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 trauma patient receives 2 L of IV crystalloid. The hemoglobin falls from 13.0 to 11.4 g/dL (130 to 114 g/L), and the vital signs are now stable. Which explanation is most likely?
AA sudden failure of the bone marrow
BA laboratory error in the first sample
CIncreased red cell production
DDilution of the blood by the IV fluid
2.Lab values & electrolytesHard
A patient with heart failure has a BNP that was very high on admission and is now falling after diuretic treatment. What does this most likely show?
ALess stretch on the heart, consistent with improvement
BThat the diuretic must be stopped at once
CWorsening heart failure that needs more fluids
DNew kidney failure caused by the diuretic
3.Arterial blood gas interpretationMedium
A practice gas shows PaCO₂ 23 mmHg (3.1 kPa). Use the rule that in chronic respiratory alkalosis HCO₃⁻ falls by 5 mmol/L for every 10 mmHg that PaCO₂ is below 40, starting from HCO₃⁻ 24 mmol/L, to find the expected HCO₃⁻.
Practice exercise (made-up values, not clinical guidance). A 91 kg patient is to be loaded intravenously with phenytoin sodium to reach a phenytoin level of 15 mg/L. The volume of distribution is 0.7 L/kg and the salt factor is S = 0.92. Find the loading dose LD of phenytoin sodium in mg.
91 kgVd = 0.7 L/kgC = 15 mg/LS = 0.92
LD = mg
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A 60 kg patient on regular gentamicin doses is at steady state. The target is a peak of 10 mg/L and a trough of 1 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?
60 kgVd = 0.3 L/kgCpeak = 10 mg/LCtrough = 1 mg/L
D = mg
7.Clinical nursingMedium
Which signs suggest a blood transfusion reaction in the first minutes?
ASlow, steady rise in haemoglobin over the next day
BFever, chills, back pain and low blood pressure
CImproved colour and energy
DMild hunger
8.Clinical nursingHard
A nurse is unsure whether a prescribed dose of a high-risk drug is correct. What is the safest action?
AAsk the patient whether the dose looks the same as usual and give it
BGive the dose because the doctor has signed the prescription
CGive half the dose as a compromise until the doctor is available
DWithhold it and check with the prescriber or pharmacist first
3Solutions
1.Lab values & electrolytes
Answer: D · Dilution of the blood by the IV fluid
Why this is the answer
IV fluids dilute the blood, so the hemoglobin falls even without further bleeding. Ongoing bleeding must still be considered, so vital signs, wounds and repeat hemoglobin values are monitored.
Answer
D · Dilution of the blood by the IV fluid
2.Lab values & electrolytes
Answer: A · Less stretch on the heart, consistent with improvement
Why this is the answer
A falling BNP means the ventricles are under less strain, which fits an improving fluid status. It is read together with symptoms, weight and examination.
Answer
A · Less stretch on the heart, consistent with improvement
3.Arterial blood gas interpretation
Answer: HCO₃⁻ = 15.5 mmol/L
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.
Change in PaCO₂
|23 − 40| = 17 mmHg → 1.7 × 10 mmHg
Expected HCO₃⁻
HCO₃⁻ = 24 − 5·1.7 = 15.5 mmol/L
Answer
HCO₃⁻ = 15.5 mmol/L
4.Arterial blood gas interpretation
Answer: AGc = 22.75 mmol/L
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.
Anion gap
AG = 144 − (106 + 21) = 17
Correction for albumin
17 + 0.25·(40 − 17) = 22.75 mmol/L
Answer
AGc = 22.75 mmol/L
Above about 12 after correction: a high anion gap.
5.Clinical pharmacokinetics
Answer: LD = 1039 mg
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 = 0.7·91 = 63.7 L
Amount of active drug needed
Vd·C = 63.7·15 = 955.5 mg
Loading dose
LD = 955.5/(0.92·1) = 1039 mg
Answer
LD = 1039 mg
In practice the dose is rounded to a convenient amount.
6.Clinical pharmacokinetics
Answer: D = 162 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 = 0.3·60 = 18 L
Dose
D = 18·(10 − 1) = 162 mg
Answer
D = 162 mg
In practice the dose is rounded to a convenient amount.
7.Clinical nursing
Answer: B · Fever, chills, back pain and low blood pressure
Why this is the answer
An acute haemolytic or febrile reaction can cause fever, rigors, loin or back pain, breathlessness and hypotension. Stop the transfusion and call for help.
Answer
B · Fever, chills, back pain and low blood pressure
8.Clinical nursing
Answer: D · Withhold it and check with the prescriber or pharmacist first
Why this is the answer
Nurses must understand any drug they administer. If a dose seems wrong, they should check with the prescriber or pharmacist and document the action.
Answer
D · Withhold it and check with the prescriber or pharmacist first