By J. Brady Scott, MSc, RRT, RRT-ACCS, AE-C, FAARC, FCCP

Associate Professor, Department of Cardiopulmonary Sciences, Division of Respiratory Care, College of Health Sciences, Rush University, Chicago

Noninvasive ventilation (NIV) is a commonly used modality in adult acute care.1 When used to treat clinical conditions such as acute cardiogenic pulmonary edema and acute exacerbation of chronic obstructive pulmonary disease, NIV has been shown to reduce the need for intubation and improve mortality.1-6 Although numerous studies have been done to evaluate the effect of NIV on outcomes such as morbidity, mortality, and costs, the focus largely has been on the clinical indications or the “who” of NIV. This article examines the other aspects of NIV that might affect the modality’s success or failure.

The Interface

An essential consideration for NIV success is the mask interface. The mask interface separates NIV from invasive mechanical ventilation and is designed to allow for the delivery of positive pressure ventilation (PPV) without an endotracheal or tracheostomy tube. Masks are made from clear, hard plastic and are constructed with cushions and a soft inner lip to assure a seal around the face to allow for PPV. Ideally, the cushion and soft inner lip make the mask comfortable and tolerable for the patient and reduce facial skin breakdown.

Several types of masks are commercially available. Clinicians need to select the interface based on purpose, facial characteristics, staff experience/preference, patient preference (when applicable), and compatibility with the ventilator used to deliver NIV. In general, oronasal masks (covering the mouth and nose) and total face masks (covering mouth, nose, and eyes) are recommended when treating acute respiratory failure.1 The helmet interface has emerged as a promising option as well.7,8 Regardless of the interface used, clinicians need to be specifically trained on mask interfaces to assure proper fit and interface/ventilator compatibility.

Proper mask fitting can be challenging. Sizing guides are available to help clinicians find the appropriately sized mask based on individual patient characteristics. Using sizing guides can help reduce waste and cost and can improve patient compliance. In hirsute patients and those with facial irregularities, clinicians may need to change the interface completely (e.g., oronasal to total face mask) if an adequate seal cannot be made.9 Current NIV mask designs allow clinicians to maintain an adequate seal without overtightening the straps. Overtightening a mask to correct leaks should be avoided since this can lead to facial skin breakdown. If skin breakdown is of concern, clinicians should assess for proper fit, adjust the forehead arm of the mask (if applicable), consider changing the interface altogether, or add skin barriers.1,9

Beyond proper fit, clinicians must use an interface compatible with the ventilator circuit configuration in use. Using an incorrect interface could result in carbon dioxide (CO2) rebreathing. Single-limbed circuits should be fitted with an interface that includes an exhalation/leak port. Dual-limbed circuits do not require a leak port because of the exhalation limb and valve. Usually, masks are colored and packaged differently to identify their intended use. Clinicians should check the interface and circuit compatibility with the ventilation device being used before placing the patient on NIV.1,9,10

Delivering NIV: Bi-Level vs. Critical Care Ventilators

When NIV is clinically indicated, clinicians must choose the device capable of delivering it. In the acute care setting, both bi-level and critical care ventilators can be used to provide NIV. Bi-level ventilators have single-limb configurations and are designed specifically to operate in the presence of leaks. The leak flushes the circuit of exhaled gases through the leak port in the mask interface or circuit. Because of the absence of an exhalation limb of the circuit, mask interfaces used on bi-level ventilators should include an anti-asphyxiation valve. The anti-asphyxiation valve allows a patient to breathe fresh gas in the event of device failure.1,9-11

Critical care ventilators have dual-limb circuit configurations and do not require an anti-asphyxiation valve in the interface. Rebreathing of exhaled gases is minimized because of the exhalation limb and valve. Although historically poor at providing NIV, modern critical care ventilators now have NIV modes that may activate leak compensation and deactivate unnecessary and problematic alarms.10,12,13

The decision to use a bi-level or critical care ventilator is based on modes, leak-compensation abilities, trigger/cycle settings, monitoring capabilities, portability, and familiarity. Bench and clinical studies have shown that bi-level ventilators outperform most critical care ventilators in the presence of a leak.10 That said, some critical care ventilators have performed comparatively well.10 It also should be noted that some newer-generation devices have not yet been evaluated and may compare well to bi-level devices. More studies are needed to assess NIV performance in newer-generation mechanical ventilators. Clinicians responsible for initiating and managing NIV should be aware of the capabilities of devices they have available.

Modes

Several modes can be used for NIV.1,10,12 Mode terminology differs between devices, which is of no consequence as long as the clinician understands the differences.1 For example, in bi-level devices, the spontaneous/timed (S/T) mode is used commonly to support patients in acute hypercapnic and hypoxemic respiratory failure.1,10,12 In that mode, clinicians set an inspiratory pressure, termed inspiratory positive airway pressure (IPAP), which is functionally similar to pressure support (PS). Clinicians also set an expiratory pressure, termed expiratory positive airway pressure (EPAP), which is functionally similar to positive-end expiratory pressure (PEEP). Although similar, the relationship between IPAP and EPAP is different than that of PS and PEEP. For example, the setting of 12 cm H2O of IPAP and 6 cm H2O of EPAP on a bi-level device results in a PS level of 6 cm H2O. The peak inspiratory pressure would be 12 cm H2O. On a critical care ventilator, 12 cm H2O of PS and 6 cm H2O of PEEP results in an additive peak inspiratory pressure of 18 cm H2O. The amount of inspiratory support given to the patient on the critical care ventilator would be 6 cm H2O more than on the NIV device.1,9,10

Leak

Although devices have leak compensation capabilities, leak is a significant cause of NIV failure. The ability to compensate for leaks varies between devices, since some appear to compensate better than others.10,11,13 When leaks are present, clinicians should prioritize proper mask fitting. The amount of leak that each device can tolerate is not currently known. If trigger and cycle asynchrony occurs, the leak is unacceptable and needs to be resolved.

Trigger and Cycle

Setting proper trigger and cycle criteria on NIV can improve patient-ventilator synchrony and increase compliance with the modality. On bi-level ventilators, the trigger and cycle criteria are auto-adaptive and based on internal algorithms, preventing the need for clinicians to set them. On critical care ventilators, trigger and cycle have to be set manually by the clinician. Generally, flow-trigger is used and has been shown to reduce trigger delays.14 The best way to set trigger is unknown, but as with invasive mechanical ventilation, setting it as sensitive as possible without causing auto-triggering probably is best.10 The cycle criteria on critical care ventilators when PS is used are as a percentage of peak inspiratory flow. When large leaks are present, the cycle criteria may need to be adjusted to allow the breath to terminate. Backup safety systems may cycle the breath to exhalation after a set time at IPAP or PS, and this may vary between devices. If pressure control modes are used on critical care ventilators for NIV, the cycle setting is inspiratory time.10 The central point with trigger and cycle criteria is that some devices require adjustments, whereas others do not.

Monitoring, Portability, and Familiarity

Both bi-level and critical care ventilators have sophisticated alarm and monitoring capabilities. Alarms should be set carefully to provide maximum safety while simultaneously reducing nuisance alarms.10,15 Scalar graphics, pressure readings, and other values should be assessed similarly to patients on invasive mechanical ventilation in an effort to improve patient-ventilator synchrony and reduce harm.16 Most modern NIV devices can connect to remote monitors, allowing facilities to use NIV outside of the intensive care or emergency department environment. However, using NIV to treat patients with acute respiratory failure outside of those environments has not been well-studied and should be avoided when possible.10,17

If a patient requires transport while on NIV, portability will need to be considered. Devices vary significantly regarding their battery life. Some devices are equipped only with internal batteries, while others have optional external batteries to supplement internal batteries. Either way, when a compressor is in use, battery life may be shortened significantly.18

Finally, clinicians responsible for the initiation and subsequent NIV management should be familiar with the NIV device and its adjuncts. Clinical training programs designed to familiarize clinicians with NIV should include comprehensive training on the nuances between devices, appropriate sizing of mask interfaces, humidity, aerosol delivery techniques, and strategies to improve patient compliance.

Summary

A substantial amount of literature informs clinicians about proper patient selection regarding NIV. However, to be successful, clinicians also should consider how devices and settings can affect NIV success. Those responsible for initiating and managing NIV should be well-trained in all aspects of the modality beyond the clinical indications.

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