6 Mechanical Ventilation

Why and how support works

Mechanical ventilation supports a person while the underlying cause of breathing failure—such as asthma, COPD, pneumonia, or another illness—is treated. It may support oxygenation, carbon-dioxide removal, or both. Decisions to start or change support depend on the whole clinical picture, not a single measurement.

During natural breathing, pressure changes draw air into the lungs; a ventilator instead pushes gas in under positive pressure. Oxygenation, or getting oxygen into the blood, is influenced mainly by inspired oxygen concentration, written as $\mathrm{FiO_2}$, and positive end-expiratory pressure, written as $\mathrm{PEEP}$.

Ventilation, or removing carbon dioxide, depends mainly on alveolar ventilation, which is affected by tidal volume, breathing frequency, and dead space. Minute ventilation is calculated as:

Here, $V_T$ is tidal volume. Common ventilator settings include $\mathrm{FiO_2}$, $\mathrm{PEEP}$, respiratory rate, tidal volume or inspiratory pressure, and inspiratory flow or time. PEEP helps keep alveoli open at the end of expiration, but excessive pressure can impair venous return or overdistend the lungs.

Ventilator modes

Ventilator modes describe how breaths are delivered. In volume-targeted modes, the ventilator targets a set tidal volume, while airway pressure varies with lung mechanics. In pressure-targeted modes, it targets inspiratory pressure, while tidal volume can vary as airway resistance or lung stiffness changes.

Assist-control provides a minimum rate and also supports breaths triggered by the patient. Pressure support assists patient-triggered breaths without setting a mandatory rate, so it is often used during spontaneous breathing trials.

Noninvasive and invasive support

Noninvasive ventilation (NIV) supplies positive pressure through a fitted mask or similar interface, without an endotracheal tube. CPAP provides one continuous pressure. Bilevel NIV uses higher pressure during inspiration than during expiration, supporting ventilation as well as oxygenation.

NIV can help selected patients with acute respiratory failure, including many with COPD exacerbation and respiratory acidosis. It requires close reassessment. It may be unsafe or ineffective when a patient cannot protect the airway, has worsening consciousness or hemodynamic instability, or cannot manage secretions. A trial of NIV must not delay needed intubation.

Invasive ventilation delivers support through an endotracheal tube or tracheostomy. It may be needed when noninvasive support fails, or when a patient cannot protect the airway or sustain breathing. A typical lung-protective approach bases tidal volume on predicted body weight rather than actual weight. Clinicians monitor delivered tidal volume and pressures, and individualize settings to the disease and the patient's response.

Monitoring and alarm response

Assess the patient and ventilator together; displayed values cannot replace bedside assessment. Regular assessment includes work of breathing, comfort, mental status, chest movement, breath sounds, oxygen saturation, and hemodynamic status.

Monitor delivered tidal volume, respiratory rate, minute ventilation, $\mathrm{FiO_2}$, $\mathrm{PEEP}$, and relevant ventilator waveforms. Use blood gases when indicated to assess oxygenation, carbon dioxide, and pH. Also check endotracheal-tube position and security, circuit connections, humidification, and the skin under masks or tube holders.

Peak pressure and, when indicated, plateau pressure provide information about the respiratory system. Rising peak pressure can reflect increased airway resistance, such as from bronchospasm or secretions, or reduced lung compliance. Plateau pressure is measured during an inspiratory hold in a passive patient; when airflow stops, it better reflects pressure in the respiratory system.

Auto-PEEP is trapped air from incomplete exhalation. Expiratory flow that has not returned to baseline before the next breath can be a clue. Auto-PEEP is particularly important in obstructive diseases such as asthma or COPD.

For a high-pressure alarm, promptly assess the patient, tubing, and airway for causes such as coughing, biting or kinking, secretions, bronchospasm, or reduced lung compliance. For a low-pressure or low-volume alarm, check for disconnection, leaks, or displacement of the airway. Treat alarms as signals to assess, not as problems to silence without investigation.

Complications and safeguards

Potential complications include ventilator-associated lung injury from excessive stretch or repeated alveolar opening and collapse; pneumothorax; and hypotension from increased chest pressure. Prolonged high $\mathrm{FiO_2}$ can cause oxygen-related lung injury. An artificial airway can be associated with infection, airway injury, and ventilator-associated pneumonia. Immobility also increases risks such as weakness, pressure injury, and blood clots.

Risk reduction includes lung-protective settings, using the lowest $\mathrm{FiO_2}$ that achieves the prescribed oxygenation goal, appropriate airway and oral care, minimizing unnecessary sedation, mobility when appropriate, and daily assessment of readiness to reduce or stop support.

Sudden deterioration is an emergency. Assess the patient immediately and check the airway, circuit, oxygen supply, and ventilator. Sudden desaturation, hypotension, or rising pressures can indicate airway obstruction, disconnection, Auto-PEEP, or pneumothorax. Get urgent help and follow local emergency procedures; only trained clinicians should provide manual ventilation or change life-support settings.

Mechanical ventilation supports oxygenation and/or carbon-dioxide removal; it does not treat the underlying disease. Safe care combines bedside assessment with monitoring of oxygenation, ventilation, delivered volumes, pressures, and alarms, while watching for complications and reassessing the need for support.

• Positive end-expiratory pressure (PEEP)