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 week after a splenectomy, a patient's platelet count is 650 × 10³/µL. What is the best interpretation?
AA laboratory error, because platelets fall after splenectomy
BA sign of new leukemia that needs immediate chemotherapy
CA sign of iron overload from the surgery and transfusion
DAn expected rise after splenectomy that increases clot risk
2.Lab values & electrolytesHard
A patient in DKA is receiving IV fluids and insulin. The glucose has fallen to about 220 mg/dL (12.2 mmol/L), but the anion gap is still high. What change would the nurse expect?
AStop the insulin infusion because the glucose is near normal
BGive an IV bolus of sodium bicarbonate at once
CAdd dextrose to the IV fluids and continue the insulin
DStop the IV fluids and start oral fluids only
3.Arterial blood gas interpretationMedium
A practice gas shows PaCO₂ 78 mmHg (10.4 kPa). Use the rule that in acute respiratory acidosis HCO₃⁻ rises by 1 mmol/L for every 10 mmHg that PaCO₂ is above 40, starting from HCO₃⁻ 24 mmol/L, to find the expected HCO₃⁻.
At sea level, a practice gas taken on 24% oxygen shows PaO₂ 60 mmHg (8.0 kPa) and PaCO₂ 33 mmHg (4.4 kPa). Use PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8 to find the A–a gradient.
Practice exercise (made-up values, not clinical guidance). An 82 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.
82 kgVd = 0.5 L/kgC = 12 mg/LS = 0.8
LD = mg
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A 65-year-old woman weighs 64 kg and has a serum creatinine of 1.5 mg/dL (133 µmol/L). Estimate the creatinine clearance with Cockcroft–Gault: CrCl (mL/min) = (140 − age) × weight (kg)/(72 × SCr in mg/dL), × 0.85 for women. The usual atenolol dose is 100 mg for a creatinine clearance of 120 mL/min, and a fraction fe = 0.9 of the drug is excreted unchanged by the kidneys. Using the Dettli dose fraction Q = 1 − fe·(1 − CrCl/normal CrCl), find the adjusted dose D. Round to the nearest whole number.
During a vesicant chemotherapy infusion, the patient reports burning and swelling at the cannula site. What should the nurse do first?
AStop the infusion straight away and follow the extravasation protocol
BFlush the cannula hard with saline
CIncrease the rate so that the infusion finishes as quickly as possible
DApply a warm pack and continue the infusion
8.Clinical nursingHard
An older patient with delirium is trying to leave the ward at night, saying he must go home. What is the best initial approach?
ALet him leave, because adults can always do as they wish whatever their state
BPhysically restrain him in bed until the morning so that he cannot fall
CCalmly reassure and redirect him while the team assesses his capacity and safety
DGive a sedative straight away without a prescription
3Solutions
1.Lab values & electrolytes
Answer: D · An expected rise after splenectomy that increases clot risk
Why this is the answer
The spleen stores about a third of the platelets. After splenectomy, the count often rises (reactive thrombocytosis), which can increase the risk of thrombosis. The count is monitored, and the patient is assessed for signs of clots.Checked against:Platelet count · MedlinePlus (NIH) (2025); Platelet Tests · MedlinePlus (NIH) (2024)
Answer
D · An expected rise after splenectomy that increases clot risk
2.Lab values & electrolytes
Answer: C · Add dextrose to the IV fluids and continue the insulin
Why this is the answer
The glucose usually falls before the ketoacidosis resolves. Dextrose is added so insulin can continue, clearing ketones without causing hypoglycemia, until the anion gap closes. Stopping insulin too early lets ketoacidosis return.
Answer
C · Add dextrose to the IV fluids and continue the insulin
3.Arterial blood gas interpretation
Answer: HCO₃⁻ = 27.8 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
|78 − 40| = 38 mmHg → 3.8 × 10 mmHg
Expected HCO₃⁻
HCO₃⁻ = 24 + 1·3.8 = 27.8 mmol/L
Answer
HCO₃⁻ = 27.8 mmol/L
4.Arterial blood gas interpretation
Answer: A–a gradient = 69.87 mmHg
Derived values: calculated serum osmolality is 2·Na⁺ + glucose + urea with all values in mmol/L (2·Na⁺ + glucose/18 + BUN/2.8 with glucose and BUN in mg/dL); normal is about 275–295 mOsm/kg. The P/F ratio is PaO₂ divided by FiO₂ as a fraction. The alveolar oxygen pressure is PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8 mmHg at sea level, and the A–a gradient is PAO₂ − PaO₂.
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 = 0.5·82 = 41 L
Amount of active drug needed
Vd·C = 41·12 = 492 mg
Loading dose
LD = 492/(0.8·1) = 615 mg
Answer
LD = 615 mg
In practice the dose is rounded to a convenient amount.
6.Clinical pharmacokinetics
Answer: D = 38 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.
Creatinine clearance (Cockcroft–Gault)
CrCl = (140 − 65)·64/(72·1.5)·0.85 = 37.78 mL/min
Dose fraction (Dettli)
Q = 1 − 0.9·(1 − 37.78/120) = 0.3833
Adjusted dose
D = 0.3833·100 = 38.33 ≈ 38 mg
Answer
D = 38 mg
In practice the dose is rounded to a convenient amount and checked against the product information.
7.Clinical nursing
Answer: A · Stop the infusion straight away and follow the extravasation protocol
Why this is the answer
Suspected extravasation of a vesicant needs the infusion stopped immediately. Local protocols then guide aspiration through the cannula, specific treatments, elevation, documentation and specialist review.
Answer
A · Stop the infusion straight away and follow the extravasation protocol
8.Clinical nursing
Answer: C · Calmly reassure and redirect him while the team assesses his capacity and safety
Why this is the answer
De-escalation, reorientation and supervision come first. If he lacks capacity, the team must act in his best interests using the least restrictive measures, following local law and policy.
Answer
C · Calmly reassure and redirect him while the team assesses his capacity and safety