Respiratory Care and Oxygen Therapy: cram pack

Oxygen therapy & respiratory calculations

Nasal cannula

FiO₂ ≈ 21% + 4% × L/min (1–6 L/min)

Cylinder duration

min = (psi − 200) × factor / (L/min); D 0.16, E 0.28, H 3.14 L/psi

Minute and alveolar ventilation

V̇E = V_T·f, V̇A = (V_T − V_D)·f

Predicted body weight

men 50 + 0.91 × (cm − 152.4); women 45.5 + 0.91 × (cm − 152.4)

P/F ratio (Berlin, PEEP ≥ 5)

mild > 200–300, moderate > 100–200, severe ≤ 100 mmHg

Oxygen content

CaO₂ = 1.34·Hb·SaO₂ + 0.003·PaO₂

Oxygen delivery

DO₂ = CaO₂ × CO × 10

ROX index

(SpO₂/FiO₂)/RR

Method

1. Put every value in the units of the formula: FiO₂ and SaO₂ as fractions, PaO₂ in mmHg, Hb in g/dL.
2. For a cylinder, subtract the safe residual, multiply by the cylinder factor and divide by the flow.
3. Round cylinder times and flows down, never up.
4. For the P/F ratio, check the PEEP first, then divide PaO₂ by FiO₂ and compare with 300, 200 and 100 mmHg.
5. For ventilation, subtract the dead space for alveolar ventilation and use the predicted body weight for tidal volumes.

Arterial blood gas interpretation

Normal ranges

pH 7.35–7.45, PaCO₂ 35–45 mmHg (4.7–6.0 kPa), HCO₃⁻ 22–26 mmol/L

ROME

Respiratory: PaCO₂ opposite to pH; Metabolic: HCO₃⁻ equal to pH

Winter’s formula

PaCO₂ = 1.5·HCO₃⁻ + 8 ± 2

Metabolic alkalosis

PaCO₂ ≈ 0.7·HCO₃⁻ + 21

Anion gap

AG = Na⁺ − (Cl⁻ + HCO₃⁻) ≈ 8–12

Albumin-corrected gap

AG + 2.5·(4.0 − alb g/dL) = AG + 0.25·(40 − alb g/L)

Delta ratio

(AG − 12)/(24 − HCO₃⁻)

P/F ratio

PaO₂/FiO₂

Alveolar gas equation

PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8

Serum osmolality

2·Na⁺ + glucose + urea (2·Na⁺ + glucose/18 + BUN/2.8)

Method

1. Read the pH: below 7.35 is acidosis, above 7.45 alkalosis.
2. Find the value that explains it: PaCO₂ moving opposite to the pH is respiratory, HCO₃⁻ moving with the pH is metabolic.
3. 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.
4. In a metabolic acidosis, compare PaCO₂ with Winter’s formula, 1.5·HCO₃⁻ + 8 ± 2, and calculate the anion gap Na⁺ − (Cl⁻ + HCO₃⁻).
5. Check a high anion gap with the delta ratio and, when albumin is low, with the corrected gap.

Clinical assessment scores

Glasgow Coma Scale

GCS = E (1–4) + V (1–5) + M (1–6) = 3–15

APGAR

5 signs × 0–2 = 0–10; 7–10 reassuring, 4–6 moderately abnormal, 0–3 low

Braden scale

5 × (1–4) + 1 × (1–3) = 6–23; ≤ 18 at risk

Morse fall scale

falls 25, diagnosis 15, aid 15/30, IV 20, gait 10/20, mental status 15

NEWS2 risk

0–4 low; single 3 low–medium; 5–6 medium; ≥ 7 high

Mean arterial pressure

MAP = (SBP + 2·DBP)/3 = DBP + PP/3

Pulse pressure and shock index

PP = SBP − DBP, SI = HR/SBP

Body mass index

BMI = kg/m² = 703·lb/in²

Method

1. Find the score sheet that the question uses and its range (GCS 3–15, APGAR 0–10, Braden 6–23, Morse 0–125).
2. Score every item separately, matching the description to its level.
3. Add the item scores and check that the total lies inside the range.
4. Apply the cut-offs: for NEWS2 also look for any single parameter scoring 3.
5. For derived values use MAP = (SBP + 2·DBP)/3, PP = SBP − DBP, SI = HR/SBP and BMI = kg/m².

Med-surg: cardiovascular & respiratory

Method

1. Name the problem: pump failure, blocked artery, rhythm, clot, airway narrowing, infection or a device problem.
2. Check the danger signs first: hypoxia, hypotension, chest pain, new confusion, a cold pulseless limb.
3. Match the oxygen to the patient: 94–98% for most, 88–92% for those at risk of CO₂ retention.
4. Link each drug to its test and its risk: warfarin and INR, heparin and aPTT and platelets, diuretics and potassium.
5. Choose the safe first action, then escalate and reassess.