Cardiovascular and hematological responses to voluntary apnea in humans
Abstract: This thesis deals with cardiovascular and hematological responses to voluntary apnea inhumans, with a special focus on O2 usage and storage. Humans, and many other air?breathinganimals, respond to apnea (breath holding) with a collection of interacting cardiovascularreflexes, which are collectively called the diving response. In humans, the main characteristics ofthe diving response are a reduction in heart rate (bradycardia), decreased cardiac output,peripheral vasoconstriction and increased arterial blood pressure. Another response duringapnea in mammals, more recently also observed in man, is a transient increase in hemoglobinconcentration across a series of apneas, probably due a reduction in spleen size. There may alsobe long?term effects on erythropoiesis in the apneic diver, as suggested by high hemoglobinlevels observed in divers. The focus of the included studies are the short transient divingresponse (I), the more slowly occurring transient hematological changes to apnea, most likelyrelated to a reduction in spleen size (II), and the possible effects of repeated apnea on serumerythropoietin concentration (III).I) The aim was to study the effects of body immersion on the O2?conserving effect of thehuman diving response. The results showed that, regardless of body immersion, apnea with faceimmersion 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 istriggered and conserves O2 even during whole?body immersion, which has previously only beenobserved during apnea without whole?body immersion.II) The aim was to study hematological responses to voluntary repeated maximal?durationapneas in divers and non?divers. Increases in hemoglobin concentration were found across aseries 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, whichsuggests a possible training effect of their extensive apnea?specific training. In contrast, physicalendurance 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 erythropoietinconcentration. 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 maximumincrease in erythropoietin of 24 % was found in untrained subjects after 15 maximal durationapneas, preceded by 1 min of hyperventilation. This suggests a possible erythropoietic effect ofapnea?induced hypoxia, which may connect the increased resting hemoglobin found in divers totheir apnea?specific training.It was concluded from these studies that man responds to apnea with a series of differentadjustments in order to limit O2 usage and increase O2 storage: The classical diving response iseffectively restricting O2?consumption also during full immersion, the spleen related hemoglobinincrease occurs in both divers and non?divers with different levels of physiological training, butis more prominent in divers, and finally, the observed high levels of hemoglobin concentration indivers may be related to enhanced erythropoiesis during dive training.
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