Respiratory · 8 min
Respiratory — last-night quick guide
Lung volumes, V/Q, lung-protective ventilation, OLV, COPD/asthma anesthesia.
Lung volumes & capacities
| Term | Definition | Adult value |
|---|---|---|
| TV | Tidal volume — normal breath | 500 mL |
| IRV | Inspiratory reserve — beyond TV | 3000 mL |
| ERV | Expiratory reserve — below TV | 1100 mL |
| RV | Residual volume — can't exhale | 1200 mL |
| FRC | Functional residual = ERV + RV | 2300 mL — equilibrium |
| VC | Vital capacity = TV + IRV + ERV | 4600 mL |
| TLC | Total lung = VC + RV | 5800 mL |
Rule
FRC eaten quickly
FRC drops 20% supine, another 20% in pregnancy/obesity, more in GA + paralysis. Low FRC → desaturate FAST after pre-O2 — preoxygenate with 100% × 3 min OR 8 vital-capacity breaths. Apneic oxygenation via nasal cannula 15 L during intubation = extra time.
Rule
Lung-protective ventilation
TV 6-8 mL/kg PREDICTED body weight (not actual!), PEEP 5-10, plateau <30 cmH2O, driving pressure (P-plat − PEEP) <15 cmH2O, RR 10-14 to keep PaCO2 35-45. PROVHILO: high PEEP not better in non-obese. Permissive hypercapnia OK in ARDS/asthma if pH >7.25.
OLV — one-lung ventilation
| Setting | Value | Why |
|---|---|---|
| TV | 5-6 mL/kg PBW | Smaller — only one lung |
| PEEP | 5-10 (dependent lung) | Recruit |
| FiO2 | Start 1.0, titrate down | Hypoxia common |
| RR | 12-16 | Maintain MV |
| PaO2 goal | >60 mmHg | Acceptable on OLV |
Rule
OLV hypoxia ladder
(1) Verify DLT position (fiberoptic), (2) ↑ FiO2 to 1.0, (3) PEEP 5-10 to dependent lung, (4) CPAP 5-10 to operative (non-dependent) lung, (5) intermittent two-lung ventilation, (6) clamp pulmonary artery (last resort, surgeon).
Watch out
COPD anesthetic
Avoid bronchospasm triggers (β-blockers, NSAIDs in aspirin-sensitive, cold air, histamine releasers). Use ketamine + sevoflurane (bronchodilators). Permissive hypercapnia. Slow extubation. Watch auto-PEEP (long expiratory phase needed).
Watch out
Asthma intraop bronchospasm
Deepen anesthesia (volatile, propofol bolus); albuterol 4-8 puffs via ETT; subQ epi 0.3 mg if severe; magnesium 2 g IV. Don't extubate awake on bronchospasm. Avoid β-blockers, desflurane (airway irritant).
Mnemonic — Silent chest + accessory + speech
Severe asthma — intubate when…
Severe asthma intubation indications
- 1Silent chest — no wheezing because no airflow
- 2Mental status change — CO2 narcosis, hypoxic
- 3Accessory muscle use — diaphragm fatiguing
- 4Speech limited — single words
- 5Heart rate >120 with rising lactate
- 6Exhaustion — paradoxical breathing
- 7Drop in PEFR <25% predicted
Ketamine is the induction agent of choice — bronchodilator + maintains drive. Avoid β-blockers; permissive hypercapnia + low rate to avoid breath-stacking.