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 with HIV on antiretroviral therapy has an undetectable viral load. What does this mean?
AThe CD4 count must now be dangerously low too
BThe treatment can now be stopped without any risk
CThe treatment is suppressing the virus very well
DThe virus has been completely cured and removed
2.Lab values & electrolytesHard
A patient with chronic hyponatremia (sodium 118 mEq/L) is being treated. Why must the sodium be raised slowly?
ARapid correction causes hypokalemia, which is the only risk
BRapid correction makes the sodium fall again even further
CIt can cause osmotic demyelination, a serious brain injury
DSlow correction is needed only to reduce hospital costs
3.Arterial blood gas interpretationMedium
A practice gas shows PaCO₂ 64 mmHg (8.5 kPa). Use the rule that in acute respiratory acidosis HCO₃⁻ rises by 1 mmol/L for every 10 mmHg that PaCO₂ is above 40, starting from HCO₃⁻ 24 mmol/L, to find the expected HCO₃⁻.
A practice gas shows a metabolic acidosis: pH 7.13, PaCO₂ 31 mmHg (4.1 kPa), HCO₃⁻ 10 mmol/L. Compare the measured PaCO₂ with Winter’s formula. Click your interpretation.
pH 7.13PaCO₂ 31 mmHg (4.1 kPa)HCO₃⁻ 10 mmol/L
1appropriate compensation
2added respiratory acidosis
3added respiratory alkalosis
5.Clinical pharmacokineticsMedium
Practice exercise (made-up values, not clinical guidance). After an IV dose of gentamicin, the level is 8 mg/L at 1 h and 2 mg/L at 5 h. Assume first-order elimination. Find the half-life t½.
C1 = 8 mg/L (1 h)C2 = 2 mg/L (5 h)
t½ = h
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A 95 kg patient takes oral theophylline (F = 1) every 8 h. The clearance is 0.03 L/kg/h. Each dose is 450 mg of theophylline. Find the average steady-state level Css,avg.
CL = 0.03 L/kg/hτ = 8 hS = 1F = 195 kgD = 450 mg
Css,avg = mg/L
7.Clinical nursingMedium
Why must levodopa for Parkinson's disease be given at the exact times prescribed in hospital?
AEarly doses cause severe low blood glucose and sweating attacks
BLate doses cause dangerous high potassium
CThe drug only works if taken at midnight
DDelays can quickly worsen rigidity, immobility and swallowing
8.Clinical nursingHard
A patient with back pain develops numbness around the anus and genitals and new difficulty passing urine. Which condition should be suspected?
ASimple muscle strain
BCauda equina syndrome
CConstipation
DKidney stones
3Solutions
1.Lab values & electrolytes
Answer: C · The treatment is suppressing the virus very well
Why this is the answer
An undetectable viral load shows that antiretroviral therapy is suppressing viral replication. The virus remains in the body, so treatment must continue. People with a sustained undetectable viral load do not transmit HIV sexually.
Answer
C · The treatment is suppressing the virus very well
2.Lab values & electrolytes
Answer: C · It can cause osmotic demyelination, a serious brain injury
Why this is the answer
When hyponatremia develops slowly, brain cells adapt. Raising the sodium too fast shrinks them and can damage the myelin of nerve cells (osmotic demyelination). Many guidelines limit the rise to about 8–10 mEq/L in 24 hours.Checked against:Osmotic demyelination syndrome · MedlinePlus (NIH) (2026)
Answer
C · It can cause osmotic demyelination, a serious brain injury
3.Arterial blood gas interpretation
Answer: HCO₃⁻ = 26.4 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
|64 − 40| = 24 mmHg → 2.4 × 10 mmHg
Expected HCO₃⁻
HCO₃⁻ = 24 + 1·2.4 = 26.4 mmol/L
Answer
HCO₃⁻ = 26.4 mmol/L
4.Arterial blood gas interpretation
Answer: added respiratory acidosis
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
1.5·10 + 8 = 23 → 21–25 mmHg
Compare
measured: 31 mmHg → added respiratory acidosis
Answer
added respiratory acidosis
5.Clinical pharmacokinetics
Answer: t½ = 2 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.
Elimination rate constant
k = ln(8/2)/4 = 1.386/4 = 0.3466 h⁻¹
Half-life
t½ = 0.693/0.3466 = 2 h
Answer
t½ = 2 h
6.Clinical pharmacokinetics
Answer: Css,avg = 19.74 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.
Answer: D · Delays can quickly worsen rigidity, immobility and swallowing
Why this is the answer
Levodopa has a short effect, so missed or late doses can lead to severe stiffness, falls, swallowing problems and, rarely, a dangerous hyperpyrexia-like state. Parkinson's medicines are treated as time-critical.
Answer
D · Delays can quickly worsen rigidity, immobility and swallowing
8.Clinical nursing
Answer: B · Cauda equina syndrome
Why this is the answer
Saddle anaesthesia, bladder or bowel dysfunction and leg symptoms with back pain suggest cauda equina syndrome. It needs an emergency MRI and surgical review to prevent permanent loss of function.