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 who is breathless during a transfusion has hypotension, fever and bilateral lung infiltrates, with no signs of fluid overload. Which complication is most likely?
ATransfusion-related acute lung injury (TRALI)
BA mild febrile nonhemolytic reaction
CTransfusion-associated circulatory overload
DA simple allergic reaction with hives
2.Lab values & electrolytesHard
Four patients had a thyroidectomy earlier today. Which result should the nurse act on first?
AGlucose 145 mg/dL (8.0 mmol/L) after surgery
BCalcium 7.2 mg/dL (1.8 mmol/L) with tingling lips
CSodium 136 mEq/L on IV fluids after surgery
DHemoglobin 12.1 g/dL (121 g/L) after surgery
3.Arterial blood gas interpretationMedium
A practice arterial blood gas shows pH 7.43, PaCO₂ 22 mmHg (2.9 kPa) and HCO₃⁻ 14 mmol/L. The primary disorder is respiratory alkalosis. How far is it compensated? Click your answer.
pH 7.43PaCO₂ 22 mmHg (2.9 kPa)HCO₃⁻ 14 mmol/L
1uncompensated
2partially compensated
3fully compensated
4.Arterial blood gas interpretationHard
Practice values: Na⁺ 138, Cl⁻ 113, HCO₃⁻ 11 mmol/L and albumin 19 g/L. Find the albumin-corrected anion gap AGc = AG + 0.25·(40 − albumin).
Practice exercise (made-up values, not clinical guidance). An 86 kg patient is to be loaded intravenously with phenytoin sodium to reach a phenytoin level of 20 mg/L. The volume of distribution is 0.7 L/kg and the salt factor is S = 0.92. Find the loading dose LD of phenytoin sodium in mg.
86 kgVd = 0.7 L/kgC = 20 mg/LS = 0.92
LD = mg
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 6 h. What is the level C after 14 h?
R = 100 mg/hCL = 4 L/ht½ = 6 ht = 14 h
C = mg/L
7.Clinical nursingMedium
A patient with sickle cell disease is admitted with a painful crisis. What is a key early nursing priority?
AApplying cold packs to the painful areas
BGiving prompt, effective analgesia
CDelaying pain relief until a diagnosis is confirmed
DRestricting oral fluids
8.Clinical nursingHard
Four call bells ring at the same time. Which patient should the nurse see first?
AA patient asking what time they will be discharged
BA patient who has just become breathless after starting an IV antibiotic
CA patient who needs help to go to the toilet and has rung twice
DA patient asking for pain relief for chronic back pain that they rate as 3/10
3Solutions
1.Lab values & electrolytes
Answer: A · Transfusion-related acute lung injury (TRALI)
Why this is the answer
TRALI is acute lung injury within 6 hours of transfusion, caused by donor antibodies activating neutrophils in the lungs. It causes hypoxia, bilateral infiltrates and often hypotension and fever. TACO, in contrast, causes hypertension and signs of fluid overload.
Answer
A · Transfusion-related acute lung injury (TRALI)
2.Lab values & electrolytes
Answer: B · Calcium 7.2 mg/dL (1.8 mmol/L) with tingling lips
Why this is the answer
Hypocalcemia after thyroid surgery, from parathyroid injury, can progress to tetany and laryngospasm. Tingling lips are an early sign, so the nurse assesses the airway and Chvostek and Trousseau signs and reports at once. The other results are expected after surgery.Checked against:Calcium Blood Test · MedlinePlus (NIH) (2024)
Answer
B · Calcium 7.2 mg/dL (1.8 mmol/L) with tingling lips
3.Arterial blood gas interpretation
Answer: fully 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 7.43: within 7.35–7.45
PaCO₂ 22 mmHg: low (35–45)
HCO₃⁻ 14 mmol/L: low (22–26)
Compensation
fully compensated
Answer
fully compensated
4.Arterial blood gas interpretation
Answer: AGc = 19.25 mmol/L
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 = 138 − (113 + 11) = 14
Correction for albumin
14 + 0.25·(40 − 19) = 19.25 mmol/L
Answer
AGc = 19.25 mmol/L
Above about 12 after correction: a high anion gap.
5.Clinical pharmacokinetics
Answer: LD = 1309 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 = 0.7·86 = 60.2 L
Amount of active drug needed
Vd·C = 60.2·20 = 1204 mg
Loading dose
LD = 1204/(0.92·1) = 1309 mg
Answer
LD = 1309 mg
In practice the dose is rounded to a convenient amount.
6.Clinical pharmacokinetics
Answer: C = 20.04 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 = 0.693/6 = 0.1155 h⁻¹
Level after the given time
C = 25·(1 − e−0.1155·14) = 20.04 mg/L
Answer
C = 20.04 mg/L
7.Clinical nursing
Answer: B · Giving prompt, effective analgesia
Why this is the answer
A sickle cell crisis needs rapid analgesia, often opioids, within a short target time, plus hydration, oxygen if hypoxic, warmth and monitoring for complications. Cold can trigger more sickling.
Answer
B · Giving prompt, effective analgesia
8.Clinical nursing
Answer: B · A patient who has just become breathless after starting an IV antibiotic
Why this is the answer
Breathlessness soon after a new IV antibiotic suggests possible anaphylaxis, an airway and breathing emergency. The infusion should be stopped and the patient assessed immediately.
Answer
B · A patient who has just become breathless after starting an IV antibiotic