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 taking furosemide has a potassium of 2.9 mEq/L. Which finding would the nurse expect?
ATingling around the mouth with a positive Chvostek’s sign
BMuscle weakness and leg cramps
CTall, peaked T waves on the ECG
DDiarrhea and abdominal cramping from overactive bowels
2.Lab values & electrolytesHard
A patient with no kidney problems and a normal ECG has a potassium of 6.2 mEq/L. The laboratory comments that the sample was hemolyzed. What is the best interpretation?
AThe result may be falsely high; assess the patient and arrange a repeat sample
BThe potassium is certainly normal and needs no follow-up
CThe patient has kidney failure
DTreat immediately with insulin and glucose without further checks
3.Arterial blood gas interpretationMedium
A practice gas shows PaCO₂ 61 mmHg (8.1 kPa). Use the rule that in chronic respiratory acidosis HCO₃⁻ rises by 3.5 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⁺ 141, Cl⁻ 109 and HCO₃⁻ 11 mmol/L. Calculate the delta ratio (AG − 12)/(24 − HCO₃⁻), using AG = Na⁺ − (Cl⁻ + HCO₃⁻). Give two decimals.
Na⁺ 141 mmol/LCl⁻ 109 mmol/LHCO₃⁻ 11 mmol/L
delta ratio =
5.Clinical pharmacokineticsMedium
Practice exercise (made-up values, not clinical guidance). A patient receives an IV bolus of 170 mg of tobramycin. The level back-extrapolated to time zero is 10 mg/L and the half-life is 2.5 h. Find the volume of distribution Vd.
D = 170 mgC₀ = 10 mg/Lt½ = 2.5 h
Vd = L
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A 61-year-old man weighs 73 kg and has a serum creatinine of 1.3 mg/dL (115 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 ciprofloxacin dose is 500 mg for a creatinine clearance of 100 mL/min, and a fraction fe = 0.6 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.
Which factor is most likely to slow wound healing?
ASmoking
BAdequate fluid intake
CA diet rich in protein
DGood blood glucose control
8.Clinical nursingHard
A patient starting a DOAC is taught about missed doses. Which statement shows a need for further teaching?
A"I will tell other doctors and my dentist I take an anticoagulant."
B"I will report any blood in my urine."
C"If I forget a dose, I'll take two the next day to catch up."
D"I will take it at the same time each day."
3Solutions
1.Lab values & electrolytes
Answer: B · Muscle weakness and leg cramps
Why this is the answer
Low potassium causes muscle weakness, cramps, reduced bowel movement (constipation, even ileus) and arrhythmias. Peaked T waves are seen in hyperkalemia, and perioral tingling suggests low calcium.Checked against:Heart failure - fluids and diuretics · MedlinePlus (NIH); Low blood potassium · MedlinePlus (NIH) (2025)
Answer
B · Muscle weakness and leg cramps
2.Lab values & electrolytes
Answer: A · The result may be falsely high; assess the patient and arrange a repeat sample
Why this is the answer
Red cells are rich in potassium. When they break in the tube (from a tight tourniquet, fist clenching or a small needle), potassium leaks out and the result is falsely high. The nurse still assesses the patient and repeats the test, because true hyperkalemia must not be missed.Checked against:Potassium Blood Test · MedlinePlus (NIH) (2024)
Answer
A · The result may be falsely high; assess the patient and arrange a repeat sample
3.Arterial blood gas interpretation
Answer: HCO₃⁻ = 31.35 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
|61 − 40| = 21 mmHg → 2.1 × 10 mmHg
Expected HCO₃⁻
HCO₃⁻ = 24 + 3.5·2.1 = 31.35 mmol/L
Answer
HCO₃⁻ = 31.35 mmol/L
4.Arterial blood gas interpretation
Answer: delta ratio = 0.6923
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 = 141 − (109 + 11) = 21
Delta ratio
(21 − 12)/(24 − 11) = 9/13 = 0.6923
Answer
delta ratio = 0.6923
Below 1: an added normal anion gap acidosis.
5.Clinical pharmacokinetics
Answer: Vd = 17 L
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 = 170/10 = 17 L
Answer
Vd = 17 L
6.Clinical pharmacokinetics
Answer: D = 385 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 − 61)·73/(72·1.3) = 61.61 mL/min
Dose fraction (Dettli)
Q = 1 − 0.6·(1 − 61.61/100) = 0.7697
Adjusted dose
D = 0.7697·500 = 384.8 ≈ 385 mg
Answer
D = 385 mg
In practice the dose is rounded to a convenient amount and checked against the product information.
7.Clinical nursing
Answer: A · Smoking
Why this is the answer
Smoking causes vasoconstriction and reduces tissue oxygen, slowing healing and increasing infection risk. Poor nutrition, diabetes, infection and steroids also impair healing.
Answer
A · Smoking
8.Clinical nursing
Answer: C · "If I forget a dose, I'll take two the next day to catch up."
Why this is the answer
Doubling up on an anticoagulant increases the bleeding risk. Patients follow the specific missed-dose advice for their drug and never take a double dose.
Answer
C · "If I forget a dose, I'll take two the next day to catch up."