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
Apart from the lithium level, which tests are monitored regularly in long-term lithium therapy?
ATroponin and creatine kinase (CK)
BKidney function and thyroid function
CINR and the platelet count
DLiver enzymes and serum amylase
2.Lab values & electrolytesHard
The nurse teaches a patient who is starting warfarin. Which statement shows a need for further teaching?
A"I will keep my intake of green leafy vegetables about the same"
B"If I miss a dose, I will take two tablets the next day"
C"I will have my INR checked as often as I am told to"
D"I will tell my dentist and pharmacist that I take warfarin"
3.Arterial blood gas interpretationMedium
A practice gas shows PaCO₂ 25 mmHg (3.3 kPa). Use the rule that in chronic respiratory alkalosis HCO₃⁻ falls by 5 mmol/L for every 10 mmHg that PaCO₂ is below 40, starting from HCO₃⁻ 24 mmol/L, to find the expected HCO₃⁻.
A practice gas shows a metabolic acidosis: pH 7.29, PaCO₂ 26 mmHg (3.5 kPa), HCO₃⁻ 12 mmol/L. Compare the measured PaCO₂ with Winter’s formula. Click your interpretation.
pH 7.29PaCO₂ 26 mmHg (3.5 kPa)HCO₃⁻ 12 mmol/L
1appropriate compensation
2added respiratory acidosis
3added respiratory alkalosis
5.Clinical pharmacokineticsMedium
Practice exercise (made-up values, not clinical guidance). Vancomycin is given as a continuous infusion to a patient whose vancomycin clearance is 4.5 L/h. What infusion rate R in mg/h gives a steady-state level of 25 mg/L?
CL = 4.5 L/hCss = 25 mg/L
R = mg/h
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). An 82 kg patient has a digoxin level of 0.2 mcg/L; the target is 0.8 mcg/L. The volume of distribution is 7 L/kg and the tablets have F = 0.7. What extra loading dose LD raises the level to the target?
82 kgVd = 7 L/kgC = 0.2 → 0.8 mcg/LF = 0.7
LD = mcg
7.Clinical nursingMedium
Why is an urgent brain scan needed before thrombolysis for a suspected stroke?
ATo measure the size of the ventricles
BTo check for brain tumours only
CTo rule out bleeding in the brain
DTo confirm the patient's age
8.Clinical nursingHard
A nurse teaches a patient with reflux disease. Which statement shows a need for further teaching?
A"I will raise the head of my bed with blocks."
B"A big meal just before bed is fine if I take my tablet."
C"I will cut down on alcohol."
D"I will try to lose some weight over the next months."
3Solutions
1.Lab values & electrolytes
Answer: B · Kidney function and thyroid function
Why this is the answer
Long-term lithium can cause hypothyroidism and impair kidney function, including nephrogenic diabetes insipidus. Creatinine/eGFR and TSH are therefore checked regularly, together with sodium and the lithium level.
Answer
B · Kidney function and thyroid function
2.Lab values & electrolytes
Answer: B · "If I miss a dose, I will take two tablets the next day"
Why this is the answer
Doubling a missed warfarin dose can raise the INR and cause bleeding; the patient should follow the prescriber's instructions, usually taking the missed dose the same day or skipping it. A steady vitamin K intake, regular INR checks and telling other providers are correct.
Answer
B · "If I miss a dose, I will take two tablets the next day"
3.Arterial blood gas interpretation
Answer: HCO₃⁻ = 16.5 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.
Change in PaCO₂
|25 − 40| = 15 mmHg → 1.5 × 10 mmHg
Expected HCO₃⁻
HCO₃⁻ = 24 − 5·1.5 = 16.5 mmol/L
Answer
HCO₃⁻ = 16.5 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
1.5·12 + 8 = 26 → 24–28 mmHg
Compare
measured: 26 mmHg → appropriate compensation
Answer
appropriate compensation
5.Clinical pharmacokinetics
Answer: R = 112.5 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.
Infusion rate
R = 4.5·25 = 112.5 mg/h
Answer
R = 112.5 mg/h
6.Clinical pharmacokinetics
Answer: LD = 492 mcg
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 = 7·82 = 574 L
Rise needed
0.8 − 0.2 = 0.6 mcg/L
Loading dose
LD = 574·0.6/(1·0.7) = 492 mcg
Answer
LD = 492 mcg
In practice the dose is rounded to a convenient amount.
7.Clinical nursing
Answer: C · To rule out bleeding in the brain
Why this is the answer
A CT scan distinguishes haemorrhagic from ischaemic stroke. Clot-busting drugs are only given when bleeding has been excluded and other criteria are met.
Answer
C · To rule out bleeding in the brain
8.Clinical nursing
Answer: B · "A big meal just before bed is fine if I take my tablet."
Why this is the answer
Large late meals increase reflux even when a PPI is taken. Raising the head of the bed, losing weight and reducing alcohol are correct measures.
Answer
B · "A big meal just before bed is fine if I take my tablet."