Sleep was manually staged according to standard criteria. A central apnea was defined as absence of oronasal airflow during sleep for > 10 s associated with absent respiratory effort. A central hypopnea was defined as any reduction in oronasal airflow for > 10 s associated with in phase thoracoabdominal movement and > 2% fall in Spo2. Obstructive apnea was defined as cessation of oronasal airflow for > 10 s in the presence of out of phase thoracoabdominal effort. An obstructive hypopnea was defined as a fall in oronasal airflow for > 10 s with out-of-phase thoracoabdominal movement associated with > 2% fall in Spo2. A mixed apnea was defined using the above criteria, when a central apnea included or terminated with obstructive components. Mixed apneas were classified as obstructive events. Patients were described as having CSA if > 80% of all respiratory events were central in origin with total central AHI > 5/h. OSA was described as AHI > 5/h with < 20% central in type. No SDB was defined as an overall AHI < 5/h. The ventilation, apnea and cycle lengths were determined during a period of continuous cyclic central apneas in stages 1 or 2 sleep and the average taken of 10 cycles as previously described. in detail
Sympathetic nervous system activity was estimated from overnight UNE. Subjects were asked to void prior to sleep. Subsequent overnight urine and first morning voided samples were collected into acidified containers of 6 mol/L hydrochloric acid (20 mL) and stored at 4°C. Urinary norepinephrine was determined by high-performance liquid chromatography with fluorescent detection, and concentrations were expressed as nanomol per millimol creatinine to adjust for effects of urine volume and renal function. Values for normal subjects in our laboratory are 13.4 ± 5.6 nmol/mmol creatinine.
The data were expressed as mean ± SD, and paired and unpaired t tests and one-way analysis of variance were used to compare groups; p < 0.05 was assumed to indicate significance.