3.3 Ventilator Monitoring & Waveform Analysis
Key Takeaways
- Peak inspiratory pressure (PIP) reflects airway resistance PLUS compliance; normal range is roughly 20–30 cmH2O in volume ventilation
- Plateau pressure (Pplat), measured with an inspiratory-hold, reflects compliance alone and should stay ≤30 cmH2O
- High PIP with normal Pplat means an airway RESISTANCE problem (bronchospasm, secretions, kinked or bitten tube, water in circuit)
- High PIP with high Pplat means a COMPLIANCE problem (ARDS, tension pneumothorax, pulmonary edema, atelectasis, right mainstem intubation)
- Transairway pressure (PIP − Pplat) isolates resistance; driving pressure (Pplat − PEEP) tracks ARDS injury risk
- Auto-PEEP (intrinsic PEEP) is trapped gas from incomplete exhalation; detect it when the expiratory flow waveform fails to return to zero before the next breath
- Treat auto-PEEP by lengthening expiratory time: lower the rate, lower VT, bronchodilate, and add applied PEEP at ~50–75% of the measured auto-PEEP
- High-pressure alarms come from resistance/compliance problems; low-pressure alarms come from circuit disconnects, leaks, or cuff deflation
The Two Pressures You Must Separate
Continuous monitoring is a core RT responsibility, and the TMC exam tests it through pressures and waveforms. Peak inspiratory pressure (PIP) is the maximum pressure during gas delivery; it lumps together airway resistance and lung compliance. Plateau pressure (Pplat) is measured during a brief inspiratory-hold (no flow), so it strips out resistance and reflects compliance — alveolar distending pressure — alone.
| Measurement | Reflects | Target | How obtained |
|---|---|---|---|
| PIP | Resistance + compliance | 20–30 cmH2O | Peak of inspiration (VCV) |
| Pplat | Compliance only | ≤30 cmH2O | Inspiratory-hold maneuver |
| Auto-PEEP | Trapped-gas pressure | 0 (ideal) | Expiratory-hold maneuver |
| Driving pressure | Lung stress in ARDS | <15 cmH2O | Pplat − PEEP |
| Transairway pressure | Airway resistance | <10 cmH2O | PIP − Pplat |
Resistance vs. Compliance — The Marquee Concept
This single comparison generates more exam questions than any other monitoring topic. When the high-pressure alarm sounds, do an inspiratory-hold and read Pplat:
High PIP + NORMAL Pplat → airway RESISTANCE problem. Extra pressure is needed to drive gas past a narrowed or blocked airway, but once gas reaches the alveoli the distending pressure is normal. Causes and fixes:
- Bronchospasm (asthma, COPD) → bronchodilator
- Secretions / mucus plug in the tube → suction
- Kinked, bitten, or migrated endotracheal tube → reposition, bite block
- Water condensate in the circuit → drain the circuit
High PIP + HIGH Pplat → lung/chest-wall COMPLIANCE problem. Both pressures rise because the lungs themselves are stiffer. Causes:
- ARDS, pulmonary edema, atelectasis, pneumonia
- Tension pneumothorax (an emergency)
- Abdominal distension (ascites, obesity, gastric insufflation)
- Right mainstem intubation — only one lung is being inflated
Worked example: PIP jumps from 28 to 48 cmH2O. If Pplat is unchanged at 22, suspect a resistance problem (suction/bronchodilate). If Pplat also climbed from 18 to 38, suspect a compliance problem (rule out tension pneumothorax first).
Auto-PEEP (Intrinsic PEEP)
Auto-PEEP is gas trapped when exhalation is cut short by the next mandatory breath, raising end-expiratory pressure above the set PEEP. It is the classic complication of COPD, asthma, and fast or high-volume ventilation.
- Causes: rate too high, expiratory time too short (I:E approaching 1:1), obstructive disease with a long expiratory time constant, high minute ventilation.
- Detection: the expiratory flow waveform does not return to the zero baseline before the next inspiration; an expiratory-hold then shows total PEEP > set PEEP. Auto-PEEP = total PEEP − set PEEP.
- Consequences: missed triggers and asynchrony (the patient must overcome auto-PEEP just to trigger), hypotension from reduced venous return, and barotrauma.
- Treatment: lengthen expiratory time — lower the rate, lower VT, extend I:E to 1:3 or 1:4, treat bronchospasm, and add applied PEEP at ~50–75% of measured auto-PEEP to ease triggering.
Alarm Triage
| Alarm | Common causes | Immediate action |
|---|---|---|
| High pressure | Bronchospasm, secretions, kinked/bitten tube, cough, pneumothorax | Assess patient, suction, check/drain circuit, bronchodilate; if compliance, rule out pneumothorax |
| Low pressure / low VT | Circuit disconnect, cuff leak, loose connection | Reconnect, trace every connection, check cuff pressure |
| High rate / high VE | Pain, anxiety, fever, metabolic acidosis, pulmonary embolism | Treat the cause; do not just sedate over it |
| Apnea | Oversedation, CNS event, neuromuscular weakness | Manually ventilate, assess, verify trigger sensitivity |
When any alarm cannot be resolved quickly, disconnect and manually ventilate with a resuscitation bag on 100% oxygen while you troubleshoot — patient safety precedes machine diagnosis.
A Disciplined Troubleshooting Sequence
When the high-pressure alarm sounds, resist the urge to silence it and instead run a fixed sequence the exam expects. Look at the patient first — color, chest rise, and symmetry of breath sounds tell you in seconds whether you are dealing with a disconnected lung (pneumothorax, mainstem migration) or a comfortable patient with a kinked line. Then perform an inspiratory-hold to read the plateau pressure, because that single maneuver sorts every high-pressure cause into the resistance bin or the compliance bin and points to the fix.
If you cannot quickly find or correct the problem, the safe default is to take the patient off the ventilator and manually ventilate with a bag-valve device on 100% oxygen; the resistance you feel in the bag itself is diagnostic — a stiff, hard-to-squeeze bag confirms a compliance or obstruction problem at the patient, while an easy bag that meets no resistance points back at a leak or disconnect in the circuit.
Waveforms add a second layer of intelligence. The pressure-time curve in volume control shows a scooped, concave rise when flow is set too low for the patient's demand (flow starvation), and a late upward bow when the lung is overdistended at end-inspiration. The flow-time curve is where auto-PEEP hides, and it is also where you watch for double-triggering — two stacked breaths from a single neural effort, common in patients whose inspiratory time is set shorter than their own neural inspiratory time. The volume-time curve that fails to return to baseline reveals a circuit or cuff leak.
Tying these together, a well-prepared candidate can look at a vignette describing a waveform plus two pressures and name both the mechanism and the corrective action without ever seeing the patient — which is exactly the skill the NBRC is probing in the critical-care items.
A ventilated patient suddenly alarms with PIP 48 cmH2O (was 28) while Pplat is 22 cmH2O (unchanged). This is most consistent with:
Auto-PEEP is detected on the ventilator waveform when:
A ventilated patient abruptly develops high PIP and high Pplat, absent left-sided breath sounds, tracheal deviation to the right, and hypotension. The most likely cause is:
A ventilator low-pressure alarm activates. The most likely cause is:
Which interventions reduce auto-PEEP? (Select all that apply.)