β1-Blockade increases maximal apnea duration in elite breath-hold divers

We hypothesized that the cardioselective beta(1)-adrenoreceptor antagonist esmolol would improve maximal apnea duration in elite breath-hold divers. In elite national-level divers (n = 9), maximal apneas were performed in a randomized and counterbalanced order while receiving either iv esmolol (150 mu g.kg(-1).min(-1)) or volume-matched saline (placebo). During apnea, heart rate (ECG), beat-by-beat blood pressure, stroke volume (SV), cardiac output (CO), and total peripheral resistance (TPR) were measured (finger photoplethysmography). Myocardial oxygen consumption (M(V)over dotO(2)) was estimated from rate pressure product. Cerebral blood flow through the internal carotid (ICA) and vertebral arteries (VA) was assessed using Duplex ultrasound. Apnea duration improved in the esmolol trial when compared with placebo (356 +/- 57 vs. 323 +/- 61 s, P < 0.01) despite similar end-apnea peripheral oxyhemoglobin saturation (71.8 +/- 10.3 vs. 74.9 +/- 9.5%, P = 0.10). The HR response to apnea was reduced by esmolol at 10-30% of apnea duration, whereas MAP was unaffected. Esmolol reduced SV (main effect, P < 0.05) and CO (main effect; P < 0.05) and increased TPR (main effect, P < 0.05) throughout apnea. Esmolol also reduced M(V)over dotO(2) throughout apnea (main effect, P < 0.05). Cerebral blood flow through the ICA and VA was unchanged by esmolol at baseline and the last 30 s of apnea; however, global cerebral blood flow was reduced in the esmolol trial at end-apnea (P < 0.05). Our findings demonstrate that, in elite breath-hold divers, apnea breakpoint is improved by beta(1)-blockade, likely owing to an improved total body oxygen sparring through increased centralization of blood volume (up arrow TPR) and reduced M(V)over dotO(2). NEW & NOTEWORTHY The governing bodies for international apnea competition, the Association Internationale pour le Developpment de l'Apnee and La Confederation Mondaile des Activites Subaquatiques, have banned the use of beta-blockers based on anecdotal reports that they improve apnea duration. Using a randomized placebo-controlled trial, we are the first to empirically confirm that beta-blockade improves apnea duration. This improvement in apnea duration coincided with a reduced myocardial oxygen consumption.