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
When several samples are drawn at the same venipuncture, which tubes or bottles are filled first?
ACoagulation (citrate) tubes
BEDTA (complete blood count) tubes
CBlood culture bottles
DFluoride (glucose) tubes
2.Lab values & electrolytesHard
A patient on digoxin has a calcium of 12.1 mg/dL (3.0 mmol/L) and new nausea and a slow pulse. Why is the calcium result important?
AHigh calcium only matters if the potassium is also high
BHigh calcium increases the risk of digoxin toxicity
CHigh calcium blocks digoxin, so the drug stops working
DHigh calcium makes the kidneys clear digoxin faster
3.Arterial blood gas interpretationMedium
A practice gas shows a metabolic acidosis with HCO₃⁻ 18 mmol/L. Use Winter’s formula to find the expected PaCO₂ (the centre of the range) in mmHg.
HCO₃⁻ 18 mmol/L
PaCO₂ = mmHg
4.Arterial blood gas interpretationHard
At sea level, a practice gas taken on 30% oxygen shows PaO₂ 80 mmHg (10.7 kPa) and PaCO₂ 36 mmHg (4.8 kPa). Use PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8 to find the A–a gradient.
Practice exercise (made-up values, not clinical guidance). A 52 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.
52 kgVd = 0.7 L/kgC = 15 mg/LS = 0.92
LD = mg
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). An 86 kg patient has a phenytoin level of 10 mg/L; the target is 20 mg/L. The drug is given intravenously as phenytoin sodium (S = 0.92), and the volume of distribution is 0.7 L/kg. What extra loading dose LD raises the level to the target?
86 kgVd = 0.7 L/kgC = 10 → 20 mg/LS = 0.92
LD = mg
7.Clinical nursingMedium
When is a rapid-acting insulin such as insulin aspart usually given?
AJust before or with a meal
BOnly at bedtime
CTwo hours after finishing a meal
DOnce a week
8.Clinical nursingHard
A patient with acute pancreatitis is in severe epigastric pain radiating to the back. Which nursing care is most important?
ARestricting all fluids
BA large fatty meal to stimulate the pancreas
CStrict bed rest without observations
DAdequate pain relief, IV fluids and close fluid balance monitoring
3Solutions
1.Lab values & electrolytes
Answer: C · Blood culture bottles
Why this is the answer
Blood cultures are drawn first to reduce contamination. A standard order of draw then follows (citrate, serum, heparin, EDTA, fluoride), which prevents additives being carried from one tube into the next and distorting results.
Answer
C · Blood culture bottles
2.Lab values & electrolytes
Answer: B · High calcium increases the risk of digoxin toxicity
Why this is the answer
Hypercalcemia, like hypokalemia and hypomagnesemia, makes the heart more sensitive to digoxin and raises the risk of toxic arrhythmias. Nausea and bradycardia suggest toxicity, so the nurse holds the dose and reports the findings.
Answer
B · High calcium increases the risk of digoxin toxicity
3.Arterial blood gas interpretation
Answer: PaCO₂ = 35 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.
Winter’s formula
PaCO₂ = 1.5·18 + 8 = 35 mmHg
range: 33–37 mmHg
Answer
PaCO₂ = 35 mmHg
4.Arterial blood gas interpretation
Answer: A–a gradient = 88.9 mmHg
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.
Alveolar oxygen
PAO₂ = 0.3·713 − 36/0.8 = 213.9 − 45 = 168.9 mmHg
Gradient
A–a = 168.9 − 80 = 88.9 mmHg
Answer
A–a gradient = 88.9 mmHg
5.Clinical pharmacokinetics
Answer: LD = 593.5 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 = 0.7·52 = 36.4 L
Amount of active drug needed
Vd·C = 36.4·15 = 546 mg
Loading dose
LD = 546/(0.92·1) = 593.5 mg
Answer
LD = 593.5 mg
In practice the dose is rounded to a convenient amount.
6.Clinical pharmacokinetics
Answer: LD = 654.3 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 = 0.7·86 = 60.2 L
Rise needed
20 − 10 = 10 mg/L
Loading dose
LD = 60.2·10/(0.92·1) = 654.3 mg
Answer
LD = 654.3 mg
In practice the dose is rounded to a convenient amount.
7.Clinical nursing
Answer: A · Just before or with a meal
Why this is the answer
Rapid-acting insulins start working within minutes, so they are given with meals to match the glucose rise. Giving the dose without eating risks hypoglycaemia.
Answer
A · Just before or with a meal
8.Clinical nursing
Answer: D · Adequate pain relief, IV fluids and close fluid balance monitoring
Why this is the answer
Acute pancreatitis can cause large fluid losses and organ failure. Care focuses on fluid resuscitation, analgesia, monitoring urine output, glucose and early nutrition when possible.
Answer
D · Adequate pain relief, IV fluids and close fluid balance monitoring