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 taking furosemide has a potassium of 2.9 mEq/L. Which finding would the nurse expect?
ATingling around the mouth with a positive Chvostek’s sign
BTall, peaked T waves on the ECG
CDiarrhea and abdominal cramping from overactive bowels
DMuscle weakness and leg cramps
2.Lab values & electrolytesHard
The nurse teaches a patient with severe neutropenia after chemotherapy. Which statement shows a need for further teaching?
A"I'll check my temperature rectally to be accurate"
B"I'll call the clinic if my temperature rises"
C"I'll wash my hands before eating and after the toilet"
D"I'll avoid people who have a cold or the flu"
3.Arterial blood gas interpretationMedium
Practice values: Na⁺ 142 mmol/L, glucose 6.5 mmol/L and urea 22.7 mmol/L. Calculate the serum osmolality Osm = 2·Na⁺ + glucose + urea to the nearest whole number.
Na⁺ 142 mmol/Lglucose 6.5 mmol/Lurea 22.7 mmol/L
Osm = mOsm/kg
4.Arterial blood gas interpretationHard
A practice gas shows a respiratory acidosis: pH 7.21, PaCO₂ 70 mmHg (9.3 kPa), HCO₃⁻ 27 mmol/L. Use the rules ‘HCO₃⁻ rises 1 mmol/L per 10 mmHg (acute) or 3.5 mmol/L per 10 mmHg (chronic)’ to classify it. Click your answer.
pH 7.21PaCO₂ 70 mmHg (9.3 kPa)HCO₃⁻ 27 mmol/L
1acute
2chronic
3acute on chronic
5.Clinical pharmacokineticsMedium
Practice exercise (made-up values, not clinical guidance). A 99 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.
99 kgVd = 0.5 L/kgC = 10 mg/LS = 0.8
LD = mg
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A patient receives vancomycin IV every 12 h; the vancomycin clearance is 3 L/h. Each dose is 750 mg of vancomycin. Find the average steady-state level Css,avg.
CL = 3 L/hτ = 12 hS = 1F = 1D = 750 mg
Css,avg = mg/L
7.Clinical nursingMedium
After thyroid surgery, a patient has tingling around the mouth and muscle cramps. Which electrolyte problem should be suspected?
AHypocalcaemia
BHypermagnesaemia
CHyperkalaemia
DHypernatraemia
8.Clinical nursingHard
A patient is recognised by the team as being in the last days of life. Which approach is correct?
AAvoid talking with the family about what is happening
BStop all medicines, including pain relief and anti-sickness drugs
CReview and stop treatments that no longer help, and focus on comfort
DContinue all routine blood tests and observations four-hourly
3Solutions
1.Lab values & electrolytes
Answer: D · Muscle weakness and leg cramps
Why this is the answer
Low potassium causes muscle weakness, cramps, reduced bowel movement (constipation, even ileus) and arrhythmias. Peaked T waves are seen in hyperkalemia, and perioral tingling suggests low calcium.Checked against:Heart failure - fluids and diuretics · MedlinePlus (NIH); Low blood potassium · MedlinePlus (NIH) (2025)
Answer
D · Muscle weakness and leg cramps
2.Lab values & electrolytes
Answer: A · "I'll check my temperature rectally to be accurate"
Why this is the answer
Rectal temperatures, suppositories and enemas are avoided in neutropenia, because injury to the rectal mucosa can let bacteria into the blood. Prompt reporting of fever, hand hygiene and avoiding sick contacts are correct.
Answer
A · "I'll check my temperature rectally to be accurate"
3.Arterial blood gas interpretation
Answer: Osm = 313 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 = 2·142 + 6.5 + 22.7 = 313.2 ≈ 313 mOsm/kg
Answer
Osm = 313 mOsm/kg
4.Arterial blood gas interpretation
Answer: acute
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 HCO₃⁻
acute: 24 + 1·3 = 27 mmol/L
chronic: 24 + 3.5·3 = 34.5 mmol/L
Compare
measured: 27 mmol/L → acute
Answer
acute
5.Clinical pharmacokinetics
Answer: LD = 618.8 mg
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 = 0.5·99 = 49.5 L
Amount of active drug needed
Vd·C = 49.5·10 = 495 mg
Loading dose
LD = 495/(0.8·1) = 618.8 mg
Answer
LD = 618.8 mg
In practice the dose is rounded to a convenient amount.
6.Clinical pharmacokinetics
Answer: Css,avg = 20.83 mg/L
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.
Damage to the parathyroid glands lowers calcium, causing perioral tingling, tetany and positive Chvostek or Trousseau signs.
Answer
A · Hypocalcaemia
8.Clinical nursing
Answer: C · Review and stop treatments that no longer help, and focus on comfort
Why this is the answer
When dying is recognised, non-beneficial interventions are stopped and anticipatory medicines for symptoms such as pain, agitation and secretions are prescribed. Clear communication with the patient and family is essential.
Answer
C · Review and stop treatments that no longer help, and focus on comfort