Non-invasive ventilation and sleep.

In this paper, we review the effects of nocturnal mechanical ventilation on sleep. Indeed, although non-invasive assisted ventilation during sleep has been applied extensively, the exact effects of this treatment on sleep quality have not been thoroughly studied. In patients with severe chronic obstructive pulmonary disease and severe restrictive ventilatory defects, the resulting respiratory failure is aggravated by the specific effects of sleep on respiration. Non-invasive mechanical ventilation can lead to improvements in both ventilation and sleep quality. However, this is not always the case. Moreover, sleep-related leaks may jeopardize the efficiency of the ventilatory assistance which in turn may result in a deterioration in sleep quality. Non-invasive mechanical ventilation, if applied during sleep, should require a monitoring procedure during sleep with the aim of obtaining the best possible effects both on ventilation and on sleep quality

Nasal mask pressure waveform and inspiratory muscle rest during nasal assisted ventilation

In mechanically ventilated patients, pressure and flow tracings can be used to assess respiratory pump muscle activity or rest. When the ventilation is delivered through an endotracheal tube, the respiratory system can be considered a one-compartment model, and activation of the respiratory muscles results in deformations and variability of the pressure tracings. This is also true when mechanical ventilation is delivered nasally. With intermittent positive-pressure ventilation delivered through a nasal mask (nIPPV), we have recently shown that the glottis can interfere with ventilation even in the absence of diaphragmatic surface electromyographic (EMG) activity. On the basis of our observations, we suggested that when mechanical ventilation is delivered through a nasal means of access, the respiratory system cannot be considered a one-compartment model. To confirm this hypothesis, we submitted one healthy subject to nIPPV while his glottis was continuously monitored through a fiberoptic bronchoscope and his diaphragmatic activity was monitored with a bipolar esophageal electrode. During wakefulness or sleep, we observed irregularities in the nasal mask pressure waveform, in nasal mask peak pressure, and in actual VT despite the absence of respiratory pump muscle activity. These irregularities were related to significant variations in the glottic width, rather than to the reappearance of transient phasic inspiratory muscle activity. We conclude that during nIPPV, deformations in the mask pressure waveform can be induced by variations in the glottic aperture without activation of the diaphragm. Thus, when mechanical ventilation does not bypass the glottis, the respiratory system does not behave like a one-compartment model, and EMG remains the only reliable technique for assessing diaphragmatic muscle activity

Effects of hypocapnic hyperventilation on the response to hypoxia in normal subjects receiving intermittent positive-pressure ventilation.

OBJECTIVE: To confirm the hypothesis that the ventilatory response to hypoxia (VRH) may be abolished by hypocapnia. METHODS: We studied four healthy subjects during intermittent positive-pressure ventilation delivered through a nasal mask (nIPPV). Delivered minute ventilation (Ed) was progressively increased to lower end-tidal carbon dioxide pressure (PETCO(2)) below the apneic threshold. Then, at different hypocapnic levels, nitrogen was added to induce falls in oxygen saturation, a hypoxic run (N(2) run). For each N(2) run, the reappearance of a diaphragmatic muscle activity and/or an increase in effective minute ventilation (E) and/or deformations in mask-pressure tracings were considered as a VRH, whereas unchanged tracings signified absence of a VRH. For the N(2) runs eliciting a VRH, the threshold response to hypoxia (TRh) was defined as the transcutaneous oxygen saturation level that corresponds to the beginning of the ventilatory changes. RESULTS: Thirty-seven N(2) runs were performed (7 N(2) runs during wakefulness and 30 N(2) runs during sleep). For severe hypocapnia (PETCO(2) of 27.1 +/- 5.2 mm Hg), no VRH was noted, whereas a VRH was observed for N(2) runs performed at significantly higher PETCO(2) levels (PETCO(2) of 34.0 +/- 2.1 mm Hg, p < 0.001). Deep oxygen desaturation (up to 64%) never elicited a VRH when the PETCO(2) level was < 29.3 mm Hg, which was considered the carbon dioxide inhibition threshold. For the 16 N(2) runs inducing a VRH, no correlations were found between PETCO(2) and TRh and between TRh and both Ed and E. CONCLUSION: During nIPPV, VRH is highly dependent on the carbon dioxide level and can be definitely abolished for severe hypocapnia

Effectiveness of controlled and spontaneous modes in nasal two-level positive pressure ventilation in awake and asleep normal subjects

STUDY OBJECTIVES: The purpose of the present study was to compare in awake and asleep healthy subjects, under nasal intermittent positive pressure ventilation (nIPPV) with a two-level intermittent positive pressure device (two-level nIPPV), the efficacy of the controlled and spontaneous modes, and of different ventilator settings in increasing effective minute ventilation (VE). PARTICIPANTS: Eight healthy subjects were studied. SETTING: In the controlled mode, inspiratory positive airway pressure (IPAP) was kept at 15 cm H2O, expiratory positive airway pressure (EPAP) at 4 cm H2O, and the inspiratory/expiratory (I/E) time ratio at 1. The respirator frequencies were 17 and 25/min. In the spontaneous mode experiment, IPAP was started at 10 cm H2O and progressively increased to 15 and 20 cm H2O; EPAP was kept at 4 cm H2O. MEASUREMENTS AND RESULTS: We measured breath by breath the effective tidal volume (VT with respiratory inductive plethysmography), actual respiratory frequency (f), and effective VE. Using the controlled mode, effective VE was significantly higher on nIPPV than during spontaneous unassisted breathing, except in stage 2 nonrapid eye movement sleep at 17/min of frequency; increases in f from 17 to 25/min led to a significant decrease in VT reaching the lungs, during wakefulness and sleep; effective VE was higher at 25 than at 17/min of frequency only during sleep; periodic breathing was scarce and apneas were never observed. Using the spontaneous mode, with respect to awake spontaneous unassisted breathing, two-level nIPPV at 10 and 15 cm H2O of IPAP did not result in any significant increase in effective VE either in wakefulness or in sleep; only IPAP levels of 20 cm H2O resulted in a significant increase in effective VE; during sleep, effective VE was significantly lower than during wakefulness; respiratory rhythm instability (ie, periodic breathing and central apneas) were exceedingly common, and in some subjects extremely frequent, leading to surprisingly large falls in arterial oxygen saturation. CONCLUSIONS: It appears that two-level nIPPV should be used in the controlled mode rather than in the spontaneous mode, since it seems easier to increase effective VE with a lower IPAP at a high frequency than at a high pressure using the spontaneous mode. We suggest that the initial respirator settings in the controlled mode should be an f around 20/min, an I/E ratio of 1, 15 cm H2O of IPAP, and EPAP as low as possible

Effects of intermittent negative pressure ventilation on effective ventilation in normal awake subjects

Rationale: Previous studies have shown that an increase in inspiratory pressure during nasal intermittent positive pressure ventilation (IPPV) does not result in increased effective minute ventilation ((V) over doe E) due to glottic interference. Study objectives: To test the consequences of increases in negative pressure ventilation (NPV) on (V) over dot E. Material and methods: Eight healthy awake subjects underwent NPV delivered by an iron lung. First, NPV was started at a respirator frequency (f) of 15 cycles per minute with an inspiratory negative pressure (INP) of - 15 cm H2O (F15-P15). Then,f was increased to 20 cycles per minute and INP was kept at - 15 cm H2O. Next,f was kept at 20 cycles per minute and INP was reduced to - 30 cm H2O (F20-P30). Finally,f was decreased to 15 cycles per minute and INP was kept at - 30 em H2O. At each step and for each breath, effective tidal volume (VT), (V) over dot E, and end-tidal carbon dioxide pressure were measured. In three subjects, the glottis width was assessed using fiberoptic bronchoseopy. Results: From spontaneous breathing to the first step of NPV (F15-P15), we observed an inhibition of the phasic inspiratory diaphragmatic electromyogram concomitant to a significant increase in (V) over dot E (p < 0.0005). For the group as a whole, the increase in mechanical ventilation (from F15-P15 to F20-P30) resulted in significant increases in VT and (V) over dot E leading to hypocapnia (p < 0.0005). Moreover, the glottis width did not decrease with the increase in mechanical ventilation. Conclusions: We conclude that in normal awake subjects, NPV allowed a significant increase in (V) over dot E. These results differ from those previously obtained with nasal IPPV in which the glottic width interferes with the delivered mechanical ventilation

Strength of the respiratory and lower limb muscles and functional capacity in chronic stroke survivors with different physical activity levels

BACKGROUND: The assessment of strength and its relationships with functional capacity could contribute to more specific and effective disability management of stroke survivors. OBJECTIVE: To compare and investigate associations between measures of strength and functional capacity of 98 chronic stroke survivors, stratified into three groups, according to their physical activity levels. METHOD: The physical activity levels were classified as impaired, moderately active, and active, based on their Human Activity Profile (HAP) scores. Strength was assessed by the maximal inspiratory (MIP) and expiratory (MEP) pressures and by the residual deficits (RDs) of work of the lower limb and trunk muscles, whereas functional capacity was evaluated by the distance covered during the six-minute walking test (6MWT). RESULTS: One-way analyses of variance revealed significant differences between the groups, except between the active and moderately active groups regarding the RDS of the hip and knee flexors/extensors and ankle dorsiflexors (2.91<F<8.62; 0.001<p<0.01). Differences between the groups were found for the 6MWT (F=10.75; p<0.001), but no differences were found for the MIP and MEP measures (0.92<F<2.13; 0.13<p<0.40). Significant, negative, and fair correlations were observed between the RDS of the hip and knee muscles and the 6MWT (0.30<r<-0.43; p<0.01) and the HAP (-0.28<r<-0.41; p<0.01). Moderate to good correlations were found between the 6MWT and the HAP (r=0.50; p<0.0001). There were no significant correlations between measures of respiratory strength and any of the investigated variables (-0.11<r<0.12; 0.26<p<0.56). CONCLUSIONS: Lower strength deficits and higher functional capacity were associated with higher physical activity levels. However, the moderately active and active groups demonstrated similar strength deficits

Reference equations for the six-minute walk distance based on a Brazilian multicenter study

BACKGROUND: It is important to include large sample sizes and different factors that influence the six-minute walking distance (6MWD) in order to propose reference equations for the six-minute walking test (6MWT). OBJECTIVE: To evaluate the influence of anthropometric, demographic, and physiologic variables on the 6MWD of healthy subjects from different regions of Brazil to establish a reference equation for the Brazilian population. METHOD: In a multicenter study, 617 healthy subjects performed two 6MWTs and had their weight, height, and body mass index (BMI) measured, as well as their physiologic responses to the test. Delta heart rate (∆HR), perceived effort, and peripheral oxygen saturation were calculated by the difference between the respective values at the end of the test minus the baseline value. RESULTS: Walking distance averaged 586±106m, 54m greater in male compared to female subjects (p<0.001). No differences were observed among the 6MWD from different regions. The quadratic regression analysis considering only anthropometric and demographic data explained 46% of the variability in the 6MWT (p<0.001) and derived the equation: 6MWDpred=890.46-(6.11×age)+(0.0345×age2)+(48.87×gender)-(4.87×BMI). A second model of stepwise multiple regression including ∆HR explained 62% of the variability (p<0.0001) and derived the equation: 6MWDpred=356.658-(2.303×age)+(36.648×gender)+(1.704×height)+(1.365×∆HR). CONCLUSION: The equations proposed in this study, especially the second one, seem adequate to accurately predict the 6MWD for Brazilians