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 spironolactone asks about using a salt substitute. What should the nurse explain?
ASalt substitutes protect against the potassium loss caused by this drug
BMost salt substitutes contain potassium and can raise the level too high
CSalt substitutes are safe because they contain only sodium chloride
DSalt substitutes are fine as long as the patient drinks more water
2.Lab values & electrolytesHard
A patient with a stable INR on warfarin starts rifampin (rifampicin) for tuberculosis. Two weeks later the INR is 1.3. What is the most likely explanation?
ARifampin contains a lot of vitamin K
BRifampin speeds up warfarin metabolism
CTuberculosis causes the INR to fall
DRifampin raises the INR, so it is wrong
3.Arterial blood gas interpretationMedium
A practice gas shows a metabolic acidosis with HCO₃⁻ 8 mmol/L. Use Winter’s formula to find the expected PaCO₂ (the centre of the range) in mmHg.
HCO₃⁻ 8 mmol/L
PaCO₂ = mmHg
4.Arterial blood gas interpretationHard
A practice gas shows a metabolic alkalosis with HCO₃⁻ 38 mmol/L. Use expected PaCO₂ = 0.7·HCO₃⁻ + 21 to find the expected PaCO₂.
HCO₃⁻ 38 mmol/L
PaCO₂ = mmHg
5.Clinical pharmacokineticsMedium
Practice exercise (made-up values, not clinical guidance). A 61 kg patient is to be loaded with digoxin tablets to reach a level of 0.8 mcg/L. The volume of distribution is 7 L/kg and the tablets have a bioavailability F = 0.7. Find the total oral loading dose LD in mcg.
61 kgVd = 7 L/kgC = 0.8 mcg/LF = 0.7
LD = mcg
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A theophylline infusion is started without a loading dose. The half-life in this patient is 8 h. How long t does it take to reach 95% of the steady-state level?
t½ = 8 h95% Css
t = h
7.Clinical nursingMedium
A patient without diabetes is started on high-dose prednisolone. Why might capillary blood glucose be monitored?
ASteroids make glucose meters inaccurate
BSteroids always cause hypoglycaemia
CSteroids destroy the pancreas
DSteroids can raise the blood glucose
8.Clinical nursingHard
A nurse teaches a patient with hypertension about their medicines. Which statement shows a need for further teaching?
A"I'll have my blood pressure checked regularly."
B"I'll get up slowly from sitting."
C"I'll keep taking my tablets even if I feel well."
D"I'll stop my tablets once my readings are normal."
3Solutions
1.Lab values & electrolytes
Answer: B · Most salt substitutes contain potassium and can raise the level too high
Why this is the answer
Spironolactone blocks aldosterone, so the kidneys keep potassium. Many salt substitutes are made of potassium chloride, and combining them with spironolactone can cause dangerous hyperkalemia, especially with kidney disease or ACE inhibitors.Checked against:Spironolactone · MedlinePlus (NIH) (2018); High potassium level · MedlinePlus (NIH) (2025)
Answer
B · Most salt substitutes contain potassium and can raise the level too high
2.Lab values & electrolytes
Answer: B · Rifampin speeds up warfarin metabolism
Why this is the answer
Rifampin induces liver enzymes that break down warfarin, so warfarin's effect falls and the INR drops, increasing clot risk. The INR is monitored closely and the dose adjusted; the reverse happens when rifampin is stopped.
Answer
B · Rifampin speeds up warfarin metabolism
3.Arterial blood gas interpretation
Answer: PaCO₂ = 20 mmHg
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.
Winter’s formula
PaCO₂ = 1.5·8 + 8 = 20 mmHg
range: 18–22 mmHg
Answer
PaCO₂ = 20 mmHg
4.Arterial blood gas interpretation
Answer: PaCO₂ = 47.6 mmHg
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.
Expected PaCO₂
0.7·38 + 21 = 47.6 mmHg
Answer
PaCO₂ = 47.6 mmHg
The rise has a limit: PaCO₂ rarely goes above about 55 mmHg from compensation alone.
5.Clinical pharmacokinetics
Answer: LD = 488 mcg
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 = 7·61 = 427 L
Amount of active drug needed
Vd·C = 427·0.8 = 341.6 mcg
Loading dose
LD = 341.6/(1·0.7) = 488 mcg
Answer
LD = 488 mcg
In practice the dose is rounded to a convenient amount.
6.Clinical pharmacokinetics
Answer: t = 34.58 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 = 0.693/8 = 0.08664 h⁻¹
Time to the given fraction of steady state
t = −ln(1 − 0.95)/0.08664 = 2.996/0.08664 = 34.58 h
Answer
t = 34.58 h
That is 4.322 half-lives.
7.Clinical nursing
Answer: D · Steroids can raise the blood glucose
Why this is the answer
Glucocorticoids increase glucose production and insulin resistance, causing steroid-induced hyperglycaemia. Glucose monitoring identifies patients who need treatment.
Answer
D · Steroids can raise the blood glucose
8.Clinical nursing
Answer: D · "I'll stop my tablets once my readings are normal."
Why this is the answer
Hypertension usually needs long-term treatment; stopping because readings are normal often makes blood pressure rise again. Patients should discuss any changes with their prescriber.
Answer
D · "I'll stop my tablets once my readings are normal."