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
Why is a baseline platelet count taken before starting heparin therapy?
ABecause heparin increases the platelet count to dangerous levels
BSo that the INR can be calculated from the platelet count later
CBecause heparin is given at a dose based on the platelet count
DSo a later fall from heparin-induced thrombocytopenia is detected
2.Lab values & electrolytesHard
After lying on the floor for many hours, a patient has a CK of 40,000 U/L, a potassium of 6.1 mEq/L and dark urine. Which prescribed treatment is the priority?
AHigh-dose NSAIDs for the muscle pain
BFluid restriction to protect the heart from overload
CLarge-volume IV fluids with cardiac monitoring
DAn oral potassium supplement with each meal
3.Arterial blood gas interpretationMedium
A practice arterial blood gas shows pH 7.38, PaCO₂ 23 mmHg (3.1 kPa) and HCO₃⁻ 13 mmol/L. The primary disorder is metabolic acidosis. How far is it compensated? Click your answer.
pH 7.38PaCO₂ 23 mmHg (3.1 kPa)HCO₃⁻ 13 mmol/L
1uncompensated
2partially compensated
3fully compensated
4.Arterial blood gas interpretationHard
Practice values: Na⁺ 140, Cl⁻ 98 and HCO₃⁻ 15 mmol/L. Calculate the delta ratio (AG − 12)/(24 − HCO₃⁻), using AG = Na⁺ − (Cl⁻ + HCO₃⁻). Give two decimals.
Na⁺ 140 mmol/LCl⁻ 98 mmol/LHCO₃⁻ 15 mmol/L
delta ratio =
5.Clinical pharmacokineticsMedium
Practice exercise (made-up values, not clinical guidance). After an IV dose of vancomycin, the level is 30 mg/L at 1 h and 13.6 mg/L at 9 h. Assume first-order elimination. Find the half-life t½.
C1 = 30 mg/L (1 h)C2 = 13.6 mg/L (9 h)
t½ = h
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A 59 kg patient takes oral aminophylline (S = 0.8, F = 1) every 12 h. The theophylline clearance is 0.05 L/kg/h. What dose D in mg gives an average steady-state level of 10 mg/L?
A patient's low potassium does not improve despite potassium replacement. Which other electrolyte should be checked?
ASodium
BPhosphate
CMagnesium
DChloride
8.Clinical nursingHard
After a high spinal cord injury, a patient has low blood pressure with a slow heart rate and warm, dry skin. What is most likely?
ANeurogenic shock
BHypovolaemic shock
CCardiac tamponade
DSeptic shock
3Solutions
1.Lab values & electrolytes
Answer: D · So a later fall from heparin-induced thrombocytopenia is detected
Why this is the answer
Heparin-induced thrombocytopenia is diagnosed partly by a fall of about 50% or more from the baseline count, usually 5–14 days after starting. A baseline count and regular checks make this possible.Checked against:Heparin-Induced Thrombocytopenia · StatPearls (NCBI Bookshelf) (2023)
Answer
D · So a later fall from heparin-induced thrombocytopenia is detected
2.Lab values & electrolytes
Answer: C · Large-volume IV fluids with cardiac monitoring
Why this is the answer
Rhabdomyolysis releases myoglobin, which can cause acute kidney injury, and potassium, which can cause arrhythmias. Early aggressive IV fluids, a target urine output and cardiac monitoring are priorities. NSAIDs and potassium would worsen kidney injury and hyperkalemia.Checked against:Rhabdomyolysis · MedlinePlus (NIH) (2025)
Answer
C · Large-volume IV fluids with cardiac monitoring
3.Arterial blood gas interpretation
Answer: fully compensated
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 7.38: within 7.35–7.45
PaCO₂ 23 mmHg: low (35–45)
HCO₃⁻ 13 mmol/L: low (22–26)
Compensation
Clinically, also check the expected compensation: Winter’s formula expects PaCO₂ 25.5–29.5 mmHg, so the measured 23 mmHg points to an additional respiratory alkalosis (a mixed disorder). The answer above is the ROME exam classification.
fully compensated
Answer
fully compensated
4.Arterial blood gas interpretation
Answer: delta ratio = 1.667
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 = 140 − (98 + 15) = 27
Delta ratio
(27 − 12)/(24 − 15) = 15/9 = 1.667
Answer
delta ratio = 1.667
Between 1 and 2: a pure high anion gap acidosis.
5.Clinical pharmacokinetics
Answer: t½ = 7.009 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(30/13.6)/8 = 0.7911/8 = 0.09889 h⁻¹
Half-life
t½ = 0.693/0.09889 = 7.009 h
Answer
t½ = 7.009 h
6.Clinical pharmacokinetics
Answer: D = 442.5 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 = 0.05·59 = 2.95 L/h
Dose
D = Css,avg·CL·τ/(S·F) = 10·2.95·12/(0.8·1) = 442.5 mg
Answer
D = 442.5 mg
7.Clinical nursing
Answer: C · Magnesium
Why this is the answer
Hypomagnesaemia increases renal potassium loss and makes hypokalaemia hard to correct. Magnesium must be replaced as well.
Answer
C · Magnesium
8.Clinical nursing
Answer: A · Neurogenic shock
Why this is the answer
Neurogenic shock from cervical or high thoracic injury causes vasodilation and bradycardia because sympathetic outflow is lost. Hypovolaemic shock usually causes tachycardia and cool, pale skin.