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 iron-deficiency anemia started oral iron. Which result is usually the earliest sign that treatment is working?
AA fall in the white cell count
BA rise in the reticulocyte count
CA rise in the platelet count
DA fall in the mean cell volume (MCV)
2.Lab values & electrolytesHard
A patient on hemodialysis receives epoetin, but the hemoglobin has not risen in 2 months. Which results should be checked first?
ATSH and the free T3 level
BAmylase and lipase levels
CFerritin and transferrin saturation
DTroponin and creatine kinase
3.Arterial blood gas interpretationMedium
A practice arterial blood gas shows pH 7.29, PaCO₂ 52 mmHg (6.9 kPa) and HCO₃⁻ 24 mmol/L. The primary disorder is respiratory acidosis. How far is it compensated? Click your answer.
pH 7.29PaCO₂ 52 mmHg (6.9 kPa)HCO₃⁻ 24 mmol/L
1uncompensated
2partially compensated
3fully compensated
4.Arterial blood gas interpretationHard
A practice gas shows a metabolic alkalosis with HCO₃⁻ 40 mmol/L. Use expected PaCO₂ = 0.7·HCO₃⁻ + 21 to find the expected PaCO₂.
HCO₃⁻ 40 mmol/L
PaCO₂ = mmHg
5.Clinical pharmacokineticsMedium
Practice exercise (made-up values, not clinical guidance). An 87 kg patient is to be loaded intravenously with aminophylline to reach a theophylline level of 12 mg/L. The volume of distribution is 0.5 L/kg and the salt factor is S = 0.8. Find the loading dose LD of aminophylline in mg.
87 kgVd = 0.5 L/kgC = 12 mg/LS = 0.8
LD = mg
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A theophylline infusion runs at 25 mg/h with no loading dose. The patient's clearance is 2.5 L/h and the half-life is 6 h. What is the level C after 14 h?
R = 25 mg/hCL = 2.5 L/ht½ = 6 ht = 14 h
C = mg/L
7.Clinical nursingMedium
Which oxygen device delivers a precise, fixed concentration of oxygen?
AVenturi mask
BNasal cannula
CNon-rebreathe mask
DSimple face mask
8.Clinical nursingHard
A patient with a central venous catheter develops fever and rigors shortly after the line is flushed, with no other obvious source. What should be suspected?
ANormal response to line use
BAn allergic reaction to the saline flush
CA catheter-related bloodstream infection
DA pulmonary embolism
3Solutions
1.Lab values & electrolytes
Answer: B · A rise in the reticulocyte count
Why this is the answer
Once iron is available, the marrow releases more young red cells, so the reticulocyte count rises within about 1–2 weeks. The hemoglobin rises more slowly over weeks, and iron stores take months to refill.Checked against:Reticulocyte Count · MedlinePlus (NIH) (2024)
Answer
B · A rise in the reticulocyte count
2.Lab values & electrolytes
Answer: C · Ferritin and transferrin saturation
Why this is the answer
Iron deficiency is the most common reason for a poor response to erythropoiesis-stimulating agents, because the marrow needs iron to make hemoglobin. Ferritin and transferrin saturation are checked, and IV iron is often given in dialysis patients. Infection and inflammation can also blunt the response.Checked against:Anemia of Chronic Kidney Disease · StatPearls (NCBI Bookshelf) (2024); Erythropoietin Stimulating Agents · StatPearls (NCBI Bookshelf) (2024)
Answer
C · Ferritin and transferrin saturation
3.Arterial blood gas interpretation
Answer: uncompensated
Reading a blood gas (ROME): first the pH: below 7.35 is acidaemia, above 7.45 alkalaemia. Then find the value that explains it. PaCO₂ (normal 35–45 mmHg, 4.7–6.0 kPa) is the respiratory part and moves Opposite to the pH in a respiratory disorder; HCO₃⁻ (normal 22–26 mmol/L) is the metabolic part and moves in the same direction as the pH (Respiratory Opposite, Metabolic Equal). The other value shows the compensation: normal means uncompensated; abnormal in the direction that pulls the pH back while the pH is still abnormal means partially compensated; a normal pH with both values abnormal means fully compensated. Ranges differ slightly between laboratories; these are practice values.
ROME, value by value
pH 7.29: acidaemia (below 7.35)
PaCO₂ 52 mmHg: high (35–45)
HCO₃⁻ 24 mmol/L: normal (22–26)
Compensation
uncompensated
Answer
uncompensated
4.Arterial blood gas interpretation
Answer: PaCO₂ = 49 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
0.7·40 + 21 = 49 mmHg
Answer
PaCO₂ = 49 mmHg
The rise has a limit: PaCO₂ rarely goes above about 55 mmHg from compensation alone.
5.Clinical pharmacokinetics
Answer: LD = 652.5 mg
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 = 0.5·87 = 43.5 L
Amount of active drug needed
Vd·C = 43.5·12 = 522 mg
Loading dose
LD = 522/(0.8·1) = 652.5 mg
Answer
LD = 652.5 mg
In practice the dose is rounded to a convenient amount.
6.Clinical pharmacokinetics
Answer: C = 8.016 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.
Steady-state level
Css = 25/2.5 = 10 mg/L
Rate constant
k = 0.693/6 = 0.1155 h⁻¹
Level after the given time
C = 10·(1 − e−0.1155·14) = 8.016 mg/L
Answer
C = 8.016 mg/L
7.Clinical nursing
Answer: A · Venturi mask
Why this is the answer
A Venturi mask entrains a set amount of room air, giving a fixed oxygen percentage regardless of the patient's breathing pattern. This makes it useful for patients at risk of hypercapnia.
Answer
A · Venturi mask
8.Clinical nursing
Answer: C · A catheter-related bloodstream infection
Why this is the answer
Rigors after flushing suggest bacteria colonising the catheter. Blood cultures are taken (often from the line and a peripheral vein) and the team decides whether to remove the line.