engleski

Sympathetic nerve response during maximal apnea with and without glossopharyngeal insufflation

Although the voluntary interruption of breathing can be followed through the ancient times of mankind, mostly due to economic reasons, in the last century many sports spread through the world whose essence was the ability to withhold breathing as long as possible. Every breath hold, since the very beginning, causes complex adaptations by nervous and cardiovascular system in order to preserve oxygen supplies and protect vital organs form hypoxia. Asphyxic conditions that builds up along advancing apneas creates strong stimulus for sympathetic nervous system. Purpose of this doctoral dissertation was to explore the response of sympathetic nervous and cardiovascular system in hypercapnic hypoxia evoked by maximal end-inspiratory dry apnea without and after additional lung-packing by glossopharyngeal insufflation procedure. Simultaneously, with measuring various hemodynamic physiological parameters we assessed muscle sympathetic nerve activity (MSNA) in peroneal nerve using the technique of microneurography. In study no. 1 subjects performed maximal end-inspiratory dry apnea and in study no. 2 subjects performed two bouts of maximal end-inspiratory dry apnea with and without lungpacking, as the control apnea, in randomized order. Results of the first study showed that repeated bouts of hypoxemia in elite divers did not lead to sustained sympathetic activation or arterial hypertension. The other important finding of the study no. 1 along with the results of study no. 2 was that prolonged apnea, with progression of arterial oxygen desaturation, resulted in profound increase of blood pressure and elevations of MSNA. Compared with the baseline the mean end-apneic sympathetic traffic was ~ 4-fold in the control subjects but around 20-fold and with no ceiling on the rise of MSNA parameters in elite divers. Also very importantly, as our work was first to investigate sympathetic nerve traffic in elite divers during maximal voluntary apnea, the results of studies no. 1 and 2. suggest that sympathetic activation during apnea is governed by different reflex mechanism depending of the apnea stages. The increase of sympathetic activation during early normoxic phase of apnea was driven mostly by baroreflex mechanism, but in late, asphyxic, phase of apnea chemoreflex engagement was predominant. One of the important results of study no. 2 is that maximal dry static apnea with additional moderate lung packing did not bring about further reduction of CO, as we expected, comparing to control apneas without this maneuver, but it did cause greater extent of sympathetic activation in early normoxic phase of apnea. We speculated that this additional MSNA increase could be a baroreflex-triggered response to vasodilatator substances released by lungs after further hyperinflation with lung packing. This issue remains to be addressed and clarified in future investigations. When apnea divers voluntarily increase intrathoracic pressure by taking deep breath followed by glosoopharyngeal insufflations the risk of hemodynamic instability and syncope, as its ultimate consequence, is much higher than without this maneuver. Findings in our study no. 3 showed that cardioinhibitory syncope was more common than low cardiac output syncope in elite divers when performing lung packing. Extreme voluntary apnea in elite divers may serve as unique human model where survival mechanisms engaged by brain facing acute profound asphyxia can be studied. These findings can be relevant in clinical conditions who bear high risk of brain hypoperfusion i.e. stroke, brain trauma, respiratory and heart failure. Maximal dry apnea with additional glossopharyngeal insufflation can be used as model to explore autonomic and cardiovascular responses with profoundly increased intrathoracic pressure ; conditions that can also be met in everyday clinical practice like artificial lungs ventilation or pulmonary hypertension. And finally, maximal breath hold diving can model (pre)syncopal events in man.

sympathetic nerve system; apnea

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