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.