Nocturnal non-invasive positive pressure ventilation: Physiological effects on spontaneous breathing
Introduction
Non-invasive positive pressure ventilation (NPPV) delivered by a facial mask is a well established and increasingly used therapeutic option for patients with chronic hypercapnic respiratory failure (HRF) due to various underlying diseases, such as chest wall deformities, neuromuscular diseases and chronic obstructive pulmonary disease (COPD) (Anon., 1999, Mehta and Hill, 2001, Hill, 2004). Here, intermittent NPPV is used at home predominantly during night while patients usually breathe spontaneously during daytime for most of the time.
In patients with chronic HRF, NPPV can improve gas exchange during nocturnal ventilation, but also during subsequent daytime spontaneous breathing with sustained reductions in daytime PaCO2 (Mehta and Hill, 2001). In addition, it has been shown that tidal volume (VT) during spontaneous breathing increases following intermittent NPPV (Schönhofer et al., 1997a, Schönhofer et al., 1997b, Diaz et al., 2002). However, it remains undefined how fast improvements of daytime PaCO2 and VT can be achieved by nocturnal NPPV and how stable PaCO2 values and VT are during daytime when NPPV is applied during night only. Further, it is unclear whether there is a stepwise improvement of spontaneous ventilation during daytime following nocturnal NPPV or whether spontaneous ventilation during daytime is improved promptly when NPPV is paused. Accordingly, there is no clear evidence for when blood gases should be measured during daytime in order to evaluate the success of nocturnal NPPV. Therefore, the time of PaCO2 measurements has not been given in nearly all former studies, which have been aimed at assessing the effects of nocturnal NPPV on gas exchange during daytime leading to an impaired comparability of these studies.
The aim of the present study was, therefore, to investigate short-term and long-term effects of nocturnal NPPV on the course of PaCO2 and VT during daytime spontaneous breathing. Thereby, it was hypothesised that there is a dynamic process of the adaptation on spontaneous breathing with a stepwise increase of VT associated with a stepwise decrease of PaCO2 during daytime following controlled nocturnal NPPV if aimed at maximal reduction of PaCO2.
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Materials and methods
The study protocol was approved by the Agency of Ethics of Albert-Ludwig University, Freiburg, Germany, and was performed in accordance with the ethical standards laid down 2000 in the Declaration of Helsinki. Informed written consent was obtained from all patients.
Patients characteristics and ventilator settings
Twenty-four patients were eligible for the study: 12 patients who received NPPV and 12 controls. Patients were separated in two groups: COPD (Table 1) and restrictive ventilatory disorders (Table 2). Accordingly, six patients in the NPPV group and six controls suffered from COPD, and six patients in the NPPV group (three with tuberculosis sequelae, two with obesity hypoventilation syndrome and one with Curschmann–Steinert disease) as well as six controls (two with tuberculosis sequelae, one
Discussion
In the present study, PaCO2 was significantly reduced in the early morning following nocturnal controlled NPPV immediately after cessation of NPPV when compared to PaCO2 prior to NPPV. In contrast, VT and fb after switching from NPPV to spontaneous breathing after successful establishment of NPPV were comparable when compared to baseline data. Therefore, the drop of PaCO2 at this time could not be explained by augmentation of spontaneous ventilation at this time, but was directly attributed to
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