P132 Carbon dioxide sensitivity in patients with hyperventilation syndrome

Abstract

Purpose: This study was undertaken to investigate carbon dioxide sensitivity in patients with Hyperventilation Syndrome (HVS) compared to healthy controls. Relevance: Many body systems may be affected by chronic hyperventilation and symptoms may be wide-ranging. There is no obvious cause, no gold standard test and not all patients with HVS demonstrate characteristic low resting PaCO2. No previous studies were found that reported the breath-hold time of HVS subjects at functional residual capacity, subjects' end-tidal carbon dioxide at the breakpoint of breath-hold, the subjective experience of breath-hold or the effect of carbon dioxide inhalation on the perception of breathing discomfort in HVS patients. Participants: Five HVS patients, diagnosed by a respiratory physician, with no organic cause for breathlessness and referred for assessment (females, aged 21-70) and five healthy controls (females, aged 21-28) were studied. Methods: Breath-hold tests at total lung capacity (TLC) and functional residual capacity (FRC) were carried out. Incremental inhalation of CO2 was performed, whilst breathing frequency and volume was unconstrained (free) and when 'fixed' by a breathing circuit and metronome. Components of minute ventilation were recorded via inductive plethysmography, in addition to end-tidal carbon dioxide (PETCO2). A 100mm visual analogue scale (VAS) was used to obtain a measure of breathing discomfort during breath-hold tests and CO2 inhalation. The breathlessness experience associated with each CO2 inhalation was assessed with the previously published D-12 questionnaire. Analysis: TLC and FRC breath-hold time, VAS and PETCO2 at the limit of tolerance during breath-holds and CO2 inhalation were all recorded for analysis. The mean and S.D. of each of these parameters for both groups were calculated and an unpaired t test performed. Results: The HVS group demonstrated a lower breath-hold time at TLC (35 vs. 67 seconds, p = 0.04) and could not hold their breath at FRC compared with controls. During the incremental CO2 inhalation tests there was a borderline significant increase in PETCO2 at the limit of tolerance in the HVS group during free breathing (1.6 kPa, p = 0.08), but not fixed (1.9 kPa, p = 0.2). Controls described feelings of air hunger following both inhalation tests: HVS patients tended to rate higher on work and effort descriptors. Conclusions: These data suggest that patients with HVS may be more sensitive to changes in CO2 than controls. Implications: Breath-hold time at TLC and FRC may be useful in identifying patients with HVS and monitoring response to physiotherapeutic intervention. Breakpoint PETCO2 during incremental CO2 inhalation requires further validation in larger cohorts.