Inductive plethysmography to control volume-targeted ventilation for leak compensation.

OBJECTIVE: Volume-targeted ventilation is associated with poor compensation for leaks yet is frequently used in neuromuscular patients, many of whom experience mouth leaks. Our objective was to test the feasibility and efficacy of a prototype ventilator designed to achieve leak compensation by feed-back control of the actual tidal volume based on either the volume blown to the expiratory circuit (VTEVC-compensation) or the tidal volume measured by inductive plethysmography (VTplet-compensation). DESIGN AND SETTING: Randomized, cross-over, physiological study in a physiological ward of a university teaching hospital. PARTICIPANTS: Nine normal individuals. INTERVENTION: Subjects tested volume-targeted assist-control ventilation without compensation, with VTEVC-compensation, and with VTplet-compensation. Tests were done with the mouth closed, with mouth leaks during inspiration, and with mouth leaks during expiration. MEASUREMENTS AND RESULTS: With inspiratory mouth leaks compared to mouth closed, the delivered volume remained unchanged without leak compensation, increased to 143 +/- 30% with VTEVC-compensation, and increased to 132 +/- 17% with VTplet-compensation; the expired tidal volume decreased to 46 +/- 24% without compensation, 66 +/- 20% with VTEVC-compensation, and 68 +/- 23% with VTplet-compensation. With expiratory mouth leaks, the ventilator-delivered volume and expired tidal volume were unchanged with no compensation and with VTplet-compensation; they increased with VTEVC-compensation. CONCLUSION: Leak compensation can be achieved during volume-targeted ventilation. VTEVC-compensation and VTplet-compensation were equally effective in compensating for inspiratory leaks, and VTplet-compensation also performed well when expiratory leaks occurred.

Twitch mouth pressure for detecting respiratory muscle weakness in suspicion of neuromuscular disorder.

Twitch mouth pressure using magnetic stimulation of the phrenic nerves and an automated inspiratory trigger is a noninvasive, non-volitional assessment of diaphragmatic strength. Our aims were to validate this method in patients with suspected neuromuscular disease, to determine the best inspiratory-trigger pressure threshold, and to evaluate whether twitch mouth pressure decreased the overdiagnosis of muscle weakness frequently observed with noninvasive volitional tests. Maximal inspiratory pressure, sniff nasal pressure, and twitch mouth pressure were measured in 112 patients with restrictive disease and suspected neuromuscular disorder. Esophageal and transdiaphragmatic pressures were measured in 64 of these patients to confirm or infirm inspiratory muscle weakness. Magnetic stimulation was triggered by inspiratory pressures of -1 and -5 cmH2O. The -5 cmH2O trigger produced the best correlation between twitch mouth pressure and twitch esophageal pressure (R2 = 0.86; P <0.0001). The best association of noninvasive tests to predict inspiratory muscle weakness was sniff nasal pressure and twitch mouth pressure. Below-normal maximal inspiratory pressure and sniff nasal pressure values suggesting inspiratory muscle weakness were found in 63/112 patients. Only 52 of these 63 patients also had abnormal twitch mouth pressure. In conclusion twitch mouth pressure measurement is a simple, noninvasive, nonvolitional technique which may help to select patients with suspected neuromuscular disorder for invasive inspiratory-muscle investigation.

Bench evaluation of flow limitation detection by automated continuous positive airway pressure devices.

STUDY OBJECTIVE: Automatic continuous positive airway pressure (CPAP) devices that adjust the pressure delivered to the patient are now available to treat sleep-disordered breathing. Sophisticated auto-CPAP devices can detect and correct flattened inspiratory flow contours (FIFCs) associated with subtle upper airway obstruction. However, evaluations of their performance are made difficult by differences across patients and devices. We performed a bench study of five commercially available auto-CPAP devices using a breath waveform simulator to evaluate sensitivity for detecting flattened inspiratory flow. DESIGN: Five degrees of FIFC were simulated. In addition, normal and abnormal flow contours from patients published in the literature were evaluated. MEASUREMENTS AND RESULTS: One device showed autotriggering leading to CPAP increases, and another device varied the CPAP level independently from the presence of an FIFC. The three remaining devices differed regarding the detection of FIFCs and the means used to increase CPAP. CONCLUSION: Based on the characteristics of each patient, physicians must choose among devices with different thresholds of FIFC detection and different pressure responses to detection. Therefore, physicians need details on the algorithms used in auto-CPAP devices. Manufacturers should supply detailed algorithms.

Transdiaphragmatic pressure control of airway pressure support in healthy subjects.

We designed a new servoventilator that proportionally adjusts airway pressure to transdiaphragmatic pressure (Pdi) generated by the subject during inspiration. Each cycle is triggered by either a preset Pdi increase or a preset inspiratory flow value (whichever is reached first), whereas cycling-off is flow-dependent. We evaluated the servoventilator in seven healthy subjects at normocapnia and three levels of hypercapnia (normocapnia + 3, + 6, and + 9 mm Hg) comparatively with spontaneous breathing. Triggering was by Pdi in six subjects and flow in one. At all end-tidal carbon dioxide pressure levels, time from onset of diaphragm electromyographic activity to inspiratory flow was similar with and without the servoventilator. Airway pressure increased proportionally to Pdi variation during servoventilator breathing. Flow, tidal volume, respiratory rate, intrinsic positive end-expiratory pressure, and esophageal and transdiaphragmatic pressure-time products increased significantly with hypercapnia with and without the servoventilator. Breathing pattern parameters were similar in the two breathing modes, and no differences were found for intrinsic positive end-expiratory pressure or gastric pressure variation during exhalation. Esophageal and transdiaphragmatic pressure-time products were lower with than without the servoventilator. The Pdi-driven servoventilator was well synchronized to the subjects effort, delivering a pressure proportional to Pdi and reducing respiratory effort at normocapnia and hypercapnia.

Measuring participation in children with disabilities using the Assessment of Life Habits.

The objectives of this study were: (1) to examine the psychometric properties of the Assessment of Life Habits (LIFE-H) for children; and (2) to draw a profile of the level of participation among children of 5 to 13 years of age with various impairments. The research team adapted the adult version of the LIFE-H in order to render it more appropriate for the daily life experiences of children. Content validity was verified by an expert panel of 29 people, made up of parents, paediatric clinicians, and researchers. Reliability and construct validity of the LIFE-H for children (interview-administered form) was tested during an experiment that comprised three sessions of interviews with a group of 94 parents of children with disabilities (36 males, 58 females; mean age 8y 10mo [SD 2y 6 mo]; diagnostic groups: cerebral palsy, myelomeningocoele, sensory-motor neuropathy, traumatic brain injury, and developmental delay). Overall, the LIFE-H showed high intrarater reliability with intraclass correlation coefficient values of 0.78 or higher for 10 out of 11 categories. The correlations between the LIFE-H and the tools used in pediatric rehabilitation varied, and categories with similar constructs generally led to higher correlations. The psychometric properties of the LIFE-H are appropriate and its content allows a complete description of participation among children with disabilities.