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
How should a blood sample for serum ammonia be handled?
AShaken firmly to mix it before it is sent away
BLeft at room temperature for several hours first
CKept warm and sent with the next routine batch
DPlaced on ice and sent to the laboratory at once
2.Lab values & electrolytesHard
A patient's creatinine was 0.9 mg/dL (80 µmol/L) on admission and is 1.3 mg/dL (115 µmol/L) 36 hours later. Using the common KDIGO criteria, what does this show?
AA laboratory error, because creatinine cannot change within 2 days
BAcute kidney injury, because it rose by at least 0.3 mg/dL within 48 hours
CNo kidney injury, because the creatinine is still within the normal range
DChronic kidney disease, because creatinine has risen above baseline
3.Arterial blood gas interpretationMedium
A practice gas shows a metabolic acidosis with HCO₃⁻ 16 mmol/L. Use Winter’s formula to find the expected PaCO₂ (the centre of the range) in mmHg.
HCO₃⁻ 16 mmol/L
PaCO₂ = mmHg
4.Arterial blood gas interpretationHard
Practice values: Na⁺ 140, Cl⁻ 99 and HCO₃⁻ 18 mmol/L. Calculate the delta ratio (AG − 12)/(24 − HCO₃⁻), using AG = Na⁺ − (Cl⁻ + HCO₃⁻). Give two decimals.
Na⁺ 140 mmol/LCl⁻ 99 mmol/LHCO₃⁻ 18 mmol/L
delta ratio =
5.Clinical pharmacokineticsMedium
Practice exercise (made-up values, not clinical guidance). After an IV dose of vancomycin, the level is 35 mg/L at 1 h and 8.8 mg/L at 13 h. Assume first-order elimination. Find the elimination rate constant k.
C1 = 35 mg/L (1 h)C2 = 8.8 mg/L (13 h)
k = h⁻¹
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A patient's gentamicin half-life is 2.5 h. The target is a peak of 10 mg/L and a trough of 0.5 mg/L. Assuming rapid IV doses and first-order elimination, what dosing interval τ lets the level fall from the peak to the trough? Round to one decimal place.
t½ = 2.5 hCpeak = 10 mg/LCtrough = 0.5 mg/L
τ = h
7.Clinical nursingMedium
After a laparoscopic cholecystectomy, a patient has pain in the shoulder tip. What is the most likely cause?
AA broken collarbone from being moved onto the table
BGas used during the operation irritating the diaphragm
CA heart attack caused by the stress of surgery
DAn allergic reaction to the dressing
8.Clinical nursingHard
A patient with a small leg wound has severe pain out of proportion to the skin changes, rapidly spreading redness and signs of sepsis. What should be suspected?
AA superficial bruise
BAn allergic reaction to the dressing
CSimple cellulitis that will settle with oral antibiotics
DNecrotising fasciitis
3Solutions
1.Lab values & electrolytes
Answer: D · Placed on ice and sent to the laboratory at once
Why this is the answer
Ammonia can rise in a standing sample as red cells and proteins break down. The sample is usually placed on ice and analyzed promptly to avoid a falsely high result.
Answer
D · Placed on ice and sent to the laboratory at once
2.Lab values & electrolytes
Answer: B · Acute kidney injury, because it rose by at least 0.3 mg/dL within 48 hours
Why this is the answer
KDIGO defines AKI as a creatinine rise of at least 0.3 mg/dL (26.5 µmol/L) within 48 hours, a rise to 1.5 times baseline within 7 days, or urine output below 0.5 mL/kg/h for 6 hours. A result inside the reference range can still be a significant rise.
Answer
B · Acute kidney injury, because it rose by at least 0.3 mg/dL within 48 hours
3.Arterial blood gas interpretation
Answer: PaCO₂ = 32 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 = 1.5·16 + 8 = 32 mmHg
range: 30–34 mmHg
Answer
PaCO₂ = 32 mmHg
4.Arterial blood gas interpretation
Answer: delta ratio = 1.833
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 = 140 − (99 + 18) = 23
Delta ratio
(23 − 12)/(24 − 18) = 11/6 = 1.833
Answer
delta ratio = 1.833
Between 1 and 2: a pure high anion gap acidosis.
5.Clinical pharmacokinetics
Answer: k = 0.115 h⁻¹
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.
Elimination rate constant
k = ln(35/8.8)/12 = 1.381/12 = 0.115 h⁻¹
Answer
k = 0.115 h⁻¹
6.Clinical pharmacokinetics
Answer: τ = 10.8 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.
Rate constant
k = 0.693/2.5 = 0.2773 h⁻¹
Dosing interval
τ = ln(10/0.5)/0.2773 = 2.996/0.2773 = 10.8 h
Answer
τ = 10.8 h
In practice the interval is rounded to a convenient value such as 8, 12, 24 or 36 h, and the levels are checked again.
7.Clinical nursing
Answer: B · Gas used during the operation irritating the diaphragm
Why this is the answer
Carbon dioxide used to inflate the abdomen can irritate the diaphragm, causing referred shoulder-tip pain. It usually settles within a few days with mobilisation and analgesia.
Answer
B · Gas used during the operation irritating the diaphragm
8.Clinical nursing
Answer: D · Necrotising fasciitis
Why this is the answer
Necrotising fasciitis spreads rapidly along fascia; severe pain, systemic toxicity and fast progression are key warning signs. It is a surgical emergency needing urgent debridement and IV antibiotics.