A mechanical ventilator is a medical device that helps move air into and out of a patient’s lungs when the patient cannot breathe well enough on their own. It matters because cells need oxygen for metabolism and must remove carbon dioxide to keep blood chemistry stable. Ventilators are used in operating rooms, intensive care units, emergency care, and during some severe lung illnesses.
They combine physics, physiology, sensors, and control systems to deliver safer, more predictable breathing support.
A ventilator pushes a controlled mixture of air and oxygen through tubing into the airway using positive pressure. Clinicians set values such as tidal volume, respiratory rate, oxygen fraction, pressure limit, and positive end expiratory pressure to match the patient’s needs. Sensors measure pressure, flow, and volume so the machine can adjust breaths and warn staff about leaks, blockages, or changing lung stiffness.
The key challenge is to support gas exchange while avoiding lung injury from excessive pressure, excessive volume, or too little pressure at the end of exhalation.
Key Facts
- Minute ventilation = tidal volume x respiratory rate
- Alveolar ventilation = (tidal volume - dead space) x respiratory rate
- Compliance = change in volume / change in pressure, C = ΔV / ΔP
- Airway resistance = pressure difference / flow, R = ΔP / Q
- FiO2 is the fraction of inspired oxygen, with room air at about FiO2 = 0.21
- PEEP keeps airway pressure above atmospheric pressure at the end of exhalation to help prevent alveolar collapse
Vocabulary
- Tidal volume
- The amount of gas delivered to or removed from the lungs in one normal ventilator breath.
- PEEP
- Positive end expiratory pressure is the pressure left in the lungs at the end of exhalation to help keep alveoli open.
- FiO2
- FiO2 is the fraction of oxygen in the gas mixture being delivered to the patient.
- Lung compliance
- Lung compliance describes how easily the lungs expand for a given change in pressure.
- Airway resistance
- Airway resistance describes how strongly the airways oppose airflow through the breathing circuit and lungs.
Common Mistakes to Avoid
- Confusing pressure with volume: pressure is the force per area pushing gas, while volume is the amount of gas delivered, and either one can be high even when the other is not.
- Ignoring dead space when estimating useful ventilation: some inhaled gas stays in tubes and conducting airways, so not all tidal volume reaches the alveoli for gas exchange.
- Assuming more oxygen is always better: high FiO2 can be lifesaving, but prolonged exposure to very high oxygen levels can contribute to oxygen toxicity and lung damage.
- Forgetting that alarms reflect patient and circuit conditions: a high pressure alarm may mean coughing, a blocked tube, or stiff lungs, while a low pressure alarm may indicate a leak or disconnection.
Practice Questions
- 1 A ventilator delivers a tidal volume of 500 mL at a respiratory rate of 12 breaths per minute. What is the minute ventilation in L/min?
- 2 A patient has a tidal volume of 450 mL, anatomical dead space of 150 mL, and respiratory rate of 16 breaths per minute. What is the alveolar ventilation in L/min?
- 3 A ventilator shows rising peak airway pressure but the set tidal volume has not changed. Explain two possible causes and how lung compliance or airway resistance could be involved.