One Minute Heart Rate Variability Quantification in Airway Obstruction Model

Following approval by the Institutional Review Board, the study was performed in a prospective cross-over, longitudinal cohort manner. Healthy volunteers aged between 20 and 40 years were included. Each participant signed an informed consent. Exclusion criteria were respiratory or cardiac pathology, diabetes, a body mass index lower than 18 or higher than 26 kg/m2, chronic medical conditions with significant involvement of the sympathetic or parasympathetic systems (e.g. diabetes), obstructive sleep apnea, use of medications with either cardiovascular or respiratory effects, and ECG abnormalities including heart rate other than sinus rhythm, atrioventricular conduction blocks, intrafascicular conduction delays or prolonged QT interval (the time between consecutive Q and T waves on the electrocardiogram).

Initially, each volunteer underwent an ultrasound evaluation (SonoSite™ M-Turbo™ ultrasound machine, SonoSite, Bothell, Washington, USA) of the lungs using a high frequency linear probe at mid clavicular lines to exclude pneumothorax, an apical view of the heart using a curve-linear probe to exclude pericardial effusion. A twelve lead ECG was obtained and analyzed to exclude rate or conduction abnormalities. Noninvasive blood pressure and oxygen saturation were recorded in all subjects throughout the study to identify and prevent any complication.

A three lead ECG and spirometry were obtained in supine position with the upper body raised by 30 degrees. Data was collected using a Datex AS/3 monitor (Datex Ohmeda Medical Equipment, GE Healthcare, USA). The data was recorded using a digital to analog acquisition card (NI-6008, National Instruments™, Austin, Texas, USA) and a Biosignal Logger of National Instruments™ Biomedical Workbench™ at a sampling rate of 500 Hertz (Hz). All experiments were performed at the same time of day (early afternoon), and under the same conditions (same place and experimental setup). Volunteers were instructed to refrain from smoking for 4 hours prior to participating in the study.

The airway obstruction was simulated by an 18 cm long, 4 mm internal diameter endotracheal tube, connected to a spirometry adaptor and an antimicrobial filter. During the obstructed breathing phase, the volunteers were directed to seal their lips tightly around the filter to prevent air leak and encouraged to reach a peak pressure of 30-40 cm H2O, using the instantaneous display on the spirometry monitor. Three sets were recorded for each volunteer; each set was comprised of one minute of normal unobstructed breathing that served as control, immediately followed by one minute of obstructed breathing. Following each set, the volunteers were allowed at least one minute of rest period to recover and return to their baseline breathing before the next set.

HRV analysis A detailed description of the means to measure and evaluate the significance of HRV can be found in the European Society of Cardiology and the North American Society of Pacing Electrophysiology Task Force on heart rate variability, standards of measurement and physiological interpretation and clinical use guidelines14. Briefly, the raw ECG signal was preprocessed (including high pass filtering to remove Baseline wandering and ECG feature identification) and the R-R intervals were extracted from the raw ECG signal using ECG Features Extractor of National Instruments™ Biomedical Workbench™ with threshold adjust factor of 0.1, a rough highest heart rate of 60 beats per minute, QRS frequency of 10-25 Hz and middle QRS onset and offset.