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 can a patient with liver failure have a raised INR without taking warfarin?
ABilirubin directly destroys the clotting factors
BThe liver makes most clotting factors, so production falls
CThe kidneys stop clearing the clotting factors from the blood
DAlbumin is a clotting factor, and its level falls
2.Lab values & electrolytesHard
A patient admitted with heavy alcohol use and malnutrition has a glucose of 52 mg/dL (2.9 mmol/L) and low magnesium. Which prescribed treatment should be given before the IV glucose?
AIV thiamine
BIV potassium
CIV insulin
DOral iron
3.Arterial blood gas interpretationMedium
A practice arterial blood gas shows pH 7.26, PaCO₂ 39 mmHg (5.2 kPa) and HCO₃⁻ 17 mmol/L. The primary disorder is metabolic acidosis. How far is it compensated? Click your answer.
pH 7.26PaCO₂ 39 mmHg (5.2 kPa)HCO₃⁻ 17 mmol/L
1uncompensated
2partially compensated
3fully compensated
4.Arterial blood gas interpretationHard
A practice gas shows a metabolic alkalosis with HCO₃⁻ 46 mmol/L. Use expected PaCO₂ = 0.7·HCO₃⁻ + 21 to find the expected PaCO₂.
HCO₃⁻ 46 mmol/L
PaCO₂ = mmHg
5.Clinical pharmacokineticsMedium
Practice exercise (made-up values, not clinical guidance). After an IV dose of gentamicin, the level is 9 mg/L at 1 h and 1.6 mg/L at 6 h. Assume first-order elimination. Find the elimination rate constant k.
C1 = 9 mg/L (1 h)C2 = 1.6 mg/L (6 h)
k = h⁻¹
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A vancomycin infusion runs at 100 mg/h with no loading dose. The patient's clearance is 4 L/h and the half-life is 8 h. What is the level C after 19 h?
R = 100 mg/hCL = 4 L/ht½ = 8 ht = 19 h
C = mg/L
7.Clinical nursingMedium
Which group of features is typical of nephrotic syndrome?
AHeavy proteinuria, low serum albumin and oedema
BSevere loin pain and fever
CPolyuria, thirst and weight loss
DBlood in the urine, high blood pressure and reduced urine output only
8.Clinical nursingHard
A patient has a painful, punched-out ulcer on the toes, with a cold, pale foot and weak foot pulses. Which type of ulcer is most likely?
AA burn
BA venous ulcer
CAn arterial ulcer
DA pressure ulcer from bed rest
3Solutions
1.Lab values & electrolytes
Answer: B · The liver makes most clotting factors, so production falls
Why this is the answer
The liver makes most clotting factors, including the vitamin K–dependent ones. In severe liver disease their production falls, so the INR rises. A raised INR is one of the markers of how severe liver failure is.Checked against:Prothrombin Time Test and INR (PT/INR) · MedlinePlus (NIH) (2024)
Answer
B · The liver makes most clotting factors, so production falls
2.Lab values & electrolytes
Answer: A · IV thiamine
Why this is the answer
People with alcohol use disorder and malnutrition are often deficient in thiamine. Giving glucose first can use up the remaining thiamine and precipitate Wernicke encephalopathy, so thiamine is usually given before or with glucose. Hypoglycemia must still be treated promptly.
Answer
A · IV thiamine
3.Arterial blood gas interpretation
Answer: uncompensated
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 7.26: acidaemia (below 7.35)
PaCO₂ 39 mmHg: normal (35–45)
HCO₃⁻ 17 mmol/L: low (22–26)
Compensation
Clinically, also check the expected compensation: Winter’s formula expects PaCO₂ 31.5–35.5 mmHg, so the measured 39 mmHg points to an additional respiratory acidosis (a mixed disorder). The answer above is the ROME exam classification.
uncompensated
Answer
uncompensated
4.Arterial blood gas interpretation
Answer: PaCO₂ = 53.2 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
0.7·46 + 21 = 53.2 mmHg
Answer
PaCO₂ = 53.2 mmHg
The rise has a limit: PaCO₂ rarely goes above about 55 mmHg from compensation alone.
5.Clinical pharmacokinetics
Answer: k = 0.3454 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(9/1.6)/5 = 1.727/5 = 0.3454 h⁻¹
Answer
k = 0.3454 h⁻¹
6.Clinical pharmacokinetics
Answer: C = 20.18 mg/L
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.
Steady-state level
Css = 100/4 = 25 mg/L
Rate constant
k = 0.693/8 = 0.08664 h⁻¹
Level after the given time
C = 25·(1 − e−0.08664·19) = 20.18 mg/L
Answer
C = 20.18 mg/L
7.Clinical nursing
Answer: A · Heavy proteinuria, low serum albumin and oedema
Why this is the answer
In nephrotic syndrome, damaged glomeruli leak protein, so albumin falls and fluid moves into tissues, causing oedema. Cholesterol is often raised too.
Answer
A · Heavy proteinuria, low serum albumin and oedema
8.Clinical nursing
Answer: C · An arterial ulcer
Why this is the answer
Arterial ulcers are usually on the feet or toes, painful and punched-out, with signs of poor perfusion. Venous ulcers are usually around the ankle, shallow and associated with swelling and skin staining.