Adaptive Non-invasive Ventilation to Abolish Tidal Flow Limitation

The partial pressure of carbon dioxide measures the efficacy of ventilation. Reducing PCO2 is the aim of nocturnal non-invasive ventilation in stable COPD patients with chronic hypercapnia. PCO2 is normally measured non-invasively using a transcutaneous probe during overnight studies.

Hypercapnia is defined as PtcCO2 > 45 mm Hg. The aim of non-invasive ventilation in stable COPD patients is to reduce the time spent with PCO2 levels above the physiological range.

Peripheral oxygen saturation in a non-invasive measure of oxygenation, which is routinely evaluated in sleep and ventilation trials.

Desaturations are common complications in respiratory disorders. A desaturation is commonly defined by a reduction in SpO2 < 90%. The investigators computed the percent of night time in which SpO2 was < 90% and the Oxygen Desaturation Index (number of desaturations per hour).

Ineffective efforts are defined as inspiratory efforts that are not supported by the ventilator. They are indicative of patient-ventilator asynchrony, they may be associated with tidal expiratory flow limitation and they increase work of breathing and cause discomfort.

Sleep quality is a key outcome during overnight studies. Nocturnal non-invasive ventilation may affect sleep quality in two ways: 1) it may improve it by reducing sleep-related respiratory problems; 2) it may worsen it if the ventilation mode causes discomfort to the patient. Sleep quality was evaluated by full laboratory polysomnography. Sleep stages were identified on epochs lasting 30 seconds by a trained operator blind to the study arms. Sleep quality was quantified using the following parameters: Sleep efficiency (percent of night time asleep), percentage of sleep time spent in phase 3 sleep and in REM sleep, number of awakenings per hour.

The expiratory positive airway pressure (EPAP) is the parameter that is adjusted in the experimental arm. The ventilator automatically titrates EPAP to the minimum value able to abolish expiratory flow limitation.

Tidal volume, respiratory rate, minute ventilation, and inspiratory to expiratory time ratio are the breathing pattern parameters that the respiratory control system adjusts to achieve an adequate gas exchange while minimizing work of breathing. Non-invasive ventilation interacts with and supports breathing pattern.

The difference between mean inspiratory and expiatory reactance is measured using the forced oscillation technique. The difference between mean inspiratory and expiratory reactance is an index of tidal expiratory flow limitation. Values > 2.8 cmH2O*s/L are indicative of tidal expiratory flow limitation.

Mean inspiratory resistance is measured using the forced oscillation technique. Inspiratory resistance is an index of airway obstruction, which increases work of breathing and contributes to the development of flow limitation.

Mean inspiratory reactance is measured using the forced oscillation technique.

During full laboratory polysomnography the respiratory activity is monitored and sleep related respiratory events are analysed, such as apneas, hypopnea, with a breakdown into central, peripheral or mixed events.