CAPNOGRAPHY

CO₂ identified in expired gas by spectrographic properties (using IR, Raman, or mass spectroscopy)

Hazards of Capnography:

• Old in-line IR detectors can heat up, causing facial thermal injury

• May fail to detect disconnection if machine disabled by secretions, or alarms disabled

CAPNOGRAPHS

Figure 8-2. Normal capnograph (normal expired CO₂ waveform).

Figure 8-3. Normal and abnormal flow volume loops.

MECHANICAL VENTILATION: PROTECTIVE STRATEGIES

Current Strategies to Prevent Volutrauma, Barotrauma, Atelectrauma, Tracheal Ischemia & O₂ Toxicity

• Alveolar overdistention (volutrauma), rather than excessively ↑ airway pressure (barotrauma), may be more injurious to lung

• Smaller tidal volumes (6 mL/kg) are recommended with greater respiratory rate

• Higher PaCO₂ levels allowed (permissive hypercapnia) in treating acute lung injury & ARDS

Specific Settings

• TV 6 mL/kg (prevents volutrauma: trauma from overdistention of alveoli)

• Plateau pressure <30 cm H₂O (prevents barotrauma: trauma from excessive press.)

• PEEP >6–10 cm H₂O (prevents atelectatic trauma: repeated alveolar closure at end-expiration)

• FiO₂ <50% to prevent O₂ toxicity

BIPAP VS. CPAP

• CPAP = delivery of continuous positive air pressure throughout respiratory cycle

• BiPAP = two levels of positive air pressure, inspiratory (triggered by insp effort) & expiratory (present throughout the rest of the respiratory cycle) (expiratory pressure is lower to facilitate exhalation)

DISCONTINUING MECHANICAL VENTILATION

SIMV Plus PS is a Common Weaning Mode in Many ICUs

• Start with full support (IMV ≈ 10), plus PS 10–15 cm H₂O, PEEP 5–10

• Decrease by 1–2 breaths per minute till IMV = 0

• Now wean PS 1–2 cm H₂O at a time until PS/PEEP is 10/5 or 5/5 cm H₂O or lower (for more deconditioned pts, you may need to go as low as PS/PEEP = 2/5)

• At the same time gradually reduce FiO₂ according to SaO₂ or PaO₂ (more sensitive to oxygenation changes)