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 HIV on antiretroviral therapy has an undetectable viral load. What does this mean?
AThe treatment is suppressing the virus very well
BThe virus has been completely cured and removed
CThe CD4 count must now be dangerously low too
DThe treatment can now be stopped without any risk
2.Lab values & electrolytesHard
A patient with severe hypertriglyceridemia has a measured sodium of 128 mEq/L, but a measured serum osmolality is normal, and the patient has no symptoms. What is the most likely explanation?
AHyponatremia caused by severe hyperglycemia in diabetes
BPseudohyponatremia, an artifact caused by very high lipids
CHyponatremia caused by drinking far too much water
DTrue hyponatremia from SIADH that needs fluid restriction
3.Arterial blood gas interpretationMedium
A practice gas shows PaCO₂ 51 mmHg (6.8 kPa). Use the rule that in chronic respiratory acidosis HCO₃⁻ rises by 3.5 mmol/L for every 10 mmHg that PaCO₂ is above 40, starting from HCO₃⁻ 24 mmol/L, to find the expected HCO₃⁻.
A practice gas shows a metabolic alkalosis with HCO₃⁻ 44 mmol/L. Use expected PaCO₂ = 0.7·HCO₃⁻ + 21 to find the expected PaCO₂.
HCO₃⁻ 44 mmol/L
PaCO₂ = mmHg
5.Clinical pharmacokineticsMedium
Practice exercise (made-up values, not clinical guidance). A 51 kg patient is to be loaded intravenously with aminophylline to reach a theophylline level of 12 mg/L. The volume of distribution is 0.5 L/kg and the salt factor is S = 0.8. Find the loading dose LD of aminophylline in mg.
51 kgVd = 0.5 L/kgC = 12 mg/LS = 0.8
LD = mg
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A 100 kg patient has a theophylline level of 6 mg/L; the target is 12 mg/L. The drug is given intravenously as aminophylline (S = 0.8), and the volume of distribution is 0.5 L/kg. What extra loading dose LD raises the level to the target?
100 kgVd = 0.5 L/kgC = 6 → 12 mg/LS = 0.8
LD = mg
7.Clinical nursingMedium
Which statement about storing insulin is correct?
AInsulin should be kept in direct sunlight
BAll insulin should be stored in the freezer to keep it fresh
CInsulin never expires once opened
DUnopened insulin is kept in the fridge and must not be frozen
8.Clinical nursingHard
A patient with known prostate cancer reports new back pain, leg weakness and difficulty passing urine. What should be suspected?
AMetastatic spinal cord compression
BSimple mechanical back pain
CHypokalaemia
DA urinary tract infection spreading to the back
3Solutions
1.Lab values & electrolytes
Answer: A · The treatment is suppressing the virus very well
Why this is the answer
An undetectable viral load shows that antiretroviral therapy is suppressing viral replication. The virus remains in the body, so treatment must continue. People with a sustained undetectable viral load do not transmit HIV sexually.
Answer
A · The treatment is suppressing the virus very well
2.Lab values & electrolytes
Answer: B · Pseudohyponatremia, an artifact caused by very high lipids
Why this is the answer
Very high lipids or proteins take up part of the plasma volume, so some laboratory methods report a falsely low sodium. The osmolality and the sodium measured by direct methods are normal. Treating this "hyponatremia" would be harmful.
Answer
B · Pseudohyponatremia, an artifact caused by very high lipids
3.Arterial blood gas interpretation
Answer: HCO₃⁻ = 27.85 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
|51 − 40| = 11 mmHg → 1.1 × 10 mmHg
Expected HCO₃⁻
HCO₃⁻ = 24 + 3.5·1.1 = 27.85 mmol/L
Answer
HCO₃⁻ = 27.85 mmol/L
4.Arterial blood gas interpretation
Answer: PaCO₂ = 51.8 mmHg
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
0.7·44 + 21 = 51.8 mmHg
Answer
PaCO₂ = 51.8 mmHg
The rise has a limit: PaCO₂ rarely goes above about 55 mmHg from compensation alone.
5.Clinical pharmacokinetics
Answer: LD = 382.5 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 = 0.5·51 = 25.5 L
Amount of active drug needed
Vd·C = 25.5·12 = 306 mg
Loading dose
LD = 306/(0.8·1) = 382.5 mg
Answer
LD = 382.5 mg
In practice the dose is rounded to a convenient amount.
6.Clinical pharmacokinetics
Answer: LD = 375 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 = 0.5·100 = 50 L
Rise needed
12 − 6 = 6 mg/L
Loading dose
LD = 50·6/(0.8·1) = 375 mg
Answer
LD = 375 mg
In practice the dose is rounded to a convenient amount.
7.Clinical nursing
Answer: D · Unopened insulin is kept in the fridge and must not be frozen
Why this is the answer
Unopened insulin is refrigerated; in-use pens or vials are usually kept at room temperature for a limited time stated by the manufacturer. Frozen or overheated insulin must be discarded.
Answer
D · Unopened insulin is kept in the fridge and must not be frozen
8.Clinical nursing
Answer: A · Metastatic spinal cord compression
Why this is the answer
Back pain with neurological symptoms in a patient with cancer suggests metastatic spinal cord compression. It is an oncological emergency needing urgent MRI and treatment to preserve function.