Key Takeaways
- Peak inspiratory pressure (PIP) reflects total airway resistance + lung compliance; normal 20-30 cmH2O
- Plateau pressure (Pplat) reflects lung compliance only; should be ≤30 cmH2O to prevent barotrauma
- High PIP with normal Pplat = increased airway resistance (bronchospasm, secretions, kinked tube)
- High PIP with high Pplat = decreased lung compliance (ARDS, pneumothorax, pulmonary edema, atelectasis)
- Auto-PEEP (intrinsic PEEP) occurs when expiration is incomplete; common in COPD and high-rate ventilation
- Auto-PEEP is detected by an expiratory flow waveform that does not return to zero baseline before the next breath
- Patient-ventilator asynchrony includes trigger asynchrony, flow asynchrony, and cycle asynchrony
- Common high-pressure alarm causes: bronchospasm, secretions, patient biting tube, coughing, kinked tube, tension pneumothorax
Ventilator Monitoring & Waveform Analysis
Continuous monitoring of the mechanically ventilated patient is a core responsibility of the respiratory therapist. The TMC exam heavily tests your ability to interpret pressure, flow, and volume waveforms, differentiate between airway resistance and compliance problems, and troubleshoot ventilator alarms.
Key Pressure Measurements
| Pressure | What It Reflects | Normal Range | Measured When |
|---|---|---|---|
| PIP (Peak Inspiratory Pressure) | Total resistance + compliance | 20-30 cmH2O | Peak of inspiration during volume ventilation |
| Pplat (Plateau Pressure) | Lung/chest wall compliance ONLY | ≤30 cmH2O | End-inspiratory hold (no flow); reflects alveolar pressure |
| PEEP (set) | End-expiratory pressure | Set by clinician | End of expiration |
| Auto-PEEP (intrinsic PEEP) | Trapped air pressure | 0 cmH2O (ideal) | End-expiratory hold maneuver |
| Driving Pressure | Pplat minus PEEP | <15 cmH2O | Correlates with ARDS outcomes; lower is better |
| Transairway Pressure | PIP minus Pplat | <10 cmH2O | Reflects airway resistance |
Differentiating Resistance vs. Compliance Problems
This is one of the MOST commonly tested ventilator concepts on the TMC exam:
High PIP + Normal Pplat = Airway Resistance Problem
The increased pressure is needed to push gas through narrowed or obstructed airways, but once the gas reaches the alveoli (Pplat), the pressure normalizes. Causes:
- Bronchospasm (asthma, COPD exacerbation)
- Secretions in the ETT or airway
- Kinked or compressed ETT
- Patient biting the ETT
- Water in the ventilator circuit
High PIP + High Pplat = Compliance Problem
Both pressures are elevated because the lungs (or chest wall) are stiffer and require more pressure to inflate. Causes:
- ARDS (reduced lung compliance)
- Tension pneumothorax
- Pulmonary edema
- Atelectasis
- Abdominal distension (ascites, obesity)
- Pleural effusion
- Right mainstem intubation (only one lung being ventilated)
Auto-PEEP (Intrinsic PEEP)
Auto-PEEP occurs when exhalation is incomplete before the next mechanical breath begins, trapping air and creating positive end-expiratory pressure above the set PEEP level.
Causes of Auto-PEEP:
- High respiratory rate (insufficient expiratory time)
- Short expiratory time (high I:E ratio, such as 1:1)
- Obstructive lung disease (COPD, asthma — prolonged expiratory time constant)
- High minute ventilation
- Inadequate expiratory flow
Detection:
- Expiratory flow waveform does not return to zero baseline before the next breath
- End-expiratory hold maneuver reveals total PEEP higher than set PEEP
- Auto-PEEP = Total PEEP - Set PEEP
Consequences of Auto-PEEP:
- Increases the work of triggering the ventilator (patient must overcome auto-PEEP + trigger threshold)
- Can cause hemodynamic compromise (decreased venous return, hypotension)
- Increases risk of barotrauma (pneumothorax)
- Leads to patient-ventilator asynchrony (missed triggers)
Treatment of Auto-PEEP:
- Decrease respiratory rate (increase expiratory time)
- Decrease tidal volume (shorter inspiration)
- Increase expiratory time (lower I:E ratio, e.g., 1:3 or 1:4)
- Bronchodilator therapy (if bronchospasm is contributing)
- Consider adding external PEEP (50-75% of measured auto-PEEP) to reduce triggering effort
Common Ventilator Alarm Causes and Responses
| Alarm | Common Causes | Immediate Actions |
|---|---|---|
| High Pressure | Bronchospasm, secretions, kinked tube, biting, coughing, pneumothorax | Assess patient, suction, check circuit, provide bronchodilator |
| Low Pressure | Circuit disconnect, cuff leak, loose connection | Reconnect circuit, check all connections, assess cuff pressure |
| Low VT / Low VE | Circuit leak, cuff deflation, patient fatigue, decreased respiratory drive | Check for leaks, assess cuff, evaluate patient |
| High VE / High Rate | Pain, anxiety, fever, metabolic acidosis, PE | Assess and treat underlying cause; consider sedation |
| Apnea | Oversedation, CNS event, neuromuscular weakness | Manually ventilate, assess patient, check trigger sensitivity |
A ventilated patient suddenly has high-pressure alarms. PIP is 48 cmH2O (was 28) and Pplat is 22 cmH2O (unchanged). This is MOST consistent with:
Auto-PEEP is detected on the ventilator waveform when:
A ventilated patient develops sudden onset of high PIP, high Pplat, absent breath sounds on the left, tracheal deviation to the right, and hypotension. The MOST likely cause is:
Which of the following are appropriate interventions for treating auto-PEEP? (Select all that apply)
Select all that apply
A ventilator low-pressure alarm is activated. The MOST likely cause is:
What is the recommended maximum plateau pressure (Pplat) to prevent ventilator-induced lung injury?