Monitoring, Troubleshooting & Emergence
Monitoring, Troubleshooting & Emergence
Clay Freeman, DNP, CRNA
Pressure Monitoring
What pressure actually represents
Airway pressure reflects the interaction of:
- Inspiratory flow
- Airway resistance
- Lung and chest wall compliance
- Breathing circuit characteristics
Pressures displayed on the ventilator are typically measured at the machine,
not directly in the trachea. Circuit compliance, gas compression, humidity,
and turbulence all influence reported values.
Clinical implication
High airway pressure does not automatically equal high alveolar pressure.
Resistance problems (bronchospasm, secretions, kinked ETT) may elevate PIP
without increasing lung stress.
Flow Monitoring
Why flow matters
Flow monitoring provides insight into airway resistance, patient effort,
and expiratory completeness.
Auto-PEEP detection
Persistent expiratory flow at end expiration indicates incomplete lung emptying
and the presence of intrinsic PEEP (auto-PEEP).
- Common in obstructive disease
- Leads to dynamic hyperinflation
- Reduces venous return
Corrective strategies
- Decrease respiratory rate
- Decrease tidal volume
- Increase expiratory time
Volume Monitoring
Delivered vs actual volume
Tidal volume displayed by the ventilator is derived from integrated flow measurements.
Errors in flow sensing lead to errors in volume estimation.
Circuit compliance, gas compression, and sensor drift can all distort reported values.
Clinical relevance
- Unexpected low VT → leaks, circuit disconnections, cuff leak
- Unexpected high VT → compliance changes, ventilator compensation issues
Waveform Analysis
Pressure–Volume Loops
Pressure–volume loops provide insight into lung compliance and recruitment.
- Steeper slope → better compliance
- Flattening → worsening compliance
Flow–Volume Loops
Flow–volume loops assist in identifying:
- Airflow obstruction
- Leaks
- Incomplete exhalation
Compliance and Resistance
Understanding the difference
- Compliance: ability of lungs and chest wall to expand
- Resistance: opposition to airflow through airways and equipment
Clinical interpretation
- ↑ PIP + normal plateau → resistance problem
- ↑ PIP + ↑ plateau → compliance problem
Ventilator Troubleshooting: A Structured Approach
When something changes, ask:
- Is oxygenation affected?
- Is ventilation affected?
- Is pressure rising?
- Did something surgical or positional change?
Common causes
- ETT obstruction or migration
- Bronchospasm
- Atelectasis
- Pneumoperitoneum
- Positioning changes
Emergence and Ventilator Weaning
Before emergence, assess:
- Is the patient breathing spontaneously?
- Has neuromuscular blockade been reversed?
- What is the current MAC?
- What are ETCO₂ and minute ventilation?
- What ventilator mode is active?
Ventilator mode considerations
- Transition from controlled to supported modes
- Ensure apnea backup is appropriate
- Avoid excessive pressure support masking inadequate effort
Recognizing readiness
- Adequate spontaneous tidal volumes
- Stable ETCO₂
- Acceptable oxygenation on reasonable FiO₂
Breath-holding physiology
PaCO₂ rise during apnea
- Awake:
- ~7 mmHg/min initially
- ~6 mmHg/min thereafter
- Anesthetized:
- ~12 mmHg first minute
- ~3.5 mmHg/min after
SCOPE GUIDE
Strategies
- Interpret trends, not isolated numbers.
- Use waveforms to confirm your assumptions.
- When in doubt, simplify and reassess.
Clinical Optimization
- Correlate ventilator data with surgical events.
- Adjust settings proactively during emergence.
- Anticipate physiologic lag after changes.
Pearls
- Ventilator alarms are early warnings, not annoyances.
- High pressure demands explanation, not suppression.
- Emergence begins long before extubation.
