Are you puzzled by ventilator modes? We help you differentiate between invasive and noninvasive ventilation and understand the common settings for each.

Your patient in the ICU today is Mrs. J, who was intubated for hypercapnic respiratory failure yesterday after she failed a trial on bilevel positive airway pressure (BIPAP). Her ventilator settings are assist control (AC), 12; tidal volume (TV), 600; positive end-expiratory pressure (PEEP), 5; and FiO₂, 40%. You suddenly feel like you're on another planet and people are speaking a different language. In this article, we'll show you how to identify the difference between invasive and noninvasive ventilation, understand the basic mechanisms of different ventilator modes, and interpret the ventilator settings.

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Invasive ventilation

Invasive positive pressure ventilation requires that the patient be intubated byplacing an endotracheal (ET) tube to provide direct ventilation to thelungs. It's indicated for patients who aren't breathing (apneic) or breathing ineffectively, causing ventilation problems. Intubation is necessary for any patient with impending or current respiratory failure. There are no specific contraindications to mechanical ventilation when a patient isn't breathing, but facial, neck, or tracheal trauma can make oral intubation undesirable (see Indications for mechanical ventilation).

Almost all ventilators have the capability of being set to four basic modes: AC, synchronized intermittent mandatory ventilation (SIMV), airway pressure release ventilation (APRV), and pressure support (PS). Most newer ventilators can also be set to specialty modes, such as high frequency oscillatory ventilation (HFOV).

Let's take a closer look at these standard ventilator modes (see Picturing modes of mechanical ventilation).

Assist control

AC is one of the most common modes used for ventilation in the ICU. It's often used for patients who require the most support from the ventilator. When looking at the ventilator, you'll see that there are several basic settings within AC mode. These include the respiratory rate, TV, FiO₂, and PEEP (see Initial ventilator settings). Let's look at each one of these terms.

The respiratory rate is the minimum amount of breaths that the patient will beallowed to take. This rate is programmed into the ventilator, often set between 12 and 18.

TV is the amount of air that will go into the patient's lungs with each breath. This is based on the ideal body weight of the patient, most often calculated at 10 mL/kg. Some patients may require a smaller TV due to poor lung compliance (the amount ofstretch the lungs can handle without damage). TV is usually set between 400 mL for a small person and up to 800 mL for a larger person.

Measured as a percentage, FiO2 is the amount of oxygen the patient requires to maintain appropriate blood oxygen levels.

PEEP is the pressure that's applied at the end of the expiratory phase that helps keep the alveoli from snapping shut when the patient exhales. This can minimize the risk of developing atelectasis and prevent shearing force trauma to the alveoli. Shearing is caused when the alveoli are opening and shutting too quickly. PEEP can also be used to help open areas of collapsed alveoli, also known as atelectasis. PEEP is measured in centimeters of water and is often seen at levels between 5 and 10 cm H₂O.

In AC mode, the patient will have a set respiratory rate, meaning that it's a time-triggered mode. If for some reason the patient doesn't initiate a breath on his or her own after so many seconds, the ventilator will sense that the patient hasn't attempted a breath and will deliver the TV. The time between breaths is based on the set respiratory rate. For example, if the respiratory rate is set to 12 and there's no breath initiated within 5 seconds, the ventilator will give the patient a controlled, or ventilator-dependent, breath. The ventilator will give this set number of breaths every minute as long as the patient isn't attempting to breathe. The ventilator won't allow the patient to breathe less than the set amount.

If the patient is capable of taking breaths on his or her own, the ventilator will sense that the patient is taking a breath by the negative flow of air and will help facilitate the breath by delivering the set TV. The patient can breathe as many times a minute as he or she wants but will get the same TV with each breath.

The benefit of AC mode is that it can be used in both patients who are spontaneously breathing and those who aren't. It will provide the set number of breaths every minute, but also allow patients who want a higher rate to initiate breaths ontheir own. This can decrease anxiety by allowing the patient to set his or her own respiratory rate while still being supported by the full TV.

For weak or critically ill patients, the ventilator does most of the work, meaning that the patient doesn't have to do much of the work of breathing. The downside of using this mode is that every breath is the same size. If the TV is set at 600 mL, then every breath (both spontaneous and assisted) will be approximately 600 mL. After the volume has been delivered, a valve closes in the circuit and the patient is forced to exhale. However, a person's normal breathing pattern doesn't include identical breaths. If a patient wants larger breaths than the set TV, it can cause anxiety and interrupt the patient's breathing pattern, which may lead to resistance in ventilation. This can lead to tachypnea and hyperventilation, which, in turn, may result in respiratory alkalosis.

Another common issue seen with the use of AC mode is often caused by the low work of breathing. When patients have been in this mode for a period of time, it can cause weakened respiratory muscles and increase in ventilator times. Sedation, appropriate volumes, and weaning trials can often help decrease these complications.

Synchronized intermittent mandatory ventilation

SIMV is also a common mode of ventilation used in the ICU. It works on the same basic principles of AC mode-a set number of breaths will be delivered each minute, but the patient can breathe as many times a minute as he or she feels theneed to. These breaths can be patient- or ventilator-initiated, but the difference ishow TV is delivered. All ventilator-initiated breaths will have the full TV delivered, but for patient-initiated breaths, the set respiratory rate will be an independent breath and the TV won't be delivered. Thepatient will need to inhale the TV independently.

The rationale behind using SIMV instead of AC is to help work the patient's respiratory muscles by providing periods of decreased support. Remember, if the respiratory rate is set high or the patient isn't breathing spontaneously, then this mode functions identically to AC mode.

The benefit of using SIMV is seen most often in surgical patients who require ventilator support for a short period of time postoperatively. As the patient wakes up, he or she is able to take an increasing number of unassisted breaths. By using this mode, it helps determine at what point the patient is ready for extubation.

The downside of SIMV is often seenwhen it's used with weakened or critically ill patients. The increased work ofbreathing can actually cause a patient totire out and lead to longer intubation times or failed weaning attempts. Other issues include hypoventilation from the inability to take adequate TV with independent breaths and anxiety from not knowingwhat breaths will be assisted orunassisted.

Table Troubleshooting problems with mechanical ventilation

Airway pressure release ventilation

APRV is considered a rescue method of ventilation and is often used for patients who are having problems with lung compliance or difficulty with oxygenation. This is a fairly advanced and complicated mode ofventilation, most commonly used in patients who have acute respiratory distress syndrome (ARDS). APRV uses an inverse ratio to achieve higher levels of pressure, meaning that the expiratory phase is longerthan the inspiratory phase. This allowshigher levels of pressure to be held throughout the respiratory cycle, although this isn't how we normally breathe. However, compared with older modes that used an inverse ratio, APRV is much more comfortable for patients and allows for spontaneous breathing. The patient can take a breath at any point in the ventilator cycle, making the high pressures more tolerable. These high pressures combined with PEEPhelp improve and prevent areas ofatelectasis. This is one way that APRV helps improve oxygenation when other modes can't.

Improved oxygenation is the biggest benefit to using this mode. It has often been shown to significantly improve oxygenation in patients who are very difficult to oxygenate otherwise. This is commonly seen in patients with ARDS because of the decrease in lung compliance and dense areas of atelectasis. Another benefit seen over the use of other inverse ratio modes is that paralysis and heavy sedation aren't required because patients can breathe anywhere in the pressure cycle.

There are more risks with the use of APRV than with the other modes, including a higher incidence of pneumothorax and other ventilator trauma injuries because of the higher levels of pressure combined with the decrease in patient lung compliance.

High frequency oscillatory ventilation

Used when all other modes fail to improve oxygenation, HFOV isn't usually found on a traditional ventilator. This mode, along with APRV, is considered a rescue mode of ventilation and is most commonly used in adult patients with ARDS or for neonates with neonate respiratory distress syndrome or meconium aspiration. The benefit of using this type of ventilation is that it has been shown to significantly improve oxygenation when conventional methods have been unsuccessful by sustaining very high levels of PEEP almost continuously. This high level of PEEP helps provide enough pressure to reopen areas of collapsed alveoli (atelectasis), often referred to asrecruitment.

The downside of HFOV is the potential for the development of pneumothorax or other barotrauma. There's also a potential for complications from the use of paralytics, sedation, and pain medication. All three are required for patients to tolerate HFOV. This can lead to difficulty in assessing neurologic function or when transitioning the patient to a conventional mode. These patients are often critically ill and require frequent close monitoring of arterial blood gases (ABGs) and one-to-one nursing care.

Figure. Invasive ventilation usually requires weaning.

Pressure support

PS is considered a weaning mode used to assess the patient's readiness for extubation. It doesn't use a set respiratory rate and is a pressure-driven mode rather than a time-triggered one. PS requires the patient to initiate each breath and then that breath is assisted through the ET tube with a set amount of pressure. This support helps overcome the resistance of the ET tube.

When this mode is used, the pressure is often started at a high rate, such as 20 cm H₂O, and titrated to usually 8 cm H₂O before extubation. The lower the pressure, the more work the patient needs to do to pull adequate TV through the ET tube. After the patient has been weaned to the lowest amount of PS and is able to achieve adequate TV while maintaining oxygenation, it suggests that he or she will be able to be successfully extubated. PS can also be used in conjunction with SIMV as additional assistance for independent breaths.

When assessing a patient in this mode, it's important to ensure that he or she is getting adequate TV. Remember that the patient should be achieving volumes between 400 and 800 mL based on body weight. The amount of time that a patient remains in PS mode will depend on how ready he or she is for extubation. Weaning often starts with short periods of high pressures; as the patient tolerates the trial, the periods can be extended and the pressure decreased. Strong patients may do a PS trial for less than an hour and then be extubated. Patients who are weak, who suffer from chronic lung disease, or who've been intubated for longperiods of time may take several days or even weeks with daily trials to be ready for extubation.

The biggest benefit of using the PS mode is that it acts as a stepping stone between a dependent ventilator mode and extubation. This helps decrease the risk of reintubation by allowing adequate assessment of the patient's ability to breathe independently. Italso helps work the respiratory muscles to get them ready for independent breathing.

The downside of PS is that the increased work of breathing can leave the patient tired and unable to pull enough TV to maintain adequate ventilation. Poor ventilation can lead to hypercapnia and respiratory acidosis. Alarm limits should be set to detect patterns of low volumes to help decrease this risk. Tachycardia and tachypnea can also be signs that a patient may need a higher level of pressure or require rest in AC mode. Often, patients who've been on mechanical ventilation for an extended period of time have a weak diaphragm due to the decreased workload of breathing while on the ventilator. These patients may require higher levels of support and many days of weaning trials before extubation.

Noninvasive ventilation

Sometime patients don't need to be intubated but need breathing support. When respiratory failure is pending, the healthcare team will often take the least aggressive method of providing appropriate ventilation. Noninvasive ventilation can be an effective alternative to intubation. There are two different methods of noninvasive ventilation that can be used in this situation: BIPAP and continuous positive airway pressure (CPAP). Both use a mask that's placed over the nose orface delivering positive airway pressure and oxygen to help assist breathing. These methods are to be used only for a patient who's breathing spontaneously. Let's take a closer look.

Bilevel positive airway pressure

BIPAP provides positive airway pressure during both inspiration and exhalation. Thishelps assist patients who are spontaneously breathing with ventilation and gas exchange.

BIPAP is useful in assisting patients with achieving full TV, leading to improved ventilation in patients with impending respiratory failure. It can also provide supplemental oxygen along with inspiratory pressure.

BIPAP is often used with patients who are hypercapnic or who have elevated levels of carbon dioxide (CO₂). It helps improve ventilation and decrease high CO₂ levels, but can only be used in patients who are able to breathe independently. BIPAP isn't appropriate for a patient who's apneic or who has a low respiratory rate.

Continuous positive airway pressure

CPAP is a noninvasive form of PEEP. It can be provided through a ventilator as a separate mode, but can also be delivered via anindependent machine. CPAP is most commonly delivered through a small mask that's worn over the nose, but can also be provided through a full-face mask.

CPAP provides a constant end-expiratory pressure that helps keep the airway open; some machines also provide supplemental oxygen if required by the patient. Because this type of noninvasive ventilation provides constant airway pressure, it's most often used for patients with obstructive sleep apnea (OSA).

The biggest benefit of CPAP is decreasing or even eliminating the adverse reactions of OSA. The positive pressure helps prevent obstruction while the patient is sleeping and allows for effective ventilation and oxygenation. Patients most often complain about wearing the mask but, for most, the improved quality of sleep outweighs the discomfort.

Nursing considerations

As the nurse caring for an intubated patient, it's important to be aware of the different alarms you may encounter. One of the most common alarms is a high pressure alarm, which may mean that there are secretions present and the patient requires suctioning or that the patient is biting on the ET tube and may require more sedation. Most intubated patients will require some sedation and analgesia to make tolerating the ETtube more comfortable. The other common alarm is a low pressure alarm, which mayindicate that there's an air leak in the ventilator circuit or the cuff on the end of the ET tube and air is leaking past the cuff and out of the patient's mouth. Adding some air to the cuff or finding the leak in the circuit will resolve this type of alarm (see Troubleshooting problems with mechanical ventilation).

Caring for an intubated patient also requires a basic care routine and assessment skills. Each ET tube is marked in centimeters, and the position should be checked every 4hours. When checking the tube's position, it's also a good time to assess for skin integrity, the stability of the securement device, and lung sounds. Mouth care should also be provided every 4hours, and the patient's teeth should be brushed twice a dayto decrease the incidence of ventilator-acquired pneumonia.

You also need to be aware of the complications of mechanical ventilation. Two of the most dangerous are volutrauma and barotrauma. Volutrauma is often caused by a TV that's too high, causing overdistension of the alveoli and leading to edema at the level of the alveoli where oxygenation takes place. Barotrauma is caused by elevated pressure in the lungs from high levels of PEEP. Most often seen in patients who have decreased lung compliance, such as in ARDS or pulmonary fibrosis, the first signs of barotrauma are low oxygen levels, tachypnea, agitation, and high airway pressures.

For patients receiving BIPAP or CPAP, you must assess the quality and rate of respirations. If respirations change or decrease, it may be a sign of worsening respiratory failure. Lung sounds should also be assessed at regular intervals to evaluate adequate air movement.

Like invasive ventilation, there are also alarms associated with noninvasive ventilation. The most common cause of alarms is low volume due to a leak in the seal between the mask and the patient's face. Readjustment of the mask to a tighter seal will usually resolve this problem. Other alarms may be for low or high respiratory rates or low TV, meaning that the patient isn't breathing deep enough. These alarms may indicate that the patient isn't tolerating the therapy and may require intubation. ABG monitoring may be needed to determine if a patient is tolerating noninvasive ventilation.

Ready, set, go!

Invasive and noninvasive ventilator modes aren't as daunting as you may think. Ventilators have come a long way over the years and are often seen in the ICU, ED, and OR settings. When working in these areas, or in other areas that commonly use ventilators, it's important to know how to interpret the settings. Knowing the ventilator mode that your patient is on will help you identify what settings will be present and allow you to assess what the next step for your patient will be.

Indications for mechanical ventilation

* Partial pressure of oxygen in arterial blood (PaO₂) < 50 mm Hg with FiO₂ > 0.60

* PaO₂ > 50 mm Hg with pH < 7.25

* Vital capacity < two times TV

* Negative inspiratory force < 25 cm H₂O

* Respiratory rate > 35/minute

The following guide is an example of the steps involved in operating a mechanical ventilator. The nurse, in collaboration with the respiratory therapist, always reviews the manufacturer's instructions, which vary according to the equipment, before beginning mechanical ventilation.

1. Set the machine to deliver the TV required (10 to 15 mL/kg).

2. Adjust the machine to deliver the lowest concentration of oxygen to maintain normal PaO₂ (80 to 100 mm Hg). This setting may be high initially but will gradually be reduced based on ABG results.

3. Record peak inspiratory pressure.

4. Set mode (AC or SIMV) and rate according to the healthcare provider's order. Set PEEP and PS if ordered.

5. Adjust sensitivity so that the patient can trigger the ventilator with a minimal effort (usually 2 mm Hg negative inspiratory force).

6. Record minute volume and obtain ABGs to measure partial pressure of carbon dioxide, pH, and PaO₂ after 20 minutes of continuous mechanical ventilation.

7. Adjust setting (FiO₂ and rate) according to results of ABG analysis to provide normal values or those set by the healthcare provider.

8. If the patient suddenly becomes confused or agitated or begins bucking the ventilator for some unexplained reason, assess for hypoxia and manually ventilate on 100% oxygen with a resuscitation bag.

The use of ventilators has been recorded since the early 1800s, but modern ventilation was first used in the 1940s. The early mechanism was based on keeping the chest in a negative-pressure environment that was contained in a closed system such as the "iron lung." As technology advanced, so did the benefits. Healthcare providers were able to perform surgeries that weren't possible without mechanical ventilation, and many patients who previously wouldn't have survived recovered from infections such as pneumonia. However, there were also drawbacks. The equipment was large and difficult to use, most ICUs weren't able to handle more than four or five ventilated patients, and there was difficulty maintaining adequate ventilation. Today's advanced ventilators are portable and use positive pressure-the forcing of gases into the chest-instead of negative pressure. Patients are no longer placed inside the ventilator; an ET tube is all that's required.

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Learn more about it

Adams AB.Too many ventilator modes! Respir Care. 2012;57(4):653-654.

Daoud EG, Farag HL, Chatburn RL.Airway pressure release ventilation: what do we know. Respir Care. 2012;57(2):282-292.

Kacmarek RM.The mechanical ventilator: past, present, and future. Respir Care. 2011;56(8):1170-1180.

Siau C, Stewart TE.Current role of high frequency oscillatory ventilation and APRV in acute lung injury andacute respiratory distress syndrome. Clin Chest Med. 200;29(2):265-275.

Singer BD, Corbridge TC.Basic invasive mechanical ventilation. South Med J. 2009;102(12):1238-1245.