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 has a sodium of 152 mEq/L. Which findings would the nurse expect?
AWeight gain with swollen ankles
BBradycardia with peaked T waves
CMuscle twitching with a positive Trousseau’s sign
DThirst, dry mucous membranes and restlessness
2.Lab values & electrolytesHard
The nurse teaches a patient with thrombocytopenia (platelets 25 × 10³/µL) about home safety. Which statement shows a need for further teaching?
A"I'll use an electric razor to shave"
B"I'll use a soft-bristled toothbrush"
C"I'll take aspirin if I get a headache"
D"I'll blow my nose gently, not forcefully"
3.Arterial blood gas interpretationMedium
A practice gas shows PaCO₂ 30 mmHg (4.0 kPa). Use the rule that in acute respiratory alkalosis HCO₃⁻ falls by 2 mmol/L for every 10 mmHg that PaCO₂ is below 40, starting from HCO₃⁻ 24 mmol/L, to find the expected HCO₃⁻.
A practice gas shows a metabolic alkalosis with HCO₃⁻ 31 mmol/L. Use expected PaCO₂ = 0.7·HCO₃⁻ + 21 to find the expected PaCO₂.
HCO₃⁻ 31 mmol/L
PaCO₂ = mmHg
5.Clinical pharmacokineticsMedium
Practice exercise (made-up values, not clinical guidance). After an IV dose of gentamicin, the level is 6 mg/L at 1 h and 1.5 mg/L at 7 h. Assume first-order elimination. Find the half-life t½.
C1 = 6 mg/L (1 h)C2 = 1.5 mg/L (7 h)
t½ = h
6.Clinical pharmacokineticsHard
Practice exercise (made-up values, not clinical guidance). A theophylline infusion is started without a loading dose. The half-life in this patient is 8 h. How long t does it take to reach 50% of the steady-state level?
t½ = 8 h50% Css
t = h
7.Clinical nursingMedium
An arterial blood gas shows a low pH, a high PaCO2 and a normal bicarbonate. What does this show?
ARespiratory acidosis
BMetabolic alkalosis
CMetabolic acidosis
DRespiratory alkalosis
8.Clinical nursingHard
A severely hypothermic patient is in cardiac arrest. Why is resuscitation usually continued until they are rewarmed?
AHypothermia always causes brain death quickly
BAdrenaline works better in cold patients
CCold patients cannot be defibrillated at all, so CPR is the only possible treatment
DThe cold protects the brain, so recovery may be possible after long resuscitation
3Solutions
1.Lab values & electrolytes
Answer: D · Thirst, dry mucous membranes and restlessness
Why this is the answer
Hypernatremia usually means a water deficit, causing thirst, dry mouth, restlessness, agitation and, if severe, seizures. It is corrected slowly to avoid cerebral edema.
Answer
D · Thirst, dry mucous membranes and restlessness
2.Lab values & electrolytes
Answer: C · "I'll take aspirin if I get a headache"
Why this is the answer
Aspirin irreversibly inhibits platelet function and raises the bleeding risk further in thrombocytopenia, so it and other NSAIDs are avoided unless prescribed. An electric razor, gentle nose blowing and a soft toothbrush reduce minor bleeding.
Answer
C · "I'll take aspirin if I get a headache"
3.Arterial blood gas interpretation
Answer: HCO₃⁻ = 22 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.
Change in PaCO₂
|30 − 40| = 10 mmHg → 1 × 10 mmHg
Expected HCO₃⁻
HCO₃⁻ = 24 − 2·1 = 22 mmol/L
Answer
HCO₃⁻ = 22 mmol/L
4.Arterial blood gas interpretation
Answer: PaCO₂ = 42.7 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
0.7·31 + 21 = 42.7 mmHg
Answer
PaCO₂ = 42.7 mmHg
The rise has a limit: PaCO₂ rarely goes above about 55 mmHg from compensation alone.
5.Clinical pharmacokinetics
Answer: t½ = 3 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.
Elimination rate constant
k = ln(6/1.5)/6 = 1.386/6 = 0.231 h⁻¹
Half-life
t½ = 0.693/0.231 = 3 h
Answer
t½ = 3 h
6.Clinical pharmacokinetics
Answer: t = 8 h
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.
Rate constant
k = 0.693/8 = 0.08664 h⁻¹
Time to the given fraction of steady state
t = −ln(1 − 0.5)/0.08664 = 0.6931/0.08664 = 8 h
Answer
t = 8 h
That is 1 half-lives.
7.Clinical nursing
Answer: A · Respiratory acidosis
Why this is the answer
A raised PaCO2 with a low pH means the lungs are not removing enough CO2, for example from hypoventilation. A normal bicarbonate suggests it is acute.
Answer
A · Respiratory acidosis
8.Clinical nursing
Answer: D · The cold protects the brain, so recovery may be possible after long resuscitation
Why this is the answer
Hypothermia reduces the brain's oxygen needs, and good recoveries have occurred after long resuscitation. Death is usually not confirmed until the patient has been rewarmed, unless injuries are clearly not survivable.
Answer
D · The cold protects the brain, so recovery may be possible after long resuscitation