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
On the first day after major surgery, a patient's white cell count is 12.5 × 10³/µL, with no fever or wound signs. What is the best interpretation?
AA common stress response to surgery that is monitored
BA laboratory error, since surgery lowers white cells
CDefinite wound infection that needs antibiotics now
DA sign of leukemia that needs bone marrow testing
2.Lab values & electrolytesHard
A patient on digoxin has a calcium of 12.1 mg/dL (3.0 mmol/L) and new nausea and a slow pulse. Why is the calcium result important?
AHigh calcium makes the kidneys clear digoxin faster
BHigh calcium increases the risk of digoxin toxicity
CHigh calcium blocks digoxin, so the drug stops working
DHigh calcium only matters if the potassium is also high
3.Arterial blood gas interpretationMedium
A practice arterial blood gas shows pH 7.37, PaCO₂ 72 mmHg (9.6 kPa) and HCO₃⁻ 40 mmol/L. The primary disorder is respiratory acidosis. How far is it compensated? Click your answer.
pH 7.37PaCO₂ 72 mmHg (9.6 kPa)HCO₃⁻ 40 mmol/L
1uncompensated
2partially compensated
3fully compensated
4.Arterial blood gas interpretationHard
A practice gas shows a metabolic acidosis: pH 7.20, PaCO₂ 42 mmHg (5.6 kPa), HCO₃⁻ 16 mmol/L. Compare the measured PaCO₂ with Winter’s formula. Click your interpretation.
pH 7.20PaCO₂ 42 mmHg (5.6 kPa)HCO₃⁻ 16 mmol/L
1appropriate compensation
2added respiratory acidosis
3added respiratory alkalosis
5.Clinical pharmacokineticsMedium
Practice exercise (made-up values, not clinical guidance). A patient has a lidocaine clearance of 900 mL/min. What infusion rate R in mg/min keeps the steady-state level at 4 mg/L?
CL = 900 mL/minCss = 4 mg/L
R = mg/min
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A 67 kg patient on regular tobramycin doses is at steady state. The target is a peak of 10 mg/L and a trough of 0.5 mg/L, and the volume of distribution is 0.3 L/kg. Assuming rapid IV doses, what dose D raises the level from the trough to the peak?
Which measure helps reduce the risk of surgical site infection?
ARemoving hair, if needed, with clippers rather than a razor
BShaving the area with a razor the night before surgery
CChanging the wound dressing every hour
DLeaving the patient cool during surgery to slow bacterial growth
8.Clinical nursingHard
A postoperative patient on IV fluids has a sodium of 128 mmol/L and is becoming confused. Which fluid could make this worse if given in large amounts?
AHartmann's solution (similar to lactated Ringer's)
B5% glucose (D5W)
C0.9% sodium chloride
DHuman albumin solution
3Solutions
1.Lab values & electrolytes
Answer: A · A common stress response to surgery that is monitored
Why this is the answer
A mild leukocytosis is common after surgery because of tissue injury and the stress response. It is monitored along with temperature and wound assessment. A rising count after several days, or fever, suggests infection.Checked against:WBC count · MedlinePlus (NIH) (2025)
Answer
A · A common stress response to surgery that is monitored
2.Lab values & electrolytes
Answer: B · High calcium increases the risk of digoxin toxicity
Why this is the answer
Hypercalcemia, like hypokalemia and hypomagnesemia, makes the heart more sensitive to digoxin and raises the risk of toxic arrhythmias. Nausea and bradycardia suggest toxicity, so the nurse holds the dose and reports the findings.
Answer
B · High calcium increases the risk of digoxin toxicity
3.Arterial blood gas interpretation
Answer: fully 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 7.37: within 7.35–7.45
PaCO₂ 72 mmHg: high (35–45)
HCO₃⁻ 40 mmol/L: high (22–26)
Compensation
fully compensated
Answer
fully compensated
4.Arterial blood gas interpretation
Answer: added respiratory acidosis
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
1.5·16 + 8 = 32 → 30–34 mmHg
Compare
measured: 42 mmHg → added respiratory acidosis
Answer
added respiratory acidosis
5.Clinical pharmacokinetics
Answer: R = 3.6 mg/min
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.
Clearance in L/min
900 mL/min = 0.9 L/min
Infusion rate
R = 0.9·4 = 3.6 mg/min
Answer
R = 3.6 mg/min
6.Clinical pharmacokinetics
Answer: D = 190.9 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 = 0.3·67 = 20.1 L
Dose
D = 20.1·(10 − 0.5) = 190.9 mg
Answer
D = 190.9 mg
In practice the dose is rounded to a convenient amount.
7.Clinical nursing
Answer: A · 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
A · Removing hair, if needed, with clippers rather than a razor
8.Clinical nursing
Answer: B · 5% glucose (D5W)
Why this is the answer
The glucose in 5% glucose is metabolised, leaving free water that dilutes the plasma sodium further. Postoperative patients often have raised ADH, so large volumes of hypotonic fluid can cause dangerous hyponatraemia.