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
Why must vitamin B12 deficiency be ruled out before treating macrocytic anemia with folic acid alone?
AFolic acid can fix the anemia while B12-related nerve damage continues
BFolic acid makes B12 deficiency anemia much worse within a few days
CFolic acid causes iron overload when B12 levels are low
DFolic acid blocks the absorption of all B12 from the diet
2.Lab values & electrolytesHard
A patient receiving IV magnesium sulfate has a respiratory rate of 9/min and absent patellar reflexes. After stopping the infusion, which medication should the nurse expect to give?
ASodium bicarbonate
BPotassium chloride
CCalcium gluconate
DProtamine sulfate
3.Arterial blood gas interpretationMedium
A practice arterial blood gas shows pH 7.37, PaCO₂ 18 mmHg (2.4 kPa) and HCO₃⁻ 10 mmol/L. The primary disorder is metabolic acidosis. How far is it compensated? Click your answer.
pH 7.37PaCO₂ 18 mmHg (2.4 kPa)HCO₃⁻ 10 mmol/L
1uncompensated
2partially compensated
3fully compensated
4.Arterial blood gas interpretationHard
Practice values: Na⁺ 141, Cl⁻ 101, HCO₃⁻ 22 mmol/L and albumin 38 g/L. Find the albumin-corrected anion gap AGc = AG + 0.25·(40 − albumin).
Practice exercise (made-up values, not clinical guidance). An 88 kg patient receives a theophylline infusion. The clearance is 0.035 L/kg/h. What infusion rate R in mg/h gives a steady-state level of 10 mg/L?
88 kgCL = 0.035 L/kg/hCss = 10 mg/L
R = mg/h
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A 65 kg patient on regular tobramycin doses is at steady state. The target is a peak of 6 mg/L and a trough of 0.5 mg/L, and the volume of distribution is 0.25 L/kg. Assuming rapid IV doses, what dose D raises the level from the trough to the peak?
A patient with renal colic is asked to pass urine through a strainer. Why?
ATo make passing urine less painful
BTo remove bacteria from the urine
CTo measure the urine's pH and specific gravity
DTo catch any stone so that it can be analysed
8.Clinical nursingHard
An older patient with a hip fracture becomes breathless and confused, with a petechial rash on the chest two days after injury. Which complication should be suspected?
AFat embolism syndrome
BUrinary tract infection
CPressure ulcer
DCompartment syndrome
3Solutions
1.Lab values & electrolytes
Answer: A · Folic acid can fix the anemia while B12-related nerve damage continues
Why this is the answer
Folic acid can improve the blood picture in B12 deficiency but does not prevent nerve damage. Giving folate alone can therefore mask the deficiency while neurological damage progresses. Both levels are checked before treatment.
Answer
A · Folic acid can fix the anemia while B12-related nerve damage continues
2.Lab values & electrolytes
Answer: C · Calcium gluconate
Why this is the answer
Absent reflexes and slow breathing indicate magnesium toxicity. The infusion is stopped, the provider is called, the airway and breathing are supported, and IV calcium gluconate is given as prescribed to counteract magnesium.
Answer
C · Calcium gluconate
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 7.37: within 7.35–7.45
PaCO₂ 18 mmHg: low (35–45)
HCO₃⁻ 10 mmol/L: low (22–26)
Compensation
Clinically, also check the expected compensation: Winter’s formula expects PaCO₂ 21–25 mmHg, so the measured 18 mmHg points to an additional respiratory alkalosis (a mixed disorder). The answer above is the ROME exam classification.
fully compensated
Answer
fully compensated
4.Arterial blood gas interpretation
Answer: AGc = 18.5 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.
Anion gap
AG = 141 − (101 + 22) = 18
Correction for albumin
18 + 0.25·(40 − 38) = 18.5 mmol/L
Answer
AGc = 18.5 mmol/L
Above about 12 after correction: a high anion gap.
5.Clinical pharmacokinetics
Answer: R = 30.8 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 = 0.035·88 = 3.08 L/h
Infusion rate
R = 3.08·10 = 30.8 mg/h
Answer
R = 30.8 mg/h
6.Clinical pharmacokinetics
Answer: D = 89.38 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 = 0.25·65 = 16.25 L
Dose
D = 16.25·(6 − 0.5) = 89.38 mg
Answer
D = 89.38 mg
In practice the dose is rounded to a convenient amount.
7.Clinical nursing
Answer: D · To catch any stone so that it can be analysed
Why this is the answer
Collecting a passed stone allows its composition to be identified, which helps guide diet and treatment to prevent recurrence.
Answer
D · To catch any stone so that it can be analysed
8.Clinical nursing
Answer: A · Fat embolism syndrome
Why this is the answer
Fat embolism typically appears 24–72 hours after a long-bone or pelvic fracture, with the triad of respiratory distress, neurological changes and a petechial rash.