Ventilation by mechanical means is most commonly thought of as being a life supportive device. Ventilators are used by people of all ages, from premature infants to adults, who have any number of health problems that for one reason or another impair their ability to breathe normally. Mechanical ventilation can be used for such purposes as relieving the work involved with breathing in order to decrease myocardial and systemic oxygen consumption for patients who have both respiratory and cardiac failure. A diminished work of breathing also helps to decrease intracranial pressure making mechanical ventilation of great aid when used in persons with brain injuries.
Many people still think of ventilators as something exclusive to hospitals. While it is true that most ventilators are used in hospitals and other health-care settings, an increasing number of them can be found in use in the home setting.
The search for a mechanical aid to ventilation is not a new one. During the mid-1800s devices resembling steam cabinets and phone booths were pressed into use to maintain breathing by rhythmically decreasing the air pressure inside the machine. One well-known device that applied this negative-pressure principle was the iron lung, which was widely used in the United States from the late 1920s into the 1950s, particularly for polio patients. They worked. These original ventilators used the force of negative pressure to remove and replace gas from the ventilator chamber. Rather than connecting to an artificial airway, these ventilators enclosed the torso, or at times the entire 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 gas into the lungs. The cessation of the negative pressure caused the chest wall to fall and exhalation to occur. While it is an advantage that these ventilators did not require insertion of an artificial airway, they were noisy and made nursing care, difficult. This type of mechanical ventilation support device needs to be large because of the way they function. Negative pressure ventilators greatly restrict patient movement and reduce quality of life. Due to these factors, this type of mechanical ventilator has fallen into disfavor as a routine choice.
Iron Lung in use, a true-life saver during its heyday
(J. D. Johnston and the Brook Hospital Historical Site)
It was in the late 1950s that positive pressure ventilators were introduced. Like their predecessors, these modern ventilators serve to deliver breaths of oxygen-enriched air to the body and remove carbon dioxide. Most ventilators today are computer-controlled, functioning in complex ways to produce positive-pressure ventilation that more closely matches the individual patient’s breathing needs.
Mechanical ventilation is used when a person is unable to breathe adequately on his or her own. The ventilator can either completely take over respiratory function, or it can be used to support the patient’s own respiratory efforts. Respiration is the movement of atmospheric components during gas exchange. The use of a machine to assist with that complex function is an amazing feat. The process of respiration is the act in which the lungs absorb oxygen molecules and distribute them into the blood stream. Then previously gathered carbon dioxide that was created as a result of cellular metabolism is expelled. This gas exchange takes place in the small air sacs of the lungs, known as alveoli. In the course of one day, some 8,000 to 9,000 liters of air are breathed in through the nose or mouth and 8,000 to 10,000 liters of blood are pumped through the lungs by the heart (Kurtzweil, 1999).
During normal breathing, a volume of air is inhaled through the airways (mouth/nose, pharynx, larynx, trachea, and bronchial tree) into millions of tiny gas exchange sacs (the alveoli) deep within the lungs. Once there it mixes with the carbon dioxide rich gases coming from the blood. It is then exhaled back through the same airways to the atmosphere. Normally we expect this cyclic pattern to repeat at a breathing rate, or frequency, of about 12 breaths a minute (breaths/min) when we are at rest, with a higher resting rate for infants and children. It is normal for the breathing rate to increase with exercise or excitement.
One of the major factors determining whether breathing is producing enough gas exchange to keep a person alive is the ventilation, or active air movement. Ventilation is expressed as the volume of gas entering or leaving, the lungs in a given amount of time.
~ Ventilation can be calculated by multiplying the volume of gas, either inhaled or exhaled during a breath (called the tidal volume), times the breathing rate ~
This means that if we wanted to develop a machine to help a person breathe, it would have to be able to produce a tidal volume and a breathing rate which, when multiplied together, produce enough ventilation. Yet not too much ventilation, to supply the gas exchange needs of the body. During normal breathing the body selects a combination of a tidal volume that is large enough to clear the dead space and add fresh gas to the alveoli, and a breathing rate that assures the correct amount of ventilation is produced.
As it turns out, research has shown that it is possible, using specialized equipment, to keep a person alive with breathing rates that range from zero (steady flow into and out of the lungs) up to frequencies in the 100's of breaths per minute. Over this frequency range, convection and diffusion take part to a greater or lesser extent in distributing the inhaled gas within the lungs. As the frequency rates are increased, the tidal volumes that produce the required ventilation get smaller and smaller.
The goal of ventilation is aiding individuals to regain and/or optimize their own health. It is best to stick with the original design of how our bodily system should function. That limits us to two approaches to mechanical ventilation. Positive pressure ventilators involve the application of positive pressure into the airway opening. Negative pressure ventilators involve the withdrawal of pressure in order to expand the chest cavity and draw in air. The terms positive or negative pressure are in relation to the atmospheric pressure.
Inspiration is the process of moving atmospheric volume into the lungs. It can be triggered in a positive pressure ventilator by either an act of the recipient or by the machine. There are four basic types of positive pressure ventilators in use.
1. Volume cycled
2. Pressure cycled
3. Flow cycled
4. Time cycled
Volume cycled ventilators are designed to deliver a preset tidal volume, and then allow passive expiration. This is ideal for those who tend to have bronchospasm since the same tidal volume is delivered regardless of the amount of airway resistance. This type of ventilator is the most common currently used in a critical care environment.
Pressure cycled ventilators deliver gases at a preset pressure, then allow passive expiration. The benefit of this is a decreased risk of lung damage from high inspiratory pressures. The disadvantage of these ventilators is that the patient may not receive the complete tidal volume particularly if they have poor lung tissue 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 deliver a breath until a preset flow rate is achieved during inspiration. Time cycled ventilators deliver a breath over a preset time period. These types of ventilators are not used as much as the volume cycled and pressure cycled ventilators.
Positive pressure ventilators usually require an artificial airway, such as an endotracheal or tracheostomy tube. The following is a discussion of the types of artificial airways.
Types of Artificial Airways
Combitube – This is often used in field resuscitation. It contains two lumens, one that intubates the esophagus, and one that intubates the trachea. It is easy to insert quickly, and it eliminates the risk of intubating the esophagus rather than the trachea. The manual resuscitation bag is attached to the lumen that inflates the lungs, and the esophageal balloon is inflated to decrease the risk of aspiration. The combitube should only be used for emergency intubation, and should be replaced with an endotracheal tube (ETT) as soon as possible.
Laryngeal mask airway (LMA) – This is used for emergent intubation or in situations where ETT intubation has failed. The LMA looks like an ETT with an inflatable, silicone rubber collar at the bottom end. This collar surrounds and covers the supraglottic area, providing a continuous upper airway.
Endotracheal tube (ETT) – This is the most common artificial airway used for short-term (up to three weeks) airway management or mechanical ventilation. It is a long semi-rigid tube of varying diameters with an inflatable silicone rubber collar at the bottom. When inserted into the trachea it establishes an intact airway. Insertion of an ETT is indicated for airway maintenance, secretion control, oxygenation and ventilation.
Tracheostomy (trach) tube – This is the preferred artificial airway for patients requiring long-term mechanical ventilation (longer than three weeks). It is also indicated for other conditions such as upper airway obstruction or malformation, failed or repeated intubations, complications from endotracheal intubations, glottic incompetence, sleep apnea, or chronic inability to clear secretions. Resistance to airflow is less with a trach tube than an ETT because it is wider, shorter, and less curved. This reduces the work of breathing for the patient, and allows easy removal of secretions.
Tracheostomy for long-term mechanical ventilation
The artificial airway and the use of forceful pressure to move oxygen into a patient’s lungs cause more lung tissue damage then the negative pressure ventilators. The trade off of mobility and flexible make it the ventilator of choice in current practice. More recently, noninvasive positive-pressure ventilators have been developed that allow for gas exchange through a face or nose mask. This less invasive measure to assist oxygen exchange is becoming steadily more popular in those areas where it can serve as an alternative.
As many as 1.5 million Americans use mechanical ventilator support every year. Ventilators help premature babies breathe until their lungs are more fully developed. They help patients recovering from the effects of anesthesia and sedatives given before and during surgery. Patients with heart disease whose failing hearts cause fluid buildup in the lungs, accident victims, and other people with emergency health needs also may need ventilators. The following is a list of general indication for using a mechanical ventilator.
General Indications for Mechanical Ventilation
The following is a list of conditions for which ventilator support might be used.
Some individuals depend on ventilators for a long time, needing their assistance in order to perform all of their breathing. If otherwise healthy and not forced by their condition to be bedridden, these people can use portable ventilators that allow them access to the world and to be free of the confines of a health-care facility or their homes.
The FDA classifies ventilators into three groups: hospital, transport and home-use ventilators. The hospital ventilator units are typically sophisticated devices. Some ventilators have central monitoring capability, which sends patients' respiratory data directly to the nurse's station. These units generally come with a number of built-in alarm systems to alert caregivers to act on impending or immediate life-threatening changes in patients' respiration.
Mechanical ventilators come in a variety of shapes, sizes and colors.
Even the control panels differ from digital to
manual knobs and switches. (Marshall, 2001)
Manufacturers are increasing development and production of ventilators for step-down areas of hospitals. While these individuals may still require full or partial ventilatory support, their conditions have otherwise stabilized, making the continuous monitoring of intensive care unnecessary. Some health-care providers are even opening subacute care facilities for long-term patients who no longer need round-the-clock care but still require a high degree of medical support. Ventilators designed for subacute breathing assisted clients offer full ventilation capabilities but do not come with all the specialized features of full-conventional ventilators, for example they might offer fewer alarms and setting variations.
Transport ventilators are designed to withstand the excessive impacts or vibration that can occur in rescue vehicles and the higher levels of electromagnetic interference from electronic sources, such as two-way radios used for communications.
Home-use ventilators operate similarly to those designed for subacute care but are still smaller and lighter. Many types of these units can readily fit on a wheelchair and are battery operated. They also tend to be less expensive, ranging from $5,000 to $12,000 per unit, compared with $15,000 to $35,000 for a hospital unit. (Kurtzweil, 1999)
This growing market force is provoking great changes. Due to the interest from long-term portable ventilator users, manufacturers are continually striving to improve portable units by making them lighter, smaller, and able to operate longer on a single battery than current models. Some of the newer models that are still in the design stage may also offer built-in telemedicine capabilities. These ventilators would gather information on a person’s respiratory function and transport it, over computer or telephone to health-care providers at distant sites. This will allow health care workers to assess and provide care to these patients without actually seeing them in person. It is also anticipated that such capabilities will aid in reducing medical costs.
Patients with sleep apnea may be treated at home with positive airway pressure (PAP) devices. PAP therapy delivers air under pressure to a patient’s airway, with or without supplemental oxygen. A physician orders the specific amount of pressure in centimeters of water (cmH 2 O) based on the results of an individualized assessment of the patient’s needs during an overnight sleep study. There has been a recent increase in the use of PAP therapy in a variety of settings due to a heightened awareness of the syndrome and availability of sleep diagnostic centers that can confirm the diagnosis of sleep apnea.
Basic Systems used in the home includes a flow generator, tubing, headgear, and either a mask or nasal prongs and humidifier. An oxygen concentrator is optional. A flow generator provides some compressed air that is delivered to the patient’s airway. The following is a picture of a facemask airway home ventilator system.
Face Mask airway home ventilator system (MDA, 2003)
The following is a list of the three general types of PAP equipment are used.
Many ventilators are now computerized and have a user-friendly control panel. To activate various modes, settings, and alarms, the appropriate key need only be pressed. There are windows on the face panel that show the current settings and the alarm parameters. Some ventilators have dials instead of computerized keys, for example, the smaller, portable ventilators used for transporting patients.
Usually a Respiratory Therapist will be available to set up a ventilator and attend routine maintaince. As with most computerized equipment, the ventilator will go through a self-test to ensure that it is working properly when turned on. The ventilator tubing that is the component of connection to the user should be changed according to the manufacturer’s recommendations and/or the facility’s policy. That timeframe can range form 24 hours to one week. The bacteria filter component should be checked for occlusions or tears and the water/humidity traps and filters checked for condensation or contaminants. If the ventilator has a moisture trap, it should be emptied and cleaned every 24 hours as a part of routine maintenance and as needed.
Sterile suctioning of the patient’s pharynx and trachea to remove excess lung secretions may at times be necessary. The needed suction source, sterile disposable suction catheters and gloves must all be readily on hand for use as needed. Some ventilator tubing has inline suction tubing. It is now considered best practice to suction the airway only when needed. The long-standing practice of routine (every 2 hours) suctioning is no longer recommended, nor is instilling saline down the tracheal tube before suctioning in order to loosen secretions (Tatsoto, 2000).
Ventilator settings are, of course, individualized to each person’s special needs. The unit is designed to actively monitor many components of the user’s respiratory status. Alarms and parameters can be set to warn of changes.
Respiratory Rate – The number of breaths delivered by the machine per minute. Usually from 4 to 20 breaths per minute.
Tidal Volume – The volume of gas delivered during each ventilator breath. Determined by 5 to 15 cc/kg body weight.
Fractional Inspired Oxygen (FIO2) – The amount of oxygen delivered by the machine to the user. 21% to 100%.
Pressure Limit – The amount of force the machine can use to deliver each breath. Typically 10-20 cm H2O above peak inspiratory pressure with a maximum of 35 cm H2O.
Inspiratory/Expiratory Ratio (I:E) – The length of inspiration compared to the length of expiration. Typically 1:2 or 1:1.5
The term Mode designates the manner in which the machine will ventilate its user in relation to each person’s own respiratory efforts. There is no lack of ways to individualize ventilation delivery, as there is a mode for nearly every situation, plus many that can be used in conjunction with each other.
Control Ventilation (CV) - Delivers a preset volume or pressure regardless of the user’s own inspiratory efforts
Assist Control Ventilation (A/CV) - Delivers breath both in response to independent effort as well as if the user fails to do so within a preset amount of time
Synchronous Intermittent Manual Ventilation (SIMV) – Forced ventilator breaths are synchronized with the user’s respiratory effort
Pressure Support Ventilation (PSV) - Preset pressure that augments the user’s own inspiratory effort and decreases the work of breathing
Positive End Expiratory Pressure (PEEP) - Positive pressure is applied at the end of expiration
Constant Positive Airway Pressure (CPAP) - Similar to PEEP but used only with spontaneously breathing persons
High Frequency Ventilation (HFV) - Delivers small amounts of gas at a rapid rate (60-100 breaths/minute) and requires sedation/paralysis to use
Inverse Ratio Ventilation (IRV) – The I:E ratio is reversed in order to allow longer inspiration time. This requires sedation/ paralysis to use.
Mechanical ventilation has its problems. The risk of infection, particularly in a health-care setting, is high for ventilator-assisted patients. In fact, the primary cause of hospital-acquired pneumonia has been found to be mechanical ventilation. This adds up. The average cost of a ventilator-associated pneumonia is about $29,369 per patient (Byers, 2000). Disconnected tubing, tubing with too much humidity causing fluid to partially block airflow, and the build-up of the user’s own secretions are all potential dangers when using a mechanical ventilator.
Ventilators are electrical equipment and must be plugged in. Most have battery back up, but this is not designed for long-term use. Ventilators should always be plugged into an outlet that will receive generator power if there is an electrical outage. Ventilators are a method of life-support. If the ventilator stops working, that person’s life may be in jeopardy. Whenever a ventilator is in use, manual intermittent forced air capability should be right there. An ambu bag, anesthesia bag, manual resuscitation bag, or some other form of rubberized manual bag and valve device achieves this function. Home care ventilators should have a manual bag and valve set that the user themselves can utilize, if that person has the ability. This type of bag acts as a collapsible reservoir unit for oxygen or atmospheric gas, and when attached to the lumen of the artificial airway can be squeezed and released to imitate the pressure of respiratory movements.
Less serious is a problem with skin irritation from the masks of the noninvasive ventilators. This problem can interfere with patients' willingness to use the ventilators, thus preventing patients from getting the level of care they need.
Mechanical ventilation is a great stride forward in the management of disease states. By allowing the body to rest, to better focus its energy on healing, many lives have been saved over the past decades. These devices are able to meet that need, and the technology is progressively advancing to better handle the quality of life issues that go with requiring ventilator assistance. By coming to understand this valuable tool, healthcare providers can be more aware of the options that are present for the people they care for.
“Caring for the Person with ALS.” Muscular Dystrophy Association (MDA). September 2003.
Byers et al. “Analysis of Factors Related to the Development of Ventilator-Associated Pneumonia: Use of Existing Databases.” American Journal of Critical Care. 2000.
Chatburn, R. “Understanding Mechanical Ventilators.” Vent World dot Com Publication. 2003.
Johnston, J. “The Brook General Hospital.” Hospital Histories. 2003.
Kurtzweil, P. “When Machines Do the Breathing.” FDA Consumer Magazine. September 1999.
Marshall, M. “How to Survive the Surgical ICU.” University of California San Diego School of Medicine. 2001.
Tatsoto et al. “Weaning Your Patient from Mechanical Ventilation.” Nursing 2000. October 2000.