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 on phenytoin has a level of 28 mcg/mL (usual range about 10–20 mcg/mL). Which findings would the nurse expect?
ATall, peaked T waves
BNystagmus, unsteady gait and slurred speech
CHyperactive behavior and insomnia only
DHunger and weight gain
2.Lab values & electrolytesHard
A patient has fever, confusion, a platelet count of 15 × 10³/µL, anemia with fragmented red cells (schistocytes) and a raised creatinine. Clotting times are normal. Which condition should be suspected?
AVitamin K deficiency with bleeding
BDisseminated intravascular coagulation
CImmune thrombocytopenia (ITP)
DThrombotic thrombocytopenic purpura (TTP)
3.Arterial blood gas interpretationMedium
A practice gas shows a metabolic acidosis with HCO₃⁻ 18 mmol/L. Use Winter’s formula to find the expected PaCO₂ (the centre of the range) in mmHg.
HCO₃⁻ 18 mmol/L
PaCO₂ = mmHg
4.Arterial blood gas interpretationHard
A practice gas shows a metabolic alkalosis with HCO₃⁻ 35 mmol/L. Use expected PaCO₂ = 0.7·HCO₃⁻ + 21 to find the expected PaCO₂.
HCO₃⁻ 35 mmol/L
PaCO₂ = mmHg
5.Clinical pharmacokineticsMedium
Practice exercise (made-up values, not clinical guidance). After an IV dose of vancomycin, the level is 40 mg/L at 2 h and 14.9 mg/L at 12 h. Assume first-order elimination. Find the half-life t½.
C1 = 40 mg/L (2 h)C2 = 14.9 mg/L (12 h)
t½ = h
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A 54 kg patient has a phenytoin level of 4.5 mg/L; the target is 15 mg/L. The drug is given intravenously as phenytoin sodium (S = 0.92), and the volume of distribution is 0.7 L/kg. What extra loading dose LD raises the level to the target?
54 kgVd = 0.7 L/kgC = 4.5 → 15 mg/LS = 0.92
LD = mg
7.Clinical nursingMedium
Which measure helps reduce the risk of surgical site infection?
AChanging the wound dressing every hour
BShaving the area with a razor the night before surgery
CLeaving the patient cool during surgery to slow bacterial growth
DRemoving hair, if needed, with clippers rather than a razor
8.Clinical nursingHard
A patient with acute pancreatitis is in severe epigastric pain radiating to the back. Which nursing care is most important?
AStrict bed rest without observations
BAdequate pain relief, IV fluids and close fluid balance monitoring
CRestricting all fluids
DA large fatty meal to stimulate the pancreas
3Solutions
1.Lab values & electrolytes
Answer: B · Nystagmus, unsteady gait and slurred speech
Why this is the answer
Phenytoin toxicity causes nystagmus, ataxia, slurred speech and drowsiness. The nurse holds the dose and informs the prescriber. Low albumin increases the free (active) drug, so toxicity can occur even with a normal total level.
Answer
B · Nystagmus, unsteady gait and slurred speech
2.Lab values & electrolytes
Answer: D · Thrombotic thrombocytopenic purpura (TTP)
Why this is the answer
TTP causes small clots that consume platelets, break red cells (microangiopathic hemolysis) and damage the brain and kidneys, while clotting times usually stay normal. It is an emergency treated with plasma exchange; platelet transfusion is generally avoided.Checked against:Thrombotic thrombocytopenic purpura · MedlinePlus (NIH) (2025)
Answer
D · Thrombotic thrombocytopenic purpura (TTP)
3.Arterial blood gas interpretation
Answer: PaCO₂ = 35 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 = 1.5·18 + 8 = 35 mmHg
range: 33–37 mmHg
Answer
PaCO₂ = 35 mmHg
4.Arterial blood gas interpretation
Answer: PaCO₂ = 45.5 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
0.7·35 + 21 = 45.5 mmHg
Answer
PaCO₂ = 45.5 mmHg
The rise has a limit: PaCO₂ rarely goes above about 55 mmHg from compensation alone.
5.Clinical pharmacokinetics
Answer: t½ = 7.019 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.
Elimination rate constant
k = ln(40/14.9)/10 = 0.9875/10 = 0.09875 h⁻¹
Half-life
t½ = 0.693/0.09875 = 7.019 h
Answer
t½ = 7.019 h
6.Clinical pharmacokinetics
Answer: LD = 431.4 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 = 0.7·54 = 37.8 L
Rise needed
15 − 4.5 = 10.5 mg/L
Loading dose
LD = 37.8·10.5/(0.92·1) = 431.4 mg
Answer
LD = 431.4 mg
In practice the dose is rounded to a convenient amount.
7.Clinical nursing
Answer: D · Removing hair, if needed, with clippers rather than a razor
Why this is the answer
Razors cause micro-abrasions that increase infection risk, so hair is removed only if necessary with clippers just before surgery. Keeping patients warm and good skin preparation also help.
Answer
D · Removing hair, if needed, with clippers rather than a razor
8.Clinical nursing
Answer: B · Adequate pain relief, IV fluids and close fluid balance monitoring
Why this is the answer
Acute pancreatitis can cause large fluid losses and organ failure. Care focuses on fluid resuscitation, analgesia, monitoring urine output, glucose and early nutrition when possible.
Answer
B · Adequate pain relief, IV fluids and close fluid balance monitoring