Mechanical ventilation

Monitoring

• Basic Parameters & Definitions
  • $F_i{O}_2$: Fraction of inspired oxygen (Goal: $<60\%$ as soon as possible to avoid $O_2$ toxicity).
  • PEEP: Positive End-Expiratory Pressure. Maintains alveolar patency.
  • Tidal Volume (TV): Volume per breath (Goal: 6-8 mL/kg ideal body weight; 6 mL/kg in ARDS).
  • Respiratory Rate (RR): Primary tool for $C{O}_2$ clearance.
  • Minute Ventilation ($V_E$): $RR\times TV$. Determines $P_{a}C{O}_2$.
• Airway Pressure Monitoring (High-Yield)
  • Peak Inspiratory Pressure (PIP): Max pressure during inspiration. Reflects airway resistance + lung compliance.
  • Plateau Pressure ($P_{plat}$): Pressure during inspiratory hold (no airflow). Reflects lung compliance/alveolar elastic recoil only.
  • Pressure Differential Analysis:
    • $\uparrow$ PIP / $\leftrightarrow$ $P_{plat}$ ($\uparrow$ Resistance):
      • Causes: Mucus plug, bronchospasm, biting ETT, kinked tube.
    • $\uparrow$ PIP / $\uparrow$ $P_{plat}$ ($\downarrow$ Compliance): c
      • Causes: PTX, pulmonary edema, ARDS, pneumonia, right mainstem intubation, pleural effusion, abdominal compartment syndrome.
• ABG Monitoring & Adjustments
  • Oxygenation ($P_a{O}_2$):
    • Adjust via $F_i{O}_2$ and PEEP.
    • Goal: $P_a{O}_2$ 55-80 mmHg or $S_p{O}_2$ 88-95%.
  • Ventilation ($P_{a}C{O}_2$ / pH):
    • Adjust via RR and TV.
    • Goal: pH 7.35-7.45.
    • In ARDS: “Permissive hypercapnia” (pH > 7.20) is acceptable to avoid barotrauma from high TV.
• Auto-PEEP (Intrinsic PEEP)
  • Definition: Incomplete expiration causing air trapping (common in COPD/Asthma).
  • Detection: Measured via expiratory hold maneuver. Look for “flow-volume” loop failing to return to zero before next breath.
  • Management: $\downarrow$ RR, $\downarrow$ TV, or $\uparrow$ inspiratory flow rate to $\uparrow$ expiratory time.
• Complications of Mechanical Ventilation
  • Barotrauma: PTX, pneumomediastinum due to high pressures ($P_{plat}>30$ cm $H_{2}O$).
  • VAP (Ventilator-Associated Pneumonia): Occurs $>48$ hrs post-intubation. Dx via semi-quantitative culture.
  • Cardiovascular: $\uparrow$ Intrathoracic pressure $\to$ $\downarrow$ venous return (preload) $\to$ hypotension (esp. in hypovolemic pts).
  • Oxygen Toxicity: Free radical damage if $F_i{O}_2>60\%$ for prolonged periods.
• Weaning Readiness (SBT - Spontaneous Breathing Trial)
  • Criteria:
    • Cause of RF improving/resolved.
    • Hemodynamically stable (minimal/no vasopressors).
    • Adequate oxygenation ($P_a{O}_2\ge 60$ on $F_i{O}_2\le 40\%$ and PEEP $\le 8$).
    • Intact mentation/cough/gag.
  • RSBI (Rapid Shallow Breathing Index): $RR/TV(L)$. If $<105$, high likelihood of successful weaning.