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?
AExpected muscle activity from shivering in fever
BWorsening tissue perfusion that needs urgent reporting
CA normal response to IV fluids in the first hours
DImproving circulation as the fluids start to work
2.Lab values & electrolytesHard
A health care worker's hepatitis B tests show: HBsAg negative, anti-HBs positive, anti-HBc positive. What does this mean?
APast infection that has resolved, with immunity
BCurrent chronic hepatitis B infection
CImmunity from vaccination only, never infected
DNot immune and never exposed to the virus
3.Arterial blood gas interpretationMedium
A practice gas shows a metabolic acidosis with HCO₃⁻ 8 mmol/L. Use Winter’s formula to find the expected PaCO₂ (the centre of the range) in mmHg.
HCO₃⁻ 8 mmol/L
PaCO₂ = mmHg
4.Arterial blood gas interpretationHard
At sea level, a practice gas taken on 30% oxygen shows PaO₂ 88 mmHg (11.7 kPa) and PaCO₂ 42 mmHg (5.6 kPa). Use PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8 to find the A–a gradient.
Practice exercise (made-up values, not clinical guidance). A patient receives an IV bolus of 120 mg of gentamicin. The level back-extrapolated to time zero is 7 mg/L and the half-life is 2 h. Find the volume of distribution Vd.
D = 120 mgC₀ = 7 mg/Lt½ = 2 h
Vd = L
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A 43-year-old man weighs 76 kg and has a serum creatinine of 3.2 mg/dL (283 µmol/L). Estimate the creatinine clearance with Cockcroft–Gault: CrCl (mL/min) = (140 − age) × weight (kg)/(72 × SCr in mg/dL), × 0.85 for women. The usual vancomycin regimen is 1000 mg every 12 h for a creatinine clearance of 100 mL/min. Keeping the dose, the interval is lengthened in proportion to the fall in clearance. Find the new dosing interval τ. Round to one decimal place.
Which type of painkiller should a patient with liver cirrhosis usually avoid?
ANon-steroidal anti-inflammatory drugs such as ibuprofen
BHot packs applied to a painful joint
CParacetamol (acetaminophen), given in a carefully reduced and monitored dose
DLocal anaesthetic creams
8.Clinical nursingHard
Two days after a bowel resection, a patient has rising pain, fever, tachycardia and fluid that looks like faeces in the wound drain. What is most likely?
AUrinary retention
BAnastomotic leak
CConstipation
DNormal postoperative recovery
3Solutions
1.Lab values & electrolytes
Answer: B · 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
B · Worsening tissue perfusion that needs urgent reporting
2.Lab values & electrolytes
Answer: A · Past infection that has resolved, with immunity
Why this is the answer
The vaccine contains only surface antigen, so vaccination produces anti-HBs alone. A positive anti-HBc shows past or current infection; with HBsAg negative and anti-HBs positive, the infection has cleared and the person is immune.
Answer
A · Past infection that has resolved, with immunity
3.Arterial blood gas interpretation
Answer: PaCO₂ = 20 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 = 1.5·8 + 8 = 20 mmHg
range: 18–22 mmHg
Answer
PaCO₂ = 20 mmHg
4.Arterial blood gas interpretation
Answer: A–a gradient = 73.4 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.
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 = 120/7 = 17.14 L
Answer
Vd = 17.14 L
6.Clinical pharmacokinetics
Answer: τ = 37.5 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.
Creatinine clearance (Cockcroft–Gault)
CrCl = (140 − 43)·76/(72·3.2) = 32 mL/min
New interval
τ = 12·100/32 = 37.5 h
Answer
τ = 37.5 h
In practice the interval is rounded to a convenient value such as 12, 24, 36 or 48 h.
7.Clinical nursing
Answer: A · Non-steroidal anti-inflammatory drugs such as ibuprofen
Why this is the answer
NSAIDs increase the risk of GI bleeding and can trigger kidney failure and fluid retention in cirrhosis. Paracetamol is usually preferred at a lower, carefully chosen dose.
Answer
A · Non-steroidal anti-inflammatory drugs such as ibuprofen
8.Clinical nursing
Answer: B · Anastomotic leak
Why this is the answer
An anastomotic leak lets bowel contents into the abdomen, causing peritonitis and sepsis. It is a surgical emergency that needs urgent review.