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 ( $V_{E}$ ): $R R \times T V$ . Determines $P_{a} C O_{2}$ .

Capnography

Continuous, noninvasive monitoring of end-tidal carbon dioxide ( $E tC O_{2}$ ) concentration over time.
Reflects ventilation (elimination of $C O_{2}$ ), perfusion (transport of $C O_{2}$ to lungs), and metabolism ( $C O_{2}$ production).
Normal range: 35–45 mmHg.

Peak Inspiratory Pressure (PIP): Max pressure during inspiration. Reflects airway resistance + lung compliance.
Plateau Pressure ( $P_{pl a t}$ ): Pressure during inspiratory hold (no airflow). Reflects lung compliance/alveolar elastic recoil only.
Pressure Differential Analysis:
- $↑$ PIP / $\leftrightarrow$ $P_{pl a t}$ ( $↑$ Resistance):
  - Causes: Mucus plug, bronchospasm, biting ETT, kinked tube.
- $↑$ PIP / $↑$ $P_{pl a t}$ ( $↓$ Compliance): c
  - Causes: PTX, pulmonary edema, ARDS, pneumonia, right mainstem intubation, pleural effusion, abdominal compartment syndrome.
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: $↓$ RR, $↓$ TV, or $↑$ inspiratory flow rate to $↑$ expiratory time.

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.

Barotrauma: PTX, pneumomediastinum due to high pressures ( $P_{pl a t} > 30$ cm $H_{2} O$ ).
VAP (Ventilator-Associated Pneumonia): Occurs $> 48$ hrs post-intubation. Dx via semi-quantitative culture.
Cardiovascular: $↑$ Intrathoracic pressure $\to$ $↓$ venous return (preload) $\to$ hypotension (esp. in hypovolemic pts).
Oxygen Toxicity: Free radical damage if $F_{i} O_{2} > 60%$ for prolonged periods.

Criteria:
- Cause of RF improving/resolved.
- Hemodynamically stable (minimal/no vasopressors).
- Adequate oxygenation ( $P_{a} O_{2} \geq 60$ on $F_{i} O_{2} \leq 40%$ and PEEP $\leq 8$ ).
- Intact mentation/cough/gag.
RSBI (Rapid Shallow Breathing Index): $R R / T V (L)$ . If $< 105$ , high likelihood of successful weaning.

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