After reading this article you should be able to:
1. Compare the various settings and modes of mechanical ventilation.
2. Describe ventilator-related problems and the alarms they trigger.
3. Develop a plan of care for the patient on a ventilator.
While many ED nurses find ventilator care intimidating, mastering the principles of mechanical ventilation is not as difficult as you may think.
When Patricia Warner, 62, arrives at your ED, she's in acute respiratory distress—and barely moving air. Her respiratory rate is 40, and pulse oximetry shows an oxygen saturation of 80%. What's more, she has a history of COPD and symptoms of an upper respiratory infection.
Though COPD patients like Ms. Warner are often put on bi-level positive airway pressure (BiPAP), a noninvasive form of mechanical support, her healthcare team determines that she needs intubation and mechanical ventilation to survive. If you were her nurse in the ED, would you be able to provide this care with confidence?
If it's been some time since you cared for a patient like Ms. Warner—or if you've never done so—you may find it intimidating, but it does not have to be. Despite advances in equipment and strategies for providing mechanical ventilation via artificial airway, the fundamentals have not changed.
Who it's for, how it works
Mechanical ventilation is used to treat patients with respiratory failure from inadequate ventilation or oxygenation (or both), as evidenced by hypoxemia with or without hypercapnia.1,2 The 50/50 rule states that mechanical ventilation is indicated if a patient's partial-pressure of oxygen (PaO2) falls below 50 mm Hg and her partial-pressure of carbon dioxide (PaCO2) rises above 50 mm Hg.3
Always remember, though, that the 50/50 rule is only a guideline. Many patients need ventilatory support before they reach these critical values. On the other hand, some patients with severe COPD may have those values at baseline.
Mechanical ventilation is fundamentally different from normal breathing. During spontaneous breathing, the diaphragm contracts on inhalation, moving toward the abdomen, and the chest wall expands. The space inside the thorax enlarges and creates a vacuum that draws air into the lungs and helps to distribute the air evenly.
In contrast, a ventilator pushes a warm, humidified mixture of oxygen and air into the lungs and creates positive pressure in the thorax during inhalation.
In patients at risk for alveolar collapse on exhalation, a small amount of pressure can be maintained in the alveoli to hold them open. This is called positive-end expiratory pressure (PEEP), and it can improve alveolar recruitment and increasing oxygenation.
Making sense of settings
When you care for a ventilated patient, work closely with the respiratory therapist, who is responsible for maintaining the equipment and is an expert in its use. With today's sophisticated machines, what seems like a simple change may actually require multiple adjustments to optimize the settings for a particular patient.
When a patient is put on a ventilator, numerous settings, including respiratory rate, fraction of inspired oxygen (FiO2), volume or pressure control, and ventilator mode, must be selected. In addition, two adjuncts—PEEP and pressure support—are sometimes used, depending on the patient's status and which ventilation mode is chosen. To properly care for your patient, it's important to understand each of these settings.
Respiratory rate: This setting simply refers to the number of breaths per minute that the ventilator delivers. Eight to 12 bpm is a typical respiratory rate.3 Depending on the mode selected, the ventilator can provide all of the patient's ventilation, or the patient may be able to breathe spontaneously be tween ventilator breaths.
FiO2: This indicates the amount of oxygen the ventilator delivers, expressed as a percentage or a number between zero and one. FiO2 varies widely depending on the patient's condition; room air is 21% (0.21). While some patients might be adequately oxygenated with an FiO2 of less than 40% (0.40), someone with severe hypoxemia, for example, might need an initial FiO2 setting of 100% (1.00).2 Arterial blood gases and pulse oximetry values will help determine FiO2 settings.
Volume control: Traditionally, mechanical ventilation is volume controlled. This setting means the ventilator is programmed to deliver a preset volume of oxygen and air, called the tidal volume (VT), regardless of the amount of pressure required to deliver the volume (a positive pressure alarm protects patients from dangerously high pressures).
Pressure control: An alternative to volume control that's indicated for some patients, pressure control simply means that pressure is the endpoint rather than volume. Thus, inspiration ends when a preset pressure is reached, regardless of the volume delivered.
The advantage of this mode is that it allows the volume to change in response to intrathoracic pressure. The goal is to increase mean airway pressure by prolonging inspiration, ideally recruiting more alveoli than volume control ventilation. By limiting pressure, there is less risk of pressure-related injury.1
Pressure-regulated volume control (PRVC): This type of mechanical ventilation is an alternative to strict pressure control, representing an attempt to obtain the best of both volume and pressure control. PRVC adapts to changing compliance of the lungs to adjust inspiratory time and pressure to maintain a preset tidal volume.
Assist control (AC): In this mode, the ventilator supports every breath, whether it's initiated by the patient or the ventilator.3 AC is often used to allow the patient to rest, because the ventilator does all the work. This high level of respiratory support is frequently required in patients who have been resuscitated, have acute respiratory distress syndrome (ARDS), or are paralyzed or sedated.
Because AC mode results in the highest level of positive pressure in the chest, it increases the risk of barotrauma to the lungs. Anxious patients who frequently trigger the ventilator can easily hyperventilate.
Synchronized intermittent mandatory ventilation (SIMV): In this mode, not all spontaneous breaths are assisted, leaving the patient to draw some breaths on her own. For example, if your patient's ventilator is set on SIMV mode with a respiratory rate of 10 bpm, she will receive a breath roughly once every six seconds. She can also breathe on her own in between the machine-assisted breaths.
There are several advantages to this mode for patients who can tolerate it. SIMV helps preserve the strength of the respiratory musculature, decreases the risk of hyperventilation and barotrauma, and facilitates weaning. Weaning can be done by gradually decreasing the percentage of machine-assist ventilation.
Patients who need short-term ventilation benefit most from SIMV, but the choice of mode should be an individual decision based on the patient's condition and tolerance.3 No one method is best for all patients.2
Positive end-expiratory pressure (PEEP): PEEP can be used to increase oxygenation in either AC or SIMV mode. The effect of PEEP on the lungs is similar to blowing up a balloon and not letting it completely deflate before blowing it up again. Most patients are started on 5 cm H2O of PEEP.3 Some patients, such as those with ARDS or other conditions that make lungs stiff, require higher levels of PEEP to keep alveoli from collapsing and to decrease intrapulmonary shunting. It's not unusual to use 8 - 12 cm H2O in these patients. But PEEP should not exceed 20 cm H2O; higher settings increase the risk of severe lung damage, subcutaneous emphysema, and pneumothorax.4
Pressure support: Used alone or added to SIMV, this provides a small amount of pressure during inspiration to help the patient draw in a spontaneous breath. Pressure support makes it easier for the patient to overcome the resistance of the ET tube and is often used during weaning because it reduces the work of breathing. It's not necessary during AC ventilation because in that setting, the ventilator supports all of the breaths.
Responding to an alarm
Since a ventilator is, in effect, merely an air pump, an alarm simply signals that there's something wrong with the pressure, volume, or rate of air being delivered. When an alarm sounds, your role is to immediately check the patient and the equipment and figure out—and fix—what's interfering with the function of the ventilator. If you can't immediately identify the problem, disconnect the patient from the ventilator, use a manual resuscitation bag, and call for help. Often, the problem is related to the tubing.
A high-pressure alarm is the one you're most likely to hear. At worst, this alarm indicates that your patient's airway is blocked and she's no longer being ventilated or that she has a tension pneumothorax. While it's far more likely that coughing triggered the alarm, you can never assume that a high-pressure alarm went off because your patient coughed—always assess the airway! If a cough triggers the alarm, the ventilator will reset itself after a few short breaths.
A simple way to determine airway patency is to disconnect the ventilator and ventilate your patient with a manual resuscitation bag. If the airway is obstructed, you'll feel it immediately and take steps to clear it by suctioning or re-intubating the patient.
The low-pressure alarm and low exhaled tidal volume alarms are common, as well. These alarms indicate that either the ventilator did not reach the pressure it expected or that some of the air it delivered was not exhaled back into the tubing for measurement. Your response should be to look for disconnected tubing or an air leak. The most common places for leaks are around the ET tube cuff, poorly secured connections, and drainage and access ports on the tubing.
A high respiratory rate alarm can signal a change in your patient's condition, such as heightened anxiety, awakening from sedation, or pain. More commonly, though, water or kinks in the tubing trigger this alarm because air is pulsing through the tubing around the obstruction. Your response: Check the tubing and eliminate any water or kinks—and assess the status of your patient.
Finally, an apnea alarm may sound, possibly signaling that your patient has stopped breathing. Disconnected tubing, however, is a more likely cause of an apnea alarm. The most common place for the tubing to become disconnected is where it attaches to the ET or tracheostomy tube. Again, check the tubing and reconnect it, if necessary.
Vigilance wards off complications
Infection, atelectasis, barotrauma, and oxygen toxicity are all potential complications of mechanical ventilation. Good pulmonary hygiene as well as careful attention to the ventilator settings are the key to avoiding them.
Infection is a potential problem because intubation bypasses the primary mechanisms that prevent bacteria from getting into the lungs. Normally, the oral pharynx washes away bacteria with saliva, and normal flora compete with bacteria in the mouth. But a patient who's mechanically ventilated doesn't eat and may be on systemic antibiotics that suppress the flora; the absence of flora—and decreased salivary flow—predisposes her to infection. Bacteria around the ET tube cuff that get aspirated when the tubing is moved or the patient coughs can lead to pneumonia.
To lower the incidence of ventilator-associated pneumonia, the Institute for Healthcare Improvement recommends implementing the ventilator bundle, a series of interventions related to ventilator care that, when implemented together, achieve better outcomes than when implemented individually. The four components of the ventilator bundle are: elevation of the head of the bed to 30 - 45 degrees; daily "sedation vacations" and assessment of readiness to extubate; peptic ulcer disease prophylaxis; and deep venous thrombosis prophylaxis.5 Good mouth care and cleaning of Yankauer suction devices can also decrease the incidence of infections.6
Atelectasis, which is collapse of the alveoli, is a risk of positive pressure ventilation because air pushed in under positive pressure is not evenly distributed throughout the lungs. Good pulmonary hygiene and regular repositioning can help prevent atelectasis, along with the administration of PEEP to those who need it. Assess your patient's breath sounds at least once every four hours, and suction secretions as necessary.
Barotrauma (lung damage caused by high airway pressure) and volutrauma (damage caused by too much volume in the lungs) are closely related. Typically, lung damage is caused by a combination of both. It is important to monitor your patient for subcutaneous emphysema, which is an early warning sign of extrapulmonary air, and decreased breath or heart sounds, which could indicate pneumothorax or pneumomediastinum.
Oxygen toxicity is another potential complication, a possible result of maintaining a high FiO2 for too long. At high concentrations, oxygen is converted into oxygen-free radicals that can cause lung damage. Although the symptoms of oxygen toxicity may be difficult to recognize in a patient on a ventilator, they include fatigue, lethargy, weakness, restlessness, nausea, vomiting, anorexia, coughing, and dyspnea, followed by refractory hypoxemia and cyanosis.1 To avoid this complication, administer the lowest FiO2 that produces an oxygen saturation greater than 90% and a PaO2 greater than 60 mm Hg.3 But never compromise a patient's oxygenation out of fear of oxygen toxicity.
Finally, remember that for many patients, the point of mechanical ventilation is to get the patient "over the hump" of acute illness, after which ventilatory support should be rapidly withdrawn. Consider, for instance, the way that Ms. Warner's treatment progressed:
After being intubated, she received mechanical ventilation in AC mode with light sedation and was started on antibiotic therapy for her respiratory infection. After 36 hours, she was switched to SIMV mode and then weaned off the ventilator. She went on to recover fully, with ongoing care for her COPD provided by her personal physician.
As was true for Ms. Warner, mechanical ventilation can save your patient's life. Knowing how to prevent complications and troubleshoot, as needed, will help you ensure your patient's safety until ventilatory support can be successfully withdrawn.
1. Hess, D. R., & Kacmerek, R. M. (2002). Essentials of mechanical ventilation (2nd ed.). New York: McGraw-Hill.
2. Martin, L. "Chapter 10: Mechanical ventilation-Part I of III." 1999. www.lakesidepress.com/pulmonary/books/physiology/chap10a.htm (16 June 2005).
3. Joyce, D. M. "Ventilator management." 2005. www.emedicine.com/emerg/topic788.htm (22 June 2005).
4. Brower, R. G., Morris, A., et al. (2003). Effects of recruitment maneuvers in patients with acute lung injury and acute respiratory distress syndrome ventilated with high positive end-expiratory pressure. Crit Care Med, 31(11), 2592.
5. Institute for Healthcare Improvement. "Implement the ventilator bundle." 2005. www.ihi.org/IHI/Topics/CriticalCare/IntensiveCare/Changes/ImplementtheVentilatorBundle.htm (25 July 2005).
6. Centers for Disease Control and Prevention. "Guidelines for preventing healthcare-associated pneumonia, 2003: Recommendations of the CDC and the Healthcare Infection Control Practices Advisory Committee (HICPAC)." 2004. www.cdc.gov/ncidod/hip/pneumonia/default.htm (23 June 2005).