title: Assessment of ventilatory sensitivity to carbon dioxide changes during orthostasis.
abstract: Investigators have anecdotally reported changes in respiratory pattern preceding symptoms of orthostatic intolerance for several years. Evidence from recent studies support these observations, and show that alterations in respiration often precede pre-syncope during orthostasis. These observations suggest the possibility that changes in interaction between the chemo and baro-reflex control systems may produce phasic or dynamic changes in respiratory and hemodynamic parameters such that these changes are important in orthostasis intolerance. Our objective in the present study was to develop a method to obtain a quantitative index of ventilatory sensitivity to changes in inspired during orthostasis. Based on an approach previously developed by one of the authors (EB), we used, during supine and 70 degrees head up tilt, pseudo-random binary changes in inspired CO2 to quantify ventilatory sensitivity to CO2 disturbances. A pneumotach was used to measure airflow. From these recordings, we used a prediction error based systems identification algorithm to quantify ventilatory impulse response to CO2 stimulus. Results from 7 subjects showed that the integrated ventilatory response (area under impulse response curve) to CO2 was larger during tilt than that during supine (mean [std] 5.3 [2.6] Vs 3.0 [1.6] ml/min/0.01LCO2). These results suggest that the changes in ventilation due to disturbances in CO2 are likely to be larger during orthostasis than those during supine. Such an elevated response would indicate that the role of chemo-reflex control of breathing in cardiovascular instability culminating in orthostatic intolerance is likely to be important and should be further investigated.
authors.affiliation: Center for Biomedical Engineering, University of Kentucky, Lexington, KY 40506-0070, USA.
authors.forename: Letetia
authors.initials: L
authors.lastname: Richardson
authors.forename: Zbigniew
authors.initials: Z
authors.lastname: Topor
authors.forename: Divyesh
authors.initials: D
authors.lastname: Bhakta
authors.forename: Francis
authors.initials: F
authors.lastname: McCaffrey
authors.forename: Eugene
authors.initials: E
authors.lastname: Bruce
authors.forename: Abhijit
authors.initials: A
authors.lastname: Patwardhan
mesh_terms: D000328:Adult; D002245:Carbon Dioxide / Q000008:administration & dosage / Q000502:physiology*; D004244:Dizziness / Q000503:physiopathology*; D004562:Electrocardiography; D006801:Humans; D012016:Reference Values; D012119:Respiration; D016683:Supine Position / Q000502:physiology; D018667:Tilt-Table Test
chemical_list: D002245:Carbon Dioxide