Contributions to the understanding of patient-ventilator interactions in long-term non invasive ventilation
(2012)
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- (en) Positive pressure non-invasive ventilation (NIV) has been available for the treatment of chronic hypercapnic respiratory failure since the first publications by Sullivan et al, Delaubier, and Rideau in the early 1980’s. NIV was performed initially with volume-cycled ventilators, until the advent of bi-level pressure cycled ventilators, in the late 80’s, which were less cumbersome, cheaper, and in most cases perceived as more comfortable by users. The number of patients treated by NIV increased very regularly during the 1990’s, with a progressive shift in indications. As of the end of the 1990’s, Chronic obstructive pulmonary disease (COPD) and obesity-hypoventilation were clearly the most frequent indications for NIV. With the advent of bi-level pressure cycled ventilators, major changes occurred in ventilators, interfaces and monitoring techniques available. Adjustable inspiratory and expiratory triggers, adjustable rate of pressurization, built-in monitoring software and volume targeting are options currently available on many pressure cycled ventilators designed for home care. Initially, the aim of long term NIV was mainly to improve daytime arterial blood gases (mainly to correct PaCO2) and to improve nocturnal oxygen saturation (SaO2). The advent and validation of transcutaneous capnography showed however that monitoring of nocturnal PtcCO2 was necessary to ensure an optimal control of nocturnal hypoventilation: indeed, re-setting of respiratory centres through correction of nocturnal PaCO2 is probably the major physiologic explanation for the efficacy of home NIV. Also, although leaks through nasal interface were to some degree inevitable, it was shown that major leaks cause micro-arousals and disrupt patients’ sleep and conversely, that correction of mouth leaks was associated with improved sleep structure. More recently, an observational study in patients with obesity-hypoventilation treated by long term NIV showed that, in spite of satisfactory correction of nocturnal PtcCO2 and SaO2, major respiratory events could occur, often undetected by the patient, but leading to a decrease in sleep efficiency, an increase in micro-arousals and sleep disruption. Patient-ventilator asynchrony, auto-triggering, ventilator-induced periodic breathing, and of course, leaks were the major events identified. Actually, up to 55% of chronic stable patients under NIV had significant periods of patient-ventilator asynchrony, and 40% had periodic breathing. In a recent study, we showed that new options proposed on recent bi-level pressure cycled ventilators such as volume targeting could lead to detrimental changes in sleep structure, even if they did improve nocturnal PtcCO2. The present study explores four different aspects of the patient-ventilator relationship during sleep and its monitoring. First, we evaluated the impact of NIV on sleep structure assessed by polysomnography (PSG) and arterial blood gases (ABG) in patients with hypercapnic respiratory failure: this aspect has received little attention in the medical literature. Surprisingly, very little recommendations detail how ventilators should be adjusted for long term home care. Therefore, we chose – in the second part of this study - to explore how the modification of one ventilator setting (back-up respiratory frequency) could affect quality of sleep and occurrence of respiratory events. In other words, we aimed to determine whether it is better, in terms of efficacy and patient-ventilator synchronisation, to use an “S” mode (spontaneous), an “ST” (Spontaneous/Timed mode) with an intermediate back-up rate, slightly below that of the patient, or a T mode, with a ventilator respiratory rate above that of the patient. This issue has to our knowledge only been marginally studied without any consensus. Thirdly, as previously mentioned, recent ventilators designed for home care have built-in softwares which provide potentially useful information for the clinician in terms of monitoring NIV at home. Clinicians need to know if values of tidal volume, leaks, or apnea-hypopnea index recorded by built-in software are reliable and can be used as recommended by AASM. These items however have not been independently validated, and we will show that there is large variability in the reliability of the results made available for the clinician. Finally, when exploring the many difficulties in analysing polygraphic and PSG data obtained in patients under NIV, we found that one major difficulty was to determine whether decreases in flow were associated with a decrease in ventilatory drive (i.e. central events) or not: a non-invasive monitoring of respiratory effort could be an attractive tool to evaluate the respiratory effort of the patient. We chose to determine whether Pulse Transit Time could be used as a surrogate marker of inspiratory effort under NIV.
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Contal, O. (2012). Contributions to the understanding of patient-ventilator interactions in long-term non invasive ventilation. https://hdl.handle.net/2078.5/207001