compares several sources to produce an accurate medication list. It finds omissions, duplications and errors, and is especially important in older people with polypharmacy.
Answer
A · Medicines reconciliation against reliable sources, such as primary care records