Key Takeaways
- Mechanical ventilation is indicated for acute respiratory failure (PaO2 <60 mmHg on supplemental O2 or PaCO2 >50 mmHg with pH <7.25)
- Volume-controlled ventilation delivers a set tidal volume; pressure varies with compliance and resistance
- Pressure-controlled ventilation delivers a set pressure; tidal volume varies with compliance and resistance
- Assist/Control (A/C) provides a full breath for every patient trigger and a minimum guaranteed rate
- SIMV synchronizes mandatory breaths with patient effort; spontaneous breaths receive only set pressure support
- Pressure Support Ventilation (PSV) augments spontaneous breaths; patient controls rate, tidal volume, and inspiratory time
- CPAP provides continuous positive pressure during spontaneous breathing; no mandatory breaths are delivered
- BiPAP/Bilevel delivers two pressure levels: IPAP (inspiratory) and EPAP (expiratory) for non-invasive ventilation
Ventilation Fundamentals
Mechanical ventilation is arguably the most important topic for the TMC exam, accounting for approximately 30% of all questions. Understanding indications, modes, and the fundamental differences between volume and pressure ventilation is essential for success on the exam and in clinical practice.
Indications for Mechanical Ventilation
Mechanical ventilation is initiated when a patient cannot maintain adequate gas exchange despite non-invasive measures.
| Indication | Criteria |
|---|---|
| Acute hypoxemic respiratory failure | PaO2 <60 mmHg on supplemental oxygen (FiO2 ≥0.60) |
| Acute hypercapnic respiratory failure | PaCO2 >50 mmHg with pH <7.25 |
| Apnea or impending respiratory arrest | Absent or severely inadequate breathing |
| Airway protection | GCS ≤8, inability to protect airway, aspiration risk |
| Excessive work of breathing | Respiratory rate >35, accessory muscle fatigue, exhaustion |
| Post-operative | After major thoracic/abdominal surgery requiring controlled ventilation |
| Neuromuscular failure | Guillain-Barre, myasthenia gravis, high cervical spine injury |
Volume-Controlled vs. Pressure-Controlled Ventilation
| Feature | Volume-Controlled (VCV) | Pressure-Controlled (PCV) |
|---|---|---|
| Set variable | Tidal volume (VT) | Inspiratory pressure |
| Variable outcome | Airway pressure varies | Tidal volume varies |
| Flow pattern | Constant (square wave) or decelerating | Decelerating (always) |
| Guaranteed VT? | Yes — delivers set VT | No — VT depends on compliance/resistance |
| Guaranteed pressure? | No — pressure depends on compliance/resistance | Yes — delivers set pressure |
| Monitoring priority | Watch peak and plateau pressures | Watch delivered tidal volume |
| Risk | Barotrauma (high pressures) if compliance decreases | Hypoventilation if compliance decreases |
Ventilator Modes
Assist/Control (A/C) — Volume or Pressure:
- Delivers a full mechanical breath for every patient trigger AND at the set minimum rate
- If the patient does not trigger, the ventilator delivers breaths at the set rate (control mode)
- If the patient triggers, each triggered breath receives the full set VT or pressure (assist mode)
- Best for: Patients who need full ventilatory support (paralyzed, sedated, acute failure)
- Risk: Hyperventilation if the patient has a high respiratory drive (respiratory alkalosis)
SIMV (Synchronized Intermittent Mandatory Ventilation):
- Delivers a set number of mandatory breaths synchronized with patient effort
- Spontaneous breaths between mandatory breaths receive only pressure support (if set)
- Best for: Transitional mode during weaning; allows gradual resumption of spontaneous breathing
- Disadvantage: Increased work of breathing during spontaneous breaths if pressure support is low
Pressure Support Ventilation (PSV):
- Patient-triggered only — no set rate (backup rate may be set as safety feature)
- Augments each spontaneous breath with a set pressure boost
- Patient controls respiratory rate, tidal volume, and inspiratory time
- Best for: Weaning, spontaneous breathing trials, patients with adequate respiratory drive
- Requirement: Patient must have a reliable respiratory drive
CPAP (Continuous Positive Airway Pressure):
- Provides a continuous positive pressure during spontaneous breathing
- No mandatory breaths; patient breathes entirely on their own
- Non-invasive: Face mask or nasal mask (most common for sleep apnea, mild CHF)
- Invasive: Through ET tube (used as a weaning mode)
BiPAP/Bilevel Positive Airway Pressure:
- Delivers two pressure levels: IPAP (during inspiration) and EPAP (during expiration)
- Pressure support = IPAP - EPAP
- Non-invasive: Used for COPD exacerbation, cardiogenic pulmonary edema, obesity hypoventilation
- Advantages over CPAP: Provides ventilatory assistance (helps with CO2 removal) in addition to oxygenation
A patient on volume-controlled A/C ventilation has a sudden decrease in lung compliance due to worsening pneumonia. What change will the respiratory therapist observe?
Which ventilator mode delivers a FULL mechanical breath for every patient-triggered and time-triggered breath?
A patient with an acute COPD exacerbation and respiratory acidosis (pH 7.28, PaCO2 62) is responsive and cooperative but tiring. What is the MOST appropriate initial ventilatory intervention?
Match each ventilator mode to its primary clinical use.
Match each item on the left with the correct item on the right
In pressure-controlled ventilation, what happens to the delivered tidal volume if lung compliance suddenly decreases?