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 a heparin infusion has an aPTT below the target range on the protocol. What should the nurse do?
AStop the infusion and give protamine sulfate as quickly as possible
BContinue the same rate and recheck the aPTT in a few days
CAdjust the rate as the protocol directs and recheck as scheduled
DSwitch the infusion to oral warfarin without a prescription
2.Lab values & electrolytesHard
A patient with a heart murmur and fever has three sets of blood cultures, taken hours apart, all growing the same Streptococcus species. What does this suggest?
AA urinary tract infection that has spread to the skin
BPersistent bacteremia, as in infective endocarditis
CContamination of all three samples during collection
DA normal finding in patients with heart murmurs
3.Arterial blood gas interpretationMedium
A practice arterial blood gas shows pH 7.43, PaCO₂ 48 mmHg (6.4 kPa) and HCO₃⁻ 31 mmol/L. The primary disorder is metabolic alkalosis. How far is it compensated? Click your answer.
pH 7.43PaCO₂ 48 mmHg (6.4 kPa)HCO₃⁻ 31 mmol/L
1uncompensated
2partially compensated
3fully compensated
4.Arterial blood gas interpretationHard
A practice gas shows a respiratory acidosis: pH 7.30, PaCO₂ 80 mmHg (10.7 kPa), HCO₃⁻ 38 mmol/L. Use the rules ‘HCO₃⁻ rises 1 mmol/L per 10 mmHg (acute) or 3.5 mmol/L per 10 mmHg (chronic)’ to classify it. Click your answer.
pH 7.30PaCO₂ 80 mmHg (10.7 kPa)HCO₃⁻ 38 mmol/L
1acute
2chronic
3acute on chronic
5.Clinical pharmacokineticsMedium
Practice exercise (made-up values, not clinical guidance). An 89 kg patient is to receive an aminophylline infusion (salt factor S = 0.8: 80% of aminophylline is theophylline). The theophylline clearance is 0.04 L/kg/h. What aminophylline infusion rate R in mg/h gives a steady-state theophylline level of 10 mg/L?
89 kgCL = 0.04 L/kg/hCss = 10 mg/LS = 0.8
R = mg/h
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A 60-year-old woman weighs 89 kg and has a serum creatinine of 2.7 mg/dL (239 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 vancomycin regimen is 1000 mg every 12 h for a creatinine clearance of 100 mL/min. Keeping the dose, the interval is lengthened in proportion to the fall in clearance. Find the new dosing interval τ. Round to one decimal place.
Which pattern is typical of relapsing–remitting multiple sclerosis?
AA single stroke that leaves permanent weakness
BConstant weakness that starts in the feet and spreads upwards after a viral illness
CSudden severe headache with neck stiffness
DAttacks of symptoms, such as visual loss or numbness, that improve between relapses
8.Clinical nursingHard
Why is morphine more likely to cause toxicity in a patient with severe kidney impairment?
AKidney failure makes the stomach absorb more morphine
BThe kidneys normally activate morphine, so more of it becomes active
CIts active metabolites build up because the kidneys excrete them
DKidney disease makes the opioid receptors disappear
3Solutions
1.Lab values & electrolytes
Answer: C · Adjust the rate as the protocol directs and recheck as scheduled
Why this is the answer
A subtherapeutic aPTT means the patient is not fully anticoagulated and is at risk of clot extension. Weight-based heparin protocols specify a bolus and/or rate increase and when to recheck. The nurse follows the protocol and documents the change.Checked against:Partial Thromboplastin Time (PTT) Test · MedlinePlus (NIH) (2026); Partial thromboplastin time (PTT) - Medical Encyclopedia · MedlinePlus (NIH) (2025)
Answer
C · Adjust the rate as the protocol directs and recheck as scheduled
2.Lab values & electrolytes
Answer: B · Persistent bacteremia, as in infective endocarditis
Why this is the answer
Several positive cultures with the same organism, taken at different times, show continuous bacteremia, typical of infective endocarditis. Echocardiography is done and prolonged IV antibiotics are started. Contamination usually affects only one bottle.Checked against:Blood culture · MedlinePlus (NIH) (2025)
Answer
B · Persistent bacteremia, as in infective endocarditis
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 7.43: within 7.35–7.45
PaCO₂ 48 mmHg: high (35–45)
HCO₃⁻ 31 mmol/L: high (22–26)
Compensation
Clinically, also check the expected compensation: for a metabolic alkalosis PaCO₂ ≈ 0.7·HCO₃⁻ + 21 = 40.7–44.7 mmHg, so the measured 48 mmHg points to an additional respiratory acidosis (a mixed disorder). The answer above is the ROME exam classification.
fully compensated
Answer
fully compensated
4.Arterial blood gas interpretation
Answer: chronic
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 HCO₃⁻
acute: 24 + 1·4 = 28 mmol/L
chronic: 24 + 3.5·4 = 38 mmol/L
Compare
measured: 38 mmol/L → chronic
Answer
chronic
5.Clinical pharmacokinetics
Answer: R = 44.5 mg/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.
Clearance
CL = 0.04·89 = 3.56 L/h
Infusion rate
R = CL·Css/S = 3.56·10/0.8 = 44.5 mg/h
Answer
R = 44.5 mg/h
6.Clinical pharmacokinetics
Answer: τ = 38.5 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.
Creatinine clearance (Cockcroft–Gault)
CrCl = (140 − 60)·89/(72·2.7)·0.85 = 31.13 mL/min
New interval
τ = 12·100/31.13 = 38.55 ≈ 38.5 h
Answer
τ = 38.5 h
In practice the interval is rounded to a convenient value such as 12, 24, 36 or 48 h.
7.Clinical nursing
Answer: D · Attacks of symptoms, such as visual loss or numbness, that improve between relapses
Why this is the answer
MS causes demyelination in the brain and spinal cord. Many patients first have attacks such as optic neuritis or sensory symptoms, with partial or full recovery between relapses.
Answer
D · Attacks of symptoms, such as visual loss or numbness, that improve between relapses
8.Clinical nursing
Answer: C · Its active metabolites build up because the kidneys excrete them
Why this is the answer
Morphine metabolites, including morphine-6-glucuronide, are cleared by the kidneys and accumulate in renal impairment, causing sedation and respiratory depression. Alternative opioids or lower doses are often chosen.
Answer
C · Its active metabolites build up because the kidneys excrete them