Cardiovascular and hematological responses to voluntary apnea in humans

Abstract: This thesis deals with cardiovascular and hematological responses to voluntary apnea in humans, with a special focus on O2 usage and storage. Humans, and many other air‐breathing animals, respond to apnea (breath holding) with a collection of interacting cardiovascular reflexes, which are collectively called the diving response. In humans, the main characteristics of the diving response are a reduction in heart rate (bradycardia), decreased cardiac output, peripheral vasoconstriction and increased arterial blood pressure. Another response during apnea in mammals, more recently also observed in man, is a transient increase in hemoglobin concentration across a series of apneas, probably due a reduction in spleen size. There may also be long‐term effects on erythropoiesis in the apneic diver, as suggested by high hemoglobin levels observed in divers. The focus of the included studies are the short transient diving response (I), the more slowly occurring transient hematological changes to apnea, most likely related to a reduction in spleen size (II), and the possible effects of repeated apnea on serum erythropoietin concentration (III). I) The aim was to study the effects of body immersion on the O2‐conserving effect of the human diving response. The results showed that, regardless of body immersion, apnea with face immersion causes a stronger cardiovascular diving response compared to during apnea alone, leading to a smaller reduction in arterial oxygen saturation levels. Thus the diving response is triggered and conserves O2 even during whole‐body immersion, which has previously only been observed during apnea without whole‐body immersion. II) The aim was to study hematological responses to voluntary repeated maximal‐duration apneas in divers and non‐divers. Increases in hemoglobin concentration were found across a series of 3 apneas in elite breath‐hold divers, elite cross‐country skiers and untrained subjects. However a larger increase in hemoglobin was found in divers compared to non‐divers, which suggests a possible training effect of their extensive apnea‐specific training. In contrast, physical endurance training does not appear to affect the hematological response to apnea. III) The aim was to study the effects of serial voluntary apnea on the serum erythropoietin concentration. In a comparison between elite breath‐hold divers and subjects untrained in apnea, divers were found to have a 5% higher resting hemoglobin concentration. An average maximum increase in erythropoietin of 24 % was found in untrained subjects after 15 maximal duration apneas, preceded by 1 min of hyperventilation. This suggests a possible erythropoietic effect of apnea‐induced hypoxia, which may connect the increased resting hemoglobin found in divers to their apnea‐specific training. It was concluded from these studies that man responds to apnea with a series of different adjustments in order to limit O2 usage and increase O2 storage: The classical diving response is effectively restricting O2‐consumption also during full immersion, the spleen related hemoglobin increase occurs in both divers and non‐divers with different levels of physiological training, but is more prominent in divers, and finally, the observed high levels of hemoglobin concentration in divers may be related to enhanced erythropoiesis during dive training.