Noninvasive Ventilation

Chapter 3


Noninvasive Ventilation image



Non-invasive ventilation (NIV) refers to the delivery of ventilatory support without the use of an endotracheal or tracheostomy tube. Since the early 2000s, there has been resurgence in the interest and use of NIV in the intensive care unit (ICU). This chapter examines the evolution, indications, contraindications, and practical application of NIV to ensure appropriate and successful use in ICU patients.



The Evolution of NIV


NIV was the mainstay of mechanical ventilatory assistance outside the operating suite through to the mid 20th century. Traditionally, it was delivered by negative pressure devices such as the “iron lung” that was used predominantly for poliomyelitis patients with respiratory paralysis. When the polio epidemic in Denmark in 1952 created a demand for negative pressure ventilators that overwhelmed the supply of iron lungs, there was a transition to positive pressure mechanical ventilation via translaryngeal cuffed endotracheal tubes. Subsequently, in view of their much higher survival rates, invasive mechanical ventilation became the standard of care for acute respiratory failure (ARF) resulting from polio and other disorders in the ICU.


It was not until the 1980s with the development of nasal masks for continuous positive airway pressure (CPAP), used for the treatment of obstructive sleep apnea (OSA) (Chapter 80), that there was a renewed interest in NIV and specifically non-invasive positive pressure ventilation. The positive pressure did not cause the upper airway collapse commonly precipitated by negative pressure ventilators. Soon after, successful NIV use in chronic respiratory failure from a variety of neuromuscular and restrictive thoracic disorders was described.


By preserving the patient’s own upper airway defense mechanisms, NIV avoids the potential complications associated with intubation itself, including laryngeal injury. NIV has been shown to lower the risk of nosocomial infections, i.e., ventilator-associated pneumonia (VAP) (Chapter 14); improve comfort and thereby reduce need for sedation; and allow patients to eat, drink, cough, and communicate, permitting greater independence and active patient participation in medical management.


In addition, in selected populations NIV has been shown to be effective in preventing intubation in patients in ARF. This has led to an increase in its use, with a rate of 35% reported among ventilated patients in European ICUs. However, studies have shown large disparities in its utilization among ICUs with apparent underutilization in many centers. One reported reason for the reduced utilization has been lack of physician knowledge and familiarity.




Interface Used in NIV


A proper interface is paramount for NIV to be effective. There are a variety of masks that can be used for NIV. These masks include the oronasal or full-face mask, the nasal mask, nasal “pillows” consisting of soft pledgets inserted directly into the nostrils, mouthpieces held in place by lip seals resembling a snorkel, a total face mask resembling a plastic hockey goalie’s mask, and the helmet (fits over the entire head). Interfaces are available in multiple sizes and shapes with various modifications ranging from straps to custom-molded masks to optimize fit and comfort.


Each interface has its own potential advantages and disadvantages, and the choice depends ultimately on the patient. There are no data comparing the effectiveness of the different masks used for NIV. Some degree of air leak either through the mouth or around the mask is universal, and patient cooperation is needed to minimize leak. The full-face mask is often preferred when initiating NIV in patients with ARF in the ICU because these patients tend to mouth breathe. However, the full-face mask interferes with speech, expectoration, and eating and it carries the risks of claustrophobia, aspiration, and rebreathing when compared to the nasal mask. Dentures should be left in place to optimize the fitting of the mask. The nasal mask requires patent nasal passages and mouth closure to minimize air leaks. Heated humidification may minimize mouth leak and improve comfort. Humidification is usually required to prevent upper airway drying. Regardless of the interface chosen, adequate time should be spent with the patient to ensure proper fit, comfort, and acclimatization with appropriate coaching and encouragement.



Ventilators


Most NIV ventilators are now positive pressure devices, assisting ventilation by the delivery of pressurized gas to increase transpulmonary pressures and inflate the lungs. Positive pressure devices consist of the standard critical care ventilators designed for use on intubated ICU patients and portable ventilators designed specifically for non-invasive ventilation. Although traditionally these two devices varied in the features offered, the distinction between the two has blurred. Conventional ICU ventilators typically offer better alarm features, allow precise O2 concentration delivery, minimize rebreathing by having separate inspiratory and expiratory tubing, and are able to generate higher inspiratory pressures compared to portable ventilators. In contrast, portable devices are designed to be more compact, convenient, and economical, with better leak compensation and greater comfort by adjusting triggering, cycling, and inspiratory flow rise times at the expense of limited pressure-generating capabilities, often with peak pressures of 20 to 30 cm H2O. Rebreathing from the single tubing can be minimized by an expiratory valve but may increase expiratory resistance and work of breathing. In practice, the choice of ventilator used is largely influenced by local availability, expertise, and costs.



Ventilator Modes and Settings


The same modes of ventilation are available for non-invasive ventilation as they are for invasive ventilation and can be divided into volume-cycled and pressure-cycled types. Studies directly comparing the two have suggested better patient tolerance with similar rates of efficacy with pressure-cycled modes. Most randomized controlled trials on NIV in ARF have used pressure-cycled modes and in practice, NIV is largely delivered by pressure-cycled ventilation.


Portable ventilators are designed to deliver continuous positive airway pressure (CPAP) or bilevel positive airway pressure (BIPAP) with or without a backup rate (note that the similar abbreviation, BiPAP, is a registered trademark of the Respironics Corporation). CPAP delivers a constant set pressure during both inspiration and expiration to increase functional residual capacity and improve oxygenation, but it is strictly not a form of ventilatory assistance. BIPAP provides positive airway pressure in a biphasic manner. An inspiratory positive airway pressure (IPAP) is set for inspiration and a lower expiratory positive airway pressure (EPAP) is set for expiration, whereas the difference between IPAP and EPAP accounts for the degree of ventilator assistance. EPAP not only ensures flow to flush CO2 from the single ventilator tube and avoid rebreathing, but it increases functional residual capacity, stents open the upper airway to prevent apneas and hypopneas, and counterbalances intrinsic positive end-expiratory pressure (PEEP) in patients with chronic obstructive pulmonary disease (COPD). As with the standard ICU ventilator, the patient triggers it and tidal volumes can vary. A backup rate can be set (spontaneous/timed [S/T], similar to intermittent mandatory ventilation [IMV] [see Chapter 2]). It is recommended if there is any doubt regarding whether the patient will maintain spontaneous respiratory efforts (e.g., during sleep or with sedation for procedures).


Conventional ICU ventilators, like the Puritan Bennett 840 ventilator, offer a non-invasive mode similar to BIPAP, but care must be taken with nomenclature. A pressure support mode of ventilation (PSV) is chosen where a preset level of inspiratory assistance, the pressure support (PS), is delivered (i.e., added to—pressure-wise) to a preset expiratory pressure, the PEEP, when triggered by the patient. A PS of 7 cm H2O and PEEP of 5 cm H2O on the standard ICU ventilator is equivalent to an IPAP of 12 cm H2O and an EPAP of 5 cm H2O on portable BIPAP devices where PEEP is interchangeable with CPAP.


In some patients, volume-cycled modes of ventilation may be more appropriate, and clinicians should be familiar with its use. This is particularly true when higher airway pressures are required to overcome increased respiratory impedance, as in obesity hypoventilation syndrome (Chapter 80). Generally, higher initial tidal volumes of 10 to 15 mL/kg predicted body weight are needed in these patients to normalize the arterial PCO2. More recently, there has been interest in proportional assist ventilation, whereby the ventilator applies assistance in proportion to the patient’s inspiratory effort in an attempt to optimize patient-ventilator synchrony (Chapter 2).


The mode of ventilation determines the parameters that need to be set, but there is a lack of evidence and no standard guidelines for initial ventilator settings. Goals of care differ in acute and chronic respiratory failure. Prompt correction of ventilation is desired in ARF, but it is generally recommended to start with low pressure settings and titrate up slowly to allow the patient to acclimate to NIV. For example, initial settings could be an IPAP of 8 cm H2O and an EPAP of 4 cm H2O, with a backup rate of 10-12 breaths per minute. Furthermore, unlike invasive mechanical ventilation, NIV does not need to be applied continuously to be effective, although in the acute setting, most favor continued use until clinical improvement has been demonstrated. In any case, close monitoring by means of an arterial line to follow arterial blood gases and titration are required.



Monitoring


Initiation of NIV in ARF requires close monitoring by appropriately skilled staff. The site of initiation must be properly chosen. NIV offers the unique opportunity to be provided outside the ICU setting such as in the emergency department, but there needs to be adequately staffed and trained personnel in order to do this successfully. In acute respiratory failure where there is high risk of clinical deterioration requiring endotracheal intubation, NIV must be performed in an ICU environment. A multidisciplinary team involving the physician, nurse, respiratory therapist, and the patient is recommended. Ideally protocol and guidelines should be in place with regular audits to ensure quality control.


Both subjective and objective physiologic responses should be monitored, especially in the initial 2 hours, as prompt improvement has been associated with NIV success. Patients should be assessed clinically for improvements in respiratory distress, including the use of accessory muscles, tachypnea, chest wall movement, fatigue and level of consciousness, comfort, and patient ventilator synchrony. Vital signs including heart rate and respiratory rate should be monitored, and continuous pulse oximetry should be applied. Frequent arterial blood gases (via an arterial catheter) are recommended as early improvement in gas exchange is predictive of NIV success. The minute ventilation should be adjusted to improve arterial pH and PaCO2. Ideally ventilators able to monitor airway pressures, expired volumes, and airflow should be utilized. Patient tolerance and comfort should be continuously monitored, and this requires close communication with the patient. Close ICU monitoring should mean intubation is not delayed when necessary. If after 2 hours NIV is not successful, conventional intubation should be considered.

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Jul 7, 2016 | Posted by in CRITICAL CARE | Comments Off on Noninvasive Ventilation

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