INFLUENCE OF APNEA ON CARDIOVASCULAR-RESPONSES TO NECK SUCTION DURINGEXERCISE

Short-lasting neck suction (NS) is a common method to assess the carot id-cardiac baroreflex, and NS is usually applied during apnea to avoid breath-synchronous variations of heart rate (HR) and blood pressure. We hypothesized that the apnea might provoke cardiovascular effects th at could confound the HR and blood pressure responses to NS. HR and bl ood pressure responses to 10-s trains of 60-mmHg pulses of NS were stu died in six male subjects during supine rest, upright rest, isometric arm exercise at 30% of maximal voluntary contraction, and dynamic leg exercise at 100 W in the sitting position. Repeated NS sequences were performed during apnea preceded by a relaxed expiration to functional residual capacity and during eupnea. Initial HR responses to NS were s imilar during eupnea and apnea in all conditions. However, during isom etric and dynamic exercise, recordings made under eupneic and apneic c onditions differed during the second half of the NS period. During apn eic isometric arm contraction, the elevation of mean carotid distendin g pressure (MCDP) (arterial pressure at carotid level minus NS pressur e) was maintained at a 25-35% higher level than during eupneic isometr ic exercise over the last half of the NS period. In dynamic exercise, mean arterial pressure and MCDP started to increase after 3-5 s of apn eic NS, whereas they were maintained during eupnea. One to three secon ds later, HR started to drop markedly in apneic subjects, reaching val ues 20 beats/min lower than those in eupneic subjects at the end of th e NS. We conclude that cardiovascular effects of apnea may appear afte r only 8 s of apnea in dynamic exercise and therefore could confound r esponses to NS.