Non-invasive ventilators are usually much simpler than those found on the ICU, with less settings to choose from. This is because most are designed for home use. The disadvantage of this is that they also tend to be poorly equipped in terms of monitoring and alarms when used in hospital. Home NIV will not be discussed here. The recent trials of NIV in exacerbations of COPD have used pressure-controlled ventilators. BiPAP (Respironics Inc.) is a commonly used pressure control ventilator in acute respiratory units in the UK.
Acute NIV should be considered in patients with mild to moderate acute respiratory failure (hypercapnia causing a pH of 7-2-7-35). The patient should have an intact airway, protective airway reflexes and be alert enough to follow commands. A common method is to begin with the expiratory level (EPAP) at 5 cm H2O and the inspiratory level (IPAP) at 15 cm H2O. The levels are adjusted based on patient comfort, tidal volume achieved (if measured), and arterial blood gases.
The main indications for acute NIV are:
• exacerbation of COPD with a mild to moderate respiratory acidosis
• weaning from invasive ventilation.
The only condition for which there is conclusive evidence for acute NIV is acute exacerbations of COPD (see minitutorial below). NIV can also be used as a step-down treatment in patients who have been intubated and ventilated on ICU. Failure to wean exceeds 60% in COPD patients and this is a major cause of prolonged ICU stay and ICU costs. A recent randomised multicentre trial has shown that NIV is more successful in weaning than a conventional approach in patients with COPD. Patients who failed a T-piece trial (breathing spontaneously with no support) 48 hours after intubation were randomly assigned to receive either NIV immediately after extubation or conventional weaning (a gradual reduction in pressure support). The NIV group took a shorter time to wean, had shorter ICU stays, a lower incidence of hospital-acquired pneumonia, and increased 60-day survival. Other studies have reported similar findings.
Reports conflict regarding the efficacy of NIV in acute respiratory failure from other conditions such as ARDS and pneumonia. Earlier trials of NIV in pneumonia were discouraging, but a recent prospective randomised trial of NIV in community acquired pneumonia (56 patients) showed a significant fall in respiratory rate and the need for intubation. Just under one half of the patients in this study had COPD and it was carried out in an ICU with ready access to intubation. Generally, NIV failure rates are higher in non-COPD patients. NIV should never be used in acute severe asthma.
Mini-tutorial: NIV for exacerbations of COPD
An exacerbation of COPD requiring admission to hospital carries a 6-26% mortality. One study found a 5-year survival of 45% after discharge and this reduced to 28% with further admissions. Invasive ventilation in COPD carries a 50% or less survival to discharge. Ventilator-associated pneumonia is common and increases mortality still further. NIV is associated with less complications than intubation (Table 4.3).
Table 4.3 Complications of NIV versus intubation NIV Intubation
Necrosis of skin over bridge of nose Pneumonia Aspiration Barotrauma and volutrauma
Changes in cardiac output (less) changes in cardiac output complications of sedation and paralysis tracheal stenosis/tracheomalacia
Most studies of NIV in acute exacerbations of COPD have been performed in critical care areas. The majority used ICU ventilators with a face mask as opposed to custom-built NIV ventilators. The studies performed in general ward areas involved patients with a pH of > 7-29. There have been at least half a dozen prospective randomised controlled trials of NIV in acute exacerbations of COPD. The studies performed in ICUs showed a reduction in intubation rates and some also showed reduced mortality when compared with conventional medical therapy. None has directly compared NIV with intubation. A recent multicentre randomised controlled trial of NIV in general respiratory wards showed both a reduced need for intubation and reduced hospital mortality. Patients with a pH of < 7-3 after initial treatment did less well and it has been recommended that this group is managed in an ICU.
NIV should be commenced as soon as the pH falls below 7-35 because the further the degree of acidosis, the less the chances are of improvement. It should be used as an adjunct to full medical therapy, which treats the underlying cause of acute respiratory failure. However, in a 1-year prevalence study of 954 patients admitted with an exacerbation of COPD to one hospital, 25% were acidotic on arrival in the Emergency Department, but their pH was normal by the time they were admitted to a ward. This included patients with an initial pH of < 7-25, which suggests that NIV should be commenced after controlled oxygen and medical therapy has been administered.
Patients on NIV require close supervision because sudden deterioration can occur at any time. Simple measures such as adjusting the mask to reduce excessive air leaks can make a difference to the success or otherwise of treatment. Basic vital signs measured frequently give a indication of whether or not NIV is effective or failing. If NIV does not improve pH and respiratory rate in the first 2 hours, intubation should be considered. Predictors of failure of NIV in acute COPD are as follows:
• no improvement within 2 hours
• high APACHE II score (acute physiological and chronic health evaluation)
• very underweight patient
• neurological compromise
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