Influence of apnea on cardiovascular responses to neck suction during exercise

Short-lasting neck suction (NS) is a common method to assess the carotid-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 that could confound the HR and blood pressure responses to NS. HR and blood pressure responses to 10-s trains of 50-mmHg pulses of NS were studied 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 similar during eupnea and apnea in all conditions. However, during isometric and dynamic exercise, recordings made under eupneic and apneic conditions differed during the second half of the NS period. During apneic isometric arm contraction, the elevation of mean carotid distending pressure (MCDP) (arterial pressure at carotid level minus NS pressure) was maintained at a 25-35% higher level than during eupneic isometric 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 apneic NS, whereas they were maintained during eupnea. One to three seconds later, HR started to drop markedly in apneic subjects, reaching values 20 beats/min lower than those in eupneic subjects at the end of the NS. We conclude that cardiovascular effects of apnea may appear after only 8 s of apnea in dynamic exercise and therefore could confound responses to NS.