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
An older adult with chronic kidney disease starts trimethoprim-sulfamethoxazole for a urinary tract infection. Which electrolyte should be monitored closely?
AChloride, because the drug causes chloride loss
BSodium, because the drug causes heavy sodium retention
CCalcium, because the drug binds calcium in the gut
DPotassium, because trimethoprim can raise it
2.Lab values & electrolytesHard
A 68-year-old man has fatigue, a hemoglobin of 9.8 g/dL (98 g/L), an MCV of 72 fL and a low ferritin. He eats a normal diet. What is the most important next step?
ARepeat the blood count in one year
BLook for a source of blood loss in the gut
CStart vitamin B12 injections
DGive iron and do no further tests at all
3.Arterial blood gas interpretationMedium
A practice arterial blood gas shows pH 7.30, PaCO₂ 21 mmHg (2.8 kPa) and HCO₃⁻ 10 mmol/L. The primary disorder is metabolic acidosis. How far is it compensated? Click your answer.
pH 7.30PaCO₂ 21 mmHg (2.8 kPa)HCO₃⁻ 10 mmol/L
1uncompensated
2partially compensated
3fully compensated
4.Arterial blood gas interpretationHard
A practice gas shows a metabolic acidosis: pH 7.27, PaCO₂ 27 mmHg (3.6 kPa), HCO₃⁻ 12 mmol/L. Compare the measured PaCO₂ with Winter’s formula. Click your interpretation.
pH 7.27PaCO₂ 27 mmHg (3.6 kPa)HCO₃⁻ 12 mmol/L
1appropriate compensation
2added respiratory acidosis
3added respiratory alkalosis
5.Clinical pharmacokineticsMedium
Practice exercise (made-up values, not clinical guidance). A patient receives an IV bolus of 150 mg of gentamicin. The level back-extrapolated to time zero is 9 mg/L and the half-life is 2.5 h. Find the clearance CL in L/h.
D = 150 mgC₀ = 9 mg/Lt½ = 2.5 h
CL = L/h
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A vancomycin infusion runs at 60 mg/h with no loading dose. The patient's clearance is 3 L/h and the half-life is 10 h. What is the level C after 3 h?
R = 60 mg/hCL = 3 L/ht½ = 10 ht = 3 h
C = mg/L
7.Clinical nursingMedium
After removing a peripheral IV cannula, what should the nurse check?
AThat the cannula is intact, and apply pressure until bleeding stops
BThat the cannula is put back in its sterile packet for the next patient
CThat the site is left uncovered and bleeding freely
DThat the patient's blood glucose is normal before removal
8.Clinical nursingHard
A patient with lung cancer develops swelling of the face and arms, distended neck veins and breathlessness that is worse lying down. What should be suspected?
AHypothyroidism
BRight-sided heart failure from valve disease
CAn allergic reaction to food
DSuperior vena cava obstruction
3Solutions
1.Lab values & electrolytes
Answer: D · Potassium, because trimethoprim can raise it
Why this is the answer
Trimethoprim blocks sodium channels in the collecting duct, reducing potassium excretion, much like amiloride. Hyperkalemia is more likely in older adults, with kidney disease, or with ACE inhibitors, ARBs or spironolactone.
Answer
D · Potassium, because trimethoprim can raise it
2.Lab values & electrolytes
Answer: B · Look for a source of blood loss in the gut
Why this is the answer
Iron deficiency in an older man or a postmenopausal woman suggests chronic blood loss, often from the gastrointestinal tract, including colorectal cancer. Iron is started, but endoscopy or colonoscopy is needed to find the cause.
Answer
B · Look for a source of blood loss in the gut
3.Arterial blood gas interpretation
Answer: partially 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 7.30: acidaemia (below 7.35)
PaCO₂ 21 mmHg: low (35–45)
HCO₃⁻ 10 mmol/L: low (22–26)
Compensation
partially compensated
Answer
partially compensated
4.Arterial blood gas interpretation
Answer: appropriate compensation
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
1.5·12 + 8 = 26 → 24–28 mmHg
Compare
measured: 27 mmHg → appropriate compensation
Answer
appropriate compensation
5.Clinical pharmacokinetics
Answer: CL = 4.621 L/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.
Volume of distribution
Vd = 150/9 = 16.67 L
Rate constant
k = 0.693/2.5 = 0.2773 h⁻¹
Clearance
CL = 0.2773·16.67 = 4.621 L/h
Answer
CL = 4.621 L/h
6.Clinical pharmacokinetics
Answer: C = 3.755 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 = 60/3 = 20 mg/L
Rate constant
k = 0.693/10 = 0.06931 h⁻¹
Level after the given time
C = 20·(1 − e−0.06931·3) = 3.755 mg/L
Answer
C = 3.755 mg/L
7.Clinical nursing
Answer: A · That the cannula is intact, and apply pressure until bleeding stops
Why this is the answer
The nurse checks that the whole cannula has been removed, applies pressure and a dressing, and records the removal. Any signs of phlebitis are noted.
Answer
A · That the cannula is intact, and apply pressure until bleeding stops
8.Clinical nursing
Answer: D · Superior vena cava obstruction
Why this is the answer
Superior vena cava obstruction prevents venous drainage from the head and arms. It is an oncological emergency; patients are positioned upright and need urgent specialist treatment.