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 on a heparin infusion has an aPTT of 110 seconds; the protocol target is 1.5–2.5 times the control of 30 seconds. What should the nurse do?
AIncrease the infusion rate, because the result is above the target range
BFollow the heparin protocol, check for bleeding and inform the provider
CGive vitamin K as prescribed and continue the infusion
DContinue unchanged and repeat the aPTT the next morning
2.Lab values & electrolytesHard
A patient taking isoniazid for latent TB reports nausea, loss of appetite and dark urine. The ALT is five times the upper limit of normal. What is the best action?
AReassure the patient that dark urine is a harmless drug effect
BContinue the isoniazid and give it with food to settle the stomach
CDouble the dose to finish the treatment course more quickly
DHold the isoniazid and report the findings to the prescriber at once
3.Arterial blood gas interpretationMedium
A practice gas shows a metabolic acidosis with HCO₃⁻ 14 mmol/L. Use Winter’s formula to find the expected PaCO₂ (the centre of the range) in mmHg.
HCO₃⁻ 14 mmol/L
PaCO₂ = mmHg
4.Arterial blood gas interpretationHard
Practice values: Na⁺ 142, Cl⁻ 97 and HCO₃⁻ 9 mmol/L. Calculate the delta ratio (AG − 12)/(24 − HCO₃⁻), using AG = Na⁺ − (Cl⁻ + HCO₃⁻). Give two decimals.
Na⁺ 142 mmol/LCl⁻ 97 mmol/LHCO₃⁻ 9 mmol/L
delta ratio =
5.Clinical pharmacokineticsMedium
Practice exercise (made-up values, not clinical guidance). A 65 kg patient is to be loaded intravenously with aminophylline to reach a theophylline level of 15 mg/L. The volume of distribution is 0.5 L/kg and the salt factor is S = 0.8. Find the loading dose LD of aminophylline in mg.
65 kgVd = 0.5 L/kgC = 15 mg/LS = 0.8
LD = mg
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A theophylline infusion runs at 24 mg/h with no loading dose. The patient's clearance is 2 L/h and the half-life is 6 h. What is the level C after 2 h?
R = 24 mg/hCL = 2 L/ht½ = 6 ht = 2 h
C = mg/L
7.Clinical nursingMedium
Which is a recognised risk factor for postoperative nausea and vomiting?
AA short operation under local anaesthetic
BBeing male
CBeing a smoker
DUse of opioids after surgery
8.Clinical nursingHard
A patient on an opioid PCA pump has a respiratory rate of 7/min and is difficult to rouse. What should be done first?
ARecord the observations and recheck them at the next four-hourly round
BStop the PCA, rouse the patient, give oxygen and get naloxone given
CIncrease the PCA bolus dose because the patient may be in pain
DEncourage the patient to press the PCA button more often for comfort
3Solutions
1.Lab values & electrolytes
Answer: B · Follow the heparin protocol, check for bleeding and inform the provider
Why this is the answer
110 s is well above the target of about 45–75 s, so the bleeding risk is high. The nurse follows the heparin protocol, assesses for bleeding and informs the provider. Protamine is the reversal agent for heparin; vitamin K reverses warfarin.
Answer
B · Follow the heparin protocol, check for bleeding and inform the provider
2.Lab values & electrolytes
Answer: D · Hold the isoniazid and report the findings to the prescriber at once
Why this is the answer
Isoniazid can cause hepatitis, which may be severe. Symptoms such as nausea, loss of appetite and dark urine with a markedly raised ALT mean the drug should be held and the prescriber informed. Patients are taught to avoid alcohol and report these symptoms early.
Answer
D · Hold the isoniazid and report the findings to the prescriber at once
3.Arterial blood gas interpretation
Answer: PaCO₂ = 29 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 = 1.5·14 + 8 = 29 mmHg
range: 27–31 mmHg
Answer
PaCO₂ = 29 mmHg
4.Arterial blood gas interpretation
Answer: delta ratio = 1.6
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 = 142 − (97 + 9) = 36
Delta ratio
(36 − 12)/(24 − 9) = 24/15 = 1.6
Answer
delta ratio = 1.6
Between 1 and 2: a pure high anion gap acidosis.
5.Clinical pharmacokinetics
Answer: LD = 609.4 mg
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 = 0.5·65 = 32.5 L
Amount of active drug needed
Vd·C = 32.5·15 = 487.5 mg
Loading dose
LD = 487.5/(0.8·1) = 609.4 mg
Answer
LD = 609.4 mg
In practice the dose is rounded to a convenient amount.
6.Clinical pharmacokinetics
Answer: C = 2.476 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 = 24/2 = 12 mg/L
Rate constant
k = 0.693/6 = 0.1155 h⁻¹
Level after the given time
C = 12·(1 − e−0.1155·2) = 2.476 mg/L
Answer
C = 2.476 mg/L
7.Clinical nursing
Answer: D · Use of opioids after surgery
Why this is the answer
Known risk factors include opioids, female sex, being a non-smoker, previous PONV or motion sickness and longer general anaesthetics. High-risk patients may be given preventive antiemetics.
Answer
D · Use of opioids after surgery
8.Clinical nursing
Answer: B · Stop the PCA, rouse the patient, give oxygen and get naloxone given
Why this is the answer
Over-sedation and slow breathing indicate opioid toxicity. Stop the opioid, support the airway and breathing, give oxygen and naloxone as prescribed, and monitor closely.
Answer
B · Stop the PCA, rouse the patient, give oxygen and get naloxone given