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 with sepsis has a lactate of 2.1 mmol/L on admission and 4.3 mmol/L two hours later, despite IV fluids. What does this suggest?
AWorsening tissue perfusion that needs urgent reporting
BImproving circulation as the fluids start to work
CA normal response to IV fluids in the first hours
DExpected muscle activity from shivering in fever
2.Lab values & electrolytesHard
A patient who had pituitary surgery has fatigue and cold intolerance. The free T4 is low and the TSH is low. What does this pattern suggest?
AHyperthyroidism from overactivity of the thyroid gland
BSecondary (central) hypothyroidism from pituitary failure
CPrimary hypothyroidism from failure of the thyroid gland
DNormal thyroid function with a laboratory error in T4
3.Arterial blood gas interpretationMedium
Practice values: Na⁺ 129 mmol/L, glucose 4.1 mmol/L and urea 6.5 mmol/L. Calculate the serum osmolality Osm = 2·Na⁺ + glucose + urea to the nearest whole number.
Na⁺ 129 mmol/Lglucose 4.1 mmol/Lurea 6.5 mmol/L
Osm = mOsm/kg
4.Arterial blood gas interpretationHard
A practice gas shows a respiratory acidosis: pH 7.32, PaCO₂ 64 mmHg (8.5 kPa), HCO₃⁻ 32 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 7.32PaCO₂ 64 mmHg (8.5 kPa)HCO₃⁻ 32 mmol/L
1acute
2chronic
3acute on chronic
5.Clinical pharmacokineticsMedium
Practice exercise (made-up values, not clinical guidance). A patient receives an IV bolus of 160 mg of gentamicin. The level back-extrapolated to time zero is 10 mg/L and the half-life is 3 h. Find the clearance CL in L/h.
D = 160 mgC₀ = 10 mg/Lt½ = 3 h
CL = L/h
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A patient's gentamicin half-life is 4 h. The target is a peak of 7 mg/L and a trough of 1 mg/L. Assuming rapid IV doses and first-order elimination, what dosing interval τ lets the level fall from the peak to the trough? Round to one decimal place.
t½ = 4 hCpeak = 7 mg/LCtrough = 1 mg/L
τ = h
7.Clinical nursingMedium
Under the Mental Capacity Act 2005 (England and Wales), which principle applies?
AA diagnosis of dementia means the person lacks capacity for all decisions
BEveryone over 75 is assumed to lack capacity
CCapacity is decided by the next of kin, who must sign a form for every decision
DA person must be assumed to have capacity unless it is shown that they lack it
8.Clinical nursingHard
A patient due a blood transfusion is not wearing an identity band. What should the nurse do?
AAsk the patient to state their name and date of birth, then start straight away
BNot start until a correct identity band is in place and identity is confirmed
CCheck the name on the bed and proceed
DAsk another patient to confirm who the patient is
3Solutions
1.Lab values & electrolytes
Answer: A · Worsening tissue perfusion that needs urgent reporting
Why this is the answer
A rising lactate despite fluids suggests ongoing tissue hypoperfusion and worsening sepsis or septic shock. The nurse reports it at once; further fluids, vasopressors and source control may be needed, and lactate is remeasured.
Answer
A · Worsening tissue perfusion that needs urgent reporting
2.Lab values & electrolytes
Answer: B · Secondary (central) hypothyroidism from pituitary failure
Why this is the answer
In primary hypothyroidism, a low T4 causes a high TSH. A low T4 with a low or inappropriately normal TSH means the pituitary (or hypothalamus) is not producing enough TSH, so the TSH cannot be used alone to monitor treatment.
Answer
B · Secondary (central) hypothyroidism from pituitary failure
3.Arterial blood gas interpretation
Answer: Osm = 269 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 = 2·129 + 4.1 + 6.5 = 268.6 ≈ 269 mOsm/kg
Answer
Osm = 269 mOsm/kg
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 + 1·2.4 = 26.4 mmol/L
chronic: 24 + 3.5·2.4 = 32.4 mmol/L
Compare
measured: 32 mmol/L → chronic
Answer
chronic
5.Clinical pharmacokinetics
Answer: CL = 3.697 L/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.
Volume of distribution
Vd = 160/10 = 16 L
Rate constant
k = 0.693/3 = 0.231 h⁻¹
Clearance
CL = 0.231·16 = 3.697 L/h
Answer
CL = 3.697 L/h
6.Clinical pharmacokinetics
Answer: τ = 11.2 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 = 0.693/4 = 0.1733 h⁻¹
Dosing interval
τ = ln(7/1)/0.1733 = 1.946/0.1733 = 11.23 ≈ 11.2 h
Answer
τ = 11.2 h
In practice the interval is rounded to a convenient value such as 8, 12, 24 or 36 h, and the levels are checked again.
7.Clinical nursing
Answer: D · A person must be assumed to have capacity unless it is shown that they lack it
Why this is the answer
The Act begins by presuming capacity. Capacity is decision-specific and time-specific, and people must be given all practicable help to make their own decision.
Answer
D · A person must be assumed to have capacity unless it is shown that they lack it
8.Clinical nursing
Answer: B · Not start until a correct identity band is in place and identity is confirmed
Why this is the answer
Positive patient identification, using an identity band checked against the blood unit and prescription, is essential before every transfusion. Wrong blood incidents can cause fatal ABO incompatibility reactions.
Answer
B · Not start until a correct identity band is in place and identity is confirmed