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 chronic kidney disease receives an erythropoiesis-stimulating agent such as epoetin alfa. Which assessments are most important?
ASerum glucose and ketones
BSerum amylase and lipase
CHemoglobin and blood pressure
DLiver enzymes and bilirubin
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?
ALisinopril lowers potassium
BSeveral medicines that raise potassium are being taken together
CPotassium supplements have no effect on the serum level
DSpironolactone causes potassium loss
3.Arterial blood gas interpretationMedium
A practice gas shows a metabolic acidosis with HCO₃⁻ 10 mmol/L. Use Winter’s formula to find the expected PaCO₂ (the centre of the range) in mmHg.
HCO₃⁻ 10 mmol/L
PaCO₂ = mmHg
4.Arterial blood gas interpretationHard
A practice gas shows pH 7.25, PaCO₂ 21 mmHg (2.8 kPa) and HCO₃⁻ 9 mmol/L, with Na⁺ 142 and Cl⁻ 103 mmol/L. Use the anion gap and the delta ratio to click the best interpretation.
Practice exercise (made-up values, not clinical guidance). Vancomycin is given as a continuous infusion to a patient whose vancomycin clearance is 85 mL/min. What infusion rate R in mg/h gives a steady-state level of 20 mg/L?
CL = 85 mL/minCss = 20 mg/L
R = mg/h
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). An 87 kg patient has a digoxin level of 0.4 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?
87 kgVd = 7 L/kgC = 0.4 → 0.8 mcg/LF = 0.7
LD = mcg
7.Clinical nursingMedium
Within which time window is IV thrombolysis generally given for acute ischaemic stroke?
AWithin 4.5 hours of symptom onset
BOnly after 24 hours
CWithin 48 hours of the onset of symptoms
DAt any time within a week
8.Clinical nursingHard
A patient has a painful, punched-out ulcer on the toes, with a cold, pale foot and weak foot pulses. Which type of ulcer is most likely?
AA pressure ulcer from bed rest
BAn arterial ulcer
CA burn
DA venous ulcer
3Solutions
1.Lab values & electrolytes
Answer: C · Hemoglobin and blood pressure
Why this is the answer
Epoetin stimulates red cell production. Hemoglobin is monitored, because raising it too high increases the risk of hypertension, stroke and thrombosis. Blood pressure is monitored, and iron stores are checked, since iron is needed for a good response.Checked against:Erythropoietin Stimulating Agents · StatPearls (NCBI Bookshelf) (2024)
Answer
C · Hemoglobin and blood pressure
2.Lab values & electrolytes
Answer: B · 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
B · Several medicines that raise potassium are being taken together
3.Arterial blood gas interpretation
Answer: PaCO₂ = 23 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 = 1.5·10 + 8 = 23 mmHg
range: 21–25 mmHg
Answer
PaCO₂ = 23 mmHg
4.Arterial blood gas interpretation
Answer: pure high-gap 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.
Anion gap
AG = 142 − (103 + 9) = 30
Delta ratio
(30 − 12)/(24 − 9) = 1.2
pure high-gap acidosis
Answer
pure high-gap acidosis
5.Clinical pharmacokinetics
Answer: R = 102 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.
Clearance in L/h
85·60/1000 = 5.1 L/h
Infusion rate
R = 5.1·20 = 102 mg/h
Answer
R = 102 mg/h
6.Clinical pharmacokinetics
Answer: LD = 348 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 = 7·87 = 609 L
Rise needed
0.8 − 0.4 = 0.4 mcg/L
Loading dose
LD = 609·0.4/(1·0.7) = 348 mcg
Answer
LD = 348 mcg
In practice the dose is rounded to a convenient amount.
7.Clinical nursing
Answer: A · Within 4.5 hours of symptom onset
Why this is the answer
Thrombolysis with alteplase is generally given within 4.5 hours of onset after a scan excludes haemorrhage. Knowing when the patient was last seen well is therefore essential.
Answer
A · Within 4.5 hours of symptom onset
8.Clinical nursing
Answer: B · An arterial ulcer
Why this is the answer
Arterial ulcers are usually on the feet or toes, painful and punched-out, with signs of poor perfusion. Venous ulcers are usually around the ankle, shallow and associated with swelling and skin staining.