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
Which instruction is correct for a patient taking an oral potassium chloride supplement?
ATake it with food and a full glass of water, and do not crush extended-release tablets
BUse a salt substitute as well, so less of the tablet is needed each day
CCrush the extended-release tablets and mix them with a little water if needed
DTake it on an empty stomach with only a small sip of water to speed absorption
2.Lab values & electrolytesHard
A patient with a platelet count of 35 × 10³/µL is prescribed an intramuscular injection. What should the nurse do?
AGive the injection as prescribed and massage the site firmly
BAsk the prescriber whether another route can be used
CGive the injection and apply a heat pack
DGive the injection with a longer needle
3.Arterial blood gas interpretationMedium
A practice gas shows a metabolic acidosis with HCO₃⁻ 12 mmol/L. Use Winter’s formula to find the expected PaCO₂ (the centre of the range) in mmHg.
HCO₃⁻ 12 mmol/L
PaCO₂ = mmHg
4.Arterial blood gas interpretationHard
At sea level, a practice gas taken on 21% oxygen shows PaO₂ 63 mmHg (8.4 kPa) and PaCO₂ 51 mmHg (6.8 kPa). Use PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8 to find the A–a gradient.
Practice exercise (made-up values, not clinical guidance). A patient has a lidocaine clearance of 600 mL/min. What infusion rate R in mg/min keeps the steady-state level at 2.5 mg/L?
CL = 600 mL/minCss = 2.5 mg/L
R = mg/min
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A 70 kg patient takes oral theophylline (F = 1) every 8 h. The clearance is 0.05 L/kg/h. What dose D in mg gives an average steady-state level of 12 mg/L?
How does CPAP help a patient with obstructive sleep apnoea?
AIt removes excess carbon dioxide from the lungs by gentle suction
BIt delivers bronchodilator drugs deep into the lungs overnight
CIt strengthens the breathing muscles during the day
DIt keeps the upper airway open during sleep with continuous air pressure
8.Clinical nursingHard
Which patient should a ward nurse assess first at the start of a shift?
AA patient with new-onset confusion and a respiratory rate of 28/min
BA patient asking for a laxative
CA patient with chronic back pain scored 4/10
DA patient awaiting discharge with stable observations
3Solutions
1.Lab values & electrolytes
Answer: A · Take it with food and a full glass of water, and do not crush extended-release tablets
Why this is the answer
Oral potassium can irritate the esophagus and stomach, so it is taken with food and plenty of fluid, and extended-release forms are swallowed whole. Most salt substitutes contain potassium chloride and can add to the risk of hyperkalemia.Checked against:Potassium (drug information) · MedlinePlus (NIH) (2024)
Answer
A · Take it with food and a full glass of water, and do not crush extended-release tablets
2.Lab values & electrolytes
Answer: B · Ask the prescriber whether another route can be used
Why this is the answer
IM injections can cause bleeding into the muscle when platelets are low. The nurse asks for another route, such as oral or IV. If an injection is unavoidable, firm pressure without massage is applied afterwards.Checked against:Bleeding and Bruising (Thrombocytopenia) and Cancer Treatment · National Cancer Institute (NIH) (2022)
Answer
B · Ask the prescriber whether another route can be used
3.Arterial blood gas interpretation
Answer: PaCO₂ = 26 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 = 1.5·12 + 8 = 26 mmHg
range: 24–28 mmHg
Answer
PaCO₂ = 26 mmHg
4.Arterial blood gas interpretation
Answer: A–a gradient = 22.98 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.
Clearance in L/min
600 mL/min = 0.6 L/min
Infusion rate
R = 0.6·2.5 = 1.5 mg/min
Answer
R = 1.5 mg/min
6.Clinical pharmacokinetics
Answer: D = 336 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.
Clearance
CL = 0.05·70 = 3.5 L/h
Dose
D = Css,avg·CL·τ/(S·F) = 12·3.5·8/(1·1) = 336 mg
Answer
D = 336 mg
7.Clinical nursing
Answer: D · It keeps the upper airway open during sleep with continuous air pressure
Why this is the answer
Continuous positive airway pressure acts as a splint for the upper airway, preventing collapse and apnoeas. Patients should bring their CPAP machine into hospital.
Answer
D · It keeps the upper airway open during sleep with continuous air pressure
8.Clinical nursing
Answer: A · A patient with new-onset confusion and a respiratory rate of 28/min
Why this is the answer
New confusion with tachypnoea suggests acute deterioration, such as sepsis or hypoxia. Patients with stable chronic problems can wait.
Answer
A · A patient with new-onset confusion and a respiratory rate of 28/min