Ventilators

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Ventilators

Definition

A ventilator is a device used to provide assisted respiration and positive-pressure breathing.

Purpose

Ventilators are used to provide mechanical ventilation for patients with respiratory failure who cannot breathe effectively on their own. They are also used to decrease myocardial gas consumption or intracranial pressure, provide stability of the chest wall after trauma or surgery, and when a patient is sedated or pharmacologically paralyzed.

Description

Different types of ventilators can be programmed to provide several modes of mechanical ventilation. A brief overview of each type and mode follows.

Negativepressure ventilators

The original ventilators used negative pressure to remove and replace gas from the ventilator chamber. Examples of these include the iron lung, the Drinker respirator, and the chest shell. Rather than connecting to an artificial airway, these ventilators enclosed the body from the outside. As gas was pulled out of the ventilator chamber, the resulting negative pressure caused the chest wall to expand, which pulled air into the lungs. The cessation of the negative pressure caused the chest wall to fall and exhalation to occur. While an advantage of these ventilators was that they did not require insertion of an artificial airway, they were noisy, made nursing care difficult, and the patient was not able to ambulate.

Positive-pressure ventilators

Postivepressure ventilators require an artificial airway (endotracheal or tracheostomy tube) and use positive pressure to force gas into a patient's lungs. Inspiration can be triggered either by the patient or the machine. There are four types of positive-pressure ventilators: volume-cycled, pressure-cycled, flow-cycled, and time-cycled.

VOLUME-CYCLED VENTILATORS. This type delivers a preset tidal volume then allows passive expiration. This is ideal for patients with acute respiratory distress syndrome (ARDS) or bronchospasm, since the same tidal volume is delivered regardless of the amount of airway resistance. This type of ventilator is the most commonly used in critical care environments.

PRESSURE-CYCLED VENTILATORS. These ventilators deliver gases at a preset pressure, then allow passive expiration. The benefit of this type is a decreased risk of lung damage from high inspiratory pressures, which is particularly beneficial for neonates who have a small lung capacity. The disadvantage is that the tidal volume delivered can decrease if the patient has poor lung compliance and increased airway resistance. This type of ventilation is usually used for short-term therapy (less than 24 hours). Some ventilators have the capability to provide both volume-cycled and pressure-cycled ventilation. These combination ventilators are also commonly used in critical care environments.

FLOW-CYCLED VENTILATORS. Flow-cycled ventilators deliver oxygenation until a preset flow rate is achieved during inspiration.

TIME-CYCLED VENTILATORS. Time-cycled ventilators deliver oxygenation over a preset time period. These types of ventilators are not used as frequently as the volume-cycled and pressure-cycled ventilators.

Modes of ventilation

Mode refers to how the machine will ventilate the patient in relation to the patient's own respiratory efforts. There is a mode for nearly every patient situation; plus, many different types can be used in conjunction with each other.

CONTROL VENTILATION (CV). CV delivers the preset volume or pressure regardless of the patient's own inspiratory efforts. This mode is used for patients who are unable to initiate a breath. If it is used with spontaneously breathing patients, they must be sedated and/or pharmacologically paralyzed so they don't breathe out of synchrony with the ventilator.

ASSIST-CONTROL VENTILATION (A/C) OR CONTINUOUS MANDATORY VENTILATION (CMV). A/C or CMV delivers the preset volume or pressure in response to the patient's inspiratory effort, but will initiate the breath if the patient does not do so within a preset amount of time. This mode is used for patients who can initiate a breath but who have weakened respiratory muscles. The patient may need to be sedated to limit the number of spontaneous breaths, as hyperventilation can occur in patients with high respiratory rates.

SYNCHRONOUS INTERMITTENT MANDATORY VENTILATION (SIMV). SIMV delivers the preset volume or pressure and preset respiratory rate while allowing the patient to breathe spontaneously. The vent initiates each breath in synchrony with the patient's breaths. SIMV is used as a primary mode of ventilation as well as a weaning mode. (During weaning, the preset rate is gradually reduced, allowing the patient to slowly regain breathing on their own.) The disadvantage of this mode is that it may increase the effort of breathing and cause respiratory muscle fatigue.(Breathing spontaneously through ventilator tubing has been compared to breathing through a straw.)

POSITIVE-END EXPIRATORY PRESSURE (PEEP). PEEP is positive pressure that is applied by the ventilator at the end of expiration. This mode does not deliver breaths but is used as an adjunct to CV, A/C, and SIMV to improve oxygenation by opening collapsed alveoli at the end of expiration. Complications from the increased pressure can include decreased cardiac output, lung rupture, and increased intracranial pressure.

CONSTANT POSITIVE AIRWAY PRESSURE (CPAP). CPAP is similar to PEEP, except that it works only for patients who are breathing spontaneously. The effect of CPAP (and PEEP) is compared to inflating a balloon but not letting it completely deflate before inflating it again. The second inflation is easier to perform because resistance is decreased. CPAP can also be administered using a mask and CPAP machine for patients who do not require mechanical ventilation but who need respiratory support (for example, patients with sleep apnea).

PRESSURE SUPPORT VENTILATION (PSV). PS is preset pressure which augments the patient's spontaneous inspiration effort and decreases the work of breathing. The patient completely controls the respiratory rate and tidal volume. PS is used for patients with a stable respiratory status and is often used with SIMV during weaning.

INDEPENDENT UNG VENTILATIO (ILV). This method is used to ventilate each lung separately in patients with unilateral lung disease or a different disease process in each lung. It requires a double lumen endotracheal tube and two ventilators. Sedation and pharmacologic paralysis are used to facilitate optimal ventilation and increase comfort for the patient on whom this method is used.

HIGH FREQUENCY VENTILATION (HFV). HFV delivers a small amount of gas at a rapid rate (as much as 60-100 breaths per minute). This is used when conventional mechanical ventilation would compromise hemodynamic stability, during short-term procedures, or for patients who are at high risk for lung rupture. Sedation and/or pharmacologic paralysis are required.

INVERSE RATIO VENTILATION (IRV). The normal inspiratory:expiratory ratio is 1:2, but this is reversed during IRV to 2:1 or greater (the maximum is 4:1). This method is used for patients who are still hypoxic, even with the use of PEEP. Longer inspiratory time increases the amount of air in the lungs at the end of expiration (the functional residual capacity) and improves oxygenation by reexpanding collapsed alveoli. The shorter expiratory time prevents the alveoli from collapsing again. This method requires sedation and therapeutic paralysis because it is very uncomfortable for the patient.

Ventilator settings

Ventilator settings are ordered by a physician and are individualized for the patient. Ventilators are designed to monitor most components of the patient's respiratory status. Various alarms and parameters can be set to warn healthcare providers that the patient is having difficulty with the settings.

RESPIRATORY RATE. The respiratory rate is the number of breaths the ventilator will deliver to the patient over a specific time period. The respiratory rate parameters are set above and below this number, and an alarm will sound if the patient's actual rate is outside the desired range.

TIDAL VOLUME. Tidal volume is the volume of gas the ventilator will deliver to the patient with each breath. The usual setting is 5-15 cc/kg. The tidal volume parameters are set above and below this number and an alarm sounds if the patient's actual tidal volume is outside the desired range. This is especially helpful if the patient is breathing spontaneously between ventilator-delivered breaths since the patient's own tidal volume can be compared with the desired tidal volume delivered by the ventilator.

OXYGEN CONCENTRATION (FIO₂). Oxygen concentration is the amount of oxygen delivered to the patient. It can range from 21% (room air) to 100%.

INSPIRATORY:EXPIRATORY (I:E) RATIO. As discussed above, the I:E ratio is normally 1:2 or 1:1.5, unless inverse ratio ventilation is desired.

PRESSURE LIMIT. Pressure limit regulates the amount of pressure the volume-cycled ventilator can generate to deliver the preset tidal volume. The usual setting is 10-20 cm H₂O above the patient's peak inspiratory pressure. If this limit is reached the ventilator stops the breath and alarms. This is often an indication that the patient's airway is obstructed with mucus and is usually resolved with suctioning. It can also be caused by the patient coughing, biting on the endotracheal tube, breathing against the ventilator, or by a kink in the ventilator tubing.

FLOW RATE. Flow rate is the speed with which the tidal volume is delivered. The usual setting is 40-100 liters per minute.

SENSITIVITY/TRIGGER. Sensitivity determines the amount of effort required by the patient to initiate inspiration. It can be set to be triggered by pressure or by flow.

SIGH. The ventilator can be programmed to deliver an occasional sigh with a larger tidal volume. This prevents collapse of the alveoli (atelectasis) which can result from the patient constantly inspiring the same volume of gas.

Operation

Many ventilators are now computerized and have a user-friendly control panel. To activate the various modes, settings, and alarms, the appropriate key need only be pressed. There are windows on the face panel which show settings and the alarm values. Some ventilators have dials instead of computerized keys, e.g., the smaller, portable ventilators used for transporting patients.

The ventilator tubing simply attaches to the ventilator on one end and to the patient's artificial airway on the other. Most ventilators have clamps that prevent the tubing from draping across the patient. However, there should be enough slack so that the artificial airway isn't accidentally pulled out if the patient turns.

Ventilators are electrical equipment so they must be plugged in. They do have battery back up, but this is not designed for long-term use. It should be ensured that they are plugged into an outlet that will receive generator power if there is an electrical power outage. Ventilators are a method of life-support. If the ventilator should stop working, the patient's life will be in jeopardy. There should be a bag-valve-mask device at the bedside of every patient receiving mechanical ventilation so they can be manually ventilated if needed.

Maintenance

When mechanical ventilation is initiated, the ventilator goes through a self-test to ensure it is working properly. The ventilator tubing should be changed every 24 hours and another self-test run afterwards. The bacteria filters should be checked for occlusions or tears and the water traps and filters should be checked for condensation or contaminants. These should be emptied and cleaned every 24 hours and as needed.

Health care team roles

The respiratory therapist is generally the person who sets up the ventilator, does the daily check described above, and changes the ventilator settings based on the physician's orders. The nurse is responsible for monitoring the alarms and the patient's respiratory status. The nurse is also responsible for notifying the respiratory therapist when mechanical problems occur with the ventilator and when there are new physician orders requiring changes in the settings or the alarm parameters. The physician is responsible for keeping track of the patient's status on the current ventilator settings and changing them when necessary.

Training

Training for using and maintaining ventilators is often done via hands-on methods. Critical care nurses usually have a small amount of class time during which they learn the ventilator modes and settings. They then apply this knowledge while working with patients on the unit under the supervision of a nurse preceptor. This preceptorship usually lasts about six weeks (depending upon the nurse's prior experience) and includes all aspects of critical care. Nurses often learn the most from the respiratory therapists, since ventilator management is their specialty.

Respiratory therapists complete an educational program that specifically focuses on respiratory diseases, and equipment and treatments used to manage those diseases. During orientation to a new job, they work under the supervision of an experienced respiratory therapist to learn how to maintain and manage the ventilators used by that particular institution. Written resources from the company that produced the ventilators are usually kept in the respiratory therapy department for reference.

Physicians generally do not manage the equipment aspect of the ventilator. They do, however, manage the relation of the ventilator settings to the patient's condition. They gain this knowledge of physiology during medical school and residency.

KEY TERMS

Alveoli— Saclike structures in the lungs where oxygen and carbon dioxide exchange takes place.

Bag-valve-mask device— Device consisting of a manually compressible bag containing oxygen and a one-way valve and mask that fits over the mouth and nose of the patient.

Endotracheal tube— Tube inserted into the trachea via either the oral or nasal cavity for the purpose of providing a secure airway.

Hemodynamic stability— Stability of blood circulation, including cardiac function and peripheral vascular physiology.

Hypoxic— Abnormal deficiency of oxygen in the arterial blood.

Intracranial pressure— The amount of pressure exerted inside the skull by brain tissue, blood, and cerebral-spinal fluid.

Peak inspiratory pressure— The pressure in the lungs at the end of inspiration.

Pharmacologically paralyzed— Shortterm paralysis induced by medications for a therapeutic purpose.

Tracheostomy tube— Surgically created opening in the trachea for the purpose of providing a secure airway. This is used when the patient requires long-term ventilatory assistance.

Weaning— The process of gradually tapering mechanical ventilation and allowing the patient to resume breathing on their own.