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 burning on urination and frequency. The dipstick is positive for nitrites and leukocyte esterase, and a urine culture is ordered. When should the sample be taken?
AAt any time, as antibiotics do not matter
BBefore the first dose of antibiotic
CAfter the symptoms have fully settled
DAfter 3 days of antibiotic treatment
2.Lab values & electrolytesHard
A patient with heart failure takes lisinopril, spironolactone and a potassium supplement. Today the potassium is 6.0 mEq/L. What is the most likely explanation?
ASeveral medicines that raise potassium are being taken together
BSpironolactone causes potassium loss
CLisinopril lowers potassium
DPotassium supplements have no effect on the serum level
3.Arterial blood gas interpretationMedium
A practice gas shows PaCO₂ 75 mmHg (10.0 kPa). Use the rule that in chronic respiratory acidosis HCO₃⁻ rises by 3.5 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 alkalosis with HCO₃⁻ 39 mmol/L. Use expected PaCO₂ = 0.7·HCO₃⁻ + 21 to find the expected PaCO₂.
HCO₃⁻ 39 mmol/L
PaCO₂ = mmHg
5.Clinical pharmacokineticsMedium
Practice exercise (made-up values, not clinical guidance). A patient receives an IV bolus of 170 mg of tobramycin. The level back-extrapolated to time zero is 7 mg/L and the half-life is 2 h. Find the clearance CL in L/h.
D = 170 mgC₀ = 7 mg/Lt½ = 2 h
CL = L/h
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A 91 kg patient takes oral theophylline (F = 1) every 8 h. The clearance is 0.045 L/kg/h. What dose D in mg gives an average steady-state level of 12 mg/L?
In suspected sepsis, what should ideally be collected before the first dose of antibiotics, as long as it does not delay treatment?
AA stool sample for occult blood
BA fasting cholesterol level
CA 24-hour urine collection
DBlood cultures
8.Clinical nursingHard
A patient is recognised by the team as being in the last days of life. Which approach is correct?
AReview and stop treatments that no longer help, and focus on comfort
BAvoid talking with the family about what is happening
CContinue all routine blood tests and observations four-hourly
DStop all medicines, including pain relief and anti-sickness drugs
3Solutions
1.Lab values & electrolytes
Answer: B · Before the first dose of antibiotic
Why this is the answer
The culture sample is taken before antibiotics, so the organism can be grown and its sensitivities tested. A sample taken after antibiotics may give a false negative culture.
Answer
B · Before the first dose of antibiotic
2.Lab values & electrolytes
Answer: A · Several medicines that raise potassium are being taken together
Why this is the answer
ACE inhibitors and spironolactone both make the kidneys retain potassium, and a supplement adds more. The nurse holds the supplement, informs the prescriber and checks kidney function and the ECG.Checked against:High potassium level · MedlinePlus (NIH) (2025); Spironolactone · MedlinePlus (NIH) (2018)
Answer
A · Several medicines that raise potassium are being taken together
3.Arterial blood gas interpretation
Answer: HCO₃⁻ = 36.25 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
|75 − 40| = 35 mmHg → 3.5 × 10 mmHg
Expected HCO₃⁻
HCO₃⁻ = 24 + 3.5·3.5 = 36.25 mmol/L
Answer
HCO₃⁻ = 36.25 mmol/L
4.Arterial blood gas interpretation
Answer: PaCO₂ = 48.3 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.
Expected PaCO₂
0.7·39 + 21 = 48.3 mmHg
Answer
PaCO₂ = 48.3 mmHg
The rise has a limit: PaCO₂ rarely goes above about 55 mmHg from compensation alone.
5.Clinical pharmacokinetics
Answer: CL = 8.417 L/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.
Volume of distribution
Vd = 170/7 = 24.29 L
Rate constant
k = 0.693/2 = 0.3466 h⁻¹
Clearance
CL = 0.3466·24.29 = 8.417 L/h
Answer
CL = 8.417 L/h
6.Clinical pharmacokinetics
Answer: D = 393.1 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.
Clearance
CL = 0.045·91 = 4.095 L/h
Dose
D = Css,avg·CL·τ/(S·F) = 12·4.095·8/(1·1) = 393.1 mg
Answer
D = 393.1 mg
7.Clinical nursing
Answer: D · Blood cultures
Why this is the answer
Blood cultures taken before antibiotics are more likely to grow the organism, which allows targeted treatment later. Antibiotics should still be given promptly and must not be delayed.
Answer
D · Blood cultures
8.Clinical nursing
Answer: A · 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
A · Review and stop treatments that no longer help, and focus on comfort