CBET Ventilators and Respiratory Therapy Equipment — Questions and Answers
Question 1: A ventilator is set to Volume-Controlled Ventilation (VCV) at 500 mL tidal volume. The high peak pressure alarm activates with every breath. Plateau pressure measured during an inspiratory hold is normal. What is the most likely cause?
- Decreased lung compliance (stiff lungs)
- Increased airway resistance (e.g., bronchospasm or secretions) (Correct answer)
- Auto-PEEP from insufficient expiratory time
- A leak in the ventilator circuit
Correct answer: Increased airway resistance (e.g., bronchospasm or secretions)
In VCV, peak pressure reflects both airway resistance and lung compliance, while plateau pressure (measured during no-flow inspiratory hold) reflects compliance only. A high peak pressure with a NORMAL plateau pressure indicates increased airway resistance — the lungs can accept the volume, but the airway is obstructed. If compliance were decreased, both peak AND plateau would be elevated.
Question 2: What is the primary physiological purpose of Positive End-Expiratory Pressure (PEEP) in a mechanically ventilated patient?
- To increase minute ventilation by extending inspiratory time
- To prevent alveolar collapse (atelectasis) at end-expiration and improve oxygenation (Correct answer)
- To reduce the work of breathing by lowering peak inspiratory pressure
- To deliver supplemental oxygen above the set FiOâ‚‚
Correct answer: To prevent alveolar collapse (atelectasis) at end-expiration and improve oxygenation
PEEP maintains positive airway pressure throughout the expiratory phase, keeping alveoli open and preventing end-expiratory collapse. This recruits more alveolar surface area for gas exchange, improving oxygenation (PaOâ‚‚). It does not change minute ventilation directly, reduce peak pressure, or affect FiOâ‚‚ delivery.
Question 3: During annual PM on a mechanical ventilator, which test is MOST critical for patient safety regarding oxygen delivery?
- Verification of the humidifier thermostat accuracy
- Calibration and accuracy check of the oxygen sensor/analyzer (Correct answer)
- Testing the exhalation valve seal
- Measuring peak inspiratory flow rate
Correct answer: Calibration and accuracy check of the oxygen sensor/analyzer
The oxygen sensor (typically a galvanic fuel cell) degrades over time and can drift, causing the ventilator to deliver incorrect FiOâ‚‚. Delivering too little Oâ‚‚ causes hypoxia; too much causes oxygen toxicity. Calibrating and verifying oxygen sensor accuracy is a mandatory safety check. While exhalation valve and humidifier checks are important, an FiOâ‚‚ error can be immediately life-threatening.
Question 4: In Pressure-Controlled Ventilation (PCV), what variable is directly set by the clinician and remains constant despite changes in the patient's lung mechanics?
- Tidal volume delivered
- Peak inspiratory flow rate
- Inspiratory pressure above PEEP (Correct answer)
- Minute ventilation
Correct answer: Inspiratory pressure above PEEP
In PCV, the ventilator delivers gas until the set inspiratory pressure is reached and then maintains that pressure for the set inspiratory time. The pressure is constant regardless of lung compliance or resistance. Tidal volume, however, will vary — if lungs become stiffer, less volume is delivered at the same pressure. This is the fundamental difference from VCV.
Question 5: A ventilator's continuous high-pressure alarm is sounding. The patient is stable on manual (bag) ventilation. After checking for obvious circuit kinks and confirming the ETT is not kinked, what should the BMET check next on the ventilator itself?
- The flow trigger sensitivity setting
- The exhalation valve for sticking or obstruction (Correct answer)
- The inspiratory filter for occlusion
- The PEEP valve spring tension
Correct answer: The exhalation valve for sticking or obstruction
If the exhalation valve sticks closed or becomes obstructed, exhaled gas cannot escape and pressure builds continuously in the circuit during exhalation, triggering the high-pressure alarm persistently. The inspiratory filter causing high pressure would be more intermittent (only during inspiration), and flow trigger/PEEP settings affect baseline parameters rather than causing sustained pressure spikes.
Question 6: What is the primary purpose of a heated wire ventilator circuit compared to a standard circuit?
- To reduce the internal diameter of the circuit to increase flow velocity
- To prevent rainout (condensation) in the circuit tubing by keeping temperature above the dew point (Correct answer)
- To sterilize inspired gas before delivery to the patient
- To measure inspiratory temperature for feedback control of FiOâ‚‚
Correct answer: To prevent rainout (condensation) in the circuit tubing by keeping temperature above the dew point
Humidified gas cools as it travels through circuit tubing, dropping below the dew point and condensing (rainout). Water pooling in the circuit can obstruct flow, alter delivered tidal volume, and harbor pathogens. Heated-wire circuits warm the tubing wall to prevent condensation. They do not sterilize gas, change FiOâ‚‚, or alter circuit diameter.
CBET Ventilators and Respiratory Therapy Equipment
A ventilator is set to Volume-Controlled Ventilation (VCV) at 500 mL tidal volume.
The high peak pressure alarm activates with every breath.
Plateau pressure measured during an inspiratory hold is normal.
What is the most likely cause?