Cardiovascular responses to apnea during dynamic exercise.

Breath holding maneuvers induce hypoxia, hypercapnia, and various cardiovascular responses typically including increases in total peripheral resistance, mean arterial pressure (MAP) and decreases in heart rate (HR). During dynamic exercise these responses may have a generally negative impact on performance. Moreover, they deserve particular attention in cardiovascular risk subjects. In 26 healthy sport students we studied the HR and MAP effects induced by the combination of dynamic exercise (cycle ergometry, 30 W and 250 W) with 20 s of either respiratory arrest (mouth piece pressure held constant at 20 mm Hg), free breathing, or rebreathing, i. e. periods of unimpeded breathing leading to similar levels of hypercapnia and hypoxia as the respiratory arrest. The measurements yielded no major differences between the conditions of rebreathing and free breathing. In contrast, 20 s of apnea led to a marked increase in MAP and a HR depression at both levels of exercise intensity. Additionally, there was a delayed MAP recovery after this stimulus. The present findings show that breath holding has marked effects on MAP and HR during dynamic exercise, which are essentially independent of the resulting hypoxia and of increases in intrathoracic pressure. The key factor seems to be an increase in total peripheral resistance, probably including a vasoconstriction in the exercising muscles.

Administration of 100% oxygen in diving accidents--an evaluation of four emergency oxygen devices.

As the use of oxygen enhances the resorption of gas bubbles in decompression illness, it is recommended and generally accepted that the inspired oxygen concentration in emergency treatment of diving accidents has to be as close to 100% as possible. Therefore, several emergency oxygen devices are offered to the diving community but only with little data in literature on the efficacy of these devices. We tested four emergency oxygen devices with respect to efficacy of oxygen supply and breathing comfort at rest. Nine blinded volunteers had to breathe from the four systems with face mask and mouthpiece as well. Gases were measured with mass spectrometry during a 3 min interval from a capillary port close to the subject. The results showed that none of the systems was able to deliver 100% oxygen all the time, but in three systems inspiratory oxygen values were achieved, although in one system the nitrogen wash-out was slowed due to air contamination during inspiration. The fourth tested system frequently supplied the subjects simply with air while breathing at rest. We conclude from our study that it is difficult to achieve oxygen levels close to 100% in practice. Even in a perfectly working system, the interface between device and subject is a source of entrained air, especially when oxygen breathing has to be performed over a longer period of time. In addition, two of four systems had conceptional problems to supply the subjects with pure O2 during inspiration. None of the tested systems was perfectly designed to serve in such emergencies.