Non-invasive ventilation – a primer

One of the questions for the last case is whether non-invasive ventilation (NIV) would be an appropriate intervention in a patient with severe hypoxemia. As a reminder, NIV employs a mask fitting over the nose and/or mouth and provides continuous positive airway pressure (CPAP) with or without positive inspiratory pressure (often referred to as BiPAP).

The most common diagnoses to use NIV in include CHF and COPD.

In CHF, the physiological rational for CPAP is that it reduces preload, decreases transmural ventricular pressure, and decreases afterload. Does it actually improve patient-centred outcomes? This 2013 Cochrane Review reviewed 32 studies with 2916 patients who received CPAP or BiPAP in addition to standard medical therapy compared to standard medical therapy alone found decreased mortality (RR 0.66) and need for intubation (RR 0.52) in the NIV group.

The COPD patient is another who may benefit from NIV where positive-pressure relieves the fatigued diaphragm and decreases work of breathing. This theoretical benefit has translated into improved outcomes in multiple RCTs and meta-analyses. This 2004 Cochrane Review showed decreased mortality (11% vs 21%) and need for intubation (16% vs 33%) when NIV was added to standard therapy.

The evidence base for the use of NIV is strongest in patients with CHF and COPD, but there is a body of evidence suggesting it may be beneficial in the following patient groups:
AsthmaThis small RCT of 30 patients showed improved FEV1 and decreased hospital admission (18 vs 62%) in the group receiving BiPAP.
Immunocompromised patients – The theoretical benefit in this population is avoiding intubation and its associated nosocomial infections. This 2001 RCT in NEJM showed decreased need for intubation and mortality. However, patients who fail NIV tend to have higher mortality. Risk factors for failure include higher APACHE II scores, need for vasopressors, and low P/F ratio.
Pneumonia – One of the most common reasons for intubation and ICU admission, but increased secretions are considered to be a relative contraindication to NIV. A Cohrane Review showed decreased need for intubation and mortality in patients with pneumonia who received NIV in addition to standard therapy. Care should be taken to only consider NIV in those patients who are able to clear their secretions on NIV.
ARDS – This is a generally sick patient population and there is limited data behind the use of NIV. A small RCT of 40 patients compared NIV vs supplemental oxygen in patients with ARDS, P/F ratio >200 and found decreased need for intubation in the NIV group along with a trend towards improved mortality.

A general principle to guide the use of NIV is consideration of a timed trial. It may be that CHF and COPD patients do so well with NIV because their underlying exacerbations show relatively rapid improvement with appropriate medical therapy. This is contrasted to pneumonia/ARDS where the natural history of resolution is over days-weeks even with appropriate therapy. Prolonged NIV comes with the cost of device-associated complications like pressure ulcers, but also importantly the possibility of delaying intubation. If your patient on NIV is not improving in the first 4-6 hours, you may have to first reconsider your diagnosis and second consider intubation and mechanical ventilation.

This post is a very brief superficial review of non-invasive ventilation. This CMAJ clinical practice guideline provides some more framework and references.

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