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 had a right mastectomy with removal of axillary lymph nodes. Where is the best place to draw blood?
AThe right arm
BThe left arm
CThe right hand
DEither arm equally
2.Lab values & electrolytesHard
A patient on lithium has had vomiting and diarrhea for 3 days and has been taking ibuprofen. They now have a coarse tremor and confusion. The lithium level is 2.2 mEq/L. What is the best interpretation and action?
AA normal level; continue lithium as prescribed
BLithium toxicity; hold lithium and report urgently
CIbuprofen side effects only; give it with food
DA low level; ask for the lithium dose to be raised
3.Arterial blood gas interpretationMedium
Practice values: Na⁺ 139 mmol/L, glucose 31.2 mmol/L and urea 5.7 mmol/L. Calculate the serum osmolality Osm = 2·Na⁺ + glucose + urea to the nearest whole number.
Na⁺ 139 mmol/Lglucose 31.2 mmol/Lurea 5.7 mmol/L
Osm = mOsm/kg
4.Arterial blood gas interpretationHard
A practice gas shows a respiratory acidosis: pH 7.27, PaCO₂ 67 mmHg (8.9 kPa), HCO₃⁻ 30 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 7.27PaCO₂ 67 mmHg (8.9 kPa)HCO₃⁻ 30 mmol/L
1acute
2chronic
3acute on chronic
5.Clinical pharmacokineticsMedium
Practice exercise (made-up values, not clinical guidance). A 63 kg patient receives a theophylline infusion. The clearance is 0.03 L/kg/h. What infusion rate R in mg/h gives a steady-state level of 10 mg/L?
63 kgCL = 0.03 L/kg/hCss = 10 mg/L
R = mg/h
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A 59-year-old man weighs 57 kg and has a serum creatinine of 3.2 mg/dL (283 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 tobramycin regimen is 120 mg every 8 h for a creatinine clearance of 120 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.
Why are patients advised to rinse their mouth after using an inhaled corticosteroid?
ATo stop the drug from being absorbed in the lungs
BTo reduce the risk of oral thrush
CTo make the drug work faster
DTo prevent tooth decay from the sugar in it
8.Clinical nursingHard
Which task is most appropriate for a registered nurse to delegate to a trained healthcare assistant (unlicensed assistive personnel)?
AAssessing a patient who has just reported new chest pain
BTeaching a patient to inject insulin for the first time
CGiving a dose of intravenous antibiotic
DMeasuring and recording vital signs on a stable patient
3Solutions
1.Lab values & electrolytes
Answer: B · The left arm
Why this is the answer
After axillary node removal, venipuncture and blood pressure readings are usually avoided on that side to reduce the risk of lymphedema and infection. The opposite arm is used whenever possible, according to policy.
Answer
B · The left arm
2.Lab values & electrolytes
Answer: B · Lithium toxicity; hold lithium and report urgently
Why this is the answer
A level of 2.2 mEq/L with coarse tremor and confusion is lithium toxicity. Fluid loss and NSAIDs both reduce lithium excretion by the kidneys. The dose is held, the provider is informed at once, and fluids and close monitoring are expected; severe cases may need dialysis.
Answer
B · Lithium toxicity; hold lithium and report urgently
3.Arterial blood gas interpretation
Answer: Osm = 315 mOsm/kg
Derived values: calculated serum osmolality is 2·Na⁺ + glucose + urea with all values in mmol/L (2·Na⁺ + glucose/18 + BUN/2.8 with glucose and BUN in mg/dL); normal is about 275–295 mOsm/kg. The P/F ratio is PaO₂ divided by FiO₂ as a fraction. The alveolar oxygen pressure is PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8 mmHg at sea level, and the A–a gradient is PAO₂ − PaO₂.
Calculated osmolality
Osm = 2·139 + 31.2 + 5.7 = 314.9 ≈ 315 mOsm/kg
Answer
Osm = 315 mOsm/kg
4.Arterial blood gas interpretation
Answer: acute on 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 + 1·2.7 = 26.7 mmol/L
chronic: 24 + 3.5·2.7 = 33.45 mmol/L
Compare
measured: 30 mmol/L → acute on chronic
Answer
acute on chronic
5.Clinical pharmacokinetics
Answer: R = 18.9 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 = 0.03·63 = 1.89 L/h
Infusion rate
R = 1.89·10 = 18.9 mg/h
Answer
R = 18.9 mg/h
6.Clinical pharmacokinetics
Answer: τ = 47.9 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 − 59)·57/(72·3.2) = 20.04 mL/min
New interval
τ = 8·120/20.04 = 47.91 ≈ 47.9 h
Answer
τ = 47.9 h
In practice the interval is rounded to a convenient value such as 12, 24, 36 or 48 h.
7.Clinical nursing
Answer: B · To reduce the risk of oral thrush
Why this is the answer
Inhaled steroid deposited in the mouth and throat can cause oral candidiasis and a hoarse voice. Rinsing and using a spacer reduce this.
Answer
B · To reduce the risk of oral thrush
8.Clinical nursing
Answer: D · Measuring and recording vital signs on a stable patient
Why this is the answer
Routine observations on a stable patient are within the role of a trained assistant, with the nurse remaining accountable and reviewing the results. Assessment of new symptoms, IV medicines and initial teaching need registered nurse skills.
Answer
D · Measuring and recording vital signs on a stable patient