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 receives subcutaneous enoxaparin (a low-molecular-weight heparin). Which statement about monitoring is correct?
AThe INR is used to adjust each dose, as with warfarin
BNo blood tests of any kind are ever needed
CNo routine aPTT; platelets and kidney function are checked instead
DThe aPTT must be checked every 6 hours to adjust the dose
2.Lab values & electrolytesHard
The nurse teaches a patient with chronic kidney disease about a potassium-restricted diet. Which statement shows a need for further teaching?
A"I'll check food labels and ask my dietitian about portion sizes"
B"I can leach potatoes by soaking them before cooking them"
C"Dried apricots and raisins will be a good snack for me now"
D"I'll limit bananas, oranges and potatoes in my meals"
3.Arterial blood gas interpretationMedium
A practice arterial blood gas shows pH 7.52, PaCO₂ 51 mmHg (6.8 kPa) and HCO₃⁻ 40 mmol/L. The primary disorder is metabolic alkalosis. How far is it compensated? Click your answer.
pH 7.52PaCO₂ 51 mmHg (6.8 kPa)HCO₃⁻ 40 mmol/L
1uncompensated
2partially compensated
3fully compensated
4.Arterial blood gas interpretationHard
Practice values: Na⁺ 142, Cl⁻ 94 and HCO₃⁻ 9 mmol/L. Calculate the delta ratio (AG − 12)/(24 − HCO₃⁻), using AG = Na⁺ − (Cl⁻ + HCO₃⁻). Give two decimals.
Na⁺ 142 mmol/LCl⁻ 94 mmol/LHCO₃⁻ 9 mmol/L
delta ratio =
5.Clinical pharmacokineticsMedium
Practice exercise (made-up values, not clinical guidance). A patient receives an IV bolus of 120 mg of gentamicin. The level back-extrapolated to time zero is 8 mg/L and the half-life is 3.5 h. Find the clearance CL in L/h.
D = 120 mgC₀ = 8 mg/Lt½ = 3.5 h
CL = L/h
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A 70 kg patient has a theophylline level of 6 mg/L; the target is 10 mg/L. The drug is given intravenously as aminophylline (S = 0.8), and the volume of distribution is 0.5 L/kg. What extra loading dose LD raises the level to the target?
70 kgVd = 0.5 L/kgC = 6 → 10 mg/LS = 0.8
LD = mg
7.Clinical nursingMedium
Modern wound care generally aims to keep a healing wound bed:
AMoist but not wet
BSoaked in antiseptic solution every day
CCompletely dry and open to the air
DTightly packed with dry gauze
8.Clinical nursingHard
A patient on metformin with reduced kidney function is due a CT scan with IV contrast. Why might metformin be withheld?
AThere is a risk of lactic acidosis if kidney function worsens
BMetformin reacts with the contrast to cause severe hyperglycaemia
CMetformin causes severe bleeding during scans
DMetformin makes the images blurry
3Solutions
1.Lab values & electrolytes
Answer: C · No routine aPTT; platelets and kidney function are checked instead
Why this is the answer
Enoxaparin has a predictable effect, so the aPTT is not used to monitor it. Platelet counts help detect heparin-induced thrombocytopenia, and kidney function matters because the drug is cleared by the kidneys. Anti-Xa levels may be used in pregnancy, obesity or kidney disease.
Answer
C · No routine aPTT; platelets and kidney function are checked instead
2.Lab values & electrolytes
Answer: C · "Dried apricots and raisins will be a good snack for me now"
Why this is the answer
Dried fruits such as apricots and raisins are very high in potassium and should be limited in kidney disease with hyperkalemia. Limiting high-potassium foods, leaching vegetables and reading labels are correct strategies.
Answer
C · "Dried apricots and raisins will be a good snack for me now"
3.Arterial blood gas interpretation
Answer: partially 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 7.52: alkalaemia (above 7.45)
PaCO₂ 51 mmHg: high (35–45)
HCO₃⁻ 40 mmol/L: high (22–26)
Compensation
partially compensated
Answer
partially compensated
4.Arterial blood gas interpretation
Answer: delta ratio = 1.8
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 = 142 − (94 + 9) = 39
Delta ratio
(39 − 12)/(24 − 9) = 27/15 = 1.8
Answer
delta ratio = 1.8
Between 1 and 2: a pure high anion gap acidosis.
5.Clinical pharmacokinetics
Answer: CL = 2.971 L/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.
Volume of distribution
Vd = 120/8 = 15 L
Rate constant
k = 0.693/3.5 = 0.198 h⁻¹
Clearance
CL = 0.198·15 = 2.971 L/h
Answer
CL = 2.971 L/h
6.Clinical pharmacokinetics
Answer: LD = 175 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.
Volume of distribution
Vd = 0.5·70 = 35 L
Rise needed
10 − 6 = 4 mg/L
Loading dose
LD = 35·4/(0.8·1) = 175 mg
Answer
LD = 175 mg
In practice the dose is rounded to a convenient amount.
7.Clinical nursing
Answer: A · Moist but not wet
Why this is the answer
A moist wound environment speeds epithelial cell migration and healing. Too much moisture causes maceration, so dressings are chosen to manage exudate.
Answer
A · Moist but not wet
8.Clinical nursing
Answer: A · There is a risk of lactic acidosis if kidney function worsens
Why this is the answer
Iodinated contrast can worsen kidney function, and metformin can then accumulate and cause lactic acidosis. Local policy guides when to stop and restart it.
Answer
A · There is a risk of lactic acidosis if kidney function worsens