Mechanical Ventilation, Positive Pressure
A mechanical ventilator moves air in and out of a patient’s lungs. Although the equipment serves to ventilate a patient, it doesn’t ensure adequate gas exchange. Mechanical ventilators may use either positive or negative pressure to ventilate patients.
Positive-pressure ventilators cause inspiration while increasing tidal volume (VT). The inspiratory cycles of these ventilators may vary in volume, pressure, time, or frequency. For example, a volume-cycled ventilator—the type most commonly used—delivers a preset volume of air each time, regardless of the amount of lung resistance. A pressure-cycled ventilator generates flow until the machine reaches a preset pressure regardless of the volume delivered or the time required to achieve the pressure. A time-cycled ventilator generates flow for a preset amount of time. A high-frequency ventilator uses high respiratory rates and low VT to maintain alveolar ventilation.
Equipment
Oxygen source ▪ air source that can supply 50 psi ▪ mechanical ventilator ▪ humidifier ▪ ventilator circuit tubing, connectors, and adapters ▪ condensation collection trap ▪ spirometer, respirometer, or electronic device to measure flow and volume ▪ in-line thermometer ▪ gloves and other personal protective equipment ▪ handheld resuscitation bag with reservoir ▪ suction equipment ▪ sterile distilled water ▪ equipment for arterial blood gas (ABG) analysis ▪ soft restraints, if indicated ▪ oximeter or capnography device ▪ oral care products ▪ Optional: chlorhexidine oral care product, as prescribed.
Mechanical Ventilation Glossary
Although a respiratory therapist usually monitors ventilator settings based on the doctor’s order, you should understand all of the following terms.
Assist-control mode: The ventilator delivers a preset tidal volume (VT) at a preset rate; however, the patient can initiate additional breaths, which trigger the ventilator to deliver the preset VT at positive pressure.
Continuous positive airway pressure (CPAP): This setting prompts the ventilator to deliver positive pressure to the airway throughout the respiratory cycle. It works only on patients who can breathe spontaneously.
Control mode: The ventilator delivers a preset VT at a fixed rate regardless of whether the patient is breathing spontaneously.
Fraction of inspired oxygen (FIO2): This is the amount of oxygen delivered to the patient by the ventilator. The dial or digital display on the ventilator that sets this percentage is labeled by the term oxygen concentration or oxygen percentage.
Inspiratory-expiratory (I:E) ratio: This ratio compares the duration of inspiration to the duration of expiration. The I:E ratio of normal, spontaneous breathing is 1:2, meaning that expiration is twice as long as inspiration.
Inspiratory flow rate (IFR): The IFR denotes the VT delivered within a certain time. Its value can range from 20 to 120 L/minute.
Minute ventilation or minute volume (VE): This measurement results from the multiplication of respiratory rate and VT.
Peak inspiratory pressure (PIP): Measured by the pressure manometer on the ventilator, PIP reflects the amount of pressure required to deliver a preset VT.
Plateau pressure: Airway pressure measured after a 0.5 second pause at the end of inspiration, plateau pressure indicates the stiffness of the lungs.
Positive end-expiratory pressure (PEEP): In PEEP mode, the ventilator is triggered to apply positive pressure at the end of each expiration to increase the area for oxygen exchange by helping to inflate and keep open collapsed alveoli.
Pressure support: A ventilator adjunct, pressure support delivers positive pressure, supplementing the spontaneous breath of a patient on the ventilator. It’s used to reduce the work of breathing associated with the artificial airway or increase the volume of the patient’s spontaneous breath.
Respiratory rate: The number of breaths per minute delivered by the ventilator; also called frequency.
Sensitivity setting: A setting that determines the amount of effort the patient must exert to trigger the inspiratory cycle.
Sigh volume: A ventilator-delivered breath that’s 1¼ times as large as the patient’s VT.
Synchronized intermittent mandatory ventilation (SIMV): The ventilator delivers a preset number of breaths at a specific VT. The patient may supplement these mechanical ventilations with his own breaths, in which case the VT and rate are determined by his own inspiratory ability.
Tidal volume (VT): This refers to the volume of air delivered to the patient with each cycle, usually 8 to 10 mL/kg.
Preparation of Equipment
In most facilities, respiratory therapists assume responsibility for setting up the ventilator. If necessary, however, check the manufacturer’s instructions for setting it up. In some cases, you’ll need to add sterile distilled water to the humidifier and connect the ventilator to the appropriate gas source.
Plug the ventilator into the electrical outlet and turn it on. Adjust the settings on the ventilator, as ordered. (See Mechanical ventilation glossary.) Make sure the ventilator’s alarms are set, as ordered, and that the humidifier is filled with the sterile distilled water. Attach a capnographic device to measure carbon dioxide levels to confirm placement of the endotracheal (ET) tube and detect any disconnection from the ventilator and other complications.
Implementation
Verify the doctor’s order.
If the patient isn’t already intubated, prepare him for intubation. (See “Endotracheal intubation,” page 260.)
Gather and prepare the necessary equipment.
Confirm the patient’s identity using with at least two patient identifiers according to your facility’s policy.1
When possible, explain the procedure to the patient and his family to help reduce anxiety and fear. Assure the patient and his family that staff members are nearby to provide care.
Make sure the patient is being adequately oxygenated, using manual ventilation, if necessary.
As the patient’s condition allows, perform a complete physical assessment, and draw blood for ABG analysis to establish a baseline.
Assist with intubation (if necessary) and then connect the ET tube to the ventilator. Observe for chest expansion, and auscultate for bilateral breath sounds to verify that the patient is being ventilated. Monitor the patient’s pulse oximetry.
Suction the patient using a closed-suction catheter, if necessary.
Monitor the patient’s ABG values after the initial ventilator setup (usually 20 to 30 minutes), after changes in ventilator settings, and as the patient’s clinical condition indicates to determine whether the patient is being adequately ventilated and to avoid oxygen toxicity. Be prepared to adjust ventilator settings based on ABG analysis.
Check the ventilator tubing frequently for condensation, which can cause resistance to airflow and which may also be aspirated by
the patient. As needed, drain the condensate into a collection trap. Keep the circuit closed during condensate drainage to prevent bacterial contamination.5 Don’t drain the condensate into the humidifier because the condensate may be contaminated with the patient’s secretions. Also avoid accidental drainage of condensation into the patient’s airway when moving the tubing or the patient.5
Inspect heat and moisture exchangers, and replace inserts or filters contaminated by secretions. Note humidifier settings. The heated humidifier should be set to deliver an inspired gas temperature of 91.4°F (33°C) plus or minus 3.6°F (2°C) and should provide a minimum of 30 mg/L of water vapor with routine use to an intubated patient.6
If you’re using a heated humidifier, monitor the inspired air temperature as close to the patient’s airway as possible. The inspiratory gas shouldn’t be warmer than 98.6°F (37°C) at the opening of the airway. Check that the high temperature alarm is set no higher than 98.6°F and no lower than 86°F (30°C).6 Observe the amount and consistency of the patient’s secretions.
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