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 an absolute neutrophil count of 0.4 × 10³/µL. Which measure is appropriate?
AStrict hand hygiene, temperature checks and no visitors with infections
BTaking temperatures rectally because readings are more accurate
CBringing fresh flowers into the room to lift the patient’s mood
DSharing a room with another patient who is being treated for pneumonia
2.Lab values & electrolytesHard
A patient has macrocytic anemia with low vitamin B12 and antibodies to intrinsic factor. What does this suggest, and how is it usually treated?
AIron deficiency, treated with oral iron tablets
BFolate deficiency, treated with folic acid only
CHemolytic anemia, treated with blood transfusion
DPernicious anemia, treated with B12 injections
3.Arterial blood gas interpretationMedium
Practice values: Na⁺ 138 mmol/L, glucose 23.7 mmol/L and urea 11.9 mmol/L. Calculate the serum osmolality Osm = 2·Na⁺ + glucose + urea to the nearest whole number.
Na⁺ 138 mmol/Lglucose 23.7 mmol/Lurea 11.9 mmol/L
Osm = mOsm/kg
4.Arterial blood gas interpretationHard
Practice values: Na⁺ 145, Cl⁻ 105 and HCO₃⁻ 13 mmol/L. Calculate the delta ratio (AG − 12)/(24 − HCO₃⁻), using AG = Na⁺ − (Cl⁻ + HCO₃⁻). Give two decimals.
Na⁺ 145 mmol/LCl⁻ 105 mmol/LHCO₃⁻ 13 mmol/L
delta ratio =
5.Clinical pharmacokineticsMedium
Practice exercise (made-up values, not clinical guidance). A patient has a lidocaine clearance of 550 mL/min. What infusion rate R in mg/min keeps the steady-state level at 4 mg/L?
CL = 550 mL/minCss = 4 mg/L
R = mg/min
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A patient's tobramycin half-life is 2 h. The target is a peak of 6 mg/L and a trough of 1 mg/L. Assuming rapid IV doses and first-order elimination, what dosing interval τ lets the level fall from the peak to the trough? Round to one decimal place.
t½ = 2 hCpeak = 6 mg/LCtrough = 1 mg/L
τ = h
7.Clinical nursingMedium
Within which time window is IV thrombolysis generally given for acute ischaemic stroke?
AOnly after 24 hours
BAt any time within a week
CWithin 48 hours of the onset of symptoms
DWithin 4.5 hours of symptom onset
8.Clinical nursingHard
Which action is most important in protecting a patient with severe neutropenia from infection?
AStrict hand hygiene by everyone who has contact with the patient
BGiving routine intramuscular injections
CKeeping fresh flowers and plants in the room to help the patient's mood
DTaking rectal temperatures regularly
3Solutions
1.Lab values & electrolytes
Answer: A · Strict hand hygiene, temperature checks and no visitors with infections
Why this is the answer
Severe neutropenia carries a high risk of infection. Hand hygiene, early recognition of fever and protection from sources of infection are key. Flowers can carry organisms, rectal procedures can injure the mucosa, and a roommate with pneumonia is an infection risk.Checked against:Low white blood cell count and cancer · MedlinePlus (NIH) (2025)
Answer
A · Strict hand hygiene, temperature checks and no visitors with infections
2.Lab values & electrolytes
Answer: D · Pernicious anemia, treated with B12 injections
Why this is the answer
Pernicious anemia is an autoimmune condition in which antibodies block intrinsic factor or destroy the cells that make it, so B12 is not absorbed. Treatment is lifelong B12, usually by injection (very high oral doses are an alternative).Checked against:Pernicious anemia · MedlinePlus (NIH) (2026)
Answer
D · Pernicious anemia, treated with B12 injections
3.Arterial blood gas interpretation
Answer: Osm = 312 mOsm/kg
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.
Calculated osmolality
Osm = 2·138 + 23.7 + 11.9 = 311.6 ≈ 312 mOsm/kg
Answer
Osm = 312 mOsm/kg
4.Arterial blood gas interpretation
Answer: delta ratio = 1.364
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 = 145 − (105 + 13) = 27
Delta ratio
(27 − 12)/(24 − 13) = 15/11 = 1.364
Answer
delta ratio = 1.364
Between 1 and 2: a pure high anion gap acidosis.
5.Clinical pharmacokinetics
Answer: R = 2.2 mg/min
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/min
550 mL/min = 0.55 L/min
Infusion rate
R = 0.55·4 = 2.2 mg/min
Answer
R = 2.2 mg/min
6.Clinical pharmacokinetics
Answer: τ = 5.2 h
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.
Rate constant
k = 0.693/2 = 0.3466 h⁻¹
Dosing interval
τ = ln(6/1)/0.3466 = 1.792/0.3466 = 5.17 ≈ 5.2 h
Answer
τ = 5.2 h
In practice the interval is rounded to a convenient value such as 8, 12, 24 or 36 h, and the levels are checked again.
7.Clinical nursing
Answer: D · 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
D · Within 4.5 hours of symptom onset
8.Clinical nursing
Answer: A · Strict hand hygiene by everyone who has contact with the patient
Why this is the answer
Hand hygiene is the single most effective measure to protect neutropenic patients. Rectal procedures and IM injections are avoided because they can cause mucosal or tissue injury and bleeding.
Answer
A · Strict hand hygiene by everyone who has contact with the patient