Methods
Subjects
Twelve nonsmoking subjects with type I diabetes who were training for an Ironman triathlon and 10 nonsmoking control subjects without diabetes matched for age, sex, and training volume were recruited for this study. The protocol was reviewed and approved by the University of Arizona Institutional Review Board, all participants provided written informed consent before study, and all aspects of the study were performed according to the Declaration of Helsinki.
Protocol
Subjects were requested to come into the laboratory in a fasting state. Venous blood samples were collected at rest for measurement of HbA1c and blood glucose. Resting measures of cardiac output, blood pressure (systolic blood pressure (SBP), diastolic blood pressure (DBP), and mean arterial blood pressure), and pulmonary function (forced vital capacity (FVC), forced expiratory flow at 1 s (FEV1), and forced expiratory flow at 50% of the forced vital capacity (FEF50)) were performed before exercise. All exercise tests were performed on the same cycle ergometer (Corival Lode BV, Groningen, The Netherlands). The workload for the exercise testing protocol was subject-specific, with the initial and incremental increase in workload differing between subjects to accommodate body size and training differences. The cycle ergometry test initiated with a workload ranging from 25 to 55 W after 1 min of unloaded pedaling (0 W) on the cycle ergometer. Workloads increased incrementally every 3 min until exhaustion was reached as determined by the subject's inability to maintain a pedal rate between 60 and 80 rpm, an RER > 1.15, or an RPE of 18 of 20. Total exercise duration ranged between 15 and 21 min.
After peak exercise, subjects completed a recovery phase of 3 min of pedaling at the initial workload and 5 min of stationary recovery while remaining seated on the bike. Oxygen uptake (VO2), carbon dioxide production (VCO2), respiratory rate, tidal volume, and minute ventilation were continuously monitored and averaged for data analysis every 3 s during all stages of the exercise test. For flow and gas exchange analyses, a Medical Graphics CPX/D (St. Paul, MN) metabolic cart was interfaced with a Perkin-Elmer MGA-1100 mass spectrometer (Perkin-Elmer 1100, Welsley, MA) as described previously. Before and during exercise, the subject's HR was monitored with a 12-lead ECG (Marquette Electronics, Milwaukee, WI).
Assessment of Cardiovascular Function
Cardiac output was assessed with a previously validated 8- to 10-breath acetylene rebreathe technique using a 5-L rebreathe bag containing 0.7% C2H2 and 9% He. Briefly, a pneumotachograph was connected to a nonrebreathing Y valve (Hans Rudolph, Kansas City, MO) with the inspiratory port connected to a pneumatic switching valve that allowed for rapid switching from room air to the test gas mixture. Gases were sampled using a mass spectrometer (Perkin-Elmer) that was integrated with custom analysis software for the assessment of Q. The volume of gas used to fill the rebreathe bag was determined by the tidal volume of the subject with 500 mL added to the subject's tidal volume to ensure the bag did not collapse at the end of inspiration. The bag volume was adjusted during exercise accordingly. Consistent bag volumes were ensured using a timed switching circuit, which, given a consistent flow rate from the tank, resulted in the desired volume. The switching circuit and tank were checked before each test for accurate volumes. At the end of a normal expiration (end-expiratory lung volume), the subjects were switched into the rebreathe bag and instructed to nearly empty the bag with each breath for 10 consecutive breaths. After each cardiac output maneuver, the rebreathe bag was emptied with a suction device and refilled immediately before the next maneuver. At the start of each maneuver, there was no residual gas in the dead space of the apparatus or from the exhaled air from the subjects, as determined through gas sampling with the mass spectrometer.
Blood pressure was obtained using the auscultation technique, with the same technician performing all measures. Stroke volume was calculated from Q and HR. Mean arterial pressure (MAP) was calculated using the equation: MAP = DBP + 1/3(SBP − DBP), where DBP is diastolic blood pressure and SBP is systolic blood pressure. Systemic vascular resistance was calculated using the formula: (MAP − pulmonary capillary wedge pressure (assumed to be 10 mm Hg)) × 80)/Q.
Assessment of Airway Function
Airway function was assessed by having subjects perform a maximal expiratory flow volume maneuver in triplicate at rest, during each stage of exercise, and into recovery as described previously. We have previously shown that this maneuver can be reproducibly performed in various populations during exercise, including patients with asthma and patients with heart failure. All subjects were carefully instructed on performing the maximal expiratory flow volume maneuver with a special emphasis on taking a gradual but maximal inspiration before the forced exhalation.
Assessment of Glucose and HbA1c
HbA1c was measured by high-performance liquid chromatography, and glucose was determined by glucose oxidation, both at the University of Arizona Medical Center Pathology Laboratory.
Statistical Analysis
All statistical analyses were performed using the SPSS statistical software package (v.12; SPSS, Inc., Chicago, IL). All data were found to have normal distribution, and a two-sample paired t-test was used to examine group differences at rest and during exercise. Pearson correlation coefficients were used to describe linear relationships between selected variables. An α level of 0.05 used to determine statistical significance. All data are presented as mean ± SD.