Ventilators & ARDS – Core Vocabulary

Ventilator Basics: Manipulating Blood Gases

• Key Gold-Star Slide (most testable fact)
  • To raise PaO2 → increase PEEP and/or increase FiO2.

• To lower PaCO2 → increase respiratory rate (RR) and/or increase tidal volume (VT).
  • Reverse statements are also true (e.g., ↓RR → ↑PaCO2).

• Question-style practice examples
  • “If tidal volume is increased, what happens to PaCO2 — It falls.

ARDS Overview & Epidemiology

• Incidence rises with age; largely a hospital-acquired problem.

• Occurs in ~10–15 % of all ICU admissions.

• Develops in ≈20 % of mechanically-ventilated pts; ~80 % of ARDS pts need mechanical ventilation.

• Mortality approximately 30–35 % (varies by study).

Etiology / Precipitants

• Direct lung insults
  • Aspiration (common in hospital).
  • Chest trauma.
  • Ventilation-induced injury (wrong settings).

• Indirect systemic insults (hyper-inflammatory)
  • Sepsis (esp. bacterial).
  • Pancreatitis.
  • Major transfusion, shock, etc.

• Top three culprits: bacterial pneumonia, systemic sepsis, aspiration.

Normal Alveolar Fluid Balance vs Injury

• Dry, patent alveoli maintained by:

  1. Intravascular proteins → oncotic pull.

  2. Interstitial lymphatics → fluid removal.

  3. Tight inter-epithelial junctions.

• In ARDS
  • Cytokine storm → neutrophil influx.
  • Proteases & ROS damage capillary + alveolar endothelium.
  • Proteins leak → oncotic gradient lost → interstitial & alveolar edema.
  • ↓Compliance + impaired gas exchange.

• Phases of lung injury
  • Exudative (< 6 d). • Proliferative (7–14 d). • Fibrotic/chronic (> 14 d; lifelong impact for some).

• Pneumocyte reminder: Type I (95 %) = gas exchange; Type II = surfactant production.

Berlin Definition of ARDS

• Timing: new/worsening resp sxs ≤ 1 week of clinical insult.

• Imaging: bilateral opacities not fully explained by effusion, lobar collapse, or nodules.

• Origin of edema: not cardiac/volume overload (echo usually required).

• Oxygenation (on PEEP ≥ 5 cm H2O):
  P/F ratio = PaO2 / FiO2 (FiO2 as decimal)
  • Mild: 200 \le \text{P/F} \le 300.
  • Moderate: 100 \le \text{P/F} < 200.
  • Severe: \text{P/F} < 100.

Imaging Hallmarks

• CXR: diffuse, bilateral “bat-wing/butterfly” edema; often central.

• CT: widespread ground-glass & consolidation.

• Contrast with
  • Pneumonia → typically unilateral, lower lobe, focal.
  • Large pleural effusion → unilateral opaque hemithorax, absent lung markings.

Core Management Mnemonic: 3 P’s + V

1. Prone positioning.

2. PEEP↑ (optimize according to tables).

3. Paralysis (short course).

4. V = low tidal-Volume ventilation (lung-protective).

• Additional/possible rescue: ECMO.

ARDSNet & Lung-Protective Ventilation

• Landmark multicenter RCT (861 pts) → ↓mortality & ↑vent-free days.

• Low tidal volume protocol
  • V_{T}=4–8\text{ mL kg}^{-1} "Predicted Body Weight" (≈ ideal body wt).
  • Start at 8 mL/kg, down-titrate to 6 mL/kg as tolerated.
  • Target plateau pressure < 30 cm H2O.
  • Use published PEEP/FiO2 tables to reach oxygenation goals.
  • Permissive hypercapnia accepted if pH tolerable.

• Why report “ventilator-free days to 30”? → Adjusts for deaths (deceased = 0 days).

Adjunct / Rescue Vent Modes

• APRV (Airway Pressure Release Ventilation)
  • Prolonged high pressure (recruitment) with brief releases.
  • Mixed evidence; avoid in severe COPD/asthma (air trapping) & some cardiogenic shock (↓preload).

• HFOV (Oscillator)
  • Very small V_T, very high R; high mean airway pressure to keep alveoli open.
  • Better pediatric data; adult RCTs → no mortality benefit → ATS 2017 recommends against routine use.

Prone Positioning Details

• Physiology: ↓dorsal compression, ↑perfusion of dependent lung, homogenizes ventilation.

• Timing/“dose”: initiate early, maintain ≈17 h sessions for benefit.

• Absolute/major contraindications
  • Unstable spine or fractures requiring traction.
  • Severe shock/CPR in progress.
  • Elevated ICP or low CPP.
  • Recent tracheal surgery / fresh sternotomy (< 2 wk).

• Relative: late pregnancy, large abdominal wound, anterior chest tubes (kinking risk), fresh pacemaker, etc.

• Practicalities: multi-person maneuver, meticulous padding, most pts paralyzed while prone.

Neuromuscular Blockade (“Short-Course Paralysis”)

• Theoretical benefits: ↑chest wall compliance, synchrony with vent, ↓peak pressures, permissive hypercapnia ease.

• Early trials ➜ mortality benefit; recent data more mixed → current practice = 24-h trial in severe ARDS or when dyssynchrony persists despite sedation.

ECMO for Severe ARDS

• VV ECMO (lung support only) preferred; VA ECMO adds cardiac support.

• Circuit: drain venous blood → oxygenator (adds O2, removes CO_2) → return to venous system.

• Emerging data: improves oxygenation, ↑vent-free days, ↓renal failure, ↓mortality when used in experienced centers.

• Consider when P/F < 100 despite optimal conventional & adjunctive therapy.

Ethical / Practical Pearls & COVID Connections

• COVID-19 pandemic highlighted value of early proning & adherence to lung-protective strategy.

• ARDS survivors may face chronic fibrotic lung disease → long-term rehab needs.

• Resource-intense therapies (paralytics, proning teams, ECMO) raise allocation & training issues; institutional protocols critical.

Rapid-Fire Exam Pearls

• Single best answer for improving oxygenation on vent settings: ↑PEEP (if already on ≥60 % FiO2).

• First ventilator change to fight hypercapnia: ↑RR.

• Severity grading requires PEEP ≥ 5 cm H2O.

• Plateau pressure target < 30 cm H2O to avoid barotrauma.

• Predicted ≠ actual body weight; use height-based formula (see MDCalc).

• 3 P’s + V = Prone, PEEP, Paralysis, low Volume.