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 taken hydrochlorothiazide for several years. Which laboratory change is a recognized effect of this drug?
AA rise in serum uric acid
BA fall in serum calcium
CA fall in blood glucose
DA rise in serum potassium
2.Lab values & electrolytesHard
A patient has a sodium of 150 mEq/L, a glucose of 180 mg/dL and a BUN of 28 mg/dL. Using the formula 2 × Na + glucose/18 + BUN/2.8, what is the calculated serum osmolality?
AAbout 300 mOsm/kg, which is normal
BAbout 285 mOsm/kg, which is normal
CAbout 358 mOsm/kg, which is high
DAbout 320 mOsm/kg, which is high
3.Arterial blood gas interpretationMedium
A practice gas shows a metabolic acidosis with HCO₃⁻ 8 mmol/L. Use Winter’s formula to find the expected PaCO₂ (the centre of the range) in mmHg.
HCO₃⁻ 8 mmol/L
PaCO₂ = mmHg
4.Arterial blood gas interpretationHard
A practice gas shows a respiratory acidosis: pH 7.25, PaCO₂ 73 mmHg (9.7 kPa), HCO₃⁻ 31 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 7.25PaCO₂ 73 mmHg (9.7 kPa)HCO₃⁻ 31 mmol/L
1acute
2chronic
3acute on chronic
5.Clinical pharmacokineticsMedium
Practice exercise (made-up values, not clinical guidance). A patient has a lidocaine clearance of 700 mL/min. What infusion rate R in mg/min keeps the steady-state level at 2.5 mg/L?
CL = 700 mL/minCss = 2.5 mg/L
R = mg/min
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). An 80 kg patient on regular gentamicin doses is at steady state. The target is a peak of 7 mg/L and a trough of 1 mg/L, and the volume of distribution is 0.25 L/kg. Assuming rapid IV doses, what dose D raises the level from the trough to the peak?
80 kgVd = 0.25 L/kgCpeak = 7 mg/LCtrough = 1 mg/L
D = mg
7.Clinical nursingMedium
What happens in third-degree (complete) heart block?
ANo impulses from the atria reach the ventricles, so they beat independently
BThe ventricles beat very fast and regularly
CThe atria beat chaotically with no P waves, and the ventricular rate is irregular
DEvery impulse reaches the ventricles, but each one is slightly delayed
8.Clinical nursingHard
A nurse teaches a patient with heart failure about self-care. Which statement shows a need for further teaching?
A"I will take my water tablet in the morning."
B"I will weigh myself each morning after passing urine."
C"I will report it if I need more pillows to sleep."
D"I only need to weigh myself once a month."
3Solutions
1.Lab values & electrolytes
Answer: A · A rise in serum uric acid
Why this is the answer
Thiazides reduce uric acid excretion and can trigger gout. They also tend to lower potassium, sodium and magnesium, raise calcium (by reducing its excretion) and raise blood glucose and lipids.Checked against:Hydrochlorothiazide · StatPearls (NCBI Bookshelf) (2026)
Answer
A · A rise in serum uric acid
2.Lab values & electrolytes
Answer: D · About 320 mOsm/kg, which is high
Why this is the answer
Osmolality = 300 + 10 + 10 = 320 mOsm/kg, above the typical range of about 275–295 mOsm/kg. The high sodium is the main cause, which points to water loss or too little water intake.Checked against:Osmolality blood test · MedlinePlus (NIH) (2025)
Answer
D · About 320 mOsm/kg, which is high
3.Arterial blood gas interpretation
Answer: PaCO₂ = 20 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 = 1.5·8 + 8 = 20 mmHg
range: 18–22 mmHg
Answer
PaCO₂ = 20 mmHg
4.Arterial blood gas interpretation
Answer: acute on chronic
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 + 1·3.3 = 27.3 mmol/L
chronic: 24 + 3.5·3.3 = 35.55 mmol/L
Compare
measured: 31 mmol/L → acute on chronic
Answer
acute on chronic
5.Clinical pharmacokinetics
Answer: R = 1.75 mg/min
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 in L/min
700 mL/min = 0.7 L/min
Infusion rate
R = 0.7·2.5 = 1.75 mg/min
Answer
R = 1.75 mg/min
6.Clinical pharmacokinetics
Answer: D = 120 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 = 0.25·80 = 20 L
Dose
D = 20·(7 − 1) = 120 mg
Answer
D = 120 mg
In practice the dose is rounded to a convenient amount.
7.Clinical nursing
Answer: A · No impulses from the atria reach the ventricles, so they beat independently
Why this is the answer
In complete heart block the atria and ventricles are dissociated, and a slow escape rhythm drives the ventricles. Patients may be dizzy or collapse and often need pacing.
Answer
A · No impulses from the atria reach the ventricles, so they beat independently
8.Clinical nursing
Answer: D · "I only need to weigh myself once a month."
Why this is the answer
Daily weights at the same time with the same scales detect fluid retention early. A rapid gain, worsening breathlessness or needing more pillows should be reported.