ventilators and how they work

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Ventilators and How They WorkSource: The American Journal of Nursing, Vol. 80, No. 12 (Dec., 1980), pp. 2202-2205Published by: Lippincott Williams & WilkinsStable URL: http://www.jstor.org/stable/3462506 .

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Ventilators

And How They Work Any patient with a condition that alters the pattern or process of ventilation may require ventilatory assistance. This includes patients, such as Mr. Bryan, who have lost their normally negative intrapleural pressure because of a pneumothorax or flail chest. In this instance, a positive pressure ventilator makes ventilation possible.

Patients with adult respiratory distress syndrome (ARDS) may also need ventilatory assistance. Damage to the alveolar-capillary membrane, impaired surfactant production, and progressive congestive atelectasis occur in ARDS and cause parts of the lungs to become increasingly stiff and noncompliant and, therefore, harder to ventilate. When there are regional areas of the lungs that are hypoventilated or atelectat- ic the total surface area available for gas exchange is reduced, and blood is shunted back to the heart without being oxygenated. Mechanical ventilation with positive end-expiratory pressure (PEEP) helps to open these alveoli and keep them open.

Patients, such as Ms. Jones with COPD, must work hard to force air out of their chronically overinflated and inelastic lungs. At times, the extra work and oxygen needed to sustain labored breathing far exceed the oxygen taken in by the extra effort. A ventilator greatly de- creases this work load.

Any patient who has had tho- racic or abdominal surgery, such as Mr. Robertson, might consciously suppress ventilation because of the incisional discomfort that accom- panies breathing. Obese or debili- tated patients may also hypoventi- late postoperatively. Short-term ventilatory support may provide

prophylaxis against this, especially in the patient who is at high risk.

Finally, some patients' respiratory regulatory mechanisms are not functioning normally. This may be due to fever, increased intracranial pressure, neuromuscular diseases, or severe neurological depression from drugs, carbon dioxide narcosis, or acid-base disturbances. In patients with these problems, the ventilator initiates and controls ventilation.

Nurses caring for patients who are being mechanically ventilated must understand the following:

* Why the patient requires ventilatory assistance

* How the particular ventilator with its special settings alters the patient's physiology and ventilatory pattern and control

* What dangers and possible complications accompany the use of the ventilator

* The nursing assessments and interventions necessary for compre- hensive patient care.

There are a variety of mechanical ventilators available for patients who need them. One distinction in classifying respirators is between negative and positive pressure ventilators.

Negative pressure ventilators mimic the normal mode of ventilation wherein air is "sucked" into the lungs. An example is the iron lung. Enclosed within a vacuum chamber, the chest wall is pulled out- ward. Ambient air rushes in through the upper airway which is outside of the sealed chamber. These ventilators are not used often. They are bulky, hinder nursing care, and require a fairly flexible chest wall. The negative intrapleural pressure must be undisturbed.

Positive pressure ventilators are more practical. They inflate the lungs by pushing air into them. This reverses the pressure dynamics of normal inspiration, but is necessary in situations in which the negative intrapleural pressure is lost or the lungs are stiff and noncompliant. Unfortunately, this provides the ba- sis for some of the most serious complications of mechanical ventilation, namely pneumothorax, decreased venous return and cardiac output, and water retention. When positive- pressure ventilation is used (without any added expiratory pressure) expiration is passive, returning the airway pressure to atmospheric.

There are three basic types of positive pressure ventilators-pressure cycled, volume cycled, and time cycled. Each terminates inspiration based on a different princi- ple.

Pressure-cycled ventilators (Bennett PR-1 and PR-2; Bird Mark 7, the IPPB machine) push gas into the lungs until a predetermined pressure is reached within the tra- cheobronchial tree. When this pressure is reached, the driving force of the ventilator shuts off. Tidal volume may vary from breath to breath.

Volume-cycled ventilators (Bennett MA-1 and MA-2, Bourns Bear 1, Ohio 560) push air into the lungs until a certain predetermined tidal volume is delivered before ter- minating inspiration. Maximum inspiratory pressure can vary with each breath. A "safety valve" or setting for a maximum pressure limit above which the ventilator will no longer continue the inspiratory cycle is also set. This prevents the respirator from pushing a high tidal volume into

2202 American Journal of Nursing/December 1980



obstructed or noncompliant lungs, which could damage or even rupture lung tissue.

Time-cycled ventilators such as the BABYbird or Bourns infant ventilator are used for neonates and infants. Other time-cycled ventilators, such as the Emerson Postopera- tive Ventilator or Engstrom are used for adults. Time-cycled ventilators terminate inspiration after a preset time. Tidal volume is regulated by adjusting the length of inspiration and the flow rate of the pressurized gas. For instance, if a patient's lungs are not compliant, the flow rate can be increased to maintain effective tidal volume while leaving the inspi- ratorv time unchanged. Tidal volume and maximum inspiratory pressure may vary from breath to breath.

Many of the other characteris- tics of time-cycled ventilators are similar to those of volume-cycled ventilators. Since the airway pressures generated by a high flow rate could become excessive and dangerous, time-cycled ventilators also have fail-safe pressure limits beyond which the ventilator ceases to push gas into the lungs.

Volume-cycled ventilators are most often used for adult patients who require ventilatory assistance because they are more versatile and easier to use than pressure- or time- cycled respirators.

The major drawback to the use of pressure-cycled machines for patients who require ventilator assistance is that the volume of gas the patient receives is variable. If an anxious, restless, or uncomfortable pa-

tient shuts off air inflow, he might not receive a volume large enough to suf- ficiently expand alveoli. In addition, if the patient develops such problems as pulmonary infiltrates and edema, the lungs will become less compliant. This will cause the ventilator to terminate inspiration with gradually less volume, setting up a vicious cycle involving atelectasis and further re- ductions in compliance.

The following discussion of the various functions and settings for ventilators and the ways in which they can control ventilation is pri- marily focused on volume-cycled respirators, although it is possible to adapt pressure-cycled respirators for many of these functions. Time- cycled ventilators are used infre- quently for adults. However, many of the time-cycled ventilators are also capable of providing the functions discussed here.

Tidal volume. This is the amount of air exchanged with each breath. The appropriate amount is estimated as 10 to 15 cc./kg. of body weight. For individuals of average weight, this translates to between 500 and 1000 cc.

Respiratory Rate. The rate is usually set between 10 and 14 breaths/minute. Slower rates with higher tidal volumes are preferred, since this limits the amount of time that positive pressure is applied into the lungs.

Inspiratory/expiratory ratio. Mechanical ventilators are most helpful to the patient when they closely approximate normal breathing patterns. During normal breathing, the expiratory phase (expiration plus the pause between breaths) is about twice as long as the inspiratory phase. This allows for optimal and passive emptying of the lungs. Ven- tilators can be programmed to maintain this approximate ratio by controlling the flow rate of the gas being pushed into the lungs during inspiration. It is safer to appropriately limit the inspiratory time when using a positive pressure ventilator, since this will decrease the mean airway pressures and the risks that accompany higher pressures.

Oxygen concentration. The percentage of oxygen used will vary with the patient's arterial blood gases. Concentrations of 50 percent or less are considered relatively safe for

American Journal of Nursing/December 1980 2203



prolonged ventilatory support. (See page 2213.) Every effort is made to use the lowest concentration possible in order to avoid alveolar damage.

Volume-cycled ventilators can deliver specific oxygen concentrations between 21 percent (similar to room air) and 100 percent. Pressure- cycled ventilators lack the ability to deliver precisely controlled levels of oxygen. They can deliver 100 percent oxygen or air mix, a concentration of about 40 percent.

Pressure. Volume-cycled ventilators have a pressure limit dial. This shows the upper limit of pressure the machine will use to push the tidal volume. It is usually set at about 10 cm. of water pressure above that needed to deliver the desired tidal volume. The higher the pressure, the greater the risk of pneumothorax and other problems due to increased in- trathoracic pressure. On the other hand, the use of higher pressures is sometimes the only way to ventilate the stiff lungs of patients with ARDS.

Volume-cycled ventilators also have a system pressure gauge, a dial which fluctuates with the pressure in the patient's airways during the ventilatory cycle. Zero on this dial cor- responds to atmospheric pressure in the airways. The peak pressure reg- istered by the dial is the level of pressure it takes to finally deliver the tidal volume. The resting pressure between breaths on this system's pressure gauge indicates the presence and amount of positive end-expiratory pressure. When no end-expiratory pressure is being maintained, the dial rests at zero between breaths. When a patient attempts to breathe spontaneously, his negative inspiratory effort registers as a swing past zero to subatmospheric pressure.

With pressure-cycled ventilators, the pressure setting is important because it regulates the machine. There are also two gauges for pressures. One gauge the operator sets for a maximum inspiratory pressure, or the pressure at which inspiration will end, usually 20 to 30 cm. of water pressure. The other dial shows a continuous reading of the actual pressure within the system. It rises and falls with each ventilation.

Sighs. A normal individual sighs periodically. Sighing provides larger tidal volumes and will inflate

more alveoli than normal breaths and, therefore, help to prevent atelectasis. This hyperinflation is also thought to stimulate surfactant production. Most volume-cycled ventilators have an optional mechanism that mimics sighing. It breaks the pattern of ventilation by periodically delivering a breath that is 200 to 300 cc. greater than the usual tidal volume. Sighing may not be used when patticularly large tidal volumes or positive end-expiratory pressures are being delivered, since these maneu- vers also serve to combat atelectasis.

The ventilator is usually programmed to deliver six sighs per hour, one approximately every ten minutes. They may be given in mul- tiples of two or three if desired. A pressure limit is also set for the delivery of sighs. It may be the same as or slightly higher than the tidal volume pressure limit. There is also a control to manually trigger a sigh. This is often used during suctioning.

Spirometer. This is a large bellows-like container that measures ex- pired tidal volume. Tidal volume can be easily read from the spirometer's labeled graduations. This is also helpful in quickly gauging the tidal volume of spontaneous ventilations during intermittent mandatory ventilation.When the patient receives any ventilator breath, it is important to note whether the exhaled tidal volume is equal to the amount set on the ventilator.

Two technical problems com- monly cause the spirometer to malfunction. First, the rubber bellows may become damp and stick as a result of condensation in the system. If this happens, the spirometer must then be dried and replaced. Move- ment of the spirometer bellows may also be blocked if the metal arm that supports the ventilator tubings is inadvertently pushed against the dipstick that rises from the top of the spirometer as its bellows rises. Read- justment of the support arm will remedy this.

Ventilator Alarms

It is unrealistic that a nurse or respiratory therapist can be in con- stant attendance. Alarms can alert to potentially dangerous situations

for the ventilator-dependent patient. A spirometer alarm goes off

when the exhaled tidal volume is less than an amount present on the spirometer. Disconnection of tubes within the ventilator system or disconnection of the ventilator from the patient could cause this. A significant air leak around the endotracheal or tracheostomy cuff could also cause a drop in delivered tidal volume. The high pressure and low pressure alarms will help to identify the un- derlying reason for the spirometer alarm going off.

A high pressure alarm sounds whenever the preset limit is reached, and delivery of the full tidal volume is aborted. Any of the conditions that increase airway pressure could cause this to happen.

A low pressure alarm sounds if the pressure within the system drops precipitously. This occurs with disconnection from the patient or gas source or disconnection of tubings within the machine. It serves as a backup for the spirometer alarm.

Alarms must NEVER be turned off, even temporarily for suctioning. The chance of the nurse's becoming distracted and leaving the alarm off is too great. On many ventilators, the alarms can be temporarily bypassed or silenced for a period of up to two minutes for suctioning. After that given period of time, the alarm system automatically becomes functional again.

Alarms also signal malfunction and overheating of the devices that warm the air, so that the patient's airway is not accidentally burned.

Controlling Ventilation

With volume-cycled respirators, the extent of mechanical control over ventilation can be altered by setting the ventilator to be either sensitive or insensitive to the patient's spontaneous inspiratory effort.

Assisted ventilation is when the machine is set to be sensitive to a certain strength of negative inspiratory effort. This is useful for patients who have spontaneous but inade- quate ventilation. When an inspiratory effort is strong enough to trigger the ventilator, the full preset tidal volume is delivered. Assisted ventilation can foster spontaneous breathing and allow a patient to alter his ven-

2204 American Journal of Nursing/December 1980



tilation according to need. It is dis- advantageous when an anxious or tachypneic patient hyperventilates and becomes alkalemic.

Controlled mechanical ventilation (CMV) is when the ventilator is set to ignore any inspiratory efforts of the patient. The ventilator delivers only the number of breaths for which it has been set. The patient is said to be "locked out." This is useful for an apneic patient, an extremely anxious patient, a heavily sedated patient, or a patient for whom negative inspiratory effort is contraindicated, such as Mr. Bryan, who had a flail chest. The major disadvantage is that it does not allow the respiratory rate to change with changing patient needs, and it prevents respiratory system compen- sation for metabolic acid-base imbal- ances.

Controlled and assisted ventilation can be combined. Some ventilators can be programmed to deliver controlled breaths at a given fail-safe frequency that takes over if the patient fails to trigger assisted breaths fast enough. This is especially useful when a patient is subject to fatigue and hypoventilation.

Intermittent mandatory ventilation, or IMV, is another variation in the amount of ventilator control. With IMV, a certain number of breaths of a preset tidal volume is delivered. Between those breaths the patient may take spontaneous breaths of whatever tidal volume he wants and is able to draw. The same level of humidified, oxygenated air is available during his spontaneous breaths as during his mandatory breaths. The number of programmed breaths can be gradually reduced until the patient can be dis- continued from the ventilator. IMV encourages patients to use and strengthen their respiratory muscles while providing periodic deep breaths that help to prevent atelectasis. Although IMV was originally designed for weaning patients from respirators, it is often used as a pri- mary mode of ventilatory support.

The major difference between IMV and assisted ventilation is that with assisted ventilation, the patient only starts the breath; the ventilator then delivers the tidal volume. With IMV, the patient independently de- termines the tidal volume of each spontaneous breath.

Synchronized intermittent mandatory ventilation (SIMV), also called intermittent demand ventilation (IDV), is available on some ventilators. This is similar to IMV except that when a mandatory breath with its preset tidal volume is due, it is delivered in synchrony with the next spontaneous inspiratory effort, much in the manner that an assisted breath is delivered. This is theoretically safer since it eliminates the possibility that the ventilator could initiate a mandatory inspiration just at the peak of a spontaneous inspiration and overdistend the lung as well as dis- rupt the smoothness and comfort of breathing.

Modes of Expiratory Pressure

Most volume-cycled ventilators can be programmed to deliver various levels of pressure during or at the end of expiration. Some can apply expiratory resistance, sometimes called expiratory retard, which pro- longs expiration and holds small airways open, much like pursed-lip breathing. Airway pressure is al- lowed to return to atmospheric pressure at the end of expiration. This facilitates better emptying of the lungs in conditions where the functional residual capacity is increased. (Functional residual capacity is the amount of air left in the lungs after a normal expiration; it increases in bro- nchitis, asthma, and emphysema.)

Functional residual capacity is decreased in some conditions such as ARDS. Hypoxemia develops because blood circulates around collapsed alveoli and then returns to the heart unoxygenated. Positive pressure can be applied at the end of expiration so that the pressure in the airways never drops below a selected level, usually between 5 and 15 cm. of water pressure. This is thought to hold small airways and alveoli open and thereby increase functional residual capacity and limit atelectasis. Positive end- expiratory pressure (PEEP) is applied in conjunction with endotracheal intubation and mechanical ventilation. By opening more alveoli for gas exchange, PEEP is thought to decrease intrapulmonary shunting, increase PaO2, and allow lower concentrations of oxygen to be used.

Mr. Bryan, the patient with the flail chest and possible aspiration

pneumonitis, was a good candidate for developing ARDS. Over the first 18 hours of controlled ventilation, his arterial blood gases showed progressive hypoxemia, despite the fact that overall ventilation was adequate to maintain a normal PaCO2. His volume-cycled ventilator was set at 14 breaths/minute, 1000 cc. tidal volume, and 50 percent oxygen. To avoid administering oxygen in excess of 50 percent, PEEP at 6 cm. was applied. His blood gases before and after PEEP are shown below:

pH PaCO2 HCO3 PaO2 SaO2

Before 7.44 37 mm.'Hg 24 mEq./l. 59 mm. Hg 91%

After 7.45 36 mm. Hg 24 mEq./l. 83 mm. Hg 95%

PEEP, however, is no panacea. Increasing the airway pressure further intensifies the problems that can result from elevated intratho- racic pressure.

When positive end-expiratory pressure is used with patients who are breathing spontaneously, it is called continuous positive airway pressure or CPAP. CPAP has been successfully used for some time in the treatment of infant respiratory distress syndrome, delivered via either an endotracheal tube or a special head-enclosing chamber. CPAP is now also used on adult patients. It can be applied by way of an airtight face mask, but this is uncomfortable and even dangerous as it can pro- mote gastric distension with possible aspiration. It is preferable to deliver CPAP using an endotracheal tube while the patient breathes oxygen- enriched, humidified air from a closed tubing system that has a one- way valve. Like PEEP, CPAP holds small airways and alveoli open. Since CPAP is used with spontaneous breathing, it is associated with significantly lower mean intratho- racic pressures and fewer problems from high airway pressures.

Most pressure-cycled respirators lack the means to readily deliver various types of expiratory resistance and end-expiratory pressure. Some, however, can be modi- fied to do this. When used for continuous ventilation, pressure-cycled ventilators can also be equipped with spirometers and alarms.

American Journal of Nursing/December 1980 2205



ventilators and how they work

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