Breathing during cardiac arrest following exercise: a new function of the respiratory system?

We have found in four sheep that, following a muscular exercise, minute ventilation is maintained for 34-131 s during a cardiac arrest (CA), at a magnitude (from 28.2 and 54.7 l min(-1)) similar to the level of ventilation (and thus proportional to the metabolic rate) preceding the period of asystole. Breathing was maintained despite the lack of pulmonary blood flow and the cessation of the muscle contractions, leading to a dramatic reduction in alveolar FCO(2) (1.9 ± 1%). Secondly, swings in arterial blood pressure (ABP) were observed (pulse pressure of 31 ± 3 Torr) in phase with breathing movements in place of the cardiac activity. This "protective" response, deprived from any role in blood gas homeostasis, as circulation is virtually abolished, is not predictable from the traditional respiratory control feedback systems thought to be involved in exercise. We are presenting the view that this response, dissociated from the pulmonary gas exchanges, is the expression of a rudimentary defense mechanism aimed at limiting the consequences of an acute failure of the cardiac pump by the thoraco-abdominal pump.