This chapter is most closely associated with the demands of the 2017 edition of the CICM Ventilation work-based competency assessment. According to this document, a trainee who “describes the role and limitations of noninvasive ventilation” would pass as at least “acceptable”, though it is not clear what that minimum might look like. Judging by what the college questions have historically asked about, it is approximately half a page of handwritten text. The main points regarding NIV are summarised in the NIV chapter from the Required Reading section for the Part Two exam – there are only a handful of important points. Generally speaking, it appears the college examiners have never expected any more from the trainees than what is contained there, and these key elements are reproduced below in a greyish box of sorts, as if to somehow emphasise their importance.
Physiological basis of the treatment effect
- Positive pressure ventilation, in general, has benefits which are common to both NIV and IPPV.
- Tight-fitting mask ensures accurate delivery of prescribed oxygen concentration
- Positive airway pressure decreases airway closure in OSA
Advantages when compared to invasive ventilation
- Decreased cost
- Better tolerated (no need for sedation)
- More convenient
- Better availability outside of the ICU setting (eg. domiciliary)
- Ability to interrupt therapy for breaks allows easier weaning from mechanical support
- Does not require airway skills to commence
Disadvantages when compared to invasive ventilation
- More difficult to manage with an uncooperative patient
- Cannot be used in physically restrained patients (what if they vomit? They cannot remove the mask)
- Prevents the effective clearance of secretions and impairs physiotherapy access for suctioning
- Cannot be performed on patients with a decreased level of consciousness
- Mask-face interface is difficult to manage: "one size fits all" masks do not, in fact, fit all; patients with unusual anatomy or little facial soft tissue (eg. in cachexia) will have more difficulty
- Mask leak is uncomfortable and decreases the effectiveness of the therapy
- Work of breathing may be increased (i.e. mandatory mechanical breaths are usually impossible or dangerous)
Probably the best-suited single resource for this discussion would be the excellent 2003 article by Laurent Brochard. This paper goes into some considerable detail to explore the evidence for the various indications for NIV, such as acute pulmonary oedema and COPD. This has obvious exam value but is not the topic of this CICM Part One - level chapter, where physiological and practical effects of NIV are examined. With the effects of positive pressure ventilation having been thoroughly dissected elsewhere, this chapter will focus only on what is uniquely special about NIV, wherever possible trying to steer away from overlaps with invasive mechanical ventilation. The question it is supposed to answer is, “why NIV and not intubation”, or “why NIV and not high-flow humidified oxygen?” In summary, one might say that NIV is a complementary therapy, one which co-exists along a spectrum with invasive ventilation, and which can never completely replace it.
Advantages of NIV as compared to invasive mechanical ventilation
Domiciliary use is possible
Airway skills are not required to apply NIV. Though one cannot say that the use of mask interfaces and ventilator circuits is something which could be left to completely unskilled individuals, it is beyond argument that the use of NIV requires less medical prowess than intubation and invasive mechanical ventilation. This is well demonstrated by the fact that the real era of popularity for NIV began with home-based ventilation of OSA patients via a nasal mask. When Delaubier et al (1987) went on to use it to prolong the lifespans of Duchenne’s muscular dystrophy patients, the convenience of this technique became truly apparent. Prior to this, tracheostomy or negative pressure ventilation were the only options available for patients with chronic hypercapnic respiratory failure.
Decreased cost as compared to invasive ventilation
In the delightful atmosphere of financially ignorant Australian medicine, it is possible to operate successfully as a trainee while remaining totally oblivious to the costs of health care interventions (at least for much of the time). However, at the department head level, these things begin to keep you up at night. It is often said that NIV saves the department money, and this would be an important advantage to mention.
It is, of course, important to acknowledge that NIV does not “save” costs as such, because patients who require invasive ventilation will still require invasive ventilation (there’s often no way of getting around that). However, the availability of NIV may prevent intubation in a select group of patients, whose stay in hospital will, therefore, require fewer nursing and medical resources.
Realistically, the equipment cost of NIV is virtually the same as invasive ventilation. It is often the same ventilator and the same humidified circuit. Reusable masks are no cheaper than single-use masks if you consider the cost of sterilising them between patients (unless, of course, you don’t). Cost savings for hospital inpatients are therefore mainly in terms of personnel, as one does not require to have an airway expert, 1:1 nurse or dedicated physiotherapy staff. This translates into decreased Intensive Care Unit room use, per life saved with NIV. Plant et al (2003) were able to demonstrate this with some British pounds – the cost savings were £645 per death avoided. In clinical trials where the patients were randomised to either have NIV or some sort of “standard management” which presumably consisted mainly of invasive ventilation, allocation to the group receiving NIV was associated with a reduction in costs of £49,362. “A typical UK hospital providing a non-invasive ventilation service will avoid six deaths and three to nine admissions to intensive care units per year”, concluded the authors.
Presumably, in Australia, this would be somewhat different. Here NIV tends to happen in ICU (so ICU admission would not be prevented per se), but the savings in personnel costs would probably still be reproduced. In any case, various authors (for example McCurdy et al, 2012) have been able to demonstrate that NIV outcomes are not improved by changing the location of care (i.e. ward vs. ICU) and so the potential remains for healthcare cost savings.
Delivery of a precisely controlled oxygen concentration and airway pressure
A major contribution to the success of NIV in the management of respiratory failure is the ability to deliver a controllable amount of oxygen. Up until the tight mask is strapped on, the hypoxic patient is tortured with various variable-performance oxygen delivery devices. They claw at the non-rebreather bag, ventilate their cheek with misplaced nasal prongs, or take huge rapid breaths well in excess of even the flow rate expected of Venturi masks. In short, only with the application of a tightly sealed interface does the patient finally get the concentration of oxygen which their doctor prescribed for them.
The same can be said for pressure. The NIV mask, for all its flaws, is a tight-fitted thing which seals the airway together with the ventilator circuit. Pressure is therefore applied, which – to be fair- is also applied using HFNP, but – without the finicky dependence on such things as mouth opening and nostril diameter. In general, having prescribed a pressure level with NIV, the clinician can walk away with the expectation that the ventilator will deliver that pressure, or at least alarm loudly if it cannot.
Prevention of airway collapse
The first uses of “modern” NIV were for the purpose of mechanically displacing the soft palate off the posterior pharynx of the obstructing patient. Sullivan et al (1981) first reported on this use of positive pressure as a “pneumatic splint for the nasopharyngeal airway” in five OSA patients. In the ICU, this has some role to play – insofar as ICU patients are still expected to sleep occasionally, and if they have obstructive sleep apnoea then they are also expected to obstruct.
Transition from invasive mechanical ventilation
The conversion from using an artificial airway to using one’s own is not straightforward for everybody. Occasionally, re-intubation occurs as a result of poor tolerance of conventional oxygen therapy. In these circumstances, non-invasive ventilation can be a useful stepping stone, if one (for whatever reason) does not wish Without elaborating extensively on the evidence for this (eg. Ferrer et al, 2009), it will suffice to say that non-invasive ventilation can act as the bridge to respiratory normality, preventing reintubation in some groups of patients.
NIV is better tolerated
If one examines the theme closely, it would in fact be totally unreasonable to say that NIV is better tolerated than invasive ventilation. Invasive ventilation is very well tolerated by comatose intubated people who are under the effects of anaesthetic. Not infrequently, patients can be invasively ventilated while remaining comfortable with minimal or no sedation. Therefore instead it would be more accurate to say that non-invasive ventilation is more comfortable for the patient by virtue of their larynx remaining unmolested.
With that caveat, the following things can be usefully said about the tolerability of NIV:
- It does not require sedation or muscle relaxant to commence
- Mandatory modes of ventilation are generally not used, and patient-triggered ventilation is by its nature more comfortable
- Unlike invasive ventilation, one is not committed to the process: NIV is easily discontinued and re-commenced, allowing breaks from ventilation.
- Unlike (most) invasive ventilation, sedation is not required - though it is “safe and feasible” according to many review authors (eg. Hilbert et al, 2012)
Limitations of non-invasive ventilation
NIV interfaces are uncomfortable
This flies in the face of the “NIV is better tolerated” argument, but is nonetheless correct. NIV masks are generally uncomfortable appliances to wear. Even the helmet interface (Rodriguez et al, 2013) has some of these problems. Generally, the following are frequently heard complaints about the NIV interface:
- It is tight-fitting, and creates pressure areas
- It is noisy. Apart from ventilator alarms, patients report problems sleeping because of the incessant ventilator sounds. This is surprisingly well out of hand - inside helmet interfaces, Cavaliere et al (2004) reported noise levels as high as 100 dB, comparable with using a lawn mower or farm tractor. It is somewhat lower with nasal and oronasal interfaces (70 dB), comparable with music in the loungeroom of a dinner party. People who have tried to fall asleep at dinner parties will agree that this is more tolerable, but still far from ideal.
- It is hot and humid in there. The humidifier, set to invasive standards, delivers gas at 37°C, with 100% humidity. Obviously one does not need this level of humidification, but some “conditioning” of the ventilator gases is still required (Rodriquez et al, 2012).
- The air leak is generally quoted as the most annoying feature. Girault et al 92009) reported that in their group this was the most prevalent cause for “mask failure” during NIV, i.e. the patient becoming sufficiently irate with their mask to require a mask change.
These issues are not trivial. Antonelli et al (2001) found that 10% of their NIV patients were intolerant of NIV while desperately needing it, which led to their being intubated.
NIV as a time-wasting exercise which delays intubation
Without exploring the empirical evidence in any great detail, it would be important to note that the time spent setting up the ventilator circuit for NIV may take one or two nursing staff away for five minutes. This time, instead, should be used to set up for intubation if the patient is profoundly distressed and will clearly not benefit from NIV. It is clearly not easy to tell who will benefit and who will not, but posthoc analysis of such events generally points to broad indicators of greater disease severity. For example, in the case of acute lung injury from the case series by Rana et al (2006) the following features were strongly associated with failure of NIV:
- Shock (100% of the shocked patients ended up requiring intubation)
- Acidosis (SBE less than -4.0)
- Extensive bilateral Xray changes
- Severe hypoxia – a PaO2/FiO2 ratio of around 150 appears to be the threshold; patients with ratios around 140-150 successfully sailed through on NIV while those with ratios closer to 110-100
Most disturbingly (though not reaching statistical significance because of the numbers), in the Rana study the non-survivors (those who failed NIV, got intubated and then died anyway) had a longer delay before intubation (14 hours vs 10 hours). One surmises that in these people the clinician’s pointless fascination with adjusting NIV settings blinded them to the obvious need for invasive ventilation.
NIV cannot (should not) be used with physical restraints
It is generally believed that in order to safely undergo NIV, one needs to be able to remove one’s own mask when it becomes necessary to do so, eg. if one needs to vomit. If one is for whatever reason prevented from doing so, aspiration and asphyxiation may result. It follows from this argument, that patients on NIV should not be physically restrained. This is a widespread belief and has been incorporated in various departmental guidelines. As such, it aligns very well with the goals of the confused agitated patient, whose main objective is to remove their mask.
With that said, Devlin et al (2007) surveyed some 790 physicians of whom 72% used physical hand restraints more than 25% of the time for patients undergoing NIV. Onodera et al (2016) performed a prospective study and found that physical restraints were used in 14% (six) of their 43 patients, with four of them also being sedated. There does not appear to be any data regarding the rates of catastrophe with this small group, or with other physically restrained patients on NIV.
Aerophagia, nausea, vomiting and aspiration
Because the lower oesophageal sphincter being an unreliable goalkeeper, the stomach may insufflate with inspiratory gas during non-invasive ventilation. 20-25 cm H2O of pressure is generally quoted as the limit for this sphincter, (Meining et al in 2004, performing manometry in patients with GORD). In spite of this, the use of pressures below this limit will still occasionally result in air being swallowed because of the fact that the patients will cough, ventilate dyssynchronously, and otherwise do things which elevate their airway pressure. This is often underdiagnosed as it is asymptomatic, but occasionally patients will present with truly ridiculous volumes of swallowed air (Kim et al, 2015) or with complications such as gastric perforation (Nishimura et al, 2016). This was viewed as a sufficiently worrying possibility that in the early days of NIV (eg. in the 1989 trial by Meduri et al), all patients had nasogastric tubes on free drainage or wall suction while undergoing NIV. These days, even though contemporary papers give the incidence of gastric insufflation as 30-40% (Carron et al, 2009) the incidence of “major” aspiration appears to be much lower (it is given as 5% in the same paper, but it is not clear as to where the author got this figure). This may be because vomiting and nausea are widely recognised as contraindications for NIV, and so few patients with high aspiration risk are subjected to this therapy.
Unsuitability of NIV for upper gastrointestinal surgery patients
It is a widely believed fact, that upper gastrointestinal surgical patients (eg. Ivor Lewis oesophagectomy patients) are forbidden any sort of positive pressure via the upper airway, lest they burst their fragile anastomotic stitches. This, as far as one can see, has never been properly explored experimentally - perhaps on the grounds that it would be unethical to randomise patients to a therapy which everybody believes will produce fatal complications. However, this was not a deterrent to Jaber et al (2004) who published a prospective case series of patients undergoing positive pressure ventilation following abdominal surgery. “Esophageal and GI surgery were not contraindications in the present study”, the authors bravely insisted. The NIV pressure was titrated up from 8 cm H2O in 2-3cm increments until tidal volumes of 10ml/kg were reliably achieved. In their cohort, they included eight oesophagectomy or gastrectomy patients. There were no complications with pressures around 12 cm H2O. An even older study on even worse patients (Joris et al, 1997, morbidly obese bariatric surgery patients) also found that the practice was safe, provided one decompressed the stomach with a nasogastric tube and kept the pressures within a reasonable range.
On the other hand, NIV-induced damage to the non-operative upper GI tract is also a well-documented thing. Van de Louw et al (2002) reported on the case of a patient who had a fatal perforation of the oesophagus following the commencement of NIV with modest pressures (5 of PEEP, 15 of pressure support). That patient had a nephrectomy, i.e. as far as anybody could tell the surgeons were staying well away from the gastrointestinal tract. One might also point to the relative paucity of other such case reports. However, the availability of less PEEPy therapies such as high flow nasal prongs makes it difficult to argue with the surgeons.
NIV may prevent the clearance of secretions
Peter Gay (2009), in his massive homage to the complications of non-invasive ventilation, mentioned impaired secretion clearance and mucus plugging as one of the complications. Unfortunately, no reference is offered in support of this, only to say that it is infrequent among the complications of NIV. On the other hand, there is model evidence (Andersen et al, 1979) and experimental human evidence (Confalonieri et al, 1999) that NIV actually improves the clearance of secretions by “increasing collateral airflow to obstructed lung regions”. In the latter study, of the 28 patients with severe community-acquired pneumonia on NIV, only one needed to be intubated for uncontrollable respiratory secretions.
So which is it? Does NIV make secretion clearance easier or more difficult? It must in all fairness be said of the Confalonieri study, that in the NIV group five other patients were intubated for reasons not described any better than “worsening hypoxemia”, and that this study only had 28 patients in each arm. A much larger and more influential document is the 2007 IDSA/ATS guidelines statement (Mandell et al, 2007), which mentions that the inability to expectorate secretions makes NIV much less useful in suppurative lung disease. It is also a belief shared by the CICM examiners, judging by their answer to Question 1 from the second Fellowship paper of 2015. Beyond coughing up your own muck, airway suctioning manoeuvres by physiotherapists are also rendered more difficult by the interfering presence of a mask, and where clever attachments for introducing catheters are available, one tends to find the process breaks the seal and produces so much leak as to lose all benefit from the positive pressure.
NIV is unsuited for comatose patients or those with respiratory arrest
It is difficult to argue with the idea that an unprotected airway is a reasonably strong indication for invasive mechanical ventilation, and that forcing pressurised gas into the upper air passages of a patient who is comatose is a recipe for gastric insufflation and aspiration. For these reasons, NIV is generally not used in patients who are unconscious, or unable to generate respiratory effort independently. A decreased level of consciousness is almost universally listed in the contraindications for NIV. As one author put it, “although there is no scientific basis for this recommendation, the rationale was that coma patients are at risk for pulmonary aspiration by virtue of their depressed sensorium and blunted cough mechanism”.
On the other hand, Diaz et al (2005) took the viewpoint that NIV might be an elegant and well-tolerated solution to patients with hypercapnia as the main cause of coma. They were able to successfully rescue 76 comatose patients over five years (and a further 19 had to be intubated because or respiratory arrest). To ameliorate the risk from gastric distension, a nasogastric tube was inserted before NIV was commenced. Within about four hours on average, about 80% of the patients were no longer comatose.
In view of these findings, there has been some high-profile support for a trial of NIV prior to intubation. For instance, the official ATS guidelines (Rocheberg et al, 2017) support the use of NIV in such patients, quoting the Diaz trial in their statement - “We recommend a trial of bilevel NIV in patients considered to require endotracheal intubation and mechanical ventilation, unless the patient is immediately deteriorating”. In short, though this dogma begs to be challenged, for the exam-going trainee it is important to note that of the CICM examiners, all historical answers to NIV questions have quoted a low GCS as a contraindication for NIV.