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 a low clinical probability of deep vein thrombosis has a normal (negative) D-dimer. What is the best interpretation?
AA pulmonary embolism is now certain to be present
BThe test is useless and should always be ignored
CA DVT is unlikely and can usually be ruled out
DA DVT is confirmed and anticoagulation is needed
2.Lab values & electrolytesHard
A patient on clozapine reports a sore throat and fever. The ANC is 400/µL (0.4 × 10⁹/L). What should the nurse do?
AHalve the dose and give it with a throat lozenge
BGive the next dose and recheck the ANC next month
CHold the clozapine and report the result at once
DGive the dose with food to protect the stomach
3.Arterial blood gas interpretationMedium
A practice gas shows PaCO₂ 69 mmHg (9.2 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⁻.
Practice values: Na⁺ 146, Cl⁻ 114 and HCO₃⁻ 16 mmol/L. Calculate the delta ratio (AG − 12)/(24 − HCO₃⁻), using AG = Na⁺ − (Cl⁻ + HCO₃⁻). Give two decimals.
Na⁺ 146 mmol/LCl⁻ 114 mmol/LHCO₃⁻ 16 mmol/L
delta ratio =
5.Clinical pharmacokineticsMedium
Practice exercise (made-up values, not clinical guidance). A 65 kg patient is to be loaded intravenously with phenytoin sodium to reach a phenytoin level of 15 mg/L. The volume of distribution is 0.7 L/kg and the salt factor is S = 0.92. Find the loading dose LD of phenytoin sodium in mg.
65 kgVd = 0.7 L/kgC = 15 mg/LS = 0.92
LD = mg
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A 42-year-old woman weighs 67 kg and has a serum creatinine of 3.2 mg/dL (283 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 levetiracetam dose is 1000 mg for a creatinine clearance of 120 mL/min, and a fraction fe = 0.66 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.
Why are patients with atrial fibrillation often given anticoagulants?
ATo slow the heart rate directly and prevent palpitations
BTo reduce the risk of stroke from clots in the atria
CTo convert the rhythm back to normal
DTo lower the blood pressure
8.Clinical nursingHard
A nurse teaches a patient with diabetes about foot care. Which statement shows a need for further teaching?
A"I will walk barefoot at home to toughen my feet."
B"I will wear well-fitting shoes."
C"I will see the podiatrist about hard skin."
D"I will check my feet every day for cuts or blisters."
3Solutions
1.Lab values & electrolytes
Answer: C · A DVT is unlikely and can usually be ruled out
Why this is the answer
D-dimer is very sensitive, so a negative result in a patient with low clinical probability makes DVT or PE unlikely. A positive result is not specific, because surgery, infection, pregnancy and cancer also raise D-dimer; imaging is then needed.Checked against:D-Dimer Test · MedlinePlus (NIH) (2024)
Answer
C · A DVT is unlikely and can usually be ruled out
2.Lab values & electrolytes
Answer: C · Hold the clozapine and report the result at once
Why this is the answer
An ANC below 500/µL is severe neutropenia, and with fever and sore throat it suggests a serious infection. Clozapine is held, the provider is told immediately, and infection is assessed and treated. A nurse never changes a dose without a prescription.
Answer
C · Hold the clozapine and report the result at once
3.Arterial blood gas interpretation
Answer: HCO₃⁻ = 26.9 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
|69 − 40| = 29 mmHg → 2.9 × 10 mmHg
Expected HCO₃⁻
HCO₃⁻ = 24 + 1·2.9 = 26.9 mmol/L
Answer
HCO₃⁻ = 26.9 mmol/L
4.Arterial blood gas interpretation
Answer: delta ratio = 0.5
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 = 146 − (114 + 16) = 16
Delta ratio
(16 − 12)/(24 − 16) = 4/8 = 0.5
Answer
delta ratio = 0.5
Below 1: an added normal anion gap acidosis.
5.Clinical pharmacokinetics
Answer: LD = 741.8 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 = 0.7·65 = 45.5 L
Amount of active drug needed
Vd·C = 45.5·15 = 682.5 mg
Loading dose
LD = 682.5/(0.92·1) = 741.8 mg
Answer
LD = 741.8 mg
In practice the dose is rounded to a convenient amount.
6.Clinical pharmacokinetics
Answer: D = 473 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 − 42)·67/(72·3.2)·0.85 = 24.22 mL/min
Dose fraction (Dettli)
Q = 1 − 0.66·(1 − 24.22/120) = 0.4732
Adjusted dose
D = 0.4732·1000 = 473.2 ≈ 473 mg
Answer
D = 473 mg
In practice the dose is rounded to a convenient amount and checked against the product information.
7.Clinical nursing
Answer: B · To reduce the risk of stroke from clots in the atria
Why this is the answer
In AF the atria do not contract effectively, so blood can pool and clot, especially in the left atrial appendage. A clot can travel to the brain, so anticoagulation reduces stroke risk.
Answer
B · To reduce the risk of stroke from clots in the atria
8.Clinical nursing
Answer: A · "I will walk barefoot at home to toughen my feet."
Why this is the answer
Diabetic neuropathy and poor circulation mean small injuries can become ulcers. Patients should avoid walking barefoot, inspect their feet daily and get professional foot care.
Answer
A · "I will walk barefoot at home to toughen my feet."