What Is Mechanical Ventilation?
Mechanical ventilation, in the healthcare setting or home, helps patients breathe by assisting the inhalation of oxygen into the lungs and the exhalation of carbon dioxide. Depending on the patient’s condition, mechanical ventilation can help support or completely control breathing.
Essential information about ventilators:
Rate, volume, sensitivity, flow, limits and measures of breathing
The clinician determines appropriate ventilator settings according to the condition and needs of the patient
Ventilators offer a variety of modes that determine how and when breath is delivered to the patient
Audible or visual alarms help monitor ventilator function and settings
Discussions about ventilation use the following terms:
Rate of breaths
Also referred to as respiratory rate, breathing rate, or frequency; can be a ventilator setting or respiratory status the ventilator tracks as the patient breathes
Volume of breaths
Usually expressed in milliliters (mL); frequently referred to as tidal volume or “VT”; can be a ventilator setting or respiratory status the ventilator tracks as the patient breathes
Sensitivity of breathing
Concerns how much inspiratory or “trigger” effort is required from the ventilator to recognize that the patient is trying to inhale; this can refer to a ventilator setting, but can also be used to describe the ventilator’s responsiveness to the patient’s breathing effort
Flow of breaths
Also referred to as peak flow or inspiratory flow; usually describes a ventilator setting but can be respiratory status the ventilator tracks as the patient breathes
Controls or limits on breathing volume, pressure or time
The ventilator can limit or control the inspiratory pressure, volume or time during breath delivery
Measured or mandatory breaths
Also referred to as mechanical breaths, describes breaths initiated by the ventilator delivered according to a consistent volume or pressure
Lung compliance and airway resistance
Lung compliance refers to the elasticity, stretch or ease with which the lung expands to receive volume
Airway resistance refers to the resistance encountered as oxygen enters the airway and to how easily the lung lets in air
The clinician determines appropriate ventilator settings according to the condition and needs of the patient. The settings include:
The measure of oxygen the ventilator is delivering during inspiration.
The number of breaths delivered by the ventilator per minute.
The volume of gas/air delivered with each breath.
This alerts the ventilator when to recognize the start of a patient’s spontaneous breath (or breathing effort). When the ventilator recognizes the patient’s effort, it triggers a response, either to provide a mechanical breath or to support a spontaneous one.
The flow of gas/air (flow rate) used to deliver each mechanical breath to the patient.
Inspiratory and expiratory times
The total time required for one complete respiratory cycle. Typically, patients are comfortable with an expiratory time two to three times longer than the inspiratory time.
The manner in which the ventilator ends the inspiratory phase of the breath and allows the patient to exhale. Ventilator breaths can be volume cycled, time cycled or flow cycled.
This setting restricts the volume, pressure or time air is delivered to the patient during the inspiratory phase.
Ventilator manufacturers offer combinations of modes and breath types that characterize how and when a breath is delivered to the patient.
Ventilator modes most commonly found on the home care ventilator include:
All breaths delivered by the ventilator will control either volume or pressure. The ventilator delivers the same measured breath every time, whether the breath is patient initiated or ventilator initiated, based on the rate setting.
Continuous positive airway pressure ventilation (CPAP)
All breathing is initiated and sustained by the patient. The ventilator delivers no machine (mandatory) breaths. The ventilator controls the delivered oxygen concentration and delivers as much flow and volume as necessary to meet the patient’s inspiratory demands. The patient decides the tidal volume and number of spontaneous breaths.
This mode also allows the patient to breathe at a continuous, elevated airway pressure that can improve oxygenation (see PEEP/CPAP).
The ventilator can also apply positive pressure during spontaneous inspirations taken during CPAP mode to reduce the patient’s work to breathe.
Synchronized intermittent mandatory ventilation (SIMV)
The ventilator synchronizes machine breath delivery with the patient’s spontaneous breath efforts. This mode is a combination of set mandatory machine breaths synchronized with the patient’s own spontaneous breaths.
Pressure control ventilation (PCV or PC)
This is a type of mandatory breath that can be used in either A/C or SIMV modes and targets a specific pressure during inspiration. The delivered flow rate varies according to the patient’s demand and own lung characteristics, such as lung compliance and airway resistance. The delivered tidal volume also varies with changes in compliance and resistance. In PC mode, the clinician also sets a specific time for inspiration or inspiratory time.
Pressure support ventilation (PSV or PS)
This is a type of spontaneous breath that can be used in either CPAP or SIMV modes and targets a set inspiratory pressure, much like PC. But the PS inspiration ends as the lung gets full and the delivered flow decreases to a specific valve set by the clinician. The patient decides the respiratory rate and inspiratory time as well as the flow rate and tidal volume.
Positive end expiratory pressure (PEEP)
Mechanical positive pressure is applied at the end of exhalation to prevent the lungs from emptying completely and returning to a “zero” reading. The benefit of positive pressure at the end of exhalation is increased lung volume for improved oxygenation.
Ventilators offer audible and visual alarms to alert the caregiver to changes in key patient and ventilator functions and settings. These alarms prompt a timely response, safeguarding the patient and proper functioning of the ventilator. Note: It is critical that whenever an alarm occurs, the caregiver evaluates the patient first before checking the ventilator.
High airway pressure alarms
These are also referred to as high inspiratory pressure (HIP) alarms. This alarm setting also provides a pressure limit function.
If the ventilator pressure reaches the set limit, an audible or visual alarm activates. The ventilator will temporarily stop the inspiration, allowing the patient to exhale immediately.
The alarm detects abnormally high inspiratory pressure and may activate in response to:
- Kinks in the patient circuit or tracheostomy tube
- Water in the ventilator circuit
- Increased mucus or other secretions blocking the airway
- Bronchospasm, or narrowing of the patient’s airway
- Coughing, gagging or “fighting” the ventilator breath
- Pneumonia or other changes in lung condition that affect airway resistance or lung compliance
After determining the condition that triggered the alarm, do whatever is needed to fix the situation, which could be suctioning the patient, repositioning the tube or adjusting the alarm settings. If the patient’s condition has worsened because of pneumonia or other illness, contact the physician promptly.
Low airway pressure alarms
These are also referred to as low inspiratory pressure (LIP) alarms.
These alarms are in response to:
- Decrease in lung pressure due to a change in lung or patient condition
- Increase in patient demand for oxygen because of agitation, pain or discomfort
- Change in lung compliance or airway resistance
Low pressure alarms can also activate if there’s an air leak out of the breathing circuit caused by:
- Patient-ventilator disconnection
- Improper inflation of the tracheostomy tube cuff
- Poorly fitting noninvasive nose masks or prongs
- Loose circuit and tubing connections
- Water condensate in the circuit
High and low rate alarms
A low or high rate alarm will trigger an audible and/or visual alert. An agitated or fatigued patient can have an increase in respiratory rate. Sedated patients or patients with impaired neuromuscular function can have a decreased respiratory rate.
High and low volume alarms
In addition to a high respiratory rate, high volume alarms may indicate increased patient demand for gas/air because of pain, anxiety or improper ventilator settings.
Low volume alarms are typically caused by air leaks. In pressure-based ventilation, these alarms may indicate worsening airway resistance or lung compliance.